Mixed AggregAte (MAA): A Single Concept for All Dipolar

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Mixed AggregAte (MAA): A Single Concept for All Dipolar Organometallic Aggregates. 2. Syntheses and Reactivities of Homo/HeteroMAAs Florence Mongin*,† and Anne Harrison-Marchand*,‡ †

Equipe Chimie et Photonique Moleculaires, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite de Rennes 1, Bât. 10A, Case 1003, Campus de Beaulieu, Avenue du General Leclerc, 35042 Rennes Cedex, France ‡ Laboratoire COBRA de l0 Universite de Rouen, INSA de Rouen, CNRS, UMR 6014 & FR 3038, IRCOF, Rue Tesniere, 76821 Mont St Aignan Cedex, France

bS Supporting Information

CONTENTS 1. Introduction 2. Formation and Main Features of MAAs 2.1. Formation of HomoMAAs and HeteroMAAs 2.1.1. Alkali Metal,Alkali Metal-MAAs 2.1.2. Ate MAAs 2.2. Main Features of MAAs 2.2.1. Lower- and Higher-Order Ate Compounds 2.2.2. 1,2-Migration of Ate Compounds 2.2.3. Migratory Aptitudes of Ligands from Ate Compounds 2.2.4. LiX Effect on Organometals 2.2.5. Contacted and Solvent-Separated Ion Pairs 2.2.6. Remark 3. Reactions of HeteroMAAs Involving Metal Oxidation/ Reduction Processes 3.1. One-Electron Transfers from HeteroMAAs 3.1.1. One-Electron Transfers from Li,Al-HeteroMAAs 3.1.2. One-Electron Transfers from Li,Ti-HeteroMAAs 3.1.3. One-Electron Transfers from Mg,V-HeteroMAAs 3.1.4. One-Electron Transfers from M,Mn-HeteroMAAs (M = Li, MgX) 3.1.5. One-Electron Transfers from M,Fe-HeteroMAAs (M = Li, MgX) 3.1.6. One-Electron Transfers from M,Co-HeteroMAAs (M = Li, MgX) r 2013 American Chemical Society

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3.1.7. One-Electron Transfers from Mg,Ni-HeteroMAAs 3.1.8. One-Electron Transfers from Li,Cu-HeteroMAAs 3.2. Two-Electron Transfers from HeteroMAAs 3.2.1. Two-electron transfers from Li,Cr-HeteroMAAs 3.2.2. Two-Electron Transfers from M,Mn-HeteroMAAs (M = Li, MgX) 3.2.3. Two-Electron Transfers from M,Fe-HeteroMAAs (M = Li, MgX) 3.2.4. Two-Electron Transfers from Li,Co-HeteroMAAs 3.2.5. Two-Electron Transfers from Mg,Ni-HeteroMAAs 3.2.6. Two-Electron Transfers from Li,Cu-HeteroMAAs 4. Reactions of MAAs Involving Nucleophile Ligand Transfer Processes (Nu-MAAs) 4.1. Reactions of Nu-HomoMAAs 4.1.1. Nu-Li,Li-HomoMAAs 4.1.2. Nu-Na,Na-HomoMAAs 4.1.3. Nu-Mg,Mg-HomoMAAs 4.2. Reactions of Nu-HeteroMAAs 4.2.1. Nu-Li,Na- and -Li,K-HeteroMAAs 4.2.2. Nu-M,Mg-HeteroMAAs (M = Li, Na, K) 4.2.3. Nu-M,Al-HeteroMAAs (M = Li, Na, K) 4.2.4. Nu-M,Ti-HeteroMAAs (M = Li, K, MgX) 4.2.5. Nu-Mg,Cr-HeteroMAAs 4.2.6. Nu-M,Mn-HeteroMAAs (M = Li, MgX) 4.2.7. Nu-Na,Fe-HeteroMAAs 4.2.8. Nu-Mg,Co-HeteroMAAs 4.2.9. Nu-M,Ni-HeteroMAAs (M = Li, MgX) 4.2.10. Nu-M,Cu-HeteroMAAs (M = Li, MgX) 4.2.11. Nu-M,Zn-HeteroMAAs (M = Li, Na, K, MgX) 5. Reactions of MAAs Involving Base Ligand Transfer Processes (Base-MAAs) 5.1. Reactions of Base-HomoMAAs 5.1.1. Base-Li,Li-HomoMAAs

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Chemical Reviews 5.1.2. Base-Na,Na-HomoMAAs 5.2. Reactions of Base-HeteroMAAs 5.2.1. Base-Li,Na- and -Li,K-HeteroMAAs 5.2.2. Base-Na,K-HeteroMAAs 5.2.3. Base-M,Mg-HeteroMAAs (M = Li, Na, K) 5.2.4. Base-M,Al-HeteroMAAs (M = Li, Na, K) 5.2.5. Base-Na,Cr-HeteroMAAs 5.2.6. Base-M,Mn-HeteroMAAs (M = Li, Na) 5.2.7. Base-M,Fe-HeteroMAAs (M = Li, Na, MgX) 5.2.8. Base-Li,Co-HeteroMAAs 5.2.9. Base-Li,Cu-HeteroMAAs 5.2.10. Base-M,Zn-HeteroMAAs (M = Li, Na, K, MgX) 6. Conclusion Author Information Biographies Acknowledgment Abbreviations Used References

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1. INTRODUCTION Mixing two organometallic reagents leads to synergic mixed aggregates or ate complexes for which we suggest (as well as for oligomers) the name “Mixed AggregAte” or MAA. The first category we will here consider for their behavior/reactivity are the “homoMAAs”, which contain only one type of metal but different ligands, such as those employed for enantioselective ligand transfers or as (unimetal) superbases. We will then turn to those containing at least two different metals, the “heteroMAAs”, for which known examples are mixed-alkali metal superbases and ate compounds. Wittig used the term “ate” to refer to the bimetal combinations containing a metal as part of a complex anion. Organo(bi)metallic and related ate complexes are the result of an interaction between a species for which the metal is a Lewis acidic (vacant orbital) and a second metallic species such as an organometal, an alkoxide, or an amide, with an anionic fragment behaving as a Lewis base.1 For example, if MeLi is added to a solution of Me2Mg in Et2O, the ether molecules around Mg are replaced by Me anions to form ate complexes of the type Me2+nMgLin. Wittig and co-workers introduced different ate compounds such as Ph3ZnLi and Ph3MgLi, the first magnesate, by direct combination of the homometallic compounds2 and observed that more stable adducts (Ph3HgLi < Ph3CdLi < Ph3MgLi < Ph3ZnLi < Ph3BeLi) are in general obtained with a smaller central metal/ a more electropositive group I metal.2 Wittig rationalized the unique reactivities or synergies that ate compounds exhibit (e.g., Bu2Zn and i-Pr2Zn only react with CO when combined with t-BuOK3) in terms of anionic activation,4 an idea shared by Tochtermann.1a Both ate complexes and combinations of alkali metal compounds are the sum/association of different substances or elements and have thus been called “aggregate” by Caubere. In addition, such associations may evolve from one kind to the other, notably as a function of the solvent, which can interact with the ionic species and cause structure modifications, and they can exhibit different behaviors.5 In accordance, ionization of dimer Li amides to form triple ions was for example observed upon treatment with hexamethylphosphoramide (HMPA).6 Similarities between ate complexes and combinations of alkali metal compounds

Scheme 1

Scheme 2

Scheme 3

Scheme 4

were also proposed in 2006 by Mulvey, who compared the deprotonating abilities of Bu3MgNa 3 TMEDA (TMEDA = N,N,N0 ,N0 tetramethylethylenediamine) and 1:1 BuLi 3 t-BuOK.7 This review is thus intended to depict the special chemistry exhibited when an alkali or alkaline earth metal compound (Li, Na, K, Mg) is combined either with another alkali metal compound (Li, Na, K) or with a compound containing a group 2 (Mg, Ca), group 4 (Ti), group 5 (V), group 6 (Cr), group 7 (Mn), group 8 (Fe), group 9 (Co), group 10 (Ni), group 11 (Cu), group 12 (Zn), or group 13 (Al) element as Lewis acidic center (the references published after 2010 have not been included). Note that borates have been excluded from the contents of this review. The review will be divided into four main sections [in addition to the introduction (section 1) and conclusion (section 6)]. Generalities concerning the formation and main features of the MAAs will be presented in section 2. Section 3 will collect examples of reactions for which central metal oxidation/reduction processes have been proposed; it will be divided into two subsections depending on whether a one-electron transfer (ET) (Scheme 1)1c or a two-electron transfer (Scheme 2)1c has been proposed. The next sections will illustrate the main synergies exhibited by the MAAs.8 Thus, section 4 will concern the nucleophilic transfer of a ligand (6¼H) from a MAA to a carbon site (Scheme 3),1c and it will be divided into Nu-homoMAAs and Nu-heteroMAAs depending on whether the MAA possesses identical or different metals, respectively. Reduction by hydride transfer and metal-mediated catalysis have been excluded from the contents of this review. Section 5 will gather the different MAAs used for deprotometalation purpose (Scheme 4),1c and comparisons with the MAAs used for halogen/metal exchange will be given. Note that the formulas used (e.g., Ph3ZnLi) will refer to the overall compositions but do not necessarily reflect the stoichiometries of the actual species present in solution.

2. FORMATION AND MAIN FEATURES OF MAAS 2.1. Formation of HomoMAAs and HeteroMAAs

2.1.1. Alkali Metal,Alkali Metal-MAAs. Li,Li-, Na,Na-, and K,K-HomoMAAs. In 1936, the first PhNa 3 NaCl suspensions 7564

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Chemical Reviews were prepared in benzene by reaction of Na with PhCl at 1530 °C9 and were, for example, used to prepare benzylsodium.10 PhK 3 KCl was prepared similarly using K and employed to metalate methylbenzenes.11 The access to the corresponding alkyl species RM 3 MCl (M = Na, K) proved more difficult.9b However, reacting alkyl and phenyl halides with Li resulted in highly more soluble RLi 3 LiX species.12 The formation of similar 1:1 alkyllithium 3 LiX complexes was more recently suspected after reaction between BuLi and LiBr (or LiI), EtLi and LiBr, and c-HexLi and LiBr in hydrocarbons. Such associations allow one to deactivate the alkyllithiums involved, but their initial reactivity can be restored upon addition of Et2O.13 Aggregation involving both organolithium compounds and Li alkoxides has been known for many years14 and considered as being responsible for the increasing carbon nucleophilicity/ basicity in the resulting combinations (increased polarization of the CLi bond). Brown and co-workers documented in 1962 the solubilization of EtOLi in benzene by EtLi and suggested the formation of a complex between both species.15 Lochmann and co-workers isolated 1:1 complexes between t-BuOLi, on the one hand, and BuLi, i-PrLi, or t-BuLi, on the other hand.16 Other mixed-ligand aggregate formations have been documented,17 for example, between organolithiums and LiCl,18 Li amides,19 Li phenolates,20 or LiSCN21 and between Li amides and LiCl,22 LiBr,23 or Li enolates.24 Na,Na complexes have also been documented.25 Li,Na-, Li,K-, and Na,K-HeteroMAAs. Metalmetal exchanges were performed (albeit in general not quantitatively)26 in apolar or polar solvents using different alkali metal alkoxides, and mixed-metal combinations such as BuNa (+t-BuOLi) from BuLi and t-BuONa,27 BuK (+t-BuOLi) from BuLi and t-BuOK,27a,28 and BuK (+t-PentOLi) from BuLi and t-PentOK29 were prepared. t-PentOK, MeEt2COK, Et3COK, and K-()-(1R)menthoxide were then employed for their better solubility in hydrocarbons.30 The stability of the adducts increases with the decreasing bulkiness of the organolithium and with the increasing bulkiness of the alkoxide.31 Unlike pure BuK, which attacks THF even at 100 °C, 1:1 BuLi 3 t-BuOK mixture is stable in THF at 75 °C and for short times at 50 °C.32 NMR,33 ESR,34 theoretical,35 and X-ray36 studies on mixed Li,Na and Li,K species have been reported. Similarly, combinations of Li enolates37 or Li dialkylamides38 with heavier alkali metal alkoxides give rise to the corresponding heavier derivatives. Generally, both metals are shared in a common structure.39 The solubility of these species can be increased by forming chelates (BuNa is for example solubilized upon addition of TMEDA)27b or ate compounds (e.g., BuNa is solubilized (and in the same time stabilized) by combination to (EtO(CH2)2O)2Mg).40 2.1.2. Ate MAAs. The tendency of organobimetallics to form ate complexes, as well as their stability, depends not only on the metals involved but also on the ligand size and charge repartition.41 Organometallics such as Li,42 Mg,43 and Zn44 compounds tend to dissociate in the presence of a strong complexing agent. For example, R2Mg-type compounds evolve in the presence of macrocyclic multidentate donors L with redistribution to LMgR+ and organomagnesates R3Mg. Mixed-metal compounds such as calciates, titanates, vanadates, chromates, manganates, ferrates, cobaltates, nickelates, cuprates, and zincates can be obtained by reaction of a corresponding inorganic salt (or a chelate) with polar organo-, amido-, and/or alkoxometallic compound (metathesis approach). It is of interest to note that salts

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such as LiCl are simultaneously formed in the syntheses performed according to this scheme. Alternative ways to metal organo, amido, and/or alkoxo ate compounds, such as magnesates, calciates, manganates, ferrates, nickelates, and zincates, use the corresponding monometallic compounds (cocomplexation approach). The syntheses of mixed-ligand organo, amido, and/or alkoxo heteroMAAs are in general performed by mixing precursors possessing different ligands in appropriate ratios. For instance, the tris(pyridyl) Al(III) complex MeAl(2-py)3Li 3 THF (THF = tetrahydrofuran) was prepared from 2-lithiopyridine and MeAlCl2 in a 3:1 molar ratio.45 Similarly, H3PhAlLi, H2Ph2AlLi, and HPh3AlLi were prepared from H3Al and PhLi, H2PhAl and PhLi, and HPh2Al and PhLi, respectively.46 Ligand redistribution reactions are also described for this purpose. For example, the syntheses of mixed ligands H3PhAlLi, H2Ph2AlLi, and HPh3AlLi were performed by mixing H4AlLi and Ph4AlLi in appropriate ratios.46 The structural data of these compounds have been summarized.47 It is important to note that, in the presence of O2 during the preparation of metal ate compounds such as magnesates,48 aluminates,49 manganates,50 and zincates,51 complex structures with oxo or peroxo cores can be formed, and some of them were evidenced by X-ray crystallography.7,52 Due to possible β-elimination from mixed compounds, encapsulation of hydrides represents another possibility.53 M,Mg- and M,Ca-HeteroMAAs (M = Li, Na, K, MgX). Wittig and co-workers studied Ph2Mg 3 PhLi systems and isolated mixed complexes such as Ph3MgLi2 (1:1 stoichiometry), the first M, Mg-heteroMAA, called magnesate. More generally, merging a diorganomagnesium and an organolithium compound stoichiometrically results in the formation of a Li trialkylmagnesate, whereas a 1:2 ratio leads to the formation of a Li tetraalkylmagnesate.54 The same is true with Na55 or MgX56 instead of Li. The diorganomagnesium can be formed in situ from an organolithium and a Grignard reagent.57 By using different amounts of Li reagent at low temperatures, Seitz and Brown identified in solution Mg ate complexes of at least three stoichiometries (Me3MgLi, Me4MgLi2, and Me5MgLi3) in ratios depending on the nature of R and the solvent (the same is true for the corresponding zincates).58 Alternately, a magnesate can be obtained by mixing an alkali organometal compound and a Mg halide in a 3:1 or a 4:1 ratio59 or even by reducting a dialkylmagnesium with Li.60 Mulvey and co-workers developed the synthesis of numerous mixed Li,Mg amides such as (Bn(Me2N(CH2)2)N)4MgLi2,61 (Bn2N)4MgLi2,61 (Bn2N)3MgLi 3 pyridine,61 HMDS3MgLi48b [HMDS = (Me3Si)2N], HMDS3MgLi 3 THF,62 HMDS3MgLi 3 pyridine,62 ((c-Hex)2N)3MgLi 3 THF,62 and t-Bu(HMDS)2MgLi.63 Na,Mg amides have also been prepared.63,64 Upon treatment with an alcohol (1 equiv), Li magnesates such as DA3MgM (DA = diisopropylamino, M = Li, Na) were converted to [{RO(DA)2MgM}2] (R = Bu, Oct).65 It is important to note that β-hydride elimination can occur when possible.54a,66 Alternative ways exist to obtain a given ligand-mixed magnesate. For example, Screttas and Micha-Screttas realized that the complex formed between Ph2Mg and EtO(CH2)2ONa, presumably (Ph2(EtO(CH2)2O)2MgNa2)n, is identical to that obtained from PhNa and (EtO(CH2)2O)2Mg.67 A similar conclusion was given for mixtures of BuLi with (EtO(CH2)2O)2Mg, and EtO(CH2)2OLi with Bu2Mg.68 Addition of Na or K alkoxides to R2Mg-type compounds in benzene leads to alkoxy-substituted organomagnesates; 1:1 organomagnesates also form by reaction of R2Mg with other salts such as tetrabutylammonium halides 7565


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Scheme 5

and Li amides, but reactions with KH can lead to R3MgK (+MgH2).69 The syntheses of calciates are less common.70 Mixed-metal amides HMDS3CaM (M = Li,71 K,72 MgHMDS73) were nevertheless prepared from the corresponding monometal amides; the Ca,K compounds were used to generate mixed-metal enolates from ketones such as 2,4,6-trimethylacetophenone.74 Higherorder Bn4CaLi2 3 2TMEDA was isolated as crystals in toluene by mixing together at room temperature (rt) the monometal organometallic reagents.75 Alternately, calciates can be obtained from CaI276 or Ca(OSO2-4-tolyl)2.77 M,Al-HeteroMAAs (M = Li, Na). If an ether solution of Me3Al is treated with MeLi, the ether solvating molecule is replaced by an Me carbanion to form Me4AlLi M,Al-heteroMAA or aluminate.78 The latter can be also prepared by adding 4 equiv of MeLi to AlCl3.79 Bu(i-Bu)3AlLi, which has been used for the reduction of substituted cyclohexanones, was prepared by mixing solutions containing equimolar quantities of (i-Bu)3Al and BuLi.80 The stereoselectivity of the reactions of α-lithio benzyl methyl sulfoxides with electrophiles depends upon the nature of the electrophile; it has been noted that their transmetalation with Et3Al occurs with inversion.81 Et4AlNa was synthesized from Et3Al and Na.82 Tetraamino Li,Al compounds were either prepared (i) by transmetalation of an Al halide with 4 equiv of the Li amide or (ii) by reaction of H4AlLi with 4 equiv of amine.83 It is pertinent to mention that ligand coupling of Li aluminates (and zincates) can be induced by VO(Oi-Pr)2Cl.84 M,Ti-HeteroMAAs (M = Li, MgX). It has been postulated that cheap (i-PrO)4Ti can be used to generate Li or Mg M,Ti-heteroMAAs, also called titanates, at low temperatures, for example, by reaction with MeLi,85 dihalomethyllithiums,86 allyllithiums,87 allylMgCl,87 functionalized phenyllithiums,88 and Li enolates.89 It was more recently observed that (i-PrO)4Ti can behave differently when treated with BuLi or alkylmagnesium bromides. Indeed, Kulinkovitch and co-workers showed that, in the presence of carboxylic esters or other unsaturated substrates, treating (i-PrO)4Ti with 2 equiv of EtMgBr results in the formation of a Ti(II)-ethylene complex, a transformation rationalized by the formation of an unstable diethyltitanium compound that rapidly loses ethane.90 According to the same authors, using only 1 equiv of EtMgBr allows one to generate (i-PrO)3Ti.91 It is also reported by Eisch and co-workers that (i-PrO)2Ti is formed (+2 equiv of i-PrOLi) using 2 equiv of BuLi, by alkylative reduction of the same Ti(IV) source and the subsequent and spontaneous reductive elimination of the two Bu groups as alkene and alkane.92 On the basis of EPR experiments, the authors

proposed a stabilization of the trimeric structure [(i-PrO)2Ti]3 in solution by coordination with i-PrOLi to form a trimeric biradical93 (Scheme 5). These results show that it is important to obtain more structural information about the species obtained by reacting Li or Mg compounds with Ti sources such as (RO)4Ti, (RO)3TiCl, (R2N)3TiCl, and (RO)2TiCl2.94 In addition, when Ti compounds are prepared from Li or Mg compounds and (RO)3TiCl or (R2N)3TiCl, salts are present in the reactions mixtures and, as pointed out by Reetz, can have an impact on reactions, for example, with the possible formation of Li or Mg titanates.95 Titanates can also be prepared from an inorganic Ti salt.96 For example, titanate(III) [(C6Cl5)4TiLi(THF)4] was synthesized in 2002 from TiCl3 and C6Cl5Li and its structure determined by X-ray diffraction.97 Li,V-HeteroMAAs. M,V-heteroMAAs such as Li tetraarylvanadates(III) can be obtained from VCl3 or VBr3 upon treatment with 4 equiv of an organolithium.98 For the same purpose, it is also possible to combine a triarylvanadium with a stoichiometric quantity of an organolithium.99 Vanadates(II) can be prepared from vanadocene and an organolithium.98b Using an excess of the Li compound results in the formation of higherorder vanadates.100 Note that R4V(III)Li and R4V(II)Li2 easily decompose to give R2V.98b Li,Cr-HeteroMAAs. Several organochromates(II)101 and -(III)102 have been prepared by adding Li compounds to CrCl2 and CrCl3, respectively. The order of the chromate formed largely depends on the stoichiometry; it is, for example, possible to generate Bu4CrLi, Bu5CrLi2, and even Bu6CrLi3, from CrCl3.102,103 M,Mn-HeteroMAAs (M = Li, MgX). M,Mn-heteroMAAs, or manganates, can be either prepared from an inorganic salt104 or by combining an organomanganese compound with a polar organometallic.105 Li triorganomanganates can be, for example, prepared by treating MnI2 with 3 equiv of an organolithium compound in Et2O.106 A less expensive and more efficient way employs soluble MnBr2 3 2LiBr as Mn source to perform the transmetalation of organolithium or organomagnesium compounds to salt-stabilized organomanganese halides.104g,107 In THF, soluble MnCl2 3 2LiCl can be used for the transmetalation of organomagnesium compounds.108 Li,Fe-HeteroMAAs. A M,Fe-heteroMAA, higher-order Li ferrate(II) Me4FeLi2, was isolated as Me4FeLi2 3 2Et2O and Me4FeLi2 3 1.25dioxane in 1973 by adding 5 equiv of MeLi to FeCl3 at a controlled temperature (Scheme 6).109 Alternative ways employ FeCl2.110 Alkyl cyanoferrates, cyanocobaltates, and cyanonickelates were also generated from M(CN)2 by transmetalation of MeLi or BuLi in an ether at 78 °C.111 7566


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Scheme 7

Scheme 8

The approach using an inorganic Fe salt was used to prepare ferrates(II)112 but also ferrates(III).113 To obtain ferrocenebased Fe(0)-ate complexes, ferrocene was treated with Li in the presence of either ethylene or 1,5-cyclooctadiene.114 Acylferrates, generated either by addition of organolithiums to Fe(CO)5, or by consecutive treatment of Fe(CO)5 with Na and acyl chloride, were converted into aldehydes and ketones by reaction with acids and alkyl halides, respectively.115 Li,Co-HeteroMAAs. Li,Co-heteroMAAs, or Li cobaltates, are in general obtained by transmetalation of alkyllithiums using CoCl2 (Scheme 7).116 M,Ni-HeteroMAAs (M = Li, MgX). The syntheses of low-order Me3NiLi and Me3NiMgBr have been described from NiCl2.117 Ni(II) compounds react with organolithiums in excess to give tetraorganonickelates(II) such as Ph4NiLi2 and Me4NiLi2.118 Fr€ohlich and co-workers have similarly treated nickelocene with 1,4-dilithiobutane in Et2O to afford higher-order ((CH2)4)2NiLi2 3 Et2O, a compound resistant to β-elimination.119 Li,Cu-HeteroMAAs120. Gilman and co-workers observed in 1952 that yellow MeCu insoluble in Et2O dissolves upon addition of 1 equiv of MeLi (Scheme 8).121 The Me2CuLi thus formed was the first Gilman reagent. Using CuCN instead of CuI with a RLi (or a Grignard) reagent results in the formation of lower-order RCu(CN)Li.122 The introduction of a second R0 ligand (different or identical) is also possible by addition of a second equiv of an organolithium (R0 Li), leading to higher-order (cyano-Gilman or Lipshutz) RR0 Cu(CN)Li2 (Scheme 9).123 The latter are more reactive than the corresponding lower-order cuprates.124 If R0 Li is a Li amide such as TMPLi, studies show that a Gilman-type cuprate forms instead in this case, with the exclusion of LiCN.125 Due to their high-lying HOMO, organocuprates are sensitive to oxidative agents such as O2, CuCl2 3 TMEDA, nitrobenzenes, or PhI(OAc)2 and yield the dimer of the organic residue. This possibility has been developed for synthesis, with the generation of CC and, more recently, CN bonds, in symmetrical and unsymmetrical derivatives.126 M,Zn-HeteroMAAs (M = Li, Na, K, MgX). Compounds of the form R3ZnM (R = Et,127 Me;128 M = alkali metal) have been known since 1858, when Wanklyn129 prepared R3ZnNa by reacting R2Zn with Na in Et2O.9b The reactions of K or Li with Et2Zn gave similar products (Scheme 10).129 Np3ZnK (Np = neopentyl), Np3ZnNa, and (Me3SiCH2)3ZnK were more recently obtained using the same approach.130

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Scheme 9

Scheme 10

Wittig and co-workers documented in 1951 the preparation of Ph3ZnLi in Et2O using the cocomplexation approach (from Ph2Zn and PhLi).2 On the basis of spectroscopic studies, Waack and Doran similarly suggested in 1963 the formation of a Li,ZnheteroMAA, or Li zincate, from Et2Zn and Ph2PentCLi.131 Schlenk and Holtz reported earlier as 1917 a similar access to Et3ZnLi (from Et2Zn and EtLi);132 nevertheless, whereas benzene seems to favor aggregates with high molecular weights,133 the addition of THF to such solutions results in deaggregation.134 In contrast, Westerhausen and co-workers observed that (Me3Si)2CHLi and ((Me3Si)2CH)2Zn only form a zincate after addition of Lewis bases able to bind to the electropositive cation.135 The metathesis approach (alternative to the cocomplexation136) in general employs ZnCl2,137 or its TMEDA chelate,138 and a Li or Mg compound; t-Bu3ZnLi was, for example, prepared by this way. The method using a Zn salt offers the advantage of avoiding prior knowledge of two concentrations.139 Mixed-ligand zincates are prepared using similar procedures by assuming that the group exchange is slow; for example, RMe2ZnM (M = Li, MgCl) can be prepared by simply mixing Me2Zn and RM.139 Since the outer shell of the Zn atom in a triorganozincate is filled with 16 electrons, an additional ligand can coordinate to Zn, leading to a favorable 18 electron state. Indeed, studies show that R3ZnLi but also R4ZnLi2 can be prepared from R2Zn and RLi in a 1:1 and 1:2 mole ratio, respectively.78a,117b,137a From Me4ZnLi2, an Me group can be replaced by a CN or SCN ligand when treated with Me3SiCN and Me3SiNCS, to afford Me3Zn(CN)Li2 and Me3Zn(SCN)Li2, respectively.140 Zn diamides such as HMDS2Zn cannot be prepared from R2Zn (R = Me, Bu) and the corresponding amine, and thus, R(HMDS)2ZnM (M = Li, Na, K) are the results of reactions between HMDSM and R2Zn 3 HMDS. By reacting ZnCl2 with HMDSLi (2 equiv), HMDS2Zn can be prepared and converted to solvent-separated141 [Na(12-crown-4)2][HMDS3Zn] in an arene solvent containing the crown ether.51a The method cannot be used to prepare HMDS3ZnLi, due to the failure of HMDS2Zn to associate with HMDSLi,7 whereas it is possible to access HMDS3ZnM (M = Na, K) using HMDSNa or HMDSK.142 Hevia, Mulvey, and co-workers showed in 2008 that transamination reactions using DAH, HMDSH, and (R)-N-benzyl-α-methylbenzylamine are possible, starting from t-Bu2(TMP)ZnNa 3 TMEDA, with concomitant loss of TMPH,143 but more common ways to prepare alkyldiaminozincates employ the cocomplexation approach.144 It is interesting to note that cocomplexation furnishing a zincate can be accompanied by unexpected reactions due to special synergic effects. The group of Mulvey for example showed that N,N0 -diisopropylethylenediamine, when treated by a 1:1 mixture of R2Zn (or R = t-Bu, Me) and BuLi in hexane containing 7567


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TMEDA (1 equiv), affords [RZn(i-PrNCHdCHN-i-Pr)Li(TMEDA)], a reaction that could involve a metalation/ β-hydride elimination/remetalation sequence.145 2.2. Main Features of MAAs

2.2.1. Lower- and Higher-Order Ate Compounds. When nonalkali metals such as Mg, Al, Fe, Co, or Zn are present in heteroMAAs, either lower- or higher-order ate compounds are possible, depending on the number of ligands around the central metal. The higher-order (or highly coordinated) ate compounds are generally more reactive than the lower-order ones. For example, lower-order cuprates R2CuLi obtained from CuI and RLi (2 equiv; R = Me, Pr, Bu, Ph) are less reactive in coupling reactions than presumably R2Cu(SCN)Li2 prepared from CuSCN and RLi (2 equiv), and the reactivity of the former is restored upon addition of 1 equiv of LiSCN.146 Another example concerns the deprotometalation of toluene employing Na magnesates, a reaction possible using DABCO-activated (DABCO = 1,4-diazabicyclo[2.2.2]octane) Bu4MgNa2 (the supramolecular storage of discrete Bu4Mg anions in interstices within the polycationic network was proposed as responsible for the anion activation) but not using DABCO-activated Bu3MgNa (ion-pair structure).55b The same is true for zincates,147 for which 1H NMR helps to compare metal reagents. For example, the field shift values of 1.08 and 1.44 ppm observed in THF at 20 °C for the Me signals in Me3ZnLi and Me4ZnLi2, respectively, are an indication of the more anionic character of Me4ZnLi2 over Me3ZnLi.140 However, if the presence of higher-order zincates has been confirmed by NMR and X-ray studies, the equilibrium between Me3ZnLi and MeLi, on the one hand, and Me4ZnLi2, on the Scheme 11

Table 1. Reactions of Organometals with α-Chloroorganolithiums

RMLn

R0

1 2

Cp2Bu2Ti Ph3V

Ph Me

13a 56a

67 b

b 22

3

Ph2Cr

Me

76a

b

b

a

4

b

b

b

entry

yields (%)

4

Ph2Mn

Me

82

5

Bu2Mn

Ph

97a

b

6

Ph2Fe

29

7

Ph2Co

12a

b

70

8

Ph2Ni(PEt3)2

9a

b

65

9

Ph2CuLi

35a

b

b

a

0

Me

39

a

RCH(D)SiMe2R is obtained (>95% D incorporation) by quenching with D2O. b Not determined.

other hand, lies far to the left in solution because of Coulombic repulsion (Scheme 11).148 The presence of coordinating/ chelating ligands such as TMEDA can help to dissolve species and thus modify the position of the equilibrium.149 2.2.2. 1,2-Migration of Ate Compounds. 1,2-Migration of ligands from nonalkali metals observed with higher-order ate compounds is a consequence of their unstability/higher reactivity. Negishi and co-workers examined reactions of organometals containing a wide variety of transition metals, with various α- and γ-halo organolithiums; they found that for most of the metals of the first transition series, CC bond formation can be promoted this way (Table 1). Whereas 1,2-migration products are obtained in high yields using V, Cr, and Mn organometals, those with Ti, Fe, Co, Ni, and Cu do not form satisfactorily due to competitive reactions.150 2.2.3. Migratory Aptitudes of Ligands from Ate Compounds. The migratory aptitudes of ligands from nonalkali metals of ate compounds has been the subject of some studies. Alkynyl,151 CN,123b,152 2-thienyl,153 and Me154 are well-known examples of nontransferable or “dummy” organoligands of cuprates.155 For example, the mixed-ligand cuprates R(Me)Cu(CN)Li2 (R = alkyl, alkenyl, aryl) preferentially transfer their R group in substitution reactions with secondary bromides or iodides;154a in ring-openings of mono-, di-, and trisubstituted epoxides;154a and in conjugate additions to α,β-unsaturated ketones;154b several factors such as aggregation state, extent of backbonding between a ligand and Cu, and stability of both R(Me)Cu(CN)Li2 and MeCu(CN)Li are proposed to explain this selectivity. Singer and Oehlschlager described in 1991 the unexpected behavior of Me(PhMe2Si)Cu(CN)Li2.156 Indeed, even if displacement of MeLi is observed when PhMe2SiLi is added to solutions of Me(PhMe2Si)Cu(CN)Li2 or Me2Cu(CN)Li2, Me(PhMe2Si)Cu(CN)Li2 preferentially transfers its PhMe2Si ligand in conjugate addition, epoxide ring-opening, and primary alkyl bromide substitution reactions, but in different ratios. A qualitative hypothesis derived from the HSAB principle and frontier orbital theory is proposed to rationalize the results observed experimentally. Bertz and co-workers documented in 1996 the efficiency of BuCu(CH2SiMe3)Li 3 LiI, combining in CH2SiMe3 both a dummy group due to its stabilizing effect in relation with the β-Si and an activating group favoring the migration of the neighbor ligand.157 Nakamura and Mori summarized in 2000 the reasons for the selective transfer of an alkyl, alkenyl, or aryl group over an alkynyl group in the course of the 1,4-addition of mixed cuprates MeCu(Y)Li (Scheme 12).158 In 2003, Bottoni et al. reported an investigation on the boron to carbon 1,2-shift of different RR0 R00 (ClCH2)BLi species.159 On the basis of experimental results and theoretical calculations at the DFT (density functional theory) and MP2 levels, the authors proposed a model where the observed migrations are the result of an interplay between two opposite factors, a steric effect (it favors the most sterically demanding migrating groups) and a charge effect (associated with the partial carbanionic nature of the migrating organo ligand, it favors the less substituted migrating groups).

Scheme 12

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Chemical Reviews Wright and co-workers showed that MeAl(2-py)3Li 3 THF behaves as a pyridyl-transfer agent when it reacts with CuCl to afford [{Cu(2-py)}3]∞.45 Nevertheless, a generalization is delicate; for example, Me3BuAlLi essentially butylates 2-methylcyclopentanone, but mainly methylates 4-tert-butylcyclohexanone.160 2.2.4. LiX Effect on Organometals161. The presence of Li salts can affect the ionic strength of a system, but can also modify the nature of the reacting species (aggregation change). For example, combined with Li halides, organolithiums exist both in the crystal and in solution as mixed aggregates and can differ from ate complexes by the absence of charge separation;162 of course, such a phenomenon has a strong impact on the organolithium reactivity. A possible explanation for the beneficial effect of LiCl in reactions such as direct insertion of Zn into organic halides163 and subsequent reaction of such generated organozinc halides164 has recently been given by Koszinowski and B€ohrer.165 Using anion-mode electrospray ionization (ESI) to study LiClmediated Zn insertion reactions into various organic halides, the authors observed the formation of organozincates [as well as free RZnCl2 (R = Me, Bu, s-Bu, t-Bu, 2-thienyl) and related polynuclear complexes from ZnCl2 and RLi (1 equiv)]. The authors attributed the efficiency of these reactions to the formed zincates, since the latter have been described as more reactive species than the corresponding diorganozinc compounds.166 Similar LiCl (or other Li halide)-mediated enhancements were for example also reported for Grignard167 and Cr-mediated reactions,168 as well as for alkylalkyl Negishi cross-couplings.169 In addition, the stability of moderately polar reagents such as organomanganese halides is increased in the presence of Li salts by formation of ate complexes, and subsequent reactions such as addition to aldehydes and ketones are favored.104g,107 The presence of Li salts has an important impact on asymmetric synthesis and catalysis results. They represent a drawback in some reactions (e.g., when LiCl is present during enantioselective addition of phenylmetals to prochiral electrophiles170) but an advantage in others (e.g., if Li triflate is present in the aldol addition of a γ-hydroxy ester162). 2.2.5. Contacted and Solvent-Separated Ion Pairs. Bimetal compounds can be divided into contacted ion pairs (CIPs) and separated ion pairs (SIPs). The metal and ligand components, but also the solvent and additives (e.g., TMEDA),171 determine whether solvent-SIPs or CIPs are formed.51e,172 For example, the use of HMPA can reduce the formation of aggregates and favor the formation of solvent-SIPs.173 By mixing MeLi and Me2Zn at 0 °C in Et2O/toluene containing diglyme, structures corresponding to solvent-SIPs [Li(diglyme)2][Me3Zn] were evidenced both in the solid state and in solution; in contrast, if diglyme is replaced by N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine (PMDTA), CIPs [MeZnMe2Li(PMDTA)] were identified instead.174 The knowledge of such structural features is fundamental, since close proximity of the two metals (CIP) is required in reactions such as alkali-metal-mediated metalations. The ability of Me4BLi and Me4AlLi to form CIPs or solventSIPs has been examined. Me4AlLi is present predominantly as CIPs in Et2O and as solvent-SIPs in 1,2-dimethoxyethane (DME). Me4BLi is less inclined to form CIPs.175 Na salts such as Et4AlNa and Bu4AlNa are essentially in the form of CIPs in noncoordinating solvents such as hydrocarbons, whereas in basic enough coordinating solvents such as THF, Et2O, or DME they exist mainly as solvent-SIPs. Compared with Li salts, Na salts are dissociated more easily in the form of solvent-SIPs. For example,

REVIEW

while Bu4AlLi exists mainly in Et2O as CIPs, Et4AlNa in the same solvent is largely in the form of solvent-SIPs.176 On the basis of 1H and 7Li NMR studies of ether solutions of MeLi 3 Me4BLi, MeLi 3 Me4AlLi, Me4AlLi 3 Me4BLi, Me3Al 3 Me4AlLi, Me3Al 3 Me4BLi, and EtLi 3 Et4AlLi mixtures, Williams and Brown inferred that alkyl group exchange is slow on the NMR time scale for all these systems, except for Me3Al 3 Me4AlLi, for which the rate-determining process appeared to be the formation of solvent-SIPs (Li+//Me4Al) from CIPs (Me4AlLi).78b Li organocuprates are known to form solvent-SIPs in THF. In contrast, CIPs predominate in Et2O due to poorer coordinating capabilities.177 On the basis of a larger range of species on the ESI mass spectra, Koszinowski and B€ohrer presumed that the equilibration between CIPs and SIPs of Li organozincates in THF proceeds too fast to be resolved by NMR spectroscopy, even at low temperatures.178 2.2.6. Remark. Bimetallic combinations such as alkali MAAs,179 magnesates,180 aluminates,180e,181 chromates,182 manganates,183 cuprates,184 and zincates,180e,185 have been used for polymerization purposes, but this topic will not be detailed in this review.

3. REACTIONS OF HETEROMAAS INVOLVING METAL OXIDATION/REDUCTION PROCESSES HeteroMAAs for which one metal is not at its highest possible oxidation state (heteroMAAs that only contain alkali or alkaline earth metals and some others are not concerned) can take advantage of this situation through one-ET (Scheme 1) or two-ET (Scheme 2).1c In this way, they can not only attack aldehydes, ketones, epoxides, and SN1-active alkyl halides but also SN1-inactive alkyl halides, 1-alkenyl, aryl, and 1-alkynyl halides, phenyl isocyanate, and azomethines, as well as in some cases electrophilic heteroarenes, nitriles, esters, amides, and CO2.186 Thus, ET from the metal was, for example, advanced to explain the reaction of organomanganates with CO2 to give the corresponding carboxylic acids (Table 2)104b and with acid chlorides (Tables 3 and 4 and Scheme 13),104g,106108 acid anhydrides,187 or phosgene188 to give the corresponding ketones.189 Manganates, and in particular higher-order R4Mn(MgX)2, are efficient in these acylation reactions, reacting instantaneously in THF at 78 °C.190 3.1. One-Electron Transfers from HeteroMAAs191

Examples of ate compound reactions for which a mechanism involving a single electron transfer (SET) (Scheme 1) has been proposed will be presented in this part. It has to be noted that the formation of aggregates favors SET with possible stabilization by delocalization over the whole aggregate (“cluster effect”).5a The ET ability of Mn, Fe, and Co ate compounds has been evaluated. By using in situ attenuated total reflectance IR spectroscopy, Uchiyama and co-workers monitored the transformation Table 2. Reactions of Organomanganates with CO2

entry

7569

R3MnM

RCO2H

yield (%)

1

Bu3MnMgCl

BuCO2H

2

Ph3MnLi

PhCO2H

86 89

3 4

(mesityl)3MnLi (Me2CdCH)3MnLi

mesitylCO2H Me2CdCHCO2H

76 96

5

(BuCtC)3MnLi

BuCtCCO2H

83

6

(Pr2CdCHCH2)3MnMgCl

Pr2C(CO2H)vinyl

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of benzophenone to the corresponding ketyl radical by transitionmetal ate complexes: the decrease of the CdO stretching vibration and the increase of a new absorption assigned to the CO stretching vibration of the newly generated ketyl species allowed the authors to propose an electron-release reaction from Me3MnLi, Me3FeLi, and Me3CoLi. Electrochemical measurements based on differential pulse voltammograms were applied for quantitative estimation of the ET abilities of these ate compounds (Table 5) and showed high oxidation potentials intermediate between those of SmI2 and Mg. A catalytic ET system composed of such a transition-metal ate complex and Mg was discovered (Scheme 14) and proved effective for the desulfonylation of N-phenylsulfonyl amides, the cleavage of O-allyl groups, the reduction of NO2 groups, the partial reduction of diketones, and the reductive coupling of diphenyliodonium salt.192 The tunability of the ET ability of ate complex was demonstrated through changing factors such as the type and number of ligands and the type of countercations. Ligands with lower σ-donating ability were found to reduce the ET ability. In contrast, dianion-type ate complexes have higher oxidation potentials (more negative) than the corresponding monoaniontype ate complexes. 3.1.1. One-Electron Transfers from Li,Al-HeteroMAAs. From H4AlLi, a one-ET has been proposed as a key step in the reaction pathway of the reduction of iodides or bromides.193 3.1.2. One-Electron Transfers from Li,Ti-HeteroMAAs. By using 1:2 (i-PrO)2Ti 3 i-PrOLi [generated in THF from (i-PrO)4Ti and 2 equiv of BuLi], Eisch and Gitua observed unusual reactions such as the conversion of benzonitrile to the corresponding isopropyl phenyl ketone (92% yield after 24 h Table 3. Reactions of Organomanganates with Acid Chlorides

R0

entry

R

yield (%)

1

BuCtC

Bu

7075

2

Bu

Hept

7580

contact at 25 °C followed by hydrolysis) and the clean and rapid deoxygenation of cis-stilbene oxides to give the more stable transstilbene (91% yield).194 On this basis, and taking account of the sensitivity of these reactions to steric hindrance, SET processes from (i-PrO)4TiLi2 have been proposed (Scheme 15) to explain these unexpected results. 3.1.3. One-Electron Transfers from Mg,V-HeteroMAAs. Oshima and co-workers proposed a mechanism involving a SET from a vanadate to explain the V-catalyzed cross-coupling reactions of aryl Grignard reagents with alkyl halides.195 Putative Ph4VMgBr, formed in THF from VCl3 and PhMgBr (4 equiv), reacts in situ with cyclohexyl bromide to afford the coupled product in 58% yield (Scheme 16). 3.1.4. One-Electron Transfers from M,Mn-HeteroMAAs (M = Li, MgX)196. Organomanganates(II) can react either as ligand-donating (see section 4.2.6) or as one-electron-donating reagents depending on the substrates employed. Toward 2-cyclohexenone, organomanganese halides react to give mixtures of three products resulting from 1,2-addition, 1,4-addition, and β-reductive dimerization. In contrast, the 1,2-addition products are generally no more obtained using symmetrical organomanganeses or organomanganates prepared from organolithium or -magnesium compounds.197 Results obtained by Cahiez and Alami using THF as solvent are summarized in Table 6. It is important to note that the manganates for which β-elimination is possible give the β-dimerization products in good yields to the detriment of the expected 1,4-adducts (entries 1315). Using an organomagnesium in the presence of a Mn salt (catalytic amount) also allows for the β-reductive dimerization product from 2-cyclohexenone, but the scope of the reaction is limited.197 To favor 1,4-adducts, alkylidene malonic esters that are strong Michael acceptors have been employed with success.198 Oshima and co-workers used 2-cyclohexenone to attempt reactions with mixed-ligand R2R0 MnMgX199 and observed the Table 5. Estimation of the ET Abilities of Ate Compounds

Table 4. Reactions of Organomanganates with Acid Chlorides a

entry

R0

R

yield (%)

1

Bu

Hept

97

2

Bu

Me2CdCH

95

3 4

Me2CdCH

Ph Hept

92 87

5

Ph

Bu

95

6

BuCtC

Bu

95

entry

metal reagent

E° (V)a

1 2

SmI2 Me3FeLi

2.33 2.50

3

Me3MnLi

2.56

4

Me3CoLi

2.60

5

Mg

3.05

Measured in 0.1 Bu4NClO4 in THF vs Ag/AgCl at 0 °C.

Scheme 14

Scheme 13

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Scheme 15

Scheme 16

Table 6. Reaction of 2-Cyclohexenone with Different Mn Reagents Prepared from RM

entry

Mn reagenta

yields (%)

entry

10

1 2

yields (%) 59 1

17b

a a

3

PhS

17

27

a

5

4

Ph

51

8

a

30

50

5

Me

50

Me > Bu > HexCtC > Me3SiCH2) (see section 4.2.6), the authors proposed for these 1,4-additions a radical process mediated by oneET from the manganate(II) reagent to the carbonyl substrate. Pinacol couplings also observed by treating aromatic aldehydes with manganates were supposed to proceed similarly (Scheme 17). The same authors documented a method for the preparation of dihydrobenzofuran, indoline, and 2-alkoxytetrahydrofuran

derivatives by Bu3Mn(II)-induced radical cyclization of allyl 2-iodophenyl ether, N,N-diallyl-2-iodoaniline, and 2-iodoethanal acetal, respectively.200 A similar reaction mechanism was proposed to explain the formation of 3-isopropenyl-2,3-dihydrobenzofuran by treatment of 2-iodophenyl prenyl ether with a 7571


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Scheme 18

Scheme 19

Scheme 20

Bu3Mn(II), the latter being generated from MnCl2 and 3 equiv of either BuLi or BuMgBr (Scheme 18). 3.1.5. One-Electron Transfers from M,Fe-HeteroMAAs (M = Li, MgX). The first mechanistic suggestions for the Feassisted CC coupling reactions of Grignard reagents advanced in the course of the original investigations by Kharasch and coworkers201 and further by Kochi and co-worker,191a,202 involve SET steps and radical intermediates. A radical pathway is still topical for Fe-based couplings of aryl Grignard reagents with alkyl halides, in which the reduced Fe catalyst enters a oneelectron redox pathway and an alkyl radical is formed from the alkyl halide.203 One-ETs were suspected in the 1970s to explain peculiar results observed in the course of reactions of Grignard reagents with ketones performed in the presence of FeCl3.204 A one-ET process was similarly proposed to explain the behavior of Me4FeLi2 toward 2-cyclohexenone giving a dihydro dimer (Scheme 19).186,205 Upon treatment with Me3FeLi, substrates with a nonpolarized CC multiple bond such as (Z)-stilbene can be rearranged to (E)-stilbene. In the light of the byproducts formed, a mechanism involving a one-electron oxidative addition was proposed (Scheme 20).186 Oshima and co-workers reported in 1998 the synthesis of dihydrobenzofurans from allyl 2-halophenyl ethers upon FeCl2catalyzed reactions with PhMgBr. On the basis of experiments

using the corresponding Fe ate compounds, a mechanism involving a SET was proposed (Scheme 21).206 3.1.6. One-Electron Transfers from M,Co-HeteroMAAs (M = Li, MgX).207 A pathway involving a Co(II)/Co(III) couple has been suggested for the Me4CoLi2-mediated conversion of ethyl benzoate to α,α-dimethyl benzyl alcohol and for the “1,4addition” of Me4CoLi2 to 2-cyclohexenone (Scheme 22).116a On the basis of studies performed with different degrees of methylation of the transition metal, an oxidative addition/reductive elimination sequence has rather been advanced for the corresponding reaction of Me3FeLi.186 Oshima and co-workers proposed a mechanism involving a SET from a 17-electron cobaltate to rationalize Co-catalyzed phenylative cyclizations.196c,208 The stoichiometric reaction of the bromo acetal depicted in Scheme 23 with a Co complex, prepared from CoCl2 3 dppe [dppe = 1,2-bis(diphenylphosphino)ethane] and PhMgBr (4 equiv), affords the phenylated product in 31% yield, whereas using 2 or 3 equiv of PhMgBr does not allow its formation. Improved yields were obtained using catalytic amounts of CoCl2 3 dppe.209 A mechanism involving a similar SET from a 17-electron cobaltate was proposed for Co-catalyzed MizorokiHeck-type reactions of alkyl halides or epoxides with styrenes.210 3.1.7. One-Electron Transfers from Mg,Ni-HeteroMAAs. Kambe and co-workers developed a Ni-catalyzed three-component cross-coupling reaction of alkyl halides, 1,3-butadienes, and ArMgX. In this reaction, alkyl radical species are generated in situ from the alkyl halide by SET from a nickelate complex (Scheme 24).211 This possibility was extended in 2009 to arylalkynes and to enynes.212 3.1.8. One-Electron Transfers from Li,Cu-HeteroMAAs. SN2 substitution reactions of an organocopper(I) reagent with an alkyl halide in general proceed in pathways involving Cu(I)/ Cu(III) couples (see section 4.2.10). For secondary iodides that lose stereochemistry during these reactions, a single-electron process has rather been proposed in these special cases.213 3.2. Two-Electron Transfers from HeteroMAAs

Examples of ate compound reactions for which a mechanism involving a two-electron process (Scheme 2) has been proposed will be presented in this part. 3.2.1. Two-electron transfers from Li,Cr-HeteroMAAs. Hosomi and co-workers discovered in 2001 new reactivities for Cr(III) ate-type reagents easily prepared from CrCl3 and BuLi. Bu5CrLi2 reacts with allylic and propargylic phosphates to generate 7572


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Scheme 21

Scheme 22

Scheme 23

Scheme 24

the corresponding allyl- and propargylchromium reagents that further react with a variety of electrophiles to afford the corresponding adducts in high yields.102 Bu6CrLi3 similarly behaves as a formal two-electron reductant with ketones and esters bearing a leaving group at the α-position to produce enolates that are intercepted with a variety of electrophiles with high selectivities (Table 8).103 Such enolates are also capable of reacting with oxiranes to afford γ- and β-hydroxy esters, depending on the Lewis acid used as promoter.214

3.2.2. Two-Electron Transfers from M,Mn-HeteroMAAs (M = Li, MgX). In 1976, Cahiez et al. disclosed the possible reduction in THF of alkenyl bromides or iodides, as well as aryl chlorides or bromides, using i-PrMgCl in the presence of a catalytic amount of MnCl2.215 The authors proposed the in situ formation of unstable i-Pr3MnMgCl and its decomposition by β-elimination to H3MnMgCl; reaction of the latter with the halide would then afford an unstable Mn(IV) intermediate, able 7573


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Table 8. Reaction of Bu6CrLi3 with Ketones and Esters Bearing a Leaving Group at the α-Position and Subsequent Electrophilic Interception

entry

R1, R2, R3, X

conditions

yield (%)

syn/anti ratio

1

Et, Me, H, Br

78 °C, 1 h

PhCHO (CH(OH)Ph)

88

68:32

2

Ph, Me, H, OAc

20 °C, 1 h

EtCHO (CH(OH)Et)

82

75:25

3

i-PrCHO (CH(OH)-i-Pr)

77

71:29

4

MeCO2CH2Br (CH2CO2Me)

69

5

OctI (Oct)

60

6

Ph, Me, Me, OAc

20 °C, 1 h

PhCHO (CH(OH)Ph)

85

7 8

EtO, Me, H, Br

40 °C, 1 h

HexCHO (CH(OH)Hex) Bu2C(O) (C(OH)Bu2)

88 80

HCtCCH2Br (CH2CtCH)

62

EtO, Me, Me, Br

20 °C, 1 h

HexCHO (CH(OH)Hex)

95

9 10 11

c-HexCHO (CH(OH)c-Hex)

79

12

2-furylCHO (CH(OH)-2-furyl)

96

13

Bu2C(O) (C(OH)Bu2)

97

14

2-cyclohexenone (CH(CH2)3C(O)CH2)

60a

15 16

HCtCCH2Br (CH2CtCH) allylBr (allyl)

87 86

17

NCCH2Br (CH2CN)

82

18

MeC(O)CH2CO2Me (C(OH)Me(CH2CO2Me)

74

PhCHdNTs (CH(NHTs)Ph)

88

19 a

electrophile (E)

78 °C, 1 h

80:20

Yield of 1,2-adduct was 38%.

Scheme 25

Scheme 26

to provide the reduced product by reductive elimination (Scheme 25). The same year, they reported homocouplings of alkenyl halides by reaction with BuLi in the presence of catalytic MnCl2.216 The reaction seems to proceed via a Li manganate(II)

according to the mechanism given in Scheme 26. The method has since been improved using O2 as oxidant.217 Cross-couplings proved possible between Li triorganomanganates, prepared by transmetalation of the corresponding Li compounds, and halides. Corey and Posner found in 1970 that 7574


Chemical Reviews Me3MnLi and, to a lesser extent, Bu3MnLi can react with alkyl, vinyl, allyl, and phenyl bromides or iodides (Table 9, entries 110).117a,218 Oshima and co-workers reported reactions using an allylmanganate219 (Table 9, entries 1114); they also showed that Mg manganates including (R3Si)3MnMgMe are suitable for stereospecific coupling reactions with vinyl halides (Br, I), vinyl sulfides, enol phosphates, enol triflates, allyl sulfides, or allyl ethers,220 and they proposed a mechanism (Scheme 27) to explain these reactions.220b Mn-catalyzed reactions of Grignard reagents have since been described.221 Kauffmann and co-workers studied the effect of neighboring groups on vinyl bromides during cross-coupling reactions with higher-order R4MnLi2.110b,116c,222 By performing competitive experiments between pairs of substrates, the authors observed that OH, OMe, and CN accelerate the reactions (Table 10). Table 9. Reaction between Li Manganates and Halides

REVIEW

On this basis, they favored the second mechanism depicted in Scheme 28. Me4FeLi2 behaves similarly.218,222,223 Hosomi and co-workers evidenced in 1997 a new reactivity of organomanganese ate complexes as reductant.224 Ketones (or esters and amides) bearing a leaving group (OAc, OSiR3, Br, I) at the α-position can be reduced to afford the corresponding ketone enolates, which were trapped by aldehydes. The authors proposed an oxidative addition of the substrate to the ate complex followed by a reductive elimination of the ligands on Mn (a process different from that of a halogen/metal exchange) to explain the reductive generation of enolates from α-acetoxypropiophenone (Scheme 29). The authors emphasized the similarity of the reaction with previously described organocupratemediated generation of enolates from α-halogeno ketones reported by Posner and co-workers225 and with Bu3MnLimediated partial reductive dimerization of cyclohexenone at the β-position during 1,4-addition reactions described by Cahiez and co-workers.197 A more recent study performed with iodomethyl sulfides showed that the reaction mechanism depends on the reaction temperature.226 Indeed, a halogen/metal exchange proceeds at very low temperature, while at higher temperature oxidative addition followed by β-elimination or reductive elimination becomes predominant. Allyl and propargyl Mn reagents have also been generated by the direct reduction of the corresponding bromides using either Bu3MnLi or Bu4MnLi2 at low temperatures.227 The organomanganese derivatives prepared using Bu4MnLi2 were quenched with aldehydes, ketones, epoxides, and chlorosilanes. The authors associated their characteristic reactivity (e.g., favored Table 10. Competitive Experiments Using R4MnLi2

Scheme 27

entry

R4MnLi2 (x)

yields (%)

1

Me4MnLi2 (0.67)

14

78

2

Bu4MnLi2 (1)

10

74

3

Me4MnLi2 (1)

H) in the presence of Li (1R,2S)-N-pyrrolidinylnorephedrate were developed.293 It was noted that when the acetylidealkoxide mixture generated at low temperature is warmed to rt to establish 7581


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aggregate equilibration (giving a single 2:2 acetylidealkoxide mixture)294 prior to cooling and reaction, a higher enantioselectivity is obtained.292 As for c-PrCtCLi,294 it was observed by employing either PhCtCLi295 or BuLi296 in the presence of a chiral Li alkoxide that the enantioselectivities are significantly dependent on the structures in solution, with the best results obtained with stoichiometries favoring a C2-symmetric 2:2 RLiR*OLi mixture. Jiang and Feng documented in 2002 the use of the chiral C2-symmetric diamino diol shown in Table 22 to perform the addition of different Li acetylides to the same ketone. The enantioselectivity obtained using c-PrCtCLi (99% ee) proved higher than those using PhCtCLi (75%), BuCtCLi Table 19. Addition of BuM to PhCHO in the Presence of Camphor-Based Chiral Ligands (PhCHOBuMligand 1:4:5.2)

(53%), TBDMSOCH2CtCLi (49%), and t-BuCtCLi (11%). The same ligand was also employed to perform alkynylations of benzaldehyde (Table 22).297 That mixed aggregate formation induced by Li additives is at the origin of the outcome of aldol reactions and Michael additions was suggested early on by Seebach.162 The presence of Li alkoxides and phenoxides in aldol reactions between benzaldehyde and the Li enolates derived from tert-butyl propionate or cyclohexanone modifies the ratio of stereoisomers.298 Majewski and Gleave noted that aldol reactions can lose stereoselectivity if BuLi, used to generate DALi, is contaminated with water or if LiBr is added to the reaction mixture.299 Chiral Li amides have been employed to achieve enantioselective aldol reactions162,298,300 (see for example Schemes 48162,298 and 49300a) but also carboxylation (Scheme 50)301 and protonation.302 As Li alkoxides, Li amides play an important role in the addition of Li reagents to aldehydes. This was evidenced in 1984 by Eleveld and Hogeveen, who employed ligands prepared from (S)-α-methylbenzylamine to perform enantioselective additions of BuLi to benzaldehyde (Table 23).303 In the case Table 21. Addition of BuLi to PhCHO in the Presence of Li Fencholates (PhCHOBuLiligand ratio 1:1:1 and 1:1:3)

entry

Table 20. Addition of BuM to PhCHO in the Presence of Camphor-Based Alkoxides

a

X

yielda (%)

eea (%)

1

H

73 (30)

8 (14)

2

SiMe3

86 (99)

66 (76)

3

t-Bu

81 (99)

55 (62)

4

SiMe2-t-Bu

84 (99)

51 (56)

5

Me

76 (92)

24 (28)

Values for the 1:1:3 ratio are in parentheses.

Table 22. Addition of RCtCLi to PhCHO in the Presence of a Chiral Dialkoxide

X, R1, R2

entry

M

ee (%) in Et2O or THF

1 2

OM, H, Ph

Li MgCl

1 (R) or 1 (R) 48 (R) or 13 (R)

3

OMgCl, Me, Ph

MgCl

8 (R) or 43 (S)

1

c-Pr

85

15

4

OMgCl, Ph, Me

MgCl

18 (R)

2

Ph

82

30

5

OMgCl, Ph, Ph

MgCl

9 (R) or 10 (S)

3

Bu

84

21

6

OM, Ph, 2-OMC6H4

Li

33 (R)

4

TBDMSOCH2

84

99

MgCl

49 (S)

5

t-Bu

80

53

7

entry

R

yield (%)

ee (%)

Scheme 47

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Scheme 48

Table 24. Addition of BuLi to PhCHO in the Presence of Chiral Li Amides (PhCHOBuLiligand ratio 1:1.8:4)

entry

Scheme 49

R

R0

X

ee (%) 72 (S)a

1

(S)-PhCH(Me)

Ph

OMe

2

(R)-PhCH(i-Pr)

Ph

OMe

75 (S)

3

(S)-2-LiC6H4CH(Me)

Ph

OMe

8 (S)

4 5

Me

Ph i-Pr

N(CH2)4

7 (R) 7 (R)

Ph

OMe

2 (S)

Ph

N(CH2)4

26 (S)

OMe

82 (S)b

6 7

i-Pr

8

Scheme 50

a

Maximum 72% ee in 1:1 DMMEt2O; the 90% ee previously obtained by Eleveld and Hogeveen could not be reproduced. b 91% ee in the presence of DMM.

Table 25. Addition of BuLi to R0 CHO in the Presence of Chiral Li Amides (RCHOBuLiligand ratio 1:1.8:4) Table 23. Addition of BuLi to PhCHO in the Presence of Li Amides Possessing Two Chiral Centers (PhCHOBuLiligand ratio 1:2.7:4)

entry

R, Ar

yield (%)

ee (%)

R

1

(S)-PhCH(Me)

2

i-Pr

3

(S)-PhCH(Me)

4

i-Pr

5

(S)-PhCH(Me)

R0 Ph

ee (%) 72 91

c-Hex

91 99

3-Pent

90

THF, 90 °C

72

14

6

i-Pr

2

Et2O, 100 °C

65

12

7

(S)-PhCH(Me)

i-Pr

96

3

DMM, 100 °C

82

7

THF, 90 °C

63

40

8 9

i-Pr (S)-PhCH(Me)

t-Bu

99 58

10

1

4

H, Ph

H, 2-Py

5

Et2O, 100 °C

73

19

6 7

DMM, 100 °C THF, 90 °C

62 51

37 54

DMM, 100 °C

47

65 68

H, 2-anisyl

8

THF, 90 °C

77

10

Et2O, 100 °C

83 (80)a

74 (81)a

11

DMM, 100 °C

90

83

1:1 DMMEt2O, 120 °C

83

90

DMM, 100 °C

74

14

9

OMe, Ph

12 13 a

solvent, temp

entry

OLi, Ph

i-Pr

65

11

Scheme 51

At 120 °C.

of (R)-2-methoxy-1-phenylethyl (S)-1-phenylethyl Li amide (Table 23, entry 9), Hilmersson and Davidsson showed in 1995 that the 1:1 complex between the Li amide and BuLi (both 6Li), which is strong at low temperatures, is responsible for the asymmetric induction observed. They also observed the presence of more reactive tetrameric BuLi, explaining the incomplete enantioselectivity obtained.304 Davidsson and coworkers also performed similar reactions using Li amides as chiral additives in Et2O at 116 °C (Table 24) and noted that the asymmetric induction is controlled by the chiral center between

R0 and X. They discarded a possible stereoselective autoinduction, the bidentate chiral Li amides employed being better complexation agents for BuLi than monodentate alkoxides.305 Employing 1:1 Et2ODMM as solvent, the most efficient Li amides were used to allow enantioselective transfers of BuLi to other aldehydes (Table 25). The results show that the steric hindrance of the R0 group of the aldehyde has a strong impact on the enantioselectivity. The presence of the Li amide accelerates 7583


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Table 28. Addition of RLi to PhCHOa in the Presence of Chiral Li Amides Bearing a Thioether/Ether Function

Scheme 52

Table 26. Addition of BuLi to PhCHO in the Presence of Chiral Li Amides Bearing an Alkoxy Group (PhCHOBuLiligand ratio 1:4.5:10)

a

In Et2O. b In 1:1 Et2ODMM. pentanetoluene.

c

In 1:1 Et2OTHF.

d

In 1:1

Table 27. Addition of BuLi to PhCHO in the Presence of Chiral Li Amides (PhCHORLiligand ratio 1:2:4)

entry 1

R, R0

(S)-PhCH(Me), (R)-CH(Ph)CH2OMe Et2O

3 4

i-Pr, (S)-CH(Ph)CH2OEt

5

i-Pr, (S)-CH(Ph)CH2OMe

Et2O Et2O

3-Pent, (S)-CH(Ph)CH2OMe

Et2O

4 (S) 34 (S)

1:1 THFEt2O 23 (S) 28 (S)

1:1 THFEt2O 17 (R)

10 11 12

3-Pent, (S)-CH(Ph)CH2OEt

13

c-Hex, (R)-CH(Ph)CH2OMe

Et2O 22 (S) 1:1 THFEt2O 14 (S) Et2O

30 (R)

1:1 THFEt2O 49 (S)

14 16

Et2O 30 (S) 1:1 THFEt2O 13 (S)

i-Pr, (S)-CH(Ph)CH2O-i-Pr

8

15

45 (R)

1:1 THFEt2O 55 (R)

6

9

ee (%)

1:1 THFEt2O 55 (S)

2

7

solvent

c-Hex, (R)-CH(Ph)CH2OEt

Et2O

43 (R)

1:1 THFEt2O 10 (R)

the addition reaction and is the origin of the high enantioselectivity. Due to the slow formation of mixed dimers, it is important

a For results using other aldehydes, see the Supporting Information (Table S1). b ArCHORLiligand ratio 1:4.5:10. c ArCHORLi ligand ratio 1:2:4.

to have obtained them before introduction of the aldehyde; indeed, both [BuLi]4 and the Li amide dimer can also add to the aldehyde (Scheme 51).306 Davidsson and co-workers reported in 2000 the addition of BuLi to benzaldehyde in the presence of the Li amides depicted in Scheme 52. The moderate enantioselectivities obtained (40 and 30% ee) were attributed to the lack of available site for the substrate.307 Using different solvents, Hilmersson and co-workers attempted the enantioselective addition to benzaldehyde of BuLi complexed to chiral Li amides bearing an internal coordinating alkoxy group (Table 26).308 A similar study performed using lithioacetonitrile instead of BuLi showed important changes in stereoselectivity as a function of the solvent (Table 27).309 Li amides bearing an internal ether group have been compared with the corresponding ligands containing a thioether in the reaction 7584


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Table 29. Addition of BuLi to PhCHO in the Presence of Chiral Li Amides Bearing PPh2, SPh, or OPh

entry

R, X

ee (%)

1

i-Pr, PPh2

93

2

i-Pr, SPh

68

3

Bn, PPh2

82

4

Bn, SPh

68

5

Ph, PPh2

98

6

Ph, SPh

98

7

Ph, OPh

96

Scheme 53

Table 32. Addition of BuLi to 2-Tolualdehyde in the Presence of 3-Aminopyrrolidine-Based Chiral Li Amides (ArCHOBuLiLi amide ratio 1:2.5:1.5)

ee (%) entry 1

R0

R Si-i-Pr3

2 3

78 °C

116 °C

Ph

60 (S)

76

t-Bu 2-furyl

75 (S) 59

83 74

4

TBDMS

t-Bu

54 (S)

5

Me

t-Bu

6 (R)

6

Bu

t-Bu

25 (R)

Table 31. Addition of BuLi to ArCHO in the Presence of 3-Aminopyrrolidine-Based Chiral Li Amides (ArCHOBuLiLi amide ratio 1:2.5:1.5)

entry 1

R Bn

2 3 4 5

CH2-t-Bu CH2-1-naphthyl

Ar

yield (%)

ee (%)

Ph

60

20 (R)

2-anisyl

61

37

2-tolyl

57

49 (R)

Ph Ph

50 56

17 (R) 18 (R)

6

CH2-2-anisyl

Ph

54

3 (R)

7

C(O)Ph

2-anisyl

54

4

8

c-Hex

2-tolyl

63

67 (R)

9

c-Pent

2-tolyl

63

63 (R)

10

CHPh2

2-tolyl

70

73 (R)

of alkyllithiums with aldehydes such as benzaldehyde; surprisingly (because OLi interaction is supposed to be stronger than that of SLi), the latter gave higher enantioselectivities (compared with pure Et2O, 1:1 Et2OTHF gives higher ee values).

R, R0

entry

Table 30. Addition of BuLi to R0 CHO in the Presence of Chiral Li Amides Bearing an Ether Function

yield (%)

ee (%)

1

Ph, Bn

57

49 (R)

2

Ph, CHPh2

77

73 (R)

3

Ph, c-Hex

72

67 (R)

4

Ph, c-Pent

63

63 (R)

5

Me, c-Hex

72

63 (R)

6 7

Me, CHPh2 MeOCH2, c-Hex

66 80

64 (R) 51 (R)

8

2-anisyl, c-Hex

71

60 (R)

9

1-naphthyl, c-Hex

65

50 (R)

10

2-naphthyl, c-Hex

67

76 (R)

11

2-naphthyl, CHPh2

98

64 (R)

12

2-naphthyl, (R)-CH(Me)Ph

95

77 (R)

13

2-naphthyl, (S)-CH(Me)Ph

91

51 (S)

14 15

H, (R)-CH(Me)Ph H, (S)-CH(Me)Ph

45 66

80 (R) 74 (S)

The use of Li amides bearing a thioether group has been extended to alkynylation (Table 28).310 Replacing the thioether by a phosphine proved equivalent and even superior in some examples (Table 29).311 If the chiral amides depicted above form in ethers 1:1 mixed dimers with BuLi, they form in hydrocarbons 2:1 Li amide 3 BuLi mixed trimers.312 Williard and co-workers analyzed their use in pentane to alkylate aldehydes (Table 30). The authors identified a 2:1 Li amide 3 Li alkoxide mixed trimer and suggested that this complex exhibits a product-induced chirality inhibition phenomenon that is detrimental to the asymmetric addition.313 Duhamel and co-workers first showed in 1997 that several chiral 3-alkylaminopyrrolidine Li amides coming from 4-hydroxyL-proline can be used to perform asymmetric addition of BuLi to aromatic aldehydes (Table 31314 and Scheme 53315). The structure of the chiral amide used as ligand for the asymmetric addition of the alkyllithium to 2-tolualdehyde was then examined. The authors observed the importance of a second asymmetric center, an α-methylbenzyl group, on the lateral 3-amino group, giving access, depending on its configuration, to one or the other of the produced alcohol (Table 32).315,316 Using efficient Li amides as ligand, different aldehydes (Table 33)315 and organolithiums (Table 33315,317 and Table 34302) were tested: 2-tolualdehyde and 2-anisaldehyde gave the highest enantioselectivities, but it is also possible to use BuLi, MeLi, and, to a lesser extent, PhLi. Running the model reaction in about stoichiometric amounts 7585


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(ArCHOBuLiLi amide ratio 1:1.2:1.2) does not alter the results, showing that the aggregate is more reactive than the free alkyllithium. Table 33. Addition of RLi to R0 CHO in the Presence of 3-Aminopyrrolidine-Based Chiral Li Amides (R0 CHORLiLi amide ratio 1:2.5:1.5)

entry

R

1

Bu

2 3

yield (%)

Ph

80

42 (R)

1-naphthyl 2-anisyl

65 63

55 67 (R)

ee (%)

4

2-naphthyl

69

33 (R)

5

t-Bu

32

17

Ph 2-anisyl

76 90

57 (R) 76

6

Me

7a 8

Ph

9 a

R0

Pr

32

28 (S)

Bu

60

18 (S)

With (R)-N(Li)CH(Ph)CF3 instead of N(Li)-c-Hex.

Table 34. Addition of RLi to 2-Tolualdehyde in the Presence of 3-Aminopyrrolidine-Based Chiral Li Amides (2-tolyl-CHORLiLi amide ratio 1:2.2:1)

entry 1

a

R Me

yield (%) a

100 (100)

ee (%) 58 (5)a (R)

2

Bu

81

70 (R)

3

t-Bu

77

0

4

Ph

75

58 (S)

In the presence of 2.2 equiv of LiBr.

Using high-field multinuclear NMR, Maddaluno and co-workers identified the three-dimensional arrangements of the 1:1 6Li amide 3 Bu6Li complexes in coordinating solvents (THF, Et2O) as a result of both steric repulsions and aggregation forces (Scheme 54, left). Depending on the configuration of the lateral chiral group, the binding of MeLi can take place along the “exo” or the “endo” face of this structure.316,318 The authors proposed an interpretation accounting for the sense and degree of induction experimentally observed for which the selective docking of the carbonyl compound on one specific Li site would be responsible for a steric differentiation between the two faces of the prochiral center (Scheme 54, right).318a,d The results obtained by DFT computations on the complex between 3-N-methylamino-N-methylpyrrolidine Li amide and MeLi support the norbornyl-like folding adopted by the pyrrolidine ring in the model inferred from the experimental NMR data.319 They also suggested that the formation of the mixed aggregates is under kinetic control.316 The docking of the aldehyde on one Li of the mixed aggregate has been analyzed through DFT calculations on simple models (Scheme 55).320 On the basis of both chemical and spectroscopic grounds, it was observed that the aggregation between Li amide and nucleophile has to be performed at 20 °C before the introduction of the electrophile at 78 °C.321 NMR studies showed that toluene, which disfavors Li amide 3 alkyllithium mixed aggregates, is not an adequate solvent for these reactions.318d In addition, the detrimental effect observed using LiX (X = Cl, Br) as additive was rationalized on the basis of NMR data, which showed that the Li amide 3 MeLi mixed aggregate can be converted to 1:1 Li amide 3 LiX.322 Chiral Li amide 3 vinyllithium mixed aggregates were similarly employed to vinylate aromatic aldehydes in enantioselectivities up to 70% in THF at 78 °C (Table 35).318c In 2008, a set of new 3-amino heterocycles was synthesized and evaluated, and a 3-aminotetrahydrofuran derivative was identified as the best ligand. It resulted from this study that the sense of induction does not depend on the absolute configuration at the 3-position of the pyrrolidine ring, but at the first stereogenic carbon of the lateral N-chain (Tables 36 and 37) when the latter is present. Concerning the reactivity, the authors described the complexes as assemblings of three independent layers: the heart of the

Scheme 54

Scheme 55

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Table 35. Addition of (E)-Me3SiCHdCHLi to ArCHO in the Presence of a 3-Aminopyrrolidine-Based Chiral Li Amide

entry 1 2

Ar

yield (%)

ee (%)

Ph 2-tolyl

57 36

46 61 29

3

4-F3CC6H4

44

4

4-anisyl

31

48

5

2-ClC6H4

46

50

6

2,5-(MeO)2C6H3

33

59

7

2-EtOC6H4

39

52

8

2-Cl-6-FC6H3

55

32

2-naphthyl 1-naphthyl

45 50

55 70

9 10

Scheme 56

Table 38. Addition of a Benzyllithium to ArCHO in the Presence of a Chiral Li Pyrrolidide

entry

Table 36. Addition of BuLi to 2-Tolualdehyde in the Presence of a 3-Aminopyrrolidine-Based Chiral Li Amide (2-tolyl-CHORLiLi amide ratio 1:2.5:1.5)

Ar

yield (%)

ee (%)

1

Ph

54

68 (S)

2

4-tolyl

57

62 (S)

3

2-tolyl

60

67 (S)

4 5

4-anisyl 4-ClC6H4

46 49

56 (S) 65 (S)

Scheme 57

entry

R, R0

yield (%)

ee (%)

1

H, (R)-CH(Me)-1-naphthyl

74

75 (R)

2

H, (R)-CH(Me)-2-naphthyl

72

80 (R)

3

H, (S)-CH(Me)-2-naphthyl

68

76 (S)

4

H, (R)-CH(Me)-c-Hex

58

74 (R)

5

H, (R)-CH(Me)-t-Bu

54

66 (R)

6

69

67 (S)

7

H, (R)-CH(Ph)CH2N(CH2)4 H, (S)-CH(Ph)CH2N(CH2)5

56

46 (R)

8 9

H, (R)-CH(Ph)CH2N(CH2)5 CH2NMe2, (R)-CH(Me)Ph

63 58

66 (S) 80 (R)

CH2NMe2, (S)-CH(Me)Ph

54

66 (S)

10

Table 37. Addition of BuLi to 2-Tolualdehyde in the Presence of Different Chiral Li Amides (2-tolyl-CHORLiLi amide ratio 1:2.5:1.5)

entry

X, R0

yield (%)

ee (%)

1

NMe, (R)-CH(Me)Ph

62

79 (R)

2

NMe, (S)-CH(Me)Ph

56

65 (S)

3

S, (R)-CH(Me)Ph

68

58 (R)

4 5

S, (S)-CH(Me)Ph O, (R)-CH(Me)Ph

70 66

46 (S) 80 (R)

6

O, (S)-CH(Me)Ph

71

80 (S)

structure contains the reacting NLiCLi quadrilateral and is surrounded on one side by the heterocycle (shield) and on the other side by both the lateral N-chain and the aldehyde, the former influencing the position of the latter (selector) (Scheme 56).323 In the course of the asymmetric synthesis of 3-aryl-3,4dihydroisocoumarins, Singh and co-workers employed in 2010 the Li amide depicted in Table 38 to perform trappings of a 2-substituted benzyllithium with aromatic aldehydes in moderate enantioselectivities.324 Reactions with Imines. Itsuno and co-workers reported in 1991 enantioselective addition reactions of BuLi to N-(trimethylsilyl)benzaldehyde imine in the presence of Li alkoxides generated from chiral alcohols or diols (Scheme 57). The use of a Grignard reagent in place of an organolithium led to both lower yield and selectivity. A TS (Scheme 58) was proposed to rationalize the enantioselectivities observed.325 7587


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Scheme 58

Scheme 59

Table 39. Addition of Li Acetylides to Cyclic N-Acyl Ketimines in the Presence of the Li Alkoxide of Quinine

Scheme 60

The proline-derived Li salt of (2S,20 S)-2-hydroxymethyl-1[(pyrrolidin-2-yl)methyl]pyrrolidine was employed in 1995 as chiral ligand for the asymmetric addition of BuLi to N-borylbenzaldehyde imine. The reaction performed at 78 °C afforded the expected amine in 72% yield and 33% ee (R).326 LiBr was tested as additive in the addition of Li (and Mg) alkyl (and allyl) reagents to chiral imines coming from 1-(2-methoxyphenyl)ethylamine, and this attempt resulted in slightly higher diastereoselectivities.327 The Li alkoxide of quinine was employed in order to perform an enantioselective Li acetylide addition to the cyclic N-acyl ketimine shown in Table 39. Among the N-protections tested, including benzyl and Me groups, 9-anthrylmethyl proved the best. Using 4-methoxybenzyl, other β-aminoalkoxides were employed, but with lower ee values: (1R,2S)-ephedrine (6%, S), (1R,2S)-N-methylephedrine (10%, R), quinine (59% ee, S), dihydroquinine (64%, S), cinchonidine (26%, S), quinidine (55%, R), dihydroquinidine (39%, R), and 9-epiquinine (28%, S). The enantioselectivity of the reaction proved to vary with the reaction temperature and the concentration.328 In the course of their synthesis of the HIV-1 non-nucleoside reverse transcriptase inhibitor DPC 963, Nugent and co-workers found that chiral Li alkoxides such as the (+)-carene derivative shown in Scheme 59 are suitable to perform the enantioselective addition of c-PrCtCLi to an unprotected N-acylketimine in THF.329 HMDSLi has been identified as an efficient proton scavenger for the formation of both the Li alkoxide and c-PrCtCLi since it is not involved in mixed aggregates.330 In the light of NMR studies, Collum and co-workers reported in 2004 the species generated upon treatment of quinazolinones

and phenylacetylene with HMDSLi. They showed, for the 1,2-addition of PhCtCLi to quinazolinones, a rate law consistent with the addition of a disolvated PhCCLi monomer to a Li quinazolinide 3 PhCtCLi mixed dimer. In the case of the methoxymethyl ether-protected quinazolinone, the same reaction implicates instead trisolvated PhCtCLi monomers.331 Conjugate Additions. Chiral Li amides have been employed to achieve Michael additions to nitroolefins162 (see, for example, Scheme 60298). The presence of PhOLi can have an impact on 1,2- to 1,4addition product ratios, as for allyllithium additions to α,βethylenic aldimines.332 More recent studies concern the use of mixed aggregates between chiral 3-aminopyrrolidine Li amide and Li ester enolate for enantioselective conjugate addition to α,β-unsaturated esters (Scheme 61).333 The results were rationalized in 2009 on the basis of NMR data and theoretical calculations. The 1:1 mixed aggregates formed in THF between the Li enolate and the amide bearing the (S)- and (R)-chain proved to be endo and exo complexes, respectively; the former could lead to the (+)-enantiomer and the latter to the ()-enantiomer (Scheme 62).334 Reactions with Halides. Jackman and Dunne studied the role of Li salts on the regiochemistry of the methylation using methyl tosylate of the Li enolate salt of isobutyrophenone. They observed that Li tosylate generated during the reaction (but also added LiClO4) can increase the rates of both the C- and O-methylation (as well as the O- vs C-methylation); these results were attributed to a possible formation of mixed aggregates335 more reactive than the initial tetramer lithioisobutyrophenone.336 In search of an efficient method to perform couplings between Li acetylides and crotyl chloride, Yamamoto and co-workers discovered that the presence of LiI provides a solution (Scheme 63).337 The intervention of mixed aggregates with Li salts can have an inhibiting effect on one reaction and the opposite effect on another, depending on whether the reaction occurs more readily via an ionization pathway or on an aggregate framework. It has been, for example, shown that the dehydrohalogenation of aryl 7588


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Scheme 61

Scheme 62

Scheme 63

halides using Li piperidide is inhibited by Li halides; in contrast, rate studies of the N-alkylation of Li diphenylamide with bromobutane in THF/hydrocarbon mixtures indicated the different effect of LiBr.23,338 Using UVvis spectroscopy and coupled equilibria, AbuHasanayn and Streitwieser characterized the mixed aggregate formed between the Li enolate of 4-(phenylsulfonyl)isobutyrophenone and LiBr in THF and noted that the rate of alkylation of the enolate was inhibited by LiBr.339 The rate constants for the alkylation in THF of mixed aggregates between Li enolates [coming from 4-(phenylsulfonyl)isobutyrophenone and 6-phenyl-2-benzyl-αtetralone] and HMDSLi have been similarly studied and were found to be substantially smaller than those of the monomeric enolates.340 Seebach and Wasmuth described in 1981 their surprise when observing that the alkylation of an aspartic acid derivative could occur without loss of optical activity and already suggested, among the possible explanations given, the formation of mixed aggregates.341 LiBr was presented by some authors as an essential ingredient in the control of the diastereoselectivity of chiral amine-mediated alkylation of Li enolates342 and in the control of the enantioselectivity of Li enolate protonation in the presence of chiral additives.342c,343 In 1990, an enantioselective alkylation at the α-position of cyclohexanone and 1-tetralone after formation of their Li enolates using a chiral Li amide in the presence of LiBr was published (Table 40).344 Coldham and co-workers developed an access to a variety of N-alkyl345 and N-Boc345b,346 2-substituted pyrrolidines by dynamic resolution of the corresponding racemic 2-lithio compounds in the presence of chiral prolinol-based Li alkoxides. A thermodynamic preference for one of the diastereomeric complexes is responsible for the asymmetric induction observed. Reactions with Alkenes. Lochmann et al. investigated in 1972 the effect of Li alkoxides on the reaction rate of the addition of BuLi to 1,1-diphenylethene347 and noted that Li

Table 40. Alkylation of Li Enolates Generated Using a Chiral Li Amide in the Presence of LiBr

entry

substrate

RX

yield (%)

ee (%)

cyclohexanone

BnBr PhCHdCHCH2Br

63 60

92 87

allylBr

41

80

1-tetralone

BnBr

89

92

5

PhCHdCHCH2Br

93

88

6

MeI

71

88

1 2 3 4

Table 41. Addition of RMgX to Ketones and Aldehydes in the Presence of Carbohydrate Alkoxides

entry

RMgX

R1, R2

yield (%)

ee (%) 65 (R)

1

MeMgI

Ph, c-Hex

88

2

MeMgBr

Ph, Et

95

70 (R)

70

24 (R)

3 4

7589

EtMgBr

Ph, Me

45

27 (S)

5 6

c-HexMgBr

Ph, H Ph, Me

81 50

24 (R) 28 (S)

7

PhMgBr

c-Hex, Me

60

26 (S)

8

Me, Et

70

10 (S)

9

Ph, H

60

9 (R)


Chemical Reviews ()-(R)-menthoxide has the strongest impact.31 Klumpp and co-workers observed in 1994 that the reaction of s-BuLi with ethylene in benzene only produces 3-methylpentyllithium when 3-methoxypropyllithium is present.348 4.1.2. Nu-Na,Na-HomoMAAs. Morton and Brachman observed that the presence of Na alkoxides accelerates the Wurtz CC couplings between organosodiums and alkyl halides,349 and rearrangement reactions are also accelerated.350 NaCl was similarly found to activate 2-sodioanisole toward PentCl, and PhCl was found to be even better.351 A similar alkylation acceleration of carbanions, generated from Ph3CH and Ph2CH2 in the presence of K+-doped H2NNa 3 t-BuONa, was observed by Caubere and co-workers. The latter attributed this result to an “aggregative activation” in which the insertion of cations, different from the ones present in a complex base, increases the aggregates dissymmetry and thus modify the efficiency of reagents.352 4.1.3. Nu-Mg,Mg-HomoMAAs. Several studies were devoted to the impact of Mg salts on alkylation reactions using Mg reagents.353 It was, for example, shown that MgBr2 does not

REVIEW

catalyze addition of ethylmagnesium reagents to 3-pentanone but suppresses the tendency of in situ formed alkoxides to give enolization or reduction byproducts.353b Inch and co-workers reported in 1969 the stereoselective addition of Grignard reagents to ketones and aldehydes in Et2O containing carbohydrates such as 1,2:5,6-di-O-isopropylidene-α-D-glucofuranose (Table 41).354 Meyers and Ford described in 1974 a similar study, but using a chiral oxazoline-based Table 42. Addition of RMgX to Ketones and Aldehydes in the Presence of a Chiral Oxazoline-Based Mg Alkoxide

entry

Scheme 64

R1, R2

R Me

yield (%)

ee (%)

1

Ph, Et

80

25 (S)

2

c-Hex, Ph

90

11 (S)

3

Ph, H

92

12 (R)

4

Hex, H

86

17 (R)

5

t-Bu, H

80

12 (R)

6

Me, Et

98

9 (S)

7 8

c-Hex, Me t-Bu, H

98 95

13 (S) 21 (S)

Ph

Scheme 65

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Chemical Reviews

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Mg alkoxide in THF; under these conditions, lower enantioselectivities were obtained (Table 42).355 Baggett and co-workers proposed in 1982 a stereochemical interpretation of these results (Scheme 64).356 Table 43. Addition of RMgX to a Nitrone in the Presence of Bromomagnesium (2S,3R)-4-Dimethylamino-1,2-diphenyl3-methyl-2-butoxide

entry

RMgX

additive

1

EtMgCl

2 3

EtMgBr

MgBr2

solvent

yield (%)

ee (%)

THF

65

43 (S)

THF THF

67 42

52 (S)a 71 (S)

4

Et2O

54

75 (S)

5

MgBr2

Et2O

26

77 (S)

6

MgBr2

DME

33

90 (S)

7

EtMgI

THF

84

50 (S)

8b

MeMgBr

MgBr2

THF

41

80 (S)c

DME

40

60 (S)

9

The value was 57% ee (R) using Et2Zn at rt. b 78 to 35 °C. c The value was 66% ee (R) using Me2Zn without additive at rt.

a

Scheme 66

Weber and Seebach first achieved in 1992 highly enantioselective additions of Grignard reagents to ketones in the presence of equimolar amounts of Mg TADDOLate (Scheme 65). The best results were obtained using THF as solvent, without steric hindrance at the reacting centers, and employing alkyl bromides instead of chlorides. The authors observed that the absence of Mg halide from the reaction mixture (using Et2Mg instead of EtMgX) slows down the reactions and that the Mg TADDOLate accelerates the reaction but is deactivated by the alkoxide formed.357 Enantioselective additions of Grignard reagents to a nitrone in the presence of bromomagnesium (2S,3R)-4-dimethylamino1,2-diphenyl-3-methyl-2-butoxide were reported by Ukaji et al. in 1993. Unlike the organomagnesiums, which attack from the si-face of the nitrone, the corresponding dialkylzincs attack from the re-face (Table 43).358 In the course of the synthesis of chiral α-amino-2-alkylthiazoles, Tejero and co-workers studied the alkylation of N-benzyl-α-(2-thiazolyl)nitrone using Grignard reagents in the presence of chiral alkoxides and obtained the higher enantioselectivity employing D-glucose diacetonide and 0.5 equiv of ZnBr2 (Scheme 66).359 4.2. Reactions of Nu-HeteroMAAs

4.2.1. Nu-Li,Na- and -Li,K-HeteroMAAs. Wittig and coworkers observed in 1955 that PhNa becomes soluble and unable to metalate Et2O in the presence of 1 equiv PhLi, giving Ph2NaLi. This “phenyllithium sodium” proved capable of outperforming PhLi in reactions such as nucleophilic addition to benzophenone.360 It was identified in 1988 as the ate compound (Ph4Li)(Na 3 TMEDA)3.361 Different behaviors were exhibited from other RLi 3 RNa combinations.362 Heavy alkali metal alkoxides exhibit a stronger impact on the reactivity of organolithiums than Li alkoxides due to metal interchange. It was observed in 1982 that 2-alkynyl ethers, metalated at their activated position by employing Li bases, could be converted to the corresponding α-metalated allenic ethers in THF by employing t-BuOK in the presence of HMPA (Scheme 67).363 It also results from the different studies on the reactivity of species generated by deprotonation using a Li,K base that they do not behave as Li reagents.34 The reaction of organolithiums with alkyl364 or aryl364a bromides and iodides (Wurtz reaction), sulfides,365 and epoxides364c was facilitated in the presence of tertiary alkoxides of heavier alkali metals; the rates of reactions with different alkyl bromides are dependent on the identity of the alkoxide (e.g., Et2MeCONa proved more efficient than t-BuONa) and increase with the R0 OM 3 RLi (M = Na, K) ratio (for example, in the reaction of octyl bromide with 1:x BuLi 3 t-PentONa, the yield of dodecane increases from 50% for x = 1 to 86% for x = 4).364b,366

Scheme 67

Scheme 68

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Scheme 69

Scheme 70

Table 44. Addition of RLi to α,β-Unsaturated Amides and Effect of t-BuOK

entry 1

R1, R2 Me3Si, Me

R

Pent THF, 78 °C, 1 h

no

74 traces

Et2O, 55 °C, 2 h

yes

5

THF, 78 °C, 1 h

no

40

2 3

solvent, conditions t-BuOK yields (%)

Pr

42

Et2O, 55 °C, 1 h

yes

5 6

Me3Si, Ph(Me)CH Bu

THF, 78 °C, 3 h Et2O, 55 °C, 0.5 h

no yes

57 56

7

Ph, Me

THF, 78 °C, 1 h

4

no

83

yes

Bu

no

66

yes

Pent

no

62

Et

8 9 10 11 12 13 14

Ph, Ph

60

Bu

THF, 78 °C, 1 h

54 69

yes

no yes

73 75

51

Oku and co-workers studied the effect of t-BuOK on the basicity of halogenolithiocarbenoids and their reaction as carbene sources at low temperatures. Upon treatment of the Li carbenoid prepared from 7,7-dibromonorcarane by Li/Br exchange, with t-BuOK (not with t-BuOLi) in THF at 85 °C, an insertion product was obtained (Scheme 68, left); the reason advanced is the formation of either a thermally labile α-bromo K carbenoid or a mixed Li,K compound. A similar acceleration was observed in the reaction of the second Li carbenoid depicted in Scheme 68 (right) with cyclohexene.367 PhLi28 or other Li reagents368 can add to ethylene in the presence of t-BuOK. Inagaki and co-workers observed the ethylation of hydrocycloalk[b]indoles by ethyl ethers upon treatment by a large excess of 1:1 BuLi 3 t-BuOK (Scheme 69); the indole dianion formed under these conditions reacts with ethylene, generated in situ by β-elimination from Et2O, before hydrolysis.369 In the course of studies concerning the addition of Li reagents to Me3Si- and Ph-substituted α,β-unsaturated amides, Asaoka and co-workers noted an inversion of the addition mode of n-alkyllithiums in the presence of t-BuOK, affording the 2-substituted (contra-Michael) adducts (Table 44).370 4.2.2. Nu-M,Mg-HeteroMAAs (M = Li, Na, K)371. In the absence of transition elements,209g,372 Li magnesates react with a large range of electrophiles, such as aldehydes, DMF, CO2, and, to a lesser extent, ketones, iodoalkanes, allylBr, and Me3SiCl.57a,c,d,373

The limitations concerning the reactions with carbonyl compounds were attributed to β-H transfer and enolization (with competitive reduction and self-aldolisation reactions, respectively).374 Mixed Li,Mg compounds, generated by halogen/metal exchange using i-PrMgCl 3 LiCl, have in addition been intercepted with tetramethylthiuram disulfide (Me2NCS2)2 (leading to the corresponding dithiocarbamates),375 with N-fluorobenzenesulfonimide (PhSO2)2NF376 or N-fluoro-2,4,6-trimethylpyridinium tetrafluoroborate (affording the corresponding fluorides),377 and by electrophilic amination using N-chloroamines378 and triazenes.379 Higher-order Na magnesate, prepared from BuNa and (EtO(CH2)2O)2Mg in a 2:1 ratio, can behave as a reducting agent; indeed, whereas trapping with CO2 works quantitatively, interception with benzophenone produces both the addition and reduction derivatives (70:30 ratio), and reaction with CoCl2 leads to Co (Scheme 70).40 Reactions with Aldehydes. The addition of organolithiums and diorganomagnesiums to carbonyl compounds leads to the formation of metal alkoxides that can have an effect on the outcome of the reactions (e.g., reduction, enolization, aldol condensation, pinacol formation, MeerweinPonndorfVerley, Oppenauer, and Tishchenko reactions).380 Additives such as t-BuOK behave similarly.381 Screttas and Steele observed, in addition to the expected Ph(Bu)CHOH, the formation of PhC(O)Bu by reacting Bu2Mg with PhCHO in methylcyclohexane containing Li ()-menthoxide [Ph(Bu)CHOH and PhC(O)Bu were obtained in 25 (∼8% ee) and 44% yield, respectively, when 4 equiv of Li alkoxide was employed]. The competitive formation of ketone was rationalized on the basis of an Oppenauer oxidation of the alkoxide by hydride transfer to PhCHO.382 α-Dialkylaminoalkoxides (e.g., BuCH(OM)NMe2), generated by addition of organolithium and -magnesium reagents to dialkylformamides, are similarly converted to higher amides [BuC(O)NMe2] by in situ Oppenauer-type oxidation in the presence of Mg and Li alkoxides, respectively. The authors ascribed to the alkoxide a catalytic role in the Oppenauer oxidation as well as a stabilization effect for the dialkylaminoalkoxide, preventing its decomposition to the corresponding enamine.383 Knochel and co-workers reported in 2007 a Mg variant of the Oppenauer oxidation as a way to synthesize aryl and metallocenyl ketones. The Mg alkoxide formed by addition of a Mg reagent to an aldehyde can be oxidized to give a ketone in the presence of benzaldehyde as hydride acceptor. They observed that employing mixed-metal RMgCl 3 LiCl favors the reaction, a result attributed to the enhanced solubility of the Mg alkoxides (Table 45).384 Asymmetric addition of achiral Mg reagents including mixedmetal compounds to achiral aldehydes or ketones was reviewed in 2009.385 The addition of different organometallics to 7592


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Table 45. Synthesis of Aryl and Metallocenyl Ketones from RMgX 3 LiCl, R0 CHO, and PhCHO

entry

R0

R

yield (%)

1

Ph

2

4-NCC6H4

90

3

4-anisyl

95

4 5

2-NCC6H4 2-anisyl

92 95

6

3-Py

89

7

5-Br-3-Py

94

8

3-benzothienyl

85

9

5-I-2-thienyl

92

6

Me2O, 123 °C, 1 h

84

43 (R)

94 91

7

THF, 110 °C, 1 h

91

59 (R)

10

9-phenanthryl

11 12 13

Ph 2-NCC6H4 4-BrC6H4

14 15

Ph

Table 46. Addition of Alkylmetals to PhCHO in the Presence of the Li Salt of (S,S)-2-Hydroxymethyl-1-[(1-methyl-2pyrrolidyl)methyl]pyrrolidine

2-BrC6H4

98 entry

alkylmetal

solvent, conditions

yield (%)l

ee (%)

1

BuCu

Et2O, 78 °C, 1 h

22

0

2

Et2Zn

Et2O, 78 to 0 °C, 3 h

76

0

3

Et3Al

Et2O, rt, 6 h

a

4

BuMgBr

Et2O, 78 °C, 1 h

90

47

5

Bu2Mg

Et2O, 78 °C, 1 h

93 (89)b

68 (73)b (R)

8

DMM, 78 °C, 1 h

87

51 (R)

9

DME, 78 °C, 1 h

96

28 (R)

toluene, 0 °C, 0.25 h

93 (94)c

36 (88)c (R)

toluene, 110 °C, 1 h

2-furyl

0 95

3-benzofuryl

ferrocenyl

79

11

Me2Mg

56

34 (R)

Ph

(CO)3MnCp

80

12

Et2Mg

74

92 (R)

16

Ph

(CO)3CrPh

85

13

Pr2Mg

90

70 (R)

17

4-anisyl

73

14

i-Pr2Mg

59

40 (R)

18

4-PhC6H4

69

15

i-Bu2Mg

81

42 (R)

19

4-tolyl

65

20 21

4-Me2NC6H4 4-DC6H4

92 72

22

4-ClC6H4

40

23

4-BrC6H4

59

24

4-IC6H4

28

25

1-naphthyl

78

26

2-naphthyl

82

aldehydes in the presence of the Li salt of (S,S)-2-hydroxymethyl-1-[(1-methyl-2-pyrrolidyl)methyl]pyrrolidine as chiral ligand has been reported by Mukaiyama and co-workers from 1978. Dialkylmagnesiums allow more efficient reactions than alkylcopper, dialkylzinc, trialkylaluminum, and Grignard reagents. It was assumed that toluene, which gives better results than ether-type solvents, favors the formation of a complex between the alkylmetal and the Li alkoxide and facilitates the coordination of the aldehyde (Table 46 and Scheme 71).276b,c,386 Noyori and co-workers documented in 1988 the enantioselective alkylation of aldehydes using chiral BINOL-derived Li,Mg reagents (Table 47). For example, after treatment with dilithium (S)-binolate, Bu2Mg was found to react with benzaldehyde in 1:1 THFDME at 100 °C to afford the expected (S)-alcohol in 88% ee. The level of stereoselectivity is highly dependent on the reaction protocol; indeed, when the reagent is generated by treatment of (S)-BINOL by Bu2Mg (1 equiv) and then BuLi (2 equiv), the ee drops to 18%.387 Fleming and co-workers showed in 2002 that, whereas mixed alkoxo-organo Mg compounds are inert toward aldehydes, their corresponding Li tert-butyl magnesates react satisfactorily to furnish the expected alcohols (Table 48).388 A similar result was noted with mixed alkoxo-organo Mg compounds generated from iodo alcohols (Table 49).389

10

BnOH is obtained in 28% yield. b At 123 °C. c At 110 °C for 1 h; 70% yield and 22% ee using i-PrCHO. a

Scheme 71

Reactions with Ketones. Ashby and co-workers proposed in 1974 a mechanism to describe Li magnesate addition to ketones considering a complexation of the Li atom by the carbonyl O atom (Scheme 72).41a The stereochemistry of the reaction of magnesates with several ketones is illustrated in Table 50, and the results show that the attack occurs predominantly at the less hindered side of the CdO group.41a When the R ligand has a β-H, the reactions of RMgX or R2Mg with ketones lead to addition as well as reduction, the latter being major with bulky R and hindered ketones. Richey and DeStephano observed that reduction in reactions of dialkylmagnesiums with ketones can be lessened by first adding an appropriate salt [MeOK, (Me2NCH2)2CHOK, MeONa, MeOLi, t-BuOLi, Bu4NBr, or BnEt3NCl] to the Mg compound. The authors attributed this result to the formation of magnesate ions [e.g., [(R2(MeO)Mg)2]2‑].390 From 2-tolyl tert-butyl ketone, Chubb and Richey observed that the addition reaction is faster (12 orders of magnitude) using an organomagnesate solution prepared from MeOK and Np2Mg than with Np2Mg alone.391 In 1997, Kostas and Screttas modified the behavior of 3- and 4-(lithioxy)alkyllithiums by combining them with 7593


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Table 47. Alkylation of Aldehydes Using Chiral BINOLDerived Li,Mg Reagents

Table 49. Reaction of Li Magnesates Generated from Iodo Alcohols with Aldehydes

entry entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 a

solvent, additive (x), conditions

R0

R Et

Et2O, 78 °C THF, 100 °C THF, DME (2), 100 °C 1:1 THFDME, 100 °C DME, 78 °C 1:1 DMEhexane, 95 °C 1:1 DMEtoluene, 95 °C THF, HMPA (4), 100 °C THF, DMIa (4), 100 °C THF, TMEDA (2), 100 °C 1:1 THFDME, 100 °C

Ph

Et

4-anisyl 4-ClC6H4 (E)-PhCHdCH Ph(CH2)2 Hex i-Pr c-Hex PentCtC Ph Ph 1:1 THFDME, 100 °C

Bu t-Bu Me allyl vinyl Me3SiCtC Ph 1-naphthyl

1:1 THFDME, 100 °C

1,3-Dimethyl-2-imidazolidinone.

b

yield (%) 82 90 96 93 94 80 95 60 90 92 98 88 97 40 86 99 68 97 57 98 76 70 89 47 75 75

ee (%) 64 (S) 85 (S) 88 (S) 92 (S) 73 (S) 84 (S) 81 (S) 6 (S) 75 (S) 85 (S) 83 (S) 68 (S) 37 (S) 85 (S) 85 (S) 37b 84b 55b 45b 88 (S) 4b 17b 82b 14b 82 (S) 12b

R1

1

H

2

R2 MgX

5 6

Me

R3

yield (%)

(CH2)5C(O) (C(OH)(CH2)5)

71

2

i-PrC(O)CO2Et (i-PrC(OH)CO2Et)

71

3

Ph2C(O) (C(OH)Ph2)

64

4

Me(CH2)5CHO (CH(OH)(CH2)5Me)

73

5

PhCHO (CH(OH)Ph)

48a

6

PhS(O)2SPh (SPh)

64

7 8

PhCHdC(CN)2 (CH(Ph)CH(CN)2) i-PrC(O)CO2Et (i-PrC(OH)CO2Et)

70 46

1

H, H, H, 1

H, H, H, 2

9

(CH2)5C(O) (C(OH)(CH2)5)

60

10

PhS(O)2SPh (SPh)

57b

11

Me, H, H, 1

12

(CH2)5C(O) (C(OH)(CH2)5)

55

PhCHO (CH(OH)Ph)

57

(CH2)5C(O) (C(OH)(CH2)5)

45c

13

H, H, Me, 1

14

allyl, allyl, H, 1 PhS(O)2SPh (SPh)

54

a

BuCH(OH)Ph also obtained in 41% yield. b BuCH(OH)Ph also obtained in 33% yield. c BuC(OH)(CH2)5 also obtained in 41% yield.

Scheme 72

Table 50. Stereochemistry of the Addition of Magnesates to Several Ketones

yield (%)

PhMgCl

Ph

60

i-PrMgBr

Ph

72

Ph(CH2)2

65

PhMgCl

Ph(CH2)2

57

PhMgCl

Ph(CH2)2

57

MeMgCl

Ph(CH2)2

61

3 4

electrophile (E)

Configuration not determined.

Table 48. Reaction of Li Magnesates Generated from Alkynenitriles with Aldehydes

entry

R1, R2, R3, n

(EtO(CH2)2O)2Mg. They notably observed that the resulting species become stable in THF and can be intercepted with CO2 and ketones to afford the expected lactones and alcohols, respectively.392 Lithoxy-substituted aza alkyllithiums Li(CH2)nNMe(CH2)2OLi can be similarly stabilized and yield

the corresponding aza diols PhCH(OH)(CH2)nNMe(CH2)2OH upon trapping with benzaldehyde.393 Ipaktschi and Eckert studied the reaction of different alkyl Grignard compounds with enolizable ketones in ethers containing concentrated LiClO4. They observed increased addition yields, due to less enolization, and sometimes less reduction, in the reactions of BuMgBr with dibenzyl ketone or β-tetralone, 7594


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Table 51. Butylation of Acetophenone and Benzophenone Using Different Agents

Table 52. Reaction of Li Magnesates with Different Ketones

entry 1 entry 1

R Me

yields (%)

BuLi

62

7

0

BuMgCl

50

9

8

3

Bu2Mg

48

27

20

4

Bu3MgLi

82

0

0

Bu3MgLi 3 LiCl BuMe2MgLi 3 LiCl

99

0

0

72a

0

BuLi

58

38

BuMgCl

0b (11)c (12)c,d

56b (81)c (78)c,d

Bu3MgLi 3 LiCl

95

5

92e

0

2

5 6 7 8 9b 10b a

butylating agent

Ph

BuMe2MgLi 3 LiCl

Ph, Me

time (h)

Bu, Bu

5

yield(s) (%) 96

97

Ph, Et

3a

Ph, i-Pr

80

4a

1-naphthyl, Me

71

5a 6a

2-naphthyl, Me Ph, Ph

93 80

7a

C6H4-2-(CH2)3

77

8a

c-Hex, c-Hex

95

9

Ph, Ph

Bu, Bu

2

Ph, Ph

95

87

11

Me, Me

99

12

Et, Et

64

78 77

8 8

13 14

Ph, Me

Bu, Me Bu, Ph

5

15

Me, Bu

26

72

16

Ph, Bu

41

57

17

t-Bu, Bu

23

61

18

Me, Et

6

93 93

19

Ph, Et

Me, Et

5

7

20

Ph, i-Pr

Me, Et

5

8

92

21 22

t-Bu, Ph 1-naphthyl, Me

Me, Et Me, Et

5 5

11 4

89 94

23

2-naphthyl, Me

Me, Et

5

5

95

24

Ph, Ph C6H4-2-(CH2)3

Me, Et Me, Et

5 5

1 7

99 89

25 26

3-Py, Me

Me, Et

5

4

96

27

4-Py, Me

Me, Et

5

3

97

28

Ph, Ph

Me, i-Pr

2

0

87

12 8

88 92

29 30 a

R, R0

2a

10

The Me adduct is obtained in 26% yield. b After 2 h reaction time. c At 0 °C for 2 h. d In the presence of LiCl (1 equiv). e The Me adduct is obtained in 8% yield.

i-PrMgCl with cyclohexanone, as well as PhCtCLi with dibenzyl ketone.394 Ishihara and co-workers compared the reactivity of several butylating agents toward acetophenone and benzophenone and observed an increased nucleophilicity, compared with that of BuLi, BuMgCl, and Bu2Mg, using Bu3MgLi and above all Bu3MgLi 3 LiCl (Table 51). The presence of 2,20 -bipyridine accelerates the reaction. The method was extended to various ketones and proved suitable for mixed Li triorganomagnesates (Table 52).374,395 To favor the addition of a LiCl-complexed Grignard reagent to a ketone, Laufer and co-workers in 2008 used neodynium salt NdCl3 3 2LiCl.396 Conjugate Addition Reactions. Wittig soon realized that Ph3MgLi in reaction with benzalacetophenone mainly produces the 1,4-addition compound, whereas PhLi mainly gives 1,2-addition.2 Richey and King noticed in 1982 that the addition of 15-crown-5 to THF solutions of Et2Mg, i-Pr2Mg, and (vinyl(CH2)4)2Mg accelerates their addition to pyridine. Moreover, whereas the reactions using diorganomagnesiums or organolithiums in general lead to 1,2-addition, those in the presence of 15-crown-5 favor 1,4-addition.397 This observation was attributed to the formation of magnesate species due to the great ability of the macrocycle to coordinate Mg2+ or RMg+. 2,1,1Cryptands43a and HMPA43f lead to similar reorganization. When studying the behavior of pyridine, quinoline, and 2-cyclohexenone toward Et2Mg 3 EtLi solutions, Richey and Farkas obtained reactions faster than when using Et2Mg, but slower than when using EtLi.398 The formation of an intermediate magnesate was proposed to explain ligand-assisted nucleophilic additions of Grignard reagents to enones bearing an OM group (Scheme 73). The best 1,4- to 1,2-selectivity was obtained with M = K. Using Li, it was improved in the presence of agents able to complex Li.399 Reactions involving such a coordination-controlled conjugate addition were next developed by Fleming and co-workers, for example, starting from γ-hydroxy-α,β-alkynenitriles388 and

a

R1, R2

Me, Pr Me, Bu

In the presence of 2,20 -bipyridine (1 equiv).

-alkenenitriles (Scheme 74).400 With the latter, and using chloro Grignard reagents, efficient operations giving exomethylene hydrindane (n = 1) or decalin (n = 2) were documented (Scheme 75).401 The reaction does not necessarily require the presence of an OH group in the substrate. Indeed, the OM group can be generated in situ by addition of a Grignard reagent to a ketone, as exemplified in Scheme 76 with 3-oxo-1-cyclohexene-1carbonitrile as substrate.402 Since 2005, examples of selective allylation starting from N-Li, N-Me, N-allyl, N-Bn, and N-Ph pyridin(e)-2-(thi)ones using Bu2(allyl)MgLi (or other allyl magnesates), prepared in THF from allylMgCl and BuLi in a 1:2 ratio, were reported by Sosnicki. The 6-substituted products were formed, but when Li was present on nitrogen (and also depending on the reaction solvent and temperature), they were in situ converted to the corresponding 4-allyl derivatives through a Cope rearrangement (Table 53).403 In 2009, a one-pot procedure was reported to prepare 3,3-dialkylated derivatives of 3,6-dihydro-1H-pyridin-2ones (Table 54).403d 7595


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Scheme 73

Scheme 74

Scheme 75

Scheme 76

Asaoka and co-workers studied the addition of different 1:n BuMgCl 3 MeLi mixtures to 3-(trimethylsilyl)acrylamide and observed that the best yields of (butylated) Michael adduct are obtained using 1:3, 1:4, and 1:5 ratios, a result attributed to the formation of ate species. The reaction was extended to other α,β-unsaturated amides and carboxylic acids; the use of other Grignard reagents led the authors to propose the reactivity order t-BuMgCl 3 3MeLi > BuMgCl 3 3MeLi > PhMgBr 3 3MeLi (Table 55).404 Reactions with Epoxides. Nakata and co-workers documented in 1997 a BuLi 3 Bu2Mg-mediated ItoKodama cyclization between a phenylthio-stabilized allylic anion and an epoxide (Scheme 77).405 The role of Bu2Mg in the reaction of a 2-substituted 1,3-dithiane with a 2,3-disubstituted oxirane after deprotonation using BuLi was next studied. Unlike the lithio

derivative, which rapidly decomposes at rt, the mixed-metal species survives until it gives the desired coupled product.406 The BuLi 3 Bu2Mg-mediated dithiane coupling was extended to other electrophiles (aldehyde, allylic bromide, ester) and efficiently used as the key step of total syntheses407 such as that of methyl sarcoate, a marine natural product (Scheme 78).408 1,2-Migration. In search of new systems from metal carbenoids and alkylmetals able to undergo alkylation, Oshima and co-workers synthesized α-silylalkylmagnesiums. To this purpose, dibromomethylsilanes were involved in a Br/Mg exchange at 78 °C in THF using either Bu3MgLi, prepared from BuLi (2 equiv) and BuMgBr (1 equiv), or s-Bu3MgLi, prepared from s-BuLi (3 equiv) and MgBr2 (1 equiv). Migration of the Bu group409 (facilitated by a Cu salt in the case of Bu3MgLi) then took place by warming the reaction mixture to rt to afford 7596


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Table 53. Allylation of Different N-Substituted Pyridin(e)-2-(thi)ones Using Magnesates

entry 1

R0

R, X, R1, R2, R3 Li, S, H, H, H

allyl

conditions 0 °C, 0.2 h

R00 H

ratio

73:27

2

0 °C, 0.5 h

31

49:51

3

rt, 0.5 h

22:78

4

rt, 3.5 h

82 (48),a (62)b

0:100

rt, 0.5 h

65

0:100

5

CH2CHdCMe2

6

CH2C(Me)dCH2

7 8

Me, S, H, H, H

allyl

9

0 °C, 0.2 h

90:10

rt, 3 h

74

0:100

0 °C, 0.5 h

80

85:15

70 °C, 0.5 h

Me

75

100:0

10

CH2CHdCMe2

70 °C, 0.5 h

80

4:96

11

CH2C(Me)dCH2

70 °C, 1 h

80

98:2

allyl

70 °C, 0.5 h

88

97:3

13

CH2CHdCMe2

70 °C, 0.25 h

78

0:100

14

CH2C(Me)dCH2

70 °C, 1 h

87

95:5

allyl

0 °C, 0.7 h

12

15

Bn, S, H, H, H

Ph, S, H, H, H

16

Bn

Ph

85

20:80

70 °C, 0.5 h

96

71:24 0:100

17

CH2CHdCMe2

70 °C, 0.25 h

74

18

CH2C(Me)dCH2

0 °C, 0.3 h

82

36:64

70 °C, 0.5 h

76

66:34 0:100

19 20

Li, O, H, H, H

allyl

0 °C, 6 h

H

50

21

Me, O, H, H, H

allyl

0 °C, 0.3 h

Me

61

95:5

81

100:0

72 °C, 0.25 h

a

total yield (%)

22

allyl, O, H, H, H

allyl

70 °C, 0.3 h

allyl

83 (27)c

99:1

23

Bn, O, H, H, H

allyl

72 °C, 0.25 h

Bn

83

95:5

24

Ph, O, H, H, H

allyl

72 °C, 0.25 h

Ph

93

69:31

25

allyl, O, Me, H, H

allyl

70 °C, 0.3 h

allyl

90

98:2

26

allyl, O, H, Me, H

allyl

65 °C, 0.3 h

allyl

90

100:0

27

allyl, O, H, H, D

allyl

72 °C, 0.3 h

allyl

82

95:5

28

allyl, O, H, H, Me

allyl

70 °C, 0.3 h

allyl

89

91:9

29

allyl, O, H, H, Me3Si

allyl

72 °C, 0.3 h

allyl

81

93:7

30

allyl, O, H, H, Cl

allyl

72 °C, 0.3 h

allyl

66

87:13

31

allyl, O, H, H, SPh

allyl

72 °C, 0.3 h

allyl

76

98:2

32

allyl, O, H, H, allyl

allyl

72 °C, 0.3 h

allyl

72

92:8

33

Bn, O, H, H, allyl

allyl

72 °C, 0.3 h

Bn

85

91:9

Using 1:1 allylMgClBuLi.

b

Using 1:3 allylMgClBuLi. c Using allylMgCl (ratio: 75:25).

α-silylpentylmagnesiums, which were intercepted by electrophiles (Scheme 79).410 In contrast, using Me3MgLi (prepared by mixing MeMgBr and MeLi in a 1:2 ratio) in THF at 78 °C for 30 min led to the monomethylated derivatives R3SiCH(Me)Br.410b,411 Even if the stereoselectivities are lower than those observed in the reactions mediated by cuprates, zincates, and manganates, gem-dibromocyclopropanes also underwent Br/Mg exchanges (at the less hindered Br atom) followed by migration of the Bu group from Mg to the adjacent carbon atom with elimination of bromide and inversion of configuration (Table 56).410b

4.2.3. Nu-M,Al-HeteroMAAs (M = Li, Na, K). Reactions with CO2, Aldehydes, and Ketones. Zweifel and Steele showed in 1967 that it is possible to selectively transfer the alkenyl group of organoalanes (prepared by alkyne hydroalumination), provided that they are combined with MeLi, to different electrophiles such as CO2, acetaldehyde, and formaldehyde (Table 57). The authors proposed the vinyllithium resulting from the disproportionation of the Li aluminate as reactive species in these reactions.412 Trapping of Li enynylaluminates, generated from DIBAL-H by successive treatment with MeLi and a diyne, with CO2 has also been described by Zweifel and 7597


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co-workers (Scheme 80).413 Other Li alkenylaluminates were similarly quenched using (CH2O)n, CO2, and ClCH2OMe (Scheme 81).414 As already proposed with Li magnesates, a mechanism based on the complexation of the CdO group to Li was given by Ashby and co-workers to describe Li aluminate addition to ketones. The stereochemistry observed in the methylation of 4-tert-butylcyclohexanone using Me4AlLi and, above all, (i-Bu3)MeAlLi is different from what is observed with Mg, B, and Zn ate complexes; indeed, the reaction occurs predominantly from the most hindered axial side in Et2O, THF, and DME. In contrast, for 3,3,5-trimethylcyclohexanone, norcamphor, and Table 54. Conversion of 3,6-Dihydro-1H-pyridin-2-ones to Their 3,3-Dialkylated Derivatives

entry 1

R1, R2

total yield (%)

ratio >99:1

BnBr

92

2

allylBr

76

85:15

3

PrI

64

97:3

4

MeI

69

95:5

5

i-BuI

72

83:17

DecI BnBr

72 84

>99:1 >99:1

8

allylBr

71

86:14

9

PrI

73

97:3

BnBr

88

91:9

11

allylBr

62

80:20

12

PrI

66

95:5

6 7

10

Me, H

RX

allyl, H

allyl, Me

camphor, identical stereoselectivities are obtained (Table 58). Complexes such as Me4AlLi 3 ketone are considered in the mechanisms proposed for the alkylation (Scheme 82).41a Abenha€im and co-workers have reported since 1976 asymmetric alkylations of carbonyl compounds using chiral alkali metal alkoxytrialkylaluminates freshly prepared from R4AlM (M = Li, Na), on the one hand, and ()-N-methylephedrine (entries 111), ()-quinine (entries 12 and 13), (+)-cinchonine (entries 14 and 15), and (+)-Darvon alcohol (entry 16), on the other hand (Table 59). Differences appear in some cases if the Li alkoxytrialkylaluminate is prepared from the required trialkylaluminum and Li alkoxide.415 The same authors studied the alkylation of ()-menthyl phenylglyoxylate using Li, Na, and K alkoxytrialkylaluminates, prepared from the required trialkylaluminum, on the one hand, and the alkoxide of tert-butanol, ()-N-methylephedrine, (+)-Nmethylephedrine, or Darvon alcohol, on the other hand (Table 60). α-Alkyl mandelic acids were obtained in ee values up to 81% by using a bulky alkoxy group (even if not chiral).416 Excellent yields and diastereoselectivities were obtained by treating (1R,2S)-trans-2-phenylcyclohexanol pyruvate and phenylglyoxylate with bulky Li alkoxytrialkylaluminates (Table 61, R00 OH = t-BuCEt2OH).417 The use of chiral Li alkoxytrialkylaluminates, coming from (+)-Darvon alcohol, to alkylate methyl phenylglyoxylate proceeded in hexane in ee values up to 85%, Table 55. Addition of Different 1:n BuMgCl 3 MeLi Mixtures to α,β-Unsaturated Amides and Carboxylic Acids

Scheme 77

Scheme 78

entry

R1, R2

RMgX

n

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Me3Si, NHMe

BuMgCl

Me3Si, NHMe Me, NHPh

PhMgBra BuMgCl t-BuMgCl PhMgBr BuMgCla PhMgBr BuMgCl s-BuMgCl PhMgBr BuMgCl t-BuMgCl PhMgBr

0 1 2 3 4 5 3 3 3 3 3 3 3 3 3 3 3 3

Ph, NHPh Ph, OH

Ph(CH2)2, OH

conditions 20 °C, 2 h

rt, 2 h 0 °C, 5 h 10 °C, 1 h rt, 3 h 10 °C, 2 h rt, 3 h 15 °C, 1.5 h 15 °C, 1.5 h 40 °C, 3.5 h 15 °C, 1.5 h 30 °C, 1.5 h rt, 3 h

yield (%) 7 54 82 78 83 77 40 66 43 74 59 60 60 58 50b 67b 26b

a

Using t-BuMgCl results in the formation of the contra-Michael adduct (2-t-Bu derivative). b Isolated as Me ester.

Scheme 79

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Table 56. Reaction of gem-Dibromocyclopropanes with Bu3MgLi Followed by Electrophilic Trapping

provided that a high dilution is used (Table 62, R*OH = (+)Darvon alcohol).418 Li tetraalkylaluminates, prepared via hydroalumination in THF of vinyl compounds, were employed in enantioselective additions to ()-menthyl phenylglyoxylate, pyruvate, and glyoxalate (Table 63).419 The Et4AlNa-mediated alkylation of acetophenone, 1-phenyl-2-propanone, 4-tert-butylcyclohexanone (Table 64), and (R)-2-phenylpropanal (Scheme 83, following Cram’s rule) in different solvents was reported, too.176,420 The selective alkenyl ligand transfer from Li tetraorganoaluminates to an aldehyde was used in a synthesis of brassinolide;421 Table 58. Stereochemistry of the Addition of Aluminates to Several Ketones

a In the presence of CuCN 3 2LiCl (30 mol %). b In the presence of CuCN 3 2LiCl (3 equiv). a Large amounts of reduction product are formed. b Depending on the concentration. c About 30% total yield. d Equatorial alcohol (52% yield) formed by reduction. e Equatorial alcohol (69% yield) formed by reduction.

Table 57. Reactions of Alkenyl Aluminates with CO2, HCHO, and MeCHO

Scheme 82 entry 1

R Bu

R0 H

2 3

electrophile (E)

yield (%)

CO2 (CO2H)

78

HCHO (CH2OH)

73

MeCHO (CH(OH)Me)

68

4

c-Hex

H

CO2 (CO2H)

72

5

Me

Me

CO2 (CO2H)

76

6

Et

Et

CO2 (CO2H)

78

Scheme 80

Scheme 81

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Table 59. Alkylation of Carbonyl Compounds Using Chiral Alkali Metal Alkoxytrialkylaluminates

a

At 0 °C.

Table 60. Alkylation of ()-Menthyl Phenylglyoxylate Using Chiral Alkali Metal Alkoxytrialkylaluminates

entry

R, M

1

Me, Li

R*OH

solvent

t-BuOH

hexane

yield (%), ee (%)a 76, 25 (R)

2

()-N-methylephedrine

3

(+)-N-methylephedrine

69, 17 (S)

t-BuOH

76, 27 (R)

5


72, 49 (R)

6

(+)-N-methylephedrine

4

Et, Li

7

Et, Na

8

Et, K

9

Et, Li

10 d

11

77, 38 (R)

75, 0.2 (R)


toluenehexane benzenehexane

74, 43 (R)


benzenehexane

70, 23 (R)b 69, 53 (R)c

Et, Na Et, Li

(+)-Darvon alcohol

12d d

rac-Darvon alcohol

13

14d d

15

d

Et, Li

71, 52 (R)

t-PentOH

16

t-BuEt2COH

17d

t-Bu-i-Pr2COH

43:57 hexaneEt2O

73, 73 (R)e

23:77 hexaneEt2O

76, 78 (R)

23:77 hexaneEt2O

77, 75 (R)

Et2O

74, 81 (R)

hexane

71, 43 (R)f

43:57 hexaneEt2O

75, 70 (R)

72, 57 (R)

a

The main enantiomer is the one predicted by the Prelog rule. b Values of 72%, 17% ee(R) using Et4AlLi. c Values of 76%, 27% ee (R) using Et4AlNa. At 50 °C for 4 h then rt for 16 h. e Values of 74%, 71% ee (S) in hexane starting from (+)-menthyl phenylglyoxylate and 76%, 24% ee (S) using Et4AlLi in hexane starting from (+)-menthyl phenylglyoxylate. f Values of 71%, 22% ee (R) using Et4AlLi. d

in the syntheses of dolicholide, dolichosterone, and 6-deoxodolichosterone (Scheme 84);422 and to access homodolicholide and homodolichosterone (Scheme 85).422

Yamamoto and co-workers have documented since 1980 regioselective reactions of heterosubstituted allylic aluminates with both carbonyl compounds and reactive halides to provide 7600


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Table 61. Alkylation of (1R,2S)-trans-2-Phenylcyclohexanol Phenylglyoxylate and Pyruvate Using Bulky Li Alkoxytrialkylaluminates

entry

R

R0

1 2

Me

Et

rt 0 °C

3

Ph

Et

0 °C

4

conditions

Me

yield (%), de (%) 83, 98 (S) 82, 100 (S) 97, 100 (R) 96, 88 (R)

5

50 °C

99, 91 (R)

6

78 °C

92, 89 (R)

Table 62. Alkylation of Methyl Phenylglyoxylate Using a Chiral Li Alkoxytrialkylaluminate

entry

R

1

i-Bu

concn (mol/L)

2 3

Bu

4 5

Me

6 7 8

Et

R

0.04

95, 85 (S)

0.4

90, 70 (S)

0.04

92, 64

0.4

95, 42

0.04

68, 1 (S)

0.4

71, 99 (28) 100 100 (60)b 80 95 (43)b 99 92 94 95 (29)b 94 (35)b 99 94 (10)b 85 (10)b 71 (17)b 24 (0)b 100 (12)b 100 (0)b 78 (18)b 64 e

b

98:2

Table 87. 1,2-Migration Reaction of Na Aluminates Followed by Electrophilic Trapping

entry

R

R0

yield (%)

Me

H2O (H)

69

Hex

Et

I2 (I) H2O (H)

63 74

I2 (I)

67

5

Dec

Me

H2O (H)

69

6

Dec

Et

H2O (H)

68

1 2 3

Hex

4

a

electrophile (E)

a

Z/E ratio >99:1

Yield of 31% octyne.

Table 88. Reaction of Li Triorganoaluminates with Cyclohexene Oxide or Benzaldehyde in the Presence of BF3 3 Et2O

95% while the addition of the Li enolate is 9095% syn selective.484 The reaction with the chiral enone given in Scheme 108 leads to the 2,3-anti-3,4-anti isomer (78% selectivity) using the mixed enolate, whereas the 2,3-syn-3,4-anti isomer (82% selectivity) is formed from the Li enolate.485 The authors proposed an inverse demand DielsAlder cycloaddition to explain the different stereochemical behavior of the mixed Li, Ti enolate.486

Stefanovsky and co-workers tried to connect stereochemical data and TS geometry for Ti-mediated conjugate additions of phenylacetic acid N,N-dialkylamides and thioamides to cinnamic aldehyde, benzalacetone, chalcone, and (E)- and (Z)-methyl cinnamate.487 The putative amide and thioamide Ti ate complexes prepared from Li enolates follow in general the behavior of the Li precursor, with the thioamide showing a greater trend to 1,4-addition than the oxygen analog. The stereochemistry of the 7611


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reactions using amide Ti ate complexes was correlated with a cyclic TS (Scheme 109). Improved 1,4-selectivities were also observed for addition of mixed Li,Ti enolates to 1-phenoxycarbonylpyridinium salts to give 1,4-dihydropyridines, which easily undergo aromatization to provide 4-(2-oxoalkyl)pyridines.488 4.2.5. Nu-Mg,Cr-HeteroMAAs. Organochromium ate complexes have received little attention from synthetic chemists. Unlike (CH2dC(Me)CH2)3Cr, (CH2dC(Me)CH2)2CrCl, and (CH2dC(Me)CH2)CrCl2, the tetramethallylchromate prepared by mixing CrCl3 with CH2dC(Me)CH2MgCl (4 equiv) is capable of converting 1,6-diynes to bicyclo[4.3.0]nonadienes through [2 + 2 + 2]annulation (Table 91).489 The key step in the mechanism proposed is the allylchromation of the CtC bond triggering a cyclization (Scheme 110). This possibility has since been extended to 1,6-enynes (Table 92).490 A catalytic amount of CrCl3 combined with methallylmagnesium chloride also achieves these cyclizations. 4.2.6. Nu-M,Mn-HeteroMAAs (M = Li, MgX). Organomanganates are more stable at rt than organomanganese halides and, above all, dialkylmanganeses104g,200c,200d,491 and have been used for different purposes. Oshima and co-workers attempted Table 89. Reaction of Li Alkynyl Aluminates with PhSCl Followed by Electrophilic Trapping

entry 1 2

R Pent Pent

R0 Me Et

3

a

yield (%)

E/Z ratio

H2O (H) H2O (H)

43 45

96:4

allylBra (allyl)

39

electrophile (E)

4

Pent

i-Bu

H2O (H)

26

5

Pent

Oct

H2O (H)

29

6

Dec

Et

H2O (H)

44

7

THPO(CH2)2

Et

H2O (H)

27

reactions between cyclohexanecarbaldehyde and mixed-ligand R2R0 MnMgX.199b The latter were prepared in THF at 0 °C from MnCl2 by successive addition of RMgX (2 equiv) and R0 MgX (1 equiv) and treated at 78 °C with the aldehyde (Table 93).492 The reactivity order following the nucleophilicity of the anions (allyl > Ph > Me > Bu > HexCtC > Me3SiCH2), the authors proposed as the mechanism an alkyl nucleophilic attack. This reactivity order is different from that obtained for 1,4-additions performed using 2-cyclohexenone (allyl > PhS > Bu > Ph > Me ∼ Me3SiCH2 ∼ HexCtC), for which a radical mechanism has been proposed (see section 3.1.4). A similar dependency of reactivity order on the substrates (except for PhS, which is retained on Cu) had previously been observed for similar experiments carried out with organocuprates(I).151b,154b Ricci and co-workers showed that organomanganates prepared by transmetalation of organolithium and -magnesium reagents add to the CdO moiety of aldehydes and acylsilanes, notably bearing a chiral center at the α-position, to afford the corresponding alcohols.493 The α-chiral 2-phenylpropanal and corresponding acylsilane furnished the expected alcohols in good diastereoselectivities (Table 94). Scheme 97

Scheme 98

After addition of 1 equiv of BuLi at 40 °C.

Table 90. 1,2-Migration Reactions with Alkyl Transfer

entry 1

R, R0

R1, R2 H, H

Me, Me

86

Et, Et Me(CH2)3CHdCH, i-Bu

83 (87)a 80

Me3SiCtC, i-Bu

75

Me, Me

92

6

Et, Et

94 (80)a

7

Me(CH2)3CHdCH, i-Bu

79

8

Me3SiCtC, i-Bu

65

2 3 4 5

9 10 11

Me, H

H, Me

Me, Me

93

Me, Me

Et, Et Me, Me

92 83

Me(CH2)3CHdCH, i-Bu

66

12 a

yield (%)

Using Et2Zn instead of Et3Al. 7612


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Scheme 99

Scheme 100

Scheme 101

Scheme 102

(RCO2)2MeMnLi have been efficiently used as methylating agents for ketones (Scheme 111). In the 1,2-addition reaction to tert-butylcyclohexanone, a better diastereoselectivity (axial/ equatorial 92/8) was observed using putative (t-BuCO2)2MeMnLi than using t-BuCO2MnMe (axial/equatorial 78/22).104g It is not clear whether manganates add to ketones by the mechanism normally found for carbanions or by oxidative addition.186 Kauffmann and co-workers performed intermolecular competitive experiments using different manganates (Table 95). When aldehydeketone pairs were submitted to (Cl2CH)3MnLi in

THF at 0 °C, the selective transfer of the Cl2CH group to the aldehyde was observed (Table 95, entries 1 and 2).86 Alkylation of the CdO group of ketones proved to be facilitated by a neighboring OH or other basic group at the α- or β-position; α-(dimethylamino)acetone is for example alkylated selectively in the presence of 3-pentanone (Table 95, entries 39).86,223 A neighboring group effect was also identified for oxirane ringopening, with heteroatom-containing α-substituents favoring the reaction (Table 95, entries 10 and 11).494 Note that reaction of Bu3MnLi with oxiranes furnishes the corresponding alkenes, a reaction not observed with the alkyl Cu(I) reagents.495 Allyldimethylmanganates, prepared from Me3MnLi and Bu3(allyl)Sn, react better than Me3MnLi with epoxides (Table 96).219 Transfer of the stannyl group from (Bu3Sn)Me3MnLi2 and (Bu3Sn)Me2MnLi to cyclopentene oxide provides trans-2-tributylstannylcyclopentanol in 90 and 92% yield, respectively.199c Oshima and co-workers reported the conversion of gemdibromocyclopropanes into the corresponding disubstituted cyclopropanes by treatment with a manganate in THF and subsequent trapping with an electrophile (Table 97).496 The proposed reaction mechanism includes (1) halogen/Mn exchange at the less hindered Br, (2) alkyl migration with bromide elimination (inversion on the cyclopropane carbon), and (3) reaction with the electrophile (retention of configuration) (Scheme 112). The stereoselectivity of the reaction has been 7613


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Scheme 103

Scheme 104

Scheme 105

attributed to the bulkiness of the Mn reagent that attacks the less hindered halogen selectively. The reaction has since been extended to other substrates such as tert-butyl dibromoacetate and N,N-diethyldibromoacetamide (Table 98).497 Using (PhMe2Si)3MnLi with N,N-diethyldibromoacetamide and then aldehydes affords α,β-unsaturated carboxamides, as shown in Scheme 113. Using N,N-diethyldichloroacetamide, for which the α-proton is more easily abstracted, successive treatment with DALi, Bu3MnLi, and an electrophile leads to α-trisubstituted derivatives (Table 99). The same authors reported the treatment of 1,3-dibromoand 1,3-dichloropropene with trialkylmanganates followed by addition of an electrophile to provide homoallylic alcohols (Table 100).498 The proposed reaction mechanism includes (i) allylic halogen/manganate exchange, (ii) isomerization of the allylmanganate thus obtained, (iii) 1,2-alkyl group migration from Mn to an adjacent carbon under bromide elimination, and (iv) electrophilic trapping of the resulting allylic manganate. Unlike similar reactions using dibutylcuprate and tributylzincate, the reactions of gem-dibromocyclopropanes and other dibromides through manganates can take place using Li or Mg

reagents in the presence of a catalytic amount of Mn halide. This possibility has been efficiently employed for the synthesis of (E)-alkenylsilanes according to the mechanism depicted in Scheme 114 (with BuMgBr): the elimination of Mn and β-H liberates BuMnH, able to generate low-valent Mn(0) species; the latter can insert into one of the CBr bond of the substrate to give an organomanganese halide, which then reacts with BuMgBr (2 equiv) to regenerate the manganate.496,499 Catalytic and stoichiometric (Table 101) reactions are equally effective for the stereoselective formation of 1-trialkylsilyl-1-alkenes.499b Using tri(cyclopropyl)manganates results in conjugated dienylsilanes due to an additional ring cleavage, as shown in Scheme 115. The reaction was extended to 2-alkoxy-1,1-dibromoalkanes to afford di- or trisubstituted alkenes (Table 102) and to 2-alkoxy1,1,1-tribromoalkanes to provide tri- or tetrasubstituted alkenes (Table 103) through mechanisms involving Br/metal exchanges, alkyl 1,2-migrations from Mn to C, and eliminations of metal and β-alkoxy (Scheme 116).500 Unexpected results were obtained with substrates bearing a Ph group. For example, whereas (dibromomethyl)benzene with Bu3MnMgBr affords the corresponding styrene in a low 9% yield, using Ph3MnMgBr results in the formation of 1,1,2,2-tetraphenylethane in 76% yield, which can occur as depicted in Scheme 117. Starting from 1,2-bis(dibromomethyl)benzene, treatment with various Mg arylmanganates affords the corresponding benzocyclobutanes depicted in Table 104; the mechanism proposed includes Br/Mn exchanges, 1,2-migrations, and a reductive elimination of Mn(0) to afford the final product. Reacting 1,2-bis(bromomethyl)benzene with Mg manganates allows replacement of both bromo groups, the first by Ph or allyl (depending on the manganate) and the second by various 7614


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Scheme 106

Scheme 107

Scheme 108

Scheme 109

substituents (H, D, CH(OH)Ph). Indeed, after the Br/Mn exchange, the species formed collapse to o-quinodimethane under 1,4-elimination; after addition of a ligand from a second equivalent of manganate, electrophilic trapping becomes possible to afford difunctionalized derivatives (Table 105). Using higher-order Bu4Mn(MgBr)2 leads to other derivatives also formed through o-quinodimethane (Scheme 118).499b From

1,1-dibromo-1-octene, only halogen/metal exchange is observed using Mg tributylmanganate. Using 2-iodobenzo[b]furan, the I/Mn exchange is in contrast followed by a 1,2-migration reaction with concomitant ring-opening, giving the (E)-alkene (Scheme 119).496b Oshima and co-workers examined more than 20 years ago the behavior of Si,Mn and Sn,Mn reagents toward alkynes and 7615


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Scheme 110

Table 93. Reaction of R2R0 MnMgX with Cyclohexanecarbaldehyde

Table 91. Reaction of Tetramethallylchromate with 1,6-Diynes

entry

R

yields (%)

entry

R1, R2, R3, X

1

Bu

98

1

Me, Me, H, C(CH2OMe)2

86 (>90% d)

2

allyl

99:1

Hex, H, Br CH2OBn, H, Br

Bu Bu

MeI (Me) MeI (Me)

48 48

50 50

76:24 70:30b

(CH2)3C(O(CH2)2O), Br

Bu

MeI (Me)

48

78

100:0

10 11 12 a

R

At 78 °C.

b

No stereochemical assignment.

Scheme 128

order cuprates are obtained by regioselective addition of either Bu2CuLi or Bu(BuCtC)CuLi to this alkyne; their rearrangement occurs after addition of an alkyl- or alkenyllithium, as shown by subsequent trapping (Table 123).532 Martin and co-workers

showed in 2001 that the union between α-lithio-N-trisylpyrroline (trisyl = 2,4,6-triisopropylbenzenesulfonyl) and higher-order Li cyanocuprates in Et2O allows the formation of homoallylic amines after hydrolysis or electrophilic trapping (Table 124).533 It is 7625


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Table 112. Reaction of 1,1-Dibromoalkenes with R2CuLi Followed by Electrophilic Trapping

a

Using 5 equiv of R2CuLi.

Scheme 129

Table 113. Oxidation of the Organocopper(I) Species to Acylsilanes

a

Starting from Me3SiSi(Me2)CCl2SiMePh2, PhC(O)SiMe2SiMe3.

the

product

is

interesting to note that ring-opening is possible with a higher cuprate coming from the metalated derivatives of 2,3-dihydrofuran, N-trisylpyrroline, furan,527b THF,530 and 3,4-dihydro-2H-pyran, but does not take place in the case of N-trisyl-1,2,3,4-tetrahydropyridine and N-trisylindole,533 probably in relation with a weaker electrophilic carbenoid behavior. Gais and co-workers studied the behavior of (E)- and (Z)-αcuprated alkenyl sulfoximines, obtained either by lithiation (Li compounds stable toward elimination up to rt) and subsequent transmetalation using a Li diorganocuprate or by direct cupration using a Li diorganocuprate. To explain the formation of the trisubstituted alkenes observed, the authors infer that a higher-order organocuprate (either formed directly, or in situ generated) undergoes a stereoselective 1,2-metalate rearrangement with inversion of configuration and elimination of sulfinamide to yield after trapping enantio- and diastereopure homoallyl alcohols (Scheme 143, Tables 125 and 126).511c 4.2.11. Nu-M,Zn-HeteroMAAs (M = Li, Na, K, MgX). The reactivities of Me cuprate and zincate toward electrophiles are tightly correlated to their d-orbital energies. According to the literature, a cuprate(I) reacts as a metal-centered nucleophile because its d orbitals are at the same energy levels as the Me carbon orbitals. In contrast, an organozincate behaves as an alkyl nucleophile because of its high-lying Me carbon orbitals and low-lying d-orbitals.534 Additions to Aldehydes and Ketones. In 1951, Wittig and coworkers compared the addition reactions on benzophenone of Ph3MgLi and Ph3ZnLi and observed a moderate reactivity for the 7626


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Table 114. Electrophilic Trapping of the Organocopper(I) Species

entry 1

R vinyl

R1, R2

yield (%) 73

MeI (Me)

H, H

2

allylBr (allyl)

H, H

86

3 4

HCtCCH2Br (CHdCdCH2) PhCHOa (CH(OH)Ph)

H, H H, H

60 69

5

c-HexCHOa (CH(OH)-c-Hex)

H, H

59

6

(E)-PhCHdCHCHOa (CH(OH)CHdCHPh)

H, H

61

7

PhC(O)Me (C(OH)MePh)

H, H

52

8

vinylC(O)Me (C(OH)Me-vinyl)

H, H

68

9

(E)-PhCHdCHC(O)Ph (C(OH)PhCHdCHPh)

H, H

53

10

2-cyclohexenone (3-cyclohexenol)

H, H

40

11 12

MeC(O)C(O)Mea (C(OH)MeC(O)Me) MeC(O)Cl (C(O)Me)

H, H H, H

64 57

13 14

MeCHdCH

PhC(O)Cl (C(O)Ph)

H, H

67

H3O+ (H)

Me, H

78

15

allylBr (allyl)

Me, H

79

16

MeC(O)Cl (C(O)Me)

Me, H

71

PhC(O)Cl (C(O)Ph)

Me, H

73

allylBr (allyl)

H, Ph

83

PhCHOa (CH(OH)Ph) MeC(O)Cl (C(O)Me)

H, Ph H, Ph

83 55

17 18

CH2dC(Ph)

19 20 21

a

electrophile (E)

allylBr (allyl)

Me, H

57

22

PhCHdCH

PhCHOa (CH(OH)Ph)

Me, H

62

23

MeC(O)Cl (C(O)Me)

Me, H

73

Me3SiCl as additive.

Table 115. 1,2-Migration of Cuprates Generated from (Ph2MeSi)CHCl2 Followed by Electrophilic Trapping

a

Me3SiCl as additive. b Pyridine as additive. 7627


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Table 116. Cuprate 1,2-Migration of Diorganophosphide Anions

Table 117. 1,2-Migration of 2,3-Dihydrofuran Li Cuprates

entry

a

R

yield (%)

1

Et

59 (74)a

2

Bu

83 (91)a

3 4

Hex Oct

83 62 (74)a

5

Ph

30

6

s-Bu

0

7

t-Bu

0

Using CuI (1 equiv) and RLi (5 equiv).

latter (54% yield against 100% for the former).2 Uchiyama and co-workers observed that the trapping with aldehydes of dilithium aryltri-tert-butylzincates, prepared by halogen/metal exchange of iodo- and bromoarenes, requires several hours at rt and is less efficient than the reaction with allylBr.185b If organoligand transfer to aldehyde still works efficiently from triorganozincates at rt,535 this is less true from mixed organoamino species. Indeed, it has, for example, been observed that the presence of TMP ligands in Li arylzincates is responsible for competitive unexpected reactions, in particular in the case of π-deficient aromatics.536 In 2000, Musser and Richey studied the reaction of various ethylzincates with aldehydes and ketones in toluene (Table 127). They observed much more rapid conversion than using the corresponding diorganozinc species. If selective Et transfer works efficiently using Et3ZnLi and, to a lesser extent, Et3ZnNa and Et3ZnK due to competitive metalation of substrates having α-hydrogens, far less efficient/selective reactions are noted after replacement of an Et ligand of the zincate by an alkoxide. Large electronic and steric effects on addition rate of acetophenone substituents were revealed using Et3ZnLi, with an increased reactivity in the following order: 4-OMe < 4-Et < 4-F < H < 3-OMe < 4-Cl < 3-F < 3-Cl < 3-C(O)Me < 4-CF3 ≈ 3-CF3 < 3,5-(CF3)2 < 4-CN. The reactivity of different alkyl phenyl ketones was also compared and was found to follow the order t-Bu < i-Pr < Et < Me. On this basis, the authors proposed ET

Table 118. 1,2-Migration of Cyclic Enol Ether Cuprates Followed by Electrophilic Trapping

as probable in the reactions between Et3ZnLi and acetophenones.537 In a more recent paper, Maclin and Richey evaluated the reaction rate between Et3ZnLi and di-tert-butyl ketone as k[Et3ZnLi]0.5[ketone] for an initial concentration of zincate greater than that of the ketone and as k[Et3ZnLi][ketone]1 for an initial concentration of ketone greater than that of the zincate. They also observed that the rate of addition of Et3ZnK to di-tertbutyl ketone is ClMg+ > BrMg+ (Table 129). Zincates tolerate a large range of functional groups, but recourse to transmetalation can help to get around their moderate reactivity toward electrophiles. In the case of aldehydes and ketones, coordination of the CdO to Lewis acids can rather be employed. It is, for example, known that the addition of Mg salts helps to increase their reactivity toward organometallic reagents.543 Knochel and co-workers recently compared the reactivities of PhZnI 3 LiCl and PhZnI 3 MgCl2 3 LiCl toward 2-chlorobenzaldehyde and observed that the former furnishes the expected alcohol in 60% yield after 72 h at rt and the latter in 88% yield after only 1 h. This MgCl2-mediated activation was successfully employed for the reaction of diorganozinc compounds, being more reactive than organozinc halides, with a large range of aldehydes and ketones, N-tosyl imines, CO2, etc. The acceleration effect of MgCl2 was rationalized on the basis of a stronger Lewis acidity of MgCl2 compared with that of RZnCl and thus a stronger ability to interact with the CdO group of the carbonyl reagent in the six-membered TS (Scheme 146).544 7631


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Scheme 140

Scheme 141

Scheme 142

Table 123. 1,2-Migration of Cuprates Generated from 1-Alkynyl Sulfides Followed by Electrophilic Trapping

entry

R0

R

electrophile (E)

yield (%)

Z/E ratio

1

Bu

Bu

H2O (H)

61

>99:1

2

Bu

Bu

ZnBr2 then I2 (I)

60

>99:1

3 4

Bu Bu

Bu Bu

P(OEt)3/allylBr (allyl) Et2NCH2OBu (CH2NEt2)

58 45

>99:1 >99:1

5

Bu

Bu

HCtCCO2Et ((E)-CHdCHCO2Et)

40

>99:1

6

BuCtC

s-Bu

P(OEt)3/allylBr (allyl)

65

>99:1

7

BuCtC

(E)-PentCHdCH

H2O (H)

70 (2 isomers)

Diastereoselective additions of zincates were explained with the help of six-membered TSs (Scheme 147).545 Ashby and coworkers reported in 1974 the stereochemistry of the addition of different Li zincates to ketones. As for ate complexes of Mg and B, the attack occurs predominantly at the less hindered side of the CdO group (Table 130).41a Kondo et al. compared in 1997 the migratory aptitude of different groups by reacting R2PhZnLi toward benzaldehyde and

deduced the order Bu > Ph > Me > t-Bu.137d Selective transfers of alkenyl ligands to aldehydes have been documented in the course of the total syntheses of (+)-amphidinolide J (Scheme 148)546 and the C15C28 portion of laulimalide.547 When the aldehydes were combined with Me2Zn before reaction, the expected alcohols were isolated satisfactorily. The stereoselectivity of the addition of allenylzincates to aldehydes was studied by Mangeney and co-workers. The 7632


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Table 124. 1,2-Migration of Li Cuprates Generated from N-Trisylpyrroline Followed by Electrophilic Trappinga

entry

a

R

electrophile (E)

yield (%)

Me3Sn Me2CdCH(CH2)2

H2O (H) Me3SnCl (SnMe3) H2O (H) H2O (H) H2O (H) H2O (H) H2O (H) MeI (Me) allylBr (allyl) I2 (I) Me3SnCl (SnMe3) MeI (Me) MeI (Me)

71 75 66 75 77 68 93 92 88 92 80 37 73

1

Me

2 3 4 5 6 7 8 9 10 11 12

vinyl Ph Bu s-Bu t-Bu

Trs = trisyl.

reagents were prepared by lithiation of a protected propargylic amine and subsequent transmetalation with diorganozincs. In contrast with allenylzinc halides and alkylallenylzincs, which mainly led to the anti adducts, using the corresponding Li dialkylallenylzincates favors the formation of the syn compounds, in particular in the case of hindered alkyl groups (Table 131).548 Soai and co-workers have documented since 1987 efficient enantioselective additions of dialkylzincs to aldehydes in the presence of catalytic Li salts of chiral pyrrolidylmethanols,549 of diaminodiols derived from ephedrine,550 and of (2S,5S)-2,5dialkylpiperazine.551 Difurylzinc was also employed in the presence of amino alcohols derived from ephedrine; high enantioselectivities are obtained provided that LiCl is present in the reaction mixture.552 Corey and Hannon documented the same year both ephedrine- and proline-based catalysts for the same purpose,553 and Chelucci and co-workers reported proline-based catalysts.554 Addition of Zn compounds to aldehydes, catalyzed by Li alkoxides555 or Li amides,556 or to nitrones, catalyzed by Mg alkoxides,557 has been examplified in more recent studies studies.558

Scheme 143

Table 125. 1,2-Migration of Li Cuprates Generated from Alkenyl Sulfoximines

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Table 126. 1,2-Migration of Li Cuprates Generated from Alkenyl Sulfoximines

Table 127. Reaction of Different Ethylzincates with Aldehydes and Ketones

a

No reaction using Et2Zn. b 59% of recovered PhCHO. c 36% of recovered HexCHO and 33% of dimeric product (HexCH(OH)CH(OH)Hex) formed from HexCHO. d 60% of recovered HexCHO (D2O quench showed it was monodeuterated) and 20% of dimeric product formed from HexCHO. e 62% of recovered PhC(O)Me. f 97% of recovered PhC(O)Me. g 100% of recovered PhC(O)Me (D2O quench showed it was monodeuterated). h 99% of recovered Ph2C(O). i 38% of recovered MeC(O)-i-Pent and 23% of dimeric product formed from MeC(O)-i-Pent. j 38% of recovered MeC(O)-i-Pent and 28% of dimeric product formed from MeC(O)-i-Pent. k 55% of recovered MeC(O)-i-Pent and 40% of dimeric product formed from MeC(O)-iPent. l 49% of recovered MeC(O)-i-Pent (D2O quench showed it was monodeuterated) and 50% of dimeric product formed from MeC(O)-i-Pent. m 45% of recovered 2-cyclohexenone. n 20% of recovered 2-cyclohexenone.

Ramos Tombo and co-workers described in 1990 enantioselective additions of phenylethynylzinc bromide to alkyl and aryl aldehydes using Li ()-N-methylephedrate (Table 132).559 A similar approach using alkenylzinc bromides was reported 1 year later (Table 133).560 The method was efficiently applied by Shair and co-workers to a synthesis of ()-longithorone.561 In the course of a synthesis of efavirenz, Tan and co-workers reported in 1999 efficient chiral Zn dialkoxide-mediated enantioselective additions of alkynyllithium and -magnesium reagents to a prochiral unprotected ketoaniline (Table 134). Extension

of the procedure to 1-[1-amino-4-(2,2,2-trifluoroacetyl)-2naphthyl]-2,2,2-trifluoroethan-1-one being possible (97% ee, 91% yield), the authors underlined the importance of the presence of an unprotected aniline group adjacent to the ketone function in the substrate.562 Gosselin et al. reported in 2007 asymmetric (R)-BINOLmediated organozincate addition to ethyl 2,2,2-trifluoropyruvate (Table 135). In contrast, zincates derived from N-methylephedrine or TADDOL led to the adducts in moderate conversions and low ee values.563 7634


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Scheme 144

Scheme 146

Scheme 145 Scheme 147

Table 128. Alkylation of Ketones and Aldehydes Using Mg Trialkylzincates

a

Yields of 97% and 3% in the presence of LiCl (1 equiv). b In the presence of LiCl (1 equiv).

Table 129. Ethylation of Benzophenone Using Different Agents

entry

ethylating agent (equiv)

yields (%)

1

EtMgBr (1) + Et2Zn (1)

54

39

2

EtMgCl (1) + Et2Zn (1)

81

14

3

EtLi (1) + Et2Zn (1)

94

1

Addition Reactions to Imines. Savoia and co-workers documented in 1997 studies on the addition of organozincates to imines derived from (S)-1-phenylethylamine and ethyl (S)-valinate. Excellent diastereoselectivities are observed for the reactions of the 2-pyridylimines derived from ethyl (S)-valinate, and possible TSs are proposed.564 A further investigation showed that O-(trimethylsilyl)valinol as auxiliary provides a superior asymmetric induction, a result rationalized on the basis of the lower basicity of the O atom (Table 136).565

Table 130. Stereochemistry of the Addition of Different Li Zincates to Ketones

Almansa et al. reported in 2008 efficient examples of diastereoselective addition of triorganozincates to N-(tert-butylsulfinyl)imines (Table 137). The nature of the group (t-Bu) on the S atom of the imine proves essential.566 The use of a catalytic amount of dialkylzinc reagent as precursor of the organozincate was next efficiently studied,567 and the method was applied to the enantioselective synthesis of α-amino acids.566c The selective ethyl transfer from Et2(Me3SiCH2)ZnLi to an acyl ketimine, generated by treating 3-oxo-1-cyclohexene-1-carbonitrile with excess MeMgCl and pivaloyl chloride and leading to an hindered enamide, was described in 2006 (Scheme 149).568 Conjugate Addition Reactions. Cuprates are in general used for 1,4-addition reactions with alkyl transfer to α,β-unsaturated ketones, and the thermal stability of zincates also makes them useful for this purpose. The similar reactivities between Li diorganocuprates and triorganozincates in conjugate additions is nevertheless superficial, because Cu is a transition element and Zn belongs to the main group. The pathway for the reactions using organozincates is thus different from that of organocuprates, which involves an ET process. The TS structures in the addition reactions of Me3ZnLi and [MeLi]2 to methyl vinyl ketone were compared using DFT calculations. It was observed that, in the case of organozincates, the reaction proceeds through an open form TS, whereas alkyllithiums favor 1,2-addition through a closed TS.569 7635


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Scheme 148

Table 131. Stereoselectivity of the Addition of Allenylzincates to Aldehydes

entry 1

R2Zn

R0 CHO

yield (%)a

syn/anti ratio

Et2Zn

i-BuCHO

72

50:50

t-BuCHO PhCHO

81 82

65:35 67:33

2 3 4

Bu2Zn

i-BuCHO

77

59:41

5

Ph2Zn

i-BuCHO

75

45:55

6

t-Bu2Zn

i-BuCHO

84

84:16

7

BuCHO

78

83:17

8

c-HexCHO

87

82:18

9

t-BuCHO

81

88:12

10 11

PhCHO allylCHO

78 80

85:15 75:25

a

Yield of 86% (syn/anti 10/90) using the allenylzinc bromide, 75% (syn/anti 18/82) using the allenylbutylzinc, and 72% (syn/anti 12/88) using the allenyl-tert-butylzinc.

Table 132. Addition of PhCtCZnBr to RCHO in the Presence of Li ()-N-Methylephedrate

The reactions attempted with the ketone depicted in Table 138 show that several symmetrical Li trialkylzincates can be used.138 Regioselective 1,4-additions to α,β-unsaturated ketones were also described using Mg trialkylzincates prepared from Grignard

reagents and ZnCl2 3 TMEDA (Table 139).570 The reactivity order for the transfer of different alkyl (and silyl) groups to 2-cyclohexenone has been studied using unsymmetrical Li and Mg zincates and proved to be Np , t-Bu, Me < Ph, i-Bu < Et, Bu, i-Pr, vinyl , Me2PhSi (Table 140). This order differs from that exhibited by unsymmetrical cuprates, which transfer Np and, to a lesser extent, t-Bu more easily than the corresponding zincates. The transfer of the silyl ligand from R2(Me2PhSi)ZnLi (R = Me, Et) to a variety of unhindered or moderately hindered enones was used to produce β-silyl ketones (Table 141).137a The method has since been employed with other conjugated compounds (Table 142).571 A disadvantage of the previous system (e.g., compared with cuprates) is the loss of 2 equiv of the alkyl ligand to be transferred. Watson and Kjonaas reported in 1986 that Me2RZnLi (R = Bu, s-Bu), obtained by successively treating ZnCl2 3 TMEDA with MeLi (2 equiv) and RLi (1 equiv), selectively transfer their R group (Table 143).572 Kjonaas and Hoffer reported in 1988 a more detailed study using Me2RZnM derived from ZnCl2 3 TMEDA, MeLi (2 equiv), and RMgCl (1 equiv),573 for which the formation of side 1,2-addition and methylation compounds was quantified by GC (Table 144).139 7636


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Table 133. Addition of Alkenylzinc Bromides to Aldehydes in the Presence of Li ()-N-Methylephedrate

entry

R1

R2, R3

R4

Me, H

Me

yield (%)

ee (%)

1

Ph

68

86 (S)

2

Bu

75

78 (R)

3 4

Hex i-Bu

72 70

78 (R) 86 (R)

5

3-Pent

65

92 (R)

6

c-Hex

78

93 (R)

7

t-Bu

62

93 (S)

8

Ph

9

H, Me

78

73 (S)

Dodec

76

85 (R)

10

c-Hex

73

83 (R)

11 12

t-Bu Ph

48 68

>98 (S) 90 (S)

13

Et

69

87 (R)

14

Bu

66

86 (R)

15

Hex

70

86 (R)

16

i-Bu

65

86 (R)

17

c-Hex

73

95 (R)

18

t-Bu

19 20

Ph Hex

21

c-Hex

22

Ph

Hex, H

Me, H

H, Me

Me

Me

Ph

67

97 (S)

82 71

93 (S) 86 (R)

62

91 (R)

63

88 (S)

Ryu and co-workers reported in 2000 access to unsymmetrical 1,6-diketones by conjugate addition of organozincates derived from ketones to enones (Scheme 150).574 Conjugate addition of zincates to cycloalkenones was examined by Yamamoto and coworkers in 1987 from a Zn azaenolate derived from the acetone imines of optically active 2-methoxy-1,2-diphenylethylamines (Table 145).575 Jansen and Feringa reported in 1988 a procedure using alkoxides as nontransferable ligands for zinc complexes. When compared as alkyl transfer agents toward 2-cyclohexenone, i-Pr3ZnMgBr 3 TMEDA, i-Pr2(t-BuO)ZnMgBr 3 TMEDA, and i-Pr(t-BuO)2ZnMgBr 3 TMEDA respectively led to 80, 94, and 10% yield. Being superior, several R2(t-BuO)ZnMgBr 3 TMEDA were next used for R transfer to different enones and proved to exhibit the reactivity order i-Pr > Et > Ph (Table 146). In order to achieve enantioselective 1,4-additions, the use of 1-menthyloxymagnesium bromide as chiral alkoxide and (S,S)-()N,N0 -dimethyl-N,N0 -bis(1-phenylethyl)-1,2-ethylenediamine as chiral TMEDA analog were attempted, but ee values below 14% were obtained.576 A method using catalytic diamineZn(II) complexes has also been described for the conjugate addition of EtMgBr and i-PrMgBr to α,β-unsaturated ketones.577 Nevertheless, if it proceeds with high chemo- and regioselectivities, modest enantioselectivities (ee values up to 33%) are obtained when chiral Zn(II) complexes are employed.578 It is interesting to note that Langer and Seebach reported modest enantioselectivities (ee e 16%) for the addition of Li trialkylzincates

to 2-cyclohexenone using (S,S)-1,4-bis(methylamino)-2,3dimethoxybutane as chiral cosolvent.579 Also in order to save ligands, Knochel and co-workers reported that a polar cosolvent such as N-methylpyrrolidinone (NMP) permits the 1,4-addition of R2Zn to enones. NMP-promoted ionization of the diorganozinc, providing a more reactive pseudozincate, is proposed as reaction origin.43d,580 Vaughan and Singer showed in 1995 that Me2(PhMe2Si)ZnLi [and, to a lesser extent, (Et2N)2(PhMe2Si)ZnLi], characterized by 29Si NMR and prepared by successively adding 2 equiv of MeLi (or Et2NLi) and 1 equiv of PhMe2SiLi to ZnCl2 or ZnI2, efficiently transfers its silyl group to α,β-unsaturated ketones and esters (Table 147).581 An alternative to generate dialkyl(trialkylsilyl)zincates employs PhMe2SiLi and R2Zn (R = Me, Et).571 The method has been used by Barrett and co-workers in the course of a total synthesis of (+)-pramanicin (Scheme 151).582 In 1996, Fleming and Lee observed that the addition of Me3SiCl to enones carrying no β-substituent and acrylonitrile leads to higher conjugate addition yields (Table 148).583 A method using PhMe2SiLi in the presence of a catalytic amount of Me2Zn has also been described.584 Oestreich and co-workers reported silyl transfers from (Me2PhSi)3ZnLi to unsaturated carbonyl compounds (Table 149).585 Uchiyama et al. compared the efficiencies of lower- and higherorder zincates in intramolecular Michael additions. To this purpose, the zincates were generated by I/Zn exchange from the substrates depicted in Table 150. While the lower-order ArMe2ZnLi does not proceed at all, the higher-order ArMe3ZnLi2 show higher reactivities.140 By reacting [(t-Bu)Zn(TMP)(t-Bu)Na(TMEDA)] with benzophenone in hexane at rt, Mulvey and co-workers isolated in 2005 a 1,6-addition product coming from a t-Bu transfer; the alkylated benzophenone enolate structure was characterized crystallographically and spectroscopically.586 In the course of the total synthesis of roseophilin, F€urstner and Weintritt intercepted an enone, formed by t-BuOK-induced elimination of PhSO2H from the substrate depicted in Scheme 152, by 1,4addition of a zincate.587 Epoxide Ring-Opening. Uchiyama et al. studied in 1996 the ring-opening of epoxides using mixed Li,Zn reagents. Styrene oxide was first reacted, but without regioselectivity whatever the Li zincate used (Table 151). The intramolecular reaction of the epoxide depicted in Table 152 after zincate formation by I/Zn exchange was next explored, and it was noted that using Me3ZnLi to perform the halogen/metal interconversion leads to almost opposite regioselectivity compared with the higherorder zincates.140,588 Similar reactions were performed in order to synthesize key precursors of CC-1065/duocarmycin pharmacophore, and Me3Zn(SCN)2Li3 was used to perform a 5-exo cyclization.253c Alexakis and co-workers reported in 2004 studies on the SN2 rt opening of cyclic (Table 153) and acyclic (Table 154, Scheme 153) vinylic epoxides with Li trialkylzincates (see section 4.2.3).79 An example of zincate-promoted 1,4elimination with epoxide opening was also described in 2007. 589 Alkylation and Acylation. Noyori and co-workers observed in 1989 that the presence of Me2Zn in the reaction of Li enolates with electrophiles suppresses competitive α-proton abstraction and enhances the efficiency of enolate alkylation and acylation. Prostaglandins were synthesized by combining this protocol with organozincate conjugate addition to an enone; an example is depicted in Scheme 154.590 The method has 7637


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Table 134. Addition of Alkynyllithium and -magnesium Reagents to a Ketoaniline in the Presence of a Chiral Zn Dialkoxide

R1OH

entry

R2OH

R c-Pr

MeOH

1

(1R,2S)-ephedrine (1R,2S)-norephedrine

42

3

(1R,2S)-N-methylephedrine

81

4 5

(1R,2S)-N,N-butylenenorephedrine

83a 87

Li

28

MgCl

6

MgBr

54

7

MgI

51


c-Pr

EtOH

9

MgCl

55

NpOH

96

10

allylOH

90

11

BnOH

89

12 13

CF3CH2OH CF3CO2H

96b 89

14

t-BuCO2H

72

15

4-NO2C6H4OH

16


89 Cl(CH2)3CtCH

NpOH

17

Yield of 83%.

Ee was 99% at 0 °C. Yield of 93%. c

d

97d

Yield of 94%.

Table 136. Addition of R2R0 ZnMgX to Chiral Imines

entry entry

95c

MgCl

CH2dC(Me)CtCH b

Table 135. Addition of Organozincates to Ethyl 2,2,2Trifluoropyruvate in the Presence of (R)-BINOL

a

ee (%)

2

8

a

M

R

yield (%)a

1

Me

29

50

2

Et

74

74

3

Pr

34

76

4

Bu

35

83

5

Pent

25

83

6

vinyl

29

13

7

allyl

37

4

8 9

Ph Bn

38 36

69 99:1) 92 (>99:1)

1

ee (%)

R1

8

CH2OSiMe3

Addition to Esters and Nitriles. Li tetraorganozincates such as Me4ZnLi2 can react with ester and nitrile functions. This has been observed in the course of a study using functionalized aromatic bromides aimed at performing halogen/metal exchange reactions.140 Silylation. Mg zincates were proposed as intermediates in Zn-catalyzed nucleophilic substitution reactions of chlorosilanes with Grignard reagents.595 1,2-Migration.596 Less common than intermolecular reactions between a nucleophile and an electrophilic carbon center, intramolecular variants where the metal bears a negative charge have been achieved and are known as 1,2-metalate rearrangements. By preparing different ate compounds from α-chloro Li compounds, Negishi and Akiyoshi suggested that the 7638


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1,2-migration reactions are not restricted to borates and cuprates and can be extended to organometals containing, for example, Mg, Al, and Zn.457 A similar behavior was more recently observed by adding 2 equiv of an organolithium or -magnesium compound to an organozinc sulfinate (Scheme 155).597 Harada et al. observed that 1-bromoalkenylzincates, generated by treating 1,1-dibromoalkenes with a Li triorganozincate at 85 °C, undergo intramolecular alkylation, giving alkenes when the reaction mixture is warmed to 0 °C (Table 155). As with borates, the reactions proceed with inversion of configuration at the carbenoid carbon.598

Such a reaction has been extended to 2-substituted 1,1dibromocyclopropanes (method A). The stereoselectivities are significantly improved by performing the reactions by ZnCl 2-mediated transmetalation of the corresponding Li carbenoids, which are prepared stereoselectively, and Table 138. Reaction of R3ZnLi 3 2LiX with Bicyclopentadienone

Table 137. Addition of R2R0 ZnM to N-(tertButylsulfinyl)imines

entry

R

1

Ph, H

2 3

R, R0

0 °C, 30 min

92

0 °C, 30 min

76

I Cl

0 °C, 30 min 78 °C, 1 h

68 73

5

78 to 0 °C, 2 h

92

6 7

Cl

0 °C, 30 min 78 °C, 1 h

86a 66

Me

R00

yield (%)

MgBr 1.5

Et

99 (97:3)

MgBr 1.5

Et

85 (98:2)

8

t-Bu

Cl

78 °C, 1 h

58

92 (96:4)

9

Ph

Cl

78 °C, 3 h

15

81 (94:6)

10

EtCtC

Cl

78 °C, 3 h

0

5 6

i-Pr, Me Me, vinyl

7

Bu, Me

8

MgBr 1.5 MgBr 1.5 MgBr 1.5 MgBr 1.5

Pent i-Pr i-Pr vinyl

99 (92:8) 93 (95:5)

MgBr 1.5

Bu

94 (94:6)

Li

1.5

Bu

99 (96:4)

9

Me, Bu

Li

1.5

Bu

98 (94:6)

10

Me, Bn

MgCl 1.5

Bn

99 (90:10)

11

Me, c-Hex MgCl 1.5 c-Hex 80 (93:7) Me, 4-tolyl MgBr 2.25 4-tolyl c (20:80)

12b 13 14

4-ClC6H4, H

Me, Et Me, vinyl

MgBr 1.5 MgBr 1.5

15

4-anisyl, H

Me, Et

MgBr 2.25 Et

Me, vinyl

MgBr 2.25 vinyl

92 (93:7)

16

Et vinyl

In the presence of TMEDA.

Table 139. Reaction of α,β-Unsaturated Ketones with Mg Trialkylzincates

86 (96:4)

Me(CH2)7, H

Me, Et

MgBr 2.25 Et

75 (70:30)

18

Ph(CH2)2, H

Me, Et

MgBr 2.25 Et

81 (29:71)

19

MeO2C(CH2)3, H Me, Et

MgBr 2.25 Et

64 (81:19)

20

Ph, CO2Et

MgBr 2.25 Et

87 (91:9)

Me, Et

a

92 (98:2) 82 (88:12)

17

21 22

Ph, CO2-i-Pr

Me, Et

MgBr 2.25 Et

85 (91:9)d 82 (79:21)

23

2-furyl, H

Me, Et

MgBr 1.5

Et

99 (94:6)

24

Me, i-Bu

MgBr 1.5

i-Bu

83 (80:20)

25

Me, Pent

MgBr 1.5

Pent

90 (93:7)

26

Me, i-Pr

MgBr 1.5

i-Pr

92 (94:6)

At 78 °C for 3 h. b At rt for 1 h. c Quantitative reaction. 96% (96:4) at 100 °C. a

s-Bu

Me, Et Me, i-Pr

4

x

Bu

Et, Me Me, Pent

a

M

yield (%)

Cl

3 4 R1, R2

conditions

Br

1 2

entry

X

d

Yield of

a

At 84 °C instead of 0 °C.

Scheme 149

7639


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Table 140. Reaction of 2-Cyclohexenone with RR0 2ZnM

RR0 2ZnM

entry

a

yields (%)

1

Me3ZnLi

37

2

Me3ZnMgIa

65

3 4

Et3ZnMgBra Bu3ZnLia

70 90

5

i-Pr3ZnMgBra

81

6

t-Bu3ZnMgClb

67c

a

7

i-Bu3ZnMgBr

81

8

Np3ZnMgBrb

69

9

(vinyl)3ZnMgBra

72

10

(Me2PhSi)3ZnLia

70

11 12

Me3ZnLid Me3ZnMgId

55 8

13

BuMe2ZnLid

92 d

Table 141. Reaction of α,β-Unsaturated Compounds with RR0 2ZnM

3

14

i-PrMe2ZnMgBr

15

t-BuMe2ZnMgCld

5

68

16

t-BuMe2ZnLid

20

10

17

i-BuMe2ZnMgBrd

67

18

18

NpMe2ZnMgBrd

19 20

Ph3ZnLid (vinyl)Me2ZnMgBrd

62 87

21

(CH2dC(Me))Me2ZnMgBrd

48

22

(Me2PhSi)Me2ZnLid

76

23

MeEt2ZnLie

95% D)

Me

67 (>95% D)c 51 (95% D)c 97 (92% D)

8 b

9

OC(O)NPh2

10b b

60 (93% D)

65 (30% D)a

11 12

Oct, H

OMs

Me3SiCH2 Bu

13

c-Hex, H

OMs

Bu

Li

95 (91% D)

14

(CH2)4CH(OMe)2, H

OMs

Bu

Li

91 (100% D)

15

(CH2)5

Cl

Bu

Li

95 (86% D)

16

(CH2)5

Cl

Bu

MgCl

79 (81% D)

a

Li

Competitive formation of Ph(CH2)2CH(D)CtCCH2SiMe3 in 30% yield ( Na > K. In addition, the dimetalation is more favored in the presence of Na or K alkoxides than in the presence of Li alkoxides. Other substrates than benzene are similarly dimetalated at 60 °C: thiophene at its 2,5-positions, tert-butylphenoxide at its 2,6-positions, and isopropylbenzene and secbutylbenzene at their 3,5-positions.688 A study performed from tert-butylbenzene shows that the metalation site, which is the 7652


Chemical Reviews

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Table 169. Lithiation of Chiral Ferrocenyl Imidazolines Using t-BuLi: Impact of DALi

entry

electrophile (E)

yield (%)

dr

entry

electrophile (E)

yield (%)

dr

1

MeI (Me)

66

1:12

5

MeI (Me)

41

1:31

2 3

Ph2C(O) (C(OH)Ph2) Ph2PCl (PPh2)

50 50

1:12 1:7

6

Ph2C(O) (C(OH)Ph2)

41

1:21

4

(4-tolylS)2 (S(4-tolyl))

53

1:17

Table 170. Lithiation of 2-Substituted Pyridines Using BuLi 3 DMAELia

R, R0

entry

conditions A

electrophile (E), conditions B

yield (%)

Me3SiCl (SiMe3), 0 °C

74656a

2

DCl in D2O (D), 0 °C

85656a

3

(MeS)2 (SMe), 0 °C

84656a

4 5

MeC(O)Et (C(OH)(Me)Et), 0 °C Et2NC(O)Cl (C(O)NEt2), 0 °C

74656a 65656a

6

MeI or Me2SO4 (Me), THF, 0 °C

70656a

7

EtI or Et2SO4 (Et), THF, 0 °C

64 or 71656a

8

HexI or HexBr (Hex), THF, 0 °C

60 or 30656a

9

c-Hex(CH2)3I (c-Hex(CH2)3), THF, 0 °C

52656a

10

Bu3SnCl (SnBu3), THF, 78 °C

78658b

11

pyridine (2-Py), THF, 0 °C to rt

64658a

12 13

pyrimidine (4-pyrimidyl), THF, 0 °C to rt pyrazine (pyrazyl), THF, 0 °C to rt

62658a 60658a

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

quinoline (2-quinolyl), THF, 0 °C to rt 2-MeO-pyridine (2-MeO-6-Py), THF, 0 °C to rt Me3SiCl (SiMe3), THF, 0 °C Me3SiCl (SiMe3), THF, 0 °C pyridine (2-Py), THF, 0 °C to rt C2Cl6 (Cl), THF, 78 °C to rt I2 (I), THF, 78 °C to rt t-BuCHO (CH(OH)-t-Bu), THF, 78 to 0 °C CO2 (CO2H) CBr4 (Br), THF, 78 to rt Ph2PCl (PPh2), THF, 78 °C PhC(O)NMe2 (C(O)Ph), THF, 78 °C PhCHO (CH(OH)Ph), THF, 78 °C t-BuC(O)Cl (C(O)-t-Bu), THF, 78 °C CO2 (CO2H) CO2 (CO2H) CO2 (CO2H)

25658a 58658a 52656b 80658a 75658a 59658d 61658d 90658c 62658j 89658h,n 50658l 78658l 75658l 65658f,m 71658g,i 26658g,i 37658k 46658k 59658k

1

a

n

MeO, H

Me2N, H MeS, H Ph2P, H Cl, H Cl, 3-F-4-SiMe3 Ph, H 2-anisyl, H 3-anisyl, H 4-anisyl, H Ph3CN((CH2)2)2N, H F3C, H F3C, 3-Cl F, 3-Cl-4-SiMe3 F, 3-F-4-SiMe3 F, 3-F-4-SiEt3

4

hexane, 0 °C, 1 h

2 2 4 2

hexane, 0 °C, 1 h hexane, 0 °C, 1 h

3 4 3 4 3 3 8 4 6 2 3

hexane, 78 °C, 1 h hexane, 75 °C, 0.75 h hexane, 0 °C, 1 h toluene, 40 °C, 1 h hexane, 0 °C, 1 h

hexane, 0 °C, 1 h

toluene, 20 °C, 2.5 h Et2O, 75 °C, 2 h hexane, 75 °C, 0.7 h hexane, 75 °C, 2 h hexane, 75 °C, 6 h

For other examples, see the Supporting Information (Table S2).

4-position using PentNa alone, moves to the 3-position in the presence of Na and K alkoxides. Dimetalation at the 3,5-positions is observed at 60 °C and favored using tertiary Na and K

alkoxides, whereas monometalation is promoted using cyclic alkoxides.685b Cumene, butylbenzene, toluene, and p-cymene behave similarly, with dimetalation using PentNa accelerated and 7653


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Table 171. Lithiation of 3-Substituted Pyridines Using BuLi 3 DMAELi

R, R0

entry

n

conditions A

electrophile (E), conditions B DCl in D2O (D), THF, 78 °C

66659a

2

Me3SiCl (SiMe3), THF, 78 °C

80659a

3

(MeS)2 (SMe), THF, 78 °C

83659a

4 5

t-BuCHO (CH(OH)-t-Bu), THF, 78 °C PhC(O)NMe2 (C(O)Ph), THF, 78 °C

60659a 69659a

6

I2 (I), THF, 78 °C

66659a

7

CBr4 (Br), THF, 78 °C

47659a

8

C2Cl6 (Cl), THF, 78 °C

60659a

1

Cl, H

hexane, 60 °C, 1 h

3

OMe, H

3

hexane, 0 °C, 0.5 h


88655a

10

2-anisyl, SMe

3


(MeS)2 (SMe), THF, 78 to 20 °C

70658l

11

2-anisyl, Cl

3


C2Cl6 (Cl), THF, 78 to 20 °C

72658l


Me3SiCl (SiMe3), THF, 78 to 20 °C (MeS)2 (SMe), THF, 78 °C

65658l 90658m

14


90658m

15


90658m

16

I2 (I), THF, 78 °C

80658m

17

t-BuC(O)Cl (C(O)-t-Bu), THF, 78 °C

50658m

18

t-BuCHO (CH(OH)-t-Bu), THF, 78 °C

72658m


(9)a 59659b

C2Cl6 (Cl), THF, 95 °C (PhS)2 (SPh), THF, 95 °C

(15)a 50659b (6)a 57659b

9

12 13

19

N((CH2)2)2NCPh3, H

SMe, H

8


6

20 21

PhCHO (CH(OH)Ph), THF, 95 °C

22 0

entry

R, R , E

0

0

n

conditions A

0

0

electrophile (E ), conditions B

(37)a 8659b yield (%)

23 24

Ph, H, H

3

hexane, 0 °C, 1 h

(MeS)2 (SMe), 78 °C to rt CBr4 (Br), 78 °C to rt

77658h 79658h

25

2-anisyl, H, H

3


(MeS)2 (SMe), THF, 78 °C to rt

84658l

26

C2Cl6 (Cl), THF, 78 °C to rt

82658l

27

Ph2PCl (PPh2), THF, 78 °C to rt

50658l

28

3-anisyl, H, H

3



70658l

29

4-anisyl, H, H

3



75658l

30

N((CH2)2)2NCPh3, H, SMe

8


CBr4 (Br), THF, 78 to 20 °C

72658m

I2 (I), THF, 78 to 20 °C PhC(O)NMe2 (C(O)Ph), THF, 78 to 20 °C

74658m 87658m

31 32 a

yield (%)

Yield of the regioisomer functionalized at C6.

favored in the presence of PentC(Me)2ONa but repressed in the presence of a cyclic hexoxide. Both lateral and ring metalation are observed with butylbenzene, toluene, and p-cymene, but only ring metalation is seen in the case of cumene.689 By using t-BuONa as additive in the course of PentNamediated metalation of tert-butylbenzene, Benkeser and coworkers observed an increased yield as well as a favored isomerization of initially formed 3-tert-butylphenylsodium to 4-tert-butylphenylsodium.690 The authors rationalized these results by the formation of mixed aggregates with which radical mechanisms have more chance to proceed.5a Caubere and co-workers later showed that combination of H2NNa with Na 2-methyl-2-alkoxides enhances the basic and nucleophilic properties of the reagent, presumably through solubilization.5a,691 A series of H2NNa 3 RONa (among them tertiary Na alkoxides and Na salts of diethylene glycol monoalkyl

ethers) reagents were developed, prepared in situ from H2NNa and alcohols, with specific properties. Using 2:1 combinations led, for example, from weak acids to carbanions that were subsequently trapped692 (see examples in Scheme 171693 and Table 182694) and to amides further converted to diazaphospholanes695 and phosphinites.696 H2NNa 3 t-BuONa was employed as well for elimination and isomerization reactions, in replacement of less accessible bases.693b,697 The nature of the solvent has an important impact on the outcome of the reactions, with the more polar increasing the reactivity of the bases; this was interpreted by solvent insertion into the aggregates, increasing the ion separation and thus the anionic reactivity.5a The authors observed that the reactivity of PhBr with various nucleophiles such as thiolates, amides, and enolates in THF or DME is greater when such nucleophiles are generated using H2NNa 3 t-BuONa or H2NNa 3 Et(O(CH2)2)ONa instead of H2NNa.698 7654


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Table 172. Lithiation of 4-Substituted Pyridines Using BuLi 3 DMAELi

entry

R

n

conditions A


4

hexane, 78 °C, 1 h

yield (%)

DCl in D2O (D), THF, 78 °C

58659a

2


85659a

3


64659a

4


51659a

5

PhC(O)NMe2 (C(O)Ph), THF, 78 °C

56659a

6

I2 (I), THF, 78 °C

70659a

7 8

CBr4 (Br), THF, 78 °C C2Cl6 (Cl), THF, 78 °C

49659a 44659a

1

Cl

9

OMe

3

hexane, 78 °C, 1 h


78655a

10

NMe2

2

hexane, 0 °C, 1 h

DCl in D2O (D), 78 °C

70665

11

PhC(O)NMe2 (C(O)Ph), 78 °C

65665

12

Ph2PCl (PPh2), 78 °C

90665

13


90665

14


94665

15 16

I2 (I), THF, 78 °C Bu3SnCl (SnBu3), 78 °C

81665 70665

C2Cl6 (Cl), 78 °C to rt

70660a,b

18

(MeS)2 (SMe), 78 °C

85660a,b

19

CBr4 (Br), 78 °C

60660a,b

20

Bu3SnCl (SnBu3), 78 °C

50660a,b

21

Ph2PCl (PPh2), 78 °C

80660a,b

hexane, 0 °C, 1 h

(MeS)2 (SMe), 78 °C to rt

80658h


CBr4 (Br), 78 °C to rt (MeS)2 (SMe), THF, 78 °C to rt

83658h 70658l

25

I2 (I), THF, 78 °C to rt

65658l

26

MeC(O)Et (C(OH)(Me)Et), THF, 78 °C to rt

68658l

17

22 23 24

N(CHdCH)2

Ph


3

4

2-anisyl

3

27

3-anisyl

3



80658l

28

4-anisyl

3



70658l

29

N((CH2)2)2NCPh3

8



91658m

30


89658m

31 32

CBr4 (Br), THF, 78 °C I2 (I), THF, 78 °C

95658m 77658m

33


62658m

34

t-BuC(O)Cl (C(O)-t-Bu), THF, 78 °C

81658m

35


65658m

C2Cl6 (Cl), 78 °C

75660c

37

CBr4 (Br), 78 °C

62660c

38

Bu3SnCl (SnBu3), 78 °C

87660c

36

39 entry

N(CH2)4

CF3

hexane, 0 °C, 2 h

2

Et2O, 75 °C, 2 h

4 R, E

0

n

conditions A

41658g

CO2 (CO2H)

0

0

0

electrophile (E ), conditions B

yield (%)

40

N(CHdCH)2, Cl

3


C2Cl6 (Cl), 78 °C

86660a,b

41

N(CHdCH)2, SMe

3


(MeS)2 (SMe), 78 °C

95660a,b

42 43

N((CH2)2)2NCPh3, Cl

8


C2Cl6 (Cl), THF, 78 to 20 °C CBr4 (Br), THF, 78 to 20 °C

78658m 53658m

44

I2 (I), THF, 78 to 20 °C

69658m

45

t-BuCHO (CH(OH)-t-Bu), THF, 78 to 20 °C

61658m

C2Cl6 (Cl), 78 °C

90660c

46

N(CH2)4, Cl

2

hexane, 0 °C, 2 h

7655


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Table 173. Lithiation of (S)-Nicotine Using BuLi 3 DMAELi

entry

electrophile (E)

yield(s) (%)

C2Cl6 (Cl)

87 (4)a

2

C2Br2Cl4 (Br)

67 (21)a

3

I2 (I)

55 (4)a

4 5

PhMe2SiCl (SiMe2Ph) (MeS)2 (SMe)

92 70 (6)a

6

Bu3SnCl (SnBu3)

55

7

HCO2Et (CHO)

26

C2Cl6 (Cl)

53 56

1

a

R H

8

SiMe3

9

Si(allyl)Me2

C2Cl6 (Cl)

10

SiMe2Ph

C2Cl6 (Cl)

43 0

entry

R, E

electrophile (E )

11

H, Cl

C2Cl6 (Cl)

yield (%) 72

12

(MeS)2 (SMe)

53

13

Bu3SnCl (SnBu3)

42

14

HCO2Et (CHO)

57

Yield for the 2-substituted regioisomer.

Table 174. Lithiation of Dipyridylpiperazines Using BuLi 3 DMAELi

The aryne intermediates of these reactions were directly involved in different other kinds of trapping.699 Pyridynes,700 strained cycloalkynes, and cycloalka-1,2-dienes701 were similarly generated and used for synthesis purpose. Studies performed on 1-chlorocyclohexene showed that the nature of the main reaction intermediate formed depends on the nature of the base and nucleophile, with H2NNa 3 ketone enolates generating 1,2cyclohexadiene702 and H2NNa 3 t-BuONa generating cyclohexyne.703 Complex reducing agents were similarly obtained by activating NaH with alkoxides.5b,704 5.2. Reactions of Base-HeteroMAAs705

Abilities for different ate compounds to deprotonate diphenylmethane were compared by Wittig and co-workers, who found the order Ph3BeLi < Ph3ZnLi < Ph3CdLi < Ph7Zn2Li3 ≈ Ph3MgLi.2

It is relatively recently that this pioneering work opened the way to a systematic study of ate compounds as deprotonating agents, but the idea of combining two metals appeared a long time ago. 5.2.1. Base-Li,Na- and -Li,K-HeteroMAAs. Wittig and coworkers early observed an enhanced deprotometalating ability of mixed Li,Na and Li,K bases over those only containing Li.360a,362,706 Eberhardt employed the product prepared from BuLi and KH in hexane, to which he attributed the formula [BuHLi]K, to perform the reaction of alkylbenzenes with ethylene (or propylene). Unlike KH and BuLi, it is capable of metalating benzylic hydrocarbons at rt in hexane. From toluene, subsequent reaction with ethylene allows the formation of a mixture containing propylbenzene (59%), indane (10%), 1-ethylindane (14%), and 3-phenylpentane (6%). The mechanism 7656


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Table 175. Lithiation of Pyridine and Quinoline Using BuLi 3 DMAELi

entry

substrate

n

conditions A

1

pyridine

2

78 °C, 1 h


2

yield (%)


80


80

3

MeI (Me), THF, 78 °C

60

4


72

5 6

PentCHO (CH(OH)Pent), THF, 78 °C PhCHO (CH(OH)Ph), THF, 78 °C

50 80

7

Me2C(O) (C(OH)Me2), LiBr, THF, 78 °C

35

8

MeC(O)Et (C(OH)(Me)Et), LiBr, THF, 78 °C

67

9

(CH2)5C(O) (C(OH)(CH2)5), THF, 78 °C

70

10

HC(O)NMe2 (CHO), THF, 40 °C

40

11 12

quinoline

78 °C, 0.5 h

4

13 14


65

Me3SiCl (SiMe3), Et2O, 78 °C

65

(MeS)2 (SMe), Et2O, 78 °C t-BuCHO (CH(OH)-t-Bu), THF, 78 °C

55 25

15

PhCHO (CH(OH)Ph), Et2O, 78 °C

45

16

MeC(O)Et (C(OH)(Me)Et), THF, 78 °C

25

17

Ph2C(O) (C(OH)Ph2), Et2O, 78 °C

40

Table 176. Lithiation of Furo[2,3-c]pyridine Using BuLi 3 DMAELi

entry

electrophile (E)

yield (%)

1

C2Cl6 (Cl)

53

2

Me3SiCl (SiMe3)

75

3

electrophile (E0 )

yield (%)

(MeS)2 (SMe)

73

Bu3SnCl (SnBu3)

74

4

PhCHO (CH(OH)Ph)

60

5 6

C2Cl6 (Cl)

85

shown in Scheme 172 was proposed to rationalize their formation. When compared with previously reported monometal reagents, the bimetallic compound favors the formation of cyclized derivatives.707 About 20 years after Morton, the groups of Wofford (still in search of polymerization initiators),708 Lochmann,27a,31 and Schlosser28,709 independently observed that organolithiums can be greatly activated (more than using a polar solvent) through deaggregation and polarization of the Cmetal bond by addition of stoichiometric amounts of K or Na alkoxide. One example is the behavior of alkyllithiums such as BuLi or 2-ethylhexyllithium toward toluene. Depending on the presence or not of t-PentOK, toluene is either stable in heptane or

electrophile (E00 ), conditions

yield (%)

C2Cl6 (Cl), 78 °C to rt (MeS)2 (SMe), 78 °C

72 39

deprotonated at its benzylic position. Due to the strength of the OLi bond, a metalmetal exchange takes place, as evidenced in the course of the formation of resonance-stabilized organometallic species (enhanced metal mobility) for which the intermediate organopotassiums precipitate, whereas the Li alkoxides remain in solution.27a,30b,710 From toluene (or isobutene), pure BnK (or 2-methallylpotassium) can be converted to BnLi (or 2-methallyllithium) by treatment with LiBr (1 equiv) and removal of insoluble KBr.711 Another example is the formation of highly reactive metal hydrides by reaction of H2 with t-BuOM (M = Na, K)-activated BuLi in hexane containing TMEDA712 and of K derivatives of aromatic hydrocarbons used as solvents upon treatment 7657


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Table 177. Lithiation of Alkylpyridines Using BuLi 3 DMAELi

entry

R1, R2, R3, R4

n

electrophile (E), conditions

yield (%)


91668a

2


90668a

3

Me2NC(O)Ph (C(O)Ph), THF, 78 °C

60668a

4


75668a

5

MeC(O)Et (C(OH)(Me)Et), THF, 78 °C

50668a

6

Bu3SnCl (SnMe3), THF, 78 °C

85670

7 8

C2Cl6 (Cl), THF, 78 °C CBr4 (Br), THF, 78 °C

70668a 65668a

9

I2 (I), THF, 78 °C

68668a

1

H, H, Me, H

3

10

H, H, Et, H

3


72655a

11

H, H, Bn, H

3


95655a

12

Me, H, Me, H

8

(MeS)2 (SMe), 78 °C

93668b

4

Me3SiCl (SiMe3), 78 °C

70668b

14

t-BuCHO (CH(OH)-t-Bu), 78 °C

65668b

15 16

C2Cl6 (Cl), 78 °C CBr4 (Br), 78 °C

75668b 88668b

17

I2 (I), 78 °C

74668b

18

Bu3SnCl (SnMe3), 78 °C

65668b

(MeS)2 (SMe), 30 °C

83668b

20

C2Cl6 (Cl), 30 °C

82668b

21

CBr4 (Br), 30 °C

85668b

13

19

Me, H, Me, Cl

8

22

Me, H, Me, SMe

8

(MeS)2 (SMe), 30 °C

73668b

23 24

Me, H, Me, D Me, H, Me, Bu

8 8

(MeS)2 (SMe), 78 °C (MeS)2 (SMe), 78 °C

90668b 93668b

25

H, Me, H, H

2

(MeS)2 (SMe), 78 °C to rt

90668c

26

C2Cl6 (Cl), 78 °C to rt

80668c

27

CBr4 (Br), 78 °C to rt

70668c

28

Bu3SnCl (SnMe3), 78 °C to rt

80668c

29

PhC(O)NMe2 (C(O)Ph), 78 °C to rt

78668c

30

PhCHO (CH(OH)Ph), 78 °C to rt

60668c

31 32

MeC(O)Et (C(OH)(Me)Et), 78 °C to rt CBr4 (Br), THF, 78 °C to rt

68668c 80668c

Bu3SnCl (SnMe3), THF, 78 °C to rt

78668c

H, Me, H, Cl

2

33 34

H, Me, H, SMe

2

CBr4 (Br), THF, 78 °C to rt

80668c

35

H, Me, Me, H

5


65668c

36

C2Cl6 (Cl), THF, 78 °C to rt

83668c

37

CBr4 (Br), THF, 78 °C to rt

80668c

3

PhC(O)NMe2 (C(O)Ph), THF, 78 °C to rt

75668c

3

C2Cl6 (Cl), THF, 78 °C to rt CBr4 (Br), THF, 78 °C

70668c 71668d

38 39 40

Me, Me, H, Cl Me, H, H, Cl

with different K alkoxide-activated organolithiums.30a Superbases have thus been identified by adding 13710a,b equiv of Na or K alkoxide to alkyllithiums (increasing reactivity with the amount of alkoxide). The formation of PhK as main product is for instance observed by treating benzene (in excess and/or using Et2O) by BuLi (or Bu(Et)CHCH2Li) in

the presence of t-BuOK28 or t-PentOK.713 Such K alkoxides were identified as additives more able to enhance the reactivity of organolithiums than polar ethers.28 Higher ratios of dimetalated (and also monometalated) benzene are obtained using BuLi activated by more soluble or more bulky K tertalkoxides.710b,714 7658


Chemical Reviews

REVIEW

Benzene was next successfully replaced by other substrates such as phenol (O,2-dimetalation) using either more soluble Li,K mixtures without TMEDA or BuLi 3 TMEDA (2 equiv) in the presence of t-BuOK in hexane.715 Indeed, if reactions using these mixed-metal bases as suspensions in apolar solvents have been reported,716 homogeneous solutions can be alternately employed either using THF as solvent, provided that the reaction temperature is maintained below 50 °C, or after addition of TMEDA to a suspension in an apolar solvent. High reactivities for Li,Na27b and Li,K29 combinations can be reached under these conditions. For example, Schleyer and co-workers found that adding TMEDA to “BuK”, prepared in situ from BuLi and t-BuOK in alkanes,36 allows the conversion of ethylene to vinylpotassium in hexane at 25 °C, whereas the same reagent in hexane or THFhexane does not react at temperatures between 90 and 20 °C.717 Deprotonative metalation combined with metal hydride elimination, using 1:1 BuLi 3 t-PentOK (in excess) in hexane, Table 178. Lithiation of Phthalans Using BuLi 3 DMAELi

has been used by Clark and Schleyer since 1976 to synthesize resonance-stabilized anions from cyclic olefins, dienes, and aromatics;718 1:1 BuLi 3 t-BuOK (LIC-KOR) is also a suitable reagent for the deprotometalation of resonance-active hydrocarbons.719 Selective proton abstractions from benzylic positions were achieved for numerous substrates (Table 183),28,368,711b,720 the outcome of the reactions depending on the length of the alkyl chain and the substitution on the aromatic ring.721 An important role of π-arene/ metal bonding in the reactivity of LIC-KOR was advanced in 2005 by Masson and Schlosser.722 Systems benefiting from two or three benzylic sites can be dideprotonated using LIC-KOR;723 subsequent trapping with alkyl halides of such benzylmetals possessing a good reducing ability is difficult and leads to competitive formation of dimers by one-electron transfer to the alkyl halide.724 Compounds containing both α- and βhydrogens, such as ethyl- and isopropylbenzene, also give high yields of dimeric products upon treatment with BuLi 3 t-PentOK; styrene radical anions arising from one-electron oxidation of styrene dianions are proposed to rationalize the dimer formation.718d Scheme 167

entry

R

electrophile (E)

yield (%)

(MeS)2 (SMe)

81

2

Me2SO4 (Me)

74

3

i-PrI (i-Pr)

62

1

H

4

Me3SnCl (SnMe3)

71

5 6

PhCHO (CH(OH)Ph) (()-camphor (bornyl)

69 52

cis/trans ratio

Scheme 168

7

Me

Me2SO4 (Me)

66

66:33

8

i-Pr

Bu3SnCl (SnBu3)

61

60:40

Me2SO4 (Me)

75

70:30

t-Bu

(MeS)2 (SMe)

81

35:65

9 10

Scheme 169

Table 179. Lithiation of Pyridines Using BuLi 3 PMLi

Scheme 170

entry 1

R0

R 2-Cl

2 3

yield (%)

ee (%)

Ph

60

53

4-anisyl 4-ClC6H4

61 56

45 23

t-BuCHO

62

35

5

2-F

Ph

74

30

6

3-Me

Ph

47

39

4

Scheme 171

Scheme 166

7659


Chemical Reviews

REVIEW

Table 180. Lithiation of Pyridines Using Me3SiCH2Li 3 DMAELi

a

Electrophile added at 0 °C. b Reverse addition. c Reaction for 2 h. d Reaction performed in hexaneTHF at 0 °C for 4 h.

The proton abstraction from both the OH and Me groups of cresols to give dianions can be achieved under heterogeneous725 or, above all, homogeneous715 conditions; trapping occurs at C only, except with dialkyl sulfates (trapping at both C and O). Similarly, the dideprotonation of isopropenylacetylene was evidenced using successively LIC-KOR (2 equiv) in THF at 80 °C and chlorotrimethylsilane.726 Extensive studies using LIC-KOR were performed by the group of Schlosser, employing various substrates such as triphenylmethane, diphenylmethane, and alkylbenzenes. Different 1:1 RLi 3 t-BuOK (R = Me, Bu, s-Bu, t-Bu) as well as different 1:1 BuLi 3 R0 OK (R0 = Pr, 3-Pent, Et3C, Et2NO) were compared for their reactivity toward cumene both in pentane at 25 °C and in THF at 50 °C. The more powerful reagent, BuLi 3 t-BuOK, also allowed through deprotometalation the formation of K derivatives of other hydrocarbons (+t-BuOLi). The different results obtained show that BuK is not the deprotonating species, and support a kind of interaction with the alkoxide at the origin of the reactivity enhancement.30c,727 The metalation of olefins containing allylic CH bonds using LIC-KOR occurs chemoselectively and regioselectively at Me rather than at methylene centers (no reaction at methine centers), and cis-olefins react faster than the corresponding trans isomers.728 The experiments performed led to the ruling out of a simple metathetical Li/K exchange in LIC-KOR; indeed, BuK is known to attack both the allylic and vinylic protons in alkenes, whereas LIC-KOR only gives the allyl metal derivatives.729 The use of LIC-KOR to metalate alkenes bearing allylic sites has been largely developed (Tables 184709a,729a,730 and 185709a,730a,730u,730x,730ad,730ae,730aj730al,731). Both ends of the allylic systems can react; in basic solvents such as THF, the reactions take place when possible at an unsubstituted terminal

position, whereas in nonpolar solvents branched products can be competitively formed.728a,732 If the organopotassium product is quenched by an electrophile immediately after its formation, the configuration of the unsaturated bond is not affected (a cis-2alkene gives an endo-alkyl substituted allylmetal species, and a trans-olefin affords an exo-alkyl stereoisomer).730ae Alternatively, stereoselective reactions can be reached by recourse to torsional (Z/E) equilibration to the more stable species (in general the endo-isomer, irrespective of the nature of the precursor; Scheme 173728b,730f,730u,733) under thermal or catalytic conditions before the electrophilic interception.728a,730ae In contrast, from arylallyl species, the exo form is favored (Scheme 174734).730ae Conjugated and homoconjugated dienes are similarly deprotonated at their allylic position to afford pentadienyl (or longer)type organopotassiums.735 The regiochemistry of the electrophilic trapping depends on the substituents and on the electrophile used;736 the functionalization in general occurs at the terminal position when halotrialkylsilanes, fluoro(dimethoxy)borane, and chlorotri(isopropoxy)titanium are employed (Table 186737 and Scheme 175731i). When unsubstituted, the generated species adopt a W-form; in contrast, in the presence of alkyl groups at the 1-, 2-, and 4-positions, the U-shape is favored for steric reasons (Scheme 176).738 The nature of the groups present on the alkenes can also modify the α vs γ regioselectivity of the reactions (Scheme 177739 and Table 187730d,m,v). Paraffinic but also ethereal solvents are tolerated in the reactions using such Li,K bases provided that the reaction temperatures are adjusted between 100 and 50 °C.709a Polymetalation is often observed when Li,K bases are employed.740 Dienes benefiting from more than one allylic position such as 2,3-dimethylbutadiene,730h,741 2-methyl-2-butene,742 2,5-dimethylhexa-2,4-diene,743 and 2,3-dimethyl-2-butene744 can 7660


Chemical Reviews

REVIEW

Table 181. Reaction of Substituted Epoxides with Organolithium- and Organomagnesium-Modified TMPLi

entry

yields (%) (E/Z ratio)

PhLi

93

2

4-anisylLi

70

15

3

2-furylLi

32

60

4 5

2-thienylLi vinylLi

39 73 (98:2)

59

1

Dec

Dec

6

Me2CdC(Me)Li

85 (E)

7

(E)-MeCHdCHLi

82 (98:2)

8

(E)-HexCHdCHLi

84 (99:1)

9

(E)-PhCHdCHLi

72 (98:2)

10

(Z)-MeCHdCHLi

70 (90:10)

11

(Z)-HexCHdCHLi

80 (91:9)

12b 13

(Z)-HexCHdCHMgCl (E)-Bu(Me)CHdCHLi

67 (91:9) 85 (91:9)

(Z)-Bu3SiO(CH2)2CHdCHLi

70 (92:8)

Me3SiCH2Li

71 (92:8)

i-Pr

Me3SiCH2Li

71 (98:2)

t-Bu

Me3SiCH2Li

63 (100:0)

18

Dec

Me3SiCH(Pent)Li

74 (E)

19

vinyl(CH2)6

Me3SiCH(Pent)Li

75 (E)

20 21

Ph(CH2)2 Bu3SiO(CH2)3

Me3SiCH(Pent)Li Me3SiCH(Pent)Li

70 (98:2) 65 (E)

14 15a

Dec

16a 17a

22

c-Hex

Me2PhSiCH(Pent)Li

72 (95:5)

23b

Dec

BuLi

73 (90:10)

BuMgCl

69 (96:4)

24b a

R0 M

R

In THF. b In Et2O.

be dimetalated using LIC-KOR. When compared with Li reagents, LIC-KOR favors the formation of linearly conjugated dianions over cross-conjugated ones. Examples are given in Table 188.741 De Jong and Brandsma have reported since 1982 propargylic metalations using LIC-KOR in THF at 75 °C, and the subsequent conversion of the organometallic species to 2,3-disubtituted thiophenes.745 Thieno[2,3-b]thiophenes (or thieno-1,4dithiines) have also been generated from 1,3-diynes (or 1,4diynes) using LIC-KOR and CS2.746 It is known that LIC-KOR and, sometimes more efficiently, LIC-NaOR (1:1 BuLi 3 t-BuONa) are able to metalate vinylic CH bonds. The presence of heteroatoms such as in cyclic vinyl ethers, 4H-pyrans, and 1,4-dihydropyridines favors the reaction (Table 189).717,730ab,730ad,747 Starting from twisted styrenes, it has been noted by Katsumura and co-workers that vinyl hydrogens can be more easily abstracted by LIC-KOR than benzyl hydrogens.748 Metalations of 1-heteroalkyl-1,3-butadienes using LIC-KOR followed by interception with isothiocyanates, either followed by S-alkylation749 or by hydrolysis to afford 2-imino-5,6-dihydro-2H-thiopyrans,750 were reported (Scheme 178).

In some cases, LIC-MOR (M = Na, K) is capable of metalating sp3 sites (Table 190),720e,729a,751 either activated as 2-substituted 1,3-dithianes751d or unactivated as 3-methoxytricyclo[2.2.1.0]heptane.729a They have also been used to generate carbenes.752 In the course of a study on the enantioselective deprotonation of cyclohexene oxide, Hilmersson and co-workers observed that mixed Li,Na chiral amides are more reactive than the corresponding single Li and Na amides.33c Used in hexane, LIC-KOR is capable of deprotonating benzene to afford, after subsequent trapping with CO2, benzoic acid in 77% yield. The superiority over BuLi (97:3 cis/trans ratio against 64:36) in the presence of t-BuOK.785

Mordini and co-workers documented in 1991 the metalation of N,N,3-tris(trimethylsilyl)propenamine using 1:1 DALi 3 t-BuOK followed by quenching with aldehydes and ketones. The unexpected formation of 2-aza-1,3-dienic products is explained by a 1,4-trimethylsilyl shift through an intramolecular nucleophilic substitution at Si (Scheme 184).786 Ricci et al. showed that bis(trimethylsilyl)methylimines can be selectively deprotonated 7666


Chemical Reviews

REVIEW

Table 187. Allylic Deprotonation Using LIC-KOR Followed by Electrophilic Trapping

entry 1a

a

R, R0 , R1, R2 Me (E), H, H, H

n

solvent, conditions

electrophile (E)

yield (%)

FB(OMe)2 then H2O2 (OH)

50730d

b

pentane, 25 °C, 6 h THF, 50 °C, 24 h

MeI (Me)

63730v

THFHexH, 50 °C, 10 h

Me3SiCl (SiMe3)

45730m

2

Me (E), H, H, SiMe3

2

3

Me, Me, H, H

1

c

Using s-BuLi instead of BuLi. Excess of substrate. Using BuI leads to mixtures of α- and γ-functionalized derivatives. b

c

Table 188. LIC-KOR-Mediated Dimetalation of 2,3-Dimethylbutadiene

entry

electrophile (E)

yield (%)

1

BuBr (Bu)

60

2 3

DodecBr (Dodec) OctadecBr (Octadec)

74 75

4

(CH2)2O ((CH2)2OH)

52

at C1 or C3, depending on the nature of the electrophile employed (Scheme 185).787 The benzylic position of benzenes bearing another substituent is in general selectively attacked using 1:1 DALi 3 t-BuOK or TMPLi 3 t-BuOK; the method has since been extended to pyridines and related compounds (Tables 208720h,751m,758f,758m,758q,758ac,761j,788 and 209788e). t-BuOK-activated TMPLi allows efficient allylic attacks of alkenes (Table 210);730x,789 from allyl isothiocyanate, using DALi 3 t-BuOK affords, after subsequent quenching with MeI, N-methyl-2-(methylthio)pyrrole in 55% yield.790 Both DALi 3 t-BuOK and, above all, TMPLi 3 t-BuOK are suitable for the functionalization of sensitive substrates; with additional PMDTA, the latter proves, for example, capable of functionalizing a substrate bearing heavy halogens at its less hindered position (Table 211).658k,751m,761a,761e,761k,791 cis-Stilbenes are selectively metalated at vinyl sites when treated by LIC-KOR or TMPLi 3 t-BuOK in THF at low temperatures to afford, after cis/trans isomerization, the functionalized derivatives (Table 212).792 5.2.2. Base-Na,K-HeteroMAAs. Na,K bases have also been employed (Table 213),729,747e,793 albeit far less than the corresponding Li,K bases. Secondary and tertiary K alkoxides were used for their accelerating effect on organosodium-mediated metalation of olefins684,686a and alkylbenzenes.685b,689 5.2.3. Base-M,Mg-HeteroMAAs (M = Li, Na, K).7 Trialkylmagnesates are efficient halogen/metal exchange agents. Oshima and co-workers reported in 2000 the efficiency of Bu3MgLi and, above all, i-PrBu2MgLi in I/Mg and Br/Mg exchange of aromatic, alkenyl, and alkyl halides.57ac,410 Further studies were performed;57cf,373,403e,679,794 they notably show that these reagents are suitable to functionalize various halides, including iodoazulenes and heteroaromatic (pyridine, thiophene, quinoline, and diazine) halides (Br, I). An alternative way, developed by Knochel and co-workers, employs LiCl to convert alkylmagnesiums into powerful halogen/

Mg exchange agents. i-PrMgCl 3 LiCl is particularly efficient, reacting with a large range of aryl and heteroaryl bromides,375,379,795 aryl and heteroaryl iodides,379,795e,795g,795i,795m,796 alkenyl iodides and bromides,796c,797 alkyl iodides and bromides,798 and even aryl sulfoxides.795l,799 s-Bu2Mg 3 LiCl is employed for the same purpose with electron-rich aryl bromides.800 The high reactivity of these halogen/Mg exchange agents was attributed to LiCl-mediated breaking of the Grignard reagents polymeric aggregates and subsequent higher Mg coordination number and more strongly electron-donating ligands. Ionic [Li(THF)4]+[i-Pr2MgCl 3 THF] was proposed as reactive species to explain the high reactivity of i-PrMgCl 3 LiCl in THF. Additives such as 1,4-dioxane or 15-crown-5 [known to complex Mg(II)] also favor the reaction of i-PrMgCl, probably by increasing the concentration of i-Pr 2 MgCl (Scheme 186).800 To suppress competitive aromatic deprotometalation during Li/Cl and Li/Br exchange reactions of ω-(halophenoxy)alkoxides, Screttas and co-workers successfully performed the reactions in the presence of (EtO(CH2 )2 O)2 Mg. 801 Richey and King observed in 1982 that addition of 15-crown-5 greatly accelerates (≈105) the fluorene metalation by Et2Mg in Et2O. Kinetic analysis of the reaction in THF indicated that metalation is first-order in 15-crown-5 but third-order in Et2Mg (k ≈ 4 102 L4 mol4 s1 at 26 °C); the authors proposed Mg ate complexes such as Et3Mg as possible deprotonating species.397 More generally, reactions in benzene between a diorganomagnesium R2Mg and an acidic hydrocarbon ZH (1,2,3,4-tetraphenyl-1,3-cyclopentadiene,44c fluorene,802 or indene802) in the presence of various macrocyclic coordinating agents form solutions of RMg(macrocycle)+,Z. Evidence for deprotonation of chloro- and fluorobenzene using combinations of R2Mg and a cryptand or alkoxide in Et2O at rt has also been obtained by trapping of benzyne to give ethylbenzene (maximum yield of 81 and 80%, respectively, using MeOK as additive).803 A particularly striking demonstration of the crown ether activation was published by Bickelhaupt and co-workers with the deprotonation of 5-bromo-1,3-xylylene-15-crown-4 depicted in Scheme 187.804 By mixing equimolar amounts of TMP2Mg and HMDSLi, the formation of the sterically crowded Li,Mg amide is followed by an unexpected deprotonation of one of the Me groups of the HMDS ligand to give ((Me3Si)NSiMe2CH2)(TMP)MgLi; this result shows that such structures can exhibit important chemical reactivity changes when compared with classical monometal amides.805 The deprotonation of thioanisole was attempted using 2:1 and 1:1 BuNa 3 (EtO(CH2)2O)2Mg in methylcyclohexane at 85 °C; after 2 h, a moderate 20% conversion was observed in the first 7667


Chemical Reviews

REVIEW

Table 189. DeprotonationTrapping Sequences of Different Substrates Using LIC-MOR (M = Na, K)a

a

For other examples, see the Supporting Information (Table S5). b In the presence of TMEDA (1 equiv). c Using (Me2N(CH2)2O)KLi instead of t-BuOK. d Excess of substrate. e s-BuLi instead of BuLi. f After transmetalation with LiBr. g A mixture of the 1- and 2-substituted products was obtained in a 87:13 ratio. h A mixture of the 1- and 2-substituted products was obtained in a 29:71 ratio. i Then desilylation and O-benzylation.

Scheme 178

7668


Chemical Reviews Table 190.

a

a

REVIEW

Mono- and Dideprotonation of Different Substrates Using LIC-MOR (M = Na, K)

For other examples, see the Supporting Information (Table S6). b Using s-BuLi instead of BuLi. c Excess of substrate. d Using t-BuLi instead of BuLi. 7669


Chemical Reviews

REVIEW

case. In contrast, if the previous 2:1 mixture is treated with K in the presence of a substrate capable of undergoing metalation, a Na displacement occurs, and 1,3-xylene, for example, is metalated at the Me group (73% yield).40 Deprotonation of toluene, thioanisole (Me group), and dibenzofuran is also noted when PhNa and PhK are combined with (EtO(CH2)2O)2Mg.67a In addition, metalation experiments of aromatic compounds

using 1:2 mixtures of Ph2Mg and metal 2-ethoxyethoxide in benzene indicate after carbonation an increasing reactivity in the order Li < Na (Table 214).67b Table 192. LIC-KOR-Mediated Metalation of Substituted Biphenyls Followed by Electrophilic Trapping

Scheme 179

entry

Scheme 180

1

R

n

H

2

electrophile (E) CO2 (CO2H and C(O))

53

15

2

Me2SiCl2 (SiMe2)

3

Br2 (Br)

31

49

4

I2 (I)

60

3

FB(OMe)2 then H2O2 (OH)

38

30

CO2 (CO2H)

41a

5 6 a

yield(s) (%)

OH

8

72

Isolated as the bis(lactone).

Table 191. LIC-KOR-Mediated Metalation of Substituted Benzenes Followed by Electrophilic Trapping

R, R0 or substrate

entry

n

solvent, conditions

electrophile (E)

hexane, 25 °C, 1 h

CO2 (CO2H)

65753e

hexane, 35 to 15 °C

(MeS)2 (SMe)

90753a

3

1

THF, 50 °C, 0.5 h

Me3SiCl (SiMe3)

60753e

4

hexane, 25 °C, 2 h

CO2 (CO2H)

56753e

THF, 20 °C, 1.52 h

MeI (Me) Me3SiCl (SiMe3)e

88753a 90753a

1 2

H, H

1

a

5 6

b

c

3,5-di(t-Bu) 2-CtCH

2

d

7

(MeS)2 (SMe)

78753a

8

Se (f)

75753a

THF, 70 °C

Se (f,g)

80753c

10

Te (f,g)

80753c

11

Me3SiCl (SiMe3)e,g

-753c

12

allylBr (allyl)h

753c

13 14

I2 (I)h Br2 (Br)h

71753b 59753b

C2Cl6 (Cl)g

59753b

9

15 16

2-CtCH

2

THF, 70 °C, 1 h

17

3-CtC-t-Bu

1

THF, 80 °C, 2 h

18

2,5-(CH2)6

3

hexane, rt, 0.3 h

Me3SiCl (SiMe3)e

95753d 78753d

Me3SiCl (SiMe3)

62753g

19

Bu3SnCl (SnBu3)

53753g

20

BrF2CCBrF2 (Br)

60753g

CO2 (CO2H) CO2 (CO2H)

24753h 51753h

21 22

2,5-di(t-Bu) 3,4-C(Me)2CH2C(Me)2

23

3,4-C(Me)2C(Me)2C(Me)2

24

1 1

cyclohexane, 75 °C, 25 h hexane, 25 °C, 24 h

1

hexane, 25 °C, 2 h

55753h

3,4-C(Me)2OC(Me)2

i

g

25 a

yield (%)

b

c

d

e

HC(O)NMe2 (CHO)

50753f

CO2 (CO2H)

79753f f

In the presence of TMEDA. Excess of substrate. Base in excess. Only 1 equiv of t-BuOK. Difunctionalized product. Benzoselenophene (and benzotellurophene) are obtained by trapping with Se (and Te) in the presence of HMPA and subsequent addition of t-BuOH. g After transmetalation with LiBr. h After transmetalation with MgBr2. i Under sonication. 7670


Chemical Reviews

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Table 193. LIC-KOR-Mediated Metalation of Substituted Benzenes Followed by Electrophilic Trappinga

a

For other examples, see the Supporting InformationI (Table S7). b Then acidic workup to allow deprotection. c Using t-BuLi instead of BuLi. d In the presence of PMDTA (1 equiv). e Using s-BuLi instead of BuLi. f Using MeLi instead of BuLi. g Using t-BuONa instead of t-BuOK. h Excess of substrate. i Then esterification using CH2N2. j Cyclization to fluorenone. k BuLi 3 t-BuOK or t-BuLi 3 t-BuOK 3 TMEDA. 7671


Chemical Reviews

REVIEW

Table 194. LIC-KOR-Mediated Metalation of Substituted Naphthalenes Followed by Electrophilic Trapping

entry

R

n

solvent, conditions

yield(s) (%)

1a

2-F

1

THF, 75 °C, 2 h

CO2 (CO2H)

59 (1-E), 25 (3-E)759e

2b 3

2-CF3 1-OMe

1 1

THF, 75 °C, 2 h THF, 75 °C, 2 h

CO2 (CO2H) CO2 (CO2H)

21 (1-E)759d,e 81 (2-E)759b

2

THF, 78 °C, 0.2 h

Me3SiCl (SiMe3)

80 (2-E)758i

Me3SnCl (SnMe3)

89 (2-E)758i

4 5 6

2-OMe

1

THF, 75 °C, 0.75 h

CO2 (CO2H)

92 (3-E)759e

7

1-OCH2OMe

2

THF, 78 °C, 0.2 h

Me3SiCl (SiMe3)

90 (2-E)758i

Me3SnCl (SnMe3)

97 (2-E)758i

1-OCH2OMe-5-OMe

2

THF, 78 °C, 0.2 h

Me3SiCl (SiMe3)

93 (2-E)758i

THF, 78 to 50 °C

Me3SnCl (SnMe3) MeI (Me)

98 (2-E)758i 48 (2-E)759c

THF, 80 °C, 1 h

Me3SiCl (SiMe3)c

86 (2-E)753d,759a

8 9

a

electrophile (E)

10 11

1-CO2H

4

12

1-CtCH

3

Using MeLi instead of BuLi.

b

c

Using t-BuLi instead of BuLi. Difunctionalized product.

Scheme 181

Castaldi and Borsotti patented in 1992 the metalation of 3-substituted [CF3 (Scheme 188), OMe, and NMe2] (trifluoromethyl)benzenes using Li magnesates.806 Bu3MgLi, TMP3MgLi, Bu2(TMP)MgLi, Bu(TMP)2MgLi, and even higher-order Bu4MgLi2 and Bu3(TMP)MgLi2 were later employed to functionalize aromatic fluorides including heterocycles,807 as well as chloro-808 and triazolopyridines809 (Table 215). The presence of TMEDA, the replacement of Bu with TMP ligands, and the use of higher-order magnesates in general result in improved conversions. The intermediate arylmagnesates prove stable enough to be successfully involved in Pd-catalyzed cross-coupling reactions, a reaction normally problematic from the corresponding aryllithiums. For substrates sensitive to nucleophilic attacks, the generated arylmetal species can be converted to symmetrical dimer before electrophilic trapping (Table 216). The dimer formation is explained by a deprotonation giving a sterically congested magnesate, which stabilizes through 1,2-migration.807,808 For heterocycles such as thiophenes810 and furans,811 H/Mg permutations occur next to S and O, respectively. Whereas Bu3MgLi (0.33 equiv) is reactive enough to deprotonate thiophenes within 30 min using THF at rt, recourse to highly coordinated Li magnesate Bu4MgLi2 allows one to shorten to 1.5 h the reaction times required with furans (Table 217). Even if it is in general less obvious to functionalize oxazoles through deprotonative metalation due to competitive ring-opening, the 2-substituted derivatives can be obtained in moderate to good yields through a similar scenario (Table 218).812 The deprotometalationfunctionalization sequence was more recently extended to N-methyl- and N-tert-butylpyridine carboxamides (Table 219)813 and to functionalized benzenes (Table 220).814 Sosnicki used Bu3MgLi to abstract two protons from 3,6-dihydro-1H-pyridin-2-ones in order to reach the corresponding

3,3-dialkylated products. 3,5-Dialkylated lactams are concomitantly formed in some cases due to allylic conjugation (Table 221).403d The cooperativity obtained by pairing Li amide and Mg diamide in TMP3MgLi, coming from Bu3MgLi and TMPH, was employed in 2001 to dideprotonate ferrocene, giving [{Fe(C5H4)2}3{(TMP)2Mg3Li2(TMPH)2}]. Replacing Li with Na similarly resulted in [{Fe(C5H4)2}3{(TMP)2Mg3Na2(TMPH)2}].815 The reactivity of the corresponding DA3MgM (M = Li, Na) toward phenylacetylene was investigated, leading to mixed-metal acetylido-amido magnesates [(PhCtC)(DA)2 MgLi]2 (1 equiv of base used) and [(DA)Mg(CtCPh)2 Na(TMEDA)]2 (2 equiv). 816 The conversion of benzene to [(TMP)Mg(TMP)(Ph)Na(TMEDA)] is accomplished in 44% yield upon treatment in hexane reflux by [(TMP)Mg(TMP)(Bu)Na(TMEDA)], a base synthesized by reacting TMPH with a 1:1 mixture of BuNa and Bu2Mg in the presence of the chelating auxiliary.817 Toluene is similarly deprotomagnesated at C3 to afford [(TMP)Mg(TMP)(tolyl)Na(TMEDA)] in 58% yield.818 In both cases, the Bu ligand is consumed during the reaction. It proves to be also the case starting from furan819 (metalation at C2), bis(benzene)chromium820 (metalation exclusive to one ring, using Na or K as alkali metal), and bis(toluene)chromium821 (metalation at C4 exclusive to one ring). The presence of TMEDA, which can, for example, help to dissolve species, is mandatory in some cases, e.g., to allow the conversion of bis(toluene)chromium. Other alkali metal ligands such as PMDTA have also been employed, as for [(TMP)Mg(CH2SiMe3)(TMP)K(PMDTA)]mediated anisole ortho-metalation. The study established that the base reacts kinetically through its TMP ligand to furnish [(Me3SiCH2)Mg(2-anisyl)(TMP)K(PMDTA)] which, in turn, 7672


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Table 195. LIC-KOR-Mediated Mono- or Dimetalation of Heterocycles Followed by Electrophilic Trapping

a

Reaction performed in the presence of 2 equiv of TMEDA. b Formation of the acetate. c Then subsequent cyclization. d In the presence of DAH (10 mol %). e The 2-, 3-, and 4-metalated pyridines are obtained in about 4:1:4 ratios. f Excess of substrate. g The 2-, 3-, and 4-metalated pyridines are obtained in about 6:1:3 ratios. h The 2-, 3-, and 4-metalated pyridines are obtained in about 2:3:15 ratios. i After addition of LiBr.

Table 197. Reaction of α,β-Unsaturated Acetals with LIS-KOR Followed by Electrophilic Trapping

Table 196. LIC-KOR-Mediated Conversion of 2,3-Epoxyamines to Hydroxyaziridines

entry

R1, R2, R3

yield (%)

entry

R1, R2, R

electrophile (E)

yield (%)

1

Me, H, H

85

1

Me, H, Me

H2O (H)

96764c

2 3

Bu, H, H (CH2)5, H

85 96

2 3

H, H, H

D2O (D) MeI (Me)

95764a 85764a

4

(CH2)2CHdCMe2, Me, H

88

4

EtI (Et)

80764a

5

Pr, H, Me

40

5

t-BuCHO (CH(OH)-t-Bu)

50764a

6

Me3SiCl (SiMe3)

90764a

Me3SiCl (SiMe3)

82764a


75764b

reacts thermodynamically through its alkyl ligand to afford [(TMP)Mg(2-anisyl)(TMP)K(PMDTA)] and Me4Si.822 Subjecting benzene (0.5 equiv) to a reagent prepared from TMPNa, t-Bu(TMP)Mg, and TMEDA leads to 1,4-dimetalated complex [1,4-{(TMP)Mg(TMP)Na(TMEDA)}2C6H4] (46% yield).823 TMP2BuMgNa, without TMEDA, is capable of dideprotonating benzene at its 1- and 4-positions and toluene at

7 8

H, Me, H

its 2- and 5-positions (maximal distances between the negative charges on the ring), giving products of the formula [(C6H4)(TMP)6Mg2Na4] and [(C6H3Me)(TMP)6Mg2Na4], respectively.824 In addition to the synergy, what is remarkable 7673


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Table 198. Reaction of α,β-Unsaturated Acetals with LIS-KOR Followed by Electrophilic Trapping

R1, R2, R

entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

H, H, Et

Me, H, Et

H, Me, Et H, H, CH2CMe2CH2OH Me, H, CH2CMe2CH2OH H, Me, CH2CMe2CH2OH (CH2)2CHdCMe2, H, Me

electrophile (E)

yield (%)

PhCHO (CH(OH)Ph) t-BuCHO (CH(OH)-t-Bu) (E)-MeCHdCHCHO (CH(OH)CHdCHMe) (MeO)2C(O) (CO2Me) PhCO2Et (C(O)Ph) Ph2C(O) (C(OH)Ph2) Me2C(O) (C(OH)Me2) t-Bu2C(O) (C(OH)-t-Bu2) t-BuC(O)Me (C(OH)(t-Bu)Me) CO2 (CO2H) MeI (Me) BuI (Bu) Me3SiCl (SiMe3) (CH2)2O ((CH2)2OH) (CH2CHMe)O (CH2CH(OH)Me) (CH2CH(Et))O (CH2CH(OH)Et) (CH2CH((CHCH2)O)O (CH2CH(OH)((CHCH2)O)Me) Bu3SnCl (SnBu3) (E)-CH2dCMeCHdCHCMe2CHO (CH(OH)CMe2CHdCHCMedCH2) Me3GeX (GeMe3) Et3GeX (GeEt3) Ph3GeX (GePh3) t-BuCHO (CH(OH)-t-Bu) Me3SiCl (SiMe3) Bu3SnCl (SnBu3) Me3GeX (GeMe3) Et3GeX (GeEt3) Et3GeX (GeEt3) Bu3GeX (GeBu3) Et3GeX (GeEt3) Et3GeX (GeEt3) H2O (H)

61766b 70764b 90766a 70766a 65766a 64766b 91766a 74766b 74766b 48766a 80764b 60764b 80764b 70766c 80766c 65766c 55766c 64766e 83766f 80766g 55766g 67766g 80764b 60764b 81766e 70766g 97766g 70766g 90766g 50766g 67766g 77763,766d

Scheme 182

is the regioselectivity of the reactions involving toluene, for which the most acidic hydrogen belongs to the Me group. The base prepared by adding TMPH (3 equiv) to a 1:1 mixture of BuK and Bu2Mg, and consisting of an in situ mixture of TMP2Mg and TMPK, converts benzene and toluene into [(Ph)6(TMP)12Mg6K6] (72% yield) and [(tolyl)6(TMP)12Mg6K6] (81% yield), respectively.825 These conversions, starting from furan,819 benzene, and toluene,824,825 correspond to encapsulations with formation of “inverse crown compounds”.52b Such an encapsulation also takes place by reacting ferrocene with DA3MgNa (2 equiv), prepared by adding DAH (6 equiv) to a 1:1 mixture of BuNa and (s-)Bu2Mg (2 equiv each), to give the 1,10 ,3,30 -tetrayl derivative [(DA)8Mg4Na4{Fe(C5H3)2}] in

42% yield.826 The yields can be improved using 4 equiv of base (72% yield), and the reaction can be extended to ruthenocene (53% yield) and osmocene (62% yield).827 Adjusting the alkyl component can result in different regioselectivities. From toluene, replacing the Bu ligand of TMP2BuMgNa by Me3SiCH2 leads to a switch from ortho meta to metameta.828 A change is also noticed when furan is subjected to [(TMP)Mg(TMP)(CH2SiMe3)Na(TMEDA)], with formation of mixed mono- and dideprotonated [{(2-C4H3O)5(2,5-C4H2O)3(Me3SiCH2)Mg3Na6(TMEDA)3}2] against monometalated [{(2-C4H3O)6(Mg2(TMEDA)){Na2(THF)3}}∞] using [(TMP)Mg(TMP)(Bu)Na(TMEDA)].829 Thiophene is more prone to deprotonation reaction than furan 7674


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Table 199. LIS-KOR-Mediated Conversion of α-Phenyl Acetals to Propargyl Alcohols

entry

R

1

H

electrophile (E)

yield (%)


72769

2

PhCHO (CH(OH)Ph)

95769

3

t-BuC(O)Me (C(OH)(t-Bu)Me)

64769

4

t-Bu2C(O) (C(OH)-t-Bu2)

57769

5

Ph2C(O) (C(OH)Ph2)

83769

6

(CH2)2O ((CH2)2OH)

68766c

7 8

(CH2CH(Me))O (CH2CH(OH)Me) Bu3SnCl (SnBu3)

50766c 92766e


87769

10

PhCHO (CH(OH)Ph)

81769

11

(E)-MeCHdCHCHO (CH(OH)CHdCHMe)

38769

12

t-BuC(O)Me (C(OH)(t-Bu)Me)

68769

13

(CH2)2O ((CH2)2OH)

93766c

14

(CH2CH(Me))O (CH2CH(OH)Me)

93766c

9

Me

Scheme 183

Table 200. DALi 3 t-BuOK-Mediated Metalation of 1-(Phenylseleno)alkenes Followed by Electrophilic Trapping

entry

R, R0

1

H, H

electrophile (E)

yield (%)

MeI (Me)

98

2

DecBr (Dec)

94

3


92

4

(CH2)5C(O) (C(OH)(CH2)5)

80

5 6

Bu, H

(CH2)2O ((CH2)2OH) MeI (Me)

92 85

7

H, Bu

MeI (Me)

85

8

i-Pr, H

MeI (Me)

80

DecBr (Dec)

88


95

9 10

and is converted to tris(α-magnesated) [(TMEDA)(C4H3S)3MgNa(TMEDA)] either by using [(TMP)Mg(TMP)-

(CH2SiMe3)Na(TMEDA)] (1 equiv) or, also in a good yield, by reacting BuNa, (Me3SiCH2)2Mg, TMEDA, and thiophene (1:1:2:3 ratio) in hexane. In contrast, tetrahydrothiophene provides [(TMP)Mg(TMP)(2-C4H7S)Na(TMEDA)] in moderate yield upon treatment with [(TMP)Mg(TMP)(CH2SiMe3)Na(TMEDA)]; stirring at rt for 3 weeks leads to [(1,4-{(TMP)Mg}2(C4H4)){(TMP)Na(TMEDA)}2],830 a complex containing a dimetalated butadiene fragment already observed by treatment of THF with the same base.831 The reactivity of Bu3MgNa and Bu4MgNa2 toward 2,4,6trimethylacetophenone was investigated by Hevia and co-workers. As expected with a bulky mesityl group, the additions to the CdO group are minimized, favoring the deprotonation reaction to afford mixed Na,Mg enolates.832 What contrasts with the Al, Cr, Mn, Fe, Co, Cu, and Zn ate bases is that the presence of a bulky amino group in a magnesate base is not mandatory. This can be exemplified with the magnesation of N-methylindole, which occurs regioselectively at C2 upon treatment with TMEDA-activated Bu4MgNa2 in hexane, as demonstrated by subsequent Pd-catalyzed cross-coupling with iodobenzene (Scheme 189).833 Knochel and co-workers started developing in 2006 the use of (“turbo-Grignard”) Li,Mg bases. TMPMgCl 3 LiCl, prepared 7675


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Table 201. DALi 3 t-BuOK-Mediated Metalation of Bis(phenylseleno)acetals Followed by Electrophilic Trapping

entry

R

1 2

Me

electrophile (E)

yield (%)

MeI (Me) BuI (Bu)

98 95

3

i-PrI (i-Pr)

70

4

DecBr (Dec)

95

5

BnBr (Bn)

88

6

77

7

Me2C(O) (C(OH)Me2) (CH2)5C(O) (C(OH)(CH2)5)

8


90

(CH2)2O ((CH2)2OH) BnBr (Bn)

82 71


63

BnBr (Bn)

98


84

9 6

i-Pr

7 8 9

Dec

76

Table 202. DALi 3 t-BuOK-Mediated Ring-Opening of Oxiranes

a

Table 203. DALi 3 t-BuOK-Mediated Ring-Opening of Alkoxy-Substituted Oxiranes

Using LIC-KOR instead of DALi 3 t-BuOK.

from i-PrMgCl 3 LiCl and TMPH, is suitable to functionalize sensitive heterocycles and benzenes bearing functional groups, provided that the temperature is adapted (Table 222).795f,l,m,796g,799,834 The method allows full functionalization of all the four thiophene positions starting from 2,5-dichlorothiophene (Table 223)835 and the elaboration of functionalized ferrocenes (Table 224);836 it was extended to magnesationamination sequences.834g,837 It is worth knowing that TMPMgCl 3 LiCl can be alternately prepared from TMPLi and MgCl2 in THF (1 day of stirring).838 According to Knochel and co-workers, the efficiency of the reactions is in relation with the presence of LiCl, which breaks up

oligomeric aggregates of Mg amides. This possibility was more recently supported by structural studies performed by Mulvey and co-workers, which show that TMPMgCl 3 LiCl exists in the crystal as [(THF)(TMP)MgCl2Li(THF)2].838 In the solid state, its DA analog DAMgCl 3 LiCl, prepared by mixing DALi with MgCl2 in THF, exists as [{(DA)MgCl2Li(THF)2}2]. In 2010 it appeared that the solid-state structures of TMPMgCl 3 LiCl and DAMgCl 3 LiCl are not retained in THF solution and that Li- and Mg-containing species do not strongly form CIPs. The solventseparated nature of TMPMgCl 3 LiCl and DAMgCl 3 LiCl distinguishes them from most of the mixed alkyl-amido species, which are contact ion-pair arrangements. It also suggests distinct mechanisms for both kinds of bases, explaining different reactivity features.839 Knochel and co-workers then realized that Mg diamides complexed with LiCl of the formula TMP2Mg 3 2LiCl, prepared by reacting either TMPLi (2 equiv) with MgCl2 or TMPMgCl 3 LiCl (1 equiv) with TMPLi, are more suitable to abstract protons from more moderately activated arenes (Table 225).795f,796g,799b,834c,834f,834h,834k,840 Among the different functional groups tolerated in these reactions, OP(O)(NMe2)2 appears as a strong ortho-directing one due to its ability to activate the base MgN bond by forming a transient ate species (Table 226). In addition, different subsequent functionalizations are possible by nucleophilic substitution of the OP(O)(NMe2)2 group.841 Bases synthesized from 2,2,4,6,6-pentamethylpiperidine and tert-butyl(isopropyl)amine can be alternately employed (Table 227).842 5.2.4. Base-M,Al-HeteroMAAs (M = Li, Na, K). Dollinger and Howell studied the carbanion-mediated ring-opening of 2-methyleneoxetanes. They reported in 1998 such reactions using BuLi or PhLi in the presence of Me3Al (Scheme 190).843 Since 2004, Uchiyama and co-workers have documented aromatic deprotoaluminations. i-Bu3Al(TMP)Li, prepared by mixing i-Bu3Al and TMPLi in THF, was identified on the basis of NMR, theoretical calculations, and X-ray crystallography studies. It is the Li,Al reagent of choice for the rt metalation of anisole (TMP acting as the active base), as demonstrated by subsequent trapping with I2. Other substrates were functionalized, and in the case of PhC(O)N-i-Pr2, other kinds of trapping were considered (Table 228).844 DFT calculations using a model reaction, in situ FT-IR spectroscopy, and X-ray crystallography allowed the authors to propose the mechanism shown in Scheme 191 to depict the reaction. The aluminate base seems to function as an amido rather than as an alkyl base but, unlike its zincate analogs (see section 5.2.10), without the successive quench of alkyl ligands by in situ formed TMPH. The base can thus be described as kinetic 7676


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Table 204. DALi 3 t-BuOK-Mediated Ring-Opening of Benzyl Oxiranyl Ethers

R1, R2, R

entry

solvent, conditions

1781a 2781a

Pent, H, OMe Pent, H, H

THF, 50 °C, 15 h THF, 50 °C, 15 h

33

84 (Z/E 40/60)

3781a

Pent, H, F

THF, 50 °C, 15 h

28

35 (Z/E 30/70)

4781a

Pent, H, Me

pentane, 25 °C, 15 h

27

21 (Z/E 84/16)

5781a

Pent, H, t-Bu

THF, 50 °C, 15 h

24

42 (Z/E 48/52)

6781e

H, Pent, H

THF, 78 °C, 0.5 h

70

7781e

H, Pr, H

THF, 78 °C, 0.5 h

68

8781e

Me, Me2CdCH(CH2)2, H

THF, 78 °C, 0.5 h

69

9781e 10781e

H, TBDPSOCH2, H H, MeOCH2, H

THF, 78 °C, 0.5 h THF, 78 °C, 0.5 h

64 50

11781e

H, Ph3COCH2, H

THF, 78 °C, 0.5 h

66

12781e

H, Bn2NCH2, H

THF, 78 °C, 0.5 h

64

13781e

H, Et2NCH2, H

THF, 78 °C, 0.5 h

75

14781e

H, (CH2)5NCH2, H

THF, 78 °C, 0.5 h

59

Table 205. DALi 3 t-BuOK-Mediated Ring Isomerization of Oxiranyl Ethers

entry

R0

R H

45

Pent

65

3

CH2OTBDMS

53

Pent

53

CH2OTBDMS

55

Pent CH2OTBDMS

55 50

4

vinyl

Ph

5 6 7

CtCH

Table 206. DALi 3 t-BuOK-Mediated Ring-Opening of Aziridyl Ethers

R, R0

entry

yield (%)

1

Bu, OCH2OMe

56783

2

MeOCH2O, OCH2OMe

51783

MeOCH2O, Ph

70783

(CH2)2 CH2

46783b 42783b

Et, Et

48783b

3 4 5 6

a

Yield of 64% using THF at 50 °C.

Table 207. Reaction of DALi 3 t-BuOK with Arylethyl Oxiranes

yield (%)

2

1

a

yield(s) (%)

[MeC(O)i-Pr is selectively converted to the kinetically preferred enolate].845 The regioselectivity was explained by a coordinative approximation effect between functional group and counter Li+ ion, enabling initial complex formation and orienting the ate base ligand toward aromatic ortho H atom.844b The structure of the deprotometalated PhC(O)N-i-Pr2 was obtained more recently by Mulvey and co-workers using X-ray crystallography.846 The scope of putative i-Bu3(TMP)AlLi was extended to aliphatic compounds bearing ether and carbamate functions. Good regioselectivities were obtained (α-substitution favored) after subsequent interception using aldehydes (Table 229). For allyl (2-methoxyphenyl) ether, the high stereoselectivity observed was found to be linked to the presence of the OMe group, able to coordinate Li+ and thus to fix the relative position and direction of the aldehyde with respect to the allylaluminate.844b In 2006, Mulvey and co-workers introduced [i-Bu2Al(i-Bu)(TMP)Na(TMEDA)], synthesized in hexane from TMPNa, 7677


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Scheme 184

Scheme 185

i-Bu3Al, and TMEDA and identified by X-ray crystallography. In the reaction with phenylacetylene (1 equiv), [(i-Bu2)Al(i-Bu)(TMP)Na(TMEDA)] functions as a TMP base, leading to the formation of [(i-Bu2)Al(CtCPh)(i-Bu)Na(TMEDA)2] (21% yield with an additional equiv of TMEDA). The authors also obtained crystals by treating PhC(O)N-i-Pr2 with i-Bu3Al(TMP)Li (Uchiyama and co-workers’ Li aluminate), prepared in situ in hexane containing TMEDA (1 equiv). [(i-Bu)2Al{CH2N(Me)(CH2)2NMe2}{PhC(O)N-i-Pr2} 3 Li{2-(1-C(O)Ni-Pr2)C6H4}] was isolated (43% yield) and identified by X-ray crystallography, showing a deprotonation of both benzamide and TMEDA by contacted Li-activated Al-attached TMP and i-Bu bases (Scheme 192).847 i-Bu3Al(TMP)Li was more recently found to function as a dual amido-alkyl base toward TMEDA, yielding [(i-Bu)2Al{CH2N(Me)(CH2)2NMe2}2Li] (activation of both TMP and i-Bu ligands by intramolecular communication between Li and Al). In contrast, i-Bu2Al(TMP)2Li acts as a TMP base in the deprotonation of TMEDA and PMDTA, affording [(i-Bu)2Al(TMP){CH2N(Me)(CH2)2NMe2}Li] and [(i-Bu)2Al(TMP){CH2N(Me)(CH2)2N(Me)(CH2)2NMe2}Li], respectively.848 Reacting TMPK with i-Bu2Al(TMP) and TMEDA in a 1:1:1 stoichiometry in hexane results in the formation through CH bond activation of [(i-Bu)Al(i-Bu)(TMP*)K(TMEDA)], where TMP* represents a CH3- and NHdideprotonated ligand; the reaction can be regarded as a selfdeprotonation, with one TMP anion reacting with the other.849 In 2007, the same authors reported the synthesis in hexane of [(i-Bu2)Al(i-Bu)(TMP)Li(L)] (L = TMPH, Et3N, and PhC(O)N-i-Pr2). When the benzamide-containing aluminate is treated by 1,4-dioxane, the preferential deprotonation of the cyclic diether (which is presented as the result of a cooperation between metals in a CIP system) is followed by its fragmentation (38% yield); a pathway is proposed to explain this result (Scheme 193). It is interesting to note that the presence of benzamide, which strongly coordinates Li, is required to observe dioxane fragmentation.850 The same year, Mulvey and co-workers revisited the behavior of i-Bu3Al(TMP)Li toward PhC(O)N-i-Pr2 and observed a marked solvent dependency of the reaction efficiency: the reaction, easy in bulk THF, becomes negligible using stoichiometric THF and does not work in bulk hexane or benzene. The

isolation of solvent-separated [Li(THF)4][i-Bu4Al] in THF, a species not capable of deprotonating PhC(O)N-i-Pr2, shows that the dismutation of i-Bu3Al(TMP)Li is possible, also furnishing putative i-Bu2Al(TMP)2Li.846 Unlike i-Bu3Al(TMP)Li, which is stable in THF,844a i-Bu2Al(TMP)2Li is able to metalate THF,851 its sulfur analog,851 and TMEDA,848 all at their α position. Wunderlich and Knochel reported in 2009 the synthesis of two Li,Al compounds, TMP3Al 3 3LiCl and (t-Bu(t-Bu(i-Pr)CH)N)3Al 3 3LiCl, from AlCl3 and the corresponding Li amides (3 equiv). Their good solubility in THF, due to the presence of LiCl, allows their use as deprotonating agents for a large range of aromatics (Table 230).795m,834j,852 Owing to the strong Lewis acidic character of Al, regioselectivities next to O-containing substituents are obtained. 5.2.5. Base-Na,Cr-HeteroMAAs. Albores et al. isolated Na chromate(II) [(TMP)Cr(CH2SiMe3)(TMP)Na(TMEDA)] (identified by X-ray crystallography) from homoleptic (Me3SiCH2)2Cr [made by metathesis between Me3SiCH2Li and (THF)2CrCl2], TMPNa, and TMEDA, showing a dismutation process. When (Me3SiCH2)2Cr is treated with TMPNa (2 equiv), TMPH (1 equiv), and benzene (0.5 equiv), Na chromate [C6H4(TMP)6Cr2Na4] is obtained through a direct Cr/H exchange (Scheme 194).853 5.2.6. Base-M,Mn-HeteroMAAs (M = Li, Na). The organometallic chemistry of Mn(II) is distinct from that of a typical transition metal due to a significant ionic contribution to the MnC bonds. The reduced influence of covalency and the 18-electron rule result in compounds possessing typical reactivity and structural diversity.854 Bimetallic compounds in which Mn(II) is combined with an alkali metal met a recent development as strong bases. In order to avoid thermic β-H decomposition491 and coordination of a fourth ligand to Mn,105a,b Mulvey and co-workers chose (Me3SiCH2)2Mn and TMPLi as Mn(II) and Li source, respectively. [(Me3SiCH2)Mn(CH2SiMe3)(TMP)Li(TMEDA)], isolated in 67% yield by mixing together (Me3SiCH2)2Mn, TMPLi, and TMEDA, can be used for the 2-fold Li-mediated manganation of ferrocene (1- and 10 -position), a substrate inert toward conventional organomanganese reagents.855 The authors showed that the mixed Li,Mn base functions, on the whole, as a tribasic dialkyl-monoamido reagent toward ferrocene, affording [{Fe(C5H4)2}3Mn2Li2(TMEDA)2] in 83% yield (Scheme 195). Both the isolated base and the metalated species were characterized by X-ray crystallography. The previous Li,Mn base being unable to deprotonate toluene, a complementarity Na-mediated manganation was developed. Substituting TMPNa for TMPLi in the previous base synthesis 7678


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Table 208. Benzylic Metalation of Aromatic Compounds Followed by Electrophilic Trappinga

a

For other examples, see the Supporting Information (Table S8). b Using t-BuOK (2 equiv). c Then esterification. d Then deprotection.

protocol does not result in the formation of [(Me3SiCH2)Mn(CH2SiMe3)(TMP)Na(TMEDA)]. Crystalline monoalkylbisamido product [(TMP)Mn(CH2SiMe3)(TMP)Na(TMEDA)] is obtained instead (probably by dismutation of the former since putative all-alkyl coproduct [(Me3SiCH2)3MnNa(TMEDA)] concomitantly forms and remains in solution), and its yield can be improved to 53% by introduction of an extra equivalent of TMPH. Treatment of benzene using this base (0.5 equiv) at hexane reflux

produces [(TMP)Mn(Ph)(TMP)Na(TMEDA)]. During this process, the stronger base CH2SiMe3 bridging the Na and Mn atom is selectively expelled, and there is retention of the molecular connectivity of the [(TMP)Mn(TMP)Na(TMEDA)] unit. In the absence of diamine, a mixture of BuNa (4 equiv), TMPH (6 equiv), and (Me3SiCH2)2Mn (2 equiv), equating to (Me3SiCH2)2(TMP)6Mn2Na4 after evolution of four BuH and two Me4Si, converts benzene (or toluene) to dimetalated compound 7679


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Table 209. Benzylic Double and Triple Metalation of Azines Followed by Electrophilic Trapping

Table 210. Benzylic Metalation of Dienes Followed by Electrophilic Trapping

a

Table 211. Metalation of Different Aromatic Compounds Using R2NLi 3 KOR Followed by Electrophilic Trapping

Then trapping with i-BuCHO.

[(C 6H4)(TMP)6Mn2 Na 4]856 (or [(TMP)6(3,5-Mn2C6H3CH3)])828 (Scheme 196). Remarkably, toluene can be similarly dideprotonated (C3C5) using the corresponding mixture of BuNa (4 equiv), TMPH (6 equiv), and (Me3SiCH2)2Mg (2 equiv), whereas the reaction takes place at the 2,5-positions using [(TMP)Mg(TMP)(Bu)Na].824 [(TMP)Mn(CH 2 SiMe 3 )(TMP)Na(TMEDA)] (1 equiv) can ortho-metalate anisole and PhC(O)N-i-Pr2 in hexane,857 affording [(TMP)Mn(2-tolyl)(TMP)Na(TMEDA)] and [(Me3SiCH2)Mn{2-[C(O)N-i-Pr2]C6H4}(TMP)Na(TMEDA)], respectively, as crystalline products (Scheme 197). Coupling the phenylmanganates with PhI under PdCl2 3 dppf catalysis generates the expected unsymmetrical biaryl compounds in respective yields of 98 and 66%. The base on the whole behaves as an alkyl base toward anisole, generating Me4Si, and as an amido base toward the benzamide, liberating TMPH. Note that the same benzamide induces ortho-metalation through alkyl (t-Bu) basicity when subjecting to related zincate base [(t-Bu)Zn(t-Bu)(TMP)Na(TMEDA)].858 The same authors attempted the use of DA ligand-containing manganates, which can undergo thermally induced β-H elimination reactions.859 The outcome of the reaction using a

a

In the presence of PMDTA.

mixture of BuNa (1 equiv), (Me3SiCH2)2Mn (1 equiv), and DAH (3 equiv) with ferrocene in hexanetoluene is temperature-dependent, with hydrido product [(DA)4H2Mn2Na2(toluene)2] and trinuclear ferrocenophane [{Fe(C5H4)2}3{(DAH)2(DA)2Mn3Na2}] respectively obtained at reflux and 0 °C (Scheme 198). 7680


Chemical Reviews Table 212. Metalation of Stilbenes Using Li,K Bases Followed by Electrophilic Trapping

REVIEW

Scheme 187

Table 214. Metalation Using 1:2 Ph2Mg 3 EtO(CH2)2OM Followed by Conversion to Carboxylic Acids

a

After Fries rearrangement.

Table 213. Metalation of Different Substrates Using Na,K and Na,Na Bases Followed by Electrophilic Trapping

a

In the presence of THF (1 equiv).

Scheme 188

Scheme 186

Among metal amides complexed by LiCl, TMP2Mn 3 2MgCl2 3 4LiCl (stable at rt, and prepared in THF at 0 °C from MnCl2 3 2LiCl and commercially available TMPMgCl 3 LiCl in a 2:1 ratio) is suitable for the directed manganation of functionalized arenes and heterocycles. The generated diarylmanganese species either react with a large range of electrophiles (Table 231), or are successively converted to Cu(I) species and treated with Li amides to afford, after oxidative amination, various arylamines.860 5.2.7. Base-M,Fe-HeteroMAAs (M = Li, Na, MgX). Albores et al. reported in 2009 the synthesis in good yield of heteroleptic Fe(II) [(Me3SiCH2)2Fe(TMP)Na(TMEDA)] by adding TMPNa to (Me3SiCH2)2Fe 3 TMEDA. Mixing the latter with BuNa and TMPH in a 2:4:8 molar ratio and then adding benzene (1 equiv)

result in the formation of an Fehost inverse crown complex [(C6H4)(TMP)6Fe2Na4], isolated in 39% yield, through dideprotonation of benzene (Scheme 199).853 In the course of their studies aimed at developing different LiCl-solubilized metal TMP bases, Wunderlich and Knochel recently described a Fe(II) base, TMP2Fe 3 2MgCl2 3 4LiCl, able to convert 1,3-disubstituted arenes into the corresponding diaryl Fe(II) derivatives at rt. Subsequent cross-coupling with alkyl iodides (or bromides) and benzylic chlorides promoted by 4-fluorostyrene affords polyfunctionalized arenes (Table 232).860b,861 Using benzothiophene as substrate, the group of Daugulis noted that reaction times longer than 10 min led to extensive decomposition.862 5.2.8. Base-Li,Co-HeteroMAAs. The use of TMP-based mixed Li,Co bases to functionalize aromatics was reported in 2010 (Table 233). The generated arylmetal compounds can be trapped by different electrophiles but hardly tolerate the presence of functional groups. Electrophile-dependent coformation of symmetrical dimers, which represents a drawback for the reaction, can be reduced in some cases employing 2 equiv of base. The dimer formation becomes important using (π-deficient) 2-methoxypyridines as substrates and even major using allylBr as electrophile (reaction probably initiated 7681


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Table 215. Magnesate-Mediated Metalation of Aromatic Compounds Followed by Electrophilic Trapping

Using Bu3MgLi 3 TMEDA. b After reflux for 18 h in the presence of PdCl2 3 dppf [dppf = 1,10 -bis(diphenylphosphino)ferrocene, 1 mol %]. c Et2O instead of THF.

a

Table 216. Magnesate-Mediated Metalation of Azines Followed by Treatment with I2

by one-electron transfer from the Co(II) ate compound to the electrophile).863 5.2.9. Base-Li,Cu-HeteroMAAs. Gais and co-workers reported in 2008 the deprotonation of (Z)- and (E)-alkenyl sulfoximines at the α-position upon treatment with Gilman-type organocuprates; the resulting Cu derivatives undergo a 1,2metalate rearrangement before trapping. The authors propose

a precomplexation of the cuprate base by the sulfoximine group at the origin of the easy metalation.511c Uchiyama and co-workers reported in 2007 the first cuprate-mediated aromatic deprotonation. Using benzonitrile as substrate, several amidocuprates was tested: whereas Gilman-type cuprates fail in giving the 2-iodo derivative in satisfying yields after subsequent trapping with I2, Lipshutz-type cuprates allow one to obtain better conversions. The deprotonationiodination sequence using Me(TMP)Cu(CN)Li2 (2 equiv) was next extended to other aromatic compounds, and the reactivity of the arylcuprate coming from PhC(O)N-i-Pr2 toward electrophiles was studied (Table 234).125,864 TMP 2 CuLi was more recently employed for the deprotonative cupration of different aromatics including heterocycles; among the electrophiles used, aroyl chlorides give the best results (Table 235). 865 5.2.10. Base-M,Zn-HeteroMAAs (M = Li, Na, K, MgX)7,866. If Te/Zn permutations have likewise been reported,140 zincates have first demonstrated good potential as chemoselective halogen/metal exchange agents. The sterically more hindered Br atom of 1,1-dibromoalkenes is predominantly exchanged using Bu3ZnLi.867 Me3ZnLi can be used to functionalize iodobenzenes, substituted at the 4-position (OMe, CO2Me, NO2) or not,868 and N-protected indoles;869 allyl, Ph, and benzoyl groups and alcohol functions were introduced by this way. tert-Butyl was presented in 1997 as a better nontransferable group than Me, and t-Bu3ZnLi as 7682


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Table 217. Magnesate-Mediated Metalation of Heteroaromatics Followed by Electrophilic Trapping

a After reflux for 18 h in the presence of PdCl2 3 dppf (3 mol %). b After reflux for 18 h in the presence of PdCl2 3 dppf (1 mol %). c After treatment with BuLi (1 equiv) at rt.

Table 218. Bu3MgLi-Mediated Metalation of Oxazoles Followed by Electrophilic Trapping

a

After reflux for 18 h in the presence of PdCl2 3 dppf (3 mol %).

a good alternative to perform the halogen/metal exchange of functionalized alkyl halides and iodobenzenes.137d,613,870 Me4ZnLi2 and other higher-order zincates proves more efficient

than Me3ZnLi to perform halogen/metal exchange on bromobenzenes.140,588 More recently, t-Bu4ZnLi2 was reported as an efficient reagent for the chemoselective functionalization of bromo and iodo aromatic substrates, even when substituted with electrophilic functional groups and some alkenyl and alkyl iodides.185b,593 Bu4ZnLi2 proves superior in the pyridine series.871 A theoretical study on the halogen/Zn exchange using organozincates shows that the exchange reactions could proceed through a hypervalent halogen-type TS.872 An alternative way developed by Knochel and co-workers uses a nucleophilic catalysis to allow diorganozinc-promoted halogen/Zn exchanges of aromatic and heteroaromatic iodides.873 Zn being less electropositive than Mg, deprotonation using zincates are difficult to predict from the results observed with magnesates. Wittig and co-workers studied the behavior of Ph3ZnLi toward fluorene in Et2O. Quenching with CO2 after 10 days, followed by acidic workup, affords diphenyleneacetic acid in a low 16% yield, as opposed to 47% with Ph3MgLi.2 Richey and co-workers observed in 2000 that the presence of coordinating agents allows the Et2Zn-mediated deprotonation of 1,2,3,4-tetraphenylcyclopentadiene in benzene, a result attributed to small amounts of organozincates.44c According to Stalke and co-workers, the close proximity of the two metals facilitates deprotozincation.174 It has been found that amidozincates are more reactive toward deprotometalation than 7683


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Table 219. Magnesate-Mediated Metalation of Pyridine Carboxamides Followed by Electrophilic Trapping

a

Using 0.67 equiv of base. b The 4-butylated product is formed in 100 and 48% yield using Bu3MgLi and Bu2(TMP)MgLi, respectively. c Reaction time of 1 h instead of 2 h. d Using Bu(TMP)3MgLi2. e The 4,40 -dimer is formed in 33% yield. f The 3,5-disubtituted derivative is isolated in yield ranging from 15 to 20%. g Using Ni(acac)2 and PPh3 (5 mol.% each). h Using Bu4MgLi2.

alkylzincates, a result explained by the preferred kinetic reactivity of ZnN bonds in comparison with ZnC bonds. On their own, Zn compounds are ineffectual metalating agents. When combined with an alkali metal component, they exhibit a synergy that greatly enhances their zincation power. t-Bu2 (TMP)ZnLi, prepared from t-Bu 2Zn (t-BuLi + ZnCl2 ) and TMPLi by Kondo and co-workers, combines two t-Bu groups with a hindered and basic TMP group. When used in THF, heteroleptic TMP-zincate is able to deprotonate functionalized or sensitive aromatics such as alkyl benzoates and π-deficient heterocycles (Table 236).661a,874 That t-Bu is behind the amino group in the migratory preference found applications in the deprotonation of 2- and 3-bromopyridine. Optional regioselectivities were obtained by varying the zincate amino group and the solvent (Scheme 200).875 Michl and co-workers reported in 2002 the use of TMP-zincate for the regioselective metalation of 3-alkylpyridine as BF3 complexes. The reaction occurs at the less hindered of the two activated positions (Scheme 201).876 The deprotonative zincation of 3-substituted bromobenzenes [OMe, Cl, F, CF3, CN, C(O)N-i-Pr2, CO2Et, CO2-t-Bu] was investigated by focusing on the nature of the base (Table 237, entries 18). For bromobenzenes bearing OMe, Cl, F, CN,

C(O)N-i-Pr2, or CF3 at C3, the reaction using t-Bu2(TMP)ZnLi takes place chemo- and regioselectively (at C2, except in the case of hindered CF3, for which it occurs at C6). Reactions from CO2 Et- and CO2 -t-Bu-substituted benzenes are still chemoselective, but not regioselective. The nature of the alkyl ligands of the zincates influences importantly the reactivities of the resultant arylzincates. Indeed, when Me 2(TMP)ZnLi is used to deprotonate the same 3-substituted bromobenzenes, the benzyne formation cannot be avoided, and raising the temperature of the reaction mixtures in the presence of 1,3-diphenylisobenzofuran results in the formation of the corresponding DielsAlder adducts (Scheme 202). These results were rationalized by theoretical calculations that show that the activation energy for benzyne formation using t-Bu-zincates is much higher than with Me-zincates, due to a more difficult interaction between the dialkylzinc moiety and Br.613,877 The study was extended to para-disubstituted benzenes for which one of the substituents is Br, Cl, or OTf, and similar reactions were described (Table 237, entries 914, and Scheme 203).613 It is interesting to note that in the presence of hexane containing TMEDA, reacting chlorobenzene with [(t-Bu)Zn(t-Bu)(TMP)Na(TMEDA)] results in [{1-Zn(t-Bu)}{2-N(Me)(CH2)(CH2)2NMe2}+C6H4] through addition of TMEDA to the benzyne formed.878 7684


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Table 220. Magnesate-Mediated Metalation of Functionalized Benzenes Followed by Electrophilic Trapping

a

NMR yield of deuteration using D2O. b In the presence of PdCl2 3 dppf (5 mol %). c Reaction under reflux.

Table 221. Reaction of 3,6-Dihydro-1H-pyridin-2-ones with Bu3MgLi Followed by Trapping with R0 X

entry

R

R0 X

total yield (%)

ratio

1 2 3 4 5 6 7 8 9 10 11 12

Me

BnBr allylBr PrI BnBr allylBr PrI BnBr allylBr PrI BnBr allylBr PrI

94 76 81 72 60 70 98 89 87 98 94 72

>99:1 83:17 96:4 >99:1 89:11 95:5 >99:1 90:10 97:3 99:1 92:8 75:25

allyl

Bn

Ph

Scheme 189

Structural information about TMP-zincate was obtained by Mulvey and co-workers in 2006. By preparing the base in hexane containing THF (1 equiv), crystals were isolated, and their structure was identified as [(t-Bu)Zn(t-Bu)(TMP)Li(THF)]. If THF is replaced by PhC(O)N-i-Pr2, [(t-Bu)Zn(t-Bu)(TMP)Li{(Od)(Ph)CN-i-Pr2}] is similarly isolated and identified. Both are described as ion-contacted zincates, and the latter is presented as a possible directed ortho-metalation precomplex (coordination of the substrate to Li) in relation with its easy deprotonation in the presence of 1 equiv of TMEDA.879 When PhC(O)N-i-Pr2 is treated by [(t-Bu)Zn(t-Bu)(TMP) Li(TMEDA)] (0.5 equiv) in hexane at rt, bis(carboxamide) 7685


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Table 222. Reaction of Different Aromatic Compounds with TMPMgCl 3 LiCl Followed by Electrophilic Trappinga

7686


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Table 222. continued

a For other examples, see the Supporting Information (Table S9). b Obtained by Pd-catalyzed cross-coupling after transmetalation with ZnCl2. c In the presence of CuCN 3 2LiCl. d Then OH deprotection using TFA. e The corresponding lactone is isolated. f Then NH2 deprotection using KF, HCl. g Then subsequent Negishi coupling with Me3SiCtCZnCl.

[(t-Bu)Zn{2-[1-C(O)N-i-Pr2]C6H4}2Li(TMEDA)] is produced in 72% yield. Surprisingly, [(t-Bu)Zn{2-[1-C(O)N-i-Pr2]C6H4}(TMP)Na(TMEDA)] is obtained instead (40% yield) using the corresponding Na base, [(t-Bu)Zn

(t-Bu)(TMP)Na(TMEDA)] (Scheme 204). The identity of the alkali metal having an effect on the outcome of the reaction, the authors ruled out the possibility of the zincate bases reacting as solvent-SIPs [M 3 (solvent)x][(t-Bu)2(TMP)Zn].858 Unlike 7687


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Table 223. TMPMgCl 3 LiCl-Mediated Sequences from 2,5-Dichlorothiophene

electrophile 1 entry 1

electrophile 2

(E1), yield (%)

10, 0.5

PhS(O)2SMe (SMe), 92

2 3

condtn 2 (°C, h)

25, 0.5 30, 0.5

NCCO2Et (CO2Et), 76

4 5 6

25, 0.5

7

electrophile 3 condtn 3 (°C, h)

(E2), yield (%) HC(O)NMe2 (CHO), 95 4-MeOC6H4Ia (4-anisyl), 84b 4-EtOC(O)C6H4Ia (4-EtOC(O)C6H4), 95 4-ClC6H4C(O)CN (4-ClC6H4C(O)), 76 allylBrc (allyl), 85 NCCO2Et (CO2Et), 87b PhS(O)2SMe (SMe), 73b

30, 0.5

8 40, 1

t-BuC(O)Clc (C(O)-t-Bu), 67b

9 10 11 12 13 14 a

Boc2O (CO2-t-Bu), 82 PhC(O)Clc (C(O)Ph), 78 t-BuC(O)Clc (C(O)-t-Bu), 75 TsCN (CN), 73

20, 0.5 78, 0.75 50, 0.5 30, 0.25

Boc2O (CO2t-Bu), 79 4-NCC6H4Ia (4-NCC6H4), 84 4-ClC6H4Ia (4-ClC6H4), 77 HC(O)NMe2 (CHO), 86

Pd-catalyzed cross-coupling after transmetalation with ZnCl2.

50, 3

b

(E3), yield (%)

allylBrc (allyl), 93 2-F3CC6H4Ia (2-F3CC6H4), 70 4-ClC6H4Ia (4-ClC6H4), 81 3,5-F3CC6H4Ia (3,5-F3CC6H4), 57

NCCO2Et (CO2Et), 92

electrophile 4 (E4), yield (%)

condtn 4 (°C, h)

30, 1 50, 0.75 50, 0.75

50, 0.5

HC(O)NMe2 (CHO), 85 HC(O)NMe2 (CHO), 79 allylBrc (allyl), 85

CH2dC(CO2Et)CH2Br (CH2C(dCH2)CO2Et), 87

For two steps. c In the presence of CuCN 3 2LiCl.

Table 224. Reaction of Functionalized Ferrocenes with TMPMgCl 3 LiCl Followed by Electrophilic Trapping

entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

R1, R2, R3

conditions

electrophile (E)

yield (%)

CO2Et, H, H

10 °C, 2.5 h

CO2-t-Bu, H, H

10 °C, 2.5 h

CN, H, H

0 °C, 67 h

I2 (I) allylBr (allyl)a PhC(O)Cl (C(O)Ph)a t-BuCHO (CH(OH)-t-Bu) EtOC(O)Cl (CO2Et)b t-BuCHO (CH(OH)-t-Bu) PhC(O)Cl (C(O)Ph)a Boc2O (CO2-t-Bu) t-BuCHO (CH(OH)-t-Bu) allylBr (allyl)a allylBr (allyl)a Me3SnCl (SnMe3) allylBr (allyl)a CH2dNEt2+, TfO (CH2NEt2) NCCO2Et (CO2Et) I2 (I) allylBr (allyl)a NCCO2Et (CO2Et)

60 72 70 60 67 82 80 78 62 75 76 34 49 48 50 40 48 60

c

CO2MgCl, H, H CO2Et, CO2Et, H

10 °C, 2.5 h 0 °C, 2 h

CO2-t-Bu, CO2-t-Bu, H CO2Et, CO2Et, CO2Et

0 °C, 2 h 10 °C, 1 h

a

In the presence of CuCN 3 2LiCl. b By Pd-catalyzed cross-coupling after transmetalation with ZnCl2. c By treatment of the corresponding carboxylic acid with MeMgCl. 7688


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Table 225. Reaction of Different Aromatic Compounds with TMP2Mg 3 2LiCl Followed by Electrophilic Trappinga

a For other examples, see the Supporting Information (Table S10). b Yield of 7% using TMPMgCl 3 LiCl, 77% using (t-Bu(c-Hex)N)2Mg 3 2LiCl, and 90% using (t-Bu(i-Pr)N)2Mg 3 2LiCl. c In the presence of CuCN 3 2LiCl. d By Pd- and Ru-catalyzed cross-coupling after transmetalation with ZnCl2. e By Pd-catalyzed cross-coupling after transmetalation with ZnCl2. f In the presence of CuCN 3 2LiCl after transmetalation with ZnCl2.

anisole, trimethyl(phenoxy)silane is converted to [(t-Bu)Zn{CH2SiMe2OPh}(TMP)Li(THF)] (52% yield) through lateral zincation when treated with [(t-Bu)Zn(t-Bu)(TMP)Li(THF)].880 The reactivity of Bu2(TMP)ZnLi 3 TMEDA toward ferrocene (FcH) was explored in 2005 by Mulvey and co-workers. When ferrocene is treated with [(Bu)Zn(TMP)(Bu)Li(TMEDA)], produced by mixing stoichiometrically TMPLi, Bu2Zn, and TMEDA in hexane, and identified by X-ray diffraction, mixtures of [(Fc)2Zn(TMEDA)] and [Li(THF)4][(Fc)3Zn] are isolated (the latter is favored using an excess of ferrocene, both were identified by X-ray diffraction). A fast disproportionation of

hypothetical Bu(TMP)(Fc)ZnLi 3 TMEDA to give both Fc2Zn 3 TMEDA and TMP2Bu2ZnLi2 3 TMEDA is proposed to explain the results observed.881 The behavior of [(t-Bu)Zn(t-Bu)(TMP)Li(THF)] toward anisole was reported by Mulvey and co-workers in 2006. When treated by the mixed base (1 equiv) in hexane, anisole is converted to ortho-metalated [(t-Bu)Zn(2-anisyl)(TMP)Li(THF)]. Two aryl groups are present on Zn in [(2-anisyl)Zn(2-anisyl)(TMP)Li(THF)], the compound obtained by using 0.5 equiv of base for a longer reaction time (Scheme 205). In the absence of THF, metalation takes place, but one anisole is 7689


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Table 226. Reaction of Different OP(O)(NMe2)2-Substituted Aromatics Followed by Electrophilic Trapping

a

By Pd-catalyzed cross-coupling after transmetalation with ZnCl2. b In the presence of CuCN 3 2LiCl after transmetalation with ZnCl2. c By Pd- and Ru-catalyzed cross-coupling after transmetalation with ZnCl2.

retained on Li, as evidenced by the isolation of [(t-Bu)Zn(2-anisyl)(TMP)Li(O(Me)Ph)]. These results show that the base functions on the whole as an alkyl base in these cases. The conversion of [(t-Bu)Zn(2-anisyl)(TMP)Li(O(Me)Ph)] to [(2-anisyl)Zn(2-anisyl)(TMP)Li(THF)] upon addition of THF going through [(t-Bu)Zn(2-anisyl)(TMP)Li(THF)], the authors ruled out a mechanism based on a complex-induced proximity effect.882 It has later been shown that the ability of R2(TMP)ZnLi to deprotonate depends on the R ligands. Unlike the Li,Zn base bearing two t-Bu ligands, the corresponding dimethylzincate is not capable of metalating anisole. Indeed, (2-anisyl)Me2ZnLi reacts with TMPH to furnish both anisole and Me2(TMP)ZnLi in benzene or THF, whereas (2-anisyl)(t-Bu)2ZnLi produces (2-anisyl)(TMP)(t-Bu)ZnLi in

benzene. In the same paper, the role of the solvent in the position of the disproportionation equilibrium between [MeZnMe(2-anisyl)Li(THF)2] (favored in THF) and [(2-anisyl)4ZnLi2(THF)2] (favored in hexane) is underlined.883 The origin of the chemoselectivity observed using TMPzincate bases was investigated in 2007 by Uchiyama et al. In the light of TS structures obtained for both directed ortho-metalation and 1,2-addition to benzonitrile and methyl benzoate, they observed that the 1,2-addition pathways for Me 2 (Me2 N)ZnLi and [MeLi]2 are different. Whereas low activation barriers for 1,2-addition are possible through the double coordination to Li atoms of CX multiple bonds in the case of (MeLi)2 , the barrier to 1,2-addition is high—higher than the barrier to directed ortho-metalation 7690


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Table 227. Reaction of Different Aromatic Compounds with Li,Mg Bases Followed by Electrophilic Trapping

a

In the presence of CuCN 3 2LiCl. b Using TMPMgCl 3 LiCl. c By Pd-catalyzed cross-coupling after transmetalation with ZnCl2. d The corresponding lactone is isolated.

Scheme 190

in the case of zincates. The LUMO components are localized completely on the Li atom of the zincate, and the central Zn metal cannot be involved in the activation of the CdO or CN group. 884 In 2006, Uchiyama et al. confirmed [(t-Bu)Zn(t-Bu)(TMP)Li(THF)] as structure at the crystal state. Gas-phase calculations indicate that the symmetrical isomer [(TMP)Zn(t-Bu)2Li(THF)] is 17 kcal/mol higher in energy, and NMR studies support the presence of the unsymmetrical structure in solution. Employing Me2(Me2N)ZnLi and Me2O as models to perform DFT calculations on the reaction pathway and NMR monitoring of the reaction resulted in the kinetic preference of N-ligand over C-ligand transfer.885 Kondo et al. documented in 2007 a study on the reaction of Et2(TMP)ZnLi with

N,N-diisopropylnaphthamide in THF. DFT calculations performed in order to explain the dual alkyl/amino basicity observed with the formation of Et(2-C10 H6 C(O)N-i-Pr 2 )2ZnLi 3 2THF reveal that direct alkyl basicity is kinetically disfavored. On the basis of theory and experiment, the authors propose for reactions performed in THF a stepwise mechanism where TMPH liberated during the first step is recycled (Scheme 206), a mechanism that could be prevented in the presence of strong metal-coordinating ligands. Further reaction with elimination of the final equivalent of EtH was found possible, but using less hindered PhC(O)N-i-Pr2 . 886 Additional structural elements supporting a two-step mechanism were more recently obtained. 883 7691


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Table 228. Metalation of Different Substrates Using i-Bu3Al(TMP)Li Followed by Electrophilic Trapping

a

Yield of 12% using Me3Al(TMP)Li for 2 h, 87% using Et3Al(TMP)Li for 2 h, 88% using i-Bu3Al(TMP)Li for 2 h. b Yield of 0% using Me3Al(TMP)Li, Et3Al(TMP)Li, or t-Bu3Al(TMP)Li. c Significant decomposition of the CN group when R3Al(TMP)Li (R = Me, Et, t-Bu) is used. d Modification of the iBu group to alkoxides or amides; of TMP to DA or HMDS; or of Li to K, MgCl, Mg(t-Bu), or Zn(t-Bu) gives a complex mixture. e Using hexane, toluene, CH2Cl2, and lower coordinative ethers results in decreased yields. f In the presence of catalytic amounts of Pd2(dba)3 and P(t-Bu)3. g Generation of the corresponding benzyne in THF at rt (identified by interception with 1,3-diphenylisobenzofuran in 100% yield).

Scheme 191

Et2(TMP)ZnLi was employed by the same authors in 2008 to achieve the ortho-metalation of PhOC(O)NR2 at rt without observing the anionic Fries rearrangement, which is a spontaneous reaction using classical Li bases. The side reaction can be totally suppressed with R = Et (Scheme 207).887 The same base was later employed to generate a key iodide in the course of the synthesis of IKK2 inhibitors (Scheme 208).888 Purdy and George showed in 1992 that Np3ZnK, Np3ZnNa, and (Me3SiCH2)3ZnK are all able to metalate benzene, the

relative order of reactivity being Np3ZnK ≈ Np3ZnNa . (Me3SiCH2)3ZnK.130 It is known that changing the alkali metal of Kondo’s base t-Bu2(TMP)ZnLi from Li to Na decelerates the deprotozincation in THF.885 Nevertheless, it seems that this reactivity can be restored using different reaction conditions. In 2005, Mulvey and co-workers synthesized this Na analog as its TMEDA adduct from t-Bu2Zn, TMPNa, and TMEDA in hexane and obtained [(t-Bu)Zn(TMP)(t-Bu)Na(TMEDA)] as the structure using 7692


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X-ray diffraction. When benzene was treated by this reagent (1 equiv) in hexane at rt for 30 min and the solution cooled at 20 °C, crystals were isolated in 51% yield and identified as [(t-Bu)Zn(TMP)(Ph)Na(TMEDA)]. On the basis of DFT calculations (favorable thermodynamic factors), the authors presented the reagent as an alkyl base.889 In 2008, Nobuto and Uchiyama studied the reaction theoretically and concluded that the reaction proceeds through a stepwise mechanism, with kinetic deprotonation involving the TMP ligand in the first step.890 There is no ate complex formation between t-Bu2Zn and TMPNa in pure hexane. However, upon addition of benzene (0.5 equiv), stirring for 2 days, and addition of TMEDA, 1,4diphenylenezinc [1,4-{(t-Bu)Zn(TMP)Na(TMEDA)}2C6H4] is isolated in 39% yield. In contrast, when TMEDA is added before benzene, monodeprotonated benzene complex [(t-Bu)Zn(TMP)(Ph)Na(TMEDA)] is isolated instead (Scheme 209).823 [(t-Bu)Zn(TMP)(t-Bu)Na(TMEDA)] is a remarkable reagent in terms of regioselectivity. Whereas PhNMe 2 is ortho-metalated using classical Li bases such as organolithiums, Table 229. Metalation of Aliphatic Compounds Using i-Bu3Al(TMP)Li Followed by Electrophilic Trapping

R0

yield (%)

α (erythro:threo)/γa ratio

MeOCH2

Bu

97

>99 (87:13)/1

i-Pr2NC(O)

Ph Bu

77 70

97 (56:44)/3 >99 ()/1

Ph

96

85 (82:17)/15

5

Me2NC(O)

Ph

63

79 (58:42)/21

6

2-anisyl

Ph

82

>99 (99:1)/1

entry 1 2 3

R

4

a

the reaction using the mixed Na,Zn base occurs at the meta site to produce [(t-Bu)Zn(TMP)(3-Me2NC6H4)Na(TMEDA)], a result evidenced by analysis of crystals suitable for X-ray diffraction. A similar metalation (at the 5-position) was observed starting from 3-tolylNMe2 (Scheme 210).891 The reagent is also capable of dimetalation. Whereas naphthalene leads to [(t-Bu)Zn(C10H7)(TMP)Na(TMEDA)] upon treatment with [(t-Bu)Zn(TMP)(t-Bu)Na(TMEDA)] in hexane at rt, [(t-Bu)2Zn2(2,6-C10H7)(TMP)2Na2(TMEDA)2] is isolated in a similar yield using the same base at hexane reflux (Scheme 211).892 Upon treatment with [(t-Bu)Zn(TMP)(t-Bu)Na(TMEDA)] in hexane at rt, (trifluoromethyl)benzene is converted to a complex mixture from which the ortho- and metaregioisomers of [(t-Bu)Zn(F3CC6H4)(TMP)Na(TMEDA)] (identified as CIPs by X-ray crystallography) as well as the para-isomer (identified by NMR) form in a 20:11:1 ratio. Ortho-regioisomer [Na(TMEDA)2][(t-Bu)2(C6H4CF3)Zn] (adopting a SIP arrangement), already observed when performing the reaction at rt, becomes exclusive at 0 °C. It is attributed to the kinetic intermediate of the deprotozincation,893 the latter proceeding according to a two-step mechanism, as suggested by Nobuto and Uchiyama on the basis of theoretical calculations.890 By subsequent reaction with TMPH, this fourth compound is converted to a mixture of the three thermodynamic regioisomers already observed at rt.893 Scheme 194

E/Z ratios of γ-products were not determined.

Scheme 192

Scheme 193

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Table 230. Metalation of Aromatics Using (t-Bu(t-Bu(i-Pr)CH)N)3Al 3 3LiCl or TMP3Al 3 3LiCl Followed by Electrophilic Trapping

a

In the presence of CuCN 3 2LiCl (stoichiometric). b Using TMP3Al 3 3LiCl instead of (t-Bu(t-Bu(i-Pr)CH)N)3Al 3 3LiCl. c In the presence of Pd(dba)2 (5 mol %) and P(2-furyl)3 (10 mol %). d In the presence of CuCN 3 2LiCl (0.25 equiv). e In the presence of CuCN 3 2LiCl (stoichiometric) after transmetalation using ZnCl2. f In the presence of Pd(dba)2 (5 mol %) and P(2-furyl)3 (10 mol %) after transmetalation using ZnCl2. g After transmetalation using ZnCl2.

Scheme 195

When [(t-Bu)Zn(TMP)(t-Bu)Na(TMEDA)] is employed to deprotonate toluene in hexane at rt, a statistical mixture of the

meta- and para-regioisomers of [(t-Bu)Zn(TMP)(tolyl)Na(TMEDA)], which are the most stable products, is obtained.894 7694


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Scheme 196

Scheme 197

Scheme 198

Scheme 199

Scheme 201

Even BnOMe produces an ortho-zincated intermediate, [(t-Bu)Zn(TMP)(C6H4CH2OMe)Na(TMEDA)], when treated similarly (Scheme 212).895 Treated by [(t-Bu)Zn(TMP)(t-Bu)Na(TMEDA)] in hexane, N-methylindole and -pyrrole are regioselectively attacked at C2. Conway et al. observed the autodismutation of putative t-Bu(N-Me-2-indolyl)(TMP)ZnNa 3 TMEDA, affording [(N-Me2-indolyl)2Zn(TMEDA)] and TMP2(t-Bu)2ZnNa2 3 TMEDA (identified by X-ray diffraction and NMR, respectively) (Scheme 213).833 Upon treatment with [(t-Bu)Zn(TMP)(t-Bu)Na(TMEDA)] (0.5 equiv) in hexane containing TMEDA, 3-tolunitrile leads to

Scheme 200

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Table 231. Metalation of Aromatics Using TMP2Mn 3 2MgCl2 3 4LiCl Followed by Electrophilic Trapping

a

In the presence of catalytic CuCN 3 2LiCl. b In the presence of catalytic Pd(PPh3)4. c The lactone is obtained. d In the presence of catalytic CuCN 3 2LiCl after transmetalation using ZnCl2.

Table 232. Metalation of Aromatics Using TMP2Fe 3 2MgCl2 3 4LiCl Followed by Electrophilic Trapping

a

Without 4-fluorostyrene. 7696


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Table 233. Metalation of Aromatics Using a Li,Co Base Followed by Electrophilic Trapping

a

Reaction performed using 1 equiv of base and with trapping at 50 °C. b Not determined. c Reaction time of 2 h.

Table 234. Metalation of Aromatics Using Different Li Cuprates Followed by Electrophilic Trapping

a

At 78 °C instead of 0 °C. b At 40 °C instead of 0 °C.

[(3-tolylCN)2Na(TMEDA)2]+[{3-Me-6-(Zn(t-Bu)2)C6H3CN}2Na(TMEDA)2] (49% yield) with two nitrile ligands zincated at C6 present in the anionic moiety and two neutral nitrile ligands found in the cationic one. To explain why the released TMPH cannot reaminate the Zn center, the authors suggest the absence of an attached Na

site to which it could “precoordinate” due to the presence of rival Lewis bases. The behavior of 1-cyanonaphthalene is different, with the isolation of CIP [{1-NCC10H6-2-(Zn(t-Bu)2)}Na(TMEDA)2]. When treated similarly, t-BuCN is converted to SIP [(t-BuCN)2Na(TMEDA)2][t-Bu3Zn] through dismutation.896 7697


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Table 235. Metalation of Aromatics Using Putative TMP2CuLi Followed by Electrophilic Trapping

Table 236. Mono- or Dimetalation of Aromatics Using t-Bu2(TMP)ZnLi Followed by Electrophilic Trapping

a

Using 1 equiv of base. b Using 4 equiv of base. c Yields of 19 and 24% using 1 equiv of base.

The synergy exhibited by [(Me3SiCH2)Zn(CH2SiMe3)(TMP)Na(TMEDA)] is unprecedented: the base is able to α-deprotozincate both THF (Scheme 214) and tetrahydropyran over days at rt to afford [(Me3SiCH2)Zn(C4H7O)-

(TMP)Na(TMEDA)] and [(Me 3 SiCH 2 )Zn(C 5 H 9 O)(TMP)Na(TMEDA)], respectively, without subsequent cleavage (stabilization of the carbanion with the α-silyl substituent). 897 7698


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Scheme 202

Scheme 203

Scheme 204

Scheme 205

Scheme 206

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Scheme 208

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Scheme 212

Scheme 213

Table 237. Metalation of Substituted Benzenes Using t-Bu2(TMP)ZnLi Followed by I2 Interception

Scheme 214

Mulvey and co-workers reported in 2003 the synergic deprotozincation of toluene at the thermodynamic site upon reaction with HMDS3ZnK,898 formed in situ from HMDSK and HMDS2Zn, to produce [{(Bn)(HMDS)2ZnK}n]. [{(3-TolylCH2)Zn(HMDS)2K}n] and [{(3,5-Me2C6H3CH2)Zn(HMDS)2K}n] were Scheme 209

Scheme 210

Scheme 211

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Table 238. Mono- or Dimetalation of Aromatics Using TMP2Zn 3 2MgCl2 3 2LiCl Followed by Electrophilic Trappinga

a

For other examples, see the Supporting Information (Table S11). b In the presence of CuCN 3 2LiCl. c Using catalytic Pd(dba)2 and P(2-furyl)3. d Then replacement of I by Et under Pd-catalyzed reaction with Et3In. e Yield of 83% at 25 °C for 110 h without microwave irradiation. f Yield of 85% at 25 °C for 110 h without microwave irradiation. 7701


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Table 239. Metalation of Aromatics Using (t-Bu(i-Pr)N)2Zn 3 2MgCl2 3 2LiCl Followed by Electrophilic Trapping

a

After transmetalation with catalytic CuCN 3 2LiCl. b Using catalytic Pd(dba)2 and P(2-furyl)3. c Under microwave irradiation.

Scheme 215

Scheme 216

Scheme 217

Scheme 218

similarly prepared from m-xylene and mesitylene, respectively, and identified by X-ray diffraction.899 In 2008, Mulvey and co-workers prepared and used [(Et)Zn(Et)(TMP)K(PMDTA)] to functionalize at C2 a series of 4-substituted pyridines (R = Me2N, H, Et, i-Pr, t-Bu, and Ph), giving [{K(PMDTA)}2{4-R-C5H3N-2-ZnEt2}2] in 53, 16, 7 (low yield due to competitive lateral metalation), 23, 67, 7702


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Table 240. Metalation of Aromatics Using TMPZnCl 3 LiCl Followed by Electrophilic Trappinga

a

For other examples, see the Supporting Information (Table S12). b After transmetalation with stoichiometric CuCN 3 2LiCl. c Using catalytic Pd(dba)2 and P(2-furyl)3. d After transmetalation with catalytic CuCN 3 2LiCl. e Using catalytic Pd(PPh3)4.

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Table 241. Metalation of Aromatics Using in Situ Prepared Mixture of ZnCl2 3 TMEDA and Li Compounds Followed by Electrophilic Trapping

A similar yield is obtained using 0.33 equiv of base and ZnBr2 3 TMEDA instead of ZnCl2 3 TMEDA. b Trapping step at 50 °C using catalytic PdCl2 and dppf. c Lower conversions of 10 and 20% are obtained using TMP2Zn (1 equiv) and TMPLi (1 equiv), respectively. d Performed in the presence of TMEDA (5 equiv). e Performed in hexane. f Deprotonation step performed at THF reflux. g Yields of 79, 20, and 0% using Bu(TMP)Zn2Li, Bu2(TMP)ZnLi, and Bu3ZnLi, respectively. h Yield of 100% using Bu(TMP)2ZnLi. a

Scheme 219

Scheme 220

and 51% yield, respectively. 4-Methoxypyridine is instead attacked at C3 to produce [catena-{[4-MeOC5H3N-3-ZnEt2]K(PMDTA)}n] (52% yield) (Scheme 215), a difference ascribed to the greater ability to direct metalation of the methoxy group, which efficiently competes with the pyridine N.900 Ferrocene and ruthenocene (RcH) produce monozincated [{(Fc)Zn(Me)2K(PMDTA)}∞]

and [{(Rc)Zn(Me)2K(PMDTA)}∞], respectively, when treated with [(Me)Zn(Me)(TMP)K(PMDTA)] (Scheme 216).901 If the alkyl ligand is a CH2SiMe3 group, the base turns into a reagent able to metalate ethene (Scheme 217).897 Clegg et al. synthesized in 2008 [(t-Bu)Zn(DA)(t-Bu)Li(TMEDA)] and [(t-Bu)Zn(DA)(t-Bu)Na(TMEDA)], which 7704


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Table 242. Metalation of Anisole Using Different Li,Zn Combinations Followed by I2 Interception (P = piperidino)

entry

Li,Zn combination: Li compd(s) added to ZnCl2 3 TMEDA (1 equiv)

yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

PLi (3 equiv) DALi (3 equiv) PLi (2 equiv) then TMPLi (1 equiv) DALi (2 equiv) then TMPLi (1 equiv) TMPLi (2 equiv) then PLi (1 equiv) TMPLi (2 equiv) then DALi (1 equiv) BuLi (3 equiv) TMPLi (1 equiv) then BuLi (2 equiv) TMPLi (2 equiv) then BuLi (1 equiv) TMPLi (1 equiv) then s-BuLi (2 equiv) TMPLi (2 equiv) then s-BuLi (1 equiv) TMPLi (2 equiv) then t-BuLi (1 equiv) TMPLi (1 equiv) then t-BuLi (2 equiv) Me3SiCH2Li (3 equiv) TMPLi (1 equiv) then Me3SiCH2Li (2 equiv) TMPLi (2 equiv) then Me3SiCH2Li (1 equiv)

0 16 2 38 35 55 28 20 79 5 73 44 65 0 0 74

Scheme 221

Table 243. Metalation of Aromatics Using TMP2Mg 3 2LiCl in the Presence of ZnCl2 Followed by Electrophilic Trapping

a

Under Pd catalysis. b Using TMP2Zn 3 2MgCl2 3 2LiCl. c Reaction time of 5 h instead of 2 h. d After transmetalation with stoichiometric CuCN 3 2LiCl. 7705


Chemical Reviews are DA analogs of the previously reported TMP complexes. Using such CIPs for the metalation of phenylacetylene gives the acetylides [{(t-Bu)Zn(CtCPh)2Li(TMEDA)}2(TMEDA)] and [{(t-Bu)Zn(CtCPh)2Na(TMEDA)}2], respectively.902 The synthesis of alkali metalzinc enolates [{MesC(O)Me}2{Mes(CH2d)CO}6Zn2Na2] and [(MePh)2{Mes(CH2d)CO}6Zn2K2] was reported by Baillie et al. from 2,4,6-trimethylacetophenone (MesC(O)Me) using HMDS3ZnM (M = Na, K).142 With TMP3ZnLi, all three ligands participate in the reaction to furnish [{Mes(CH2d)CO}4ZnLi2(L)2] (L = TMEDA or TMPH). In contrast, with Me2(TMP)ZnLi 3 TMEDA, only the amino ligand reacts as a base to provide [{Mes(CH2d)CO}2Li2(TMEDA)2], releasing Me2Zn 3 TMEDA and TMPH as coproducts.903 With the aim of developing bases more compatible with highly sensitive groups (e.g., NO2 and CHO) and substrates (e.g., oxazoles and oxadiazoles), Knochel and co-workers developed salt-activated neutral Zn bases. TMP2Zn 3 2MgCl2 3 2LiCl, prepared from TMPMgCl 3 LiCl and ZnCl2, allows the functionalization of activated substrates, as evidenced by subsequent metalcatalyzed or direct interception with electrophiles (Table 238, entries 136).834g,h,j,k,841b,904 According to the authors, MgCl2 and LiCl in solutions of TMP2Zn are respectively essential for the high reactivity and solubility of the base.905 For less activated substrates, microwave irradiation is used to ensure their monodeprotonation (Table 238, entries 3754)796g,906 or dideprotonation (Table 238, entry 55).796f (t-Bu(i-Pr)N)2Zn 3 2MgCl2 3 2LiCl, a base synthesized from available secondary amines, can be alternately employed (Table 239), albeit in a lower chemoselectivity.842 Knochel and co-workers reported in 2009 the synthesis of TMPZnCl 3 LiCl,907 from TMPLi and ZnCl2, and its use for the functionalization of aromatic compounds including functionalized and heterocyclic substrates. As above, the deprotozincation is realized either under classical conditions or under microwave irradiation for less activated substrates (Table 240).834i,l,908 According to the authors, TMPZnCl 3 LiCl leads to more chemoselective zincations (e.g., the functionalization of CHO- and NO2-containing aromatics) compared with TMP2Zn 3 2MgCl2 3 2LiCl. It is also interesting to note that Knochel and co-workers compared the relative kinetic basicities of PhZnI 3 LiCl, OctZnBr 3 LiCl, and BnZnCl 3 LiCl toward isopropyl alcohol, butylamine, and aniline and found the decreasing order arylzinc halide > alkylzinc halide > benzylzinc halide.909 The relative rates for the proton-transfer processes from formic acid to different tributylzincates were found to decrease in the series Bu3Zn > s-Bu3Zn > t-Bu3Zn and also by replacing the Bu groups by chloro ligands.178 Another approach employs a polar reagent such as a Li or Mg compound in the presence of a Zn compound. Mongin and co-workers showed in 2008 that the in situ prepared mixture of ZnCl2 3 TMEDA and TMPLi (3 equiv) in THF is not a Li zincate but more likely a 1:1 mixture of TMPLi and TMP2Zn (+2LiCl 3 TMEDA). Nevertheless, it is able to behave synergically, allowing the deprotonation of aromatics, including very sensitive heterocycles such as bare diazines (Table 241). The mechanism depicted in Scheme 218 (applied to N-phenylpyrrole) is advanced to explain the synergy observed.536,910 Starting from anisole, the base prepared from ZnCl2 3 TMEDA and TMPLi (3 equiv) is more efficient than different amino-alkyl mixed Li,Zn combinations (Table 242, P = piperidino).536,911 Knochel and co-workers functionalized aromatic compounds bearing sensitive functions and including heterocycles through a deprotometalation step using TMP2Mg 3 2LiCl in the presence of ZnCl2 (Table 243).796f,904c,912 Successful attempts were also

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obtained using the same base in the presence of Bu3SnCl and CuCN 3 2LiCl (Scheme 219).904c

6. CONCLUSION In this review, our aim was to present the main reactions using bimetallic combinations. Of course, the subject is not exhaustively covered because of the huge work done in this field, and also because the role of ate compounds in some reactions is still obscure.913 Trimetallic combinations such as the following are starting to emerge also. Screttas and Steele used as metalating agent a mixture of BuLi and (Me2N(CH2)2O)LiK, which combines the activating effect of a tertiary amine with that of an alkoxide. Its powerful metalating ability was demonstrated in the reaction of ethylene, hexene, and (+)-α-pinene in methylcyclohexane to give an allylic-type carbanionic species then trapped with CO2. The regioselectivity of the reaction can be modified by addition of (EtO(CH2)2O)2Mg (Scheme 220).730ad By combining catalytic amounts of a BuLi 3 (Me2N(CH2)2O)LiK mixture with (EtO(CH2)2O)2Mg,914 multiple addition of ethylene to alkylaromatics was promoted (Scheme 221).915 The role attributed to (EtO(CH2)2O)2Mg is a solubilization of the organoalkali metal, and a modification of its reactivity.392,393,801b The method was extended to aniline916 and phenol917 derivatives.915b The inclusion of (EtO(CH2)2O)2Mg in the catalyst in general restricts alkylation to the Me groups ortho to the hydroxy or amino group. Thus, the authors tried to depict the special chemistry exhibited by aggregates formed by combining an alkali or alkaline earth metal compound (Li, Na, K, Mg) with another alkali metal compound (Li, Na, K), or with a compound containing a group 2 (Mg, Ca), group 4 (Ti), group 5 (V), group 6 (Cr), group 7 (Mn), group 8 (Fe), group 9 (Co), group 10 (Ni), group 11 (Cu), group 12 (Zn), or group 13 (Al) element as Lewis acidic center. Both ate complexes and combinations containing alkali (or alkaline earth) metal compounds can behave synergically in processes involving either nucleophile ligand transfer or base ligand transfer. One origin of this synergy is the sharing between two identical reacting ligand(s) of different metals. One limit to their reactivity can be the inappropriate choice, for a given reaction, of metals and/or ligands in the combinations used but also unsuitable reaction conditions (e.g., solvent) to reach a required structural arrangement. For example, cuprates that are solvent-SIPs have been identified as unreactive in many standard organocopper reactions.177 According to Mulvey, the metalation using alkali,Mg or alkali,Zn bases is alkali metal mediation, since the substrate to be deprotonated has to enter the coordination sphere of the alkali metal. In addition, as the prebinding of the substrate lowers the entropy of the reaction, the basicity of the Mg or Zn center in enhanced compared with that of the monometallic corresponding reagents.7 ASSOCIATED CONTENT

bS

Supporting Information Tables S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, S11, and S12 containing additional examples for Tables 28, 170, 184, 185, 189, 190, 193, 208, 222, 225, 238 and 240, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

*F.M.: phone, 33 223 236 931; fax, 33 223 236 955; e-mail, fl[email protected]. A.H.-M.: phone, 33 235 522 438; fax, 33 235 522 971; e-mail, [email protected]. The authors declare no competing financial interest. 7706


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BIOGRAPHIES

Florence Mongin obtained her Ph.D. in Chemistry in 1994 from the University of Rouen under the supervision of Prof. Guy Queguiner. After a two-year stay at the Institute of Organic Chemistry of Lausanne as a postdoctoral fellow with Prof. Manfred Schlosser, she returned to the University of Rouen as an Assistant Professor in 1997 (HDR in 2003). Besides research activities concerning the functionalization of heteroaromatic compounds using lithium and magnesium reagents and the synthesis of biologically active compounds, she got involved in catalysis studies, notably for the activation of CF bonds and the transition metal CC bond formation using Grignard reagents in the presence of reactive functional groups. She has thus a strong expertise in the functionalization of aromatic and heterocyclic compounds, in the synthesis and use of metal bases, and in metalcatalyzed coupling reactions. She took up her present position in 2005 as Professor at the University of Rennes and was appointed Junior Member of the Institut Universitaire de France in 2009. Her present scientific interests include the functionalization of aromatic compounds with recourse to bimetallic bases. The synergies brought out by combining lithium and magnesium, lithium and zinc, lithium and cadmium, lithium and copper, lithium and cobalt, and lithium and iron reagents have been evidenced in the course of the last seven years. Extensions to asymmetric synthesis are currently under investigation.

Anne Harrison-Marchand obtained her Ph.D. in Chemistry in 1995 from the University of Nantes. Her doctoral research was carried out in the laboratory of Dr. J. Villieras, under the supervision of Drs. A. Guingant and J.-P. Pradere. She then spent a year and a half occupying a postdoctoral position (with a Marie-Curie Training Grant) in the group of Prof. A. Pelter at the University of

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Wales, Swansea, UK. From January 1997 to August 1999, she worked on industrial projects: first, with Rhone-Poulenc Rorer in the group of Prof. A. Barrett at the Imperial College of London (UK) and, second, with UCBPharma in the group of Prof. G. Guillaumet at Orleans (France). She was offered an Assistant Professor position (HDR in 2008) at the University of Rouen in September 1999. She has expertise in two distinct areas related to asymmetric heterocycloadditions and enantioselective nucleophilic additions of organo(bi)metallic reactants. The latter problem is currently her main focus and consists of both exploring new synthetic methodologies and understanding their stereochemical outcomes thanks to analyses of the species directly in solution.

ACKNOWLEDGMENT The authors thank Prof. Manfred Schlosser for his help through discussions and Dr. Jacques Maddaluno for his interest in and encouragement of this project. F.M. acknowledges the Institut Universitaire de France and the Agence Nationale pour la Recherche (grant ANR-BLAN-08-2-311886), and A.H.-M. thanks the Agence Nationale pour la Recherche (grant ANR-07-BLAN-0294-1). ABBREVIATIONS USED Ac acetyl acac acetylacetonate BINOL 1,1'-bi-2-naphthol Bn benzyl Boc tert-butoxycarbonyl Bu n-butyl CIP contacted ion pair DA diisopropylamino DABCO 1,4-diazabicyclo[2.2.2]octane Dec n-decyl DFT density functional theory DMAE 2-dimethylaminoethoxide DME 1,2-dimethoxyethane DMF dimethylformamide DMI 1,3-dimethyl-2-imidazolidinone DMM dimethoxymethane DMPU N,N′-dimethylpropyleneurea Dodec n-dodecyl dppe 1,2-bis(diphenylphosphino)ethane dppf 1,1′-bis(diphenylphosphino)ferrocene dppp 1,4-bis(diphenylphosphino)propane ESI electrospray ionization ESR electron spin resonance ET electron transfer Fc ferrocenyl GC gas chromatography Hept n-heptyl Hex n-hexyl HMDS (Me3Si)2N HMPA hexamethylphosphoramide HOMO highest occupied molecular orbital HSAB hard and soft acids and bases IR infrared LIC-KOR 1:1 BuLi·t-BuOK LIC-NaOR 1:1 BuLi·t-BuONa LIS-KOR 1:1 s-BuLi·t-BuOK LUMO lowest unoccupied molecular orbital 7707


Chemical Reviews MAA Ms Nf NMR NMP Non Np Oct Octadec Pent petrol. ether PM PMDTA Pr Py Rc rt SAMP SET SIP TBDMS TBDPS Tf THP TMP THF TMEDA Tridec Trs TS Ts Undec

Mixed AggregAte mesyl nonaflate (or nonafluorobutanesulfonate) nuclear magnetic resonance N-methylpyrrolidinone n-nonyl neopentyl n-octyl n-octadecyl n-pentyl petroleum ether (S)-N-methyl-2-pyrrolidine methoxide N,N,N′,N″,N″-pentamethyldiethylenetriamine n-propyl pyridyl ruthenocenyl room temperature (S)-1-amino-2-(methoxymethyl)pyrrolidine single electron transfer separated ion pair tert-butyldimethylsilyl tert-butyldiphenylsilyl triflate 2-tetrahydropyranyl 2,2,6,6-tetramethylpiperidino tetrahydrofuran N,N,N′,N′-tetramethylethylenediamine n-tridecyl trisyl (2,4,6-triisopropylbenzenesulfonyl) transition state tosyl n-undecyl

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