Oxidative Addition and Reductive Elimination at Main-Group Element

Review pubs.acs.org/CR

Cite This: Chem. Rev. 2018, 118, 3608−3680

Oxidative Addition and Reductive Elimination at Main-Group Element Centers Terry Chu and Georgii I. Nikonov* Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario L2S 3A1, Canada ABSTRACT: Oxidative addition and reductive elimination are key steps in a wide variety of catalytic reactions mediated by transition-metal complexes. Historically, this reactivity has been considered to be the exclusive domain of d-block elements. However, this paradigm has changed in recent years with the demonstration of transition-metallike reactivity by main-group compounds. This Review highlights the substantial progress achieved in the past decade for the activation of robust single bonds by maingroup compounds and the more recently realized activation of multiple bonds by these elements. We also discuss the significant discovery of reversible activation of single bonds and distinct examples of reductive elimination at main-group element centers. The review consists of three major parts, starting with oxidative addition of single bonds, proceeding to cleavage of multiple bonds, and culminated by the discussion of reversible bond activation and reductive elimination. Within each subsection, the discussion is arranged according to the type of bond being cleaved or formed and considers elements from the left to the right of each period and down each group of the periodic table. The majority of results discussed in this Review come from the past decade; however, earlier reports are also included to ensure completeness.

CONTENTS 1. Introduction 2. Cleavage of σ Bonds 2.1. H−H Bond Activation 2.1.1. Group 2 Compounds 2.1.2. Group 13 Compounds 2.1.3. Group 14 Compounds 2.2. B−H and Al−H Bond Activation 2.3. C−H Bond Activation 2.3.1. Group 13 Compounds 2.3.2. Group 14 Compounds 2.3.3. Group 15 Compounds 2.4. Si−H, Ge−H, and Sn−H Bond Activation 2.4.1. Group 13 Compounds 2.4.2. Group 14 Compounds 2.5. N−H Bond Activation 2.5.1. Group 13 Compounds 2.5.2. Group 14 Compounds 2.5.3. Group 15 Compounds 2.6. P−H and As−H Bond Activation 2.7. O−H and S−H Bond Activation 2.7.1. Group 13 Compounds 2.7.2. Group 14 Compounds 2.7.3. Group 15 Compounds 2.8. C−F Bond Activation 2.8.1. Group 2 and 13 Compounds 2.8.2. Group 14 Compounds 2.9. C−X (X = Cl, Br, I) Bond Activation 2.10. C−O and C−S Bond Activation 2.11. C−E Bond Activation 2.12. E−E Bond Activation © 2018 American Chemical Society

2.12.1. Tetrel−Tetrel Bond Activation 2.12.2. Pnictogen−Pnictogen Bond Activation 2.12.3. Chalcogen−Chalcogen Bond Activation 2.13. Other Bond Activations 3. Cleavage of CX and PX Double Bonds 4. Reversible Activation and Reductive Elimination of σ Bonds 4.1. Reversible Activation by Group 13 Compounds 4.2. Reversible Activation by Group 14 Compounds 4.3. Reversible Activation by Group 15 Compounds 4.4. Reductive Elimination from Group 12 Compounds 4.5. Reductive Elimination from Group 13 Compounds 4.6. Reductive Elimination from Group 14 Compounds 4.7. Reductive Elimination from Group 15 Compounds 5. Conclusions and Outlook Author Information Corresponding Author ORCID Notes Biographies Acknowledgments

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Received: September 19, 2017 Published: March 20, 2018 3608

DOI: 10.1021/acs.chemrev.7b00572 Chem. Rev. 2018, 118, 3608−3680

Chemical Reviews Dedication References

Review

Power and co-workers in 2005. 13 Since that seminal communication, the activation of H2 has been performed by a variety of main-group element compounds spanning the periodic table. 2.1.1. Group 2 Compounds. Currently there are no examples of Group 2 compounds in the zero oxidation state available for the activation of hydrogen at a single Group 2 atom. However, the accessibility of dimeric Mg(I) species, pioneered by Jones and co-workers,32 allowed for the cleavage of robust bonds via the magnesium centers (vide inf ra). DFT calculations revealed that addition of dihydrogen to a guanidinate supported magnesium(I) dimer to give the corresponding hydride bridged dimer is thermodynamically feasible (ΔH ∼ 24 kcal mol−1).33 However, attempts at performing this reaction with βdiketiminate and guanidinate supported magnesium dimers failed even when the reaction mixtures were heated to 80 °C under 5 atm of H2 or upon irradiation with ultraviolet light (λ = 254 nm).32,34 This lack of reactivity clearly indicates a kinetic hurdle which deserves further theoretical investigation to set guidelines for the design of more reactive systems. 2.1.2. Group 13 Compounds. The facile reaction of hydrogen with the highly Lewis acidic, antiaromatic perfluoropentaphenylborole (1), in the solution and the solid state, was described by Piers and co-workers (Scheme 1).35 The resulting

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1. INTRODUCTION Cleavage of strong σ-bonds, e.g. H−H and C−F, is the key event in many important catalytic processes. For many decades, activation of such bonds has been considered to be the exclusive domain of transition metals. This is attributed to the presence of closely spaced and partially occupied d-orbitals that allows for access to several stable oxidation states by the transition metals. However, the past decade has witnessed an impressive development in the chemistry of main-group compounds in unusually low oxidation states, such as B(I),1 Al(I),2 Si(0),3,4 and Ge(0).5−7 These species have been realized in the form of carbenoids, heavy analogues of alkenes and alkynes, and donorstabilized (e.g., carbene, phosphine, imine) atoms.8−12 These highly reactive compounds have been shown to possess highenergy (pseudo) lone pairs and available empty (usually πantibonding) orbitals, thus mimicking the electronic situation typically observed in transition-metal compounds. In 2005, Power et al. disclosed the reaction of H2 with a digermaalkyne under mild conditions to give a mixture of hydrogenated digermanes and germanes.13 This constituted the first example of the uncatalyzed addition of dihydrogen to a main-group molecule. This breakthrough inspired a series of studies on the activation of small molecules, such as amines, silanes, and boranes, with Group 13 to 15 carbenoids, heavy main-group analogues of olefins and alkynes, and related low oxidation state compounds. It was realized early on that these addition reactions bear a strong resemblance to the corresponding bond activation processes on transition metals which led to the idea of catalysis mediated by main-group element compounds. As such, the first examples of “metal-free” catalytic reactions followed and became an area of intensive research.14−16 In this Review we offer an overview of the oxidative addition of strong σ-bonds on main-group compounds and extension to the cleavage of multiple bonds. The first examples of the reverse reaction, reductive elimination, are also presented and discussed. Heterolytic activation of σ bonds by zwitterionic main-group compounds is largely excluded from the discussion, unless there is relevance to corresponding bond activation by low oxidation state systems.17 As well, heterolytic bond cleavage by Frustrated Lewis Pairs is omitted from this review as this chemistry has been covered in depth in recent reviews.18−21

Scheme 1. Activation of H2 by Perfluoropentaphenylborole 1

cis- and trans- isomers of boracyclopent-3-ene (2) are formed via the addition of hydrogen to the carbon atoms α to boron in 1. Kinetic studies and DFT calculations were performed to probe the mechanism that started with 1,2-addition of dihydrogen across the B−Cα bond of 1 followed by 1,2-hydride migration to give cis-2.36 The trans isomer of 2 was produced via a kinetically competitive ring opening, during which rotation about the B−Cα bond was unrestricted, followed by rapid cyclization and 1,2hydride migration. Bertrand and Stephan reported the preparation of a cyclic alkyl amino carbene (cAAC) aminoborylene adduct 3 in 2014 featuring a linear allenic geometry as evidenced by the crystal structure determined by X-ray diffraction analysis.37 Compound 3 readily activated dihydrogen to give the cAAC-borane adduct 4′ which was unstable and subsequently isomerized, via a 1,2hydride migration, to the final product 4 (Scheme 2). The twostep process was shown by calculations to be highly exergonic at ΔG = −18 and −12 kcal mol−1 for each step. Braunschweig and co-workers disclosed the facile activation of dihydrogen by the cAAC-supported diboracumulene 5 at room temperature to furnish the 1,2-dihydrodiborene 6 (Scheme 3).38 Diboryne 7 was also shown to activate H2; however, the reaction required heating at 80 °C for 2 days to give 8. Interestingly, no reaction with dihydrogen was observed with the bulkier derivative of 7 with isopropyl groups on the aromatic ring or the analogue supported by unsaturated N-heterocyclic carbenes, highlighting the subtle interplay of electronic and steric factors in the reaction. DFT calculations were carried out to elucidate the mechanism of H2 addition to 5. The overall reaction was found to be exothermic by 20.6 kcal mol−1 and began with very weak

2. CLEAVAGE OF σ BONDS In this section we describe the cleavage of σ-bonds by maingroup compounds, starting with the H−E bonds, proceeding to C−X bonds (X = halogen, oxygen, or sulfur), and finally concluding with addition of E−E bonds. 2.1. H−H Bond Activation

Dihydrogen activation by transition-metal complexes has been studied extensively for many decades.22,23 In contrast, the activation of H2 with main-group elements and their compounds had been underexplored until recently. Historically, there have been scattered reports of the reactions of dihydrogen with heavier Group 13 elements in the vapor phase, typically upon photoactivation, with the resulting products trapped in a frozen matrix.24−31 The activation of dihydrogen has only been demonstrated with main-group element compounds in solution and in the solid state within the past decade, first reported by 3609


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Scheme 2. H2 Activation by cAAC-aminoborylene Adduct 3

Scheme 3. H−H Bond Activation by Diboracumulene 5 and Diboryne 7

Scheme 5. Cleavage of H2 by Aluminum Compounds 11 and 13

binding of dihydrogen to one of the boron centers in 5. The hydrogen bond is subsequently cleaved to yield a hydridebridged intermediate that rearranges to the final product 6. Tetra(o-tolyl)diborane 9, prepared by Lin and Yamashita, reacted directly with hydrogen at room temperature to yield di(otolyl)hydroborane 10 (Scheme 4).39 The mechanism, inter-

and Meyer bond orders. The authors concluded that activation of dihydrogen proceeds as an asynchronous, one step reaction, encompassing the following events: (1) polarization of the H−H bond by a nucleophilic attack of aluminum on the H−H bond, (2) development of an Al−H bond near the transition state region, and (3) hydride transfer to aluminum. Correspondingly, the coordination of H−H to aluminum in the transition state is asymmetric, with the Al−H distances being 1.94 and 1.59 Å. A similar geometry in the transition state was obtained by Schoeller and Cao, who also favored a concerted mechanism with a heterolytic, asynchronous character. Later, Tokitoh et al. reported the cleavage of hydrogen with the barrelene-type dialumane 13 to the dihydroalumane dimer 15.44 The authors proposed that 13 is a masked form of 1,2diaryldialumene (14) and undergoes a retro [2 + 4] cycloaddition at room temperature to generate 14 in situ, which subsequently reacts with H2 to produce 15, as shown in Scheme 5. The activation of the H−H bond by aluminum is not just an academic curiosity but may have some practical implications. Due to their high energy density, aluminum hydrides are being intensively studied as potential hydrogen carriers for use in a future hydrogen economy.45,46 To make this application practical, one needs materials that both easily release hydrogen on demand and can be recharged, i.e. add hydrogen. In this context, learning the fundamentals of H−H bond addition to aluminum is important for the rational design of new hydrogen carriers. Dihydrogen activation by gallium-containing compounds has also been realized. Reaction of H2 with the digallene species ArDippGaGaArDipp (16, ArDipp = 2,6-(2,6-iPr2C6H3)2C6H3), which partially dissociates in solution to the monomeric

Scheme 4. Reaction of Dihydrogen with Tetra(otolyl)diborane 9

rogated via DFT calculations, starts with coordination of the H− H bond to a vacant p-orbital on one of the boron atoms. Concomitant cleavage of the B−B bond and proton migration result in the formation of a dimeric hydroborane intermediate with a hydride and o-tolyl group as bridging ligands. Finally, dissociation of the dimer followed by dimerization of the monomeric hydroborane gave 10 as the final product. Nikonov and co-workers demonstrated the activation of dihydrogen at elevated temperatures by the monomeric aluminum(I) complex DippNacNacAl: (11; DippNacNac = [DippNC(Me)CHC(Me)NDipp]−, Dipp = 2,6-iPr2C6H3) to furnish the known dihydride DippNacNacAlH2 (12, Scheme 5).40 Since the initial report, the reaction of 11 with H2 has been studied by DFT calculations by several groups.41−43 Toro-Labbé and co-workers analyzed the mechanism of H2 addition through the use of reaction force, reaction electron flux, dual descriptor, 3610


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ArDippGa:, yielded a dimeric gallium hydride derivative, [ArDippGa(μ-H)H)]2 (17), with two bridging and two terminal hydrogen atoms (Scheme 6).47 DFT calculations were

calculations, which showed that barriers for insertions into H2 and CH4 are essentially zero. Although typical Arduengo-type Nheterocyclic carbenes (NHC), with either saturated or unsaturated rings, are unreactive toward H2,52 both the cyclic alkyl amino carbene 22 and the acyclic alkyl amino carbene 24 cleaved the H−H bond at 35 °C to furnish the alkane products 23 and 25, respectively (Scheme 8).53 The difference in reactivity

Scheme 6. Dihydrogen Activation by Digallene 16

Scheme 8. Activation of H2 by Cyclic (22) and Acyclic (24) Alkyl Amino Carbenes

performed to determine whether the dimeric or monomeric species is activating dihydrogen since both are present in solution.48 The activation energy for the reaction of ArDippGa: with H2 was calculated to be approximately 50 kcal mol−1, effectively ruling out the digallene dissociation pathway due to the significant energy barrier. Therefore, it was revealed that the mechanism started with the addition of one equivalent of H2 to 16 (ΔG‡ = 23 kcal mol−1) to give the 1,2-dihydride intermediate, ArDippHGa−GaHArDipp. A second equivalent of H2 then reacted with ArDippHGa−GaHArDipp, which led to cleavage of the Ga−Ga bond and production of 2 equiv of ArDippGaH2, which subsequently dimerized to form 17. The gallium(I) β-diketiminate compound DippNacNacGa (18) oxidatively added H2 to give the gallium dihydride DippNacNacGaH2 (19).49 The closely related zwitterionic gallium(III) species DippNacNac′GatBu (20; DippNacNac′ = [DippNC( CH2)CHC(Me)NDipp]2−), prepared via dehydrohalogenation of DippNacNacGaBr(tBu) with K[CH(SiMe3)2], similarly reacted with dihydrogen.50 The HOMO of 20 resides primarily on the terminal carbon of the exocyclic alkene moiety while the LUMO is dominated by the Ga 4p character as revealed by DFT calculations. Thus, the reaction of 20 with H2 gave the 1,4addition product DippNacNacGaH(tBu) (21), consistent with the calculations (Scheme 7).

between the two types of carbenes, as revealed by theoretical studies, is correlated with their singlet−triplet energy gap, calculated to be 46 kcal mol−1 for cAACs versus 68 kcal mol−1 for NHCs, resulting in the activation energy for insertion being approximately 10 kcal mol−1 lower for the former. In addition, alkyl amino carbenes 22 and 24 have a higher lying HOMO than Arduengo-type NHCs and therefore can be regarded as nucleophilic carbenes. Calculations by Bertrand et al. further suggested that dihydrogen activation by the singlet carbene proceeded via a heterolytic mechanism.53 Donation of electron density from the carbene lone pair to the antibonding σ* orbital of H2 is the initial step, which leads to elongation and polarization of the H−H bond as well as asymmetric carbon−hydrogen distances in the transition state. The hydridic end of the polarized dihydrogen molecule then attacks the positively polarized carbene center, leading to the complete cleavage of the H−H bond to yield the alkane products. Later calculations by Ess et al. confirmed this asynchronously timed activation process.54 In addition, the absolutely localized molecular orbital (ALMO) analysis and charge decomposition calculations showed that the difference between the forward-bonding and back-bonding charge transfer stabilization in the case of H−H cleavage by cAAC 22 is small (6.4 kcal mol−1), which suggests that the alkyl amino carbene acts as an ambiphile toward H2 and not merely as a nucleophile. Wang and Ma studied computationally the addition of H2 to a variety of cyclic and acyclic silylenes and germylenes.55 Cleavage of the H−H bond was found to occur through a concerted mechanism and is more facile for silylenes than for related germylenes. The relative reactivity of these metallenes depends on the HOMO−LUMO or singlet−triplet energy gaps. NHeterocyclic silylenes and acyclic bis(arylthio)-substituted silylenes do not activate hydrogen due to their increased HOMO−LUMO gaps;55−57 albeit, the latter class of compounds has been shown to reversibly bind ethylene58 while coordination of alkynes was irreversible.59 Nevertheless, the first examples of dihydrogen activation on Si(II) centers have been demonstrated. The facile activation of dihydrogen by the stable mixed amido boryl silylene, :Si(B(N(Dipp)CH)2)(N(SiMe3)Dipp) (26), was reported by the groups of Aldridge, Jones, and Mountford (occurring even at 0 °C) to give the dihydrosilane H2Si(B(N-

Scheme 7. 1,1- and 1,4-Addition of H2 by β-Diketiminate Gallium Compounds

2.1.3. Group 14 Compounds. Containing a vacant orbital and a lone pair of electrons in a nonbonding orbital, singlet carbenes present an orbital configuration reminiscent of the frontier molecular orbitals of transition-metal fragments. Therefore, similar insertion chemistry was proposed to be viable for carbenes. Insertions of difluorovinylidene into H2, CH4, and CD4 under matrix isolation conditions (argon, 20 to 40 K) were reported in 1999.51 Interestingly, these reactions are controlled by diffusion of trapped gases rather than by activation barriers, indicating that activation barriers are less than 1 kcal mol−1. This conclusion is further supported by ab initio and DFT 3611


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Scheme 9. Dihydrogen Activation by Silylenes

The Inoue group has recently reported the first instance of hydrogen activation by a multiply bonded silicon compound under ambient conditions. The guanadinato-supported disilene 35 reacted with H2 at room temperature to furnish the disilane 36 (Scheme 10).64 This unique reactivity can be ascribed to the

(Dipp)CH)2)(N(SiMe3)Dipp) (27) in quantitative yield, the first experimentally observed activation of H2 by a silylene (Scheme 9).60 The reaction is feasible because of the reduced HOMO−LUMO gap (1.96 eV in 27 vs 4.26 eV in bis(arylthio)substituted silylenes) due to the presence of a strong σ-donor group (boryl) and more obtuse bond angle at silicon (B−Si−N angle of 118.1(1)° vs 90.52 to 100.05(2)° for the S−Si−S angle in (ArS)2Si:). Widening of the bond angle at silicon results in rehybridization of the frontier orbitals, delivering more p character to the silicon-based lone pair (HOMO) which increases its energy. Thermodynamically, the activation of H2 is calculated to be strongly exergonic (ΔG = −29.2 kcal mol−1), while kinetically the computed value of ΔG‡ (23.3 kcal mol−1) is consistent with the observed activation at or below room temperature. The silyl coordinated silylene Si(Si(SiMe3)3)(N(SiMe3)Dipp) (28), prepared by the same groups, similarly activated dihydrogen at room temperature to furnish the corresponding dihydrosilane H2Si(Si(SiMe3)3)(N(SiMe3)Dipp) (29).61 Unlike the alkyl amino carbene 22, the boryl and silyl substituted amino silylenes and related germylenes favor a sideon approach of H−H to the Group 14 center, nearly perpendicular to the B−Si−N plane, with the dominant orbital stabilization arising from donation of the lone pair to H2.54 More recently, the Inoue group discovered that the N-heterocyclic imino-ligated silepin 30, serving as a masked form of silylene 31, is also capable of activating dihydrogen to furnish 32.62 Like compounds 26 and 28, 31 bears an amido-type ligand and a donating main-group substituent (silyl), resulting in a decreased HOMO−LUMO gap sufficient for the activation of strong bonds. Filippou et al. showed that the NHC-supported cationic metallasilylene 33 can be readily hydrogenated to furnish the NHC-ligated silylium ion 34 (Scheme 9).63

Scheme 10. Activation of H2 by Disilene 35

highly trans-bent (θ = 37.86° and 39.03°) and twisted (τ = 23.1°) structure of this disilene and its very long (in fact, the longest) SiSi bond distance of 2.3124(7) Å (cf. the Si−Si single bond distances of 2.3991(7) and 2.3886(7) Å in the same compound and the typical SiSi bond distance range of 2.14−2.29 Å). DFT calculations suggested that the SiSi bond in 35 can be described as a very weak double donor−acceptor bond. Another unusual feature of this reaction is the stereospecific transhydrogenation of 35 due to the twisted geometry of the starting disilene. A plausible mechanism based on dissociation of 35 into two silylenes followed by H2 activation and Si−H insertion to give 36 was ruled out on the basis of monitoring the reaction by low temperature NMR spectroscopy and the lack of reactivity between 35 and tBu2SiH2. DFT calculations on the hydrogenation reaction further showed that stepwise syn-addition of 3612


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Scheme 11. Multiple Additions of H2 to Digermyne 37 To Give Digermene (38), Digermane (39), and Primary Germane (40)

calculations, addition of H2 to 41 gave the singly bridged species L1Ge(μ-H)GeHL1, which then rearranged via a low activation pathway to the symmetrically hydrogenated compound L1HGeGeHL1 followed by rearrangement to the most stable isomer 42, the isolated product.67 Calculations revealed that the second addition of H2 would result in cleavage of the Ge−Ge bond, giving L1GeH and L1GeH3. However, this reaction was calculated to be thermodynamically unfavorable, consistent with the observed reactivity. Unlike 41, the amido-digermyne L2GeGeL2 (43; L2 = N(Ar#)(SiiPr3), Ar# = 2,6-(C(H)Ph2)2-4-iPrC6H2) featured a Ge−Ge multiple bond as a result of the increased steric bulk on the amido ligand.68 Dihydrogen activation was also demonstrated with compound 43, furnishing the hydrido-digermene L2HGeGeHL2 (44) within 20 min at room temperature (Scheme 13). The reaction also proceeded at −10 °C; however, a longer reaction time (1 h) was needed. Akin to 41, no evidence for the formation of di- or trihydrogenation products was observed even with excess H2 at elevated temperatures. Calculations were performed, and the reaction mechanism for the addition of H2 to 43 was found to be similar to the reaction of 41 with dihydrogen.69 Dihydrogen activation could be accomplished with bulky divalent two-coordinate germylenes as reported by Power and co-workers.70 Reaction of the germylene :GeArMes2 (45; ArMes = 2,6-(2,4,6-Me3C6H2)2C6H3) with H2 afforded the tetravalent germanium dihydride ArMes2GeH2 (46) in high yield. With the more sterically encumbered :GeArDipp2 (47), reaction with dihydrogen gave the trihydride germane 40 along with elimination of HArDipp (Scheme 14). Calculations revealed that reaction of 45 and 47 with H2 likely proceeded via interaction of the σ orbital of H2 with the empty 4p orbital at the germanium atom with concomitant back-donation from the germanium lone pair to the H2 σ* orbital. With 45, H−H bond cleavage gave the energetically favored product 46. In the case of 47, the initial steps are identical; however, the bulky ArDipp groups introduced sufficient strain such that the preferred pathway is elimination of HArDipp along with production of monomeric :GeHArDipp, which then reacted with H2 to give the final product 40. In a related study by Aldridge and co-workers, heteroleptic aryl silyl germylene 48 was revealed to oxidatively add hydrogen to furnish the dihydride product 49.71 Analogous to their germanium congeners, distannynes are also capable of activating dihydrogen. As shown in Scheme 15, the tin analogue of 37, ArDippSnSnArDipp (50), reacted with H2 to give the symmetrically bridged Sn(II) hydride Ar Dipp Sn(μH)2SnArDipp (51).72 Using a bulkier terphenyl ligand, reaction of ArDipp*SnSnArDipp* (52; ArDipp* = 2,6-(2,4,6-iPr3C6H2)23,5-iPr2−C6H) with dihydrogen gave the kinetically and

dihydrogen to 35 requires a high free energy barrier of 38.2 kcal mol−1, whereas anti-addition to the twisted SiSi bond proceeds via an affordable barrier of 15.6 kcal mol−1. The first example of dihydrogen bond activation by a maingroup element compound in the condensed phase was reported by Power and co-workers in 2005 with the facile activation of H2 by the germanium alkyne analogue ArDippGeGeArDipp (37).13 Reaction of 37 with dihydrogen in hexane at room temperature and atmospheric pressure yielded a mixture of hydrogenated products 38 to 40, with the ratio of the products dependent on the equivalents of H2 in the reaction mixture (Scheme 11). Based on the products observed, initial addition of H2 to 37 afforded the digermene 38, which further reacted with a second equivalent of H2 to give the digermane 39. The presence of ArDippGeH3 (40), which lacks a Ge−Ge bond, may be accounted for by the fact that digermene 38 exists in an equilibrium with either monomeric ArDippHGe: or the hydrogen-bridged isomer ArDippGe(μH)2GeArDipp. Reaction of H2 with either the monomer or the hydrogen-bridged isomer would yield the primary germane 40 as the Ge−Ge bond is absent in both germanium species. The ease of reaction between the digermyne 37 and hydrogen can be attributed to the availability of a low-lying LUMO (which is an n+ combination), amenable for a symmetry-allowed interaction with the σ-bonding MO of H2, and a high-lying HOMO (the Ge−Ge π-bond), resulting in the transition-metal-like cleavage of the H− H bond. Alternatively, calculations suggested that heavier alkyne congeners may have a singlet biradical character.65 Thus, the enhanced reactivity of 37 may be due to the ease of hydrogen abstraction from H2. Later, Jones et al. disclosed the activation of dihydrogen by the amido-digermyne L1Ge−GeL1 (41; L1 = N(Ar*)(SiMe3), Ar* = 2,6-(C(H)Ph2)2-4-MeC6H2) (Scheme 12).66 Compound 41 Scheme 12. Activation of H2 by Amido-digermyne 41

readily reacted with H2 at room temperature in solution to yield the stable germanium hydride L1GeGe(H2)L1 (42). Remarkably, the reaction proceeded at temperatures as low as −10 °C in solution while activation of dihydrogen in the solid state was observed at 20 °C to give 42 in greater than 95% yield after 1 h. Unlike 37, compound 41 did not further react with H2 even at elevated temperatures (up to 100 °C). As revealed by theoretical 3613


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Scheme 13. Dihydrogen Activation by Amido-digermyne 43

as the calculated barrier of 41.3 kcal mol−1 for the direct addition of H2 to :SnHArDipp is prohibitively high. Jones et al. were able to prepare distannyne L2Sn−SnL2 (54), the tin analogue of 43, which similarly activated dihydrogen (Scheme 16).74 However, the reaction is slower compared to 43, proceeding to 70% completion after 24 h to yield the H-bridged distannylene L2Sn(μ-H)2SnL2 (55), isomeric of the digermene 44.

Scheme 14. H−H Bond Cleavage by Homoleptic and Heteroleptic Germylenes

Scheme 16. Dihydrogen Activation by Amido-distannyne 54

Direct reaction of divalent, dicoordinate stannylenes with dihydrogen has also been demonstrated. Although no reaction was realized between H2 and :SnArMes2 (56) even upon heating to 70 °C, the more sterically crowded derivative :SnArDipp2 (57) reacted readily with H2 (Scheme 17).75 Unlike the germanium analogue 47, the product isolated was not a Sn(IV) species, but instead the formation of divalent compound 51 was observed along with elimination of arene. The difference in reactivity between 56 and 57 can be ascribed to the lower singlet−triplet gap in the latter, consistent with the red-shift of absorption in the electronic spectra (λmax = 553 vs 600 nm, respectively) and the wide C−Sn−C angle of 117.6(8)° in 57. A similar reaction pathway to the one described above for the germanium diaryls was revealed by DFT calculations.70 The impact of electron-withdrawing and electron-donating substituents on the electronic structure of substituted germylenes, stannylenes, and plumbylenes supported by terphenyl ligands was studied by investigating their solid state structures and spectroscopic properties (Mössbauer, NMR, and UV−vis spectroscopy) and further delineated by DFT calculations.76 Electron-withdrawing ligands lead to a higher contribution of p-orbitals from the central Group 14 element in the σ-bonding toward the ligands, which is evidenced by more acute bending angles at the Group 14 atom and consequently resulting in increased s-electron character of the lone pair. As a result, the energy gap between the frontier orbitals increases, which is manifested in the hypsochromic shift in their UV−vis spectra. In contrast, electron-rich derivatives have smaller HOMO−LUMO gaps and are better bond splitters. As an illustration of this rule, reaction of H2 with the bis(boryl) stannylene complex :Sn(B(N(Dipp)CH)2)2 (58) resulted in the oxidative addition product H2Sn(B(N(Dipp)CH)2)2 (59, Scheme 18).77 The reaction occurred readily at room temperature and under 1 atm of H2 and represents the first example of simple oxidative addition of H2 to a monometallic Sn(II) system to generate a Sn(IV) product.

Scheme 15. Activation of H2 by Distannynes 50 and 52

thermodynamically more stable isomer ArDipp*SnSn(H2)ArDipp* (53). Unlike the reaction with digermyne 37, no further reaction with H2 was observed to give distannenes or distannanes, even though complete hydrogenation of 50 to ArDippSnH3 was calculated to be thermodynamically feasible.73 Instead, the calculations revealed that activation of H2 by 50 leads to ArDippSn(μ-H)SnHArDipp which promptly isomerizes to ArDippHSnSnHArDipp. The latter subsequently dissociates into 2 equiv of :SnHArDipp which then dimerizes to give 51. The reason the reaction halts at the hydride dimer is likely kinetic 3614


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Scheme 17. Activation of Dihydrogen by Diaryl Stannylene 57

2.2. B−H and Al−H Bond Activation

Scheme 18. Oxidative Addition of H2 by Bis(boryl) Stannylene 58

Although hydrogenation of Mg(I) compounds has so far been unsuccessful, the β-diketiminate supported Mg(I) dimer [MesNacNacMg−]2 (62; MesNacNac = [MesNC(Me)CHC(Me)NMes]−), Mes = 2,4,6-Me3C6H2) reacted with the NHCstabilized alane IPrAlH3 (IPr = :C(N(Dipp)CH)2) at 80 °C to furnish the hydrogenated species [MesNacNacMg(μ-H)]2 and the Al(II) dimer [IPrAl(H)2−]2 (Scheme 20).79 Related reactions between [ArNacNacMg−]2 (Ar = Mes (62), Dipp (65)) and a series of amidinate-ligated dialanes [LAl(H)(μH)−]2 also gave the hydride-bridged dimers [ArNacNacMg(μH)]2 and the Al(II) species [LAl(H)−]2. The mechanism of these formal hydrogenation reactions remains unknown. B−H bond activation by main-group element compounds has been reported recently in the literature. Pentaphenylborole 68 reacted with pinacolborane to furnish 69 as a result of B−H bond activation.80 Examination of the crude NMR spectra showed no evidence of the other conformer of 69, and a mechanism akin to the reaction between 1 and dihydrogen was proposed for the formation of 69. The monomeric aluminum(I) complex 11 also cleaved the B−H bond of pinacolborane to give the first boryl hydride derivative of aluminum 70 (Scheme 21).40 Cyclic alkyl amino carbene 22 activated pinacolborane to give borane 71,81 while the closely related carbene 72 activated catecholborane to furnish 73.82 The first example of carbene insertion into BH3 was reported by the groups of So and Mézailles.83 Addition of BH3·SMe2 to the stable carbenoid 74 gave compound 75 after one night at room temperature. Compound 75 was found to be monomeric in solution but

Main-group clusters have also been shown to activate dihydrogen. As reported by Tuononen and Power, the tin cluster Sn8(ArMes)4 (60) reacted with excess H2 to give the tetrameric tin hydride (HSnArMes)4 (61) after stirring for 4 h at 60 °C (Scheme 19).78 Calculations revealed that the frontier Scheme 19. Dihydrogen Activation by Tin Cluster 60

molecular orbitals of 60 contain both electron donating and accepting features that are required for bond activation. Consistent with the reaction conditions (higher temperature and excess dihydrogen) the Gibbs energy of activation for the reaction was found to be 32 kcal mol−1.

Scheme 20. Dehydrogenation of Alanes by Mg(I) Dimers 62 and 65

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Scheme 21. Activation of Pinacolborane by Group 13 Compounds

Scheme 22. B−H Bond Activation by Low Oxidation State Group 14 Compounds

dimerized upon crystallization. The addition of BH3 to 74 is highly exergonic with a calculated ΔG value of −59.5 kcal mol−1. Recently, the reaction between diamido carbene 76 and boranedimethyl sulfide to give the Lewis base stabilized hydrido boryl complex 77 was reported.84 Insertions of the heavier congeners of carbene into boranes are very rare. In 1992, Nöth et al. reported the insertion of in situ generated germylene into the B− H bond.85 Germylene 78 and stannylene 58 reacted with boranetrimethylamine to furnish the B−H addition products 79 and 80, respectively.71,77 The reactions are summarized in Scheme 22. Phosphenium ions [R2P]+ are isolobal with carbenes and thus can be expected to engage in oxidative addition reactions. Vidović et al. showed that the dicoordinate diamido-substituted phosphenium ion [(iPr2N)2P]+ (81) inserted into the B−H bond of aminoboranes to give four-coordinate boryl phosphonium salts 82 (Scheme 23).86 The related dications 83 and 84, stabilized by the strongly donating carbodicarbene ligand, also cleaved the B−H bond of aminoboranes to give a mixture of hydridophosphenium (85) and hydrido boryl (86) products in the ratio of 1:23 for R = iPr and 9:1 for R = Ph. These oxidative addition products were characterized by NMR spectroscopy and X-ray diffraction analysis. The first example of Al−H bond activation was reported by Driess and co-workers in 2012.87 Reaction of silylene DippNacNac′Si: (87) with 1 equiv of alane-trimethylamine at room temperature yielded the 1,1-insertion product 88 (Scheme 24). 2.3. C−H Bond Activation

hydrogen atoms on the Al−H and α-C−H moieties that led to elimination of dihydrogen via a five-membered-ring transition state. The resulting zwitterion then undergoes a barrierless rearrangement to the isolated product. Jones and co-workers have demonstrated the activation of C− H bonds with an anionic gallium(I) N-heterocyclic carbene analogue.90 Gallium hydride 93, prepared via the reaction of 92 with [HC(N(Mes)CH)2]Cl, is the result of a formal oxidative insertion of the Ga(I) center into the imidazolium C−H bond, the first example of such a reaction with gallium (Scheme 26). Mechanistically, the reaction likely proceeds via deprotonation followed by coordination. 2.3.2. Group 14 Compounds. Su and Chu investigated by B3LYP and CCSD(T) calculations the reactions of unsaturated N-heterocyclic (Arduengo-type) carbene, silylene, and germylene with CH4.91 All three species were found to be kinetically stable toward insertion into the C−H bond, as the computed activation energies are prohibitively large (>56 kcal mol−1) and increase down the group. Albeit the latter trend is expected, it is at odds with the variation of the singlet−triplet gap (ΔEST) which changes in the direction opposite to what was found for the

C−H bond activation by transition metals has emerged as an attractive method for functionalizing abundant starting materials into value added products with the potential to streamline synthetic strategies, increase atom economy, and minimize waste.88 Recent efforts have demonstrated the ability of maingroup element compounds to perform such reactions as well. 2.3.1. Group 13 Compounds. C−H activation of the sterically encumbered pentamethylcyclopentadiene was observed upon reaction with DippNacNacAl: (11) at 70 °C over the course of 3 days to give the hydrido alkyl derivative 89 (Scheme 25).40 The crystal structure of 89 revealed that the Cp* ligand is η1-coordinated. In contrast, the 1H NMR spectrum displayed equivalent methyl groups on the cyclopentadienyl ligand, suggesting fluxionality. The in situ generated bis(carbene) aluminum(I) complex 90, produced from the reaction of the bromo precursor with potassium hydride, was shown by Driess and co-workers to activate the α-C−H bond of THF to give the metalated compound 91 with concomitant elimination of H2.89 The mechanism was revealed by DFT calculations to begin with coordination of THF followed by interaction between the 3616


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Scheme 23. B−H Bond Activation by Phosphenium Cations

Nevertheless, Arduengo and co-workers first reported the activation of acidic C−H bonds by N-heterocyclic carbenes in 1999.92 Carbene 94 reacted with acetylene to give the C−H insertion product 95 while the C−H bonds of methyl phenyl sulfone and acetonitrile were similarly activated at room temperature to give 96 and 97, respectively (Scheme 27). Noteworthy is the observation that the analogous unsaturated carbene, 1,3-diadamantyl-imidazol-2-ylidene, failed to react with CH3CN, only giving the solvate as the product. Though a longer reaction time was needed, 94 also reacted with chloroform at room temperature to give the C−H activated product 98 (Scheme 27). Whittlesey et al. demonstrated the activation of nonacidic C− H bonds with ring-expanded NHCs.93,94 The singlet−triplet gap of 99 and 101 is considerably smaller compared to 94 as a result of a significantly wider N−C−N angle.95 As such, intramolecular activation of a methyl group in the flanking N-aromatic rings to furnish 100 and 102, respectively, was observed when a solution of 99 or 101 was heated at 70 °C (Scheme 28).94 These reactions are similar to the well-established ortho-metalation of aryl rings by low oxidation state or highly Lewis acidic transition-metal compounds. C−H bonds have also been activated by triazole-based Nheterocyclic carbenes. The reaction of acetonitrile with 103 at elevated temperatures gave the C−H activated product 104 (Scheme 29).96 The Bertrand group has demonstrated the ability of aryl amino carbenes to readily activate C−H bonds.97 While stable for days at −50 °C after being generated at low temperature, carbene 105 isomerized to 106 upon warming to room temperature via an intramolecular C−H insertion reaction. Additionally, biaryl amino carbenes 107 and 109 spontaneously rearranged at low temperature into the aminofluorene isomer 108 and aminodibenzofluorene isomer 110, respectively (Scheme 30).98 To shed further light on these rearrangements, DFT calculations were performed. A singlet ground state was predicted for both 107 and 109 with the corresponding triplet state higher in energy by 17.9 kcal mol−1 and 16.0 kcal mol−1, respectively. The rearrangement of 107 to 108 was predicted to be highly exothermic (ΔG = −36.2 kcal mol−1) with a transition state located 17.0 kcal mol−1 above 107, consistent with the C−H insertion reaction at low temperature. Similar values of ΔG = −27.4 kcal mol−1 and ΔG‡ = 27.5 kcal mol−1 were predicted for the rearrangement of 109 to 110.

Scheme 24. Al−H Bond Activation by 87

Scheme 25. C−H Bond Activation by Al(I) Compounds 11 and 90

Scheme 26. C−H Activation by Anionic Gallium(I) Compound 92

parent system :EH2 and related diamides :E(NH2)2 (E = C, Si, Ge). Thus, the ΔEST values in the Arduengo-type metallenes are, in kcal mol−1, 84 (C) > 65 (Si) > 54 (Ge), whereas in :CH2, :SiH2, and :GeH2 the values are −9, 21, and 23 kcal mol−1, respectively. The observed reactivity is, however, in agreement with the Hammond postulate as the hydrogenation reaction becomes less exothermic (or even endothermic for germanium) down Group 14. 3617


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Scheme 27. Activation of Acidic C−H Bonds by 94

arylisopropyl group upon heating to 50 °C to produce the cyclized product 115. The more robust diamido carbene 76 only underwent intramolecular C−H activation after heating for several hours at 100 °C to furnish 116.101 Furthermore, intermolecular C−H activation of a range of electron-rich or electron-deficient para-substitued tolyl derivatives (Scheme 33) as well as secondary benzylic C−H bonds and primary C−H bonds of α-carbonyl substrates at high temperature was performed with 76.101 Presumably due to steric hindrance, tertiary C−H bonds were not activated. A Hammett plot with a linear, positive slope was constructed with data from the activation of the toluene derivatives, reflecting the buildup of negative charge at the benzylic carbon atom. Therefore, a stepwise process is consistent with the intermolecular C−H insertion wherein 76 functioned as a nucleophile which polarized the C−H bond prior to insertion. The first example of transfer hydrogenation of a carbene by a hydrocarbon was recently realized.101 The reaction of 76 with 1,4-cyclohexadiene yielded the formally hydrogenated diamido carbene 119 along with the C−H activated product 118 and benzene (Scheme 34). C−H bond activation by carbene 120 has also been disclosed. As shown in Scheme 35, 120 activated the C−H bonds of several tolyl derivatives to give 121 at elevated temperature (80 °C).102 Compound 120 also inserted into the α-C−H bond of THF to afford 122, the least acidic C−H bond activated by a stable carbene to date (Scheme 35). The Bielawski group has recently prepared a cyclic alkyl amido carbene.103 Carbene 123, generated in situ at ambient temperature, immediately activated a C−H bond of the pendant diisopropylphenyl group to furnish 124 (Scheme 36). Similar rearrangements were observed with 76 and 114 only upon heating. Banaszak Holl and co-workers observed that reactions of silylene 125 with iodobenzene in aliphatic and ethereal solvents led to the formation of C−H activation products (CH(tBu)N)2SiRI along with an equivalent of benzene and the oxidative addition product (CH(tBu)N)2SiPhI (Scheme 37), where the ratio of products was dependent on the substrate and concentration.104 These reactions are believed to proceed by a radical mechanism initiated by iodobenzene coordination to the Lewis acidic Group 14 metallene (see also section 2.9). C−H activation of alkylamines has also been demonstrated. In the case of NEt3 and NPr3, the C−H activation product along with (CH(tBu)N)2SiPhI and the hydrido amide complexes (CH(tBu)N)2SiHNR2 were obtained. The analogous reaction with NMe2tBu resulted in a mixture of the C−H activation product, (CH(tBu)N)2SiPhI, and the ammonium salt

Scheme 28. Intramolecular C−H Activation by RingExpanded NHCs 99 and 101

Scheme 29. Activation of Acetonitrile by Triazole-Based NHeterocyclic Carbene 103

Scheme 30. Intramolecular C−H Activation by Aryl Amino Carbenes 105, 107, and 109

Turner recently reported that cyclic alkyl amino carbene 72 underwent an intramolecular C−H activation of the diisopropylphenyl group to furnish compound 111 upon heating at 110 °C (Scheme 31).99 Intermolecular C−H activation was also observed with 72 and a range of sp-, sp2-, and sp3-hybridized C− H bonds at 25 °C to afford compound 112 while a mixture of 111 and 113 was formed when 72 was heated in the presence of toluene (Scheme 31). C−H activation was favored when the reaction was performed at lower temperatures and with a large excess of toluene. C−H bond activation has also been performed with diamido carbenes prepared by Bielawski and co-workers (Scheme 32).100 Initial studies revealed that 114 reacted with its pendant 3618


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Scheme 31. Intra- and Intermolecular C−H Activation by 72

Scheme 32. Thermolysis of 114 and 76 To Give 115 and 116

Scheme 33. C−H Activation of para-Substituted Tolyl Derivatives by 76

acetylene and phenylacetylene with silylene 87 to give the 1,1addition products 126 and 127, respectively, at room temperature was reported by Driess and co-workers.107 A prohibitively high barrier for the direct C−H insertion of 41.3 kcal mol−1 was found by theoretical calculations. Instead, a two-step mechanism was proposed beginning with deprotonation of the alkyne by the basic exocylic methylene group of 87 followed by hydrogen migration to the silicon atom. The C−H bonds in pentafluorobenzene and trifluorobenzene were also activated by 87 to give compounds 128 and 129, respectively (Scheme 38).108 The NHC adduct of 87 (130) has been reported to slowly rearrange via intramolecular C−H activation above −20 °C to give the alkyl hydride 131 (Scheme 39).109 Though the mechanism is currently unknown, the authors proposed two distinct pathways. Deprotonation of an N-methyl group of the NHC ligand by the nucleophilic exocyclic methylene group of 87 followed by H[1,4]- and Si[1,3]-shifts to give 131 was the first mechanism proposed. The second pathway proceeded via direct addition of the C−H bond to the Si(II) center followed by cleavage of the dative Si−C bond to yield 131. Cui et al. reported similar insertions of the C−H bond of carbenes with silylenes generated in situ.110 Treatment of aminobromosilane 132 with lithium bis(trimethylsilyl)amide in the presence of pyridine or 4dimethylaminopyridine resulted in facile activation of the α-

[HNMe2tBu]I. While the manner in which the ammonium iodide is produced remains unclear, addition of [HNR3]I (R = Et or Pr) to 125 in the presence of the corresponding amine resulted in the formation of the hydride products. The tertiary ammonium salt [NEt4]I does not bring about this reaction, nor is the hydride product formed when 125 and HNEt2 are mixed in the absence of iodide. However, addition of 0.1 equiv of LiI (but not LiCl) to a mixture of 125 and NEt3 did yield the hydride. These experiments suggested that both the N−H function and the iodide are important for the generation of the Si−H bond. Intramolecular C−H activation by silylenes has been reported previously.105,106 More recently, examples of intermolecular C− H activations by silylenes have been disclosed. The reaction of Scheme 34. Transfer Hydrogenation of 1,4-Cyclohexadiene by 76

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Scheme 35. Activation of C−H Bonds by 120

Scheme 36. Intramolecular C−H Activation by Cyclic Alkyl Amido Carbene 123

Scheme 38. Activation of C−H Bonds by Silylene 87

C−H bond of the heterocycle to give the aminosilane 133 or 134 (Scheme 40).111 DFT calculations along with deuterium labeling studies suggest that C−H activation proceeds via a hypothetical Lewis base stabilized iminosilane. Cyclometalated compounds 135 and 136 were formed as a result of sluggish intramolecular C−H activation of one of the isopropyl groups on a 2,6-diisopropylphenyl ring when the amido boryl silylene 26 and silyl substituted silylene 28 were exposed to elevated temperatures, respectively (Scheme 41).60,61 A two-coordinate cyclic alkyl amino silylene 137, prepared by Iwamoto and co-workers, was found to activate the C−H bond of toluene upon heating to reflux to furnish the hydridobenzylsilane 138 (Scheme 42).112 Compound 137 was also shown to participate in the dehydrogenation of dihydrogenated aromatic

compounds. Thus, the reaction between 137 and 1,4-cyclohexadiene afforded dihydrosilane 139 and benzene. Similarly, 137 dehydrogenated 9,10-dihydroanthracene to give 139 and anthracene. Competitive C−H activation of 9,10-dihydroanthracene was also observed to give 140 as a minor product in the reaction.

Scheme 37. C−H Activation by Silylene 125 in the Presence of Iodobenzene

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Scheme 39. C(sp3)−H Activation by Silylene 130

reaction of cyclopentadiene with digermyne 37 and distannyne 50 at room temperature yielded the corresponding germanium (148) and tin (149) aryl cyclopentadienyl compounds along with hydrogen gas evolution (Scheme 46). Although 50 did not react with the less reactive cyclopentene, 37 reacted with 3 equiv of cyclopentene to give the same Cp-containing species 148 along with a complex mixture of products. Finally, the reaction between 37 and 5 equiv of cyclohexadiene resulted in a mixture of the known hydride 38, benzene, and novel germanorbornene 150 (Scheme 46). No reaction was observed between cyclohexadiene and 50. The resulting products were highly unusual as dimetallynes had been shown earlier to undergo cycloadditions with alkenes.119 In a later paper, experimental evidence was found to suggest that the reaction proceeded via the 1,2-addition of the doubly allylic C−H bond of cyclopentadiene across the MM bond, while in the case of cyclopentene an oxidative dehydrogenation to cyclopentadiene is proposed to occur first followed by C−H activation.120 Similarly, reaction of an excess of 1,3-cyclohexadiene with amido-digermyne 43 afforded the 1,4-bis(germylene)-substituted carbocycle 151.121 Upon heating, compound 151 released benzene and gave the hydrido-digermene 44 (Scheme 47). Overall, the reaction can be described as the C−H activation of 1,3-cyclohexadiene by 43 or the transfer hydrogenation of 43 by 1,3-cyclohexadiene. Quantum mechanical calculations were employed to probe the mechanism of the reaction, which revealed that addition of 43 to 1,3-cyclohexadiene occurred in parallel with cleavage of the Ge−Ge bond followed by a stepwise β-hydride elimination to give 44 and benzene. The germylene :Ge(CH(SiMe3)2)2 (152) was shown to insert into the α-C−H bond of aliphatic nitriles, such as acetonitrile, propionitrile, succinonitrile, and phenylacetonitrile, to give the hydrido alkyl derivatives ((Me3Si)2HC)2GeHR (Scheme 48) while the related diamido germylene :Ge(N(SiMe3)2)2 (153) does not undergo this reaction.122 This is in agreement with theoretical studies performed by Su and Chu, who showed that germylene insertion into the C−H bond is favored by bulky, electron donating groups at germanium, whereas electronegative or π-donating ligands should inhibit this reaction.123 Interestingly, the success of oxidative addition crucially depends on the choice of solvent and the presence of inorganic salts such as LiCl, MgCl2, or LiBr. For example, no reaction took place between 152 and acetonitrile in benzene, diethyl ether, or dioxane at 20 °C over the course of 1 week. On the other hand, the same reaction in THF is completed within 2 and 20 min in the presence of at least 0.2 equiv of magnesium or lithium chloride, respectively. The exact mechanism of this reaction remains unknown, but it is reasonable to assume that an alkali or alkaline earth halide forms an adduct with the Lewis acidic germylene. Related C−H

Scheme 40. α-C−H Bond Activation of N-Heterocycles by 132

Kinetically stabilized 1,2-dihydridodisilene 141, reported by Tokitoh et al., has been shown to isomerize to the cyclometalated product 142 (Scheme 43).113 The isomerization was suggested to ensue via an initial 1,2-hydrogen migration to form the intermediate silyl silylene 141′ followed by subsequent insertion of the silylene moiety into the benzylic C−H bonds of the sterically demanding CH(SiMe3)2 groups. An earlier study by du Mont et al. suggested the propensity of the bulky supermesityl-ligated germylene :GeMes*2 (143, Mes* = 2,4,6-tBuC6H2) to insert into a C−H bond of an ortho tert-butyl group at room temperature to furnish the germaindane 144 (Scheme 44).114 However, careful reinvestigation of this reaction by the Jutzi group revealed that pure :GeMes*2 is stable for several days at room temperature with no evidence of C−H activation. 115 Only after several weeks does :GeMes* 2 decompose with concomitant release of Mes*H. Nevertheless, insertion into the C−H bond does take place but only upon activation by a Lewis acid, such as GeCl2·dioxane, GeCl4, AlCl3, BBr3, and TiCl4. It is therefore safe to assume that the formation of germaindane from the germylene prepared in situ as reported by du Mont was catalyzed by residual GeCl2. Activation of the C−H bond of phenylacetylene by the germanium analogue of 87 (145) to give 146 has been disclosed; however, the reaction is accompanied by formation of the [2 + 4] cycloaddition product.116 In a separate report, 145 activated the C−H bond of trimethylsilyl diazomethane to furnish the diazogermylene NacNacGeC(N2)SiMe3 (147) after 3 days at room temperature (Scheme 45).117 A series of C−H activation reactions of cyclic olefins by dimetallynes have been reported by the Power group.118 The

Scheme 41. Intramolecular C−H Activation by Silylenes 26 and 28

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Scheme 42. C−H Activation by Cyclic Alkyl Amino Silylene 137

Scheme 43. Thermal Isomerization of 141 to give 142

ether, 1,4-dioxane) and amines accompanied by the production of benzene (Scheme 49).104,125 The presence of the phenyl halide was crucial as no reaction took place in its absence. Oxidative addition of the aryl halide to germylene is a concentration-dependent side reaction. At high concentrations of germylene and iodobenzene (0.2 M), the oxidative addition product is formed in up to 40% yield with 152 and up to 82% yield with 153. At low concentrations of iodobenzene (0.02 M) and germylene ( Br > Cl, consistent with the cleavage of the C−X bond in the rate-determining step. Interestingly, a kinetic isotope effect (KIE) study in THF returned a different value of KIE (kH/kD = 5.0 ± 0.2) for the reaction mediated by 152 in comparison to a radical process initiated by phenylazotriphenylmethane (kH/kD = 4.2 ± 0.2), which speaks against

Scheme 44. Lewis Acid Catalyzed Intramolecular C−H Activation of Germylene 143

activation of ketones by 152 promoted by MgCl2 was also reported.124 Banaszak Holl and co-workers further showed that mixtures of germylenes 152 and 153 with aryl halides PhX (X= Cl, Br, and I) can activate the C−H bonds of alkanes (butane, methylcyclohexane, cyclohexane, cyclopentane), ethers (tetrahydrofuran, diethyl Scheme 45. C−H Activation by Germylene 145

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Scheme 46. Activation of C−H Bonds in Cyclic Olefins by Dimetallynes 37 and 50

Scheme 47. Reaction of 43 with 1,3-Cyclohexadiene

Scheme 48. C−H Activation by Dialkyl Germylene 152 in the Presence of Inorganic Salts

the involvement of a free phenyl radical in the germylene reaction. With deuterated bromobenzene, a secondary isotope effect was observed (kH/kD = 1.8 ± 0.1 for C6H5Br/C6D5Br) which is in line with kH/kD ratios reported for the formation of aromatic radical anions. Construction of a Hammet plot for a series of substituted iodobenzene derivatives showed a positive slope with ρ = 0.18, indicative of a buildup of negative charge in the aromatic ring. Altogether, these data are consistent with an in-cage radical mechanism initiated by PhX coordination to the Lewis acidic germylene followed by C−H abstraction by the ipsocarbon of the phenyl halide. Further agreement with the radical

mechanism was the observation that methylcyclohexane reacted with 152 to give a radical-like selectivity of 2° and 3° products (1:7.1). Similar C−H activations of alkanes, ethers, alkenes, and alkynes were observed for mixtures of stannylenes with phenyl halides.126−128 Attempts to prepare the bis(silyl)-substituted germylene 155 by reacting silylene :Si((NCH2tBu)2C6H4-1,2) (154) with the diamido germylene :Ge(N(SiMe3)2)2 resulted in an unstable species which underwent C−H activation with one of the SiMe3 groups to furnish 156 (Scheme 50).129 The corresponding bis(silyl) derivatives for tin and lead are, however, stable likely 3623


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Scheme 49. Activation of C−H Bonds by 152 and 153 in the Presence of Iodobenzene

Scheme 51. Intramolecular C−H Activation upon Heating of 157 To Give 158 and Isobutylene

xanthene to the carbodicarbene-supported phosphenium ion 83.86 The major product of the reaction with 1,3,5-cycloheptatriene was the cycloaddition species 164 whereas C−H activation led to the minor component 165. In contrast, C−H oxidative addition of xanthene led to a single product 166. Both 165 and 166 were unstable and were characterized by NMR spectroscopy. Related amido-substituted phosphenium ions 81 and 84 do not undergo these reactions because of their reduced electrophilicity and increased singlet−triplet gap.

because of the increased propensity of these elements to remain in the +2 oxidation state. Finally, C−H activation was observed at elevated temperature by the adduct between diaryl germylene 45 and tert-butyl isocyanide (157) to give the Ge(IV) hydride cyanide complex 158 and isobutene in nearly quantitative yield (Scheme 51).130 A plausible mechanism for the reaction was studied by DFT calculations. A prohibitively high energy was found for the transition state in the direct deprotonation of the tert-butyl group. Therefore, a concerted mechanism was considered by the authors that gave a transition state with an activation energy of 26 kcal mol−1 for heterolytic C−N bond cleavage along with the production of a formally anionic germanium center and a tertbutyl cation. The remaining step of the reaction, complete dissociation of the tert-butyl cation coupled with simultaneous C−H proton transfer to germanium, is exothermic by 9 kcal mol−1. Compound 158 was also produced by the oxidative addition of HCN to 45.131 2.3.3. Group 15 Compounds. Cowley et al. reported that protonation of diphopshene 159 with HBF4·Et2O at low temperature initially gave the C−H activation product 161 which upon warming resulted in P−P bond cleavage and the formation of 162 (Scheme 52).132 The reaction likely proceeded via protonation of the PP double bond to give the phosphenium ion 160 which then activated the C−H bond of the adjacent tert-butyl group. The Cowley group further showed that the phosphenium ion 81 inserted into the C−H bond of stannocene and plumbocene to give the phosphonium salt 163.133 Vidović et al. recently demonstrated the addition of slightly hydridic C−H bonds of 1,3,5-cycloheptatriene and

2.4. Si−H, Ge−H, and Sn−H Bond Activation

Despite the analogy between H−H and Si−H activation, only a handful of reports regarding silane activation by main-group metal compounds are found in the literature. 2.4.1. Group 13 Compounds. Braunschweig and coworkers have demonstrated the facile splitting of the Si−H bond of triethylsilane with pentaphenylborole (68).134 Addition of triethylsilane to a solution of 68 resulted in quantitative yield of the kinetic product trans-167. Heating a solution of trans-167 at 60 °C for 3 days allowed for the conversion to the thermodynamic product cis-167 (Scheme 53). Activation of triethylsilane was proposed to occur via 1,2-addition across the B−Cα bond to give a ring-opened 1-bora-2,4-pentadiene product that subsequently undergoes ring closure followed by 1,2hydride migration to give the final product, analogous to the mechanism for H2 addition to 1. Under ambient conditions, the activation of triethylsilane is highly selective as a result of restricted rotation in the ring-opened intermediate due to the bulky phenyl and triethylsilyl groups. Therefore, interconversion between the two rotamers is disfavored and trans-167 is formed exclusively. Martin and co-workers later reported analogous reactions of 68 with triethylgermane and tributyltin hydride to yield the products of Ge−H and Sn−H bond activation, 168 and 169, respectively.80 The Al(I) compound 11 readily activated the Si−H bond of phenylsilane at room temperature to give silyl hydride 170 while activation of methylphenylsilane required heating at elevated temperatures to furnish 171.40 Activation of hydridic SiH4 by 20 furnished gallane 172 with a backbone appended SiH3 moiety

Scheme 50. C−H Activation by in-Situ Generated Germylene 155

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Scheme 52. C−H Bond Activation by Phosphenium Ions

triphenyltin hydride with DippNacNacGa (18) gave the 1,1addition product DippNacNacGaH(SnPh3) (173), as seen in Scheme 54.49 Oxidative addition of X−H (X = elements from Groups 13 to 15) bonds to aluminum(I) compound 11 was studied with DFT calculations by Schoeller,42 Cao,43 and Bickelhaupt,135 who in addition investigated the oxidative addition of H4Si and H3SiPh to the heavier Group 13 congeners of compound 11. Except for the addition of the C−H bond, relatively low activation barriers were found. The following reactivity trends were established: the activation energy decreases down the group, i.e. in the order C > Si, N > P, O > S, and decreases from left to right within the same period, i.e. C > N > O, Si ≈ P > S. For Group 13 elements, the activation barrier increases down the group, i.e. Al < Ga < In. Activation strain model and energy decomposition analysis carried out by Buckelhaupt et al. further suggested that the main factor controlling the height of the activation barrier is the interaction energy between the deformed reactants, which is primarily the result of stronger electrostatic and orbital interactions between these fragments. The increasing activation barriers down the Group 13 elements were explained in terms of the higher deformation energy required by the heavier analogues of aluminum and decreasing interaction energy due to stabilization of the Group 13 element lone pair down the group. Based on the analysis of geometry changes in transition states, Cao and Zhang concluded that the mechanism of X−H

Scheme 53. Activation of Triethylsilane, Triethylgermane, and Tributyltin Hydride by Pentaphenylborole 68

and a hydride bound to gallium, contrary to the addition of protic bonds to 20 discussed earlier.50 Furthermore, reaction of 3625


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Scheme 54. Activation of Si−H and Sn−H Bonds by Group 13 Compounds

Scheme 56. Si−H Bond Activation by NHC 177 on-Route toward C−N Bond Cleavage and Ring Expansion

silicon atom to the carbon atom is the final step of the reaction sequence to give 179 as the final product. Si−H bond activation has also been demonstrated with diamido carbene 76.137 The Si−H bond of a variety of alkyl, aryl, and alkoxysilanes was cleaved at room temperature by 76 while heating at 60 °C was required to activate trichlorosilane to furnish the corresponding silane product 180 (Scheme 57). Scheme 57. Activation of Silanes by Diamido Carbene 76

addition to compound 11 depends on the element X. For X = H, the addition resembles oxidative addition to transition metals, whereas for X = Al, the process can be described as a hydride transfer. For elements from Group 15 (N and P) and Group 16 (O), bearing lone pair(s) of electrons, the reaction coordinate includes initial interaction between these lone pairs and the Lewis acidic aluminum center. 2.4.2. Group 14 Compounds. Cyclic alkyl amino carbene 22 is also capable of activating a series of silanes to give silane 174 as reported by Bertrand and co-workers, with reaction conditions dependent on the steric bulk of the substrate.81 Although NHCs are unreactive toward the activation of dihydrogen and alkanes, phenylsilane reacted with carbene 175 at room temperature to afford the silane product 176 (Scheme 55). This reactivity trend likely reflects the smaller energy gap between the frontier orbitals of phenylsilane in comparison with dihydrogen.

Reactions of transient silylenes with silanes have been a common trapping technique since the early 60s.138−143 For example, Okazaki et al. reported trapping of the unstable silylene 181 with triethylsilane to give the silane 182.144 More recently, dialkyl silylene 183 reacted with triethylsilane at room temperature to give the corresponding Si−H insertion product 184 in quantitative yield.145 Alkyl amino silylene 137 added triethylsilane upon heating to 100 °C to afford the silane 185112 whereas the more π-stabilized diamido silylene 125 and its saturated analogue :Si(N(tBu)CH2−)2 did not react even at 110 °C.146 Reaction of the amido boryl germylene 78 with silane gave the silyl hydride product 186.71 The first example of Si−H oxidative addition across a Sn(II) center was demonstrated when bis(boryl) stannylene 58 reacted with silanes to furnish the silyltin(IV) hydrides 187 and 188 (Scheme 58).77 Walsh et al. performed rate constant measurements for the reactions of the parent silylene, :SiH2, and dimethylsilylene, :SiMe2, with Me2GeH2 by using the time-resolved laser absorption technique.147 The silylenes were generated by the 193 nm laser flash photolysis of silacyclopent-3-ene and 1,1dimethyl-1-silacyclopent-3-ene, respectively. The experimental rate constants showed that :SiMe2 reacted with Me2GeH2 12.5 times more slowly than with Me2SiH2.148 DFT and ab initio calculations of the insertion reactions of :SiMe2 with SiH4, MeSiH3, GeH4, and MeGeH3 revealed that the reaction proceeded via the formation of weakly bound H-bridged complexes with dimethylsilylene. The lower reactivity of :SiMe2 with Ge−H bonds was then ascribed to the higher secondary barrier for rearrangement of the initially formed complexes. Analogous time-resolved studies of the reactions of

Scheme 55. Activation of Silanes by Carbenes 22 and 175

On the other hand, phenylsilane reacted with the unsaturated NHC 177 at high temperature to give 179 via C−N bond cleavage and ring expansion (Scheme 56).136 The reaction is proposed to go through the Si−H activation intermediate 178, which was not observed directly, followed by insertion of the silyl group into the C−N bond. Hydrogen atom transfer from the 3626


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Scheme 58. Silane Activation by Silylenes, Germylenes, and Stannylenes

Scheme 59. N−H Bond Activation by Pentaphenylborole 68

spectrum of 68 and aniline along with DFT calculations suggest that the reaction proceeds via initial adduct formation.153 Reactions of aluminum, gallium, and indium atoms with ammonia led to a series of metal complexes M·NH3, HMNH2, MNH2, and H2MNH2 that were trapped and characterized in a solid argon matrix.154 The formation of monovalent amides in these reactions and their subsequent transformation into trivalent species has inspired studies on the oxidative addition of ammonia and amines to aluminum and gallium compounds in the +1 oxidation state. Nikonov and co-workers have recently shown that amines add readily to the aluminum(I) complex 11.40 Thus, the addition of tert-butyl amine is completed after the mixture is stirred for 3 days at room temperature to give 190 whereas a reaction with the more acidic aniline is completed after 16 h to yield 191. (Scheme 60). Scheme 60. Activation of Amines by Aluminum(I) Complex 11

Reaction of digallene 16, which is known to dissociate to monomeric species in solution, with ammonia at room temperature afforded ArDippHGa(μ-NH2)2GaHArDipp (192), with the gallium centers symmetrically bridged by two NH2 units and each bound to a terminal hydride.47 Ammonia activation was also observed with the zwitterionic compound 20 to produce the 1,4-addition product DippNacNacGaNH2(tBu) (193),50 while oxidative addition of the N−H bond in diethylamine occurred with 18 to give the gallium hydrido amido complex DippNacNacGaH(NEt2) (194; Scheme 61).49 2.5.2. Group 14 Compounds. Although carbenes derived from imidazoles were known to be inert toward ammonia,155 recent reports have demonstrated the ability of aminocarbenes to activate NH3. Ammonia readily reacted with cyclic (22) and acylic (24) alkyl amino carbenes to give the N−H activated products 195 and 196, respectively (Scheme 62).53 Calculations were performed and the reaction mechanism was found to be very similar to that observed with H2. The N−H bonds of a wide variety of para-substituted anilines with either electron-donating or electron-withdrawing substituents were activated with the saturated diamino carbene 94.156 Ammonia was activated by cyclic diamido carbenes 76 and 120 to give amines 197 and 198, respectively (Scheme 63).102,157 The activation of N−H bonds by 76 was extended to a wide variety of alkyl and aryl amines in a subsequent paper. By treating

:GeH2 with GeH4 to form digermane Ge2H6 and of :SiH2 with SiH4 to form disilane Si2H6 were also reported.149,150 2.5. N−H Bond Activation

In contrast to the activation of dihydrogen, N−H bond activation is more challenging for transition-metal compounds since Lewis basic amines tend to favor the formation of classical Werner-type complexes. Only in 1991 did Koelliker and Milstein report on the first example of a reversible intramolecular cleavage of the N−H bond of ammonia by an iridium complex.151 The first example of intermolecular ammonia activation by a transition-metal complex was later described by Hartwig and co-workers in 2005.152 Since the activation of ammonia by nucleophilic carbenes was reported by Bertrand in 2007,53 N−H bond activation has been performed with a variety of main-group element compounds with numerous substrates, including alkyl and aryl amines as well as the parent or substituted hydrazines. 2.5.1. Group 13 Compounds. Antiaromatic pentaphenylborole 68 reacted with aniline at room temperature to give the ring-opened amino-borane 189 (Scheme 59). Observation of a four-coordinate boron species in the low temperature 11B NMR 3627


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and it was reported that formation of the 1,4-addition product is kinetically more favorable than the 1,1-addition product due to the higher energy barrier for direct insertion into the N−H bond (48.7 kcal mol−1 for the 1,1-addition versus 26.6 kcal mol−1 for the 1,4-addition).159 On the other hand, a significant decrease in the energy barrier to 15.3 kcal mol−1 was found for the 1,1addition product when the free energy profiles were calculated for the reaction of 87 with 2 equiv of ammonia to reflect the fact that the reaction was performed with excess ammonia. This difference is due to the assistance of the second molecule of ammonia in the transition state as a proton shuttle, consistent with the results reported. Roesky also described the analogous activation of the N−H bond in hydrazine and N-methyl hydrazine with 87 to give 200 and 201, respectively (Scheme 64).160 More recently, Aldridge and Jones described the preparation of a two-coordinate, acyclic diaminosilylene 202.161 Exposure of a toluene solution of 202 to an excess of dry ammonia led to a mixture of triaminosilane 204 and a secondary amine. The authors proposed that 204 was generated via the transient diaminosilylene 203, itself formed via a σ-bond metathesis between 202 and NH3, which oxidatively adds ammonia to produce 204 as the final product. With the Ni(CO)3 adduct of silylene 87 (205), selective activation of ammonia, isopropylamine, and phenylhydrazine in the 1,4 fashion in toluene was shown by Driess et al. to form the corresponding β-diketiminate Si(II)−Ni(CO)3 complex 206 (Scheme 65).162 Although the mechanism is unclear, the authors proposed that the increased acidity of the N−H protons upon coordination of the substrate with 205 resulted in selective 1,4addition. Intermolecular deprotonation of the N−H bond with a second molecule of 205 by the exocyclic methylene group followed by proton migration afforded the products observed. The NHC-supported cationic metallasilylene 33 developed by the Filippou group also readily cleaved the N−H bond of ammonia to furnish 207.63 Lewis base-stabilized silanone NacNac′SiO(DMAP) (208) also reacted with gaseous ammonia.163 An equilibrium was found between the expected product, silahemiaminal 209, and the silanoic amide tautomer 209′, with the equiblibrium dependent on both the polarity of the solvent as well as the concentration of the solution. Similarly, the Lewis-base stabilized phosphasilene NacNac′SiPH(DMAP) (210) reacted with ammonia gas to yield the 1,2-aminophosphinosilane 211.164 Compound 210 can also cleave the N−H bonds of isopropylamine, p-toluidine, and pentafluorophenylhydrazine to give 212 (Scheme 66). Ammonia was activated at room temperature by germylene 145 to give the Ge(II) amide 213 (Scheme 67).165 Unlike the related silylene 87, 1,4-addition of ammonia was observed in the reaction with 145, consistent with DFT calculations that revealed 1,4-addition product was kinetically and thermodynamically more favorable. The barrier heights for the 1,4- and 1,1-addition were 27.5 and 67.9 kcal mol−1, respectively, whereas the 1,4addition product was calculated to be more stable than the 1,1addition product by 13.6 kcal mol−1.159 Furthermore, formation of the Ge−H bond in the putative 1,1-addition product versus formation of the stronger C−H bond in 213 could be another driving force for the selectivity observed. Similary, 145 reacted with hydrazine in a similar fashion to give the germanium hydrazide 214.166 Driess and co-workers prepared a related sixmembered, zwitterionic N-heterocyclic germylene 215 that was also found to activate ammonia to give the 1,4-addition product 216 (Scheme 67).167

Scheme 61. Activation of NH3 and Diethylamine by Gallium Compounds

Scheme 62. Oxidative Addition of Ammonia by Alkyl Amino Carbenes To Give 195 and 196

Scheme 63. Ammonia Activation by Cyclic Diamido Carbenes 76 and 120

76 with electron-rich and electron-deficient para-substituted anilines, the mechanism of N−H activation by diamido carbenes was investigated.156 From the data collected, a Hammett plot featuring a negative slope was constructed, consistent with the buildup of positive charge at the amine nitrogen atom. Therefore, an electrophilic mode of activation was suggested for 76, in contrast to the nucleophilic pathway proposed for cAACs. However, insertion of 76 into electron deficient diarylamines was the fastest, suggesting that 76 is ambiphilic and activates the N− H bonds of acidic or basic amines through nucleophilic or electrophilic pathways, respectively. Silylenes are particularly effective in the addition of N−H bonds. Roesky and co-workers demonstrated that activation of ammonia with the silylene DippNacNac′Si: (87), gave the 1,1addition product, NacNac′SiH(NH2) (199).158 The reaction of 87 with ammonia was examined computationally by Sicilia et al., 3628


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Scheme 64. Activation of Ammonia and Hydrazines by Silylenes

Scheme 67. Activation of Ammonia and Hydrazine with NHeterocyclic Germylenes 145 and 215

Scheme 65. N−H Bond Activation by Silylene 205 and Metallasilylene 33

In contrast, diaryl germylenes 45 and 47 underwent oxidative addition of ammonia to give the corresponding Ge(IV) products ArMes2GeH(NH2) (217) and ArDipp2GeH(NH2) (220), respecScheme 66. N−H Bond Activation by Lewis Base-Stabilized Silanone 208 and Phosphasilene 210

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tively (Scheme 68).70 DFT calculations for the pathway involving a single ammonia molecule revealed a high energy

transfer, mediated by a second equivalent of hydrazine that has formed an adduct with 45 beforehand via a dative Ge−N bond. Therefore, only an adduct was obtained when 45 was reacted with N,N-dimethylhydrazine. Similarly, hydrazoic acid reacted with diaryl germylene 45 to furnish the Ge(IV) species ArMes2GeH(N3).131 Likewise, aryl silyl germylene 48 oxidatively added ammonia to give the amido hydride product 221.71 Diaryl stannylenes 56 and 57 reacted with NH3 to furnish the amido bridged compounds ArMesSn(μ-NH2)2SnArMes (222) and ArDippSn(μ-NH2)2SnArDipp (223), respectively, along with elimination of the respective aryl ligand (Scheme 69).70,75 Unlike the germylene analogues, the elimination of HArMes and production of 222 is strongly favored by 20.3 kcal mol−1 whereas the oxidative addition product, ArMes2SnH(NH2), is energetically disfavored by 2.2 kcal mol−1.70 Bis(boryl) stannylene 58 has also been shown to activate ammonia (Scheme 70).77 The ammonia adduct 224 was formed upon addition of excess ammonia to a benzene solution of 58 followed by N−H bond activation to yield the amido tin(IV) hydride (H2N)SnH(B(NArCH)2)2 (225). 2.5.3. Group 15 Compounds. The activation of N−H bonds with the tricoordinate phosphorus compound 226 was recently reported by Radosevich and co-workers.169 Initially prepared by Arduengo,170 the tridentate O,N,O-donors of compound 226 occupy three adjacent coplanar sites, resulting in an unusual planar T-shaped geometry at phosphorus. The pentacoordinate hydrido amido phosphorus species 227 was formed upon condensation of NH3 onto a solid sample of 226. Related reaction of 226 with alkyl and aryl amines occurred readily at room temperature to give the corresponding phosphorus compound 228, while heating to 55 °C was required for the bulkier aryl amines to complete the reaction (Scheme 71). Kinetic studies revealed an unexpected reaction order of three in the amine (i.e., k ∼ [H2NR]3) and a negative enthalpy of activation (ΔH⧧ = −0.8 ± 0.4 kcal mol−1) which along with DFT calculations suggested that the mechanism of N−H bond activation by 226 is an entropically controlled, stepwise process initiated by electrophilic activation of the amine substrate to give an amidophosphoranide intermediate, i.e. a hypervalent anionic phosphorus(III) species. The second step is the rate-determining amine-assisted proton transfer to furnish the final hydrido amido products. Calculations of a direct addition of the N−H bond to 226 afforded an unrealistic ΔH⧧ of 50.1 kcal mol−1, which can be rationalized by the large singlet−triplet gap for the starting compound 226 (ΔEST = 62.6 kcal mol−1). Shortly after, the preparation of a diazaphosphapentalene derivative 229 featuring a bent geometry with two phosphorus atoms at the bridgehead was reported by Kinjo and coworkers.171 Gaseous ammonia was activated by 229 to give the 1-aza-2,3-diphospholene derivative 230 at ambient temperature,

Scheme 68. N−H Bond Activation of Ammonia and Hydrazines with Diaryl Germylenes 45, 47, and 48

barrier upon which a transition state could not be located. Therefore, an alternative scenario involving two NH3 molecules was considered in the calculations as an excess of ammonia was utilized in the reaction. One molecule of ammonia is coordinated to the empty 4p orbital on germanium in the calculated structure via the lone pair while the second NH3 molecule solvates the coordinated ammonia via intermolecular N--H interactions, a situation reminiscent of the mechanism leading to the amido silane 199. The oxidative addition pathway was found to be 5.7 kcal mol−1 lower in energy than the alternative elimination pathway, even though the products ArMesGeNH2 and HArMes are more energetically favored than 217. Not only that, 45 similarly activated the N−H bonds of hydrazine and N-methyl hydrazine to give the first Ge(IV) hydrazides ArMes2GeH(NHNH2) (218) and ArMes2GeH(NHNHMe) (219) (Scheme 68).168 The mechanism of N−H bond activation, as studied by DFT calculations, was found to proceed via an intermolecular proton Scheme 69. Ammonia Activation by Diaryl Stannylenes 56 and 57

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Scheme 70. Formation of NH3 Adduct 224 from 58 Followed by N−H Bond Activation To Give 225

bond activation, starting with 1,2-addition across a phosphorus− amide bond followed by subsequent intramolecular σ3-P to σ5-P tautomerization. Later, the preparation of the tridentate phosphorus compound 233 supported by the N,N-bis(3,5-di-tert-butyl-2-phenoxy)amide ligand was reported by Aldridge and Goicoechea.173 While compounds 233 and 226 are closely related, the molecular structure of 233 was found to be closer to the bent structures observed for 229 and 231. Akin to the other tricoordinate phosphorus species discussed above, 233 readily activated gaseous ammonia at room temperature to give 234 (Scheme 74).

Scheme 71. Activation of Ammonia and Alkyl and Aryl Amines by 226

Scheme 74. Activation of Ammonia by 233

formally the product of σ-bond metathesis between an N−H bond of ammonia and an endocyclic P−N bond of 229 (Scheme 72). Unlike the activation of ammonia by 226, kinetic studies revealed that the reaction is first order in 229 and ammonia.

Schulz and co-workers recently disclosed the reactivity of biradicaloid 235 with ammonia.174 The initially orange solution of 235 turned colorless upon treatment with NH3 at room temperature, yielding the ring-opened product 236 (Scheme 75).

Scheme 72. N−H Bond Cleavage by 229 To Give 230

Scheme 75. Ammonia Activation by Biradicaloid 235

Utilizing a similar strategy, the chelated σ3-phosphorus triamide 231 with an exaggerated Cs distortion was synthesized by Radosevich and co-workers.172 Ammonia as well as alkyl and aryl amines was activated by 231 to give the pentacoordinate P(V) compound 232 (Scheme 73). Monitoring the reaction by multinuclear NMR spectroscopy along with kinetic studies allowed the authors to propose a stepwise mechanism for N−H

As no intermediates were observed by 31P NMR spectroscopy, the mechanism was probed via DFT calculations. The initial step involved N−H bond activation by both phosphorus radical centers followed by the rate-determining proton migration from phosphorus to nitrogen with a calculated barrier of 53.8 kcal mol−1. The final step involved cleavage of the P−N bond to furnish the final, isolated product. Due to the large activation barrier calculated for the second step, the authors proposed that the presence of excess ammonia in the reaction mixture acted as a proton shuttle and significantly decreased the barrier.

Scheme 73. Ammonia and Alkyl and Aryl Amine Activation by 231

2.6. P−H and As−H Bond Activation

Both monomeric, monovalent aluminum (11)40 and gallium (18)49 β-diketiminate compounds cleaved the P−H bond of diphenylphosphine to yield the corresponding hydrido phosphido products 237 and 238, respectively (Scheme 76). 3631


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arene elimination products, 247 and 249, respectively. Later, the Gao group demonstrated that reaction of zwitterionic germylene 145 with diphenylphosphine resulted in 1,4-addition and the production of DippNacNacGePPh2 250 (Scheme 79).179

Scheme 76. Activation of Diphenylphosphine by Low Oxidation State Aluminum and Gallium Compounds

2.7. O−H and S−H Bond Activation

Activation of O−H and S−H bonds has been shown with many main-group element compounds and provides a facile route to well-defined compounds bearing a terminal OH or SH group. 2.7.1. Group 13 Compounds. Antiaromatic boroles have been shown to readily activate O−H bonds. Piers et al. first reported that pentaphenylborole 68 reacted with water and phenol to yield the ring opened products 251 and 252, respectively.36 Marder and co-workers later showed that similar products were obtained upon reaction of the mesityl (253) and 2,4,6-tris(trifluoromethyl)phenyl (255) derivatives of 68 as well as for the reaction of triarylborole 257 with water.180 Recently, Martin revealed that reaction of 68 with 1-naphthalenethiol yielded the S−H activation product 259. In contrast to the reaction of 68 with main-group hydrides, the S−B bond is retained and the phenyl group migrates to the carbon atom (Scheme 80).153 The reaction of DippNacNacAl (11) with isopropyl alcohol proceeded smoothly at room temperature to furnish the hydrido alkoxy product DippNacNacAlH(OiPr) (260).40 Both water and methanol were activated upon reaction with DippNacNacGa (18), giving the corresponding compounds 261 and 262.49 Hydrogen sulfide reacted with the gallium complex 20 to give the 1,4addition product 263, akin to the products of dihydrogen and ammonia activation (Scheme 81).50 2.7.2. Group 14 Compounds. Reactions of in situ generated silylenes with alcohols and water to give alkoxysilanes and siloxanes, respectively, have been used for many years to trap these reactive species.140,181 The advent of the chemistry of isolable silylenes allowed for a systematic study of the oxidative addition of O−H bonds to reduced silicon centers.

Bertrand et al. demonstrated the ability of cAACs 22 and 72 to activate the P−H bonds of phenyl and diphenylphosphine, respectively.81 NHC 175 also activated phenylphosphine to give compound 241. Furthermore, 175 reacted with PH3 to yield the phosphanyl-imidazolidine 242.175 Finally, amido carbene 76 smoothly activated primary phosphines at room temperature, while secondary phosphines required elevated temperature to complete the reaction to give the respective tertiary phosphine derivatives (Scheme 77).176 Silylene 87 reacted slowly with an excess of the parent phosphine PH 3 to give the 1,1-addition product Dipp NacNac′SiH(PH2) (244).177 In contrast, AsH3 reacted rapidly with 87 to yield the arsasilene 245. Interestingly, dissolving isolated crystals of 245 lead to an equilibrium mixture with its tautomer, the 1,1-addition product 245′. Performing the reaction at low temperature led to the realization that 245′ is the initial product obtained from the reaction of 87 with arsine, forming at −50 °C, while production of arsasilene 245 only occurred upon warming above −30 °C (Scheme 78). The drastic difference in reactivity between PH3 and AsH3 is ascribed to the higher Brønsted acidity of AsH3 versus PH3 which facilitates As− H activation by 87. In 2014, Ragogna and co-workers reported the first example of P−H bond activation by germanium and tin.178 An excess of PH3 (80 psi) reacted with both diaryl germylene 45 and stannylene 56 to give both the 1,1-addition products, 246 and 248, as well as the Scheme 77. Activation of Phosphines by Carbenes

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Scheme 78. PH3 and AsH3 Activation by Silylene 87

Scheme 79. P−H Bond Activation by Low Oxidation State Tetrylene Compounds

Scheme 80. O−H and S−H Bond Activation by Boroles

Scheme 81. O−H and S−H Bond Activation by Aluminum and Gallium Compounds

82).185 Although the mechanism is unknown, the authors proposed that the reaction proceeded via O−H bond activation to give the silanol DippNacNac′SiH(OH), the expected product of 1,1-addition, or the hydroxosilylene, DippNacNacSiOH, as a result of 1,4-addition, followed by reaction with the second equivalent of 87. Hydrogen sulfide gas reacted with 87 at low temperature to give the donor-stabilized silathioformamide 265 (Scheme 82).162 Analogous to the reaction with water, two possible intermediates were proposed. The 1,1-addition of H2S followed by proton migration to the exocyclic methylene group would give the intermediate DippNacNac′SiH(SH). Alternatively, 1,4-addition would give the amino(mercapto)silylene, DippNacNacSiSH, with subsequent protonation of the Si(II) lone pair to give 265. DFT calculations were performed to shed further light on the mechanism as attempts to observe an intermediate by low temperature NMR spectroscopy were unsuccessful. The two

West et al. described the insertion reactions of silylene 125 into the O−H bond of ethanol and water to give an alkoxysilane and silanol, respectively.182 The latter product is unstable and undergoes self-condensation to yield a disiloxane as the isolated product. Lappert and co-workers found that silylene :Si(N(CH2tBu)2C6H4-1,2) (154) can insert into the O−H bond of ethanol, forming the alkoxysilane (EtO)HSi(N(CH2tBu)2C6H41,2).183,184 The reaction between silylene 87 and water in a 1:2 ratio afforded the donor-stabilized siloxysilylene 264 (Scheme 3633


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Scheme 82. Activation of H2O and H2S by Silylene 87 and Phosphasilene 210

Scheme 84. Activation of Water and Phenols by Germylenes 145 and 211

postulated intermediates were calculated to be practically isoenergetic while the tautomer 265 is favored by −23 kcal mol−1. The reaction of phosphasilene 210 with 1 equiv of H2S gas at low temperature resulted in the selective formation of the thiosilanoic phosphine 266.164 The protons of hydrogen sulfide underwent a 1,5-addition across the phosphasilene linkage and protonated the methylene group in the backbone of the ligand. The Lappert group also found that oxidative addition of the O−H bond of ethanol to dialkyl germylene :Ge(CH(SiMe3)2)2 (152) afforded ((Me3Si)2HC)2GeH(OEt) (267) at room temperature.186 Similarly, diaryl germylene :GeArMes2 (45) reacted with water or methanol to give the Ge(IV) insertion products ArMes2GeH(OH) (268) or ArMes2GeH(OMe) (269), respectively (Scheme 83). The mechanism of O−H bond

the addition of water and methanol with Lappert’s stannylene, :Sn(CH(SiMe3)2)2 (274) to yield the hydroxyl and methoxy diorganostannanes ((Me3 Si) 2 HC) 2 SnH(OH) (275) and ((Me 3 Si) 2 CH) 2 SnH(OMe) (276), respectively (Scheme 85).189 In contrast, reaction of the less electron-rich diaryl stannylene 56 with water or methanol produced the Sn(II) species Ar Mes Sn(μ-OH) 2 SnAr Mes (277) and Ar Mes Sn(μOMe)2SnArMes (278), respectively, along with elimination of HArMes (Scheme 85). Calculations point to arene elimination via a one-step σ bond metathesis through a cyclic transition state in which a hydrogen atom from a coordinated water molecule is transferred to the aryl substituent,187 analogous to the mechanism calculated for the reaction of 56 with NH3.70 The subtle balance between the formation of Sn(II) versus Sn(IV) products is likely controlled by the donor ability of stannylene substituents. Thus, stannylene 58 bearing two strongly donating boryl groups readily underwent oxidative addition with water to give hydroxystannane (HO)SnH(B(NArCH)2)2 (279) under ambient conditions (Scheme 86).77 2.7.3. Group 15 Compounds. Analogous to the activation of N−H bonds, triamido phosphorus complex 231 activated the O−H bonds of alkyl and aryl alcohols to give 280 (Scheme 87).172 The same mechanism was found to be operative as proposed for N−H bond activation. Phosphorus(III) complex 233 similarly reacted with water to give the P(V) species 281 (Scheme 87).173 The phosphenium cation [P(NiPr2)(C(PPh3)2)]+ (282), featuring a carbodiphosphine ligand, isolobal with diaminocarbene, added the O−H bonds of water and methanol to furnish the phosphonium cation 283 (Scheme 88).190

Scheme 83. O−H Bond Activation by Dialkyl (152) and Diaryl Germylene (45)

2.8. C−F Bond Activation

Activation of C−F bonds is challenging as carbon forms the strongest single bond with fluorine (116 kcal mol−1), typically requiring the application of highly elaborate transition-metal complexes and/or forcing conditions.191−198 However, several recent reports have described the ability of main-group element compounds to cleave such robust bonds. 2.8.1. Group 2 and 13 Compounds. Crimmin et al. reported the addition of the C−F bond of polyfluorinated arenes across the Mg−Mg bond of the bimetallic complex [DippNacNacMg−]2 (65) in 2016.199 Addition of fluoroarenes to 65 resulted in the clean formation of Mg−fluoroaryl and Mg− F moieties as observed by 19F NMR spectroscopy while the 1H NMR data demonstrated a set of broad resonances indicating a single ligand environment, consistent with an asymmetric dimeric species formed following the C−F bond cleavage. The equilibrium mixture simplified into a 2:1 mixture of the solvated

insertion, discerned via DFT calculations, was found to be catalytic, aided by a second molecule of water,187 which parallels the mechanism calculated for the insertion of 45 into the N−H bond of NH3.70 Related reaction of 45 with triflic acid gave the hydrido aqua complex [ArMes2GeH(OH2)][SO3CF3], with the complexed water molecule likely stemming from trace amounts of moisture in the reaction mixture.131 Germylene 145 reacted with H2O at room temperature to form DippNacNacGeOH (270) while reactions with phenol and pentafluorophenol gave the corresponding aryloxides 271 and 272, respectively.188 N-Heterocyclic germylene 215 similarly reacted with water at room temperature to give germylene hydroxide 273 (Scheme 84).167 O−H bond activation by dialkyl and diaryl stannylenes has been demonstrated as well. Pörschke and co-workers reported 3634


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Scheme 85. O−H Bond Activation by Dialkyl (274) and Diaryl Stannylene (56)

with either 9,10-dihydroanthracene or 1,4-cyclohexadiene. Neither reaction provided any evidence for hydrogen abstraction. Furthermore, reaction of 65 with hexafluorobenzene was not inhibited in the presence of either radical trap. Therefore, this reaction can be regarded as a concerted 2-electron reduction of the C−F bond by the Mg−Mg moiety. In 2015, the Nikonov and Crimmin groups independently described the facile oxidative addition of aryl C−F bonds with Roesky’s aluminum(I) compound 11 (Scheme 90).200,201 The ease of these additions follows the trend established for C(sp2)− F bond activation by transition-metal complexes.194,195,202 That is, the cleavage of a C−F bond is facilitated by the remaining fluorine atoms whose activating ability decreases in the order o- > p- > m-. In agreement with this trend, hexafluorobenzene and pentafluorobenzene readily reacted with 11 at room temperature whereas reaction with 1,2,4,5-tetrafluorobenzene, having only one activating o-C−F and one m-C−F groups, required heating to 70 °C for 48 h. The scope of activation was extended to 1,3,5trifluorobenzene, which reacted with 11 at 100 °C over the course of 4 days. Reactions with difluoro- and monofluorobenzenes were unsuccessful as compound 11 decomposes at temperatures above 100 °C. Kinetic measurements for the addition of 1,2,3,4-tetrafluorobenzene to 11 revealed second order kinetics with the activation parameters ΔH‡ = 13.6 kcal mol−1 and ΔS‡ = −27.2 cal K−1 mol−1, consistent with a concerted oxidative addition process. Remarkably, 11 also oxidatively cleaved the more challenging primary and secondary C(sp3)−F bonds of fluoroalkanes, taking place at temperatures as low as −60 °C (Scheme 91).200,201 This result is significant as there had been only one prior example in the literature for the formal oxidative addition of a C(sp3)−F bond to a transition metal.203 However, the latter reaction was shown to proceed not via direct C−F addition but via C−H bond activation followed by rearrangement to the final product. 2.8.2. Group 14 Compounds. Kuhn et al. were the first to describe C−F bond activation by NHCs. In 1998, the nucleophilic aromatic substitution of pentafluoropyridine by 287 to give 288 was reported.204 Later, the reaction was extended to hexafluorobenzene and other electron-poor perfluoro(hetero)arenes, with the products isolated as BF4− salts upon sequestration of fluoride by addition of BF3·OEt2.205 Bertrand and co-workers reported in 2012 the additions of pentafluoropyridine and octafluorotoluene to ethynyl dithiocarbamate 290.206 In both cases, selective activation of the C−F bond at the para-position was observed accompanied by ring closure to give compounds 291 and 292. In a later paper, the same group

Scheme 86. Insertion of 58 into the O−H Bond of Water

Scheme 87. Oxidative Addition of O−H Bonds in Water and Alcohols by the Phosphorus Compounds 231 and 233

Scheme 88. O−H Bond Activation by Phosphenium Cation 282

species NacNacMg(THF)(fluoroaryl) and [NacNacMg(μ-F)(THF)]2 upon addition of THF. The reaction has a wide scope as 65 was shown to react with an array of hexafluoro-, pentafluoro- and tetrafluoroarenes (Scheme 89). In order to rule out a radical intermediate in the reaction, 65 was reacted 3635


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Scheme 89. C−F Bond Activation by Magnesium Dimer 65

Scheme 90. Oxidation Addition of C(sp2)−F Bonds by 11

Scheme 91. Oxidative Addition of C(sp3)−F Bonds by 11

activated 2 equiv of octafluorotoluene by consecutive aromatic para-C−F substitution to yield the tetrasubstituted imidazolium salt 299.208 Both 297 and the mesityl substituted derivative 298 underwent nucleophilic substitution of the weakly activated aromatic C−F bond in 1-fluoro-4-trifluoromethylbenzene at elevated temperature, the first example of such reactivity by a carbene. Utilizing the bioxazoline-derived imidazole-2-ylidene 302, Chaplin and co-workers revealed the activation of the paraC−F bond of octafluorotoluene to give the zwitterionic imidazoliumolate 303.209 The authors proposed that formation of 303 started with initial nucleophilic aromatic substitution followed by ring opening of one of the oxazoline rings by the liberated fluoride anion. Related reaction of 302 with hexafluorobenzene resulted in the formation of 304 as a mixture of two rotamers in an approximately 1:1 ratio. The formation of 304 was suggested to proceed via a mechanism analogous to that of 303. Finally, the Baker group disclosed that reaction of saturated NHCs 94 and 175 with a variety of fluorinated alkenes gave the NHC fluoroalkene products 305 and 306, respectively.210 The mechanism of the reaction likely starts with nucleophilic addition of the carbene to the fluoroolefin to give

described the insertion of the cAAC compound 293 into the para-C−F bond of pentafluoropyridine at room temperature to give 294.207 Later, Turner reported that addition of hexafluorobenzene to the cyclic alkyl amino carbene 72 gave the double C−F activation product 295, which contained two chiral centers, as a racemic mixture.99 Reaction of 72 with pentafluorobenzene resulted in chemoselective C−H activation and subsequent C−F activation to furnish 296. Lee et al. have shown that NHC 297 3636


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Scheme 92. C−F Bond Activation with Carbenes

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Scheme 93. Aryl C−F Bond Activation by Silylenes 87 and 307

zwitterionic intermediates. Ionization to the fluoride salts followed by an attack of the fluoride anion to the opposite side of the fluoroolefin gave the final products 305 and 306. The reactions discussed above are summarized in Scheme 92. By utilizing the two-coordinate silylene 87 and the threecoordinate chloro silylene 307, Roesky and co-workers have performed the activation of a variety of aryl C−F bonds.108 Hexafluorobenzene was activated by both compounds to furnish the corresponding silicon(IV) fluorides 308 and 309. Reaction of pentafluoropyridine and octafluorotoluene with 87 and 307 led to the regioselective activation at the para-position and the formation of compounds 310 to 313. While reaction of 87 with pentafluorobenzene resulted in selective addition of the C−H bond, the reaction between 307 and pentafluorobenzene led to the selective insertion into the para-C−F bond to give 314. The regioselectivity of the reaction was attributed by the authors to the lower stability of pentacoordinate silicon hydrides compared to pentacoordinate silicon fluorides. The reactivity of silylenes 87 and 307 is summarized in Scheme 93. As shown by Tacke et al., bis(amidinate) silylene 315 oxidatively added hexafluorobenzene to give the six-coordinate compound 316 at room temperature.211 The related reaction between pentafluoropyridine and amido germylene 317 at room temperature resulted in the clean formation of the para-C−F activated product 318 (Scheme 94),212 whereas its silicon analogue activated the aryl C−F bonds of hexafluorobenzene and octafluorotoluene.213 Roesky and co-workers have reported the monoactivation of the CF3 group by silylenes.214 Treatment of 87 and 307 with PhNC(CF3)2 gave the difluoro-alkene products 319 and 320, respectively, as a result of selective activation of one of the carbon−fluorine bonds rather than a three-membered silacycle as a result of [1 + 2]-cycloaddition (Scheme 95).

Scheme 94. Activation of C−F Bonds by Silylene (315) and Germylene (317) Amidinate Compounds

with the indium analogue of 18, DippNacNacIn (322, Scheme 97).217 Rapid recombination of indium and carbon-centered radicals upon homolytic cleavage of the C−X bond was proposed as the mechanism for these reactions, based on the observation of a weak but persistent EPR signal in the reaction of 322 with methyl iodide at −83 °C. In contrast to the reactivity of 322 with alkyl iodides and bromides, no reaction was observed with aryl iodides or alkyl chlorides. Earlier studies with silylenes were focused on the in situ reactions of photochemically or thermally generated silylenes with alkyl halides.140,181,218,219 For example, Timms reported the condensation of dichlorosilylene with CCl4 to give the insertion product Cl3Si−CCl3.220 Kumada et al. then showed that the photochemically generated silylene :SiPh(SiMe3) inserted into the C−Cl bonds of octyl chloride, cyclopropylcarbinyl chloride, and sec-butyl chloride to give silicon(IV) chlorides; however, with tert-butyl chloride the products obtained were isobutene and the hydrosilane (Me3Si)PhSiHCl.221 It was postulated that the reaction occurred via formation of an initial adduct between the chloroalkane and silylene, but the mechanism of the insertion process remained unclear. Similar reactions with allylic chlorides gave both the products of direct insertion into the C−Cl bond as well as the products of allylic rearrangement.222

2.9. C−X (X = Cl, Br, I) Bond Activation

Related activation of weaker C−X bonds (X = Cl, Br, and I) by main-group compounds has been known for several decades with heavier Group 14 elements.140,215 Only recently has this reactivity been extended to Group 13 elements. Fischer and co-workers found that the β-diketiminate stabilized gallium(I) complex 18 oxidatively added the C−Cl bond of tert-butyl chloride to give the chlorogallane 321 (Scheme 96).216 Activation of primary, secondary, and tertiary alkyl iodides, as well as secondary and tertiary alkyl bromides has been reported 3638


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Scheme 95. Selective Monoactivation of a CF3 Group by Silylenes 87 and 307

the latter. Interestingly, addition of 125 to (HC(tBu)N)2SiBrPh does not result in the dimer, suggesting that the oxidative addition product is not an intermediate en route to the dimer. The saturated analogue of 125 similarly reacted with an array of halogenated organic compounds to give either disilane or oxidative addition products, or a mixture of both, depending on the nature of the halocarbon.224 West proposed that the disilanes were formed as a result of an initial halophilic interaction between the silylene and halocarbon and partial charge transfer from the nucleophilic silylene to the carbon center. The Si−Si bond then forms after an attack by a second silylene molecule. Interestingly, the outcome of this reaction crucially depends on the nature of halobenzene and the reaction conditions, as a complementary study of silylene 125 in the presence of iodobenzene resulted in the C−H bond activation of aliphatic, ethereal, and amine solvents (Section 2.3.2).104 Gehrhus et al. later showed that reactions of silylene 154 with alkyl or aryl chlorides and bromides also gave dimers akin to 326.225 Insertion of stable silylenes into the C−X bond has been studied by theoretical methods. Su investigated the reaction of silylene 125 with HCCl3 and HCBr3 by DFT and confirmed the formation of a weakly bound precomplex between 125 and the haloalkane, formed by donation of a halogen lone pair into the empty p-orbital (Si−N π-antibonding) on silicon.226 The ratedetermining step was found to be the insertion into the C−X bond; albeit, the calculated barriers were prohibitively high (ΔE‡ > 45 kcal mol−1). The formation of the dimer was then accounted for by insertion of a second silylene into the Si−X bond of the first product, with a much smaller barrier for this step. Although the author dismissed West’s proposal of the attack of the second silylene on the precomplex, the corresponding pathway was not investigated. Subsequent DFT study by Joo and McKee confirmed that the reaction of silylene 125 with halomethane XCH3 (X = Cl, Br, I) occurred via a radical process initiated by halogen atom abstraction from halomethane.227 The disilane product observed by West was formed by the reaction of a methyl-substituted silyl radical, generated by the addition of a methyl radical to 125, with another equivalent of silylene followed by halogen abstraction from XCH3. Preferential formation of the 1:1 insertion product over the 2:1 addition product increases in the order chloromethane < bromomethane < iodomethane. The reaction of the parent silylene :SiH2 with ClCH3 was studied by time-resolved kinetic studies and ab initio calculations (G3 level).228 In agreement with West’s original suggestion (vide supra), it was found that the initial step is the formation of a zwitterionic complex between the silylene and chloromethane followed by chloride abstraction to give the radicals •CH3 and •SiClH2. The competitive direct insertion pathway required a high-energy barrier and thus can be ruled out. Kira’s dialkylsilylene :Si(C(SiMe3)2CH2−)2 (183) reacted with dichloromethane and (chloromethyl)cyclopropane to give

Scheme 96. C−Cl Bond Activation by Gallium(I) Complex 18

Scheme 97. C−X Bond Activation by DippNacNacIn (322)

With the development of the chemistry of stable silylenes in the mid 90s, well-defined reactivity patterns started to emerge. In 1996, Lappert and co-workers showed that the stable but reactive silylene 154 can insert into the C−I bond of methyl iodide to give 325 (Scheme 98).184 West et al. then discovered unusual Scheme 98. C−X Bond Activation by Silylenes 154 and 125

reactions of the nucleophilic silylene 125 with halocarbons. Thus, addition of HCCl3 to 125 did not lead to a simple oxidative addition of the C−Cl bond but, surprisingly, resulted in the formation of the disilane 326 (Scheme 98).223 Similar dimers were obtained in the reaction with CCl4, CH2Cl2, and benzyl chloride. However, with bulkier substrates, such as tert-butyl chloride, 1-bromonaphthaline, and 1-bromofluorene, oxidative addition products (HC(tBu)N)2SiRX were produced instead, akin to the chemistry described by Lappert. Likewise, reaction of 125 with methyl and phenyl iodide resulted in the corresponding oxidative addition products. Reaction of 125 with bromobenzene gave both the dimer and oxidative addition product (HC(tBu)N)2SiBrPh though excess bromobenzene favored 3639


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Scheme 99. Reaction of Dialkylsilylene 183 with Dichloromethane and (Chloromethyl)cyclopropane

insertion product, consistent with previous DFT calculations which suggested that the 1,4-addition of electrophiles to 87 is due to the zwitterionic character of 87.234 Germylenes and stannylenes are similarly capable of activating C−X bonds. The two-coordinate germanium(II) cation 336 readily cleaved the C−Cl of dichloromethane upon dissolution to give the germyl cation 337 as reported by Aldridge and coworkers (Scheme 101).235 Schröer and Neumann provided evidence that reaction of distannanes ClBu2Sn−SnBu2Cl with methyl iodide, leading to Bu2SnCl2 and Bu2SnMeI, proceeded via stannylene :SnBu2 as an intermediate.236 Transient stannylenes :SnR2 (R = Me, n-Bu) were also produced by photolysis of polystannanes and trapped in oxidative addition reactions with alkyl chlorides and bromides.237 Lappert and co-workers then showed that stannylene 274 can undergo oxidative addition with a variety of alkyl and aryl halides to give the Sn(IV) compounds ((Me3Si)2HC)2SnRX (338 X = I, 339 X = Br, 340 X = Cl).238 As expected, the relative rates of reaction decreased in the sequence I > Br > Cl. Likewise, the diamido stannylene :Sn(N(SiMe3)2)2 reacted with aliphatic and aromatic halides to furnish Sn(IV) products ((Me3Si)2N)2SnRX (X = Cl, Br, I).239,240 Related reactions of diamides :M(N(SiMe3)2) (M = Ge, Sn, Pb) with BrN(SiMe3)2, acid chlorides, and triflic anhydride afforded the bromides ((Me 3 Si) 2 N) 3 MBr and the acyl compounds ((Me3Si)2N)2M(C(O)R)Cl and ((Me3Si)2N)2M(C(O)CF 3)OTf. These reactions likely proceed via a radical mechanism, as an EPR study of the reaction of 274 with bromoethane and 1-bromopropane in the presence of the spintrapping reagent nitrosoduren led to the formation of a stable nitroxide radical whereas low temperature studies revealed the formation of the radical intermediate •SnBr(CH(SiMe3)2)2.215 Interestingly, oxidative addition of PhBr to :Sn(CH(SiMe3)2)2 and :Sn(N(SiMe3)2)2 is catalyzed by a trace amount of EtBr which likely behaves as an initiator of the radical reaction.241 The reaction is also sensitive to the solvent, as substitution of benzene for tetrahydrofuran resulted in a larger proportion of the dihalide byproduct R2SnBr2 (R = (Me3Si)2HC, (Me3Si)2N). Other divalent tin compounds also showed similar reactivity. As reported by the groups of Eaborn and Smith, cyclic stannylene 341 similarly activated alkyl iodides to give compound 342 (Scheme 102).242,243 Stannocene Cp2Sn underwent oxidative addition with methyl iodide, diiodomethane, and ethyl bromoacetate, whereas reactions with allyl-1-bromide, benzyl bromide, and triphenylmethyl bromide resulted in the formation of CpSnBr and the C−C coupling product of the second Cp ring with the organic radical.244 The latter reactions and the observation that these oxidative addition processes are accelerated by light again point to a radical mechanism. Other examples include oxidative addition of n-alkyl iodides to tin(II) diketonates245 and insertion of the tin analogues of Kira’s silylene 183 into C−Cl bonds of acyl chlorides.246

the double silylene insertion product 328 and an unusual 5silaspiro[4.4]nonane derivative 329, respectively (Scheme 99).229 It was suggested that the reaction proceeded via formation of a Lewis acid/base complex between the electrophilic silylene and the chloride. More recently, the Kira group also reported the oxidative addition of acyl chlorides to 183 to furnish aroyl silanes.230 Su et al. performed DFT calculations of the reaction of CCl4 with 183 and a series of other silylenes and showed that abstraction of chlorine is kinetically more favorable than abstraction of CCl3 or insertion into the C−Cl bond.231 Analogous reaction of 183 with carbon tetrahalides CX4 (X = F, Cl, Br, and I) was also studied.232 The lowest barriers were again found for the abstraction of the X atom; albeit, silylene insertion gave more thermodynamically stable products. The reactivity was found to increase in the order F ≪ Cl < Br < I. Driess and co-workers further investigated the reactivity of silylene 87 with a series of haloalkanes (Scheme 100).233 Methyl Scheme 100. C−X Bond Activation by Silylene 87

iodide reacted with 87 to furnished exclusively the 1,1-insertion product DippNacNac′SiI(Me) (330). In addition, 87 readily activated the aliphatic C−Br bond of benzyl bromide, forming the 1,1-insertion product DippNacNac′SiBr(CH2Ph) (331). While reaction of dichloromethane with 87 took several weeks to produce the 1,1-insertion product DippNacNac′SiCl(CH2Cl) (332), the reaction of 87 with CH2ClI was much faster and completed within 8 h, leading to the iodo(chloromethyl)silane 333 exclusively. Dibromomethane reacted with 87 to afford the 1,1-insertion product DippNacNac′SiBr(CH2Br) (334) with concomitant production of the dibromosilane DippNacNac′SiBr2. Treatment of 87 with perhalogenated hydrocarbons also afforded the respective 1,1-insertion products. Chloroform and 1,1,1-trichloroethane reacted with 87 to give the chloro(dichloroalkyl)silane 335 along with a small amount of the dichlorosilane DippNacNac′SiCl2. The reactions were proposed to proceed via 1,4-addition of the haloalkane to give the kinetic product followed by rearrangement to the thermodynamic 1,1-

2.10. C−O and C−S Bond Activation

The activation of C−O bonds is a highly desirable reaction, particularly in the context of processing biomass into value3640


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Scheme 101. Activation of C−X Bonds by Germylenes and Stannylenes

the Al−H bond, leading to the rate-determining aluminum hydride bond scission. Finally, hydride transfer from the HBEt3− anion to the THF molecule triggers the penultimate C−O bond cleavage, leading to the stable, isolated product 343. C−O bond activation with β-diketiminate aluminum(I) compound 11 has been demonstrated in separate reports by Nikonov and Crimmin.200,201 Tetrahydrofuran adds to 11 smoothly at room temperature to give the cyclic alkoxide derivative 344 while reaction of 11 with benzofuran required heating at 80 °C to furnish the closely related C−O activation product 345. In a subsequent report, the reaction between 11 and diethyl sulfide at 50 °C yielded the unsymmetrically substituted alkyl thiolate aluminum complex 346, the first example of C(sp3)−S oxidative addition mediated by a main-group element.248

Scheme 102. C−I Bond Activation by Cyclic Stannylene 341

added products,247 with many examples in the literature for the cleavage of the more reactive allylic and benzylic C(sp3)−O bonds by transition-metal complexes. In contrast, examples of alkyl C(sp3)−O bond cleavage are scarce in the literature. As reported by Driess and co-workers, generation of the bis(carbene) aluminum(I) complex 90 with potassium triethylborohydride in THF afforded the THF-ring-opened complex 343 as the product (Scheme 103).89 The mechanism of the reaction, as suggested by DFT calculations, starts with activation of the Al−H bond by the Lewis acid BEt3 formed in situ from K[HBEt3]. Interaction of the aluminum center with THF further weakens

2.11. C−E Bond Activation

The propensity of carbene-like, monovalent Group 13 compounds DippNacNacM: (M = Al (11), Ga (18), In (322)) to oxidatively add H−X and E−E bonds was extended to the activation of C−E bonds, where E is an element from groups 13 to 16. Schulz et al. showed that DippNacNacGa inserted into the C−In bond of InEt3, resulting in the formation of NacNacGaEt(InEt2) (347, Scheme 104).249 The same reaction carried out in the presence of the N-heterocyclic carbene ItBu (ItBu = :C(N(tBu)CH)2) led to the abnormal carbene adduct 349 with the carbene coordinated to the indium atom. Reaction of InEt3 with 2 equiv of DippNacNacGa resulted in double insertion and formation of [DippNacNacGa(Et)]2InEt (348) whereas the third insertion of DippNacNacGa into the remaining In−Et bond does not occur, likely for steric reasons (Scheme 104). A related reaction between DippNacNacGa and Cp*InEt2 lead to the production of DippNacNacGaEt2 (350) and aromatically stabilized Cp*In. The reaction is believed to start by oxidative addition of the In−Et bond to DippNacNacGa followed by reductive elimination of DippNacNacGaEt2 from the indium(III) center. The Schulz group has also described similar insertion reactions of DippNacNacM: (M = Al, Ga, In) with BiEt3 leading to Dipp NacNacMEt(BiEt2) (351 to 353) and oxidative addition of the C−Te bond to DippNacNacGa to give 354 (Scheme 105).250,251

Scheme 103. C−O and C−S Bond Activation by Aluminum(I) Compounds

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Scheme 104. Insertion of DippNacNacGa (18) into the C−In Bond

Scheme 105. Insertion of DippNacNacM into the C−Bi and C− Te Bond

lowest energy structure on the triplet potential energy surface was cyclic C5H6 whereas reaction with singlet methylene expectedly gave cyclopenta-1,3-diene as the lowest minimum. In the reaction of triplet CH2 with Si4H4, a variety of ring-opened species that relieve the strain in the tetrasilatetrahedrane core were found, with the lowest being an unusual bicycle with a trisila 3-membered ring and a carbon-inserted four-membered ring. On the singlet surface, the most stable geometry is that of a symmetric bicycle with two tetrasilacyclobutane rings and the methylene in the bridging position. 2.12.2. Pnictogen−Pnictogen Bond Activation. The majority of research on pnictogen−pnictogen bond activation by p-block elements has been dedicated to the oxidative addition of P−P bonds and, in particular, the activation of white phopshorus.253,255,256 A noticeable exception is Power’s report on the reaction of digermyne ArDippGeGeArDipp (37) with Me3SiN3 to give the non-Kekulé biradicaloid ArDippGe(μNSiMe3)2GeArDipp (355, Scheme 106).257 The X-ray crystal

2.12. E−E Bond Activation

Oxidative cleavage of E−E bonds, where E is a heavier maingroup element, are primarily centered on the additions of heavy elements from Groups 15 and 16 which form relatively weak homoelement bonds (e.g., 50 kcal mol−1 for P−P bonds and 54 kcal mol−1 for S−S bonds). Nevertheless, this chemistry attracted significant attention because it gives rise to a variety of structural forms and potential precursors to heterometallic materials. In fact, reactions of elemental phosphorus and sulfur were among the first and most extensively investigated.252,253 Since activation of the element−element bond in pnictogens and chalcogens is well established, the primary focus of this section is on more recent literature, primarily from the past decade, although some earlier examples are also included and discussed. 2.12.1. Tetrel−Tetrel Bond Activation. To the best of our knowledge, there have been no reports of tetrel−tetrel bond activation by any main-group center. However, interactions of the elusive tetrahedrane and tetrasilatetrahedrane, both having an estimated strain energy of 140 kcal mol−1, with CH2 (both singlet and triplet state) and singlet :SiH2 were studied computationally by Damrauer.254 These reactions gave rise to several stable, i.e. the minima on the potential energy surface, species for both carbenoids, with a bigger structural diversity observed for the products of reactions with tetrasilatetrahedrane. For the reaction of triplet CH2 with tetrahedrane C4H4, the

Scheme 106. N−N Bond Cleavage by Digermyne 37

structure of 355 showed a planar Ge2N2Si2 motif with a pyramidal geometry of the germanium radicals. DFT calculations for the truncated model MeGe(μ-NSiH3)2GeMe revealed a singlet ground state (with a large singlet−triplet gap of 17.51 kcal mol−1) and the absence of a Ge−Ge bond. Stannylene :Sn(N(SiMe3)2)2 reacted with azides RN3 at −30 °C to generate transient imines RNSn(N(SiMe3)2)2 which decomposed above −30 °C via C−H bond activation of the R groups.258,259 3642


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Scheme 107. P−P Bond Cleavage by Aluminum(I) Complex 11 and Gallium(I) Complex 18

Scheme 108. P−P Bond Activation by [Ga−C(SiMe3)3]4 (359)

Scheme 109. Preparation of Dithallium Diaryltetraphosphabutadienediide and Its Transformation into Diaryltetraphosphabutadiene

Schnöckel et al. were the first to report a reaction of the Al(I) compound (Cp*Al)4 with white phosphorus leading to the cleavage of all P−P bonds and formation of a mixed aluminum− phosphorus cluster (Cp*Al)6P4. The cluster can be described as two face-sharing, distorted Al4P4 cubes which lack two phosphorus atoms at opposite vertices.260 In contrast, the monomeric, two-coordinate aluminum compound DippNacNacAl (11) only added to two opposing edges of P4 to furnish the phosphorus-bridged dimer [DippNacNacAl]2(μ-P4) (356), in which each phosphorus atom retains two P−P bonds (Scheme 107).261 Further addition of the bulky DippNacNacAl does not happen, likely for steric reasons. DFT calculations indicated a significant charge transfer from aluminum to phosphorus atoms consistent with a 4-electron transfer and oxidation of each aluminum center from Al(I) to Al(III). The gallium analogue Dipp NacNacGa (18) inserted into the P−P bond of white phosphorus to give the tetraphosphabicyclobutane derivative 358.262 Recently, Nikonov et al. revealed that 11 cleaved the P−P bond of tetraphenyl diphosphine after a reaction at elevated temperature to give the aluminum bis(diphenyl phosphido) compound 357.248 Uhl and Benter discovered that Ga−C(SiMe3)3, produced in situ in solution upon dissociation of the tetrameric cluster [Ga− C(SiMe3)3]4 (359), slowly inserted into the P−P bond of P4 in boiling n-hexane to furnish a P4(Ga−C(SiMe3)3)3 heteroelement cage (360) which can be viewed as a product of triple gallium insertion across three P−P bonds stemming from the same phosphorus vertex (Scheme 108).263 Reaction of the analogous indium compound gave a different product, likely containing four

indium atoms and only one phosphorus atom; however, this compound was not fully characterized. Related oxidative addition of the P−P bond of tri(tert-butyl)cyclotriphosphane to Ga− C(SiMe3)3 afforded the galatriphosphacyclobutane (361).264 The Power group reported on the two-electron reduction of white phosphorus accompanied by the aryl group transfer from the dithallene [ArDippTl]2 (362) to furnish a thallium salt of the otherwise elusive tetraphosphabutadiene, i.e. dithallium diaryltetraphosphabutadienediide (Scheme 109).265 Remarkably, when this species was treated with iodine, the neutral diaryltetraphosphabicyclobutane and insoluble thallium iodide are formed. Of note, calculation on the parent H2P4 system revealed that the dihydridotetraphosphabicyclobutane geometry lies approximiately 20 kcal mol−1 below the dihydridotetraphosphabutadiene isomer.266 Although mechanistic details of this transformation are not known, one may speculate that it proceeds by oxidative P−P bond cleavage followed by reductive P−C bond elimination. A diverse array of reactivity modes between carbenes and white phosphorus, such as ring-opening, fragmentation, and aggregation, have been identified. Computational studies on reactions of singlet and triplet CH2 with white phosphorus revealed that a P−P insertion structure is the lowest energy species by ∼24 kcal mol−1 on the singlet potential energy surface.267 For the interaction of triplet CH2 with P4, a structure was found having the energy ∼19 kcal mol−1 higher than the insertion product. An insertion product akin to the computed structure, 366, was experimentally realized upon reaction of the electrophilic diamido carbene 365 with P4 (Scheme 110).268 3643


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cyclopropenylidene (375), a bis(carbene) P1 cation 376 (isolated as the chloride salt) was obtained. As described by Hudnall and co-workers, reaction of the electrophilic diamido carbene 76 with P4 gave the tris(Pphosphaalkenyl) phosphine 377 whereas its less electrophilic monoamido analogue 378 surprisingly afforded a P8 cluster 379 which can be described as a tetraphosphacyclobutane substituted by four phosphaalkenyl groups (Scheme 113).272 Formation of the P8 cluster 379 illustrated the possibility of aggregation beyond the parent P4 motif as a result of [2 + 2] cyclization of the tetraphosphatriene structure exemplified by 368. This was further expanded on by Bertrand et al. in the preparation of analogues of 379 decorated by carbenes 22 and 76.268 Unlike the reaction shown in Scheme 112, the derivative of 379 prepared with 22 was formed in benzene, a solvent in which P4 has a greater solubility and thus a higher ratio of P4 to carbene in solution. In all, these findings demonstrate the divergent reactivity of amidocarbenes compared to cAACs and NHCs, defining a clear pattern of reactivity that more nucleophilic carbenes affect deeper disintegration and fragmentation of the tetrahedral P4 core. Therefore, the outcome of the reactions between carbenes and white phosphorus is the function of many variables including the nature of the carbene, stoichiometry, and reaction conditions. Recently, Grützmacher and co-workers reported the reaction of imidazolium salts 380, 381, and 382 with potassium tertbutoxide and P4 to furnish phophaalkenes 383 to 385 along with small quantities of unidentified polyphosphorus compounds (Scheme 114).273 Although the combination of these reagents produces NHCs in situ, the reactivity observed is unlike the behavior described above. In order to elucidate the reaction pathway, 381 was treated with potassium tert-butoxide and P4 at −20 °C. This gave a solution with a 31P NMR spectrum that showed signals for 384, unreacted P4, and signals that are consistent with the bis(imidazoyl)-substituted tetraphosphatriene (akin to 368) and the bicyclic anion HP4−. As analogues of 384 had not been reported in previous reactions of carbenes with P4, the authors attributed its production to the presence of

Scheme 110. Reaction of White Phosphorus with Electrophilic Carbene 365

In contrast, cyclic alkyl amino carbenes and N-heterocyclic carbenes underwent a nucleophilic attack at white phosphorus to give a mixture of (E)- and (Z)-diphosphene isomers with pendant C-amino phosphaalkene substitutents which can be regarded as unusual 2,3,4,5-tetraphosphatrienes.269,270 Scheme 111 illustrates this chemistry for the case of a chiral cAAC. DFT calculations showed that, unlike the related reactions of silylenes discussed below, the carbene readily attacks P4 at a vertex of a tetrahedron as a nucleophile to give triphosphirene 369 as the first intermediate, followed by an attack of a second carbene molecule at one of the unsaturated phosphorus centers. The resulting (E)-isomer from the reaction of P4 with 367 reacted further with 2,3-dimethylbutadiene to give the product of [4 + 2] cycloaddition to the diphosphene moiety (370) which is 95% diastereoselective.269 Further variation of the carbene structure allowed for finetuning of its electrophilic and nucleophilic properties and thus the ability to cleave, fragment, and trap different phosphorus clusters (Scheme 112).271 Thus, the smaller cAAC 22 can attack the 2,3,4,5-tetraphosphatriene intermediate to give a mixture of tris(P-phosphaalkenyl) phosphine 371 (67%) and a phosphaalkene dimer 372 (12%). The proposed triphosphirene intermediate 369 can be intercepted by a more electrophilic carbene :C(tBu)N(Cy)2 (373). Therefore, addition of 373 to P4 in diethyl ether at room temperature led to the bis(carbene) adduct 374, the product of cyclopropanation of 373 with the P P bond of 369. With a smaller carbene, bis(diisopropylamino)-

Scheme 111. Reaction of White Phosphorus with cAAC 367 and Further Derivatization with 2,3-Dimethylbutadiene

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Scheme 112. Reaction of P4 with Different Nucleophilic Carbenes

Scheme 114. Reaction of Imidazolium Salts with KOtBu and P4

Scheme 113. Reaction of Electrophilic Carbenes 76 and 378 with P4

Scheme 115. Reaction of Disilene 386 with P4

t

BuOH, which is a byproduct of the deprotonation of imidazolium salts and is not separated in these reactions. This hypothesis was supported by the fact that the formation of phosphaalkenes was suppressed when the deprotonation of the imidazolium salt was done with sodium hydride. Thus, it was suggested that the phosphaalkenes were formed via an initial carbene activation of P4 and subsequent trapping of the tetraphosphatriene intermediates with tBuOH while the unidentified polyphosphides were produced upon decomposition of the HP4− anion. For heavier Group 14 element compounds, earlier studies by West et al. showed that the disilene Mes2SiSiMes2 (386) reacted with white phosphorus at 40 °C to give the butterfly shaped Si2P2 heterobicyclo[1.1.0]butane 387, though the mechanism of this reaction remained unknown (Scheme 115).274 In a similar fashion, Driess reported that phosphasilenes R2SiPR′ reacted with P4 to generate a related SiP3 structure.275

Driess and co-workers later found that reaction of silylene NacNac′Si: (87), isoelectronic with DippNacNacAl (11), with white phosphorus at room temperature proceeded via consecutive insertion into the P−P bond to give products of mono and double insertion (Scheme 116).276 This result is significant as neither Kira’s electrophilic silylene :Si(C(SiMe3)2CH2−)2 (183) nor West and Denk’s nucleophilic silylene :Si(N(tBu)CH)2 (125) displayed this reactivity, although the latter species catalyzed the transformation of P4 into a glassy, deep orange-red substance that is insoluble in THF and is believed to be red phosphorus.182 Interestingly, the related germylene Dipp NacNac′Ge: is also unreactive to white phosphorus, likely due to its lower reduction potential. Dipp

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Scheme 116. Reaction of Silylene 87 with White Phosphorus

Scheme 117. Reaction of Silylenes with White Phosphorus

hydrogen migration to one phosphorus atom and silicon binding to the other, (ii and iii) two rotational isomers of cyclotriphosphirene, (iv) a product of P4 coordination to the empty 3p orbital of silicon via one phosphorus vertex, and (v) a unique species with an intact P4 core having H2Si bound in a noncovalent fashion to one P3 face with an energy of ∼5.5 kcal mol−1. In this latter structure the distance of the silicon atom to the proximate phosphorus atoms is ∼2.6 to 2.9 Å, the H−Si−H angle is 95.9°, and the P4 cluster is slightly distorted. DFT calculations by Schoeller et al. for the truncated compound MeNacNac′Si (MeNacNac′ [MeNC(CH2)CHC(Me)NMe] 2−) suggested that unlike the typical reactions of P4 with nucleophiles (e.g., alkoxides), this reaction should be described as an electrophilic attack of silylene at a P−P edge of the phosphorus tetrahedron.280,281 The alternative nucleophilic attack at a phosphorus corner was discarded because the silicon lone pair was regarded to be inert while the direct insertion of MeNacNac′Si into the P−P bond was found to be a high energy process. Counterintuitively, electrophilic P−P bond cleavage is facilitated by about 4 kcal mol−1 through coordination of another molecule of P4 which acts as an external nucleophile toward the silylene, such that in the transition state the silicon atom adopted a distorted trigonal bipyramidal geometry. The discrepancy between the ease of silylene insertion (room temperature, 4 h) observed by Driess and the high barriers found in earlier computational studies was finally explained by Szilvási and Veszprémi, who showed that the bulky substituents at nitrogen promote the reaction by means of stabilizing dispersion interactions.282 The use of B3LYP-D/ccpVTZ, wB97X-D/ccpVTZ, and SOS-MP2/cc-pVTZ methods was important because they incorporate a proper dispersion description. Using the unencumbered model compound HNacNac′Si (HNacNac′ =

For comparison, Roesky’s silylene 390 affected even deeper disintegration of white phosphorus, producing the first example of an acyclic Si2P4 chain with 6π electrons (391, Scheme 117), resembling the carbene derivative 368 discussed above.277 Reaction of disilene 392, shown to exist in equilibrium with the corresponding silylene in solution, with P4 resulted in double insertion of the two-coordinate silylene into the phosphorus tetrahedron which yielded the silicon−phosphorus cage 393. Interestingly, a Cp* group has migrated from silicon to one of the adjacent phosphorus atoms. Utilization of the chloro-substituted amidinato silylene PhC(NtBu)2SiCl (307) resulted in further fragmentation of the P4 tetrahedron to furnish the disiladiphosphacyclobutane 394.278 Alternatively, 394 can also be prepared by reacting disilylene 395 with P4. The X-ray crystal structure of 394 revealed a planar Si2P2 ring with four equivalent Si−P bonds while the bond angles within the ring suggest a bis-ylide structure in which the two silicion atoms are formally positively charged while the two phosphorus atoms carry partial negative charge. DFT calculations revealed that the Si2P2 ring contains four single bonds and has no antiaromatic character along with two lone pairs of electrons at each phosphorus atom. The natural charges obtained from NBO analysis confirmed significant charge separation between silicon and phosphorus consistent with the zwitterionic character of the ring. Damrauer and Pusede studied computationally a related reaction of white phosphorus with the parent singlet silylene :SiH2 (with a singlet-to-triplet gap of 21 kcal mol−1).279 The most stable product is the result of silylene insertion into the P−P bond; however, the barrier for insertion is quite high. Five more stable forms with the composition H2SiP4 were located on the potential energy surface. These products can be described as (i) a structure in which the P−P bond is cleaved as a result of 3646


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Scheme 118. Activation of P4 by Distannyne 396

[HNC(CH2)CHC(Me)NH]2−), the researchers confirmed that the reaction proceeds by an electrophilic mechanism with Eact = 26.3 kcal mol−1, whereas the nucleophilic attack of silylene at P4 leads to a much high barrier (Eact = 50.0 kcal mol−1). The rate-determining step of the electrophilic mechanism is the initial silicon attack at a phosphorus atom to make a SiP double bond, accompanied by the cleavage of two P−P bonds and formation of a cyclotriphosphirene ring. For the actual molecule DippNacNac′Si, the first reaction barrier decreases to 20.3 kcal mol−1, such that it becomes even lower than the subsequent conformational change (20.6 kcal mol−1) preceding the formation of the second Si−P bond, thus becoming the formal rate-determining step. Roesky et al. reported the first example of white phosphorus activation by a low-oxidation state tin complex in 2011. The pincer-supported distannyne [(2,6-iPr2C6H3NCCH3)2C6H3Sn]2 (396) opened one P−P bond to give a tin-capped tetraphosphabicylobutane derivative 397 (Scheme 118).283 Complex 396 features a long Sn−Sn distance of 2.8981(9) Å which is more consistent with a single bond; thus, the compound is better described as a distannylene. Interestingly, while the starting complex showed asymmetric ligation of the two tin centers by the flanking imino groups, both tin centers in 397 are coordinated by both imino moieties. Weigand and co-workers have demonstrated the ability of phosphenium cations, phosphorus analogues of carbenes, to insert into the P−P bonds of the P4 tetrahedron.284 The reaction of P 4 and Ph 2 PCl activated with GaCl 3 yielded the polyphosphorus clusters [Ph2P5]+, [Ph4P6]2+, and [Ph6P7]3+ via consecutive insertion of the reactive phosphenium carbenoid [Ph2P]+ into the P−P bond of P4 (Scheme 119).285 Similarly,

and dicationic derivatives [(DmpNP) 3 (P 4 )Cl 2 ] + and [(DmpNP)3(P4)2Cl]2+.290 2.12.3. Chalcogen−Chalcogen Bond Activation. The aluminum(I) compound (Cp*Al)4 (398) reacted with excess elemental selenium and tellurium to furnish colorless [Cp*AlSe]4 (399) and pale green [Cp*AlTe]4 (400, Scheme 120).291,292 These tetrameric compounds formed (Al−E)4 heterocubane structures that were elucidated by X-ray diffraction analyses. Similar heterocubane structures of the composition [(Me3Si)3CGaE]4 (E = S, Se, Te) are produced upon the reaction of Ga(I) compound 359 with sulfur, selenium, and tellurium. Whereas the selenium and tellurium derivatives have low solubility in benzene, the sulfur analogue dissolved readily to give solutions of the dimeric species [(Me3Si)3CGa(μ-S)]2.293 The reaction of [In−C(SiMe3)3]4 with sulfur and tellurium furnished the analogous tetrameric indium sulfide and telluride, as reported by Uhl et al.294 In contrast to these cage compounds, Al(I) complex 11 and its Ga(I) analogue DippNacNacGa (18) reacted with S8 to form the eight- and four-membered heterocycles [DippNacNacAl(μS3)]2295 and [DippNacNacGa(μ-S)]2,296 respectively. Likewise, reaction of DippNacNacGa with elemental tellurium led to [DippNacNacGa(μ-Te)]2.251 DippNacNacAl also cleaved the S−S bond of diphenyl disulfide to give complex 402248 while Dipp NacNacGa reacted with diphenyl ditelluride to furnish the bis(phenyltelluride) gallium complex 403 (Scheme 121).251 Parkin et al. showed that the gallium(I) compound supported by a tripodal pyrazolylborato ligand (404) easily cleaved the E−E t

bonds in chalcogens to give terminal chalcogenides (Tp Bu2)Ga t

E (Tp Bu2 = tris(3,5-di-tert-butyl)pyrazolylhydroborate; E = S, Se, or Te) unlike the cage and dimeric structures discussed above.297,298 A similar oxidative addition of elemental selenium

Scheme 119. Reaction of Ph2Cl and GaCl3 with P4 To Yield Polyphosphorus Clusters

t

to an indium(I) pyrazolylborate (406) resulted in (Tp Bu2)In Se (Scheme 122).299 On the other hand, the anionic gallium analogue of NHC, 92, failed to react with elemental sulfur, selenium, or tellurium. The lack of reactivity was attributed to the low solubility of the chalcogen elements in THF (in the case of Se and Te) and to the relative strength of the S−S bonds in S8. However, when 92 was reacted with soluble PhE−EPh (E = Se, Te), clean oxidative addition of the E−E bond took place (Scheme 123).300 Uhl et al. reported the related cleavage of the Te−Te bond of (Me3Si)3SiTe−TeSi(SiMe3)3 by the digallane ((Me3Si)2HC)2Ga−Ga(CH(SiMe3)2)2 in toluene at 100 °C to furnish the coordinatively unsaturated gallium telluride ((Me3Si)2HC)2Ga−TeSi(SiMe3)3.301 For main-group compounds based on Group 14, Jutzi et al. reported that the nucleophilic silylene Cp*2Si: reacted with sulfur to form the dithiadisiletane [Cp*2Si(μ-S)]2.302 Similarly, West and Denk’s stable silylene :Si(N(tBu)CH)2 (125) reacted with sulfur or selenium to give the chalcogen bridged

chlorophosphines RPCl2 activated by GaCl3 or AlCl3 reacted with white phosphorus to give chloro-substituted organophosphorus cages [RP5Cl]+.286 Analogous reactions took place with amino-substituted dichlorophosphines (R2N)PCl2 (R = iPr, Cy) activated by GaCl3.287 Cyclic phosphenium cations [DippNP]22+ and [Cl2Si(Me3SiN)2P]+, generated via chloride abstraction from cyclo-1,3-diphospha-2,4-diazane [DippNPCl]2 or from Cl2Si(Me3SiN)2PCl by a Lewis acid, also inserted into the P−P bond of P4.288,289 More recently, reaction of the cyclotriphosphatriazenium cation [(DmpNP)3Cl2]+ (Dmp = 2,6dimethylphenyl) with white phosphorus furnished the mono3647


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Scheme 120. Reaction of (Cp*Al)4 (398) and [Ga−C(SiMe3)3]4 (359) with Elemental Sulfur, Selenium, and Tellurium

Scheme 121. Reaction of Diphenyl Dichalcogenide with Dipp NacNacAl and DippNacNacGa

Scheme 124. Reaction of Silylenes 125 and 411 with Sulfur and Selenium

Scheme 122. Cleavage of E−E Bonds by Pyrazolylborate Supported Ga(I) and In(I)

Scheme 123. Cleavage of E−E Bonds on a Heterocyclic Gallanide

Scheme 125. Reactions of Silylene 183 with Sulfur, Selenium, and Telerium

dimer 410 (Scheme 124).182 The analogous reaction was also observed with Lappert’s silylene :Si(N(CH2tBu)2C6H4-1,2) (154).184 In contrast, Okazaki’s bulkier silylene Tripp(Tbt)Si: (411, Tbt = 2,4,6-((Me3Si)2CH)3C6H2), generated in situ upon reduction of the dibromide precursor Tripp(Tbt)SiBr2, cleaved and fragmented S8 to give the monomeric, five-membered cyclic tetrathiosilolane Tripp(Tbt)Si(−S−)4 (412).303 A related compound 413 was formed in the reaction of dialkylsilylene 183 with elemental sulfur (Scheme 125).304 This

polysulfide can be subsequently desulfurized upon addition of triphenylphosphine, affording the terminal sulfide 414. The same 3648


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Scheme 126. Reactions of Bis(amidinato) Silylene 315 and Amido-amidinate 390 with Chalcogens

compound can also be prepared by the direct reaction of 183 with trimethylphosphine sulfide. Compound 183 also cleaved the E−E bonds of elemental selenium and tellurium to furnish terminal chalcogenides 415 and 416, respectively. The nature of the SiE bond was elucidated by X-ray diffraction analysis, UV− vis spectroscopy, and DFT calculations. As could be expected from the relative strength of the π-bond, the extent to which the SiE bond is shortened relative to the Si−E single bond decreases in the order S (9.4%) > Se (8.6%) > Te (7.6%). In contrast, amidinate-supported silylenes afforded terminal chalcogenide derivatives for all heavy Group 16 elements. Tacke et al. found that the donor-stabilized silylene 315 reacted with elemental sulfur, selenium, or tellurium to give five-coordinate silicon(IV) compounds with silicon-chalcogen double bonds (Scheme 126).305 The same silylene reacted with PhSe−SePh to give the six-coordinate product 418 as a result of Se−Se bond oxidative addition.211 Similarly, the Roesky group showed that the related amido-amidinate silylene 390 reacted with sulfur and selenium to afford the four-coordinate chalcogenides 419 and 420.306 The Driess group reported that the NHC-stabilized silylene 130 can be easily oxidized by elemental chalcogens (E = S, Se, Te) giving silanechalcogenones 421 featuring four-coordinate silicon centers.307 Related oxidations of the siloxysilylene 264 resulted in the corresponding silanoic silylesters (Scheme 127). These silicon analogues of carbonic esters contain intramolecular N → Si donor−acceptor supported SiX systems and relatively strong SiX π-bonding interactions.308 Schnöckel and co-workers showed that molecular SiS2 can be synthesized and isolated under matrix isolation conditions at very low temperatures and studied spectroscopically.309,310 Recently, bis(carbene) stabilized silylone 423 was found to induce fragmentation of elemental sulfur to produce an isolable, monomeric silicon disulfide complex featuring terminal Si−S bonds (Scheme 128).311 The cleavage of the E−E bond in chalcogens by germylenes has been intensively studied.10,312,313 These reactions usually lead to compounds containing chalcogen-bridges,314−318 terminal GeE bonds,319−324 or polychalcogenides.325,326 Comparable findings were reported for analogous reactions of stannylenes.327,328 In some cases, the products are transient and undergo subsequent transformations, as is the case for the reaction of GeMes*2 (143) with sulfur which furnished the C−H

Scheme 127. Oxidation of Silylenes 130 and 264 by Elemental Chalcogens

Scheme 128. Reaction of Silylone 423 with Sulfur

activation product 425 instead of the germathione Mes*2GeS (425′, Scheme 129).114 Parkin et al. showed that oxidative addition of elemental chalcogens to Ge(II) and Sn(II) centers supported by a tetraamino macrocycle (426 and 428) gave five-coordinate derivatives of Ge(IV) and Sn(IV) with a terminal chalcogenido group (427 and 429, Scheme 130).329,330 Oxidative additions of organochalcogenides have also been described. Thus, reactions of dichalcogenanes PhE−EPh with the Ge(II) bis(amidinate) complex 430 and amido-amidinates 433 and 434 were shown to proceed with insertion into the E−E bond and formation of the bis(chalcogenides) 431 to 432 and 435 to 436, respectively.331 Related reactions have also been demonstrated with Ge(II) bis(amides).332,333 3649


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Scheme 129. Reaction of Germylene 143 with Sulfur

Jambor and Jurkschat reported on the activation of chalcogens by the pincer-supported distannyne 437,334−336 more aptly described as a distannylene with a Sn−Sn single bond.337 These reactions led to the Sn(II) chalcogen-bridged species 438 (Scheme 131). In the case of selenium, the oxidation proceeds further to give an anhydride of triseleneoxostannoic acid, whereas with sulfur a polysulfide Sn(IV) derivative with a pentasulfide and two monosulfide bridges forms. The monosulfur-bridged dimer 438-S was also reacted with selenium and tellurium to produce mixed organotin(IV) chalcogenides.338 Jambor and Jurkschat extended their work on pincersupported distannynes (or distannylenes) to include the oxidative addition of dichalcogenides ArE−EAr (E = S, Se, Te) to the Sn(I) compound 437.339,340 These reactions proceeded with E−E bond cleavage and gave Sn(II) chalcogenides while the Sn(II) sulfide and selenide can be oxidized further to the corresponding Sn(IV) trichalcogenides (Scheme 132). A similar reactivity was also observed with the OCO pincer ligated distannynes; however, the second oxidative addition of ArSe− SeAr does not occur even upon heating to 120 °C in THF.339 The related NCN pincer compound of Bi(I), 2,6(NMe2CH2)2C6H3Bi, prepared in situ by reacting the chloride precursor 446 with K-Selectride, was also shown to cleave the PhE−EPh bonds (E = S, Se, Te), producing Bi(III) phenylchalchogenolates 447.341 Surprisingly, similar reactions of the monoarmed organobismuth(I) compound (o-(DippNCH)C6H4)Bi, generated in situ by reduction of the chloride 448, produced diorganobismuth chalcogenides 450 as a result of

Scheme 130. Activation of Elemental Sulfur, Selenium, and Tellurium with Germylenes and Stannylenes

Scheme 131. Oxidative Addition of Chalcogens to Distannyne 437

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Scheme 132. Oxidative Addition of Dichalcogenides to Distannynes 437 and 443

Scheme 133. Oxidation Addition of Dichalcogenides to Organobismuth Compounds 446 and 448

Scheme 134. Oxidative Addition of I2 and Me3SiI by 451

rearrangement of the unstable products 449 (Scheme 133).342 In contrast, the analogous antimony(I) complex [(o-(DippN CH)C6H4)Sb]4 facilitated the oxidative addition of PhE−EPh to give Sb(III) derivatives (o-(DippNCH)C6H4)Sb(EPh)2 (E = S, Se, Te). The stability of the pincer compounds of bismuth relative to the monoarmed system can be attributed to the extra coordination of the nitrogen donor in the former which prevents redistribution to the bis(ligand) complex. Schulz and co-workers reported that the Zn(I) dimer [MesNacNacZn−]2 cleaved the E−E bonds of PhE−EPh to afford Zn(II) derivatives MesNacNacZn−EPh (E = Se or Te).343 Interestingly, an analogous reaction with elemental chalcogens and chalcogenophosphines EPBu3 (E = S, Se, Te) does not occur.

mainly the silicone polymer (Me2SiO)n, to give the mixed hydrido/siloxy bridged dimer (MesNacNacMg)2(μ-H)(μ-OSiMe2H).79 The mechanism of this reaction remains unknown but likely involves a Si−O/H−Mg metathesis. The Jones group also reported on the oxidative additions of I2 and Me3SiI to the guanidinate gallium complex (DippNC(NCy2)NDipp)Ga (451) to yield gallium(III) derivatives (Scheme 134).344 Both products were spectroscopically characterized, and the structure of the silyl substituted compound 452 was confirmed by X-ray diffraction analysis. In addition to the reaction with tBuCl discussed above, the Fischer group also studied the oxidative addition of GaMe3, GaCl3, SiCl4, and Me2SnCl2 to DippNacNacGa (18, Scheme 135).216 Reactions with GaMe3 and SiCl4 afforded the corresponding gallium species 454 and 455, whereas GaCl3 and Me2SnCl2 underwent double oxidative additions to give trimetallic species 456 and 457. Related additions of Me3PbCl and Pb(OSO 2 CF 3 ) 2 afforded bimetallic compounds

2.13. Other Bond Activations

Jones et al. reported that the hydride-bridged dimer [MesNacNacMg(μ-H)]2 reacted with silicon grease, which is 3651


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The groups of West and Apeloig reported that the cyclic silylene :Si(N(tBu)CH2−)2 (462) with a saturated backbone readily underwent insertion into the Si−N bond of a second molecule of 462 to give, after dimerization, the disilene 464 (Scheme 138).350 The unsaturated analogue, Si(N(tBu)CH)2

Scheme 135. GaCl, GaMe, SiCl, and SnCl Bond Activation by DippNacNacGa (18)

Scheme 138. Formation of Disilene 464 via Insertion of Silylene 462 into a Si−N Bond

(125), does not show this reactivity. DFT calculations revealed that, due to the size of the tBu groups, 462 cannot make a donor− acceptor dimer with N → Si bonds, nor can it dimerize into a tetraaminodisilene structure. Calculations for an N-methylsubstituted analogue showed that insertion into the Si−N bond is feasible, such that the resulting silylamino silylene is only 0.5 kcal mol−1 less stable than two separate diamino silylenes and that the barrier for insertion is 10.9 kcal mol−1. Dimerization of this intermediate afforded a disilene which is 27.3 kcal mol−1 lower in energy. The reason why diaminosilylenes such as 462 do not dimerize, while silylamino analogues do, lies in the larger singlet−triplet gap of the former. For example, the ΔEST of Si(NMe2)2 was calculated to be 68.7 kcal mol−1, whereas for Si(SiH3)(NMe2) it is only 45.2 kcal mol−1. In 1998, Belzner et al. described the oxidative addition of Cl− SiCl(Mes)(2-Me2NCH2C6H4) to a diarylsilylene, generated in situ by theromolysis of the hexaaryl cyclotrisilane precursor at 90 °C.351,352 With hydrochlorosilanes, both Si−H and Si−Cl additions were observed. Shortly after, Kira et al. showed that the isolable silylene 183 oxidatively cleaved the Si−Cl bonds of dichlorodimethylsilane, tetrachlorosilane, and dichlorosilane to give the corresponding 1,2-dichlorodisilanes 465 (Scheme 139), whereas reaction with dimethylchlorosilane afforded the Si−H insertion product 466 exclusively.145,353 DFT calculations of the reaction of SiMe2 with chlorosilanes ClSiYR2 (R = Me or H; Y = H, Cl, F, Me, or SiH3) revealed stereoelectronic substituent effects, of which the most important is the nucleophilic interaction of the silylene lone pair with the σ* (Si−Y) orbital.354 The effect of the substituents at silicon on the activation energy of insertion was also investigated.355 Unlike the related reaction of silylenes with the C−Cl bonds (section 2.9), a radical pathway is much less favorable than direct insertion (ΔG‡ = 44.1 kcal mol−1 for the radical pathway vs ΔG‡ = 15.3 kcal mol−1 for the direct insertion). Calculations suggest that electron-withdrawing substituents Y in the chlorosilane facilitate the insertion pathway due to the increased interaction between the silylene lone pair and the σ* (Si−Y) orbital while the substituent R mainly exerts a steric effect. Jutzi et al. attempted the preparation of the germanone Mes*2GeO by oxidation of germylene :GeMes*2 (143) with trimethylamine N-oxide (an N−O cleavage reaction). The product isolated, however, was germaindanol 467, apparently formed as a result of the addition of a C−H bond from an ortho t Bu group across the reactive GeO bond (Scheme 140).115 A series of N−O bond cleavage reactions have been reported by Power and co-workers with the dimetallynes ArDippGe

Dipp

NacNacGaCl(PbMe3) and [DippNacNacGa(OSO2CF3)]2Pb, respectively.345 Oxidative addition of the Sn−Cl bond likely occurred in the reaction between SnCl2 and 18 as reported by Fischer et al.346 The product of this reaction is best described as a Zintl-type anionic [Sn17]4− cluster decorated by an electrophilic shell of gallium ligands, with the gallium atom being in the +3 oxidation state. Likewise, Bi(OR)3 (R = SO2CF3 or C6F5) reacted with 18 to give gallium-supported dibismuthenes, 458 and 459, with concomitant elimination of DippNacNacGa(OR)2 (Scheme 136). Scheme 136. Reaction of 18 with Bi(OR)3

The dibismuthenes exhibit short BiBi bonds of 2.8111(2) and 2.8182(4) Å.347 The reaction apparently proceeds via Bi−O oxidative addition to Ga(I) followed by elimination of Dipp NacNacGa(OR)2 from a Bi(III) intermediate to give Bi(OR). Finally, Bi(OR) adds to another equivalent of 18 with subsequent dimerization to furnish the final product. Schulz et al. reported a related reaction between 18 and stibanes Sb(NMe2)3 and SbCl3, leading to the gallium-supported distibene 460 and 461, respectively, along with elimination of Dipp NacNacGaX 2 (X = NMe 2 , Cl; Scheme 137).348,349 Interestingly, heating 460 to 120 °C resulted in disproportionation into 18 and the gallium-ligated tetraantimony analogue of bicyclo[1.1.0]butane, [DippNacNacGaNMe2]2Sb4. The more robust stibane SbEt3 does not react with 18. Scheme 137. Reaction of 18 with Stibanes

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Scheme 139. Oxidative Addition of Si−Cl and Si−H Bonds with Kira’s Silylene 183

Scheme 140. N−O Bond Cleavage by Germylene 143

Scheme 141. N−O Bond Cleavage by Dimetallynes 37 and 50

GeArDipp (37) and ArDippSnSnArDipp (50). Reaction of 50 with TEMPO gave the dimeric tin hydroxide 468 along with 2 equiv of 2,2,6,6-tetramethylpiperidine (Scheme 141).356 The production of 468 was proposed to proceed through a putative Sn(ArDipp)TEMPO intermediate where the N−O bond is homolytically cleaved to give the ArDippSn−O• and •NR2 fragments. The radicals may then abstract hydrogen from the solvent to furnish 468 and HNR2. Support for the proposed intermediate is given by the analogous reaction of 37 with TEMPO which yielded Ge(ArDipp)TEMPO as the sole product in high yield. In a separate report, 37 and 50 were reacted with pyridine N-oxide to give the ditetryldiyl ethers 469 and 470, respectively357 An adduct between pyridine N-oxide and 37 or 50 followed by pyridine elimination and rearrangement of the unsymmetric monoxide intermediate to 469 and 470 was proposed as the mechanism of the reaction.

The Lappert group also described the double insertion of silylene :Si(N(CH2tBu)2C6H4-1,2) (154) into the E−N bond of Group 14 bis(amides) :E(N(SiMe3)2)2 (E= Ge, Sn, Pb).129 The corresponding bis(silyl) derivatives are stable for the tin and lead derivatives (Scheme 142) but underwent C−H bond activation in the germanium analogue (Section 2.3.2). In 2012, the Radosevich group reported that the T-shaped phosphorus compound 226 reacted with ammonia-borane to furnish the dihydridophosphorane 473, a two-electron redox process that oxidized the phosphorus center from +3 to +5 (Scheme 143).358 Alternatively, phosphorus remained in the +3 oxidation state if one considers the redox activity of the ligand, such that the initial bis(alkoxy)amido fragment transforms into a mono(alkoxy)ketoimine ligand. Interestingly, compound 473 can hydrogenate azobenzene to 1,2-diphenylhydrazine and regenerate 226, thus closing a potential catalytic cycle. Indeed, both 226 and 473 catalyzed the transfer hydrogenation of 3653


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Scheme 142. Insertion of Silylene 154 into E−N Bonds

Scheme 144. Reaction of Ammonia-Borane with 229

several recent reports on the cleavage of multiple bonds, including CO (178 kcal mol−1), CN (147 kcal mol−1), and CS (137 kcal mol−1), by main-group compounds which circumvent this restriction. These findings defined several strategies that can be used in the activation of multiple bonds and the design of future catalytic cycles. In brief, these strategies can be categorized as (i) oxidative addition of the XY bond to a single reactive main-group center if either X or Y moiety is stable in its original subvalent state, i.e. X = CO or CNR; (ii) cleavage of XY on two low-oxidation state main-group centers, which happens, for example, in the additions to heavy analogues of alkynes; (iii) oxidative addition accompanied by migration of a substituent from the main-group center to either X or Y, which makes the whole process a feasible two-electron reduction. Braunschweig et al. reported an unusual cleavage of the CC triple bond of 2-butyne upon photolysis of the phosphinestabilized diborene 475 in diethyl ether at room temperature to give the monophosphine-stabilized homoaromatic 1,3-dihydro1,3-diborete 476 (Scheme 145).361 Interestingly, analogous reaction of 1,2-bis(trimethylsilyl)acetylene with the benzeneligated dialumene 13 resulted in cycloaddition to give the 1,2dialuminacyclobut-3-ene 47744 whereas the related digallene ArDippGaGaArDipp (16) underwent double cycloaddition of phenylacetylene to furnish 1,4-digallacyclohexa-2,5-diene 478.362 In both cases complete cleavage of the CC bond does not occur. The Braunschweig group also showed that addition of propyne to the cAAC supported diboryne 5 resulted in the triplet biradical 479.361 The localization of two electrons on the cAAC ligands allowed 479 to avoid the formation of an antiaromatic diboracyclobutadiene, such that only two electrons are delocalized within the central ring which becomes aromatic. Compound 5 is inert to 2-butyne; however, with the smaller alkyne acetylene, double cycloaddition took place to give the cAAC-ligated diborabenzene 480. The first example of formal CX bond addition to a maingroup center was likely the report by Schmidpeter and coworkers on the formation of the 2-phosphaallyl cation 483 (Scheme 146).363 They found that the P(I) cation 481 cleaved the activated CC bond of diimidazolidinylidene to afford what we would now categorize as an NHC-stabilized phosphinidene. Although the mechanism of this reaction is unavailable, one can envisage that the triphosphenium salt 481 loses an equivalent of tris(dimethylamino)phosphine at higher temperature to generate a reactive cationic phosphinidene [(Me2N)2P−P]+, isoelectronic and isolobal with silylene, that can react with the tetraaminoethylene derivative to furnish 483 via an undetected phosphiranylphosphonium intermediate 482. The first example of CO bond cleavage was reported by Jutzi et al.364,365 Nucleophilic silylene Cp*2Si: (484) cleaved the CO bond of CO2 under mild conditions, but the identity of the product obtained was dependent on the solvent used. Thus, addition of CO2 at room temperature to a solution of Cp*2Si in toluene led to the spiro compound 486 (Scheme 147), whereas in pyridine this reaction yielded the eight-membered cyclic species 487. It was suggested that the reaction proceeded via C O oxidative addition and extrusion of CO to give the reactive

Scheme 143. Reaction of Ammonia-Borane with 226

azobenzene, a rare example of transition-metal-free transfer hydrogenation. Monitoring the reaction by 31P NMR spectroscopy revealed that 473 is the resting state of the catalyst. Sakaki et al. studied this reaction by DFT calculations and concluded that direct hydrogen transfer from ammonia-borane to 226 proceeded with a very high transition state (42.7 kcal mol−1).359 A lower energy pathway was found in which a proton from the ammonia end of H3NBH3 went to the oxygen atom and the hydride from boron goes to phosphorus to give a secondary phosphine with a dangling alcohol group. The latter compound rearranges to the more stable dihydridophosphorane by a H2NBH2-assisted mechanism. Likewise, direct transfer of hydrogen from the phosphorus atom of 473 to azobenzene was calculated to be a higher energy process than a stepwise mechanism proceeding via the OH/PH intermediate. However, this description neglected several observations. First, Radosevich et al. showed that oxidative addition/reductive elimination from 226 proceeded in a stepwise fashion169 and not via a concerted three-center manner probed by Sakaki. Second, the enol intermediate suggested by the Sakaki group was computed to be approximately 8 to 10 kcal mol−1 higher in energy than the corresponding keto tautomer; thus, one can expect that the keto tautomer would be formed preferentially. If this were the case, preparation of a deuterium labeled compound (ONO)PD2 would result in deuterium incorporation into the vinylic position of the ligand backbone. This experiment was carried out by the Radosevich group, but no scrambling of deuterium was observed.358 Therefore, the transfer hydrogenation from the dihydride 473 to azobenzene should proceed directly, albeit in a stepwise manner, without involving the formation of an enol tautomer. In a related study, Kinjo and co-workers described reaction of diazadiphosphapentalene 229 with ammonia borane leading to the hydrogenation of the P−N bond (Scheme 144); however, no applications to catalysis were reported.360

3. CLEAVAGE OF CX AND PX DOUBLE BONDS Complete cleavage of an XY double bond is a four-electron process which is rarely observed for transition metals and appears to be impossible for main-group elements whose oxidation states vary in a much narrower range. Nevertheless, there have been 3654


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Scheme 145. Cleavage of CC Bonds by Group 13 Complexes

Scheme 146. CC Bond Cleavage by Triphosphenium Salt 481

Scheme 147. Activation of CO2 by Silylene Cp*2Si: (484)

silanone 485 followed by cycloaddition of a second equivalent of CO2. When the reaction was carried out in the presence of an enolizable ketone, the product was an enol-substituted silanol, presumably formed by deprotonation of ketone by the highly polar CO bond of 485.

The Jutzi group further found that Cp*2Si cleaved the CS bonds of COS and RNCS (R = Me, Ph) to give the sulfidebridged dimer [Cp*2Si(μ-S)]2 (488).364,365 In the reaction with PhNCS, the sulfide-bridged dimer is formed upon heating at 65 °C but undergoes further cycloaddition after heating to 100 °C 3655


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Scheme 148. CS Bond Activation by 484

Scheme 149. Reaction of Kira’s Dialkyl Silylene (183) with CO2, Isocyanate, and Isothiocyanate

Scheme 150. Deoxygenation of N2O or CO2 by 264

Scheme 151. Reaction of Bis(amidinato) Silylene 315 and Bis(guanidinato) Silylene 495 with N2O and CO2

carbonate-bridged disilanol (Scheme 149). The reaction likely followed a similar pathway proposed for the chemistry reported by Jutzi. DFT calculations revealed that the rate limiting step was the [1 + 2] addition of the CO bond to Si(II) to generate a silaoxacyclopropanone structure which then readily extruded CO, forming a labile silanone. The latter is intercepted by CO2 to give a carbonate intermediate and then an analogue of the spirocompound 486 described by Jutzi. Decomposition of the latter

(Scheme 148). Reaction with CS2 also proceeded easily to give a sulfide-bridged structure that is complicated by a multistep rearrangement that included the migration of one Cp* ligand to the CS2-derived carbon center and its coupling with the second Cp* to give a [3,0,1] bicyclic product. Kira and co-workers investigated the reactivity of the highly reactive silylene 183 toward heterocumulenes.366 The silylene was shown to reduce CO2 to CO, yielding, after workup, a 3656


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species furnished the final, isolated product 490. Related reactions with isocyanate DippNCO and isothiocyanate DippNCS proceeded via CX bond (X = O, C) cleavage to generate products of cyclization (491 and 492) with the intermediate silanone and silathione, respectively, akin to the corresponding reactions of Cp*2Si described above. The proposed silanethione intermediate can be independently accessed via reaction of silylene 183 with phosphine sulfide.304 In contrast, the isolable DippNacNac′ supported silylene 87 prepared by Driess et al. does not react with CO2,234 but interestingly, the related siloxysilylene 264 deoxygenated N2O and CO2 to afford the first example of a silanoic silyl ester 493 (Scheme 150).367 The reactive SiO bond is prone to methanolysis to furnish the methoxy silanol 494. Along the same lines, the Tacke group showed that the bis(amidinato) silylene 315 and related bis(guanidinato) silylene 495 deoxygenated N2O or CO2 to produce transient silylones (Scheme 151).211 In the reaction with N2O, the silylone dimerized to give cyclic disiloxanes 496 and 497, while a reaction with excess CO2 gave carbonates 498 and 499. Carbon disulfide reacted with 315 in a similar fashion to give a trithiocarbonate derivative, whereas with bis(guanidate) 495 a donor−acceptor complex formed.368 Likewise, silepin 30, which served as a masked form of the reactive silylene 31, reacted with CO2 to give the corresponding silyl carbonate.62 Okazaki et al. described the related addition of CS2 to the bulky disilene 500. It was suggested that upon heating to 60 °C the disilene dissociated into the silylene 181 which then reacted with CS2 to form a 1:1 adduct (Scheme 152).369 The second

Scheme 153. Insertion of Silylene 183 into the C−C Bond of Benzenes and a Ni(II) Norcorrole Complex

resulting in the change of the oxidation state of silicon from +2 to +4. With the more sterically hindered substrate mesitylene, the excited silylene inserts into the benzylic C−H bond. The Iwamoto and Shinokubo groups later demonstrated the regioselective insertion of 183 into the antiaromatic Ni(II) norcorrole complex to give 503.372 The amidinato silylene PhC(NtBu)2SiCl (307) reacted with adamantyl phosphaalkyne to furnish compound 504 containing a four-membered Si−C−Si−P ring, representing the first example of a triple bond cleavage on a main-group center (Scheme 154).278 The same product is also formed upon addition of phopshaalkyne to disilylene 395. Both silicon centers in 504 are nearly equidistant from both the carbon and phosphorus vertices, and the distances of the respective Si−C and Si−P bonds are intermediate between single and double bonds. It was argued that the central ring has zwitterionic character, with the positive charge localized on the Si−C−Si fragment and the phosphorus atom being negative, which allowed the compound to avoid destabilization through antiaromaticity associated with four πelectron systems. The calculation of nucleus-independent chemical shifts further corroborates the absence of antiaromatic character. The related reaction of 307 with diphenylacetylene resulted in only partial cleavage of the C−C multiple bond and formation of a disilacyclobutene ring.373 In an attempt to prepare terminal GeO and SnO species, Group 14 metallenes :GeArMes2 (45) and :SnArMes2 (56) were reacted with N2O (Scheme 155). The reaction, however, did not afford the intended products, but instead monomeric, gemdihydroxy compounds 506 and 507 were obtained.374 The formation of 506 and 507 is most likely accounted for by the generation of an (ArMes)2MO intermediate which then reacted with adventitious moisture to give the gem-dihydroxy compounds as the isolated products. Multiple bonds between heavier main-group elements exhibit much smaller gaps between the lowest unoccupied and highest occupied frontier orbitals,10 making them attractive candidates for bond activation, including cleavage of the CO bond. Thus, the disilylene (or distorted disilyne) 508 prepared by Baceiredo and Kato displayed high reactivity toward the deoxygenation of CO2 to CO, as shown in Scheme 156.375

Scheme 152. Cleavage of the CS Bond by Disilene 500

addition of 181 gave the spiro intermediate which then underwent rearrangement to afford the final 1,2,4-thiadisiletane product 501. In this case, the four electrons required for the cleavage of the CS bond are provided by the initial SiSi bond. In contrast, the more stable disilene (tBu3Si)PhSi SiPh(SitBu3) reacted with heterocumulenes to give products of [2 + 2] cycloaddition.370 Singlet silylene 181 underwent [1 + 2] addition to aromatic rings to give silacyclopropanes.371 In this process, the benzene ring transformed into cyclohexadiene (or cyclohexene in the case of double silylene addition). In contrast, Kira et al. showed that the dialkyl silylene 183, excited photochemically to the triplet state, inserted into the aromatically stabilized C−C bond of benzene, m- and p-xylenes, and various p-disubstituted-benzenes to give the corresponding silepin derivatives 502 (Scheme 153).106 In the case of substituted benzenes, the insertion occurred regiospecifically at the unsubstituted double bonds. Formally, this reaction can be written as an oxidative addition of a C−C single bond of the hypothetical cyclohexatriene form of benzene and therefore is an allowed two-electron process, 3657


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Scheme 154. Addition of Phosphaalkyne and Alkyne to Silylene 307

Scheme 155. Reaction of Germylene 45 and Stannylene 56 with N2O

Scheme 157. Reaction of Disilicon(0) Compound 510 and Disilicon(III) Compound 511 with CO2

Scheme 156. CO Bond Cleavage by Disilylene 508

515 and 514, respectively. A DFT study of the mechanism of CO2 reduction showed that the rate limiting step (ΔG‡ = 17.0 kcal mol−1) starts with CO2 coordination to a single germanium center and proceeds with the addition of the CO bond to the reactive Ge−Ge bond to give a germacarboxylate intermediate. Further cleavage of the remaining C−O bond overcomes a lower but comparable barrier (ΔG‡ = 16.3 kcal mol−1) that requires cooperative action of both germylene centers. The NO bond of a nitrosoarene was cleaved by digermyne 37 to afford the unsymmetric singlet diradicaloid oxo/imido bridged compound 516 as reported by Power and co-workers (Scheme 160).381 The X-ray crystal structure of 516 revealed a planar Ge2NO core with each germanium center displaying pyramidal geometry. The distance between the germanium atoms is ∼0.3 Å longer than an average Ge−Ge bond, indicating the near absence of a Ge−Ge bond which is consistent with the singlet diradical character of 516. A different strategy is realized in the reactions of germylene/ borane Lewis pair 517 with 4-methoxyacetophenone (Scheme 161).382 The reaction proceeded via insertion of the CO bond into the Ge-amido bond resulting in cleavage of the CO bond and formation of a C−N bond, Ge−O bond, and a dative O → B bond. The oxidation state of germanium increased by two to Ge(IV) as the second pair of electrons required for the cleavage

A similar carbonate- and oxo-bridged motif was realized by Robinson et al. in the oxidation of the NHC-stabilized Si(0) compound 510 with CO2 (Scheme 157).376 The product of this reaction can be classified as a carbene-stabilized silicon carbon mixed oxide 512. Alternatively, 512 can be obtained via CO bond activation by the Si(III) dimer 511. Amidinate-supported disilyne 395, better viewed as a basestabilized disilylene with a Si−Si single bond (2.413(2) Å),377,378 cleaved the CO bond of benzophenone to furnish the unexpected dioxasiletane 513 with a four-membered Si2O2 ring (Scheme 158).379 The mechanism of this reaction remains unknown, but the solvent (THF) was identified as a likely source of the extra protons, as repeating this reaction in THF-d8 resulted in disappearance of the 1H NMR signal of Ph2CH. Jones et al. showed that the highly distorted amido-digermyne 41, which can be better classified as a digermylene with a very long Ge−Ge single bond (2.7093(7) Å), can quantitatively reduce CO2 to CO at temperatures as low as −40 °C.380 The germanium product of this reaction is an oxo-bridged (bis)germylene 514 (Scheme 159), which unlike the silylenes described above is resistant to further oxidation by both CO2 and N2O. The digermyne 41 also easily cleaved the CS bond of CS2 and the CO bond of tBuNCO to give (bis)germylenes 3658


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Scheme 158. Cleavage of the CO Bond of Benzophenone by the Disilylene 395

independently accessed by a direct reaction of isonitrile with 521. Interestingly, related reaction of 521 with neat phenylisocyanate did not lead to either CO or CN bond cleavage but yielded, after 4 days, the product of isocyanate coupling, isolated from the mixture in a low yield.387 The gallium analogue of 521 did not react with methyl, tert-butyl, or phenyl isothiocyanates even upon heating to reflux, likely due to the smaller size of gallium compared to aluminum.388 The aluminum(I) compound DippNacNacAl (11) cleaved the CS bond of cyclic thioureas to furnish neutral monomeric aluminum sulfides (526 and 527) that were characterized by NMR spectroscopy and X-ray diffraction analysis (Scheme 163).389 DFT calculations showed that the AlS bond has multiple bond character, which was further validated upon cycloaddition of phenyl isothiocyanate to furnish 528. The chemistry of Scheme 163 was possible because the cyclic portion of the thiourea is stable in the divalent form as an N-heterocyclic carbene which in turn coordinated to the aluminum center as a two-electron donor. Compound 11 reacted in a similar manner with triphenylphosphine sulfide, cleaving the PS bond (the PS bond energy = 80 kcal mol−1) to give the sulfide Dipp NacNacAl(S)(SPPh3). Attempts to extend this approach to the activation of the C O bond of a cyclic urea resulted in the unexpected rearrangement of the β-diketiminate skeleton and formation of an Al(III) hydride supported by the DippNacNac′ ligand (Scheme 164).390 This reaction was performed at −60 °C and was remarkably facile compared to the thermal reaction of 11 with N-heterocyclic carbenes which gave a closely related product.391 While CO bond activation by 11 was unsuccessful, activation of the weaker PO bond was achieved. Reaction of 11 with 2 equiv of triphenyl or triethylphosphine oxide produced the aluminum hydroxides 531 and 532, respectively. Cleavage of the PO bond was confirmed by the production of free triphenyl and triethylphosphine confirmed by 1H and 31P NMR spectroscopy. DFT study revealed that the transient aluminum oxo species readily deprotonated the slightly acidic methyl proton in the backbone of the ligand to furnish the final products isolated. Furthermore, deprotonation of the ligand in both cases proceeded via successive bimolecular hydrogen transfers as shown by DFT calculations. Finally, the reaction of compound 11 with a cyclic guanidine resulted in cleavage of the CN bond to yield the carbeneligated amido complex 533, the first example of the oxidative

Scheme 159. CO and CS Bond Activation by Amidogermylene 41

Scheme 160. NO Bond Cleavage by 37

of the CO bond is provided by the Ge−N(amido) bond. A related reaction with iPrNCO resulted in deoxygenation of the isocyanate to give a borane-stabilized GeO → B species and an isonitrile moiety intercepted by a second equivalent of 517. DFT calculations showed that the reaction proceeded via addition of the CO bond across the Ge → B dative bond. Related migration processes occurred in the reactions of the borinium ion [Mes2B]+ with CO2 and CS2.383,384 Although the boron products of these reactions were not isolated, the organic products were identified to be [Mes−CO]+ and [Mes−C S]+, respectively, which clearly indicated that the mesityl group migrated to the carbon atom of CX2 and that the CX bond is cleaved. DFT calculations of the reaction with CO2 suggested that the immediate boron product is the labile oxide MesBO and thus the overall reaction can be classified as an oxo/aryl metathesis. Uhl et al. reported that the Al(II) compound ((Me3Si)2HC)2Al−Al(CH(SiMe3)2)2 (521), upon dissolution in CS2, is oxidized by the solvent to give the product of CS cleavage, 522, and the CS2-insertion product 523 (Scheme 162).385 Analogous cleavage of the CS bond in isothiocyanates with 521 proceeded more rapidly (1 h at RT) and afforded the bridging sulfide 524 and the product of isonitrile insertion into the Al−Al bond (Scheme 162).386,387 The latter product can be

Scheme 161. CO Bond Cleavage by Germylene/Borane Lewis Pair 517

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Scheme 162. CS Bond Cleavage by Dialane 521

Scheme 163. CS Bond Activation by NacNacAl (11)

Scheme 164. Reaction of 11 with a Cyclic Urea and Phosphine Oxides

took place at room temperature in less than 24 h, the more sterically shielded 534 only reacted with Et3PS in refluxing toluene. The indium sulfide was not prepared as the reaction with the phosphine sulfide was too slow at room temperature and the starting diindane decomposed in toluene upon reflux. Dialane 521 did not react with phosphine oxides397 but the SO bond of dimethyl sulfoxide was cleaved to give the oxo-bridged derivative ((Me3Si)2HC)2Al(μ-O)Al(CH(SiMe3)2)2.398 The gallium derivative, ((Me3Si)2HC)2Ga(μ-O)Ga(CH(SiMe3)2)2, was similarly prepared upon reacting 534 with 2 equiv of DMSO in refluxing toluene.399 The acetate-bridged digallium(II) compound [(Me3Si)2CHGa(μ-OOCCH3)]2 reacted with propylene sulfide, triethylphosphonium selenide, and triethylphosphonium telluride to give products of sulfur, selenium, and tellurium insertion into the Ga−Ga bond, [(Me 3 Si) 2 CHGa(μOOCCH3)]2(μ-E) (E = S, Se, Te).400 Related reaction of DippNacNacGa (18) with N2O led to the oxo bridged dimer 536296 while reactions of the anionic gallium heterocycle 92 with SPPh3 and SePEt3 yielded an inseparable mixture of products. However, with TePEt3, the reaction returned an extremely air sensitive, yet thermally robust dimer 537 (Scheme 166).300

addition of a CN double bond to any metal center (Scheme 165).392 The reaction mechanism was studied by DFT Scheme 165. Oxidative Addition of the CN Bond by 11 To Give 533

calculations and consisted of two separate unimolecular and bimolecular phases. The unimolecular portion corresponds to the oxidative addition of the guanidine to 11 to give an imido intermediate which subsequently rearranges in a bimolecular fashion to afford the final product 533. Uhl and co-workers revealed that dialane 521393,394 and its gallium and indium analogues, ((Me3Si)2HC)2Ga−Ga(CH(SiMe3)2)2 (534)395 and ((Me3Si)2HC)2In−In(CH(SiMe3)2)2 (535),396 reacted with triethylphosphine chalcogenides Et3PE (E = S, Se, Te) to give products of chalcogen atom insertion into the Group 13 M−M bond ((Me3Si)2HC)2M(μ-E)M(CH(SiMe3)2)2 (M = Al, Ga, In). While most of these reactions 3660


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symmetrical species 539 in a dynamic equilibrium. A stationary state was reached after 8 days at 100 °C with the ratio of 538 and 539 approximately 1:2. When the temperature was lowered to 50 °C, the relative proportion of 538 increased until both organoboron species were equally abundant in the mixture. When an independently prepared solution of 539 was heated under an atmosphere of H2, it resulted in the production of 538 that was similarly in a dynamic equilibrium with 539 and H2. The mechanism of dihydrogen activation was investigated by DFT calculations, and a modest activation barrier of 21.0 kcal mol−1 was found for the concerted 9,10-addition of H2 to give the final product. On the other hand, a prohibitively high activation barrier (60.7 kcal mol−1) was calculated for the 1,2-addition across the B−C bond as well as the subsequent [1,3]-H migration (58.1 kcal mol−1) to yield the final product. The Ozin laboratory disclosed the reversible photochemical activation of methane by aluminum atoms.402 As revealed by a combination of spectroscopic techniques (UV−vis, IR, and EPR), photolysis of aluminum atoms (368 or 305 nm) isolated in neat methane at 12 K resulted in a rapid depletion of features associated with aluminum atoms while giving rise to new signals attributed to H3CAlH. Subsequent broad-band photolysis above 400 nm resulted in the disappearance of signals for CH3AlH and recovery of about 80% of the aluminum atoms initially present with no other products detected. Later, the same laboratory investigated the behavior of aluminum atoms condensed in pure silane or in a silane/argon mixture at 12 K. A combination of UV−vis, IR, and EPR spectroscopies in conjunction with theoretical calculations supported the presence of a groundstate complex with a three-center, two-electron agostic interaction.403 Upon brief irradiation at 400 nm, this species is converted to the bent orthorhombic Si−H activation product H3SiAlH.404 Regeneration of the ground-state complex via reductive elimination of SiH4 was accomplished with photolysis at 520 nm over a much longer time period than that of the insertion reaction. In 2013, Fischer et al. reported the first example of reductive elimination from an isolable aluminum compound.405 The authors found that Cp*AlH2 eliminates Cp*H in benzene at 70 °C with concomitant production of metallic aluminum. Moreover, they observed that in toluene solutions at 110 °C Cp*2AlH exists in an equilibrium with Cp*H and Cp*Al, which precipitates out of solution as a tetramer. Cowley and coworkers studied the reductive elimination of Cp*H from Cp*2AlH and determined the activation and thermodynamic parameters for this process.406 It was found that reductive elimination was suppressed by the addition of external donors, such as DMAP or NHC, which formed four-coordinate species that were isolated and characterized. The reverse reaction, oxidative addition of Cp*H to Cp*Al, was studied both experimentally and by DFT calculations. The reaction starts with the initial proton transfer from Cp*H to the Al(I) center. The ΔG°300 of oxidative addition was measured to be −3.3 kcal mol−1, which is much lower in absolute value than the standard free energy change for the addition of Cp*H to NacNacAl(I), calculated to be −24.1 to −25.8 kcal mol−1.43 Cowley et al. attributed this difference to the greater stabilization of the Al(I) center in Cp*Al due to aromatization of the Cp* ring. On the other hand, the relatively facile elimination of Cp*H from Cp*2AlH can be ascribed to aromatization of the incipient Cp* ring in the transition state. Nikonov and co-workers found that the Al(I) compound NacNacAl (11) reacted with NacNacAlH2 (12) to give the Al(II)

Scheme 166. Reaction of 18 and 92 with N2O and TePEt3, respectively

4. REVERSIBLE ACTIVATION AND REDUCTIVE ELIMINATION OF σ BONDS Reductive elimination is the culminating step in many important industrial processes (e.g., hydrogenation of olefins, hydroformylation, Wacker oxidation) catalyzed by transition metals. The relative ease of these reactions relates to the accessibility of lower oxidation states for many transition metals. The situation is, however, much more challenging for main-group elements because their low oxidation states are more difficult to access. Currently, this is the most significant hurdle toward the development of a main-group catalyzed bond coupling process based on conventional oxidative addition/reductive elimination steps. The problem is aggravated by the fact that the low oxidation state main-group compounds required for the bond activation step are usually prepared under rather harsh conditions, such as reduction by alkali metals. Despite the associated challenges, there have been some remarkable examples of reductive elimination reactions reported recently that may define patterns for the development of future catalytic cycles. 4.1. Reversible Activation by Group 13 Compounds

Holthausen and Wagner have recently demonstrated the ability of the bis(alkynyl) 9,10-dihydro-9,10-diboraanthracene derivative 538 to eliminate dihydrogen (Scheme 167).401 Heating a THF solution of 538 at 100 °C resulted in a successive decrease of 538 along with concomitant formation of H2 and a new Scheme 167. Reversible Elimination of H2 from 538

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dimer [NacNacAl(H)−]2 (540) with the three species existing in an equilibrium (Scheme 168).40 This addition reaction is

Scheme 170. Reversible Activation of Ammonia by NHC 94

Scheme 168. Reversible Reaction between 11 and 12 To Give 540 Scheme 171. Photoswitchable Activation of Ammonia by 543

reversible as heating the reaction mixture to 50 °C regenerated the starting compounds 11 and 12. Upon returning to room temperature, the original equilibrium mixture was reproduced without any side reactions. The equilibrium between 11, 12, and 540 was studied by variable temperature 1H NMR spectroscopy which revealed the thermodynamic parameters (ΔH = −10.0 kcal mol−1, ΔS = −7.5 cal K−1 mol−1) indicating that elimination of the Al−H bond from 540 is an entropy driven reaction. As discussed in section 2.13, the related gallium(I) compound Dipp NacNacGa (18) underwent irreversible oxidative addition of GaMe3 to furnish the Ga(II) dimer 454. Interestingly, the latter species can undergo a second oxidative addition with 18 to give the trimer 541 (Scheme 169), which exists in a thermal equilibrium with the starting compounds. Therefore, at −40 °C the trimer is the only species in solution observable by NMR spectroscopy; however, at 50 °C 541 completely dissociated to 18 and 454.

addition and reductive elimination of Si−H bonds.408 The reduction of dichlorosilane 545 at room temperature yielded a mixture of hydrosilylene 546 and the dimer 547 as a result of Si− H bond activation (Scheme 172). Initially, the ratio of 546 and Scheme 172. Reversible Dimerization of Phosphine Stabilized Silylene 546

4.2. Reversible Activation by Group 14 Compounds

Reaction of carbene 94 with ammonia gave the N−H oxidative addition product 542 (Scheme 170).156 Remarkably, the ammonia activation was found to be reversible, as addition of sulfur to 542 at low temperature resulted in formation of the corresponding thiourea along with free ammonia, observed as a triplet at −0.17 ppm in the 1H NMR spectrum, upon warming. Ammonia was also observed in the headspace of a standing solution of 542 in benzene via gas chromatography and highresolution mass spectrometry. Utilizing the photoswitchable NHC 543, the Bielawski group has demonstrated the on-demand reversible activation of ammonia.407 While addition of gaseous NH3 to a solution of 543 did not result in any reaction, 543 was consumed along with concomitant appearance of 544 after the sample was subjected to UV radiation (λ = 313 nm) for 30 min (Scheme 171). Irradiation of the sample resulted in photoisomerization of 543 into the ringclosed form which readily activated ammonia to give 544, confirmed by an independent reaction of preirradiated 543 with NH3. Subsequent irradiation with visible-light (λ > 500 nm) for 100 min resulted in the regeneration of 543 along with liberated NH3, confirmed by high-resolution mass spectrometric analysis of the reaction mixture headspace. Baceiredo and Kato reported the reversible dimerization of phosphine-stabilized silylenes as a consequence of oxidative

547 was 70:30. However, the amount of 547 slowly increased until equilibrium was reached after several hours at room temperature, leading to the final molar ratio of 30:70. Further evidence that dimerization of 546 is reversible was demonstrated by dissolving pure 547 in benzene. At room temperature, 546 is slowly produced, leading to the same ratio of 546 and 547 (30:70) observed during the initial reduction of chlorosilane 545 after 7 days. The ratio of 546 and 547 is also temperature dependent, with the amount of 546 increasing concurrently with temperature. The results conclusively demonstrate that dimerization of 546 is an equilibrium process, consistent with the small Gibbs free energy of −2.13 kcal mol−1 at 20 °C. DFT calculations supported direct silylene insertion into the Si−H bond as the operating mechanism, with a calculated activation barrier of 29.1

Scheme 169. Reversible Addition of the Ga−Me Bond to 18

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kcal mol−1. With a bulkier phosphine fragment and less sterically demanding N-aryl substituent supporting the related chlorosilylene, reversible dimerization was observed, proceeding via cleavage and formation of the Si−Cl bond. In a subsequent publication, Baceiredo and Kato described the reversible insertion of silylenes 548 and 549 into the Si−H and P−H bonds at room temperature (Scheme 173).409 The starting

temperature to give the corresponding yellow tungstagermanes 557 and 558, respectively (Scheme 175).410 Interestingly, these additions are reversible when heated between 60 and 80 °C. Analogous addition of dihydrogen occurs at 60 °C to furnish 559, though the reaction is irreversible. Inoue et al. reported that an attempt to prepare the Nheterocyclic imino-stabilized silylene 31, by reacting 560 with 2 equiv of KSi(SiMe3)3, instead yielded the silepin 30 (Scheme 176).62 The latter compound is believed to form as a result of aromatic C−C bond activation of the pendant Dipp substituent by the incipient silylene, akin to the reaction of 183 with aromatic rings shown in Scheme 153. Remarkably, the authors gathered compelling evidence that this silylene insertion is reversible, allowing 30 to serve as a masked form of 31 for small molecule activation and to form a Lewis acid adduct with electrophilic B(C6F5)3. Tetrylenes :MArMes2 (M = Ge (45), Sn (56)) can insert into the C−E bond of Group 13 alkyls (E = Al and Ga), with the outcome of these reactions dependent on the nature of the reactants (Scheme 177).411 Treatment of 45 with AlMe3 and GaMe3 furnished the oxidative addition products 562 and 563, respectively. Reaction of 56 with AlMe3 did not produce a product analogous to 562 but instead gave the pentastanna[1.1.1]-propellane Sn2(Sn(Me)ArMes)3 in low yield. In contrast, the reaction of stannylene 56 with GaMe3 and GaEt3 gave the insertion products 564 and 565; however, this reaction was found to be reversible under ambient conditions. The equilibrium was studied for the reaction of 56 and GaEt3 that gave an equilibrium constant value of 8.09(6) × 10−3 kJ mol−1 and a ΔG value of 11.8(9) kJ mol−1 for the dissociation reaction at 296 K. Likewise, 45 and 56 were found to insert into the C−Zn bond of ZnMe2 to give the zinc substituted products 566 and 567 featuring a four-coordinate Group 14 metal and a two-coordinate zinc (Scheme 178).412 Interestingly, the formation of 567 was found to be reversible at room temperature in hydrocarbon solution. The equilibrium between 56 and 567 was studied, and the dissociation constant, K298diss = 0.0028(5) kJ mol−1, and the free energy of dissociation for this process, ΔG298diss = 14(4) kJ mol−1, were determined. Ragogna and co-workers found that germylene :GeArMes2 (45) inserted into the P−P bond of white phosphorus to give diphosphide 568, the first instance of P4 activation by a germanium compound (Scheme 179).413 Remarkably, the

Scheme 173. Reversible Insertion of Silylenes into the Si−H and P−H Bonds

silylenes and insertion products are in an equilibrium that is temperature dependent, with the position of the equilibrium dependent on the steric congestion around the silylene center and the bulkiness of silane used. Thus, for phenylsilane and 548 at 40 °C, the ratio of 548 and 550 is 6:94, whereas with the bulkier silylene 549 the ratio is 80:20 at 35 °C. For the encumbered combination of 549 and diphenylsilane, the insertion product was not detected due to low concentration but the existence of an equilibrium was established by observation of deuterium scrambling upon a reaction with deuterated diphenylsilane. Similarly, the stannyl substituted species 553 reacted with phenylsilane to give the disilane 550, stannyl-silyl addition product 554, and stannyl migration product 555.409 The mixture of disilanes observed can be rationalized with a series of reversible oxidative addition/reductive elimination reactions (Scheme 174). When excess (15 equiv) silane was used, the only products were 550 and trimethylstannyl(phenyl)silane, formed as a result of Sn−Si bond elimination. The excess amount of PhSiH3, relative to Me3Sn−SiH2Ph, pushed the equilibrium toward the production of 550. These experiments show that not only Si−H but also Si−Sn bonds can undergo reversible oxidative addition to the silylene center. Tobita et al. showed that the green cationic, NHC-supported tungstagermylene (556) added Si−H and B−H bonds at room

Scheme 174. Sequence of Reversible Oxidative Additions and Reductive Eliminations To Give the Products Observed upon Reaction of 553 with Phenylsilane

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Scheme 175. Reversible Oxidative Addition of Si−H and B−H Bonds to Tungstagermylene 556

Scheme 176. Reversible Insertion of Silylene 31 into an Aromatic C−C Bond

Scheme 177. Reaction of Tetrylenes 45 and 56 with Aluminum and Gallium Alkyls

Scheme 178. Reaction of Tetrylenes 45 and 56 with Dimethylzinc

phosphorus tetrahedron is released quantitatively upon short

4.3. Reversible Activation by Group 15 Compounds

exposure to UV light (254 nm) and the system can be cycled 5

In 2014, Stephan and co-workers reported that the aromatic heterocycle 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene (569) could be reduced under 4 atm of H2 to give isomeric [3.1.0]bicyclo reduction products (571 and 571′), with the structure of the major isomer confirmed by X-ray crystallography

times without appreciable reduction in the production of 45. This unusual process represents the first example of reversible activation of white phosphorus by any metal system. 3664


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Scheme 179. Reversible Activation of White Phosphorus by Diaryl Germylene 45

Scheme 182. Reversible Activation of H2 by Biradicaloid 235

(Scheme 180).414 NMR studies revealed that the reaction proceeds via reversible 1,4-addition of H2, confirmed by para-

However, 572 was not stable over the long-term, slowly releasing H2 to regenerate 235. After six months, approximately 90% of 572 and 10% of 235 was detected according to the 31P NMR spectrum. The rate of dihydrogen elimination can be increased by heating a solution of 572 above 60 °C. The activation barrier of H2 addition was found by DFT calculations to be 17.9 kcal mol−1 while the reverse reaction required a higher activation energy of 25.7 kcal mol−1.

Scheme 180. Reversible Activation of H2 by Triphosphabenzene 569

4.4. Reductive Elimination from Group 12 Compounds

Power and co-workers reported that the terphenyl-supported cadmium hydride dimer [ArDippCd(μ-H)]2 (573) is quite temperature sensitive, releasing dihydrogen after a few hours at room temperature to generate the cadmium dimer ArDippCd− CdArDipp (574, Scheme 183).415 Remarkably, the oxidation state of cadmium in this process decreases from +2 to the much rarer +1. Scheme 183. Reductive Elimination of H2 from the Cadmium Hydride Dimer 573

hydrogen experiments, to generate intermediate conformational isomers of 1,3,5-triphosphacyclohexa-1,4-dienes (570 and 570′). The hydrogenated intermediates then undergo an irreversible suprafacial hydride shift concurrent with P−P bond formation to give the final, isolated products. DFT calculations revealed that facile distortion of the planar triphosphabenzene to a boatconformation provides the requisite combination of vacant acceptor (C−P π* orbital localized on phosphorus) and donor (C−P π orbital localized on carbon) orbitals, which allows for the polar addition of dihydrogen to the hexadiene intermediates. The oxidative addition of a wide array of N−H and O−H bonds to the triamido phosphorus compound 231 disclosed by the Radosevich laboratory was discussed earlier in the review.172 Interestingly, the oxidative addition of tert-butyl amine and tertbutyl alcohol was found to be reversible (Scheme 181). Refluxing a toluene solution of either 232-tBu or 280-tBu under a dynamic nitrogen sweep gave 231 via reductive elimination. Biradicaloid 235 reacted with dihydrogen at room temperature to readily furnish the diphosphadiazane 572 (Scheme 182).174


Although elimination of SiMe4 from the ate-complexes KM(CH2SiMe3)3H (M = Ga or In) leading to Ga(I)416 and In(I)417 derivatives was reported a few decades ago, the results were not reproducible and the proposed decomposition likely follows a more complicated pathway rather than a simple reductive elimination.418 The Ga−In bimetallic compound DippNacNacGa(Et)InEt2 (347) slowly decomposed in solution via reductive elimination of the Ga−C bond from indium to furnish DippNacNacGaEt2 (350) and the indium(I) species [InEt]n which disproportionates in solution into metallic indium and InEt3 (Scheme 184).249 Reductive elimination also likely occurred in the reaction of DippNacNacGa and Cp*InEt2 which produced Dipp NacNacGaEt2 and Cp*In. The reaction can proceed by oxidative addition of the In−Et bond to Ga(I) followed by reductive elimination of the Ga−Et bond from indium. The authors also reported that heating [DippNacNacGaEt]2(μ-InEt) (348) at 80 °C led to a mixture of 18, 350, and metallic indium. Monitoring the reaction by 1H NMR spectroscopy showed that the reaction occurred by initial elimination of the In−C bond to give DippNacNacGa and DippNacNacGa(Et)InEt2 followed by decomposition of the latter as described above. Related processes took place in the thermal decomposition of NacNacM(Et)BiEt2 (M = Al, 351; M = Ga, 352; M = In, 353).250 In this case the products are NacNacMEt2, BiEt3, and metallic

Scheme 181. Reversible Oxidative Addition of tert-Butyl Amine and tert-Butyl Alcohol by 231

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Scheme 184. Reductive Elimination from Gallium β-Diketiminate Compounds

Scheme 185. Base Induced Dehydrogenation of ArTrippSnH3 (575)

Scheme 186. Preparation of Distannyne 437 via Reaction of 580 with K-Selectride

distannane 577 was observed. Analogous reaction with a 20-fold excess of the non-nucleophilic base iPr2NEt resulted after 24 h in a mixture of stannylstannylene 578 (∼50%), distannane 577 (35%), and a new species (15%) assigned as 579 on the basis of NMR spectra. Changing the amine to Et2NMe led to a mixture of isomeric compounds 578 and 579 formed in a 4:1 ratio. Related dehydrogenations were also observed with ArMesSnH3422 as well as bis(trimethylsilyl)methyltin trihydride and mesityltin trihydride.423 DFT calculations performed in the gas phase showed that dehydrogenation of ArTrippSnH3 to stannylene is only slightly exergonic (ΔG = −2.6 kcal mol−1) but becomes more thermodynamically favorable when dimerization of ArTrippSnH into distannylene is considered (ΔG = −19.5 kcal mol−1).421 Nevertheless, in the absence of a base, 575 is stable to heating in toluene at 100 °C for 24 h. Further DFT calculations for the model system PhSnH3/DMAP revealed that direct hydrogen extrusion from the adduct PhSnH3(DMAP) proceeds via a very high barrier of 46 kcal mol−1, which is comparable to the barrier of 40.8 kcal mol−1 for the dehydrogenation of ArDippSnH3 to

bismuth. Similar to the previous example, reductive elimination is driven by the weakness of the M−Bi and Bi−C bonds. Decomposition of 351 took place upon heating to 90 °C for 5 h, whereas heating for 15 hours at 120 °C is required for 352, likely because the atomic size of gallium is smaller than that of aluminum. Although 353 followed the same reductive pathway initially with the production of NacNacInEt2, further decomposition occurred to furnish a complex mixture of products. 4.6. Reductive Elimination from Group 14 Compounds

It has long been known that organotin hydrides react with bases to give polymeric products, but the mechanistic details of these reactions remained unavailable.419 Recently, Wesemann and coworkers demonstrated that addition of bases, such as pyridines, trisubstituted amines, and N-heterocyclic carbenes, to the bulky organotin trihydride Ar Tripp SnH 3 (575, Ar Tripp = 2,6(2,4,6-iPr3C6H3)2C6H3) induced elimination of dihydrogen to afford Sn(II) hydrides, such as ArTrippSnH(L) (L = added base; Scheme 185).420,421 When this reaction was performed with a catalytic amount of DMAP (1 mol %), dehydrogenation to 3666


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Scheme 187. Production of Tetraantimony Cluster 582 via Reductive Elimination of [Me3P−PMe3]2+

Scheme 188. Reductive Elimination of H2 To Give Monovalent Antimony (584) and Bismuth (586) Compounds

[ArDippSn(μ-H)]2 calculated by Schleyer and co-workers.73 In contrast, a stepwise process via deprotonation of PhSnH3 followed by hydride abstraction by the incipient ammonium salt requires only 15.8 to 22.3 kcal mol−1 (depending on the basis set used) and is, therefore, the preferred mechanism. Related elimination of dihydrogen from a transient pincersupported stannylene 581 to give distannyne 437 was suggested by Jurkschat et al. to account for the reaction of chloride precursor 580 with K-Selectride (Scheme 186).337 In this case, the change of oxidation state from Sn(II) to Sn(I) is promoted by a pendant donor group. It is of interest whether a similar mechanism is applicable to the formation of arene products upon dihydrogen addition to diarylstannylene :SnArDipp2, previously reported by Power,75 or whether a direct C−H elimination process is involved.

reaction initially generated unstable hydrides that immediately eliminated dihydrogen to furnish the final products. The same group observed that the Sb(III) ditelluride 587, obtained by the addition of diphenylditelluride to 588, slowly decomposed in solution to regenerate 588 (Scheme 189).342


The driving force for this reaction is the preferential crystallization of 587 from solution; however, further details of this interesting reversible process have not been reported.

Scheme 189. Reductive Elimination of Diphenylditelluride from Sb(III) Ditelluride 587

Burford et al. reported that reactions of Sb(OTf)3, prepared independently or in situ by reaction of SbF3 with Me3SiOTf, or [bipy2Sb][OTf]2 with PMe3 yielded the phosphorus dication [Me3P−PMe3]2+ and cationic antimony cluster [(Me3P)4Sb4]4+ (582) as shown in Scheme 187.424 Although mechanistic details are currently unavailable, it was suggested that a highly electrophilic Sb(III) cation [Sb(PMe3)3]3+ forms in situ and undergoes reductive elimination of [Me3P−PMe3]2+ to form the Sb(I) intermediate [Sb(PMe3)]+ which then assembles into the tetraantimony cluster 582. Related reaction of FSb(OTf)2 with PMe3 leads to elimination of [Me3PF]+ and formation of the same cluster 582. Similar reactivity is observed with PEt3, PnPr3, and PnBu3, giving the corresponding phosphorus dication [R3P− PR3]2+ and cationic antimony cluster [(R3P)4Sb4]4+.425 Jambor and co-workers reported that addition of K[HB(iBu)3] to the pincer-supported dichlorides of antimony (583) and bismuth (585) produced the monovalent species 584 and 586, respectively, and hydrogen gas (Scheme 188).426,427 The

5. CONCLUSIONS AND OUTLOOK Over the past decade, the cleavage of robust bonds on reduced main-group centers has evolved into a vibrant field of research which also serves as an efficient entry point into main-group catalyzed reduction and coupling chemistry. To date, most types of strong σ-bonds, including bonds as durable as C−O and C−F, have been successfully activated. The span of elements employed is as remarkable, ranging from Group 2 to Group 15 (obviating the transition metals). The final, significant challenge is the activation of C−C bonds. While not being particularly strong (83 kcal mol−1 vs 104.2 kcal mol−1 for H−H and 116 kcal mol−1 for C−F), the C−C bond is kinetically stabilized by the directional nature of the hybridized carbon orbitals and substituents at carbon. It is vitally important to explore the cleavage of C−C bonds as its microscopic reverse, reductive elimination of C−C 3667


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(redox-active) ligands. Main-group compounds with these ligands feature ligand-centered redox activity, which allows one to circumvent the unfavorable decrease of the state of the maingroup center upon reductive elimination by pushing excessive electron density to the ligand. In this context, a very promising direction for future developments is the hydrogenation and transfer hydrogenation of unsaturated substrates mediated by Group 14 elements. As discussed in section 2.1.3, addition of dihydrogen to heavy carbene analogues gave both E(II) and E(IV) derivatives. The outcome of these hydrogenation reactions can be controlled by the sterics and electronics of the substituents as well as the nature of Group 14 element. The accessibility of both oxidation states allows one to envisage that transfer of H2 to an unsaturated substrate may become possible which will allow for the use of Group 14 compounds as catalysts in this fundamental reaction. Finally, we are venturing into the risky business of prognostication. The elegant work of Bertrand and Bielawski with carbenes showed the possibility of reversible bond activation on carbon centers. If the trends outlined above persist, and we have every reason to believe that they will, one can envisage a potential catalytic coupling cycle occurring exclusively at a carbon center! We envision that carbon catalysis will come to life within a few coming decades and will eventually replace expensive and toxic late transition-metal systems that are currently utilized. Sounds bizarre? Well, 50 years ago metalcatalyzed coupling used routinely nowadays was also a mere fantasy. We hope we have convinced the reader that main-group bond activation has bright and important future applications in catalysis.

bonds, is the culminating step in most important coupling reactions. Mechanistic, experimental, and computational studies showed that in most cases cleavage of nonpolar bonds by main-group centers proceeded as a concerted oxidative addition process. On the other hand, reactions of heavy Group 14 metallenes with alkyl and aryl halides are carried out via free-radical or in-cage radical mechanisms promoted by coordination of halides to the Lewis acidic Group 14 centers. This situation is thus reminiscent of the diversity of mechanisms observed in transition-metal chemistry where both concerted and radical mechanisms are well-established and the reaction pathway is dependent on both the substrate and metal complex and may be altered by the reaction conditions (such as solvent and temperature). The propensity of main-group centers to cleave σ-bonds usually decreases down the group (both kinetically and thermodynamically). The key parameter is the singlet−triplet gap which can be significantly affected by both steric and electronic properties of the substituents bonded to the maingroup element. Strong σ-donors induce a higher contribution to the σ-bonding of the s-orbital from the central atom, which in turn results in the increased p-electron character of its lone pair, decsreased ΔEST, and hence greater ability to cleave bonds. On the other hand, strong π-donors raise the energy of the LUMO, which is the π*(E−R) orbital, resulting in increased ΔEST. The strength of π-donation from the substituent and the ability to cleave bonds can thus be fine-tuned by the ligands supporting the main-group element. Schemes 170 and 171 provide some examples in the case of N−H activation. Steric effects of ligands also have an effect on the reactivity observed. In the case of Group 14 metallenes :ER2, the repulsion between bulky R groups leads to more obtuse R−E−R angles, resulting in rehybridization of the central atom and increased p-character of the lone pair on E. As a result, the nucleophilicity and bond splitting ability of the metallene increase (e.g., the difference in reactivity of stannylenes 56 and 57 with H2). Finally, the products of bond activation are not necessarily the original compounds in higher oxidations states. This is a result of the greater stabilization of the element’s ns2 pairs and consequently the increased stabilization of lower oxidation states with the heavier elements. As such, the initial product of oxidative addition may be unstable and thus convert to the preferred oxidative state via reductive elimination. The chemistry shown in Scheme 17 illustrates this principle. Another significant breakthrough was the recent demonstration of the oxidative cleavage of multiple bonds. The scope of bond activation is so far rather limited (CO, CN, CS, P O, PS), but this research reveals outlandish but beautiful structures, such as molecular complexes of SiO2, and is promising in delivering new and unusual reactivity. To close a potential catalytic coupling cycle, chemists need to cleave bonds, manipulate (functionalize) the fragments produced, and then fulfill a productive reductive elimination that will deliver a value added product. All these steps are challenges on their own, but the final step, reductive elimination, is the largest hurdle. It is therefore rewarding that several impressive examples of this process occurring at main-group centers have been described recently. The bonds being formed in these reactions have so far been relatively weak, e.g. Al−Al, Al−C, Sb−P, and Bi−C, which certainly helps drive forward this thermodynamically uphill process. However, the take home message is that a preferred coupling reaction should feature weaker bonds at the end of the catalytic process. Another promising approach could be the application of noninnocent

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Terry Chu: 0000-0002-7845-5502 Georgii I. Nikonov: 0000-0001-6489-4160 Notes

The authors declare no competing financial interest. Biographies Terry Chu received his B.Sc. in 2009 from McMaster University, working in the laboratory of David J. H. Emslie studying organoactinide complexes. In the laboratory of Warren E. Piers at the University of Calgary, he studied organoscandium chemistry and received his M.Sc. in 2012. He then pursued his doctoral studies under the supervision of Georgii I. Nikonov at Brock University where he investigated the chemistry of low-valent main-group element compounds, receiving his Ph.D. in 2017. As a Postdoctoral Research Associate under the supervision of Benjamin L. Davis at Los Alamos National Laboratory, he is currently investigating redox flow batteries for grid scale energy storage. Georgii Nikonov received his Ph.D. degree from the Lomonosov Moscow State University (Russia) in 1995 under the guidance of Prof. Dmitry Lemenovskii. After a postdoctoral stay at the University of Nottingham (UK) with Dr. Philip Mountford, he returned to the MSU as a faculty member. In 2005, he moved to Brock University (Canada) as an Associate Professor. He was promoted to full Professor in 2010. He is interested in organometallic chemistry, transition-metal and main-group element catalysis, reaction mechanisms, nonclassical interactions, and the reactivity of main-group element compounds in low oxidation states. 3668


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