Asymmetric Catalysis Using Chiral SalenâMetal Complexes: Recent

Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

pubs.acs.org/CR

Asymmetric Catalysis Using Chiral Salen−Metal Complexes: Recent Advances Subrata Shaw† and James D. White*,‡ †

Downloaded via UNIV OF SOUTHERN INDIANA on July 18, 2019 at 11:38:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Center for the Development of Therapeutics, Broad Institute of MIT and Harvard, 415 Main Street, Cambridge, Massachusetts 02142, United States ‡ Department of Chemistry, Oregon State University, Corvallis, Oregon 97331, United States ABSTRACT: Chiral salen−metal complexes are among the most versatile asymmetric catalysts and have found utility in fields ranging from materials chemistry to organic synthesis. These complexes are capable of inducing chirality in products formed from a wide variety of chemical processes, often with close to perfect stereoinduction. Salen ligands are tunable for steric as well as electronic properties, and their ability to coordinate a large number of metals gives the derived chiral salen−metal complex very broad utility in asymmetric catalysis. This review primarily summarizes developments in chiral salen−metal catalysis over the last two decades with particular emphasis on those applications of importance in asymmetric synthesis.

CONTENTS 1. Introduction and Background 2. Chiral Salen Ligands: Synthesis and Metalation 3. Stereostructural Properties of Chiral Salen−Metal Complexes 4. Asymmetric Reactions Catalyzed by Salen−Metal Complexes A. Asymmetric Carbon−Carbon Bond Formation A.1. Asymmetric [1,2]-Addition to Carbonyls and Imines A.2. Asymmetric Conjugate Addition of Carbon Nucleophiles A.3. Asymmetric α-Alkylation of Ketone Enolates A.4. Miscellaneous Asymmetric Reactions B. Asymmetric Formation of Carbon−Heteroatom Bonds B.1. Asymmetric Oxidation of Carbon−Carbon Double Bonds B.2. Asymmetric Baeyer−Villiger Oxidation B.3. Asymmetric Opening of Small Rings B.4. Asymmetric Hydrophosphonylation B.5. Asymmetric Conjugate Addition of Heteroatom Nucleophiles B.6. Asymmetric Iodolactonization B.7. Asymmetric Iodoetherification B.8. Asymmetric C−H Functionalization C. Concomitant Formation of Carbon−Carbon and Carbon−Heteroatom Bonds C.1. Asymmetric Darzens Condensation C.2. Asymmetric Cycloaddition C.3. Asymmetric Tandem Michael-Cyclization Reactions C.4. Asymmetric Carbonyl-ene Reactions C.5. Asymmetric Carbenoid C−H Insertion © XXXX American Chemical Society

D. Asymmetric Formation of Heteroatom−Heteroatom Bonds D.1. Asymmetric Oxidation of Sulfides D.2. Asymmetric Sulfimidation D.3. Asymmetric Carbenoid Insertion into Silicon−Hydrogen Bonds E. Resolution of Racemic Compounds E.1. Oxidative Kinetic Resolution of Secondary Alcohols E.2. Asymmetric Phase-Separative Resolution of Amino Acids F. Asymmetric Catalysis by Bimetallic Salen Complexes F.1. Asymmetric Mannich Reaction F.2. Asymmetric Amination F.3. Asymmetric Conjugate Addition F.4. Asymmetric Henry and Aza-Henry Reactions F.5. Asymmetric Addition of Isocyanoacetamides to Aldehydes F.6. Desymmetric Ring Opening of Epoxides 5. Conclusion and Outlook Author Information Corresponding Author ORCID Notes Biographies References

B B C D D D Q T T U U W W Z AA AA AB AB

AF AF AG AH AH AH AJ AJ AJ AJ AK AK AL AL AL AM AM AM AM AM AM

AC AC AD AE AE AF

Received: February 1, 2019

A

DOI: 10.1021/acs.chemrev.9b00074 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

Figure 1. Representative salen ligands used in asymmetric catalysis.

1. INTRODUCTION AND BACKGROUND The term “salen” refers to a class of Schiff bases that are the products of condensation of two equivalents of an orthohydroxyl-substituted aryl aldehyde, most commonly salicylaldehyde, with a diamine. The prototype salen, N,N′-bis(salicylaldehydo)ethylenediamine (1), was first synthesized in 1889 from condensation of salicylaldehyde with ethylenediame by Combes who found that the compound formed a stable copper complex.1 This key observation set the stage for the genesis of an entire field of chemistry where N,N,O,Otetradentate bis-Schiff bases provide ligands for a large number of metal ions. Many book chapters and reviews have appeared that summarize the chemistry of salen ligands, their metal complexes, and the role that these complexes play in catalysis.2−8 This review will focus on recent advances in the synthesis and applications of chiral salen−metal complexes, with particular attention being given to applications in which the complex is a catalyst for reactions that result in useful product(s) at a high level of stereochemical enrichment. Certain aspects of chiral salen−metal complexes such as the design of new salen-type

ligands and their application in asymmetric synthesis have been reviewed, some as recently as 2012,9 but the rapid pace at which the use of these complexes in asymmetric catalysis has been expanding since the early 1990s provides new incentive for this summary.

2. CHIRAL SALEN LIGANDS: SYNTHESIS AND METALATION Salen compounds are typically synthesized as in the conventional preparation of a Schiff base, in this case by condensation of an aromatic ortho-hydroxy aldehyde with a primary diamine in a 2:1 ratio of aldehyde to diamine and usually with a method such as azeotropic distillation with benzene for removing water of condensation. In early versions, the amine component of the reaction was invariably a 1,2-diamine with stereogenic centers incorporated into the salen via the diamine. However, chiral salen ligands have been prepared in which asymmetry is also introduced via the aryl component. A set of currently known chiral salen ligands, 1−27, is shown in Figure 1. Electronic and steric properties of substituents in the aryl residues of a salen B


Chemical Reviews

Review

metal complexes. Halides such as titanium tetrachloride or zirconium tetrachloride are used in stepwise reaction with a base such as a metal hydride or an amine to deprotonate the phenolic hydroxyls.

compound can play an important role in the performance of the salen ligand as a template for asymmetric catalysis, as can extended separation of coordinating heteroatoms as in salens 24−26 derived from a 1,4-diamine scaffold. The tetradentate property of salen ligands, in which two neutral nitrogen atoms and two acidic hydroxyl functions are set in a square array, enables these substances to form coordination complexes with a broad range of metals. The metal core in these complexes may occupy various oxidation states that can be chemically quite stable, and the resulting salen−metal complex facilitates reactions such as asymmetric oxidation that would be difficult to accomplish by other methods. With a chiral salen ligand surrounding the metal core of a salen−metal catalyst, the asymmetric environment of the complex will generally steer a reaction toward one of two or more stereochemical outcomes. If catalyst turnover is rapid and background reaction(s) leading to achiral product(s) are suppressed, the level of asymmetric induction in a reaction employing chiral salen−metal catalysis can be very high. Considerable effort is often required to optimize the structure and conformation of a chiral salen ligand in order to maximize stereoinduction in a reaction catalyzed by its metal complex, and as will be seen, progress in this field has resulted from both empirical discovery and general principles of catalyst design. Five methods are commonly used for preparing metal complexes from the parent salen ligand. Complexes of early transition metals such as titanium and zirconium can be obtained by treating the ligand with a metal chloride or alkoxide (Scheme 1, method 1), although this route is sensitive to the

3. STEREOSTRUCTURAL PROPERTIES OF CHIRAL SALEN−METAL COMPLEXES The degree to which stereochemical information is relayed from a chiral salen−metal system to a reaction product depends on a number of factors. These include structural features present in the ligand, the nature of the metal core such as its oxidation state, and especially the reacting conformation of the metal complex itself. The conformation of a salen−metal complex is strongly influenced by the presence of ancillary ligands, specifically whether they are coordinating or noncoordinating, whether they are monodentate or bidentate, and whether they occupy apical or equatorial sites in the complex. As a result, minor modifications to a chiral salen−metal catalyst can dramatically alter the level of asymmetric induction. Most salen−metal complexes adopt an octahedral configuration with the metal tightly bound to the two nitrogen and two oxygen atoms and with neutral or anionic ligands occupying the two vacant sites. When noncoordinating metal counterions such as PF6− or ClO4− are present, vacant sites at the metal center are taken up by a solvent molecule. For an octahedral salen−metal complex with two coordinating ancillary ligands, there are three possible configurations. These are (a) trans with two apical ligands, (b) cis-α with two equatorial ligands, and (c) cis-β with one apical and one equatorial ligand (Figure 2). When ancillary ligands are

Scheme 1. Metalation of Salen-Type Ligands

Figure 2. Three configurations of a chiral salen−metal complex.

monodentate, salen−metal complexes usually adopt a trans configuration and catalysis occurs by replacement of an apical ligand with a reacting entity such as oxygen (Scheme 2). presence of water which can lead to the formation of μ-oxo complexes. A more reliable method for preparing complexes with transition metals is to employ the corresponding metal amide, e.g., M(NMe2)4 (Scheme 1, method 2). This reaction liberates 2 mol of dimethylamine with simultaneous removal of the acidic phenolic protons and leads to a bis-amido salen− metal species. Exposure of this material to trimethylsilyl chloride produces a bis-chloro complex. Salen−metal complexes can usually be prepared cleanly from a metal alkyl via a direct exchange reaction (Scheme 1, method 3). Metal mesitylates are often used for this purpose. A wide range of salen−metal complexes including those based on vanadium, manganese, iron, and copper have been obtained using this method.10 Metal acetates and acetylacetonates have been used to prepare salen− metal complexes (Scheme 1, method 4) in a solvent such as THF at reflux when the metal complex is sufficiently stable to survive the reaction conditions. Copper, cobalt, nickel, rhodium, and palladium salen complexes have been prepared using this method. Direct exchange with a metal halide can be employed for generating a salen−metal complex (Scheme 1, method 5) and is especially well suited to preparation of early transition

Scheme 2. Substitution Reactions of trans-Salen−Metal Complexes

Meanwhile, the electrophilic metal coordinates the substrate. For example, in salen−metal catalyzed epoxidation, an apical ligand is replaced by oxygen from a reagent such as iodosylbenzene to form a metal−oxene species while an olefinic substrate is bound to the metal core. This brings the two reactants into close proximity, and it is this proximity within a chiral environment that is primarily responsible for stereoinduction in asymmetric catalysis by salen−metal complexes. An analogous process takes place in asymmetric cyclopropanation C


Chemical Reviews

Review

The selection of substituents for the salicyl component of salen−metal complexes is a valuable means for “tuning” the catalyst not only for steric properties but also for electronic characteristics that optimize its performance. For example, incorporation of electron-donating substituents such as methoxy groups into the salicyl units can stabilize the oxidation state of the metal core in a complex that would otherwise be too unstable for practical use. For example, a “first generation” manganese(V) complex 33 used in asymmetric epoxidation underwent extensive structural refinement to produce “second generation” complex 34 where enantioselection is enhanced by steric encumbrance arising from naphthyl substituents in the salicyl components as well as by axial stereogenicity from restricted rotation around the binaphthyl units (Figure 5).

where an apical metal−carbenoid species is generated from a diazo compound and adds in intramolecular fashion to a metalcomplexed alkene. The conformation of a salen−metal catalyst is of pivotal importance when the complex is chiral. In general, a trans salen− metal complex based on a ligand such as 14 or 15 adopts one of two possible conformations: a “stepped” conformation 28 in which the five-membered ring is a half-chair or a “bowl-shaped” conformation 29 where the five-membered ring has an envelope shape (Figure 3). When a chiral diamine unit is present in the

Figure 3. Two conformations of trans salen−metal complexes.

ligand, as in 14 or 15, the equilibrium between conformers of a trans salen−metal system favors the diequatorial conformer 31 over the diaxial conformer 30 due to steric repulsion between apical ligands and diamine substituents in the latter (Scheme 3). Figure 5. First and second generation chiral manganese−salen catalysts for asymmetric epoxidation.

Scheme 3. Equilibrium between Diaxial and Diequatorial Conformers in a Salen−Metal Complex Derived from a trans1,2-Disubstituted Diamine

As described above, a chiral salen−metal complex with a trans configuration can function as an efficient catalyst for reactions involving monodentate substrates or reagents. Conversely, when the ancillary ligands are bidentate, chiral salen−metal complexes usually adopt a cis-β configuration and reactions of trans salen− metal complexes with bidentate substrates or reagents normally afford cis-β salen−metal complexes (Scheme 4). ConformaScheme 4. Synthesis of cis-β Salen−Metal Complexes

While the degree of out-of-plane deformation in conformer 28 is influenced by the steric and electronic properties of substituents in the diamine portion of the ligand as well as by the oxidation state of the central metal atom, substituents in the aromatic portion of a chiral salen complex can also play an important role in determining conformation and therefore the level of asymmetric induction. In particular, the presence of bulky substituents such as tert-butyl groups at the 3,3′,5,5′positions of the salicyl units in conformer 28 can favor approach to the metal core from a direction that steers the reaction toward one face of the substrate. For example, asymmetric epoxidation of an alkene by the stepped conformer of oxomanganese−salen complex 32 is believed to take place by a pathway in which the alkene approaches the metal via a trajectory over the downward oriented benzene ring. The opposite trajectory over the upward oriented benzenoid ring is disfavored in large part by steric obstruction due to the bulky substituents in this residue (Figure 4).

tional effects in chiral salen−metal catalysis with complexes other than trans configured structures are poorly understood, although it is known that certain metals such as zirconium, hafnium, and ruthenium as well as some second and third row transition metals favor structures having a cis-β configuration. Opportunities for asymmetric catalysis with salen−metal complexes of this class remain to be explored.

4. ASYMMETRIC REACTIONS CATALYZED BY SALEN−METAL COMPLEXES A. Asymmetric Carbon−Carbon Bond Formation

A.1. Asymmetric [1,2]-Addition to Carbonyls and Imines. A.1.1. Asymmetric Addition of Cyanide to Aldehydes. Products of asymmetric addition of cyanide to carbonyl compounds are especially valuable for their facile conversion into a variety of more complex substances.11 Research by North and Belokon established that titanium−salen complex 35 is an efficient catalyst for asymmetric addition of trimethylsilyl cyanide to benzaldehyde in a process that proved to be general

Figure 4. Direction of alkene approach to a maganese−salen catalyst in asymmetric epoxidation. D


Chemical Reviews

Review

(Scheme 5).12 The discovery ushered in a broad effort toward development of chiral salen−metal catalysts for trimethylsilylcyanation of carbonyl compounds.13−19

A series of chiral salen ligands based on trans-1,2diaminocyclohexane has been synthesized in which one of the aromatic rings carries an appended Lewis base at the 3-position. The titanium complexes formed from these ligands with titanium tetraisopropoxide catalyze cyanosilylation of aldehydes with trimethylsilyl cyanide (TMSCN) under mild conditions (Scheme 7).21 The optimal catalyst system employs salen ligand

Scheme 5. Asymmetric Cyanohydrin Synthesis from Aldehydes Catalyzed Titanium−Salen Complex 35

Scheme 7. Asymmetric Cyanosilylation of Aldehydes Catalyzed by a Bifunctional Catalyst Generated from Titanium Tetraisopropoxide and Salen Ligand 39

In related chemistry, titanium−salen complex 36 generated in situ from ligand 6 and titanium tetraisopropoxide was shown to catalyze enantioselective cyanoformylation of aldehydes using ethyl cyanoformate.20 By incorporating isopropanol into the reaction medium along with chloroform, the authors found that mononuclear titanium complex 36 is the species responsible for the stereodiscriminating step in this reaction rather than the putative bridged dimeric complex 37 (Scheme 6). Optimal Scheme 6. Asymmetric Cyanoformylation of Aldehydes with Ethyl Cyanoformate Catalyzed by the Complex Formed from Titanium Tetraisopropoxide and Salen Ligand 6

39 bearing a (diethylamino)methyl group at C3 of the aryl residue and is active even at an aldehyde:catalyst ratio of 50,000:1. At a catalyst loading of 0.05 mol %, quantitative conversion of benzaldehyde to the corresponding cyanosilylation product occurs within 10 min at ambient temperature. Both aromatic and aliphatic aldehydes undergo cyanosilylation with TMSCN under these conditions, although asymmetric induction is only moderate in some cases. Kinetic experiments performed during this work suggest a mechanism involving intramolecular cooperative catalysis in which the titanium atom in the salen complex acts as a Lewis acid to coordinate the aldehyde carbonyl while the appended Lewis base activates TMSCN and releases cyanide. Titanium−salen complex 40 based on a trans-3,4-diaminopyrrolidine scaffold, when used in combination with N,Ndimethylaniline N-oxide as an adjuvant to assist dissociation of TMSCN, gives cyanohydrins from aldehydes in moderate enantiomeric excess (Scheme 8).22 The reaction requires a relatively low catalyst loading (1 mol %) and only 1.05 equiv of TMSCN. Conversion of ligand 40 to the corresponding pyrrolidine N-oxide 41 removed the need for external aniline N-oxide additive, but asymmetric induction remained variable. Catalyst 41 produces cyanohydrins of the reversed enantiomeric series to those obtained with 40, but no explanation for this unexpected outcome was offered. Salen ligand 6 (Figure 1) forms vanadium complexes, and oxovanadium complex 42 catalyzes asymmetric cyanation of aromatic and aliphatic aldehydes with ethyl cyanoformate in the presence of imidazole as a cocatalyst (Scheme 9).23 Excellent yields of cyanohydrin carbonates in up to 97% enantiomeric excess are obtained at −20 °C. In many cases, the carbonates could be crystallized to 100% enantiomeric purity. A reaction pathway represented as 43 similar to that proposed in 38 for titanium catalyst 36 was offered by the authors. Again, attack by cyanide takes place at the more exposed re face of the aldehyde to yield products of (S) configuration. The role of imidazole as a cocatalyst is presumed to involve assistance in the release of cyanide ion from ethyl cyanoformate.

results are obtained with only 5 mol % catalyst in a chloroform− isopropanol mixture (4:1 v/v) at −20 °C. Under these conditions, a wide range of aldehydes including aromatic, α,βunsaturated, and aliphatic aldehydes are converted into their cyanohydrin ethyl carbonates in 76−91% enantiomeric excess. Transition state 38, where the re face of the aldehyde is more exposed to nucleophilic attack by cyanide, was proposed as the explanation for asymmetric induction leading to the (S) enantiomer of the O-protected cyanohydrin. E


Chemical Reviews

Review

Scheme 10. Asymmetric O-Acetyl Cyanation of Aromatic Aldehydes Catalyzed by Polymeric Oxovanadium−Salen Complex 44 with Acetic Anhydride and KCN

Scheme 8. Asymmetric Addition of TMSCN to Aldehydes Catalyzed by Titanium−Salen Complexes 40 and 41

Scheme 11. Enantioselective Addition of Ethyl Cyanoformate to Aldehydes Using Polymeric Oxovanadium−Salen Complex 44

Scheme 9. Enantioselective Cyanoformylation of Aldehydes Catalyzed by Vanadium−Salen Complex 42

producing cyanohydrin carbonates in higher optical purity. As with O-acetyl cyanation using 44 (Scheme 10), both the catalyst and cocatalytic hydrotalcite are recoverable and recyclable several times with no loss of activity. Katsuki’s manganese−salen complex 45 has been studied as a catalyst for asymmetric cyanosilylation of aromatic and aliphatic aldehydes with TMSCN (Scheme 12).26 With 0.25 mol % of 45 Scheme 12. Asymmetric Cyanosilylation of Aldehydes Catalyzed by Manganese−Salen Complex 45

In an extension of this work, polymeric oxovanadium−salen complex 44 is used as a catalyst for asymmetric O-acetyl cyanation of aldehydes in the presence of acetic anhydride with potassium cyanide as the cyanide source (Scheme 10).24 OAcetylcyanohydrins possessing an (S) configuration are obtained in up to 96% enantiomeric excess with aromatic aldehydes. The polymeric catalyst is easily recovered after use and can be recycled four times with full retention of its activity. Polymeric catalyst 44 was also employed in asymmetric cyanoformylation of aldehydes using ethyl cyanoformate as the source of cyanide (Scheme 11).25 The basic additive hydrotalcite enhances the yield of (S)-ethyl cyanohydrin carbonate products (>95%), and enantiomeric excess up to 94% is achieved. In a comparison of polymeric catalyst 44 with its monomeric counterpart 42, the former is more reactive,

in combination with 10 mol % of triphenylphosphine oxide, aromatic aldehydes are converted to (S)-cyanohydrin trimethylsilyl ethers in high yield but in only moderate and highly variable (11−67%) enantiomeric excess. As with other catalysts effecting asymmetric cyanosilylation of aldehydes with TMSCN, a double activation mechanism was envisioned in which the manganese ion of 45 activates the carbonyl oxygen while triphenylphosphine oxide promotes release of cyanide ion. F


Chemical Reviews

Review

A complex prepared in situ from bis(sulfonato)−salen ligand 46 and manganese(II) acetate is an efficient catalyst for asymmetric addition of sodium cyanide to aldehydes (Scheme 13).27 Cyanohydrins of (R) configuration are formed in up to

designed to mimic the operation of a biocatalyst by constraining the substrate within a channel where steric and electronic interactions of catalyst and reactants are conducive to good stereocontrol. Salen complexes of main group metals such as aluminum have also been used as catalysts in asymmetric cyanosilylation of aldehydes and ketones. The novel aluminum−salen complex 48 based on a dibenzobicyclo[2.2.2]octane scaffold was found to catalyze asymmetric trimethylsilylcyanation of aldehydes in the presence of tri-n-octylphosphine oxide (Scheme 15).29 At 1 mol

Scheme 13. Asymmetric Addition of Sodium Cyanide to Aldehydes Catalyzed by Salen Ligand 46 and Manganese(II) Acetate

Scheme 15. Asymmetric Trimethylsilylcyanation of Aldehydes Catalyzed by Aluminum−Salen Complex 48

99% enantiomeric excess using only 0.2 mol % of catalyst. This procedure for direct asymmetric cyanohydrin formation from aldehydes has several advantages over indirect methods including air tolerance and is applicable to a wide range of substrates. Inspired by Katsuki’s manganese−salen complex 45, Xu prepared the multistereogenic manganese−salen complex 47 in which trans-1,2-diaminocyclohexane replaces the diaminodiphenylethane scaffold core and benzyl groups replace the phenyl substituents attached to the chiral binaphthyl portion of the ligand. Asymmetric cyanosilylation of aldehydes with TMSCN in the presence of catalyst 47 and triphenylphosphine oxide affords (S)-cyanohydrins after acidic hydrolysis in 65−90% enantiomeric excess (Scheme 14).28 As previously noted with catalysts of this type and with triphenylphosphine oxide as an additive, a dual-activation mechanism is believed to operate in which the manganese ion coordinates the aldehyde carbonyl and triphenylphosphine oxide provides a Lewis base to activate TMSCN. It is asserted by the authors that complex 47 was

% of complex 48 and 10 mol % of the phosphine oxide, aromatic aldehydes with electron-donating substituents as well as electron-rich heteroaromatic aldehydes such as 2-furaldehde are transformed with TMSCN into (S)-cyanohydrins after acidic hydrolysis. Good-to-excellent enantiomeric excess (80− 92%) is observed for these products, but aromatic aldehydes bearing an electron-withdrawing substituent as substrates result in lower enantioselectivity. Chiral salen ligands based on a 1,2-diaminoethane scaffold prepared by North from four amino acids (alanine, valine, phenylalanine, and phenylglycine) form complexes with first row transition metals including titanium, vanadium, copper, and cobalt. The resulting complexes are moderately effective catalysts for asymmetric cyanosilylation of aldehydes, with oxovanadium complex 49 prepared from valine being the best (Scheme 16).30 A.1.2. Asymmetric Addition of Cyanide to Ketones. Asymmetric addition of cyanide to ketones affords products bearing a stereodefined quaternary center, and although ketones are less reactive than aldehydes, success has been achieved in this

Scheme 14. Asymmetric Cyanation of Aldehydes Catalyzed by Manganese−Salen Complex 47

Scheme 16. Asymmetric Addition of Cyanide to Aldehydes Catalyzed by C1-Symmetric Oxovanadium−Salen Catalyst 49

G


Chemical Reviews

Review

calculations led to the proposed model 54 as a rationale for its stereogenicity. In a clever extension of the process, Zhou demonstrated that a tandem Wittig-cyanosilylation sequence can be accomplished when an aldehyde reacts with an excess of the stabilized phosphorane 55 and the resulting α,β-unsaturated ketone 56 is exposed to catalyst 52 and TMSCN. Residual phosphorane 55 along with triphenylphosphine oxide generated in the Wittig reaction become partners in the asymmetric cyanosilylation of methyl ketone 56. A.1.3. Asymmeric Addition of Cyanide to Imines (Strecker Reaction). The Strecker synthesis of α-amino nitriles by addition of cyanide to an imine was discovered in 1850, but it was not until 1998 that an asymmetric Strecker reaction catalyzed by a salen−metal complex was described by Jacobsen. This was accomplished by addition of HCN to an imine in the presence of a chiral aluminum−salen complex.33 Recent advances in this area have been summarized,34 and the asymmetric Strecker reaction continues to play an important role in the synthesis of both natural and non-natural α-amino acids. C1-symmetric oxovanadium complex 57 prepared from phenylalanine catalyzes asymmetric addition of HCN to imines. In a typical example, N-benzyl benzaldimine was reacted in the presence of 10 mol % of 57 with HCN generated in situ from TMSCN and methanol to give the α-aminonitrile in 81% enantiomeric excess, as determined by 1H NMR analysis of its diastereomeric camphor sulfonate salts (Scheme 19).35

reaction with salen−metal complexes. Thus, Carpentier found that aluminum−salen complexes 50 and 51 exhibit moderate catalytic activity in asymmetric cyanosilylation of substituted acetophenones with TMSCN in the presence of catalytic N,Ndimethylaniline N-oxide (DMAO) (Scheme 17).31 Scheme 17. Cyanosilylation of Aryl Alkyl Ketones with TMSCN Catalyzed by Aluminum−Salen Complexes 50 and 51

A significant advance in this area was made when phosphoranes were found to be Lewis bases for activating the chloroaluminum complex 52 to enhance its electrophilicity. A three-component catalyst system consisting of 52, phosphorane 53, and triphenylphosphine oxide proved to be a powerful combination for asymmetric cyanosilylation of ketones with TMSCN (Scheme 18).32 In particular, the first highly enantioselective cyanosilylation of linear aliphatic ketones was achieved with this catalyst system. A detailed investigation of the mechanism of the reaction using NMR, MS, IR, and DFT

Scheme 19. Asymmetric Addition of HCN to Imines Catalyzed by Oxovanadium−Salen Complex 57

Scheme 18. Asymmetric Cyanosilylation of Ketones and Tandem Wittig Cyanosilylation of Aldehydes Catalyzed by a Phosphorane and Aluminum−Salen Complex 52

Dimeric oxovanadium−salen complex 58 derived from 5,5methylenedi-[(S,S)-[N-(3-tertbutylsalicylidine)-N′-(3′,5′-ditert-butylsalicylidene)]-1,2-cyclohexanediamine] and vanadyl sulfate followed by auto oxidation is a catalyst for the enantioselective Strecker reaction of N-benzylimines with TMSCN at −30 °C (Scheme 20).36 Both aromatic and aliphatic imines were tested as substrates, giving α-amino nitriles of (R) configuration in enantiomeric excess that ranged from 22 to 89%. A recent development in this area employs heterogeneous catalysts, prepared by covalent attachment of vanadium−salen and aluminum−salen complexes to polystyrene supports, for asymmetric Strecker synthesis of α-amino nitriles.37 Asymmetric induction in trimethylsilylcyanation of aldimines with this technique is modest, but the method has potential for improvement. A.1.4. Asymmetric Mannich Addition to Ketimines. In a comprehensive study of the asymmetric Mannich reaction of αisothiocyanate esters with ketimines, Shibasaki reported that metal complexes generated in situ from axially chiral salen ligand 21 produced 2-thionoimidazolidines 59 in good enantiomeric excess (Scheme 21).38 Strontium and magnesium complexes of 21 give the best results, with anti (E)-59 being the major isomer from the strontium complex, whereas the magnesium complex gives predominantly the syn (Z) product. H


Chemical Reviews

Review

Scheme 20. Asymmetric Addition of Cyanide to Imines Catalyzed by Dimeric Oxovanadium−Salen Complex 58

Scheme 22. Asymmetric Alkynylation of Aldehydes Catalyzed by Dimethylzinc and Polymeric Salen Ligand 60

Scheme 21. Asymmetric Mannich Reaction of αIsothiocyanate Esters with Ketimines Catalyzed by Strontium and Magnesium Complexes of Salen Ligand 21

Scheme 23. Asymmetric Addition of Terminal Alkynes to Acetyltrimethylsilane Catalyzed by Diethylzinc and Salen Ligand 6

A.1.5. Asymmetric Addition of Alkynes to Aldehydes and Ketones. Access to enantioenriched propargylic alcohols through asymmetric addition of terminal alkynes to aldehydes and ketones has made significant progress recently, due in large part to work by Cozzi who employed chiral chromium−salen and zinc−salen catalysts for the reaction.39−41 In an extension of this approach, a polymeric zinc−salen complex, prepared in situ from ligand 60 and dimethylzinc, is used as a catalyst for addition of phenylacetylene to aliphatic and aromatic aldehydes and ketones (Scheme 22).42 The resulting secondary and tertiary alcohols are obtained in good yield but in only moderate enantiomeric excess. For the addition to aldehydes, a double complexation pathway via 61 was proposed in which a complexed alkynylzinc species attacks the re face of the zinccomplexed carbonyl oxygen to deliver a product of (R) configuration. The polymeric catalyst is recycled up to four times without loss of stereoselectivity. Acylsilanes have been investigated as substrates for asymmetric addition of terminal alkynes using the zinc complex of ligand 6, prepared in situ with diethylzinc (Scheme 23).43 Enantioenriched tertiary propargylic silanols obtained from this reaction are valuable entry points to a variety of more complex

structures through modification of the three functionalities in the molecule. A zinc complex prepared in situ by reaction of dimethylzinc with Kozlowski’s salen ligand 62 bearing 1-piperidinylmethyl substituents at the C-3- and C-3′-positions of the benzenoid moieties catalyzes asymmetric addition of phenylacetylene to a range of aldehydes (Scheme 24).44 Propargylic alcohols are obtained in good yield but with only moderate enantioselectivity (53−80% enantiomeric excess). A dual activation mechanism was proposed in which an alkynylzinc nucleophile coordinated to one of the piperidine nitrogens of 63 attacks the si face of the zinc-complexed aldehyde carbonyl to produce a propargylic alcohol of (R) configuration. A.1.6. Asymmetric Addition of Phenylzinc to Aldehydes and Ketones. A recent study with salen ligand 6 describes an interesting result from reaction of 6 with aldehydes and ketones in the presence of a combination of 2 equiv of diethylzinc and 1 equiv of diphenylzinc that affords phenyl carbinols in excellent enantiomeric excess (Scheme 25).45 The reaction is successful with a wide range of aldehydes and ketones including I


Chemical Reviews

Review

Scheme 24. Asymmetric Addition of Phenylacetylene to Aldehydes Catalyzed by Dimethylzinc and Salen Ligand 62

Scheme 26. Asymmetric Alkylation of Imino Esters Catalyzed by Copper−Salen Complexes 65−68

homoallylic secondary alcohols.48 A catalytic protocol introduced by Furstner that avoids the use of stoichiometric chromium(III) as the source of chromium(II) by employing manganese(0) as a reducing agent together with trimethylsilyl chloride49 has played an important role in efforts to develop an asymmetric version of the NHK reaction. Early experiments, particularly those by Cozzi and Umani-Ronchi50 as well as Berkessel,51 suggest that developing an asymmetric NHK process affording homoallylic alcohols in high enantiomeric excess would be a challenge. However, a novel C2-symmetric salen ligand 24 based on a cis-2,5-diaminobicyclo[2.2.2]octane scaffold solves this problem for allylation of aromatic aldehydes (Scheme 27).52 Thus, chlorochromium−salen complex 69 is

Scheme 25. Enantioselective Phenyl Transfer to Aldehydes and Ketones Catalyzed by a Zinc Complex of Salen Ligand 6

Scheme 27. Asymmetric Addition of Allyl Halides to Aromatic Aldehydes Catalyzed by a Chromium−Salen Complex from 69

heteroaromatic ketones but is unproductive with substrates bearing a pyridyl group, probably due to competing coordination by the pyridine nitrogen for zinc ion. In general, reactions were run with 10 mol % of 6 at −40 °C for aldehydes and at room temperature for ketones. Phenyl transfer from zinc to the carbonyl substrate is thought to proceed via bis-cordinated zinc intermediate 64.46 A.1.7. Asymmetric Alkylation of Imines. C2-symmetric copper−salen complex 65 was shown by North to be an efficient catalyst for α-alkylation of imino derivatives of α-amino esters with an alkyl bromide. A typical example is benzylation of an imino derivative of alanine to yield an alkylated product of (S) configuration in 81% enantiomeric excess (Scheme 26). In contrast, C1-symmetrical copper−salen catalysts 66−68 where natural amino acids provide chirality of the salen scaffold give much lower enantioinduction in the same reaction.30,47 A.1.8. Asymmetric Addition of Allyl Halides to Aldehydes (Nozaki−Hiyama−Kishi Reaction). The Nozaki−Hiyama− Kishi (NHK) reaction, which effects Barbier-type addition of an allylchromium(II) species to an aldehyde, has undergone several modifications since its original inception as a route to

reduced by excess manganese(0) to the reactive chromium(II) species which catalyzes addition of allyl chlorides and bromides to aldehydes affording homoallylic secondary alcohols in excellent yield and in enantiomeric excess up to 97%. Chromium complex 69 of (R,R,R,R) configuration uniformly gives alcohols of (R) configuration. An attempt to extend this protocol to asymmetric addition of vinyl iodide to 1-naphthaldehyde using a catalytic quantity of nickel(II) chloride as an additive gave allylic alcohol (S)-70 in moderate yield and good enantiomeric excess (Scheme 28). A.1.9. Asymmetric Addition of Silyloxyallenes to Aldehydes. Chromium−salen complex 71 catalyzes asymmetric addition of silyloxyallenes to aldehydes to give α-alkylidene βhydroxy ketones in generally high enantiomeric excess and with the product geometry strongly favoring the (Z) isomer (Scheme J


Chemical Reviews

Review

gen-bonded dinuclear cobalt−salen system 73 as the active catalyst.56 Reaction of aromatic aldehydes with nitromethane under catalysis by 73 provides β-nitro alcohols with good enantioselectivity (Scheme 30). The structure of 73 was confirmed by X-ray analysis and 1H NMR experiments.

Scheme 28. Asymmetric Addition of Vinyl Iodide to 1Naphthaldehyde Catalyzed by Chromium−Salen Complex 69

Scheme 30. Asymmetric Henry Reaction of Aldehydes with Nitromethane Catalyzed by Dimeric Cobalt−Salen Complex 73 29).53 The reaction can be viewed formally as a Lewis acidcatalyzed carbonyl-ene reaction with the silyloxyallene acting as Scheme 29. Asymmetric Addition of Silyloxyallenes to Aldehydes Catalyzed by Chromium−Salen Complex 71

The concept of a hydrogen-bonding motif to enhance the catalytic activity of salen−metal complexes was pursued with cobalt complex 74 where urea moieties are attached to the aromatic rings of the ligand. Complex 74 catalyzes the Henry reaction of higher order nitroalkanes with both aromatic and aliphatic aldehydes to yield predominantly anti nitro alcohols in excellent enantiomeric excess (Scheme 31).57 Diastereoselectivity is explained with catalyst 74 by model 75 where hydrogen bonding between urea N−H functions and oxygens of the nitroalkane stabilizes the latter as its (Z)-aci tautomer. The catalyst brings this entity into close proximity with the exposed re face of the cobalt-complexed aldehyde, resulting in high enantioselectivity. The catalyst design inherent in manganese−salen complex 47 for asymmetric addition of cyanide to aldehydes (Scheme 14) has been extended to cobalt complex 76 for asymmetric Henry addition of nitromethane to aldehydes (Scheme 32).58 As with 47, the BINMOL segment of the catalyst creates a chiral pocket which encapsulates nitromethane. Benzyl substituents in the binaphthyl moieties serve as “helping hands” to guide attack at the re face of the cobalt-complexed aldehyde. A.1.10.2. Chromium−Salen Catalyzed Asymmetric Henry Reactions. Commercially available chromium−salen complex 77 has been examined as a catalyst for asymmetric Henry reactions of aldehydes with nitromethane in the presence of Hunig’s base.59 However, β-nitroethanols are obtained in only modest enantiomeric excess with this system (Scheme 33). Some aliphatic acyclic aldehydes such as n-hexanal and

an α-acylvinyl anion equivalent. The transformation provides functionalized β-hydroxy unsaturated ketones with a high degree of stereocontrol over both the resulting alkene and the secondary alcohol. Products from the reaction were converted to substituted indanones and chromenes with good transfer of configurational fidelity. A.1.10. Asymmetric Addition of Nitroalkanes to Aldehydes and Ketones (Henry Reaction). Condensation of a nitro alkane with an aldehyde or ketone to give a nitro alcohol, known as the Henry or nitroaldol reaction, is a versatile process that forges a carbon−carbon bond while introducing a nitrogen function which can be easily transformed to other functional groups. Consequently, a broad-based effort employing a wide array of catalysts has characterized the search for an asymmetric version of the Henry reaction. A review of the area appeared in 2007.54 This section groups salen−metal catalyzed asymmetric Henry reactions by the three most commonly used metalscobalt, chromium, and copper. A.1.10.1. Cobalt−Salen Catalyzed Asymmetric Henry Reactions. The first cobalt−salen catalyzed asymmetric Henry reaction was reported by Yamada and described the reaction between nitromethane and aromatic aldehydes.55 Nitro alcohols were obtained in only moderate enantiomeric excess in this work. An improved version of Yamada’s protocol involves bimetallic cooperativity and employs cobalt−salen complex 72 which undergoes spontaneous self-assembly to produce hydroK


Chemical Reviews

Review

Scheme 31. Asymmetric Henry Reaction of Aldehydes with Higher Order Nitroalkanes Catalyzed by Cobalt−Salen Complex 74

Scheme 33. Enantioselective Henry Reactions Catalyzed by Chromium−Salen Complex 77

Scheme 34. Asymmetric Henry Reaction of Aldehydes Catalyzed by Second Generation Chromium−Salen Complex 78

Scheme 32. Asymmetric Henry Reaction of Aromatic Aldehydes Catalyzed by BINMOL-Derived Cobalt−Salen Complex 76

aromatic aldehydes bearing a substituent at the ortho position such as 2-methoxybenzaldehyde as well as 1-naphthaldehyde give nitro alcohols in slightly diminished enantiomeric excess (70−75%). The benzylic substituents in complex 78 are believed to impose a steric barrier which orients the coordinated aldehyde in a manner that blocks the si face and exposes the re face of the carbonyl to attack by nitronate. The result is formation of nitro alcohols with (S) configuration ((R) in the case of furfural). Another modification made to chromium complex 77 replaces tert-butyl groups at the 5- and 5′-positions of the salicylidene unit by thiophene substituents, the purpose being to avoid steric encumbrance while retaining electron donation to the central chromium ion. In practice, chromium−salen complex 79 shows only modest improvement over 77 in catalysis of the asymmetric reaction of aldehydes with nitromethane, although some aliphatic aldehydes give nitro alcohols in enantiomeric excess as high as 83% (Scheme 35).61 As expected, reversal of the diamine configuration in the scaffold of 79 gives products of (R) configuration. A second property of the thiophene substituents in 79 enables preparation of polymeric catalyst 80 by anodic polymerization. Recoverable polymeric complex 80 promotes heterogeneous Henry reactions and can be recycled four times with only slight loss of stereoefficiency. A.1.10.3. Copper−Salen Catalyzed Asymmetric Henry Reactions. A class of chiral salen ligands exemplified by 17, 81, and 82 containing an intramolecular polyether bridge between the 5- and 5′-sites of the salicylidene moiety has been investigated as their copper complexes for catalysis of the

isobutyraldehyde failed to react, but cyclohexanecarboxaldehyde gives a β-nitro alcohol in moderate enantiomeric excess (76% ee). The inconsistency of this reaction is unexplained. A modification made to 77 by incorporating sterically demanding substituents at the 3- and 3′-positions of the salicylidene moiety results in marked improvement in asymmetric induction of this reaction by catalyst 78 (Scheme 34).60 Thus, treatment of nitromethane with both aromatic and aliphatic aldehydes in the presence of 78 affords nitro alcohols with enantiomeric excess ranging from 80 to 94%, although L


Chemical Reviews

Review

Scheme 35. Asymmetric Henry Reaction of Aldehydes Catalyzed by Thiophene-Substituted Chromium−Salen Complex 79

Scheme 37. Enantioselective Henry Reaction of Aldehydes with Nitromethane Catalyzed by Tetrahydrosalen Ligand 84 and Copper(I) Triflate−Toluene Complex

asymmetric Henry reaction of nitromethane with aromatic aldehydes (Scheme 36).62 The metal complex prepared in situ Scheme 36. Asymmetric Nitroaldol Reaction of Aldehydes Catalyzed by Salen Ligands 17, 81, and 82 in Combination with Copper(II) Chloride

hydes with nitromethane to obtain enantiomerically pure βadrenergic receptor blocking agents (S)-toliprolol, (S)-moprolol, and (S)-propanolol. A structural feature that explains the superiority of ligand 84 over 24 is hydrogen bonding between N−H functions of the ligand and the metal-complexed nitronate which orients reacting partners to favor si face attack at the aldehyde carbonyl, as shown in 85. A.1.11. Asymmetric Passerini Reaction. The three-component process that assembles an aldehyde, an isocyanide, and a carboxylic acid in one pot to give an α-acyloxy amide (Passerini reaction) is one of many “domino” reactions that have come to prominence in recent synthetic methodology.64 Chiral salen− metal complexes have been explored by Wang as catalysts for an asymmetric version of the Passerini reaction, and aluminum− salen catalyst 52 is found to give α-acyloxyamides in good enantiomeric excess from reaction of aliphatic aldehydes with both aromatic and aliphatic isocyanides and carboxylic acids (Scheme 38).65 A consideration in selecting reactants for this assembly is the propensity of each component to coordinate with the central aluminum ion of the catalyst, since only activation of the aldehyde carbonyl will lead to a Passerini product. Competition by reactants for the Lewis acidic metal will generally lead to side reactions or to deactivation of the catalyst. Replacement of the carboxylic acid component by hydrazoic acid in an asymmetric Passerini reaction leads to chiral 5(hydroxyalkyl)tetrazoles. Catalyst 50, which contains an apical methyl group on the central metal ion, has been used for synthesis of these heterocycles with good results, with Passerini products being formed in >90% enantiomeric excess in most cases (Scheme 39).66 When acrolein is used as a substrate, initial

by reaction of binaphthyl ligand 82 with copper(II) chloride dihydrate is the superior catalyst in this series. Under optimized conditions (15 mol % of 82 and 50 mol % of 2,6-lutidine), the reaction furnishes β-nitro alcohols in up to 95% enantiomeric excess with both aliphatic and aromatic aldehydes. The copper complex of 82 is sufficiently stable to be recovered and reused as many as eight times with little decrease in stereoefficiency. The catalyst was employed in an asymmetric nitroaldol condensation that led to synthesis of enantiomerically pure (R)-phenylephrine, an α-1-adrenergic receptor agonist. The C2-symmetric salen ligand 24 based on a cis-2,5diaminobicyclo[2.2.2]octane scaffold that furnishes chromium complex 69 (Scheme 27) also forms stable copper complex 83 with copper(II) acetate, but attempts to catalyze an enantioselective Henry reaction with this complex result in poor asymmetric induction. However, reduction of 24 with sodium borohydride to the corresponding diamine 84 followed by in situ preparation of its copper(I) complex with copper(I) triflate−toluene complex yields a system that is an excellent catalyst for asymmetric reaction of nitro alkanes with aromatic aldehydes (Scheme 37).63 With higher order nitro alkanes, a syn:anti ratio of nitro alcohols as high as 50:1 is obtained, with the major diastereomer being isolated in >95% enantiomeric excess. The copper(I) catalyst from 84 was employed in asymmetric nitroaldol condensation of three aryloxyacetaldeM


Chemical Reviews

Review

reaction of alkenes with α-diazo esters in the presence of chiral organometallic catalysts. A review of this area was published in 2008.68 Katsuki has been at the forefront of this effort with ruthenium−salen complexes that are efficient catalysts for asymmetric cyclopropanation of alkenes with diazoacetates.69,70 In this context, ruthenium−salen complex 87 catalyzes intramolecular cyclopropanation of trans allylic diazoacetates under irradiation to afford fused cyclopropyl γ-lactones in high yield and enantiomeric excess (Scheme 40).71 A reaction pathway using DFT calculations postulates cis-β-salen complex 88 as the species that delivers an exo cyclopropane product via a ruthenium carbenoid.

Scheme 38. Enantioselective Passerini Reaction Catalyzed by Aluminum−Salen Complex 52

Scheme 40. Asymmetric Intramolecular Cyclopropanation Catalyzed by Ruthenium−Salen Complex 87

Scheme 39. Enantioselective Synthesis of 5-(1Hydroxyalkyl)tetrazole by a Three-Component Passerini Reaction Catalyzed by Aluminum−Salen Complex 50

A theoretical study of the mechanism of asymmetric cyclopropanation of styrene with ethyl diazoacetate under ruthenium−salen catalysis using DFT methods (B3LYP[6311+G**, Lanl2dz]//B3LYP/[6-31G*, Lanl2dz]) supports the hypothesis that cyclopropanation proceeds stepwise, involving the formation of a carbenoid species followed by ring closure to regenerate the catalyst.72 Iridium−salen complexes are more labile than the corresponding rhodium and ruthenium complexes, and much research has been devoted to synthesis of a stable iridium-based catalytic system for asymmetric carbene-transfer reactions. The stable iridium−salen complex 89 bearing a tolyl group as an apical ligand was synthesized by Katsuki who showed that the system is an excellent catalyst for asymmetric cyclopropanation of conjugated and nonconjugated alkenes (Scheme 41).73,74 The latter are generally unreactive toward other salen−metal complexes. Catalyst 89 is highly selective for cyclopropanation of cis disubstituted alkenes, affording products in >93% enantiomeric excess. Cyclic olefins such as indene and benzofuran yield cis trisubstituted cyclopropanes exclusively with high enantioselectivity. A crystal structure of 89 is consistent with a mechanism in which the tolyliridium carbenoid intermediate adopts a nonplanar stepped conformation. The alkene approaches the carbenoid carbon along the iridium−nitrogen axis from the open space facing the downward oriented naphthalene ring, a trajectory that leads to a cis cyclopropane after rotation. Asymmetric cyclopropanation with iridium−salen catalyst 89 has been extended to an α-diazo δ-lactone as the carbenoid source to yield α-spirocyclopropyl lactones (Scheme 42).75

conjugate addition by hydrazoic acid to the double bond occurs but a three-component Passerini reaction follows to give a tetrazole, albeit in lower enantiomeric excess. A mechanistic hypothesis for this process supports zwitterionic azidoaluminumoxynitrilium intermediate 86 which collapses to the tetrazole. A.1.12. Asymmetric Carbocycle Formation. A.1.12.1. Asymmetric Cyclopropanation. The first asymmetric cyclopropanation employing a salen−metal catalyst was reported in 1978 by Nakamura using a cobalt−salen complex, although enantioselectivity was poor.67 As asymmetric construction of cyclopropanes assumed greater importance in the synthesis repertoire, increased attention was given to the asymmetric N


Chemical Reviews

Review

Scheme 41. Enantioselective Cyclopropanation of 1,2Disubstituted Alkenes Catalyzed by Iridium−Salen Complex 89

Scheme 43. Enantioselective Cyclopropenation of Substituted Acetylenes Catalyzed by Iridium−Salen Complex 90

Scheme 42. Synthesis of α-Spirocyclopropyl Lactones via Asymmetric Cyclopropanation Catalyzed by Iridium−Salen Complex 89

eoselectivity together with moderate-to-good enantioselectivity. It was later found that a heteroaromatic substituent in the ethylenediamine moiety of a cobalt−salen catalyst as in 91 effects asymmetric cyclopropanation of styrene derivatives in up to 96% enantiomeric excess with a cis:trans ratio as high as 99:1 for monosubstituted styrenes (Scheme 44).82 The heteroarScheme 44. Asymmetric Cyclopropanation of Styrene Derivatives with tert-Butyl α-Diazoacetate Catalyzed by Cobalt−Salen Complex 91

Cyclopropanation of aryl- and alkenyl-substituted olefins proceeds with excellent enantioselectivity (≥97% enantiomeric excess) and with high (E) selectivity regardless of the substitution pattern of the alkene substrate. Replacement of phenyl substituents in the binaphthyl units of 89 by bulkier tert-butyldiphenylsilyl (TBDPS) groups leads to iridium−salen complex 90 which catalyzes asymmetric cyclopropenation of both aliphatic 1-alkynes and arylacetylenes (Scheme 43).76 The reaction is carried out with both donor/ acceptor- and acceptor/acceptor-substituted diazo compounds such as α-aryl α-diazoacetates, α-phenyl α-diazophosphonate, 2,2,2-trifluoro-1-phenyl-1-diazoethane, and α-cyano α-diazoacetamide as carbenoid precursors. The reaction is a source of enantioenriched cyclopropenes (84−98% enantiomeric excess) containing a functionalized quaternary carbon. Asymmetric carbene transfer using less expensive metals such as cobalt was first reported by Nakamura,77 and Katsuki followed this lead with a series of papers on asymmetric cyclopropanation using cobalt−salen catalysts.78−81 With styrenes as substrates, cyclopropanation showed high diaster-

omatic substituent (imidazole or pyridine) is believed to engage the cobalt atom of the salen complex as a fifth coordinating ligand, thereby exerting a strong trans effect. This effect changes the conformation of the ligand to a degree that reverses the sense of stereoinduction seen with the catalyst lacking the heterocyclic appendage. In this case, α-substituted styrenes give cis and trans cyclopropanes in almost equal amounts. A novel cobalt−salen complex 92 has been prepared by Carreira which catalyzes asymmetric synthesis of trans disubstituted trifluoromethyl cyclopropanes from styrenes and trifluoroethylammonium chloride in the presence of triphenylarsine and sodium acetate (Scheme 45).83 The reaction proceeds with high stereoinduction and affords a valuable O


Chemical Reviews

Review

was completed using asymmetric cyclopropanation catalyzed by 93. Cobalt−salen complex 96 has been reported to catalyze asymmetric cyclopropanation of a series of fused electrondeficient nitrogen heterocycles by ethyl diazoacetate (Scheme 47).85 This dearomatization protocol provides access to unique

entry to enantioenriched trifluoromethyl-containing building blocks. Scheme 45. Asymmetric Trifluoromethylcylopropanation of Styrenes Catalyzed by Cobalt−Salen Complex 92

Scheme 47. Asymmetric Cyclopropanation of Imidazopyrazine and Imidazopyridazine Substrates Catalyzed by Cobalt−Salen Complex 96

Cobalt complex 93, prepared from salen ligand 25, affords excellent stereoselectivity in catalyzing cyclopropanation of 1,1disubstituted alkenes with ethyl diazoacetate in the presence of potassium thioacetate as an additive (Scheme 46).84 Styrenes Scheme 46. Asymmetric Cyclopropanation of 1,1Disubstituted Ethylenes Catalyzed by Cobalt−Salen Complex 93

heterocycle-substituted cyclopropanes in good diastereomeric ratio and high enantiomeric excess. The cyclopropanes were further functionalized to generate complex heterocyclic building blocks. A.1.12.2. Asymmetric Diels−Alder Cycloaddition. Salen− metal complexes have played a prominent role in catalysis of the asymmetric Diels−Alder reaction, and there have been numerous advances in the area since a review on this topic was published in 2000.86 This cycloaddition has been used extensively in complex natural product synthesis for generating a cyclohexene bearing multiple stereocenters in a single step with good enantioselectivity. Cobalt−salen complexes are effective catalysts in this context.87,88 For example, asymmetric Diels− Alder addition of functionalized diene 97 to dienophile 98 catalyzed by cobalt−salen complex 99 gives densely functionalized cycloadduct 100 in excellent yield and in 96% enantiomeric excess (Scheme 48). The substituted cyclohexene was subsequently converted by Nicolaou to (−)-platencin.89 Complex 99 has also been used by Brimble in catalysis of an Scheme 48. Asymmetric Diels−Alder Reaction Catalyzed by Cobalt−Salen Complex 99

give a high proportion of the (E) cyclopropane which is formed in high enantiomeric excess in every case. However, when a 1,1dialkyl substituted alkene is the substrate or when a heteroatom substituent is present in the alkene, both (E) selectivity and enantioselectivity are diminished. The reaction is believed to take place through the intermediacy of four-membered cobaltocycle 94 and is assisted by a trans effect due to coordination at the cobalt ion by the sulfur atom of the thioacetate. A short, stereospecific synthesis of the dual serotonin−epinephrine reuptake inhibitor (+)-synosutine (95) P


Chemical Reviews

Review

hexanes and cycloheptanes as well as to cyclobutanes, although the yield and enantioselectivity are lower in the latter case. A mechanism is proposed in which the iron center of the catalyst first forms a metal enolate with the carbonyl substrate and then activates the alkyne toward nucleophilic attack by π-complexation, as shown in 105. The presence of n-butyl substituents in the salen moiety of the catalyst is essential for good enantioselectivity in this case. An iron complex such as 104 is especially attractive as a catalyst for large-scale and industrial operations, since iron is among the most environmentally benign metals. A.1.13. Asymmetric Cyclization of α-Keto Enamides. Chromium−salen complex 77 in the presence of a trace amount of sodium carbonate catalyzes asymmetric cyclization of α-keto tertiary enamides to afford N-substituted 2-pyrrolinones bearing a hydroxylated quaternary carbon at carbon-3 (Scheme 51).96 The reaction proceeds in high yield with excellent enantioselectivity to give both monocyclic and fused pyrrolinones.

asymmetric Diels−Alder reaction of dienamine 101 with enal 102 to afford cyclohexene 103, a key intermediate in a route to tetracyclic methyllyconitine analogues (Scheme 49).90 Scheme 49. Asymmetric Diels−Alder Reaction of a Dienamine with an Enal Catalyzed by Cobalt−Salen Complex 99

A.1.12.3. Asymmetric Intramolecular Conia-ene Reaction. The intramolecular Conia-ene reaction in which an enolate adds internally to a carbon−carbon multiple bond to form a carbocycle containing an all-carbon quaternary center was reviewed in 2010,91 but until recently, all approaches toward an asymmetric version of the reaction employed a chiral dual catalytic system comprised of hard and soft Lewis acids.91−94 In 2014, salen ligand 26 containing a cis-2,5-diaminobicyclo[2.2.2]octane scaffold was found to form a stable iron trifluoroacetate complex 104 which, as a single entity, catalyzes asymmetric Conia-ene cyclization of α-functionalized ketones containing an unactivated terminal alkyne.95 Exo-methylene substituted carbocycles bearing an adjacent quaternary stereocenter are formed in good yield (>90%) and high enantiomeric excess (>90%) for cyclopentane derivatives (Scheme 50). The cyclization was extended to synthesis of homologous cyclo-

Scheme 51. Asymmetric Cyclization of α-Keto Enamides Catalyzed by Chromium−Salen Complex 77

Scheme 50. Asymmetric Conia-ene Cyclization of Alkynyl αFunctionalized Ketones Catalyzed by Iron−Salen Complex 104

A.1.14. Desymmetrization of meso Epoxides and Aziridines with Carbon Nucleophiles. A series of chiral salen ligands was synthesized from salicylaldehydes and 3,3′diformylBINOL which, in combination with titanium tetraisopropoxide, act as binuclear catalysts in asymmetric ring opening of meso epoxides and aziridines by trimethylsilyl cyanide.97 For example, treatment of symmetrically substituted meso epoxides with TMSCN in the presence of 0.1 mol % of ligand 106 and 0.2 mol % of titanium tetraisopropoxide gives vicinal trimethylsilyloxy nitriles in high yield and in up to 96% enantiomeric excess (Scheme 52). Up to 15% of an isonitrile product is observed in some cases. Opening of unsymmetrically substituted epoxides requires higher catalyst loading and generally gives the opened product in lower enantiomeric excess. Bimolecular yttrium−salen complex 107 derived from 2,2′diaminobinaphthyl catalyzes asymmetric opening of meso-Nacyl aziridines by trimethylsilyl cyanide to give β-amino nitriles (Scheme 53).98 With certain substrates, the reaction proceeds in very good yield and high enantioselectivity, but the reaction fails with aziridines derived from cycloheptene, cyclooctene, and 1,4dihydronaphthalene and also from (Z)- and (E)-stilbenes. A.2. Asymmetric Conjugate Addition of Carbon Nucleophiles. Asymmetric conjugate addition of carbon nucleophiles to α,β-unsaturated carbonyl compounds has been investigated extensively using salen−metal catalysis, and aluminum−salen complexes have emerged as favored catalysts Q


Chemical Reviews

Review

Scheme 54. Conjugate Cyanation of α,β-Unsaturated Imides Catalyzed by Binuclear Aluminum−Salen Complex 109

Scheme 52. Asymmetric Opening of Epoxides by Trimethylsilyl Cyanide Catalyzed by Titanium Tetraisopropoxide and Salen Ligand 106

Scheme 53. Asymmetric Opening of meso Aziridines by TMSCN Catalyzed by Yttrium−Salen Complex 107

results. A titanium−salen complex prepared from ligand 6 and titanium tetraisopropoxide in equimolar concentration catalyzes asymmetric addition of cyanide from TMSCN to nitro olefins (Scheme 55).103 Alkyl substituted nitro olefins with either linear Scheme 55. Asymmetric Conjugate Cyanation of Nitroolefins Catalyzed by Titanium Tetraisopropxide and Salen Ligand 6

for this reaction with weakly acidic carbon nucleophiles. Jacobsen showed that aluminum−salen complex 108 is an active catalyst for asymmetric conjugate addition of hydrogen cyanide to β-alkyl substituted α,β-unsaturated imides,99 and he later synthesized a tethered dimeric aluminum−salen complex 109 that exhibited enhanced activity and comparable enantioselectivity to 108 for this reaction (Scheme 54).100 Substrates such as β-aryl and vinyl substituted α,β-unsaturated imides which are unreactive toward HCN with monomeric catalyst 108 undergo conjugate addition of HCN when catalyzed by 109 to afford β-cyano imides in 87−96% enantiomeric excess. The bimetallic complex 109 is thought to operate via a dual activation mechanism, as shown in 110 where the imide and cyanide partners are coordinated to separate metal centers. This is one of the earliest examples to demonstrate that an entropic barrier to a reaction pathway can be lowered by a bifunctional salen−metal catalyst. A further advance in the catalysis of asymmetric conjugate addition of cyanide to α,β-unsaturated imides occurs with immobilization of aluminum−salen catalysts on solid supports, particularly on poly(norbornenes)101 and on macrocyclic cyclooctene polymers.102 The supported catalysts give good

or branched substituents furnish β-nitro nitriles in good yield and up to 84% enantiomeric excess. Cyanation of 1-nitrocyclohexene gives a 3:1 mixture favoring the anti isomer. When reaction progress was monitored by 1H NMR, formation of a stable silyl nitronate intermediate was identified which underwent hydrolysis to the nitro nitrile upon workup. Prior formation of HCN from TMSCN is therefore not a requirement for this reaction to proceed. In contrast to the titanium−salen catalyst prepared from ligand 6 and used in situ, two structurally well-defined R


Chemical Reviews

Review

complexes, bimetallic titanium−salen species 37 and monometallic vanadium−salen complex 111, were synthesized and used to catalyze asymmetric addition of trimethylsilyl cyanide to aliphatic nitro alkenes (Scheme 56).104 Under carefully

Scheme 57. Asymmetric Hydrocyanation of Nitro Olefins Catalyzed by Aluminum−Salen Complex 108

Scheme 56. Asymmetric Conjugate Addition of Cyanide to Nitro Alkenes Catalyzed by Titanium−Salen and Vanadium− Salen Complexes 37 and 111

Scheme 58. Asymmetric Conjugate Addition of Alkyl Radicals to Cyclic Enones Catalyzed by Aluminum−Salen Catalyst 114

optimized reaction conditions involving the solvent, temperature, reaction time, and vanadium counterion, conversion of the nitro alkene to β-nitro nitrile approaches 100% and enantiomeric excess is in the range 79−89%. A rationale for sterically favored si face addition of cyanide to the nitro olefin substrate with catalyst 37, which leads to a (S)-nitro nitrile, is shown in 112 where orientation of the nitro alkene is governed by a stepped conformation of the salen ligands. A similar mechanism explains the mode of addition by cyanide to aldehydes with these two catalysts. Recently, a series of aluminum−salen complexes were synthesized which, in combination with 4-phenylpyridine Noxide (4-PPNO), catalyze asymmetric hydrocyanation of nitro olefins with trimethylsilyl cyanide (Scheme 57).105 Thus, 5 mol % of aluminum complex 108 with 4-PPNO as an additive yields β-nitro nitriles in up to 90% enantiomeric excess at −15 °C. Spectroscopic studies of the reaction suggest 4-PPNO serves as both an axial ligand at the metal center of the catalyst and an activator of TMSCN for promoting delivery of cyanide to the metal complexed nitro alkene. This concept is represented in the assemblage 113. Asymmetric conjugate addition of radical intermediates to enones has received little attention, but Sibi found that β-acyloxy and β-aryl substituted exocyclic α,β-unsaturated ketones with fixed s-trans geometry undergo enantioselective addition of radicals generated from alkyl iodides and tri-n-butyltin hydride in the presence of aluminum−salen complex 114 (Scheme 58).106 Both diastereoselectivity and enantioselectivity of the

reaction are high, but the absence of an acyloxy group from the substrate results in lower stereoselectivity.107 A reaction mechanism was proposed in which the electron pair of the enone carbonyl oxygen anti to the exo double bond coordinates to the metal atom, as in 115, while the acyloxy or aryl substituent is oriented away from axial hydrogens on the cyclohexane ring. This model leaves the si face of the double bond open to nucleophilic radical addition while blocking the re face. Subsequent hydrogen transfer to the α-carbon atom is apparently controlled not by the catalyst but by the newly formed β-stereocenter where the introduced R group shields the si face. The overall result is stereoselective anti addition of R• and a hydrogen atom to the exo double bond. S


Chemical Reviews

Review

A.3. Asymmetric α-Alkylation of Ketone Enolates. Formation of chiral all-carbon quaternary stereocenters via asymmetric synthesis108 most commonly involves addition of enolates to carbon-centered electrophiles where both π-facial accessibility and enolate geometry govern stereoselectivity of the carbon−carbon bond-forming event.109,110 The first salen− metal catalyzed reaction to accomplish this construction was discovered by Jacobsen who showed that chromium complex 77 promotes asymmetric α-alkylation of tetrasubstituted tin enolates of cyclic ketones by alkyl halides. The reaction furnishes five-, six-, and seven-membered ring ketones bearing α-quaternary stereocenters in high enantiomeric excess and synthetically useful yields (Scheme 59).111 Extension of the

Scheme 60. Asymmetric Nazarov Cyclization of Dienones Catalyzed by Chromium−Salen Complex 117

Scheme 59. Asymmetric α-Alkylation of Cyclic and Acyclic Methyl Ketones Catalyzed by Chromium−Salen Complexes 77 and 116

the major diastereomer is formed in high (80−94%) enantiomeric excess. A model 118 rationalizing the torquoselectivity for hydrindenone formation places the dienone, with carbonyl oxgen coordinated to the metal ion, in a quadrant of the octahedral complex where conrotatory counterclockwise rotation, as viewed from the oxygen along the CO axis, is favored. This is due to displacement of the bulky C3 aryl substituent away from a mesityl group in the salen ligand. In contrast, clockwise rotation of the dienone creates a steric clash of the C3 substituent with this mesityl residue. A.4.2. Asymmetric Electrocarboxylation of 1-Phenylethyl Chloride. Enantioselective carboxylation of a racemic alkyl chloride has been described which generates an active catalyst electrochemically from cobalt−salen complex 119 in the presence of carbon dioxide. Under the reported conditions, 1phenethyl chloride is converted to 2-phenylpropionic acid in 37% yield and 83% enantiomeric excess (Scheme 61).115 This is the first example employing an electrochemically generated salen−metal system capable of transforming a racemic starting material into an enantioenriched product. The reaction is believed to proceed via initial electrochemical reduction of the cobalt(II) complex to [cobalt(I)]− which inserts by oxidative addition into the carbon−halogen bond of the substrate. This

process to acyclic ketones requires the formation of a geometrically defined tin enolate, a task that remains challenging,112 but modification of 77 in the form of chromium−salen complex 116 bearing thexyldimethylsilanyloxy (OSiThMe2) groups at the 4- and 4′-positions of the aryl rings together with 5 mol % of tri-n-butylmethoxystannane successfully catalyzes asymmetric α-alkylation of α-substituted methyl ketones.113 The reaction can be accomplished with a mixture of (E) and (Z) tetrasubstituted tin enolates and gives methyl ketones containing an α quaternary stereocenter in high yield and good enantioselectivity with a broad range of alkylating agents. A.4. Miscellaneous Asymmetric Reactions. A.4.1. Asymmetric Nazarov Cyclization of Dienones. Chromium complex 117 bearing 4- and 4′-mesityl substituents in the salen ligand catalyzes torquoselective and enantioselective Nazarov cyclization of conjugated dienones in which one (endocyclic) double bond is embedded in a six-membered ring, either a cyclohexene or a dihydropyran, and the other (exocyclic) olefin carries methyl and aryl groups at C2 and C3, respectively (Scheme 60).114 Disubstitued hydrindenones and their 7-oxa analogues are produced in good yield, with the former having an exclusively trans (2S,3R) configuration, whereas 7-oxa products are biased toward the cis (2R,3R) stereoisomer. In both product mixtures,

Scheme 61. Asymmetric Electrocarboxylation of Racemic 1Phenylethyl Chloride Catalyzed by Cobalt−Salen Complex 119

T


Chemical Reviews

Review

results in a cobalt(III)−salen alkyl complex which is reduced electrochemically to a reactive cobalt(II)−alkyl species. The latter reacts with CO2 stereoselectively at the metal center to give the carboxylic acid. This approach to asymmetric catalysis has broad implications beyond electrochemical fixation of CO2 and will likely find future applications in enantioselective transformations involving other salen−metal systems.

Scheme 63. Asymmetric Aerobic Epoxidation of Alkenes Catalyzed by Ruthenium(nitrosyl)−Salen Complex 121

B. Asymmetric Formation of Carbon−Heteroatom Bonds

B.1. Asymmetric Oxidation of Carbon−Carbon Double Bonds. B.1.1. Asymmetric Epoxidation of Alkenes. A groundbreaking achievement in this area occurred in 1980 with the enantioselective epoxidation of allylic alcohols by tert-butyl hydroperoxide (TBHP), titanium tetraisopropoxide, and diethyl tartrate.116 The benchmark reaction reported by Katsuki and Sharpless was followed by broad application of asymmetric epoxidation to alkenes using transition metal catalysis,117 including metal−salen complexes based on manganese,118−122 chromium,123−126 and ruthenium.127,128 The oxidants employed in these reactions were alkyl hydroperoxides, iodosylbenzene, sodium hypochlorite, and oxones, all of which result in low atom efficiency. An important advance was made in 2005 by Katsuki who described a highly enantioselective epoxidation of nonfunctionalized conjugated olefins catalyzed by di-μ-oxo-titanium trans-1,2-diaminocyclohexane-salalen catalyst 120 containing a semireduced salen ligand (Scheme 62).129 The oxidant in this

ambient conditions; in contrast to biological oxygenation, it does not require either an electron transfer agent or a sacrificial reductant. The mechanism of this epoxidation has not been fully clarified, but experimental results support the view that water is coordinated with the ruthenium ion and serves as a recyclable proton transfer agent for oxygen activation. Structural modification of 121 by replacing phenyl substituents in the binaphthyl portion of the salen ligand with 3,5dichlorophenyl groups leads to ruthenium−salen complex 122 which gives improved asymmetric induction in aerobic epoxidation of both (E) and (Z) aryl substituted alkenes without the need for irradiation (Scheme 64).133 The method is especially useful for preparing trans epoxides in high enantiomeric excess from the corresponding alkenes, an otherwise challenging transformation.

Scheme 62. Enantioselective Epoxidation of Olefins Catalyzed by Di-μ-oxo-Titanium−Salen Catalyst 120

Scheme 64. Asymmetric Aerobic Epoxidation of Conjugated Olefins Catalyzed by Ruthenium−Salen Complex 122

A novel concept134 introduced by List for enantioselective salen−metal-catalyzed epoxidation of alkenes employs counteranion-directed catalysis in which an ion-pair system consisting of an achiral manganese−salen complex and a chiral phosphate counteranion serves as a catalyst. Ionic complex 123 with iodosobenzene as oxidant catalyzes asymmetric epoxidation of a wide range of alkenes, resulting in a high yield of epoxide in good enantiomeric excess (Scheme 65).135 A mechanism for this reaction was not proposed, although a stepped conformation of the Lewis pair induced by the chiral counterion is probably the major factor responsible for asymmetric induction. B.1.2. Asymmetric Oxidation of Enol Derivatives. B.1.2.1. Asymmetric Hydroxylation and Amination of Enol Ethers. Metal−salen complexes based on manganese136−138 and ruthenium139 have been used in asymmetric functionalization at

reaction, 30% aqueous hydrogen peroxide, is not only inexpensive but is ecologically benign, since water is the only byproduct. A subsequent report describes application of the same catalyst to nonconjugated olefins that afford epoxides in good yield in up to 97% enantiomeric excess.130 Dienes containing both terminal and internal double bonds undergo chemoselective epoxidation at the less electron-rich terminal alkene; (Z)-enol esters are also good substrates for this epoxidation.131 In another important development, Katsuki found that ruthenium(nitrosyl)−salen complex 121 catalyzes asymmetric epoxidation of conjugated olefins when irradiated in the presence of oxygenated water at room temperature (Scheme 63).132 This marked the first catalytic asymmetric oxygen transfer reaction using molecular oxygen carried out under U


Chemical Reviews

Review

B.1.3. Asymmetric Aziridination of Alkenes. A claim that aziridination of alkenes with a chiral manganese−salen complex results in no asymmetric induction141 was revisited by Katsuki who described successful aziridination of styrene with a manganese−salen catalyst, initially with only low enantioselectivity.142 Katsuki subsequently prepared several salen complexes of transition metals mostly based on manganese143,144 and ruthenium145−147 as catalysts for aziridination of alkenes. Modification of first generation ruthenium−salen complex 124 by replacing phenyl substituents in the binaphthyl portion of the ligand with 3,5-dichloro-4-trimethylsilylphenyl groups enables highly enantioselective aziridination of alkenes with ruthenium(carbonyl) complex 125 as catalyst and tosyl azide as the nitrene precursor (Scheme 67).148 The reaction exhibits broad substrate

Scheme 65. Asymmetric Counterion Directed Epoxidation of Alkenes Catalyzed by Manganese−Salen Complex− Phosphonate Ion Pair 123

Scheme 67. Asymmetric Aziridination of Alkenes Catalyzed by Ruthenium−Salen Complex 125

the α-carbon of ketones via electrophilic trapping of silyl enol ethers and other derivatives. Both hydroxylation and amination with these catalysts proceed with good enantioselectivity. B.1.2.2. Asymmetric α-Halogenation of β-Keto Esters. Asymmetric introduction of fluorine into organic substrates has been a longstanding goal of interest to medicinal chemists, and chiral catalysts containing metals such as titanium, ruthenium, palladium, copper, nickel, and magnesium are known to induce enantioselective α-fluorination of β-ketoesters. In this context, cobalt−salen complex 119 prepared from cobalt(acac)2 and salen ligand 6 catalyzes fluorination of cyclic β-ketoesters with N-fluorobenzenesulfonimide (NFSI) to yield α-fluorinated product in up to 90% enantiomeric excess (Scheme 66).140 Fluorination of acyclic β-ketoesters such as

scope and a high turnover rate of the catalyst. Aziridination of α,β-unsaturated esters and amides with 125 proceeds with excellent enantioselectivity, although nonconjugated terminal olefins react more slowly and give aziridines in lower enantiomeric excess. Ruthenium−salen complex 125 with 2-(trimethylsilyl)ethanesulfonyl azide (SESN3) as the nitrene source is an excellent catalyst for enantioselective aziridination of acidsensitive vinyl ketones, amides, and esters (Scheme 68).149 The

Scheme 66. Asymmetric α-Halogenation of β-Ketoesters Catalyzed by a Cobalt−Salen Complex 119

Scheme 68. Asymmetric Aziridination of Electron-Deficient Terminal Olefins Catalyzed by Ruthenium−Salen Complex 125

reaction gives almost enantiopure N-protected aziridines in high yield even at 0.5 mol % catalyst loading. The resulting N-SES aziridines are easily deprotected by treatment with tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF) under mild conditions (0−25 °C). A synthetic application of this asymmetric aziridination was demonstrated in a short synthesis of the dopamine D3 receptor agonist (+)-PD128907.

ethyl 2-methyl-3-oxobutanoate under the same conditions affords α-fluorinated product in lower yield (64%) and enantioselectivity (71% enantiomeric excess). In an extension of this method using trifluoromethylsulfonyl chloride as the halogen source, a cyclic β-ketoester gave an α-chlorinated product in 62% yield and 88% enantiomeric excess. V


Chemical Reviews

Review

The most general catalytic system for asymmetric aziridination of alkenes appears to be ruthenium−salen complex 126 based on a trans-1,2-diphenylethylenediamine scaffold where aryl substituents in the binaphthyl portion of the ligand are 3,5di(trifluoromethyl)phenyl groups. The preferred nitrene precursor is SESN3. This combination accomplishes aziridination of conjugated, nonconjugated, and cyclic olefins in high enantiomeric excess and good chemical yield under mild conditions (Scheme 69).150 A rationale for the superior stereoefficiency of

Scheme 70. Enantioselective Baeyer−Villiger Oxidation of Cyclic Ketones Catalyzed by Cobalt−Salen Complex 119

Scheme 69. Asymmetric Aziridination of Alkenes Catalyzed by Ruthenium−Salen Complex 126

salen−metal catalysts relies on coordination of the heteroatom by a Lewis acidic metal ion in the environment of a chiral salen ligand which guides an incoming nucleophile to one terminus of the activated three-membered ring. The reaction is an important method for obtaining vicinal bifunctionalized compounds bearing adjacent stereogenic centers from a substrate that may be meso and/or achiral. Previous studies established that chromium−salen and cobalt−salen complexes can be catalysts for this transformation. B.3.1.1. Desymmetrization of meso-Epoxides and Aziridines. A titanium−salen complex prepared in situ from ligand 6 and titanium tetraisopropoxide catalyzes ring opening of cisstilbene oxide and cyclohexene oxide with aniline and 2methoxyaniline to give β-amino alcohols in enantiomeric excess as high as 99%, although asymmetric induction is variable with other cycloalkene oxides (Scheme 71).159 A dodecapolymeric

this aziridination catalyst postulates that (i) enhanced reactivity of the nitrenoid intermediate results from interaction of a lone pair of the sulfonyl oxygen on the metal nitrenoid with a ligand having a vacant orbital in close proximity to the oxygen atom and (ii) a low-lying carbon−fluorine antibonding orbital in the trifluoromethyl substituent interacts with the π-orbital of the aryl group at the 2″-position to enhance attraction of the complex for an electrophile. B.2. Asymmetric Baeyer−Villiger Oxidation. Enantioselective oxidation of prochiral ketones to esters and lactones (asymmetric Baeyer−Villiger reaction) has been explored with organometallic catalyst systems, although few of these methods are general.151−154 Katsuki found that a cationic cobalt−salen complex having a cis-β-structure is a catalyst for asymmetric Baeyer−Villiger oxidation of 3-substituted cyclobutanones with hydrogen peroxide as the terminal oxidant. γ-Lactones are produced in up to 78% enantiomeric excess from this reaction.155,156 A zirconium-based salen complex gives slightly improved enantioselectivity (up to 87% enantiomeric excess) with the same substrates; bicyclic cyclobutanones afford the corresponding lactones in up to 93% enantiomeric excess.157 A publication from the Strukul group reports that, whereas commercially available cobalt−salen complex 119 is almost inactive and nonstereoselective in Baeyer−Villiger oxidation of cyclobutanones with hydrogen peroxide in organic media, dissolution of the complex in water with the aid of a surfactant leads to moderate-to-high catalytic activity and enantioselectivity (Scheme 70).158 The enantiomeric excess of lactone product is heavily dependent on the substrate and the choice of surfactant. As an example, racemic cis-bicyclo[3.2.0]hept-2-en6-one gives a Baeyer−Villiger product in 90% enantiomeric excess and 70:1 regioisomeric ratio when TritonX114 is used as the surfactant. B.3. Asymmetric Opening of Small Rings. B.3.1. Ring Opening Desymmetrization of Epoxides and Aziridines. Ring opening desymmetrization of meso-epoxides and aziridines with

Scheme 71. Desymmetric Ring Opening of Epoxides with Aromatic Amines Catalyzed by Titanium Tetraisopropoxide and Salen Ligands 127 and 128

ligand 127 consisting of 12 conjoined units of 128 through C5 and C5′ linkages was synthesized, the titanium complex of which catalyzes asymmetric aminolysis of epoxides. Cobalt−salen complex 119 in combination with bicyclic heterocycle 129 as cocatalyst opens cyclic meso and terminal epoxides with fluoride ion generated from hexafluoroisopropyl alcohol (HFIP) and benzoyl fluoride (Scheme 72).160 Vicinal fluoro alcohols are generated in high enantiomeric excess in this reaction, especially from terminal epoxides. W


Chemical Reviews

Review

Scheme 72. Desymmetric Ring Opening of meso-Epoxides by Fluoride Catalyzed by Cobalt−Salen Complex 119

Scheme 74. Desymmetric Ring Opening of Epoxides Catalyzed by Yttrium−Salen Complex 131

The same fluoride source with cobalt complex 119 opens meso N-picolinamido aziridines, in this case with achiral tetra(dimethylamino)titanium as a cocatalyst, to generate trans βfluoro amines in enantiomeric excess up to 84% (Scheme 73).161

nitrobenzoylaziridines with trimethylsilyl azide and other nucleophiles (Scheme 75).163 Reaction of aziridines with Scheme 75. Desymmetric Ring Opening of Aziridines Catalyzed by Dual Metallic Yttrium−Salen Complex 132

Scheme 73. Desymmetric Ring Opening of meso-Aziridines by Fluoride Catalyzed by Cobalt−Salen Complex 119 and an Achiral Titanium Complex

trimethylsilyl azide yields ring opened products in nearly enantiomerically pure form, but asymmetric induction in ring opening by silyl isothiocyanates depends on the substituents on silicon. Sterically bulky tert-butyldiphenylsilyl isothiocyanate gives optimal stereoselectivity. Yttrium complex 132 with trimethylsilyl azide catalyzes regiodivergent kinetic resolution of racemic monosubstitued aziridines. A mechanism involving activation of both the electrophile (epoxide or aziridine) and the nucleophile (azide or isothiocyanate) by 132 at the two yttrium sites of the complex is supported by techniques that include molecular weight determination from vapor pressure osmometry, diffusion-ordered NMR spectroscopy (DOSY), and kinetic studies. B.3.1.2. Kinetic Resolution of Racemic Epoxides. Studies by Jacobsen in the 1990s established that chiral salen complexes of chromium164 and cobalt165 are capable of catalyzing kinetic resolution of racemic epoxides by asymmetric reaction with trimethylsilyl azide and water, respectively. The kinetic acceleration of ring opening of one epoxide enantiomer over the other in a racemate can be as high as 500 to 1. More recently,

The N-picolinamide substituent plays a crucial role in this dual activation process by coordinating titanium and promoting aziridine opening, while fluoride ion is delivered from the cobalt−salen system as shown in 130. The picolinamide residue is removed without epimerization of the fluoro amine by protection of the amine as its N-butoxycarbonyl derivative and reduction with sodium borohydride. This fluorination protocol is also applicable to kinetic resolution via desymmetric ring opening of racemic aziridines. The monomeric yttrium−salen complex 131 is reported to catalyze ring opening of epoxides by trimethylsilyl azide under solvent-free conditions, although the resulting trans azido silyl ether is produced in only moderate enantiomeric excess (Scheme 74).162 The presence of an activated yttrium−azide complex was detected in the reaction medium by in situ IR spectroscopy, but it could not be isolated. Studies by RajanBabu found that 131 has a high propensity for forming dimeric structures and that the dual yttrium complex 132 is a catalyst for ring opening of meso-epoxides and pX


Chemical Reviews

Review

cobalt−salen complex 133 at a catalyst loading as low as 0.5 mol % is found to catalyze kinetic resolution of racemic syn and anti alkoxy and azido terminal epoxides in the presence of water (Scheme 76).166 This hydrolytic kinetic resolution is conducted Scheme 76. Hydrolytic Kinetic Resolution of syn-Alkoxy and Azido Epoxides Catalyzed by Cobalt−Salen Complex 133

Figure 6. “Matched” and “mismatched” transition complexes for hydrolytic kinetic resolution of epoxides by a pair of cobalt−salen complexes.

under mild conditions and leads to a nearly enantiopure epoxide along with the diol derived from epoxide opening. This reaction was employed in enantioselective syntheses of (S,S)-reboxetine and (+)-epi-cytoxazone. Under slightly different conditions from those employing catalyst 133, cobalt−salen complex 134 catalyzes hydrolytic kinetic resolution of racemic terminal epoxides including styrene oxides and substituted β,γ-epoxy esters (Scheme 77).167

for delivering the nucleophile in a facially selective manner. It is important to note that this form of catalysis may be different from that operating in bimetallic complexes where two metal ions are present within the same ligand (see section F). The concept of catalysis by a dual metal complex in this reaction has been explored by Doyle who prepared dimeric cobalt−salen complex 137 from two units of 119 using a covalent tether. In combination with benzoyl fluoride, diazabicyclononene (138), hexafluoroisopropyl alcohol, and tert-butyl hydroperoxide, 137 catalyzes kinetic resolution of racemic terminal epoxides with ring opening by fluoride to yield vicinal fluoro alcohols in high enantiomeric excess (Scheme 78).169

Scheme 77. Hydrolytic Kinetic Resolution of β,γ-Epoxy Esters with Cobalt−Salen Complex 134

Scheme 78. Kinetic Resolution of Terminal Epoxides Catalyzed by Dual Metal Cobalt−Salen Complex 137

An explanation has been offered on the basis of computational studies and NMR for the cooperative effect in dual metal−salen systems leading to hydrolytic kinetic resolution of a terminal epoxide.168 With the two catalytic units in head-to-tail orientation and the epoxide oxygen axially bound to a hexacoordinated metal center, there are matched and mismatched transition states, 135 and 136, respectively, for attack by a nucleophile (water) bound at the second pentacoordinate metal center (Figure 6). The matched transition state has the epoxide substituent directed away from the center of the complex, leaving a face of the epoxide open for attack, as in 135, that is determined by the chiral pentacoordinate salen complex with which the nucleophile is associated. Reversal of epoxide orientation within the assembly as in 136 creates a steric clash of the epoxide substituent in one complex with the nucleophile bound to the second metal. A principle of dual-metal asymmetric induction that emerges from this picture is that one of the two metal complexes serves primarily as a bulky chiral substituent by spatially orientating the substrate and that chirality associated with the second metal complex is responsible

B.3.1.3. Enantioselective Addition of Carbon Dioxide to Epoxides. Carbon dioxide has found many applications in synthesis,170 and its asymmetric reaction with racemic epoxides to afford enantioenriched five-membered cyclic carbonates is among the most important.171 This is accomplished with the polymeric cobalt−salen catalyst 139 which possesses chirality resulting from both the diamine scaffold and the BINOL substituent linked through 5- and 5′-positions of the salen ligand (Scheme 79).172 The catalyst is easily recovered from the reaction and can be reused more than 10 times with no loss of enantioselectivity. Asymmetric copolymerization of monosubstituted epoxides with carbon dioxide generates enantioenriched polycarbonates,173,174 and a version of this process involving kinetic resolution with ring opening of cyclohexene oxide is catalyzed by cobalt−salen complex 140 and bis(triphenylphosphine)Y


Chemical Reviews

Review

B.3.2. Asymmetric Ring Opening of Oxetanes. Cobalt− salen triflate 142 catalyzes asymmetric intramolecular opening of achiral 3,3-disubstituted oxetanes with oxygen nucleophiles to afford functionalized tetrahydrofurans in high enantiomeric excess and excellent yield (Scheme 82).179 Oxetanes with a phenol substituent at the 3-position undergo ring opening to give dihydrobenzofurans bearing a C3 stereogenic quaternary center.

Scheme 79. Asymmetric Reaction of Carbon Dioxide with Racemic Propylene Oxide Catalyzed by Cobalt−Salen Complex 139

Scheme 82. Asymmetric Intramolecular Ring Opening of Oxetanes Catalyzed by Cobalt−Salen Complex 142 iminium chloride [(PPN)Cl] as cocatalyst. The reaction produces highly isotactic semicrystalline trans-poly(cyclohexene)carbonate in up to 96% enantiomeric excess (Scheme 80).175−177 A sterically bulky adamantyl substituent at C3 of the salen ligand is necessary for this reaction. Scheme 80. Asymmetric Copolymerization of Cyclohexene Oxide with Carbon Dioxide Catalyzed by Cobalt−Salen Complex 140 B.4. Asymmetric Hydrophosphonylation. Asymmetric addition of a phosphite to a substrate such as an aldehyde (the asymmetric Pudovic reaction) can be catalyzed by aluminum− salen complex 143 containing a chiral binaphthyl scaffold. The reaction produces chiral α-hydroxy phosphonates that are important materials in medicinal chemistry (Scheme 83).180 Scheme 83. Asymmeric Hydrophosphonylation of Aldehydes Catalyzed by Aluminum−Salen Complex 143

B.3.1.4. Enantioselective Homopolymerization of Epoxides. The dimeric cobalt−salen complex 141 constructed around a binaphthyl unit as a tether is a catalyst in the presence of bis(triphenylphosphine)iminium acetate [(PPN)(OAc)] for enantioselective homopolymerization and kinetic resolution of racemic terminal epoxides. The stereoregular polyether products are obtained nearly enantiopure (Scheme 81).178 Scheme 81. Asymmetric Polymerization of Propylene Oxide Catalyzed by Dicobalt−Salen Complex 141

Both aliphatic and aromatic aldehydes yield the corresponding α-hydroxy phosphonates with moderate-to-high enantioselectivity (64−86% enantiomeric excess). Katsuki’s design of complex 143 adheres to the principle that the aluminum ion makes available two open sites in a cis relationship, allowing replacement of the chlorine ligand by phosphite while an aldehyde is coordinated to the metal. The resulting intramolecular reaction is under tight stereochemical control in this array. An aluminum half-salen (salalen) complex was also prepared for asymmetric hydrophosphonylation of both aldimines and aldehydes, producing α-amino and α-hydroxy phosphonates in up to 98% enantiomeric excess.181,182

The extraordinarily high enantioselectivity associated with this catalyst system is reflected in its large stereoselectivity factor s, a measure of the rate difference in the reaction of enantiomers of the racemic epoxide. For propylene oxide, s = 370 and the corresponding isotactic polyether is obtained in 99% enantiomeric excess. Z


Chemical Reviews

Review

Scheme 85. Asymmetric Addition of Thiols to α,βUnsaturated Ketones Catalyzed by Iron−Salen Complex 147

B.5. Asymmetric Conjugate Addition of Heteroatom Nucleophiles. B.5.1. Asymmetric Conjugate Addition of Nitrogen Nucleophiles. Jacobsen’s 1999 publication on asymmetric conjugate addition of hydrazoic acid to β-alkyl substituted α,β-unsaturated imides with aluminum−salen complex 144183 led to multiple studies on salen−metal complexes as catalysts for inducing asymmetric conjugate addition by other nitrogen nucleophiles. A notable advance occurred when it was discovered that cobalt−salen complex 145 catalyzes enantioselective Michael addition of O-alkylhydroxylamines to nitro alkenes to give N-alkylhydroxy nitro amines in high yield and with generally good enantioselectivity (Scheme 84).184 The method provides a unique route to enantiomerically Scheme 84. Asymmetric Michael Addition of OAlkylhydroxylamines to Nitro Alkenes Catalyzed by Cobalt− Salen Complex 145

catalyst 147, trisubstituted conjugated enones bearing a substituent at the α position give almost exclusively the syn diastereomer which is obtained highly enantioenriched. Spectroscopic and chemical evidence supports activation of complex 147 by prior coordination of a thiol ligand at a vacant site on the iron center before complexation of the enone at the metal. This generates a trans ligand effect that fixes the enone in an orientation favoring external attack by thiol at the si face of the double bond, as represented in 149. A crossover experiment confirmed that the initially coordinated thiol is not the species responsible for sulfa-Michael addition. Modification of complex 147 by replacing hydrogens at the imine carbons of the salen ligand by n-butyl groups produces complex 150 which catalyzes enantioselective and regioselective δ-addition of sulfur nucleophiles to α,β,γ,δ-unsaturated carbonyl systems (Scheme 86).187 The addition of silver tetrafluoroborate to enhance ionization of 150 improves the proportion of δaddition over β-addition. An experiment with deuterium-labeled thiol established that both sulfur and deuterium are added syn to the si face of the γ,δ-double bond of the dienone. B.6. Asymmetric Iodolactonization. A study of iodolactonization of 4-substituted 4-pentenoic acids with cobalt−salen complex 119 as catalyst in the presence of N-chlorosuccinimide established that γ-lactones bearing a stereogenic quaternary center at C4 are obtained in up to 83% enantiomeric excess (Scheme 87).188 Although the reaction shows broad substrate tolerance that includes pentenoic acids with aliphatic and aromatic groups as well as sterically bulky substituents, enantioselectivity is markedly dependent on the electronic nature of the aryl substituent. Thus, with p-bromophenyl

enriched nitro amines and is broadly tolerant of structural variation in the nitro alkene substrate. Transition state 146 rationalizes asymmetric induction leading to (R) configuration of the addition product. The cobalt-coordinated nitro oxygen atom of the nitro alkene is placed syn to the β hydrogen, and the alkene occupies a spatial orientation that blocks the si face by a pmethoxyphenyl substituent of the salen scaffold. B.5.2. Asymmetric Conjugate Addition of Oxygen Nucleophiles. A chiral μ-oxo dimeric aluminum−salen complex catalyzes asymmetric conjugate addition of oxygen nucleophiles such as salicylaldoxime to β-alkyl substituted α,β-unsaturated imides.185 B.5.3. Asymmetric Conjugate Addition of Sulfur Nucleophiles. Iron−salen complex 147 based on a chiral cis-2,5diaminobicyclo[2.2.2]octane scaffold catalyzes asymmetric conjugate addition of thiols to α,β-unsaturated ketones. This asymmetric sulfa-Michael reaction produces β-thioketones in excellent yield and high enantiomeric excess with aliphatic and aromatic thiols as donors and with a wide range of conjugated enones as acceptors (Scheme 85).186 The method is used in enantioselective installation of the carbon−sulfur bond in Montelukast 148, whose sodium salt is the widely prescribed drug Singulair for treatment of respiratory conditions. With AA


Chemical Reviews

Review

Scheme 86. Asymmetric δ-Sulfa-Michael Conjugate Addition of Thiols to α,β,γ,δ-Unsaturated Dienones Catalyzed by Iron−Salen Complex 150

Scheme 88. Asymmetric Iodocyclization of γ-Hydroxy cisAlkenes Catalyzed by Chromium−Salen Complex 77

Scheme 87. Asymmetric Iodolactonization of 4-Pentenoic Acids Catalyzed by Cobalt−Salen Complex 119

this ionic complex so that iodonium ion attacks the more accessible re face of the alkene. B.8. Asymmetric C−H Functionalization. B.8.1. Asymmetric C−H Amination. Enantioselective allylic C−H amination with azides was initially explored by Katsuki using manganese−salen complexes bearing electron-withdrawing groups in the ligand in order to make the intermediate iminometal species more electrophilic. The assumption was that this would avoid competing aziridination.192 In a recent publication, Katsuki demonstrated that iridium−salen complexes are catalysts for asymmetric C−H insertion by nitrenes.193 He also showed that iridium−salen catalyst 156 containing bulky tert-butyldiphenylsilyl (TBDPS) groups in the binaphthyl portion of the salen ligand leads to intramolecular C−H amination of aryl sulfonyl azides. The reaction gives fivemembered sultams with high enantioselectivity (Scheme 89).194 With ortho-substituted aryl sulfonyl azides, insertion occurs at the benzylic carbon, but with extended alkyl substitution at the ortho position, insertion takes place predominantly at the

substitution, an iodolactone is obtained in 73% enantiomeric excess, whereas, with p-methoxyphenyl, the enantiomeric excess is only 22%. Subsequent research with catalyst 119 and 5substituted 4-pentenoic acids found that the (E) isomer gives an iodolactone 154 in moderate enantiomeric excess (up to 74%), whereas the (Z)-pentenoic acid results in an iodolactone of much lower enantiopurity.189 B.7. Asymmetric Iodoetherification. A study of asymmetric iodoetherification of 5-substituted cis-4-pentenols with chromium−salen complex 77 and N-chlorosuccinimide published in 2003190 was subsequently refined to show that 5iodoalkyl substituted tetrahydrofurans are prepared by this method in high diastereomeric and enantiomeric excess using only 7 mol % of catalyst (Scheme 88).191 The method was used in a synthesis of the indolizidine alkaloid swainsonine. The role of N-chlorosuccinimide in this process is assumed to be that of a reactant with iodide, releasing iodine monochloride slowly enough to minimize a background nonasymmetric reaction with free iodine. In this scenario, iodine monochloride is activated upon exposure to 77, generating zwitterionic species 155 in which iodonium ion is paired with a hexacoordinated dichlorochromium anion. The alkene substrate is organized in

Scheme 89. Regio- and Stereoselective Intramolecular C−H Amination of Aryl Sulfonyl Azides Catalyzed by Iridium− Salen Complex 156

AB


Chemical Reviews

Review

orchestrating the stereochemical outcome of an asymmetric reaction. B.8.3. Asymmetric C−H Fluorination. Manganese−salen complex 159 catalyzes benzylic fluorination with nucleophilic fluorine sources and iodosobenzene, although a maximum of 40% enantiomeric excess is obtained with this protocol (Scheme 92).197 However, the method has broad scope, exhibiting good

homobenzylic position to give six-membered sultams with very high enantioselectivity. An explanation for this counterintuitive preference for β-amination probably lies in steric effects associated with the orbital angle between the carbon−hydrogen and iridium−nitrene bonds. Further research by Katsuki found that ruthenium(carbonyl)−salen complex 157 bearing less bulky 2,6difluorophenyl groups in the binaphthyl portion of the salen ligand is a catalyst for asymmetric benzylic and allylic C−H amination using 2-(trimethylsilyl)ethanesulfonyl azide (SESN3) as the nitrene source (Scheme 90).195 Amines are produced cleanly and in good enantiomeric excess in this reaction with no competition from aziridination or isomerization of the double bond.

Scheme 92. Asymmetric Fluorination of Benzylic Carbon Catalyzed by Manganese−Salen Complex 159

Scheme 90. Asymmetric Benzylic and Allylic C−H Amination Catalyzed by Ruthenium(Carbonyl)−Salen Complex 157

functional group tolerance, and it is selective for monofluorination at a benzylic site. When an electron-withdrawing group is present in the substrate, silver fluoride can be used as the fluorine source. Density functional theory calculations on a related manganese−porphyrin system employed in aliphatic C−H fluorination support the presence of a catalytically active manganese−fluorine species and a very early transition state for the fluorine transfer step, probably via a linear manganese− fluorine−carbon trajectory.198 This would account for poor enantioselectivity with 159.

B.8.2. Asymmetric C−H Oxygenation. A dual metal system similar to that used in allylic amination and consisting of chromium−salen complex 158, palladium(II) acetate, 1,2bis(phenylsulfoxy)ethane, benzoquinone, and acetic acid catalyzes asymmetric allylic C−H oxygenation of terminal olefins to give allylic acetates with high regioselectivity but in only modest enantiomeric excess (Scheme 91).196 Importantly, Scheme 91. Asymmetric Allylic Acetoxylation of Terminal Alkenes Catalyzed by Chromium−Salen Complex 158 in a Dual Metal System

C. Concomitant Formation of Carbon−Carbon and Carbon−Heteroatom Bonds

C.1. Asymmetric Darzens Condensation. Cobalt−salen complex 160 bearing methoxy substituents at C3 sites in the salen ligand catalyzes asymmetric Darzens condensation of αbromo amides with benzaldehyde to give a mixture of cis and trans glycinamides with each isomer enantioenriched to approximately 40% (Scheme 93).199 Although stereoselection in this reaction leaves room for improvement, the Darzens condensation is a valuable reaction for assembling chiral Scheme 93. Asymmetric Darzens Condensation of αBromoamides and Benzaldehyde Catalyzed by Cobalt−Salen Complex 160

the products can be enantioenriched through enzymatic resolution to full optical purity. As with allylic amination, the reactants in this acetoxylation are believed to be a πallylpalladium sulfoxide complex, formed from the alkene, palladium acetate, and the bis-sulfoxide, together with a chromium−benzoquinone−salen complex carrying an acetate ligand that delivers acetate to the π-allyl substrate. Although some mechanistic details of the process remain unclear, this catalytic interaction of a chiral metal−salen complex with an organometallic intermediate represents a new approach to AC


Chemical Reviews

Review

rings.201 These catalysts give only slightly improved stereoselectivity compared to 161. The requirement for Hunig’s base in cycloadditions catalyzed by 161 is removed when a basic moiety is installed within the catalyst as in 164 (Scheme 95).202 This bifunctional quinine-

functionality from achiral partners. An efficient asymmetric version of the reaction remains an important, as yet unrealized, goal. C.2. Asymmetric Cycloaddition. C.2.1. Asymmetric [2 + 2] Cycloaddition of Ketenes. A concept employing dualactivation catalysis by a salen−metal complex that combines Lewis acid coordination at the metal with cooperative action by an aprotic contact-ion pair led to preparation of aluminum− salen species 161. The complex catalyzes trans-selective asymmetric [2 + 2] cyclocondensation of acyl bromides with aliphatic aldehydes to furnish 3,4-disubstituted β-lactones with high stereoselectivity (Scheme 94).200 The reaction formally

Scheme 95. Asymmetric [2 + 2] Ketene−Aldehyde Cycloaddition Catalyzed by Bifunctional Catalyst 164

Scheme 94. Trans-selective Asymmetric [2 + 2] Cyclocondensation of Acyl Halides with Aliphatic Aldehydes Catalyzed by Bifunctional Salen Complex 161

tethered cobalt−salen complex catalyzes [2 + 2] cycloaddition of ketene to aldehydes at 5 mol % catalyst loading to give C4substituted β-lactones in >99% enantiomeric excess with aliphatic and aromatic aldehydes. The cooperative intramolecular bifunctional catalysis operating with 164 uses the basic quinine nitrogen to generate an ammonium enolate of the ketene formed from the acyl bromide which is delivered to the cobalt-activated aldehyde as in 165. The resulting cobalt aldolate 166 cyclizes to furnish a β-lactone and regenerates 164. Enantiomerically enriched β-lactones are valuable compounds for accessing the corresponding β-hydroxy esters with retention of configuration. These esters are formally the products of asymmetric acetate aldol condensation for which silyl ketene acetals are generally used. C.2.2. Asymmetric Hetero-Diels−Alder Reactions. Studies of asymmetric hetero-Diels−Alder cycloaddition by Jacobsen203 and Rawal204 established that [4 + 2] addition of 1,3-dienes to aldehydes can be catalyzed by chromium−salen complexes. The resultant oxygen heterocycles are produced with good enantioselectivity. Jurczak modified chromium−salen complex 77 by incorporating bulky 3-phenylpent-3-yl substituents at the 3- and 3′-sites of the salen ligand to provide a complex 167 that catalyzes cycloaddition of Danishefsky’s diene with aldehydes to afford 2-substituted dihydro-4-pyranones after acidic hydrolysis of the intermediate dihydropyran in up to 96% enantiomeric excess (Scheme 96).205 The bicyclic diamine scaffold in chromium−salen complex 69 provides a chiral catalyst that yields enantioenriched heteroDiels−Alder products from cycloaddition of Danishefsky’s diene with a wide range of aldehydes at low temperature (Scheme

represents an asymmetric anti-aldol addition to aldehydes. Enantioselectivity is not significantly dependent on the aldehyde, since almost identical results are obtained with aldehydes possessing long aliphatic side chains with or without a carbon−carbon double bond, with β-branched aldehydes such as isovaleraldehyde, and with sterically undemanding aldehydes such as n-propanal, n-butanal, and n-pentanal. The authors propose a reaction mechanism in which the sterically more accessible lone pair of the aldehyde carbonyl binds at the free aluminum coordination site to form an octahedral complex while a ketene is generated from the acyl bromide with Hunig’s base. The reacting enolate from the ketene is generated within the catalyst sphere by attack of bromide ion on the ketene, as shown in 162. The attack would be expected to occur trans to R1 of the ketene to minimize repulsive interactions, thus leading to (E)-configured enolate contact-ion pair 163. If this species reacts with the aldehyde carbonyl via an open transition state in a staggered conformation, a trans β-lactone results as observed. The reaction is restricted to acyl bromides; acyl chlorides do not participate in this process at −70 °C. A later study describes a series of aluminum−salen bispyridinium catalysts similar to 161 but differing in the presence of substituents on the pyridinium AD


Chemical Reviews

Review

Scheme 96. Asymmetric Hetero-Diels−Alder Cycloddition of Danishefsky’s Diene with Aldehydes Catalyzed by Chromium−Salen Complex 167

Scheme 98. Asymmetric Hetero-Diels−Alder Cycloaddition of Danishefsky’s Diene with Aldehydes Catalyzed by Polymeric Chromium−Salen Complex 169

reported by Noyori.208 An extension of the process where the second step after Michael addition is heterocyclization opens an important route to new, more complex structures, and a version of this consecutive sequence has been described in which an achiral 5,5-disubstituted cyclohexane-1,3-dione reacts with a 3substituted 2-cyanoacrylate in the presence of chiral cobalt− salen complex 170. The complex is generated in situ from ligand 14 and cobalt(II) acetate tetrahydrate. The reaction product, a 2-amino-5-oxo-5,6,7,8-tetrahydro-4H-chromene, is formed in up to 89% enantiomeric excess (Scheme 99).209 Additives,

97).52 Enantioselectivity in the formation of 2-substituted dihydro-4-pyranones with 69 compares favorably with Jacobsen’s results employing catalyst 168. Scheme 97. Asymmetric Hetero-Diels−Alder Reaction of Danishefsky’s Diene with Aldehydes Catalyzed by Chromium−Salen Complex 69

Scheme 99. Asymmetric Tandem Michael Cyclization of Cyclohexane-1,3-diones and 2-Cyanoacrylates Catalyzed by Cobalt−Salen Complex 170

including acids, bases, alcohols, and phenols, were tested to improve enantioselectivity of the reaction; 3,5-dinitrosalicylic acid at 22.5 mol % loading was found to be optimal. C.4. Asymmetric Carbonyl-ene Reactions. Modification of chromium−salen complex 168 by replacing tert-butyl substituents at the 3- and 3′-positions of the salen ligand by adamantyl groups generates complex 171 that catalyzes, at 2 mol % loading, asymmetric carbonyl-ene reaction of glyoxalates with 1,1-disubstituted alkenes. The reaction produces γ,δ-unsaturated α-hydroxy esters in acceptable yield with enantioselectivity as high as 92% enantiomeric excess for acyclic 2-hydroxy-4pentenoates (Scheme 100).210 Increased steric bulk at the 3- and 3′-positions of the salen ligand of 171 was explored by Rawal who replaced tert-butyl substituents by triisobutylsilyl (TIBS) groups in cobalt complex 172. This complex is highly effective in catalyzing the asymmetric carbonyl-ene reaction of ethyl glyoxalate with 1,1disubstituted and trisubstituted alkenes (Scheme 101).211 The reaction proceeds at room temperature with catalyst loading as

Attachment of a thiophene substituent at C5 and C5′ of the salen ligand in a chromium−salen complex followed by electropolymerization of the system generates chiral polymer 169 as an insoluble powder. Heterogeneous asymmetric heteroDiels−Alder addition of Danishefsky’s diene to aldehydes carried out with 169 as catalyst gives dihydropyranones after exposure to triflic acid in moderate-to-good enantiomeric excess (Scheme 98).206,207 The recoverable polymeric catalyst shows no loss of activity for up to 15 cycles. A requirement for good enantioselectivity in all of these catalyzed hetero-Diels−Alder cycloadditions is the presence of molecular sieves in the medium. C.3. Asymmetric Tandem Michael-Cyclization Reactions. The first asymmetric tandem reaction initiated by Michael addition followed by a subsequent in situ carbon− carbon bond forming step, in this case aldol condensation, was AE


Chemical Reviews

Review

Scheme 100. Asymmetric Carbonyl-ene Reaction of Alkyl Glyoxylates with Alkenes Catalyzed by Chromium−Salen Complex 171

Scheme 102. Asymmetric Carbenoid C−H Insertion into Tetrahydrofuran and 1,4-Cyclohexadiene Catalyzed by Iridium−Salen Complexes 89 and 173

Scheme 101. Asymmetric Carbonyl-ene Reaction Catalyzed by Cobalt−Salen Complex 172

tions proceed through intermediate hydroperoxo and alkylperoxo species where the oxo ligand occupies an apical site at the metal center of the complex, as shown in 174 and 175, respectively (Figure 7). Recent studies in this area focus on catalyst modification and include supported systems as well as cooperative catalysts that lead to enhanced asymmetric induction.218−222

low as 0.1 mol % and affords unsaturated α-hydroxy esters in very high enantiomeric excess. For a trisubstituted alkene, diastereoselection at C2,C3 is predominantly anti (9:1), and for an acyclic alkene, the (E) configuration at C4,C5 is favored (22:1). C.5. Asymmetric Carbenoid C−H Insertion. Two iridium−salen complexes 89 and 173 prepared by Katsuki catalyze asymmetric insertion by carbenoids generated from αaryl α-diazoacetates or α-diazopropionate into an activated carbon−hydrogen bond such as that at the 2-position of a tetrahydrofuran or the 3-position of a cyclohexa-1,4-diene (Scheme 102).212 Products are formed in high diastereomeric and very high enantiomeric excess, although with cyclohexa-1,4dienes a minor product in some cases is a cyclopropane resulting from carbenoid addition to a double bond.

Figure 7. Metal−salen hydroperoxo and alkylperoxo species for asymmetric sulfoxidation.

An important advance due to Katsuki is the finding that ruthenium(nitroso)−salen complex 176 catalyzes asymmetric oxidation of organic sulfides with air when the aqueous medium is irradiated with visible light.223 Dialkyl and alkyl aryl sulfides furnish sulfoxides in 75−98% enantiomeric excess. Oxidation of thioacetals can be controlled to give a monosulfoxide with excellent enantioselectivity (Scheme 103). Dimeric bis(μ-oxo)titanium−salen complex 120 with a cis-β configuration at each metal center catalyzes asymmetric sulfoxidation with aqueous hydrogen peroxide in methanol. A limitation of the method due to instability of the titanium complex toward water is overcome with a hydrogen peroxide− urea inclusion complex as an oxidant (Scheme 104).224 The catalyst was employed in a total synthesis of the large nonribosomal peptide polytheonamide B where asymmetric

D. Asymmetric Formation of Heteroatom−Heteroatom Bonds

D.1. Asymmetric Oxidation of Sulfides. A recent review summarizes methods for the preparation of enantiomerically enriched sulfoxides including methods employing oxidation of sulfides with chiral salen−metal complexes.213 The first successful reaction of this type was reported by Fujita who described an oxovanadium catalyst for enantioselective oxidation of sulfides with organic peroxides including tertbutyl hydroperoxide.214 Subsequently, Jacobsen reported asymmetric sulfoxidation with a manganese−salen catalyst and hydrogen peroxide215 and Katsuki described an analogous reaction using a titanium−salen complex.216,217 These oxidaAF


Chemical Reviews

Review

Scheme 103. Asymmetric Aerobic Oxidation of Sulfides Catalyzed by Ruthenium(nitrosyl)−Salen Complex 176

Scheme 105. Asymmetric Oxidation of Alkyl Aryl Sulfides Catalyzed by Manganese−Salen Complex 177

possible under appropriate conditions. A study of asymmetric sulfoxidation of alkyl aryl sulfides by a series of iron complexes using an iodosyl arene as the terminal oxidant finds that iron− salen complex 178 catalyzes the oxidation at a loading of 0.2−2 mol %. Sulfoxides are obtained in up to 84% enantiomeric excess by this method (Scheme 106).228 Electronic and steric

Scheme 104. Diastereoselective Sulfoxidation in Synthesis of Polytheonamide B Catalyzed by Dimeric Titanium−Salen Complex 120

Scheme 106. Asymmetric Oxidation of Sulfides with Iodosobenzene Catalyzed by Iron−Salen Complex 178

properties of substituents in the salen ligand play a significant role in the stereoselectivity of this oxidation. Investigation of the reaction mechanism using 1H NMR spectroscopy revealed that an iron(iodoso)−salen complex is an active intermediate. A different approach to asymmetric sulfoxidation due to List employs an achiral iron−salen cation in combination with a chiral phosphate counteranion to generate ion-pair system 179 that catalyzes at 1 mol % loading oxidation of alkyl aryl sulfides with iodosylbenzene (Scheme 107).229 The reaction leads to sulfoxides in high yield and generally good enantioselectivity. It is particularly successful with challenging substrates where other asymmetric sulfoxidation methods fail, such as sulfides in which an aryl residue bears an electron-withdrawing substituent or with sulfides that carry a long alkyl chain. D.2. Asymmetric Sulfimidation. Early studies on chiral sulfimides are noteworthy for the absence of methods for their preparation,230,231 but this changed when it was discovered that manganese−salen complexes catalyze asymmetric sulfimidation of alkyl aryl sulfides by reaction with nitrenes.232,233 Further research found that salen-based ruthenium complexes are good catalysts for this transformation, affording chiral sulfimides with high levels of asymmetric induction.234−237 Subsequently, it was shown that half-salen (salalen) complexes 180−183 of ruthenium(dicarbonyl) are superior catalysts for asymmetric sulfimidation of alkyl aryl sulfides, producing sulfimides in up to

sulfoxidation of the monomeric unit occurs in 85% diastereomeric excess and improves to 96% after a second chromatography.225 Manganese−salen complexes catalyze asymmetric oxidation of sulfides under mild conditions, and manganese−salen complex 177 at 1% catalyst loading with iodosylbenzene as the stoichiometric oxidant gives alkyl aryl sulfoxides in up to 68% enantiomeric excess (Scheme 105).226 The electronic properties of substituents in the aryl component of the sulfide substrate have a significant effect on the enantioselctivity of this oxidation. An electron-withdrawing group such as para-nitro results in much higher asymmetric induction than an electron-donating substituent such as para-methoxy. An approach to asymmetric sulfoxidation using an achiral manganese−salen complex in association with the protein streptavidin has been published.227 Four biotinylated salen ligands were synthesized, and their manganese complexes were tested in combination with several streptavidin mutants in sulfoxidation of thioanisole with hydrogen peroxide. Although these artificial metalloenzymes give only moderate conversion to sulfoxide (up to 56%) with low enantioselectivity (maximum of 13% enantiomeric excess), the results suggest that improved asymmetric catalysis of this oxidation by a protein may be AG


Chemical Reviews

Review

Scheme 107. Asymmetric Counterion Directed Sulfoxidation Catalyzed by Ion Pair Phosphate−Iron−Salen Complex 179

Scheme 109. Asymmetric Insertion into Silicon−Hydrogen Bonds by α-Aryl and α-Alkyl Diazoacetates Catalyzed by Iridium−Salen Complex 187

alcohol is obtained almost enantiopure (Scheme 110).240 A mechanistic investigation by Corey of this oxidative kinetic

90% enantiomeric excess with irradiated tosyl azide as the nitrene source (Scheme 108).238

Scheme 110. Asymmetric Oxidative Kinetic Resolution of Racemic Secondary Alcohols Catalyzed by Manganese−Salen Complex 188 with Bis(acetoxy)iodobenzene

Scheme 108. Asymmetric Sulfimidation Catalyzed by Ruthenium−Salen and Half-Salen Complexes 87 and 180− 186

resolution using analogous 190 as catalyst indicated that the reaction proceeds through an intermediate in which the secondary alcohol is hydrogen bonded to the catalyst to generate oxomanganese(V)−salen intermediate 191 in an orientation that places the larger alcohol substituent furthest from the congested metal center. Oxidation of the alcohol in this intermediate occurs by intramolecular hydrogen transfer from carbon to a phenoxy oxygen of the salen ligand; manganese(V) is reduced to manganese(III) in the overall redox process.241 Catalyst 188 is also used with potassium acetate and Nbromosuccinimide (NBS) as the stoichiometric oxidant in oxidative resolution of racemic secondary alcohols (Scheme 111).242 The scope of the reaction under these conditions is broad, and enantiomeric excess of the resolved alcohol in most cases is high. However, the resolution of some aliphatic secondary alcohols such as (±)-1-cyclopropylethanol results in low enantioselectivity with this protocol. In another variant of this catalytic resolution of secondary alcohols, manganese−salen complex 192 is used in combination with N-bromosuccinimide and sodium hypochlorite as the stoichiometric oxidant (Scheme 112).243 The reaction is carried

D.3. Asymmetric Carbenoid Insertion into Silicon− Hydrogen Bonds. Enantioselective generation of a chiral center at silicon by asymmetric insertion into a silicon− hydrogen bond can be catalyzed by iridium−salen complex 187. Treatment of a prochiral dialkyl silane in the presence of 187 with an α-alkyl diazoacetate or α-aryl diazoacetate results in a trialkyl silane in which both diastereoselectivity and enantioselectivity approach 100% (Scheme 109).239 This is the first example of an enantioselective insertion into a silicon−hydrogen bond using an α-substituted diazoacetate. Non-prochiral trisubstitued silanes react with α-aryl diazoacetates in a similar fashion to give tetrasubstituted silanes bearing stereogenicity at the α-carbon with virtually complete asymmetric induction. E. Resolution of Racemic Compounds

E.1. Oxidative Kinetic Resolution of Secondary Alcohols. Manganese−salen complex 188 in combination with quaternary pyridinium salt 189 and bis(acetoxy)iodobenzene as stoichiometric oxidant catalyzes partial oxidation of racemic secondary alcohols to ketones, leaving residual alcohol enantioenriched. In many cases, the residual AH


Chemical Reviews

Review

Scheme 111. Asymmetric Oxidative Kinetic Resolution of Secondary Alcohols Catalyzed by Manganese−Salen Complex 188 with N-Bromosuccinimide

Scheme 113. Asymmetric Aerobic Oxidative Kinetic Resolution of Secondary Alcohols Catalyzed by Ruthenium− Salen Complex 193

Scheme 112. Asymmetric Oxidative Kinetic Resolution of Secondary Alcohols Catalyzed by Manganese−Salen Complex 192 complex which moderates reactivity at the metal center and improves stereoselectivity. A further refinement to catalyst 193 gives ruthenium complex 194 in which phenyl substituents in the binaphthyl portion of the salen ligand are replaced by methyl groups and the nitroso ligand on the metal is removed (Scheme 114).247 Ruthenium Scheme 114. Asymmetric Aerobic Oxidative Kinetic Resolution of Racemic Alcohols Catalyzed by Ruthenium− Salen Complex 194

out in a biphasic dichloromethane−water system and generally affords resolved alcohols in very high enantiomeric excess, although resolution of sterically hindered alcohols such as ortho substituted benzylic alcohols is unsuccessful. This negative result may be due to failure of an intermediate analogous to 191 to be formed or perhaps to inhibition of the intramolecular hydrogen transfer step in this intermediate. Replacement of N-bromosuccinimide by a catalytic quantity of bromine in the reaction catalyzed by 192 also effects oxidative resolution of secondary alcohols.244 Manganese complex 192 was used with bis(acetoxy)iodobenzene as terminal oxidant in a resolution of racemic secondary alcohols that led to syntheses of (R)- and (S)-lipoic acids.245 In an extension of his research with salen complexes bearing binaphthyl units for chirality augmentation of the ligand, Katsuki reports that ruthenium(nitrosyl) complex 193 catalyzes partial asymmetric aerobic oxidation of secondary alcohols upon irradiation with visible light. The reaction results in resolved alcohols at variable levels of enantioenrichment, but addition of a 1,3-diketone or an acyclic β-hydroxy ketone to the medium has a significant effect on the level of asymmetric induction (Scheme 113).246 Thus, 193 pretreated with 1,3-bis(p-bromophenyl)propane-1,3-dione results in a ratio of relative rates for oxidation of enantiomers in the racemic alcohol (kS/kR) as high as 30 and yields resolved alcohols in uniformly high enantiomeric excess. The differential rate enhancement in the oxidation is attributed to chelate formation of the 1,3-diketone with the ruthenium−salen

complex 194 improves the resolution of racemic secondary alcohols by increasing the differential rate of oxidation of enantiomers in the racemate up to 60-fold without the need for irradiation. In this reaction, added water replaces the nitroso ligand at the apical metal site in the complex and oxidation proceeds by exchange of water for the secondary alcohol. This is followed by a rate-determining single electron transfer from ruthenium to dioxygen and subsequent alcohol oxidation within the complex. No additive is needed for this oxidative resolution which is effective with both activated and unactivated secondary alcohols, although the differential reaction rate of enantiomers is sensitive to substrate concentration. With the goal of replacing expensive ruthenium in these catalysts by cheaper, more environmentally benign metals, iron−salen complex 196 was prepared by in situ reaction of iron(II) acetate with ligand 195 in which chirality is present as a 2,2-diaminobinaphthyl scaffold (Scheme 115).248 Oxidative kinetic resolution of benzoins with this catalyst using oxygen and AI


Chemical Reviews

Review

Scheme 117. Resolution of Racemic N-Benzyl Amino Acids Catalyzed by Cobalt−Salen Complex 133

Scheme 115. Asymmetric Oxidative Kinetic Resolution of Benzoins Catalyzed by Iron(II) Acetate and Salen Ligand 195

counter-extraction with aqueous sodium dithionite or with Lascorbic acid in methanol. This process reduces cobalt(III) to cobalt(II), and the recovered cobalt(II)−salen is then reoxidized with air to complete the cycle. An exchange experiment with enantiomers showed that the liquid−liquid extraction which separates complexed amino acid 197 from its noncomplexed partner is an equilibrium process operating under thermodynamic control. Computational and spectroscopic studies ascertain how the cobalt-complexed N-benzyl αamino acid enantiomer is accommodated in the chiral binding pocket of the cobalt complex. Cobalt−salen complex 133 is also employed in phaseseparative resolution of racemic N-benzyl β-amino acids.252 In this case, an iterative two-cycle liquid−liquid extraction is required for efficient enantiomer separation. The resolved amino acid is obtained in up to 93% enantiomeric excess.

2,2,6,6-tetramethylpiperidine N-oxide (TEMPO) as additive results in a differential rate of oxidation of benzoin enantiomers in the racemate of 6.7−10.6. The enantiomeric excess of resolved benzoin is 90−98%. Apart from the diaryl 1,2-diketone, the only byproduct from the reaction is water. Salen ligand 195 is also used in combination with cobalt(II) acetate to form a cobalt−salen complex in situ that catalyzes the aerobic oxidative kinetic resolution of racemic α-hydroxy aryl acetates (Scheme 116).249 Under conditions similar to those Scheme 116. Asymmetric Oxidative Kinetic Resolution of αHydroxy Aryl Acetates Catalyzed by Cobalt(II) Acetate and Salen Ligand 195

F. Asymmetric Catalysis by Bimetallic Salen Complexes

F.1. Asymmetric Mannich Reaction. Although dual metal−salen systems in which each metal is encapsulated in separate salen ligands can be effective asymmetric catalysts through cooperative effects between each of the units, rationally designed bimetallic complexes in which two metals are bound within the same salen ligand raise the prospect of new modes of asymmetric catalysis. This concept was first reduced to practice by Shibasaki, initially with homobimetallic complexes such as the stable nickel complex 198 in which chirality is derived from a 1,1′-binaphthyl-2,2′-diamine scaffold. In contrast to monometallic nickel−salen complex 199, bimetallic complex 198 catalyzes asymmetric Mannich condensation of α-nitro esters, malonates, β-keto esters, and β-keto phosphonates with imines to yield products in very high enantiomeric and diastereomeric excess (Scheme 118).253 The role of each metal in 198 was not defined in this work, although the authors believe that the nickel centers are involved cooperatively in the reaction. F.2. Asymmetric Amination. Shibasaki showed that homobimetallic nickel−salen complex 198 catalyzes, at 1−2 mol % loading, asymmetric amination of 3-substituted oxindoles with azodicarboxylates. The reaction produces 3,3-disubstituted 3-aminooxindoles, in this case of (R) configuration, in 87−99% enantiomeric excess (Scheme 119).254 The reaction is especially valuable for installing a stereogenic quaternary center at the 3position of an oxindole nucleus. Interestingly, the same reaction catalyzed by monometallic nickel−salen complex 199 gives an

used with iron catalyst 196, the cobalt−salen complex shows a differential oxidation rate of enantiomers of up to 31.9. The resolved hydroxy ester is formed in up to 99% enantiomeric excess. E.2. Asymmetric Phase-Separative Resolution of Amino Acids. Cobalt−salen complex 133 forms a lipophilic complex selectively and stoichiometrically with one enantiomer in a racemic mixture of N-benzyl α-amino acids, and the complexed enantiomer 197 can be separated from the noncomplexed enantiomer by extraction from an aqueous solution into an organic phase (Scheme 117).250,251 After phase separation, the complexed amino acid is released by reductive AJ


Chemical Reviews

Review

Scheme 118. Asymmetric Mannich Reaction of α-Nitro Acetates with Imines Catalyzed by Homobimetallic and Monometallic Nickel−Salen Complexes 198 and 199

Scheme 120. Asymmetric Conjugate Addition of β-Keto Esters to Alkynones and Nitro Alkenes Catalyzed by Homobimetallic Cobalt−Salen Complex 200

Scheme 119. Asymmetric Amination of Oxindoles Catalyzed by Homobimetallic and Monometallic Nickel−Salen Complexes 198 and 199

the β-keto ester and the inner cobalt acts as a Lewis acid to activate the nitro alkene or alkynone. F.4. Asymmetric Henry and Aza-Henry Reactions. In an extension of his research with homobimetallic salen catalysts, Shibasaki devised systematic synthetic routes to heterobimetallic complexes in which two different metals are bound within a single chiral salen ligand such as 201 (Scheme 121). The first Scheme 121. Synthesis of Heterobimetallic Catalysts 202 and 203 from Ligand 201

metal incorporated into the ligand is usually a d-transition metal such as copper with a relatively small ionic radius. This metal takes the smaller inner binding site surrounded by two nitrogen and two oxygen atoms. The second larger ion, often a lanthanide, occupies the remaining outer site surrounded by four oxygen atoms. A functional characteristic of this type of heterobimetallic complex is its behavior as a hard−soft catalytic system with the corresponding properties of a Lewis acid− Bronsted base pair. Heterobimetallic salen complex 202 with copper(II) at its inner site and samarium(III) as the outer ion catalyzes the asymmetric aza-Henry (nitro-Mannich) reaction of nitro alkanes with imines to give predominantly syn nitro amines in good-to-excellent enantiomeric excess (Scheme 122).257

(S) 3,3-disubstituted oxindole. A rationale for this curious reversal of enantioselectivity was not offered. F.3. Asymmetric Conjugate Addition. The cobalt analogue 200 of bis(nickel) complex 198 catalyzes asymmetric 1,4-addition of β-keto esters to alkynones and nitro alkenes (Scheme 120).255 With alkynones and α-substituted β-keto esters, the addition is highly (E) selective for unsaturated diketo ester products which are formed in very high enantiomeric excess. With 2-alkyl or 2-aryl nitro alkenes, the conjugate addition gives nitro keto esters with good-to-excellent diastereoselectivity, with the major diastereomer being formed in high enantiomeric excess.256 Mechanistic studies with this catalyst suggest that the outer cobalt ion forms an enolate with AK


Chemical Reviews

Review

204 whose polydentate structure coordinates a p-block ion such as gallium(III) within the inner sphere and a lanthanide ion such as ytterbium(III) in the outer cavity. The resulting heterobimetallic complex 205 catalyzes asymmetric addition of αisocyanoacetamides to aryl, heteroaryl, alkenyl, and alkyl aldehydes to give trisubstituted oxazoles with a stereogenic hydroxyalkyl substituent at carbon-2 of the heterocycle in 88− 98% enantiomeric excess (Scheme 123).260 A working

Scheme 122. Asymmetric Henry and Aza-Henry Reactions Catalyzed Heterobimetallic Salen Complexes 202 and 203

Scheme 123. Asymmetric Addition of Isocyanoacetamides to Aldehydes Catalyzed by Heterobimetallic Gallium− Ytterbium−Salen Catalyst 205

Similarly, heterobimetallic complex 203 with palladium(II) as the inner metal and lanthanum(III) at the outer site of ligand 201 catalyzes asymmetric Henry (nitroaldol) reaction of nitro alkanes with aldehydes to give nitro alcohols of predominantly anti configuration, again with good enantioselectivity for the major diastereomer.258 In an account summarizing this research, Shibasaki proposes that the two proximal metal centers in a heterobimetallic complex such as 203 work cooperatively, with the aryloxy lanthanide serving as a Bronsted base to deprotonate the nitro alkane, while the companion softer metal functions as a Lewis acid to activate the aldehyde carbonyl or the imine.259 The reactivity and selectivity of a heterobimetallic catalyst depend heavily on the metals selected for a given reaction as well as the order in which the two metals are introduced into the salen ligand. For example, the combination of copper(II) and samarium(III) in heterobimetallic complex 202 affords a catalyst that produces nitro amines from the aza-Henry reaction of nitro alkanes with imines in 66−99% enantiomeric excess,257 whereas palladium(II) and lanthanum(III) in complex 203 are the optimal pair for the asymmetric Henry reaction of nitro alkanes with aldehydes, affording nitro aldol products in 72− 92% enantiomeric excess (Scheme 122).258 In both the nitro-Mannich and nitroaldol reactions, the Brønsted basic lanthanide moiety is thought to be the agent that deprotonates the nitroethane, resulting in a nitronate bound to the lanthanide center. This places the nitronate in close proximity to its electrophilic partner which is activated by the neighboring transition metal.259 The reactivity pattern implicit in this approach opens a domain for asymmetric catalysis of bimolecular reactions that has yet to be explored in depth. F.5. Asymmetric Addition of Isocyanoacetamides to Aldehydes. In a departure from conventional scaffolds employed in chiral metal−salen complexes, the C2-symmetric trans-diamino-9,10-dihydroanthracene framework is found to be a productive source of stereoinduction in asymmetric synthesis. The diamine is condensed with ortho-vanillin to give Schiff base

hypothesis for the mechanism of this reaction has both metal ions functioning as Lewis acids by coordinating to carbonyl oxygens in the reactant pair. The activated aldehyde is first attacked by the nitrogen atom of the isocyanide to form a nitrilium ion which undergoes cyclization to furnish the oxazole. F.6. Desymmetric Ring Opening of Epoxides. A heterobimetallic gallium−titanium−salen complex 206 (5 mol %) catalyzes ring opening of meso epoxides by aryl selenols and thiols to give β-arylseleno alcohols and β-hydroxy sulfides in good yield and high enantioselectivity (Scheme 124).261,262 Synergistic cooperation between the two metals of the complex and their separate coordination with the reactant pair as shown in 207 are thought to be responsible for the activity of this catalyst. A review describing bimetallic salen complexes and their applications was published in 2014.263

5. CONCLUSION AND OUTLOOK The role of asymmetric catalysis by salen−metal complexes in organic synthesis has grown exponentially since the first exploratory studies in the 1970s, and it is tempting to think of this as a mature field. There is now a broad selection of chiral scaffolds available that can be placed within salen ligands as well as a range of ligands that can encapsulate a variety of metal ions. Furthermore, there is a better understanding of how both ligand structure and the core metal can be tuned for optimal catalytic properties. Nevertheless, much of the science underpinning asymmetric catalysis by salen−metal complexes has evolved in a highly empirical manner and there are many reactions that generate products at a satisfactory level of stereoefficiency which are far from being understood mechanistically. It is likely that that this deficiency will be repaired as more detailed structural information about metal−substrate coordination, spatial oriAL


Chemical Reviews

Review

Scheme 124. Desymmetric Ring Opening of meso-Epoxides with Aryl Selenols and Thiols Catalyzed by Gallium− Titanium−Salen Catalyst 206

Biographies Subrata Shaw was born in West Bengal, India, and graduated with a B.Sc. from the University of Calcutta in 2005, with honors in chemistry. He obtained his M.Sc. degree at the Indian Institute of Technology Kanpur in 2007. In the summer of 2006, he was a visiting M.Sc. student, supervised by Professor John E. Greedan, at McMaster University, Canada, after which he joined the graduate program at Oregon State University in 2007. His graduate studies were carried out under the supervision of Professor James D. White, and he obtained his Ph.D. in 2014. His graduate research focused on synthesis of new catalysts based on novel chiral salen frameworks for asymmetric organic transformations. The reactions of primary interest were those providing access to biologically important chiral compounds and to pharmaceutical products. In 2014, he took a postdoctoral position in the cancer drug discovery program of Professor Stephen Fesik at Vanderbilt University Medical Center. He is currently a research scientist in the Center for the Development of Therapeutics (CDoT) at the Broad Institute of MIT and Harvard. James D. White was born in Bristol, England, and obtained his undergraduate degree from Cambridge University in 1959. He completed a M.Sc. degree in chemistry at the University of British Columbia and obtained his Ph.D. at the Massachusetts Institute of Technology in 1965. In the same year, he joined the chemistry faculty of Harvard University and rose to Associate Professor in 1971, before moving to Oregon State University where he is currently Distinguished Professor Emeritus. At Oregon State, his research group completed syntheses of over 50 natural products, many containing a highly complex architecture, while exploring new methods for stereocontrol in these challenging structural environments. The latter effort included asymmetric catalysis employing metal−salen complexes. He has coauthored over 350 publications, and his work has been recognized with national and international awards that include a John Simon Guggenheim Memorial Fellowship (1988−1989), an honorary Sc.D. from Cambridge University (1995), the Centenary Medal of the Royal Society of Chemistry (1999), an Arthur C. Cope Senior Scholar Award from the American Chemical Society (2003), and the Oregon Academy of Sciences Outstanding Scientist Award (2006).

entation of reacting partners, and other aspects of transition states in these stereodefining processes become available from physical and chemical methods. In the meantime, we can expect that creative investigators will devise new catalytic systems using salen ligands with new chiral scaffolds, perhaps incorporating metals that have thus far seen little application in asymmetric synthesis. Recent developments with ligands that encapsulate pairs of different metals within a single complex may be pointing the way to new areas of asymmetric catalysis where the Periodic Table is the only limit. The prospect that chiral salen−metal complexes can be designed to mimic metalloenzymes where asymmetric catalysis with turnover rates approaching those of enzymatic reactions as, for example, in asymmetric aerobic oxidation of natural substrates is certainly conceivable. Further progress in the design and synthesis of immobilized salen−metal catalysts is likely to contribute not only to easier separation and removal of the catalyst from products but also to greatly extended reusability of the catalyst.264−268 Finally, it is important to recognize that research in the field of asymmetric salen−metal catalysis is essentially an exercise in prospecting and is not for the faint of heart. The mostly successful outcomes portrayed in this review are the consequence of many trials and disappointments, often with little guidance from principle about where to turn for a better result. While it is said that “luck favors the prepared mind”, real progress in this field is much more likely to originate from hard work than good fortune.

REFERENCES (1) Combes, A. C. R. Acad. Fr. 1889, 108, 1252. (2) Canali, L.; Sherrington, D. C. Utilisation of homogeneous and supported chiral metal(salen) complexes in asymmetric catalysis. Chem. Soc. Rev. 1999, 28, 85−93. (3) Katsuki, T. Some Recent Advances in Metallosalen Chemistry. Synlett 2003, 0281−0297. (4) Che, C.-M.; Huang, J.-S. Metal complexes of chiral binaphthyl Schiff-base ligands and their application in stereoselective organic transformations. Coord. Chem. Rev. 2003, 242, 97−113. (5) Cozzi, P. G. Metal-Salen Schiff base complexes in catalysis: practical aspects. Chem. Soc. Rev. 2004, 33, 410−421. (6) Larrow, J. F.; Jacobsen, E. N. Asymmetric Processes Catalyzed by Chiral (Salen)Metal Complexes. Top. Organomet. Chem. 2004, 6, 123− 152. (7) Baleizao, C.; Garcia, H. Chiral Salen Complexes: An Overview to Recoverable and Reusable Homogeneous and Heterogeneous Catalysts. Chem. Rev. 2006, 106, 3987−4043. (8) Matsumoto, K.; Saito, B.; Katsuki, T. Asymmetric catalysis of metal complexes with non-planar ONNO ligands: salen, salalen and salan. Chem. Commun. 2007, 3619−3627. (9) Shiryaev, A. K. Recent Advances in Chiral Catalysis Using Metal Salen Complexes. Curr. Org. Chem. 2012, 16, 1788−1807. (10) Chapman, M. R.; Henkelis, S. E.; Kapur, N.; Nguyen, B. N.; Willans, C. E. A Straightforward Electrochemical Approach to Imine-

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

James D. White: 0000-0003-0112-6636 Notes

The authors declare no competing financial interest. AM


Chemical Reviews

Review

and Amine-bisphenolate Metal Complexes with Facile Control Over Metal Oxidation State. ChemistryOpen 2016, 5, 351−356. (11) North, M.; Usanov, D. L.; Young, C. Lewis Acid Catalyzed Asymmetric Cyanohydrin Synthesis. Chem. Rev. 2008, 108, 5146− 5226. (12) North, M.; Orizu, C.; Tararov, V. I.; Ikonnikov, N. S.; Belokon, Y. N.; Hibbs, D. E.; Hursthouse, M. B.; Malik, K. M. A. First Structurally Defined Catalyst for the Asymmetric Addition of Trimethylsilyl Cyanide to Benzaldehyde. Chem. Commun. 1998, 387−388. (13) Belokon’, Y. N.; Caveda-Cepas, S.; Green, B.; Ikonnikov, N. S.; Khrustalev, V. N.; Larichev, V. S.; Moscalenko, M. A.; North, M.; Orizu, C.; Tararov, V. I.; Tasinazzo, M.; Timofeeva, G. I.; Yashkina, L. V. The Asymmetric Addition of Trimethylsilyl Cyanide to Aldehydes Catalyzed by Chiral (Salen)Titanium Complexes. J. Am. Chem. Soc. 1999, 121, 3968−3973. (14) Belokon’, Y. N.; Green, B.; Ikonnikov, N. S.; Larichev, V. S.; Lokshin, B. V.; Moscalenko, M. A.; North, M.; Orizu, C.; Peregudov, A. S.; Timofeeva, G. I. Mechanistic Investigation of the Asymmetric Addition of Trimethylsilyl Cyanide to Aldehydes Catalysed by Dinuclear Chiral (Salen)titanium Complexes. Eur. J. Org. Chem. 2000, 2000, 2655−2661. (15) Belokon’, Y. N.; North, M.; Parsons, T. Vanadium-Catalyzed Asymmetric Cyanohydrin Synthesis. Org. Lett. 2000, 2, 1617−1619. (16) Belokon’, Y. N.; Green, B.; Ikonnikov, N. S.; North, M.; Parsons, T.; Tararov, V. I. Optimized Catalysts for the Asymmetric Addition of Trimethylsilyl Cyanide to Aldehydes and Ketones. Tetrahedron 2001, 57, 771−779. (17) Belokon, Y. N.; Carta, P.; Gutnov, A. V.; Maleev, V. I.; Moskalenko, M. A.; Yashkina, L. V.; Ikonnikov, N. S.; Voskoboev, N. V.; Khrustalev, V. N.; North, M. Catalytic Asymmetric Synthesis of OAcetylcyanohydrins from Potassium Cyanide, Acetic Anhydride, and Aldehydes, Promoted by Chiral Salen Complexes of Titanium(IV) and Vanadium(V). Helv. Chim. Acta 2002, 85, 3301−3312. (18) Kim, S. S.; Song, D. H. Asymmetric Cyanohydrin Synthesis Catalyzed by Al(salen)/Triphenylphosphane Oxide. Eur. J. Org. Chem. 2005, 2005, 1777−1780. (19) Belokon, Y. N.; Maleev, V. I.; North, M.; Usanov, D. L. VO(salen)(X) Catalysed Asymmetric Cyanohydrin Synthesis: An Unexpected Influence of the Nature of Anion X on the Catalytic Activity. Chem. Commun. 2006, 4614−4616. (20) Chen, S. K.; Peng, D.; Zhou, H.; Wang, L.-W.; Chen, F.-X.; Feng, X.-M. Highly Enantioselective Cyanoformylation of Aldehydes Catalyzed by a Mononuclear Salen-Ti(OiPr)4 Complex Produced in Situ. Eur. J. Org. Chem. 2007, 2007, 639−644. (21) Wen, Y.-Q.; Ren, W.-M.; Lu, X.-B. Unsymmetric Salen Ligands Bearing a Lewis Base: Intramolecularly Cooperative Catalysis for Cyanosilylation of Aldehydes. Org. Biomol. Chem. 2011, 9, 6323−6330. (22) Lv, C.; Cheng, Q.; Xu, D.; Wang, S.; Xia, C.; Sun, W. Fine Modification of Salen Ligands-Effects on the Salen-Ti-catalyzed Asymmetric Cyanosilylation of Aldehydes. Eur. J. Org. Chem. 2011, 2011, 3407−3411. (23) Khan, N. H.; Agrawal, S.; Kureshy, R. I.; Abdi, S. H. R.; Prathap, K. J.; Jasra, R. V. Vanadium(V)-Salen Complex Catalyzed Highly Enantioselective Cyanoformylation of Aldehydes in the Presence of Imidazole as a Cocatalyst. Eur. J. Org. Chem. 2008, 2008, 4511−4515. (24) Khan, N. H.; Agarwal, S.; Kureshy, R. I.; Abdi, S. H. R.; Mayani, V. J.; Jasra, R. V. Easily Recyclable Polymeric V(V) Salen Complex for the Enantioselective O-acetyl Cyanation of Aldehydes. J. Mol. Catal. A: Chem. 2007, 264, 140−145. (25) Khan, N. H.; Agarwal, S.; Kureshy, R. I.; Abdi, S. H. R.; Pathak, K.; Bajaj, H. C. Enantioselective Cyanoformylation of Aldehydes Catalyzed with Solid Base Mediated Chiral V(V) Salen Complexes. Chirality 2010, 22, 153−158. (26) Kim, S. S.; Kwak, J. M.; George, S. C. Enantioselective Cyanosilylation of Aldehydes Catalyzed by Mn(salen) Complex/ Triphenyl Phosphine Oxide. Appl. Organomet. Chem. 2007, 21, 809− 813.

(27) Qu, Y.; Jing, L.; Wua, Z.; Wu, D.; Zhou, X. Asymmetric Cyanohydrin Formation from Aldehydes Catalyzed by Manganese Schiff Base Complexes. Tetrahedron: Asymmetry 2010, 21, 187−190. (28) Wei, Y.-L.; Huang, W.-S.; Cui, Y.-M.; Yang, K.-F.; Xua, Z.; Xu, L.W. Enantioselective Cyanosilylation of Aldehydes Catalyzed by a Multistereogenic Salen-Mn(III) Complex with a Rotatable Benzylic Group as a Helping Hand. RSC Adv. 2015, 5, 3098−3103. (29) Zeng, Z.; Zhao, G.; Zhou, Z.; Tang, C. A Novel Chiral (salen)AlIII Complex Catalyzed Asymmetric Cyanosilylation of Aldehydes. Eur. J. Org. Chem. 2008, 2008, 1615−1618. (30) Belokon’, Y. N.; Hunt, J.; North, M. Asymmetric Catalysis of Carbon-carbon Bond Forming Reactions Using Amino Acid-derived C1-Symmetrical Salen Ligands. Tetrahedron: Asymmetry 2008, 19, 2804−2815. (31) Alaaeddine, A.; Roisnel, T.; Thomas, C. M.; Carpentier, J.-F. Discrete versus In Situ-Generated Aluminum-Salen Catalysts in Enantioselective Cyanosilylation of Ketones: Role of Achiral Ligands. Adv. Synth. Catal. 2008, 350, 731−740. (32) Zeng, X. P.; Cao, Z. Y.; Wang, X.; Chen, L.; Zhou, F.; Zhu, F.; Wang, C. H.; Zhou, J. Activation of Chiral (Salen)AlCl Complex by Phosphorane for Highly Enantioselective Cyanosilylation of Ketones and Enones. J. Am. Chem. Soc. 2016, 138, 416−425. (33) Sigman, M. S.; Jacobsen, E. N. Enantioselective Addition of Hydrogen Cyanide to Imines Catalyzed by a Chiral (Salen)Al(III) Complex. J. Am. Chem. Soc. 1998, 120, 5315−5316. (34) Cai, X.-H.; Xie, B. Recent Advances in Asymmetric Strecker Reactions. ARKIVOC 2014, 205−248. (35) Belokon’, Y. N.; Hunt, J.; North, M. Asymmetric Catalysis of Carbon-Carbon Bond Forming Reactions using Amino Acid-Derived C1-Symmetrical Salen Ligands. Tetrahedron: Asymmetry 2008, 19, 2804−2815. (36) Khan, N. H.; Saravanan, S.; Kureshy, R. I.; Abdi, S. H. R.; Sadhukhan, A.; Bajaj, H. C. Asymmetric Addition of Trimethylsilylcyanide to N-Benzylimines Catalyzed by Recyclable Chiral Dimeric V(V) Salen Complex. J. Organomet. Chem. 2010, 695, 1133−1137. (37) Esteves, M. A.; Gigante, B.; Santos, C.; Guerreiro, A. M.; Baleizão, C. New Heterogeneous Catalysts for the Synthesis of Chiral Amino Acids: Functionalization of Organic Resins with Chiral Salen Complexes. Catal. Today 2013, 218−219, 65−69. (38) Lu, G.; Yoshino, T.; Morimoto, H.; Matsunaga, S.; Shibasaki, M. Stereodivergent Direct Catalytic Asymmetric Mannich-Type Reactions of α-Isothiocyanato Ester with Ketimines. Angew. Chem., Int. Ed. 2011, 50, 4382−4385. (39) Cozzi, P. G. Enantioselective Alkynylation of Ketones Catalyzed by Zn(Salen) Complexes. Angew. Chem., Int. Ed. 2003, 42, 2895−2898. (40) Cozzi, P. G.; Kotrusz, P. Highly Enantioselective Addition of Me2Zn to Aldehydes Catalyzed by ClCr(Salen). J. Am. Chem. Soc. 2006, 128, 4940−4941. (41) Cozzi, P. G. A Catalytic, Me2Zn-Mediated, Enantioselective Reformatsky Reaction with Ketones. Angew. Chem., Int. Ed. 2006, 45, 2951−2954. (42) Pathak, K.; Bhatt, A. P.; Abdi, S. H. R.; Kureshy, R. I.; Khan, N.-U. H.; Ahmad, I.; Jasra, R. V. Enantioselective Phenylacetylene Addition to Aldehydes and Ketones Catalyzed by Recyclable Polymeric Zn(salen) Complex. Chirality 2007, 19, 82−88. (43) Li, F.-Q.; Zhong, S.; Lu, G.; Chana, A. S. C. Zinc-Salen-Catalyzed Asymmetric Alkynylation of Alkyl Acylsilanes. Adv. Synth. Catal. 2009, 351, 1955−1960. (44) Shen, C.; Chen, L.; Tang, J.; Xu, M. Asymmetric Alkynylation of Aldehydes Catalyzed by Bifunctional Zinc(salen) Complex. Chin. J. Chem. 2009, 27, 413−418. (45) Shimizu, K.; Uetsu, H.; Gotanda, T.; Ito, K. Zn(salen)-Catalyzed Enantioselective Phenyl Transfer to Aldehydes and Ketones with Organozinc Reagent. Synlett 2015, 26, 1238−1242. (46) Rudolph, J.; Bolm, C.; Norrby, P.-O. New Insights into the Stereoselectivity of the Aryl Zinc Addition to Aldehydes. J. Am. Chem. Soc. 2005, 127, 1548−1552. AN


Chemical Reviews

Review

Scope and Factors Determining the Enantioselectivity. J. Am. Chem. Soc. 1978, 100, 3443−3448. (68) Pellissier, H. Recent Developments in Asymmetric Cyclopropanation. Tetrahedron 2008, 64, 7041−7095. (69) Uchida, T.; Irie, R.; Katsuki, T. Chiral (ON)Ru-Salen Catalyzed Cyclopropanation: High Cis- and Enantioselectivity. Synlett 1999, 1999, 1163−1165. (70) Uchida, T.; Irie, R.; Katsuki, T. Highly cis- and EnantiofaceSelective Cyclopropanation using (R,R)-Ru-Salen Complex: Solubility Dependent Enantioface Selection. Synlett 1999, 1999, 1793−1795. (71) Xu, Z. J.; Fang, R.; Zhao, C. Y.; Huang, J. S.; Li, G. Y.; Zhu, N. Y.; Che, C. M. Cis-β-Bis(carbonyl) Ruthenium-Salen Complexes: X-ray Crystal Structures and Remarkable Catalytic Properties Toward Asymmetric Intramolecular Alkene Cyclopropanation. J. Am. Chem. Soc. 2009, 131, 4405−4417. (72) Shi, T.; Luo, Y.; Wang, X.-L.; Lu, S.; Zhao, Y.-L.; Zhang, J. Theoretical Studies on the Mechanism, Enantioselectivity, and Axial Ligand Effect of a Ru(salen)-Catalyzed Asymmetric Cyclopropanation Reaction. Organometallics 2014, 33, 3673−3682. (73) Kanchiku, S.; Suematsu, H.; Matsumoto, K.; Uchida, T.; Katsuki, T. Construction of an Aryliridium-Salen Complex for Highly Cis- and Enantioselective Cyclopropanations. Angew. Chem., Int. Ed. 2007, 46, 3889−3891. (74) Suematsu, H.; Kanchiku, S.; Uchida, T.; Katsuki, T. Construction of Aryliridium-Salen Complexes: Enantio- and Cis-Selective Cyclopropanation of Conjugated and Nonconjugated Olefins. J. Am. Chem. Soc. 2008, 130, 10327−10337. (75) Ichinose, M.; Suematsu, H.; Katsuki, T. Efficient Construction of α-Spirocyclopropyl Lactones: Iridium-Salen-Catalyzed Asymmetric Cyclopropanation. Angew. Chem., Int. Ed. 2009, 48, 3121−3123. (76) Uehara, M.; Suematsu, H.; Yasutomi, Y.; Katsuki, T. Enantioenriched Synthesis of Cyclopropenes with a Quaternary Stereocenter, Versatile Building Blocks. J. Am. Chem. Soc. 2011, 133, 170−171. (77) Nakamura, A.; Konishi, A.; Tatsuno, Y.; Otsuka, S. A Highly Enantioselective Synthesis of Cyclopropane Derivatives Through Chiral Cobalt(II) Complex Catalyzed Carbenoid Reaction. General Scope and Factors Determining the Enantioselectivity. J. Am. Chem. Soc. 1978, 100, 3443−3448. (78) Niimi, T.; Uchida, T.; Irie, R.; Katsuki, T. Co(II)-SalenCatalyzed Highly Cis- and Enantioselective Cyclopropanation. Tetrahedron Lett. 2000, 41, 3647−3651. (79) Niimi, T.; Uchida, T.; Irie, R.; Katsuki, T. Highly Enantioselective Cyclopropanation with Co(II)-Salen Complexes: Control of Cis- and Trans-Selectivity by Rational Ligand-Design. Adv. Synth. Catal. 2001, 343, 79−88. (80) Uchida, T.; Katsuki, T. α-Diazoacetates as Carbene Precursors: Metallosalen-Catalyzed Asymmetric Cyclopropanation. Synthesis 2006, 2006, 1715−1723. (81) Uchida, T.; Saha, B.; Katsuki, T. Co(II)-Salen-Catalyzed Asymmetric Intramolecular Cyclopropanation. Tetrahedron Lett. 2001, 42, 2521−2524. (82) Shitama, H.; Katsuki, T. Synthesis of Metal-(PentadentateSalen) Complexes: Asymmetric Epoxidation with Aqueous Hydrogen Peroxide and Asymmetric Cyclopropanation (salenH2: N,N′-bis(salicylidene)ethylene-1,2-diamine). Chem. - Eur. J. 2007, 13, 4849− 4858. (83) Morandi, B.; Mariampillai, B.; Carreira, E. M. Enantioselective Cobalt-Catalyzed Preparation of Trifluoromethyl-Substituted Cyclopropanes. Angew. Chem., Int. Ed. 2011, 50, 1101−1104. (84) White, J. D.; Shaw, S. A New Cobalt-Salen Catalyst for Asymmetric Cyclopropanation. Synthesis of the Seretonin-Norepinephrine Reuptake Inhibitor (+)-Synosutine. Org. Lett. 2014, 16, 3880−3883. (85) Joshi-Pangu, A.; Cohen, R. D.; Tudge, M. T.; Chen, Y. Dearomatization of Electron-Deficient Nitrogen Heterocycles via Cobalt-Catalyzed Asymmetric Cyclopropanation. J. Org. Chem. 2016, 81, 3070−3075.

(47) Achard, T. R. J.; Clegg, W.; Harrington, R. W.; North, M. Chiral Salen Ligands Designed to Form Polymetallic Complexes. Tetrahedron 2012, 68, 133−144. (48) Okude, Y.; Hirano, S.; Hiyama, T.; Nozaki, H. Grignard-type carbonyl addition of allyl halides by means of chromous salt. A chemospecific synthesis of homoallyl alcohols. J. Am. Chem. Soc. 1977, 99, 3179−3181. (49) Fürstner, A.; Shi, N. Nozaki-Hiyama-Kishi Reactions Catalytic in Chromium. J. Am. Chem. Soc. 1996, 118, 12349−12357. (50) Bandini, M.; Cozzi, P. G.; Melchiorre, P.; Umani-Ronchi, A. The First Catalytic Enantioselective Nozaki-Hiyama Reaction. Angew. Chem., Int. Ed. 1999, 38, 3357−3359. (51) Berkessel, A.; Menche, D.; Sklorz, C. A.; Schroder, M.; Paterson, I. A Highly Enantioselective Catalyst for the Asymmetric NozakiHiyama-Kishi Reaction of Allylic and Vinylic Halides. Angew. Chem., Int. Ed. 2003, 42, 1032−1035. (52) White, J. D.; Shaw, S. Cis-2,5-Diaminobicyclo[2.2.2]octane, a New Scaffold for Asymmetric Catalysis via Salen-Metal Complexes. Org. Lett. 2011, 13, 2488−2491. (53) Reynolds, T. E.; Scheidt, K. A. Catalytic Enantioselective αAcylvinyl Anion Reactions of Silyloxyallene. Angew. Chem., Int. Ed. 2007, 46, 7806−7809. (54) Palomo, C.; Oiarbide, M.; Laso, A. Recent Advances in the Catalytic Asymmetric Nitroaldol (Henry) Reaction. Eur. J. Org. Chem. 2007, 2007, 2561−2574. (55) Kogami, Y.; Nakajima, T.; Ikeno, T.; Yamada, T. Enantioselective Henry Reaction Catalyzed by Salen-Cobalt Complexes. Synthesis 2004, 2004, 1947−1950. (56) Park, J.; Lang, K.; Abboud, K. A.; Hong, S. Self-Assembled Dinuclear Cobalt(II)-Salen Catalyst Through Hydrogen-Bonding and Its Application to Enantioselective Nitro-Aldol (Henry) Reaction. J. Am. Chem. Soc. 2008, 130, 16484−16485. (57) Lang, K.; Park, J.; Hong, S. Urea/Transition-Metal Cooperative Catalyst for Anti-Selective Asymmetric Nitroaldol Reactions. Angew. Chem., Int. Ed. 2012, 51, 1620−1624. (58) Wei, Y.-L.; Yang, K.-F.; Li, F.; Zheng, Z.-J.; Xua, Z.; Xu, L.-W. Probing the Evolution of an Ar-BINMOL-Derived Salen-Co(III) Complex for Asymmetric Henry Reactions of Aromatic Aldehydes: Salan-Cu(II) versus Salen-Co(III) Catalysis. RSC Adv. 2014, 4, 37859−37867. (59) Kowalczyk, R.; Sidorowicz, Ł.; Skarzewski, J. Asymmetric Nitroaldol Reaction Catalyzed by a Chromium(III)-Salen System. Tetrahedron: Asymmetry 2007, 18, 2581−2586. (60) Kowalczyk, R.; Kwiatkowski, P.; Skarzewski, J.; Jurczak, J. Enantioselective Nitroaldol Reaction Catalyzed by Sterically Modified Salen-Chromium Complexes. J. Org. Chem. 2009, 74, 753−756. (61) Zulauf, A.; Mellah, M.; Schulz, E. New Chiral Thiophene-Salen Chromium Complexes for the Asymmetric Henry Reaction. J. Org. Chem. 2009, 74, 2242−2245. (62) Kureshy, R. I.; Dangi, B.; Das, A.; Khan, N. H.; Abdi, S. H. R.; Bajaj, H. C. Recyclable Cu(II)-Macrocyclic Salen Complexes Catalyzed Nitroaldol Reaction of Aldehydes: A Practical Strategy in the Preparation of (R)-Phenylephrine. Appl. Catal., A 2012, 439−440, 74−79. (63) White, J. D.; Shaw, S. A New Catalyst for the Asymmetric Henry Reaction: Synthesis of β-Nitroethanols in High Enantiomeric Excess. Org. Lett. 2012, 14, 6270−6273. (64) Kazemizadeh, A. R.; Ramazani, A. Synthetic Applications of Passerini Reaction. Curr. Org. Chem. 2012, 16, 418−450. (65) Wang, S.-X.; Wang, M.-X.; Wang, D.-X.; Zhu, J. Catalytic Enantioselective Passerini Three-Component Reaction. Angew. Chem., Int. Ed. 2008, 47, 388−391. (66) Yue, T.; Wang, M.-X.; Wang, D.-X.; Zhu, J. Asymmetric Synthesis of 5-(1-Hydroxyalkyl)tetrazoles by Catalytic Enantioselective Passerini-type Reactions. Angew. Chem., Int. Ed. 2008, 47, 9454−9457. (67) Nakamura, A.; Konishi, A.; Tatsuno, Y.; Otsuka, S. A Highly Enantioselective Synthesis of Cyclopropane Derivatives Through Chiral Cobalt(II) Complex Catalyzed Carbenoid Reaction. General AO


Chemical Reviews

Review

(86) Carmona, D.; Lamata, M. P.; Oro, L. A. Recent Advances in Homogeneous Enantioselective Diels-Alder Reactions Catalyzed by Chiral Transition-Metal Complexes. Coord. Chem. Rev. 2000, 200−202, 717−772. (87) Huang, Y.; Iwama, T.; Rawal, V. H. Design and Development of Highly Effective Lewis Acid Catalysts for Enantioselective Diels-Alder Reactions. J. Am. Chem. Soc. 2002, 124, 5950−5951. (88) McGilvra, J. D.; Rawal, V. H. Carbon-Substituted Co(III) Salens as Effective Catalysts for Enantioselective Diels-Alder Reactions. Synlett 2004, 2440−2442. (89) Nicolaou, K. C.; Tria, G. S.; Edmonds, D. J.; Kar, M. Total Syntheses of (±)-Platencin and (−)-Platencin. J. Am. Chem. Soc. 2009, 131, 15909−15917. (90) Sparrow, K.; Barker, D.; Brimble, M. A. The Enantioselective Synthesis of Tetracyclic Methyllycaconitine Analogues. Tetrahedron 2011, 67, 7989−7999. (91) Dénès, F.; Pérez-Luna, A.; Chemla, F. Addition of Metal Enolate Derivatives to Unactivated Carbon-Carbon Multiple Bonds. Chem. Rev. 2010, 110, 2366−2447. (92) Corkey, B. K.; Toste, F. D. Catalytic Enantioselective Conia-Ene Reaction. J. Am. Chem. Soc. 2005, 127, 17168−17169. (93) Yang, T.; Ferrali, A.; Sladojevich, F.; Campbell, L.; Dixon, D. J. Brønsted Base/Lewis Acid Cooperative Catalysis in the Enantioselective Conia-ene Reaction. J. Am. Chem. Soc. 2009, 131, 9140−9141. (94) Matsuzawa, A.; Mashiko, T.; Kumagai, N.; Shibasaki, M. La/Ag Heterobimetallic Cooperative Catalysis: A Catalytic Asymmetric Conia-ene Reaction. Angew. Chem., Int. Ed. 2011, 50, 7616−7619. (95) Shaw, S.; White, J. D. A New Iron(III)-Salen Catalyst for Enantioselective Conia-ene Carbocyclization. J. Am. Chem. Soc. 2014, 136, 13578−13581. (96) Yang, L.; Wang, D.-X.; Huang, Z.-T.; Wang, M.-X. Cr(III)(salen)Cl Catalyzed Enantioselective Intramolecular Addition of Tertiary Enamides to Ketones: A General Access to Enantioenriched 1H-Pyrrol-2(3H)-one Derivatives Bearing a Hydroxylated Quaternary Carbon Atom. J. Am. Chem. Soc. 2009, 131, 10390−10391. (97) Maleev, V. I.; Chusov, D. A.; Yashkina, L. V.; Ikonnikov, N. S.; Il’in, M. M. Asymmetric Ring Opening of Epoxides with Cyanides Catalyzed by Chiral Binuclear Titanium Complexes. Tetrahedron: Asymmetry 2014, 25, 838−843. (98) Wu, B.; Gallucci, J. C.; Parquette, J. R.; RajanBabu, T. V. Bimetallic Catalysis in the Highly Enantioselective Ring-opening Reactions of Aziridines. Chem. Sci. 2014, 5, 1102−1117. (99) Sammis, G. M.; Jacobsen, E. N. Highly Enantioselective, Catalytic Conjugate Addition of Cyanide to α,β-Unsaturated Imides. J. Am. Chem. Soc. 2003, 125, 4442−4443. (100) Mazet, C.; Jacobsen, E. N. Dinuclear {(salen)Al} Complexes Display Expanded Scope in the Conjugate Cyanation of α,βUnsaturated Imides. Angew. Chem., Int. Ed. 2008, 47, 1762−1765. (101) Madhavan, N.; Weck, M. Highly Active Polymer-Supported (Salen)Al Catalysts for the Enantioselective Addition of Cyanide to α,β-Unsaturated Imides. Adv. Synth. Catal. 2008, 350, 419−425. (102) Madhavan, N.; Takatani, T.; Sherrill, C. D.; Weck, M. Macrocyclic Cyclooctene-Supported AlCl-Salen Catalysts for Conjugated Addition Reactions: Effect of Linker and Support Structure on Catalysis. Chem. - Eur. J. 2009, 15, 1186−1194. (103) Lin, L.; Yin, W.; Fu, X.; Zhang, J.; Ma, X.; Wang, R. Asymmetric Cyanation of Nitroalkenes Catalyzed by a Salen-titanium Catalyst. Org. Biomol. Chem. 2012, 10, 83−89. (104) North, M.; Watson, J. M. Asymmetric Addition of Cyanide to βNitroalkenes Catalysed by Chiral Salen Complexes of Titanium(IV) and Vanadium(V). ChemCatChem 2013, 5, 2405−2409. (105) Jakhar, A.; Sadhukhan, A.; Khan, N. H.; Saravanan, S.; Kureshy, R. I.; Abdi, S. H. R.; Bajaj, H. C. Asymmetric Hydrocyanation of Nitroolefins Catalyzed by an Aluminum(III) Salen Complex. ChemCatChem 2014, 6, 2656−2661. (106) Sibi, M. P.; Nad, S. Enantioselective Radical Reactions: Stereoselective Aldol Synthesis from Cyclic Ketones. Angew. Chem., Int. Ed. 2007, 46, 9231−9234.

(107) Zhao, C.; Sibi, M. P. Enantioselective and Diastereoselective Conjugate Radical Additions to α-Arylidene Ketones and Lactones. Synlett 2017, 28, 2971−2975. (108) Trost, B. M.; Jiang, C. Catalytic Enantioselective Construction of All-Carbon Quaternary Stereocenters. Synthesis 2006, 369−396. (109) Ooi, T.; Miki, T.; Taniguchi, M.; Shiraishi, M.; Takeuchi, M.; Maruoka, K. Highly Enantioselective Construction of Quaternary Stereocenters on β-Keto Esters by Phase-transfer Catalytic Asymmetric Alkylation and Michael Reaction. Angew. Chem., Int. Ed. 2003, 42, 3796−3798. (110) Park, E. J.; Kim, M. H.; Kim, D. Y. Enantioselective Alkylation of β-Keto Esters by Phase-Transfer Catalysis Using Chiral Quaternary Ammonium Salts. J. Org. Chem. 2004, 69, 6897−6899. (111) Doyle, A. G.; Jacobsen, E. N. Enantioselective Alkylations of Tributyltin Enolates Catalyzed by Cr(salen)Cl: Access to Enantiomerically Enriched All-Carbon Quaternary Centers. J. Am. Chem. Soc. 2005, 127, 62−63. (112) Guibez, F.; Xian, Y. T.; Zigna, A. N.; Balavoine, G. Regiospecifically Controlled Formation of Tetrasubstituted Tributyltin Ketone Enolates Catalyzed by a Palladium Complex. Tetrahedron Lett. 1985, 26, 3559−3562. (113) Doyle, A. G.; Jacobsen, E. N. Enantioselective Alkylation of Acyclic α,α-Disubstituted Tributyltin Enolates Catalyzed by a {Cr(salen)} Complex. Angew. Chem., Int. Ed. 2007, 46, 3701−3705. (114) Hutson, G. E.; Türkmen, Y. E.; Rawal, V. H. Salen Promoted Enantioselective Nazarov Cyclizations of Activated and Unactivated Dienones. J. Am. Chem. Soc. 2013, 135, 4988−4991. (115) Chen, B.-L.; Zhu, H.-W.; Xiao, Y.; Sun, Q.-L.; Wang, H.; Lu, J.X. Asymmetric Electrocarboxylation of 1-Phenylethyl Chloride Catalyzed by Electrogenerated Chiral [CoI(salen)]− Complex. Electrochem. Commun. 2014, 42, 55−59. (116) Katsuki, T.; Sharpless, K. B. The First Practical Method for Asymmetric Epoxidation. J. Am. Chem. Soc. 1980, 102, 5974−5976. (117) Lan, L. Metal-Catalyzed Asymmetric Epoxidation and its Related Reactions. Ph.D. Dissertation [Online], The University of Chicago, Chicago, IL, 2016. https://knowledge.uchicago.edu/handle/ 11417/173 (accessed April 17, 2018). (118) Jacobsen, E. N.; Zhang, W.; Muci, A. R.; Ecker, J. R.; Deng, L. Highly Enantioselective Epoxidation Catalysts Derived from 1,2Diaminocyclohexane. J. Am. Chem. Soc. 1991, 113, 7063−7064. (119) Brandes, B. D.; Jacobsen, E. N. Highly Enantioselective, Catalytic Epoxidation of Trisubstituted Olefins. J. Org. Chem. 1994, 59, 4378−4380. (120) Pozzi, G.; Cavazzini, M.; Cinato, F.; Montanari, F.; Quici, S. Enantioselective Catalysis in Fluorinated Media-Synthesis and Properties of Chiral Perfluoroalkylated (Salen)manganese Complexes. Eur. J. Org. Chem. 1999, 1999, 1947−1955. (121) Smith, K.; Liu, C.-H. Asymmetric Epoxidation Using a Singlybound Supported Katsuki-type (salen)Mn complex. Chem. Commun. 2002, 886−887. (122) Jeong, Y.-C.; Choi, S.; Yu, K.; Ahn, K.-H. Asymmetric Alkene Epoxidations Catalyzed by Sterically Hindered Salen-Mn(III) Complexes. Bull. Korean Chem. Soc. 2003, 24, 537−538. (123) Daly, A. M.; Renehan, M. F.; Gilheany, D. G. High Enantioselectivities in an (E)-Alkene Epoxidation by Catalytically Active Chromium Salen Complexes. Insight into the Catalytic Cycle. Org. Lett. 2001, 3, 663−666. (124) O’Mahony, C. P.; McGarrigle, E. M.; Renehan, M. F.; Ryan, K. M.; Kerrigan, N. J.; Bousquet, C.; Gilheany, D. G. Asymmetric Alkene Epoxidation with Chromium Oxo Salen Somplexes. A Systematic Study of Salen Ligand Substituents. Org. Lett. 2001, 3, 3435−3438. (125) Brandt, P.; Norrby, P. O.; Daly, A. M.; Gilheany, D. G. Chromium-salen-mediated Alkene Epoxidation: A Theoretical and Experimental Study Indicates the Importance of Spin-surface Crossing and the Presence of a Discrete Intermediate. Chem. - Eur. J. 2002, 8, 4299−4307. (126) Clarke, E. F.; McGarrigle, E. M.; Gilheany, D. G. Cr-Salen Mediated Asymmetric Epoxidation of Alkenes: Rational Complex Design and Substrate Scope of Catalyst. ARKIVOC 2005, 30−38. AP


Chemical Reviews

Review

(146) Omura, K.; Uchida, T.; Irie, R.; Katsuki, T. Design of a Robust Ru(salen) Complex: Aziridination with Improved Turnover Number Using N-Arylsulfonyl Azides as Precursors. Chem. Commun. 2004, 2060−2061. (147) Kawabata, H.; Omura, K.; Katsuki, T. Asymmetric Aziridination: A New Entry to Optically Active Non-N-protected Aziridines. Tetrahedron Lett. 2006, 47, 1571−1574. (148) Kawabata, H.; Omura, K.; Uchida, T.; Katsuki, T. Construction of Robust Ruthenium(salen)(CO) Complexes and Asymmetric Aziridination with Nitrene Precursors in the Form of Azide Compounds That Bear Easily Removable N-Sulfonyl Groups. Chem. Asian J. 2007, 2, 248−256. (149) Fukunaga, Y.; Uchida, T.; Ito, Y.; Matsumoto, K.; Katsuki, T. Ru(CO)-salen-Catalyzed Synthesis of Enantiopure Aziridinyl Ketones and Formal Asymmetric Synthesis of (+)-PD 128907. Org. Lett. 2012, 14, 4658−4661. (150) Kim, C.; Uchida, T.; Katsuki, T. Asymmetric Olefin Aziridination Using a Newly Designed Ru(CO)(salen) Complex as the Catalyst. Chem. Commun. 2012, 48, 7188−7190. (151) Bolm, C.; Schlingloff, G.; Weickhardt, K. Optically Active Lactones from a Baeyer-Villiger-Type Metal-Catalyzed Oxidation with Molecular Oxygen. Angew. Chem., Int. Ed. Engl. 1994, 33, 1848−1849. (152) Gusso, A.; Baccin, C.; Pinna, F.; Strukul, G. Platinum-Catalyzed Oxidations with Hydrogen Peroxide: Enantiospecific Baeyer-Villiger Oxidation of Cyclic Ketones. Organometallics 1994, 13, 3442−3451. (153) Colladon, M.; Scarso, A.; Strukul, G. Towards a Greener Epoxidation Method: Use of Water-Surfactant Media and Catalyst Recycling in the Platinum-Catalyzed Asymmetric Epoxidation of Terminal Alkenes with Hydrogen Peroxide. Adv. Synth. Catal. 2007, 349, 797−801. (154) Cavarzan, A.; Bianchini, G.; Sgarbossa, P.; Lefort, L.; Gladiali, S.; Scarso, A.; Strukul, G. Catalytic Asymmetric Baeyer-Villiger Oxidation in Water by Using PtII Catalysts and Hydrogen Peroxide: Supramolecular Control of Enantioselectivity. Chem. - Eur. J. 2009, 15, 7930−7939. (155) Uchida, T.; Katsuki, T. Cationic Co(III)(salen)-catalyzed enantioselective Baeyer−Villiger oxidation of 3-arylcyclobutanones using hydrogen peroxide as a terminal oxidant. Tetrahedron Lett. 2001, 42, 6911−6914. (156) Uchida, T.; Katsuki, T.; Ito, K.; Akashi, S.; Ishii, A.; Kuroda, T. New Asymmetric Catalysis by (Salen)cobalt(III) Complexes (Salen = [Bis(salicylidene)ethylenediaminato]={{2,2′-[ethane-1,2-diyl]bis[(nitrilo-κN)methylidyne]bis[phenolato-κO]}(2−)}) of cis-β-Structure: Enantioselective Baeyer-Villiger Oxidation of Prochiral Cyclobutanones. Helv. Chim. Acta 2002, 85, 3078−3089. (157) Watanabe, A.; Uchida, T.; Irie, R.; Katsuki, T. Zr[bis(salicylidene)ethylenediaminato]-mediated Baeyer−Villiger oxidation: Stereospecific synthesis of abnormal and normal lactones. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 5737−5742. (158) Bianchini, G.; Cavarzan, A.; Scarso, A.; Strukul, G. Asymmetric Baeyer-Villiger Oxidation with Co(Salen) and H2O2 in Water: Striking Supramolecular Micelles Effect on Catalysis. Green Chem. 2009, 11, 1517−1520. (159) Kureshy, R. I.; Prathap, K. J.; Agrawal, S.; Khan, N.-U. H.; Abdi, S. H. R.; Jasra, R. V. Highly Enantioselective Syntheses of Chiral βAmino Alcohols in the Presence of Chiral TiIV Schiff Base Complexes as Catalysts. Eur. J. Org. Chem. 2008, 2008, 3118−3128. (160) Kalow, J. A.; Doyle, A. G. Enantioselective Ring Opening of Epoxides by Fluoride Anion Promoted by a Cooperative Dual-Catalyst System. J. Am. Chem. Soc. 2010, 132, 3268−3269. (161) Kalow, J. A.; Doyle, A. G. Enantioselective Fluoride Ring Opening of Aziridines Enabled by Cooperative Lewis Acid Catalysis. Tetrahedron 2013, 69, 5702−5709. (162) Saha, B.; Lin, M.-H.; Rajanbabu, T. V. Exceptionally Active Yttrium-Salen Complexes for the Catalyzed Ring Opening of Epoxides by TMSCN and TMSN3. J. Org. Chem. 2007, 72, 8648−8655. (163) Wu, B.; Gallucci, J. C.; Parquette, J. R.; RajanBabu, T. V. Bimetallic Catalysis in the Highly Enantioselective Ring-opening Reactions of Aziridines. Chem. Sci. 2014, 5, 1102−1117.

(127) Takeda, T.; Irie, R.; Shinoda, Y.; Katsuki, T. Ru-Salen Catalyzed Asymmetric Epoxidation: Photoactivation of Catalytic Activity. Synlett 1999, 1999, 1157−1159. (128) Nakata, K.; Takeda, T.; Mihara, J.; Hamada, T.; Irie, R.; Katsuki, T. Asymmetric Epoxidation with a Photoactivated [Ru(salen)] Complex. Chem. - Eur. J. 2001, 7, 3776−3782. (129) Matsumoto, K.; Sawada, Y.; Saito, B.; Sakai, K.; Katsuki, T. Construction of Pseudo-heterochiral and Homochiral Di-μoxotitanium(Schiff base) Dimers and Enantioselective Epoxidation Using Aqueous Hydrogen Peroxide. Angew. Chem., Int. Ed. 2005, 44, 4935−4939. (130) Sawada, Y.; Matsumoto, K.; Katsuki, T. Titanium-Catalyzed Asymmetric Epoxidation of Non-Activated Olefins with Hydrogen Peroxide. Angew. Chem., Int. Ed. 2007, 46, 4559−4561. (131) Matsumoto, K.; Feng, C.; Handa, S.; Oguma, T.; Katsuki, T. Asymmetric Epoxidation of (Z)-Enol Esters Catalyzed by Titanium(salalen) Complex with Aqueous Hydrogen Peroxide. Tetrahedron 2011, 67, 6474−6478. (132) Tanaka, H.; Nishikawa, H.; Uchida, T.; Katsuki, T. Photopromoted Ru-Catalyzed Asymmetric Aerobic Sulfide Oxidation and Epoxidation Using Water as a Proton Transfer Mediator. J. Am. Chem. Soc. 2010, 132, 12034−12041. (133) Koya, S.; Nishioka, Y.; Mizoguchi, H.; Uchida, T.; Katsuki, T. Asymmetric Epoxidation of Conjugated Olefins with Dioxygen. Angew. Chem., Int. Ed. 2012, 51, 8243−8246. (134) Mahlau, M.; List, B. Asymmetric Counteranion-Directed Catalysis: Concept, Definition, and Applications. Angew. Chem., Int. Ed. 2013, 52, 518−533. (135) Liao, S.; List, B. Asymmetric Counteranion-Directed Transition-Metal Catalysis: Enantioselective Epoxidation of Alkenes with Manganese(III) Salen Phosphate Complexes. Angew. Chem., Int. Ed. 2010, 49, 628−631. (136) Reddeppa Reddy, D.; Thornton, E. R. A very mild, catalytic and versatile procedure for α-oxidation of ketone silyl enol ethers using (salen)manganese(III) complexes; a new, chiral complex giving asymmetric induction. A possible model for selective biochemical oxidative reactions through enol formation. J. Chem. Soc., Chem. Commun. 1992, 172−173. (137) Fukuda, T.; Katsuki, T. Mn-salen catalyzed asymmetric oxidation of enol derivatives. Tetrahedron Lett. 1996, 37, 4389−4392. (138) Adam, W.; Fell, R. T.; Stegmann, V. R.; Saha-Möller, C. R. Synthesis of Optically Active α-Hydroxy Carbonyl Compounds by the Catalytic, Enantioselective Oxidation of Silyl Enol Ethers and Ketene Acetals with (Salen)manganese(III) Complexes. J. Am. Chem. Soc. 1998, 120, 708−714. (139) Liang, J.-L.; Yu, X.-Q.; Che, C.-M. Amidation of Silyl Enol Ethers and Cholesteryl Acetates with Chiral Ruthenium(II) Schiff-base Catalysts: Catalytic and Enantioselective Studies. Chem. Commun. 2002, 124−125. (140) Kawatsura, M.; Hayashi, S.; Komatsu, Y.; Hayase, S.; Itoh, T. Enantioselective α-Fluorination and Chlorination of β-Ketoesters by Cobalt Catalyst. Chem. Lett. 2010, 39, 466−467. (141) O’Connor, K. J.; Wey, S. J.; Burrows, C. J. Alkene Aziridination and Epoxidation Catalyzed by Chiral Metal Salen Complexes. Tetrahedron Lett. 1992, 33, 1001−1004. (142) Noda, K.; Hosoya, N.; Irie, R.; Ito, Y.; Katsuki, T. Asymmetric Aziridination by Using Optically Active (Salen)manganese(III) Complexes. Synlett 1993, 1993, 469−471. (143) Nishikori, H.; Katsuki, T. Catalytic and Highly Enantioselective Aziridination of Styrene Derivatives. Tetrahedron Lett. 1996, 37, 9245− 9248. (144) Minakata, S.; Ando, T.; Nishimura, M.; Ryu, I.; Komatsu, M. Novel Asymmetric and Stereospecific Aziridination of Alkenes with a Chiral Nitridomanganese Complex. Angew. Chem., Int. Ed. 1998, 37, 3392−3394. (145) Omura, K.; Murakami, M.; Uchida, T.; Irie, R.; Katsuki, T. Enantioselective Aziridination and Amination Using p-Toluenesulfonyl Azide in the Presence of Ru(salen)(CO) Complex. Chem. Lett. 2003, 32, 354−355. AQ


Chemical Reviews

Review

(164) Larrow, J. F.; Schaus, S. E.; Jacobsen, E. N. Kinetic Resolution of Terminal Epoxides via Highly Regioselective and Enantioselective Ring Opening with TMSN3. An Efficient, Catalytic Route to 1,2-Amino Alcohols. J. Am. Chem. Soc. 1996, 118, 7420−7421. (165) Tokunaga, M.; Larrow, J. F.; Kakiuchi, F.; Jacobsen, E. N. Asymmetric Catalysis with Water: Efficient Kinetic Resolution of Terminal Epoxides by Means of Catalytic Hydrolysis. Science 1997, 277, 936−938. (166) Reddy, R. S.; Chouthaiwale, P. V.; Suryavanshi, G.; Chavana, V. B.; Sudalai, A. Co(III)(salen)-catalyzed HKR of Two Stereocentered Alkoxy- and Azido Epoxides: A Concise Enantioselective Synthesis of (S,S)-Eboxetine and (+)-Epi-cytoxazone. Chem. Commun. 2010, 46, 5012−5014. (167) Viera, I.; Manta, E.; González, L.; Mahler, G. Synthesis of Enantiomerically Enriched α,α-Disubstituted β,γ-Epoxy Esters Using Hydrolytic Kinetic Resolution Catalyzed by Salen-Co(III). Tetrahedron: Asymmetry 2010, 21, 631−635. (168) Kemper, S.; Hrobarik, P.; Kaupp, M.; Schlörer, N. E. Jacobsen’s Catalyst for Hydrolytic Kinetic Resolution: Structure Elucidation of Paramagnetic Co(III) Salen Complexes in Solution via Combined NMR and Quantum Chemical Studies. J. Am. Chem. Soc. 2009, 131, 4172−4173. (169) Kalow, J. A.; Doyle, A. G. Mechanistic Investigations of Cooperative Catalysis in the Enantioselective Fluorination of Epoxides. J. Am. Chem. Soc. 2011, 133, 16001−16012. (170) Sakakura, T.; Choi, J.-C.; Yasuda, H. Transformation of Carbon Dioxide. Chem. Rev. 2007, 107, 2365−2387. (171) Darensbourg, D. J.; Holtcamp, M. W. Catalysts for the reactions of epoxides and carbon dioxide. Coord. Chem. Rev. 1996, 153, 155−174. (172) Yan, P.; Jing, H. Catalytic Asymmetric Cycloaddition of Carbon Dioxide and Propylene Oxide Using Novel Chiral Polymers of BINOLSalen-Cobalt(III) Salts. Adv. Synth. Catal. 2009, 351, 1325−1332. (173) Nakano, K.; Hashimoto, S.; Nakamura, M.; Kamada, T.; Nozaki, K. Stereocomplex of Poly(propylene carbonate): Synthesis of Stereogradient Poly(propylene carbonate) by Regio- and Enantioselective Copolymerization of Propylene Oxide with Carbon Dioxide. Angew. Chem., Int. Ed. 2011, 50, 4868−4871. (174) Lu, X.-B.; Darensbourg, D. J. Cobalt Catalysts for the Coupling of CO2 and Epoxides to Provide Polycarbonates and Cyclic Carbonates. Chem. Soc. Rev. 2012, 41, 1462−1484. (175) Wu, G.-P.; Ren, W.-M.; Luo, Y.; Li, B.; Zhang, W.-Z.; Lu, X.-B. Enhanced Asymmetric Induction for the Copolymerization of CO2 and Cyclohexene Oxide with Unsymmetric Enantiopure SalenCo(III) Complexes: Synthesis of Crystalline CO2-Based Polycarbonate. J. Am. Chem. Soc. 2012, 134, 5682−5688. (176) Lu, X.-B.; Ren, W.-M.; Wu, G.-P. CO2 Copolymers from Epoxides: Catalyst Activity, Product Selectivity, and Stereochemistry Control. Acc. Chem. Res. 2012, 45, 1721−1735. (177) Ren, W.-M.; Zhang, W.- Z.; Lu, X.-B. Highly Regio- and Stereoselective Copolymerization of CO2 with Racemic Propylene Oxide Catalyzed by Unsymmetrical (S,S,S)-SalenCo(III) Complexes. Sci. China: Chem. 2010, 53, 1646−1652. (178) Hirahata, W.; Thomas, R. M.; Lobkovsky, E. B.; Coates, G. W. Enantioselective Polymerization of Epoxides: A Highly Active and Selective Catalyst for the Preparation of Stereoregular Polyethers and Enantiopure Epoxides. J. Am. Chem. Soc. 2008, 130, 17658−17659. (179) Loy, R. N.; Jacobsen, E. N. Enantioselective Intramolecular Openings of Oxetanes Catalyzed by (Salen)Co(III) Complexes: Access to Enantioenriched Tetrahydrofurans. J. Am. Chem. Soc. 2009, 131, 2786−2787. (180) Ito, K.; Tsutsumi, H.; Setoyama, M.; Saito, B.; Katsuki, T. Enantioselective Hydrophosphonylation of Aldehydes Using an Aluminum Binaphthyl Schiff Base Complex as a Catalyst. Synlett 2007, 2007, 1960−1962. (181) Saito, B.; Egami, H.; Katsuki, T. Synthesis of an Optically Active Al(salalen) Complex and Its Application to Catalytic Hydrophosphonylation of Aldehydes and Aldimines. J. Am. Chem. Soc. 2007, 129, 1978−1986.

(182) Suyama, K.; Sakai, Y.; Matsumoto, K.; Saito, B.; Katsuki, T. Highly Enantioselective Hydrophosphonylation of Aldehydes: BaseEnhanced Aluminum-salalen Catalysis. Angew. Chem., Int. Ed. 2010, 49, 797−799. (183) Myers, J. K.; Jacobsen, E. N. Asymmetric Synthesis of β-Amino Acid Derivatives via Catalytic Conjugate Addition of Hydrazoic Acid to Unsaturated Imides. J. Am. Chem. Soc. 1999, 121, 8959−8960. (184) Kobayashi, T.; Shimura, T.; Kurita, Y.; Katsumata, Y.; Kezuka, S. Chiral Cobalt(II)-salen-catalyzed Michael Addition of Amines to βSubstituted Nitroalkenes. Tetrahedron Lett. 2014, 55, 2818−2821. (185) Vanderwal, C. D.; Jacobsen, E. N. Enantioselective Formal Hydration of α,β-Unsaturated Imides by Al-Catalyzed Conjugate Addition of Oxime Nucleophiles. J. Am. Chem. Soc. 2004, 126, 14724− 14725. (186) White, J. D.; Shaw, S. Iron Catalyzed Enantioselective SulfaMichael Addition: A Four-step Synthesis of the Anti-asthma Agent Montelukast. Chem. Sci. 2014, 5, 2200−2204. (187) Shaw, S.; White, J. D. Regioselective and Enantioselective Addition of Sulfur Nucleophiles to Acyclic α,β,γ,δ-Unsaturated Dienones Catalyzed by an Iron(III)-salen Complex. Org. Lett. 2015, 17, 4564−4567. (188) Ning, Z.; Jin, R.; Ding, J.; Gao, L. Enantioselective Iodolactonizations of 4-Pentenoic Acid Derivatives Mediated by Chiral Salen-Co(II) Complex. Synlett 2009, 2009, 2291−2294. (189) Zhao-lun, N.; Jin-ying, D.; Ri-zhe, J.; Chuan-qing, K.; Yan-qin, C.; Lian-xun, G. Enantioselective Iodolactonizations of (E)- and (Z)-5Substituted-4-pentenoic Acid Derivatives Mediated by Chiral SalenCo(II) Complex. Chem. Res. Chin. Univ. 2011, 27, 45−48. (190) Kang, S. H.; Lee, S. B.; Park, C. M. Catalytic Enantioselective Iodocyclization of γ-Hydroxy-cis-alkenes. J. Am. Chem. Soc. 2003, 125, 15748−15749. (191) Kwon, H. Y.; Park, C. M.; Lee, S. B.; Youn, J. H.; Kang, S. H. Asymmetric Iodocyclization Catalyzed by Salen-CrIIICl: Its Synthetic Application to Swainsonine. Chem. - Eur. J. 2008, 14, 1023−1028. (192) Kohmura, Y.; Katsuki, T. Mn(salen)-catalyzed Enantioselective C-H Amination. Tetrahedron Lett. 2001, 42, 3339−3342. (193) Uchida, T.; Katsuki, T. Asymmetric Nitrene Transfer Reactions: Sulfimidation, Aziridination and C-H Amination Using Azide Compounds as Nitrene Precursors. Chem. Rec. 2014, 14, 117− 129. (194) Ichinose, M.; Suematsu, H.; Yasutomi, Y.; Nishioka, Y.; Uchida, T.; Katsuki, T. Enantioselective Intramolecular Benzylic C-H Bond Amination: Efficient Synthesis of Optically Active Benzosultams. Angew. Chem., Int. Ed. 2011, 50, 9884−9887. (195) Nishioka, N.; Uchida, T.; Katsuki, T. Enantio- and Regioselective Intermolecular Benzylic and Allylic C-H Bond Amination. Angew. Chem., Int. Ed. 2013, 52, 1739−1742. (196) Covell, D. J.; White, M. C. A Chiral Lewis Acid Strategy for Enantioselective Allylic C-H Oxidation. Angew. Chem., Int. Ed. 2008, 47, 6448−6451. (197) Liu, W.; Groves, J. T. Manganese-Catalyzed Oxidative Benzylic C-H Fluorination by Fluoride Ions. Angew. Chem., Int. Ed. 2013, 52, 6024−6027. (198) Liu, W.; Huang, X.; Cheng, M.-J.; Nielsen, R. J.; Goddard, W. A., III; Groves, J. T. Oxidative Aliphatic C-H Fluorination with Fluoride Ion Catalyzed by a Manganese Porphyrin. Science 2012, 337, 1322− 1325. (199) Belokon’, Y. N.; Hunt, J.; North, M. Asymmetric Catalysis of Carbon-carbon Bond Forming Reactions Using Amino acid-derived C1-Symmetrical Salen Ligands. Tetrahedron: Asymmetry 2008, 19, 2804−2815. (200) Kull, T.; Peters, R. Contact Ion Pair Directed Lewis Acid Catalysis: Asymmetric Synthesis of Trans-Configured β-Lactones. Angew. Chem., Int. Ed. 2008, 47, 5461−5464. (201) Meier, P.; Broghammer, F.; Latendorf, K.; Rauhut, G.; Peters, R. Cooperative Al(Salen)-Pyridinium Catalysts for the Asymmetric Synthesis of Trans-Configured β-Lactones by [2 + 2]-Cyclocondensation of Acylbromides and Aldehydes: Investigation of Pyridinium Substituent Effects. Molecules 2012, 17, 7121−7150. AR


Chemical Reviews

Review

(202) Chidara, S.; Lin, Y.-M. Reaction Rate Acceleration Enabled by Tethered Lewis Acid-Lewis Base Bifunctional Catalysis: A Catalytic, Enantioselective [2 + 2] Ketene Aldehyde Cycloaddition Reaction. Synlett 2009, 2009, 1675−1679. (203) Schaus, S. E.; Branalt, J.; Jacobsen, E. N. Asymmetric HeteroDiels-Alder Reactions Catalyzed by Chiral (Salen)Chromium(III) Complexes. J. Org. Chem. 1998, 63, 403−405. (204) Huang, Y.; Iwama, T.; Rawal, V. H. Highly Enantioselective Diels-Alder Reactions of 1-Amino-3-siloxy-dienes Catalyzed by Cr(III)-Salen Complexes. J. Am. Chem. Soc. 2000, 122, 7843−7844. (205) Chaładaj, W.; Kwiatkowski, P.; Jurczak, J. Improvement of the Reactivity and Selectivity of the Oxo-Diels-Alder Reaction by Steric Modification of the Salen-chromium Catalyst. Tetrahedron Lett. 2008, 49, 6810−6811. (206) Mellah, M.; Ansel, B.; Patureau, F.; Voituriez, A.; Schulz, E. Electropolymerized Cr-salen Complexes for the Heterogeneous Asymmetric Hetero-Diels-Alder Reaction. J. Mol. Catal. A: Chem. 2007, 272, 20−25. (207) Zulauf, A.; Mellah, M.; Guillot, R.; Schulz, E. ChromiumThiophene-salen-Based Polymers for Heterogeneous Asymmetric Hetero-Diels-Alder Reactions. Eur. J. Org. Chem. 2008, 2008, 2118− 2129. (208) Kitamura, M.; Miki, T.; Nakano, K.; Noyori, R. Conjugate Addition of Diorganozincs to α,β-Unsaturated Ketones Catalyzed by a Copper(I)-sulfonamide Combined System. Tetrahedron Lett. 1996, 37, 5141−5144. (209) Dong, Z.; Liu, X.; Feng, J.; Wang, M.; Lin, L.; Feng, X. Efficient Asymmetric Synthesis of 4H-Chromene Derivatives through a Tandem Michael Addition-Cyclization Reaction Catalyzed by a Salen-cobalt(II) Complex. Eur. J. Org. Chem. 2011, 2011, 137−142. (210) Chaładaj, W.; Kwiatkowski, P.; Jurczak, J. Enantioselective Glyoxylate-ene Reactions Catalysed by (Salen)chromium(III) Complexes. Tetrahedron Lett. 2007, 48, 2405−2408. (211) Hutson, G. E.; Dave, A. H.; Rawal, V. H. Highly Enantioselective Carbonyl-ene Reactions Catalyzed by a Hindered Silyl-Salen-Cobalt Complex. Org. Lett. 2007, 9, 3869−3872. (212) Suematsu, H.; Katsuki, T. Iridium(III) Catalyzed Diastereoand Enantioselective C-H Bond Functionalization. J. Am. Chem. Soc. 2009, 131, 14218−14219. (213) Srour, H.; Maux, P. L.; Chevance, S.; Simonneaux, G. Metalcatalyzed Asymmetric Sulfoxidation, Epoxidation and Hydroxylation by Hydrogen Peroxide. Coord. Chem. Rev. 2013, 257, 3030−3050. (214) Nakajima, K.; Kojima, M.; Fujita, J. Asymmetric Oxidation of Sulfides to Sulfoxides by Organic Hydroperoxides with Optically Active Schiff Base-Oxovanadium(IV) Catalysts. Chem. Lett. 1986, 15, 1483− 1486. (215) Palucki, M.; Hanson, P.; Jacobsen, E. N. Asymmetric Oxidation of Sulfides with H2O2 Catalyzed by (salen)Mn(III) Complexes. Tetrahedron Lett. 1992, 33, 7111−7114. (216) Saito, B.; Katsuki, T. Ti(salen)-catalyzed Enantioselective Sulfoxidation Using Hydrogen Peroxide as a Terminal Oxidant. Tetrahedron Lett. 2001, 42, 3873−3876. (217) Saito, B.; Katsuki, T. Mechanistic Consideration of Ti(salen)catalyzed Asymmetric Sulfoxidation. Tetrahedron Lett. 2001, 42, 8333− 8336. (218) Xuan, W.; Ye, C.; Zhang, M.; Chen, Z.; Cui, Y. A Chiral Porous Metallosalan-organic Framework Containing Titanium-oxo Clusters for Enantioselective Catalytic Sulfoxidation. Chem. Sci. 2013, 4, 3154− 3159. (219) Zhu, C.; Chen, X.; Yang, Z.; Du, X.; Liu, Y.; Cui, Y. Chiral Microporous Ti(salan)-based Metal-organic Frameworks for Asymmetric Sulfoxidation. Chem. Commun. 2013, 49, 7120−7122. (220) Bryliakov, K. P. Titanium Catalyzed Enantioselective Oxidation of Thioethers with Hydrogen Peroxide. Mini-Rev. Org. Chem. 2014, 11, 87−96. (221) Chen, Y.; Tan, R.; Zhang, Y.; Zhao, G.; Yin, D. Dendritic Chiral Salen Titanium(IV) Catalysts Enforce the Cooperative Catalysis of Asymmetric Sulfoxidation. ChemCatChem 2015, 7, 4066−4075.

(222) Talsi, E. P.; Rybalova, T. V.; Bryliakov, K. P. Isoinversion Behavior in the Enantioselective Oxidations of Pyridylmethylthiobenzimidazoles to Chiral Proton Pump Inhibitors on Titanium Salalen Complexes. ACS Catal. 2015, 5, 4673−4679. (223) Tanaka, H.; Nishikawa, H.; Uchida, T.; Katsuki, T. Photopromoted Ru-Catalyzed Asymmetric Aerobic Sulfide Oxidation and Epoxidation Using Water as a Proton Transfer Mediator. J. Am. Chem. Soc. 2010, 132, 12034−12041. (224) Saito, B.; Katsuki, T. Ti(salen)-catalyzed Enantioselective Sulfoxidation Using Hydrogen Peroxide as a Terminal Oxidant. Tetrahedron Lett. 2001, 42, 3873−3876. (225) Inoue, M.; Shinohara, N.; Tanabe, S.; Takahashi, T.; Okura, K.; Itoh, H.; Mizoguchi, Y.; Iida, M.; Lee, N.; Matsuoka, S. Total synthesis of the large non-ribosomal peptide polytheonamide B. Nat. Chem. 2010, 2, 280−285. (226) Choi, J. Y.; Hwang, G.-S.; Senapati, B. K.; Ryu, D. H. Efficient Asymmetric Sulfoxidation of Prochiral Sulfides Catalyzed by Chiral Salen-Mn(III) Complexes. Bull. Korean Chem. Soc. 2008, 29, 1879− 1880. (227) Pordea, A.; Mathis, D.; Ward, T. R. Incorporation of Biotinylated Manganese-salen Complexes into Streptavidin: New Artificial Metalloenzymes for Enantioselective Sulfoxidation. J. Organomet. Chem. 2009, 694, 930−936. (228) Bryliakov, K. P.; Talsi, E. P. Iron-Catalyzed Oxidation of Thioethers by Iodosylarenes: Stereoselectivity and Reaction Mechanism. Chem. - Eur. J. 2007, 13, 8045−8050. (229) Liao, S.; List, B. Asymmetric Counteranion-Directed Iron Catalysis: A Highly Enantioselective Sulfoxidation. Adv. Synth. Catal. 2012, 354, 2363−2367. (230) Cram, D. J.; Day, J.; Rayner, D. R.; von Schriltz, D. M.; Duchamp, D. J.; Garwood, D. C. Stereochemistry of Sulfur Compounds. I. Stereochemical Reaction Cycles Involving An Open Chain Sulfoxide, Sulfimide, and Sulfoximide. J. Am. Chem. Soc. 1970, 92, 7369−7384. (231) Annunziata, R.; Cinquini, M.; Colonna, S.; Cozzi, F. Synthesis of Optically Active Sulphilimines via Chiral Discrimination. J. Chem. Soc., Perkin Trans. 1 1981, 1, 3118−3119. (232) Nishikori, H.; Ohta, C.; Oberlin, E.; Irie, R.; Katsuki, T. Mnsalen Catalyzed Nitrene Transfer Reaction: Enantioselective Sulfimidation of Alkyl Aryl Sulfides. Tetrahedron 1999, 55, 13937−13946. (233) Ohta, C.; Katsuki, T. Mn(salen)-catalyzed Sulfimidation: What Are the Real Active Species in Sulfimidation? Tetrahedron Lett. 2001, 42, 3885−3888. (234) Murakami, M.; Uchida, T.; Katsuki, T. Ru(salen)-catalyzed Asymmetric Sulfimidation Using Arylsulfonyl Azide. Tetrahedron Lett. 2001, 42, 7071−7074. (235) Murakami, M.; Uchida, T.; Saito, B.; Katsuki, T. Ru(salen)catalyzed Asymmetric Sulfimidation and Subsequent [2,3]-Sigmatropic Rearrangement. Chirality 2003, 15, 116−123. (236) Tamura, Y.; Uchida, T.; Katsuki, T. Highly Enantioselective (OC)Ru(salen)-Catalyzed Sulfimidation Using N-Alkoxycarbonyl Azide as Nitrene Precursor. Tetrahedron Lett. 2003, 44, 3301−3303. (237) Uchida, T.; Tamura, Y.; Ohba, M.; Katsuki, T. Mechanism of Asymmetric Sulfimidation with N-Alkoxycarbonyl Azide in the Presence of (OC)Ru(salen) Complex. Tetrahedron Lett. 2003, 44, 7965−7968. (238) Fujita, H.; Uchida, T.; Irie, R.; Katsuki, T. Asymmetric Sulfimidation with cis-Ru(salalen)(CO)2 Complexes as Catalyst. Chem. Lett. 2007, 36, 1092−1093. (239) Yasutomi, Y.; Suematsu, H.; Katsuki, T. Iridium(III)-Catalyzed Enantioselective Si-H Bond Insertion and Formation of an Enantioenriched Silicon Center. J. Am. Chem. Soc. 2010, 132, 4510− 4511. (240) Cheng, Q.; Deng, F.; Xia, C.; Sun, W. The Mn(Salen)-catalyzed Oxidative Kinetic Resolution of Secondary Alcohols: Reaction Development and Scope. Tetrahedron: Asymmetry 2008, 19, 2359− 2362. (241) Brown, M. K.; Blewett, M. M.; Colombe, J. R.; Corey, E. J. Mechanism of the Enantioselective Oxidation of Racemic Secondary AS


Chemical Reviews

Review

Alcohols Catalyzed by Chiral Mn(III)-salen Complexes. J. Am. Chem. Soc. 2010, 132, 11165. (242) Xu, D.; Wang, S.; Shen, Z.; Xia, C.; Sun, W. Enantioselective Oxidation of Racemic Secondary Alcohols Catalyzed by Chiral Mn(III)-salen Complexes with N-bromosuccinimide as a Powerful Oxidant. Org. Biomol. Chem. 2012, 10, 2730−2732. (243) Zhang, Y.; Gao, B.; Zhou, Q.; Zhao, J. Oxidative Kinetic Resolution of Secondary Alcohols with Salen-Mn(III)/NBS/NaClO System. Catal. Lett. 2014, 144, 1797−1802. (244) Zhang, Y.; Zhou, Q.; Ma, W.; Zhao, J. Enantioselective Oxidation of Racemic Secondary Alcohols Catalyzed by Chiral Mn(III)-salen Complex with Sodium Hypochlorite as Oxidant. Catal. Commun. 2014, 45, 114−117. (245) Purude, A. N.; Pawar, K. P.; Patil, N. B.; Kalkote, U. R.; Chavan, S. P. Total Synthesis of (R)-Lipoic Acid and (S)-Lipoic Acid via An Mn (III)-salen-catalyzed Oxidative Kinetic Resolution. Tetrahedron: Asymmetry 2015, 26, 281−287. (246) Nakamura, Y.; Egami, H.; Matsumoto, K.; Uchida, T.; Katsuki, T. Aerobic oxidative kinetic resolution of racemic alcohols with bidentate ligand-binding Ru(salen) complex as catalyst. Tetrahedron 2007, 63, 6383−6387. (247) Mizoguchi, H.; Uchida, T.; Katsuki, T. Ruthenium-Catalyzed Oxidative Kinetic Resolution of Unactivated and Activated Secondary Alcohols with Air as the Hydrogen Acceptor at Room Temperature. Angew. Chem., Int. Ed. 2014, 53, 3178−3182. (248) Muthupandi, P.; Alamsetti Santosh, K.; Sekar, G. Chiral Iron Complex Catalyzed Enantioselective Oxidation of Racemic Benzoins. Chem. Commun. 2009, 3288−3290. (249) Alamsetti, S. K.; Sekar, G. Chiral Cobalt-catalyzed Enantioselective Aerobic Oxidation of α-Hydroxy Esters. Chem. Commun. 2011, 46, 7235−7237. (250) Reeve, T. B.; Cros, J.-P.; Gennari, C.; Piarulli, U.; de Vries, J. G. A Practical Approach to the Resolution of Racemic N-Benzyl α-Amino Acids by Liquid-Liquid Extraction with a Lipophilic Chiral SalenCobalt(III) Complex. Angew. Chem., Int. Ed. 2006, 45, 2449−2453. (251) Dzygiel, P.; Reeve, T. B.; Piarulli, U.; Krupicka, M.; Tvaroska, I.; Gennari, C. Resolution of Racemic N-Benzyl α-Amino Acids by LiquidLiquid Extraction: A Practical Method Using a Lipophilic Chiral Cobalt(III) Salen Complex and Mechanistic Studies. Eur. J. Org. Chem. 2008, 2008, 1253−1264. (252) Dzygiel, P.; Monti, C.; Piarulli, U.; Gennari, C. Efficient Resolution of Racemic N-benzyl β-Amino Acids by Iterative Liquidliquid Extraction with a Chiral (Salen)cobalt(III) Complex as Enantioselective Selector. Org. Biomol. Chem. 2007, 5, 3464−3471. (253) Chen, Z.; Morimoto, H.; Matsunaga, S.; Shibasaki, M. A Benchstable Homodinuclear Ni2-Schiff Base Complex for Catalytic Asymmetric Synthesis of α-Tetrasubstituted anti-α,β-Diamino Acid Surrogates. J. Am. Chem. Soc. 2008, 130, 2170−2171. (254) Mouri, S.; Chen, Z.; Mitsunuma, H.; Furutachi, M.; Matsunaga, S.; Shibasaki, M. Catalytic Asymmetric Synthesis of 3-Aminooxindoles: Enantiofacial Selectivity Switch in Bimetallic vs Monometallic Schiff Base Catalysis. J. Am. Chem. Soc. 2010, 132, 1255−1257. (255) Chen, Z.; Furutachi, M.; Kato, Y.; Matsunaga, S.; Shibasaki, M. A Stable Homodinuclear Biscobalt(III)-Schiff Base Complex for Catalytic Asymmetric 1,4-Addition Reactions of β-Keto Esters to Alkynones. Angew. Chem., Int. Ed. 2009, 48, 2218−2220. (256) Furutachi, M.; Chen, Z.; Matsunaga, S.; Shibasaki, M. Catalytic Asymmetric 1,4-Additions of β-Keto Esters to Nitroalkenes Promoted by a Bifunctional Homobimetallic Co2-Schiff Base Complex. Molecules 2010, 15, 532−544. (257) Handa, S.; Gnanadesikan, V.; Matsunaga, S.; Shibasaki, M. synSelective Catalytic Asymmetric Nitro-Mannich Reactions using a Heterobimetallic Cu-Sm-Schiff Base Complex. J. Am. Chem. Soc. 2007, 129, 4900−4901. (258) Handa, S.; Nagawa, K.; Sohtome, Y.; Matsunaga, S.; Shibasaki, M. A Heterobimetallic Pd/La/Schiff Base Complex for anti-Selective Catalytic Asymmetric Nitroaldol Reactions and Applications to Short Syntheses of β-Adrenoceptor Agonists. Angew. Chem., Int. Ed. 2008, 47, 3230−3233.

(259) Shibasaki, M.; Kanai, M.; Matsunaga, S.; Kumagai, N. Recent Progress in Asymmetric Bifunctional Catalysis Using Multimetallic Systems. Acc. Chem. Res. 2009, 42, 1117−1127. (260) Mihara, H.; Xu, Y.; Shepherd, N. E.; Matsunaga, S.; Shibasaki, M. A Heterobimetallic Ga/Yb-Schiff base Complex for Catalytic Asymmetric α-Addition of Isocyanides to Aldehydes. J. Am. Chem. Soc. 2009, 131, 8384−8385. (261) Sun, J.; Yuan, F.; Yang, M.; Pan, Y.; Zhu, C. Enantioselective Ring-opening Reaction of Meso-epoxides with ArSH Catalyzed by Heterobimetallic Ti-Ga-Salen System. Tetrahedron Lett. 2009, 50, 548−551. (262) Sun, J.; Yang, M.; Yuan, F.; Jia, X.; Yang, X.; Pan, Y.; Zhu, C. Catalytic Asymmetric Ring-Opening Reaction of Meso-epoxides with Aryl Selenols and Thiols Catalyzed by a Heterobimetallic GalliumTitanium-salen Complex. Adv. Synth. Catal. 2009, 351, 920−930. (263) Clarke, R. M.; Storr, T. The Chemistry and Applications of Multimetallic Salen Complexes. Dalton Trans. 2014, 43, 9380−9391. (264) Abdellah, I.; Martini, C.; Santos, A. D.; Dragoe, D.; Guérineau, V.; Huc, V.; Schulz, E. Calix[8]arene as new platform for Cobalt-Salen complexes immobilization and use in hydrolytic kinetic resolution of epoxides. ChemCatChem 2018, 10, 4761−4767. (265) Halder, M.; Bhanja, P.; Islam, M. M.; Bhaumik, A.; Islam, S. M. Chiral copper-salen complex grafted over functionalized mesoporous silica as an efficient catalyst for the asymmetric Henry reactions and synthesis of potent drug (R)-isoproterenol. New J. Chem. 2018, 42, 11896−11904. (266) Sun, Z.; Chen, J.; Liu, Y.; Tu, T. Chiral Titanium Coordination Assemblies: Robust Cooperative Self-Supported Catalysts for Asymmetric Ring Opening of meso-Epoxides with Aliphatic Amines. Adv. Synth. Catal. 2017, 359, 494−505. (267) Tak, R.; Kumar, M.; Menapara, T.; Choudhary, M. K.; Kureshy, R. I.; Khan, N. H. Asymmetric Catalytic Syntheses of Pharmaceutically Important β-Amino- α-Hydroxyl Esters via Enantioselective Aminolysis of Methyl Phenylglycidate. ChemCatChem 2017, 9, 322−328. (268) Tak, R.; Kumar, M.; Menapara, T.; Gupta, N.; Kureshy, R. I.; Khan, N. H.; Suresh, E. Asymmetric Hydrolytic and Aminolytic Kinetic Resolution of Racemic Epoxides using Recyclable Macrocyclic Chiral Cobalt (III) Salen Complexes. Adv. Synth. Catal. 2017, 359, 3990− 4001.

AT


Asymmetric Catalysis Using Chiral SalenâMetal Complexes: Recent

Recommend Documents

Asymmetric Catalysis Using Chiral SalenâMetal Complexes: Recent