Bruno Donnadieu

Synthesis and Characterization of a Neutral Tricoordinate Organoboron Isoelectronic with Amines

Carefully chosen carbon substituents stabilize a boron oxidation state that bears an extra electron pair. Amines and boranes are the archetypical Lewis bases and acids, respectively. The former can readily undergo one-electron oxidation to give radical cations, whereas the latter are easily reduced to afford radical anions. Here, we report the synthesis of a neutral tricoordinate boron derivative, which acts as a Lewis base and undergoes one-electron oxidation into the corresponding radical cation. These compounds can be regarded as the parent borylene (H-B:) and borinylium (H-B+.), respectively, stabilized by two cyclic (alkyl)(amino)carbenes. Ab initio calculations show that the highest occupied molecular orbital of the borane as well as the singly occupied molecular orbital of the radical cation are essentially a pair and a single electron, respectively, in the p(π) orbital of boron.

Isolation of a C5-Deprotonated Imidazolium, a Crystalline “Abnormal” N-Heterocyclic Carbene

Stretching Carbene Versatility Stable N-heterocyclic carbene (NHC) molecules are versatile catalysts for organic reactions (see the Perspective by Albrecht). Lavallo and Grubbs (p. 559) now show that these molecules can also catalyze organometallic transformations. Specifically, the carbenes induce coupling of monomeric iron olefin complexes to form clusters incorporating three or four bonded iron centers. Initial coordination of carbene to iron may facilitate formation of an iron-iron bond with a second complex. In a related development, Aldeco-Perez et al. (p. 556) prepared an NHC isomer in which the divalent carbon is shifted so that it no longer lies between the nitrogens. The compound forms stable complexes with both gold and CO2. A stable cyclic molecule has been prepared with an unusually positioned divalent carbon. The discovery two decades ago of metal-free stable carbenes, especially imidazol-2-ylidenes [N-heterocyclic carbenes (NHCs)], has led to numerous breakthroughs in organic and organometallic catalysis. More recently, a small range of complexes has been prepared in which alternative NHC isomers, namely imidazol-5-ylidenes (also termed abnormal NHCs or aNHCs, because the carbene center is no longer located between the two nitrogens), coordinate to a transition metal. Here we report the synthesis of a metal-free aNHC that is stable at room temperature, both in the solid state and in solution. Calculations show that the aNHC is more basic than its normal NHC isomer. Because the substituent at the carbon next to the carbene center is a nonbulky phenyl group, a variety of substitution patterns should be tolerated without precluding the isolation of the corresponding aNHC.

Homogeneous catalytic hydroamination of alkynes and allenes with ammonia.

Nitrogen–carbon bonds are ubiquitous in products ranging from chemical feedstock to pharmaceuticals. As ammonia is among the least expensive bulk chemicals produced in the largest volume, one of the greatest challenges of synthetic chemistry is to develop atom-efficient processes for the combination of NH3 with simple organic molecules to create nitrogen–carbon bonds. Transition-metal complexes can readily render a variety of N–H bonds reactive enough to undergo functionalization, including those of primary and secondary amines. However, with a few exceptions,[1,2] metals react with ammonia to afford supposedly inert Lewis acid–base complexes, as first recognized in the late 19th century by Werner.[3] Consequently, the homogeneous catalytic functionalization of NH3 remained elusive[4] until the recent discovery by Shen and Hartwig[5] and Surry and Buchwald[6] of the palladium-catalyzed coupling of aryl halides with ammonia in the presence of a stoichiometric amount of a base. An even more appealing process would be the addition of NH3 to carbon–carbon multiple bonds, a process that would occur ideally with 100% atom economy.[7] Although various homogeneous catalysts, including alkali metals,[8] early[9] and late transition metals,[10] and d-[11] and f-block elements,[12] have been used to effect the so-called hydroamination reaction, none of them were reported to be effective when NH3 is used as the amine partner.[13] Herein we report that cationic gold(I) complexes supported by a cyclic (alkyl)(amino)carbene (CAAC)[14] ligand readily catalyze the addition of ammonia to a variety of unactivated alkynes and allenes to provide a diverse array of linear and cyclic nitrogen-containing compounds. We showed recently that the cationic CAAC–gold complex A was very robust and exhibited unusual catalytic reactivity towards alkynes.[15] This discovery prompted us to investigate whether such a complex could activate alkynes sufficiently to enable the addition of NH3.[16] Thus, excess ammonia was condensed into a sealable NMR tube containing A (5 mol%), 3-hexyne, and deuterated benzene. Upon heating to 160 °C for 3.5 h, the clean addition of NH3 afforded the primary imine 2a, the expected tautomer of the corresponding enamine (Table 1).[17] Table 1 Catalytic hydroamination of 3-hexyne with ammonia.[a] Complex A does not have to be isolated; when it was prepared in situ from an equimolar mixture of [(CAAC)AuCl]/KB(C6F5)4 (A1), identical results were obtained. When the related silver complex [(CAAC)AgCl]/KB(C6F5)4 or NH4B(C6F5)4 was used as the catalyst, no reaction was observed, which shows the importance of gold and rules out a Bronsted acid mediated reaction.[18] Finally, as AuCl, AuCl/ KB(C6F5)4, and even [(CAAC)AuCl] do not induce the hydroamination, it is clear that the gold center can only catalyze the addition of NH3 if it is coordinated by the CAAC ligand and rendered cationic by Cl abstraction. To gain insight into the catalytic process, we performed a number of experiments: The addition of excess NH3 to complex A gave the Werner complex B instantaneously; the addition of 3-hexyne (1a; 1 equiv) to complex A gave the η2-bound alkyne complex C instantaneously (Scheme 1). Upon the exposure of a solution of C in benzene to excess NH3, 3-hexyne was immediately displaced from the gold center, and the Werner complex B was isolated in quantitative yield. This result suggests that NH3 does not add to the alkyne through an outer-sphere mechanism. Importantly, when a solution of complex B in benzene was treated at room temperature for 24 h with a large excess of 3-hexyne, the imine complex D was obtained quantitatively, even when the reaction vessel was open to a glovebox atmosphere. This experiment implies that NH3 does not dissociate from the metal by a simple ligand exchange with the alkyne. Therefore, an insertion mechanism, similar to that proposed by Tanaka and co-workers[19] and Nishina and Yamamoto[20] for gold-catalyzed hydroamination with aryl amines, is quite likely. Finally, the addition of excess NH3 to D liberated the imine 2a and regenerated complex B. From the results of these experiments, it can be concluded that the Werner complex B is the resting state of the catalyst. Indeed, B exhibits identical catalytic activity to that of A/A1. Consequently, the robust and readily available complex B (Figure 1)[21] was used in subsequent experiments. Scheme 1 Experiments to probe the reaction mechanism. The results suggest that an insertion process is involved, and that B is the resting state of the catalyst. Figure 1 Molecular structure of complex B in the solid state. (Hydrogen atoms, except those at N1, and the (C6F5)4B anion are omitted for clarity; ellipsoids are drawn at 50% probability). To test the scope of the reaction, the terminal alkyne 1b and the diaryl alkyne 1c were treated with NH3 in the presence of a catalytic amount of complex B (Scheme 2). With 1b, the reaction took place even at 110 °C to afford the Markovnikov imine 2b exclusively in 60% yield. When diphenyl acetylene (1c) was used, the 2-aza-1,3-diene 3c was formed cleanly in 95% yield. The different outcome of the reaction of 1c with respect to the results with substrates 1a,b can be rationalized by the presence of acidic benzylic hydrogen atoms in the imine. These acidic hydrogen atoms favor the formation of the enamine tautomer, which can then react further with a second molecule of the alkyne to afford 3c. Scheme 2 Catalytic hydroamination of various alkynes with ammonia. Nitrogen heterocycles are an important class of compounds that occur widely in natural products and often display potent biological activity. On the basis of the results described above, we attempted the direct synthesis of hetero-cycles from diynes and NH3. When 1,4-diphenylbuta-1,3-diyne (1d) and hexa-1,5-diyne (1e) were used, the corresponding 2,5-disubstituted pyrroles 4d and 4e were produced in 87 and 96% yield, respectively. Both products result from the Markovnikov addition of NH3, followed by ring-closing hydroamination.[22,23] The treatment of the 1,4-diyne 1f with NH3 under similar conditions led to a 3:2 mixture of the five- and six-membered heterocycles 5f and 6f in 88% yield. The six-membered ring 6f arises from two consecutive Markovnikov hydroamination reactions, whereas the formation of 5f involves an anti-Markovnikov addition of NH3 or ring-closing step. To expand the scope of the hydroamination reaction with NH3, we next tested allenes as substrates. When 1,2-propadiene (7a) was used, a mixture of mono- (8a), di- (9a) and triallylamine (10a) was obtained in excellent yield. Allyl amines are among the most versatile intermediates in synthesis and are of industrial importance. For example, the parent compound 8a, which is produced commercially from ammonia and allyl chloride, is used in antifungal preparations and the synthesis of polymers. By varying the NH3/allene ratio it is possible to control the selectivity of this reaction significantly (Table 2). In particular, the parent allylamine (8a) and triallylamine (10a) can be obtained with 86 and 91% selectivity, respectively, and further optimization of the conditions should be possible. The addition of NH3 to 1,2-dienes is not restricted to the parent allene 7a. The dialkyl-substituted derivative 7b was also converted into the corresponding allyl amines 8b–10b, with exclusive addition of the NH2 group at the less-hindered terminus; however the selectivity of this reaction for the mono-, di-, or trisubstituted amine product needs some improvement. Interestingly, even the tetrasubstituted allene 7c underwent hydroamination with ammonia. Probably because of steric factors, a different regioselectivity was observed, and only the monohydroamination product 11C was formed.[24] Table 2 Catalytic hydroamination of allenes with ammonia. The results outlined herein demonstrate that (CAAC)-gold(I) cations readily catalyze the addition of NH3 to non-activated alkynes and allenes. This reaction leads to reactive nitrogen-containing compounds, such as imines, enamines, and allyl amines, and is therefore an ideal initial step for the preparation of simple bulk chemicals, as well as rather complex molecules, as illustrated by the preparation of heterocycles 4–6. This study paves the way for the discovery of catalysts that mediate the addition of ammonia to simple alkenes, a process considered to be one of the ten greatest challenges for catalytic chemistry.[25]

Crystalline 1H-1,2,3-triazol-5-ylidenes: new stable mesoionic carbenes (MICs).

In 2001, Crabtree and co-workers first reported complex A, which features an imidazole ring bound at the C5 position (III), and not at C2 as commonly observed.[7] More recently, Huynh and co-workers[8] and Albrecht and co-workers[9a] showed that pyrazolium and 1,2,3-triazolium salts can serve as precursors to metal complexes of type B and C, which feature pyrazolin-4-ylidenes IV and 1,2,3-triazol-5-ylidenes V as the ligand, respectively. As a consequence of their lineage, these have also been referred to as N-heterocyclic carbenes (NHCs). However, as no reasonable canonical resonance forms containing a carbene can be drawn for free ligands III–V without additional charges (see V′), these ligands have been described as abnormal or remote carbenes (aNHCs or rNHCs, respectively).[10] As they are, in fact, mesoionic compounds,[11] we suggest naming this family of compounds mesoionic carbenes (MICs). There have been no reported dimerizations of MICs III and IV, which suggests that the Wanzlick equilibrium pathway for classical carbenes is disfavored;[12] this observation should lead to relaxed steric requirements for their isolation. Moreover, experimental and theoretical data suggest that MICs III–V are even stronger electron-donating species than NHCs I and II, which opens up interesting perspectives for their applications.[10] Our recent success in the isolation of a free imidazol-5-ylidene III[13] and pyrazolin-4-ylidenes IV (cyclic bent allenes),[14,15] prompted us to investigate the possibility of preparing new types of stable neutral compounds that feature a lone pair of electrons on the carbon atom.[16] Preliminary calculations (B3LYP, 6–311G(d,p); for details, see the Supporting Information) predicted that the parent MIC V is located at an energy minimum, about 32 kcalmol−1 above the regioisomeric parent 1,2,4-triazol-5-ylidene II. Furthermore, parent V is predicted to exhibit an appreciably large singlet–triplet band gap (56 kcalmol−1), which is a good predictor of carbene stability and thus of possible isolation. Herein, we report the preparation, isolation, and characterization of two free 1,2,3-triazol-5-ylidenes of type V. By analogy with the synthetic route used for preparing NHCs and the related species III and IV, 1,2,3-triazolium salts (2a,b) were targeted as precursors for the desired 1,2,3-triazol-5-ylidenes (Va,b). A sterically hindered flanking aryl substituent (2,6-diisopropylphenyl, Dipp) was selected to provide kinetic stabilization to the ensuing free ligand. 1,2,3-Triazole 1 was obtained in 83% yield from the copper-catalyzed azide–alkyne cycloaddition (CuAAC, click chemistry) of 2,6-diisopropylphenyl azide and phenylacetylene.[17] The one-pot conversion of aniline into the desired aryl azide, followed in situ by CuAAC as reported by Moses and co-workers[18] was found to be especially convenient for the synthesis of 1. Alkylation of 1 with methyl or isopropyl trifluoromethanesulfonate afforded the corresponding tri-azolium salts in moderate to excellent yields (2a and 2b, respectively; Scheme 2). Scheme 2 Synthesis of the free 1,2,3-triazol-5-ylidenes Va,b. Potassium bases have been identified as the reagents of choice for the depronation of carbene precursors, as they avoid the formation of stable carbene–alkali-metal adducts that are commonly encountered when lithium bases are used.[12,13,14a,19] Gratifyingly, triazolium salts 2a,b were cleanly deprotonated with either potassium bis(trimethylsilyl)amide or potassium tert-butoxide in ethereal solvents to afford the corresponding MICs Va and Vb in 55 and 39% yield, respectively. Deprotonation was evidenced by the disappearance of the triazolium CH signal in their 1H NMR spectra (2a: δ =8.62 ppm; 2b: δ =8.85 ppm) and the appearance of a signal at low field in the 13C NMR spectrum (Va: δ = 202.1 ppm; Vb: δ =198.3 ppm). The structure of Va was unambiguously confirmed by X-ray crystallography (Figure 1).[20] In the solid state, Va contains a planar heterocycle, characterized by bond lengths that are intermediate between those of single and double bonds; both of these features are indicative of electronic delocalization. Upon deprotonation, the C5 carbon bond angle becomes more acute (2a: 106°; Va: 100°), which is consistent with an increased s character in the σ lone pair orbital of Va compared to the C–H bonding orbital of the precursor 2a. This is in agreement with the generally observed trend for carbenes and their conjugate acids.[5] Figure 1 Molecular views (thermal ellipsoids set at 50 % probability) of 2a (top) and Va (bottom) in the solid state. For clarity, counter ions, solvent molecules, and H atoms are omitted, except for the ring hydrogen of 2 a. Selected bond lengths [A] ... In the solid state, with the exclusion of oxygen and moisture, free 1,2,3-triazol-5-ylidene Va (m.p. 50–52°C decomp.) remained stable for several days at −30°C and for a few hours at room temperature. By contrast, Vb (m.p. 110–112°C) was significantly more stable, showing no sign of decomposition after three days at room temperature in the solid state. Upon heating in a benzene solution for 12 hours at 50°C, Va decomposed to give, among other products, triazole 3 (Scheme 3; for details, see the Supporting Information). We surmise that the latter product results from a nucleophilic attack of the carbon lone pair of Va on the methyl group of a second molecule of Va, giving rise to heterocycles 4 and 5, which react together to afford the observed product 3. This apparent rearrangement is reminiscent of that recently observed in the formation of imidazol-2-ylidenes of type I from imidazol-5-ylidenes of type III that contain an electro-philic Y group.[21] In agreement with this hypothesis, MIC Vb, which contains the less-electrophilic isopropyl group at the N3 position, appears much more robust with respect to this decomposition pathway. Scheme 3 Degradation of free 1,2,3-triazol-5-ylidene Va, and analogy with the rearrangement of III into I. To evaluate the donor properties of 1,2,3-triazol-5-yli-denes, the [(Va)Ir(CO)2Cl] complex was prepared by addition of Va to [{Ir(cod)Cl}2] (cod = 1,5-cyclooctadiene), followed by treatment with an excess of carbon monoxide. The CO vibration frequencies (ν =2061 and 1977 cm−1; νavg = 2019 cm−1) are in line with those of the analogous iridium complex, previously reported by Albrecht and co-workers (νavg = 2021 cm−1),[9a] and are indicative of donor properties that are superior to those of NHCs I and II (νavg = 2022–2031 cm−1),[22] but inferior to those of MICs III (νavg = 2003–2006 cm−1)[23] and IV (νavg = 2002 cm−1).[14b] Free 1H-1,2,3-triazol-5-ylidenes, as exemplified by compounds Va,b, possess an ensemble of properties that portend to their utility. The synthesis of their precursors is short and efficient, from readily available starting materials, yet is modular and thus amenable to a wide variety of potential analogues. As with other mesoionic carbenes III and IV, the dimerization of MICs of type V has not been observed; therefore, the preparation of comparatively unhindered MICs is predicted to be viable. Their donor properties are greater than those of NHCs of type I and II, but they are nonetheless available by deprotonation using mild bases (e.g. alkoxides), thus signaling their potential for applications, such as nucleophilic organocatalysis. Free triazolylidenes V complement the rapidly growing numbers of neutral carbon-based κ1C ligands that are now available. We predict that many other classes of MICs, that are derived from a variety of heteroaromatic scaffolds, can be isolated. This endeavor is currently the object of ongoing efforts in our laboratory.

Allene formation by gold catalyzed cross-coupling of masked carbenes and vinylidenes

Addition of a sterically demanding cyclic (alkyl)(amino)carbene (CAAC) to AuCl(SMe2) followed by treatment with [Et3Si(Tol)]+[B(C6F5)4]− in toluene affords the isolable [(CAAC)Au(η2-toluene)]+[B(C6F5)4]− complex. This cationic Au(I) complex efficiently mediates the catalytic coupling of enamines and terminal alkynes to yield allenes and not propargyl amines as observed with other catalysts. Mono-, di-, and tri-substituted enamines can be used, as well as aryl-, alkyl-, and trimethylsilyl-substituted terminal alkynes. The reaction tolerates sterically hindered substrates and is diastereoselective. This general catalytic protocol directly couples two unsaturated carbon centers to form the three-carbon allenic core. The reaction most probably proceeds through an unprecedented “carbene/vinylidene cross-coupling.”

Nonmetal‐Mediated Fragmentation of P4: Isolation of P1 and P2 Bis(carbene) Adducts

Transition metals are well-known for activating small molecules and for stabilizing highly reactive species. A recent trend in the accomplishment of either of these goals is the use of nonmetals, especially stable singlet carbenes. Robinson and co-workers have reported that N-heterocyclic carbenes (NHCs) give rise to stable adducts with HBBH and Si2 , [3] which are otherwise not isolable. We have shown that cyclic (alkyl)(amino)carbenes (CAACs) can activate CO, H2 , [6] and even NH3 , [7] which is difficult when attempted with transition-metal centers. 8] White phosphorus (P4) is a small molecule that is of industrial interest as it is the classical starting material for the large-scale preparation of organophosphorus derivatives. The reactivity of P4 with transition metals has been widely studied, and is therefore an excellent model to further test if carbenes can undergo reactions in the same manner as transition metals. Our research group has already shown that the bulky rigid CAAC 1 opens P4 and simultaneously stabilizes the resulting acyclic P4 species (A, [13a] Scheme 1),

Isolation of crystalline carbene-stabilized P2-radical cations and P2-dications

The discovery in 1900 by Gomberg that the trityl radical (Ph(3)C(.)) exists at room temperature is often considered to be the beginning of radical chemistry. Since then, persistent and even room-temperature stable radicals based on second-row and heavier elements have been synthesized. However, few of them have been characterized crystallographically, because they are either too reactive or dimerize in the solid state. Here, we show that a P(2) fragment, capped with two bulky, strongly electron-releasing singlet carbenes (dicoordinate carbon compounds with only six valence electrons), can undergo one-electron oxidation, giving rise to room-temperature stable radical cations. Moreover, when N-heterocyclic carbenes are used, two-electron oxidation can also be performed, producing the corresponding stable dicationic diphosphene, which has to be regarded as a P(2)(2+) fragment coordinated by two carbenes. These results reveal a new application of stable singlet carbenes, the stabilization of paramagnetic species and electron-poor fragments.

Serendipitous Discovery of the Catalytic Hydroammoniumation and Methylamination of Alkynes

The transition-metal-catalyzed hydroamination reaction, that is, the addition of an N—H bond across a carbon-carbon multiple bond, has been widely studied.[1,2] We have recently reported that cationic gold(I) complexes,[3,4] supported by cyclic (alkyl)(amino)carbene (CAAC) ligands,[5] readily catalyze the intermolecular hydroamination of alkynes with a variety of amines,[6] including ammonia.[6c] Based on preliminary mechanistic studies, we postulated that the key step of the catalytic cycle was the formation of a tricoordinate gold complex (I), which was followed by inner-sphere C—N bond formation, as first postulated by Tanaka et al.,[7a] and Nishina and Yamamoto[7b,c] (Scheme 1). However, for other gold catalysts,[8] Che et al.[9a] and Li et al.[9b] hypothesized an outer-sphere nucleophilic attack to the alkyne complex II, a mechanism widely accepted for palladium[10] and platinum complexes.[11] Herein, our attempts to isolate a gold(I) complex of type I have led to the structural characterization of two (CAAC)(η1-alkene)AuI complexes, and to the discovery of two catalytic reactions: the intramolecular hydroammoniumation using tertiary ammonium salts, and the aminomethylation of carbon–carbon triple bonds.

Synthesis of a Simplified Version of Stable Bulky and Rigid Cyclic (Alkyl)(amino)carbenes, and Catalytic Activity of the Ensuing Gold(I) Complex in the Three-Component Preparation of 1,2-Dihydroquinoline Derivatives

A 95/5 mixture of cis and trans 2,4-dimethyl-3-cyclohexenecarboxaldehyde (trivertal), a common fragrance and flavor material produced in bulk quantities, serves as the precursor for the synthesis of a stable spirocyclic (alkyl)(amino)carbene, in which the 2-methyl-substituted cyclohexenyl group provides steric protection to an ensuing metal. The efficiency of this carbene as ligand for transition metal based catalysts is first illustrated by the gold(I) catalyzed hydroamination of internal alkynes with secondary dialkyl amines, a process with little precedent. The feasibility of this reaction allows for significantly enlarging the scope of the one-pot three-component synthesis of 1,2-dihydroquinoline derivatives, and related nitrogen-containing heterocycles. Indeed, two different alkynes were used, which include an internal alkyne for the first step.

Activation of SiH, BH, and PH Bonds at a Single Nonmetal Center

For many years, it was believed that only transition-metal centers could activate small molecules and enthalpically strong bonds. However, it has recently been shown that several nonmetallic systems are capable of some of these tasks. For example, stable singlet carbenes can activate CO, H2, [3b] and P4. [3c–e] Such reactions have long been known for transition metals. However, stable singlet carbenes can also activate NH3; [3b] a much more difficult task for transition metals. The oxidative addition of hydrosilanes, hydroboranes, and hydrophosphines at vacant coordination sites of transition metals are well-exemplified and are considered as key steps in the transition-metal-catalyzed hydrosilylation, hydroboration, and hydrophosphination of multiple bonds. Herein, we report the first examples of the activation of E H bonds (E=Si, B, P) at a single nonmetal center. On the basis of our successful results with H2, [3b] we began our study with the activation of Si H bonds. Indeed, silanes are similar to H2 in that they lack both nonbonding electron pairs and p electrons. They can bind to various metal centers to form stable Si H s complexes, which undergo subsequent oxidative addition. To test the possible activation of Si H bonds with carbenes, we treated the cyclic (alkyl)(amino)carbenes (CAACs) 1a and 1b with primary, secondary, and tertiary silanes. The addition of phenylsilane to 1a and 1b occurred readily at room temperature, and the corresponding adducts 2a,b were isolated in 91 and 83% yield, respectively (Scheme 1). As expected, in the case of the enantiomerically pure CAAC 1a, two diastereomers 2a,a’ were formed (in a 2:1 ratio), as shown by two singlets at d= 36.4 and 29.3 ppm in the Si NMR spectrum. The C NMR spectrum revealed the loss of the carbene signal and a new C H peak at d= 63.2 (2a) and 65.5 ppm (2b). The H NMR spectrum of the major isomer 2a revealed a pseudotriplet at d= 4.78 ppm (SiCH) and two doublets at d= 4.29 and 4.21 ppm corresponding to the diastereotopic hydrogen atoms of the SiH2 fragment. The structure of 2a was confirmed by X-ray crystallography (Figure 1, top), whereas the presence of a triplet at d= 4.53 ppm and a doublet at d= 4.08 ppm in the H NMR spectrum confirmed the identity of adduct 2b. CAACs 1a,b also reacted with (EtO)3SiH to afford 3a (d.r. 3:1) and 3b in 64 and 73% yield, respectively. However, when Ph2SiH2 was used, only the less bulky carbene 1b underwent insertion into the Si H bond (to give 4b in 65% yield), and a reaction time of 16 hours at 80 8C was necessary for the reaction to reach completion. Surprisingly, although it has been shown that, in contrast to CAACs, N-heterocyclic carbenes (NHCs) do not react with H2, [11] we found that imidazolidin-2-ylidene 5 also reacted at room temperature with phenylsilane to afford the Si H insertion product 6 in 88% yield (Figure 1, bottom). The formation of 6 raises the question of the mechanism of the activation of Si H bonds with carbenes. Why should NHCs react with silanes although they are inert towards hydrogen? The evident difference is the presence of low-lying vacant orbitals in silanes. In other words, the observed reactivity might be due to the Lewis acid character of silanes; indeed, several NHC–SiX4 adducts are known. [13]