Experimental and Computational Studies of the Ruthenium

Article pubs.acs.org/Organometallics

Experimental and Computational Studies of the RutheniumCatalyzed Hydrosilylation of Alkynes: Mechanistic Insights into the Regio- and Stereoselective Formation of Vinylsilanes Ruili Gao,† Dale R. Pahls,‡ Thomas R. Cundari,*,‡ and Chae S. Yi*,† †

Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881 United States Department of Chemistry, CASCaM, University of North Texas, Denton, Texas 76203-5017 United States

‡

S Supporting Information *

ABSTRACT: The ruthenium hydride complex (PCy3)2(CO)RuHCl was found to be a highly effective catalyst for the regio- and stereoselective hydrosilylation of alkynes to form vinylsilane products. (Z)-Vinylsilane products were selectively formed for sterically nondemanding terminal alkynes, while (E)-vinylsilane products resulted from sterically demanding terminal alkynes. Kinetic data were obtained from the hydrosilylation of phenylacetylene. The phosphine inhibition study showed an uncompetitive Michaelis−Menten type of inhibition kinetics. The empirical rate law rate = kobs[1]1[alkyne]0[silane]0 was established from the reaction rate as a function of both [alkyne] and [silane]. DFT calculations were performed and found that Z/E isomerization is facile via a metallacyclopropene transition state and that the isomerization occurs prior to the silane substrate binding. A detailed mechanistic scheme on the hydrosilylation reaction has been delineated on the basis of both experimental and computational data.

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INTRODUCTION Vinylsilanes are an important class of organosilicon compounds that have been widely used as the monomeric precursors for industrial production of polysilicon compounds as well as reagents for the synthesis of complex organic molecules.1 Alkyne hydrosilylation reactions using soluble transition-metal catalysts have been shown to be some of the most effective ways to synthesize vinylsilanes.2 In industrial settings, platinum catalysts such as H2PtCl6 (Speier’s catalyst) and Pt2[((H2C CH)Me2Si)2O]3 (Karstedt’s catalyst) have been widely used for the hydrosilylation reaction, but neither homogeneous3 nor heterogeneous Pt catalysts4 provide general solutions for controlling regio- and stereoselective formation of vinylsilanes. Takeuchi and co-workers first demonstrated the stereoselective formation of vinylsilane products by modulating different phosphine ligands on soluble Rh catalysts and by using solvents with different polarities.5 Trost and co-workers successfully utilized cationic Ru catalysts to promote regio- and stereoselective hydrosilylation of oxygen-functionalized alkynes.6 The authors adopted a mechanistic model via the formation of metal vinyl species proposed by Tanke and Crabtree to rationalize the highly E-selective hydrosilylation reaction.7 Despite these advances, controlling the regio- and stereoselective formation of vinylsilane products remains an unsolved © XXXX American Chemical Society

issue for the transition-metal-catalyzed alkyne hydrosilylation. Stereoselective formation of (Z)-vinylsilane products has been proven to be particularly challenging for the hydrosilylation of terminal alkynes. Even though a number of highly Z-selective catalytic hydrosilylation methods have been reported for terminal alkynes,6,8 these catalytic methods are substrate dependent and often result in either 1,1-disubstituted vinylsilanes or a mixture of (E)- and (Z)-vinylsilane products. Controlling regioselectivity is another ongoing issue for the hydrosilylation of internal alkynes. In this regard, a number of highly regioselective hydrosilylation methods have been achieved for internal alkynes by using heteroatom directing groups.9 Although high regioselectivity toward the formation of 1,1-disubstituted vinylsilane products has been achieved by using a Pt catalyst,10 factors controlling regio- and stereoselectivity have been not been clearly established, despite considerable efforts. For a number of years, the research of one of our groups has been focused on the development of ruthenium-catalyzed heterofunctionalization of unsaturated substrates.11 In particular, Yi et al. previously reported that the 16-electron Received: October 6, 2014

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dx.doi.org/10.1021/om501019j | Organometallics XXXX, XXX, XXX−XXX

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Table 1. Hydrosilylation of Alkynes Catalyzed by 1a

ruthenium hydride complex (PCy3)2(CO)RuHCl (1) is a highly effective and selective catalyst for the oxidative silylation of alkenes to form E-selective vinylsilanes.12 More recently, we discovered that complex 1 effectively catalyzes highly stereoselective alkyne to carboxylic acid coupling to form (Z)-enol esters.13 In an effort to extend the scope and utility of stereoselective formation of substituted alkene products, we have been investigating the analogous hydrosilylation of alkynes. Herein, we report the scope and mechanistic studies of the ruthenium-catalyzed stereoselective hydrosilylation of terminal alkynes to give (Z)-vinylsilane products. We disclose full details of both experimental and computational studies on the ruthenium-catalyzed alkyne hydrosilylation reaction, which provide a more complete mechanistic picture on the stereoselective formation of (Z)-vinylsilanes.

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RESULTS AND DISCUSSION At the onset of the study, we screened a number of commonly available ruthenium catalysts for the hydrosilylation of alkynes (eq 1). Thus, the mixture of PhCCH (1.0 mmol) with

HSiEt3 (2.0 mmol) in the presence of a Ru catalyst (1−2 mol %) in CH2Cl2 was stirred at room temperature for 4 h, after which the product conversion was analyzed by GC. Complex 1 was found to exhibit uniquely high activity and selectivity in yielding cis-vinylsilane product (Z)-2a over (E)- and gemvinylsilane products (E)-2a and 3a, among the screened ruthenium catalysts. Other common ruthenium catalysts such as Ru3(CO)12 and [(p-cymene)RuCl2]2 showed only modest activity, giving ∼5% of the hydrosilylation products under similar reaction conditions. Another distinctive feature of the catalyst 1 is that it mediates Z-selective formation of vinylsilanes, in contrast to the case for other Ru and Pt catalysts, which generally give E-selective vinylsilane products.3,6 Reaction Scope. The substrate scope of the hydrosilylation reaction was examined by using the catalyst 1 (Table 1). In general, the catalyst 1 was found to be highly effective in forming cis-vinylsilane products (Z)-2 without giving any significant amounts of trans- and gem-vinylsilane products (E)-2 and 3 for both aryl- and alkyl-substituted terminal alkynes. The reaction is essentially complete within 6 h at room temperature for most substrates. A mixture of cis- and transvinylsilane products, (Z)- and (E)-2, was formed with sterically more demanding terminal alkynes (entries 10−12), while transvinylsilane products (E)-2 exclusively resulted from both tertbutylacetylene and trimethylsilylacetylene (entries 13 and 14). Functional group tolerance was also demonstrated by employing a bioactive 17α-ethynylestradiol molecule, in which case, the trans-vinylsilane product (E)-2o was isolated in 67% yield (entry 15). For the unsymmetrically substituted internal alkyne PhCCCH3, the regioselective addition of the silyl group to the electron-deficient alkynyl carbon occurred to yield the product 2p (entry 16). The hydrosilylation of a diyne with HSiEt3 afforded cis-selective formation of the bis-(Z)-vinylsilane (Z)-2r over bis-(E)-vinylsilane (E)-2r in 94% combined product yield (entry 18). Notable features of the catalytic

a Reaction conditions: alkyne (1.0 mmol), silane (2.0 mmol), 1 (1 mol %), CH2Cl2 (2−3 mL), room temperature, 6 h. bIsolated yield. cAt 60 °C, 15 h.

method from a synthetic standpoint are that it predictively forms (Z)-vinylsilane products, tolerates a number of sensitive functional groups, and can be performed under very moderate reaction conditions using a relatively low catalyst loading. Isotope Labeling Studies. We performed the following kinetic experiments to probe the mechanism of the hydrosilylation reaction. First, we probed the H/D exchange pattern on the product formation. The treatment of PhCCD (1.0 mmol) with HSiEt3 (2.0 mmol) in the presence of 1 (1 mol %) in CH2Cl2 at room temperature yielded the coupling product (Z)-2a-d, which was isolated by column chromatography on silica gel. The 1H and 2H NMR of (Z)-2a-d clearly showed that the alkynyl deuterium is mostly intact on the α-carbon of the product (Z)-2a-d (eq 2) (Figure S1, Supporting Information).

The absence of significant H/D exchange between Si−H and acetylenic C−H bonds is consistent with a direct addition of the silyl group to the alkyne substrate but inconsistent with a mechanism involving acetylene-to-vinylidene rearrangement as proposed in some transition-metal-catalyzed hydrosilylation reactions.14 As will be discussed later, the DFT calculations also suggest that the formation of metal vinylidene complexes would require high-energy processes. To probe electronic effects of the alkyne substrate on the reaction rate, we next compared the rates of the hydrosilylation reaction for a series of para-substituted p-X-C6H4CCH (X = B


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OMe, CH3, H, Br, CF3). The kobs value was measured from a pseudo-first-order plot of ln[product] vs the reaction time. Surprisingly, the reaction rate was found to be quite insensitive to the electronic nature of the alkyne substrate. For example, the reaction rate from an alkyne with a para-electronwithdrawing group, p-Br-C6H4CCH, was virtually identical with that of PhCCH (kobs = 4.3 × 10−5 h−1 vs 4.0 × 10−5 h−1). To confirm the lack of electronic effects, the reaction rates for another set of terminal alkynes with electron-withdrawing and electron-releasing groups, p-CF3-C6H4CCH and p-CH3C6H4CCH, were compared, and the similar rate constants, kobs = 1.1 × 10−5 and 0.8 × 10−5 h−1, respectively, were obtained from both substrates. The lack of a discernible electronic correlation on the reaction rate indicates that the hydrosilylation reaction is insensitive to the electronic nature of the alkyne substrate. Phosphine Inhibition Study. The phosphine inhibition kinetics was measured from the reaction of PhCCH with HSiEt3. In a J. Young NMR tube, PhCCH (0.20 mmol), HSiEt3 (0.40 mmol), 1 (2 mg, 1 mol %), and C6Me6 (2 mg, internal standard) were dissolved in CDCl3 (0.5 mL), and PCy3 (predissolved in CDCl3, 0.4−1.4 mM) was added to the tube via a syringe. The reaction progress was monitored by 1H NMR in 30 min intervals at room temperature. The plot of the initial rate (vi), which was estimated from the first-order plot of ln[product] vs reaction time, as a function of [PCy3] showed an inverse dependence on [PCy3] (Figure S2, Supporting Information). The reaction rate steadily decreased upon addition of PCy3, during which time kobs steadily decreased from 4.1 × 10−5 h−1 (no PCy3) to 8.7 × 10−6 h−1 (14 μM PCy3). To further discern the nature of phosphine inhibition, a Lineweaver−Burke plot was constructed from the inhibition kinetic data. The reciprocal plots of 1/vi vs 1/[PhCCH] showed a set of nonintersecting parallel lines for all four different concentrations of PCy3 (Figure 1). Nonintersecting

Scheme 1. Uncompetitive Phosphine Inhibition Kinetics for the Ruthenium-Catalyzed Hydrosilylation of Alkynes

catalytically inactive complex L-[Ru]-S. We attribute the 14electron Ru monophosphine fragment (PCy3)(CO)RuHCl to the catalytically active species [Ru], where both the bisphosphine complex [Ru](L)2 and an alkyne-bound L-[Ru]-S complex would be treated as catalytically inactive species. A lower Vmax value due to the presence of inactive species is another characteristic feature of uncompetitive inhibition. Empirical Rate Law Determination. The empirical rate law of the hydrosilylation reaction was determined by using the initial rate method. The plot of initial rate as a function of [PhCCH] showed that the rate is independent of [PhC CH] with the rate constant kobs = 4.3 × 10−5 h−1 at a relatively low concentration range (0.06−0.20 M) (Figure 2). At a higher

Figure 2. Plot of initial rate (vi) vs [PhCCH].

alkyne concentration ([PhCCH] > 0.20 M), the rate was substantially decreased to kobs = 1.0 × 10−5 h−1, which is about 4 times lower than that at [PhCCH] = 0.20 M. Examining the empirical rate as a function of [HSiEt3] under otherwise similar reaction conditions also showed that the rate is independent of [HSiEt3], giving kobs = 0.43 × 10−4 h−1 in the range of 0.040−0.70 M (Figure S3, Supporting Information). These results indicate that the reaction rate is independent of both [PhCCH] and [HSiEt3] under the catalytic operating conditions. A considerably lower rate at a relatively high [PhCCH] (>2.0 M) suggests strong inhibition due to multiple coordination of the alkyne substrate to the catalyst. As expected, the empirical rate measurement as a function of the catalyst concentration showed a linear dependence on [1]. The kobs value for the hydrosilylation of PhCCH (0.20 M) with HSiEt3 (0.40 M) linearly increased from 0.75 × 10−5 h−1 to 6.8 × 10−5 h−1 as the catalyst concentration was increased from 1.4 to 11 mM. The linear plot of vi vs [1] established a first-order dependence on the Ru catalyst (Figure 3). The empirical rate law rate = kobs[1]1[alkyne]0[silane]0 was established from combining these rate data. The rate law is consistent with phosphine dissociation as the rate-limiting step of the catalytic cycle. These experimental data suggest the formation of an unsaturated Ru-silyl complex, which is

Figure 1. Lineweaver−Burke plot of 1/vi vs 1/[PhCCH] at no PCy3 (●), 4 μM PCy3 (▲), 9 μM PCy3 (■), and 14 μM PCy3 (◆).

phosphine inhibition is a characteristic feature of an uncompetitive phosphine inhibition in Michaelis−Menten type kinetics.15 Thus, we devised a kinetic scheme for an uncompetitive phosphine inhibition, where [Ru] is the catalytically active species, S = alkyne substrate, L = PCy3 ligand, and P = vinylsilane product (Scheme 1). The main feature of the uncompetitive inhibition kinetics involves a reversible coordination of the inhibitor L to the catalyst− substrate complex [Ru]-S in creating an equilibrium with a C


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with molecular mechanics using the universal force field (UFF)17 as the lower level of theory and DFT for the higher level of theory using the B3LYP18 exchange correlation functional with the CEP-121G(d)19 basis set. Since PMe3 is a better electronic model for PCy3 than PH3, the C−C bonds of PCy3 that are α to the phosphorus atoms were chosen as the cutoff between the two levels of theory. The rest of the complex was treated with full QM methods (B3LYP/CEP-121G(d)). The catalytic cycle of the hydrosilylation of phenylacetylene with HSiMe3 was computed. HSiMe3 was employed because the difference in size between SiEt3 and SiMe3 should be relatively small, while the smaller silane model reduces computational costs significantly by removing the conformational search necessary for HSiEt3. The Tolman steric parameters for PMe3 and PEt3, which should be a good estimate for the steric parameters for Ru-SiMe3 and Ru-SiEt3 ligands, are 118 and 132°, respectively,20 support the small difference in steric influence for the smaller model silane substrate. Particular attention was given to ruthenium complexes with relatively weakly interacting ligands (Cl, HSiMe3, agostic) trans to strongly trans influencing ligands, as these are critically important for searching stable intermediate species. Multiple isomers of each complex were considered to identify the lowest energy conformations. Our discussion of the simulation focuses on the lowest energy isomers for the various stationary points in the computed reaction coordinate. Formation of Putative Active Species. The first step toward formation of the active catalyst is loss of PCy3 from 1. Surprisingly, given the rate-limiting phosphine dissociation shown by the experimental kinetics, the thermodynamics of the reaction are calculated to be uphill by only 3.8 kcal/mol to give the 14-electron complex (PCy3)(CO)RuHCl (5) (Scheme 2).

Figure 3. Plot of the initial rate (vi) vs [1].

generated from an initial phosphine dissociation followed by the elimination of an alkene. In support of this hypothesis, we detected the formation of styrene (1−2%) from the hydrosilylation of PhCCH prior to the formation of the products. We also found that the previously reported Ru-vinyl complex (PCy3)2(CO)(Cl)RuCHCHPh (4), which was formed from the reaction of 1 with PhCCH, exhibited the same catalytic activity as 1.16 Computational Study. Even with the empirical rate law and uncompetitive phosphine inhibition kinetics, it was not possible to establish the complete catalytic cycle, in part because of the inability to detect or trap any catalytically relevant species for the hydrosilylation reaction despite concerted experimental efforts. To attain deeper insights into the reaction mechanism as well as the nature of reactive species, quantum chemical calculations were performed. To maintain the steric environment of the catalytic system at a reasonable computational cost and time, ONIOM methods were applied

Scheme 2. Computed Free Energies of Ruthenium Complexes Relevant for the Hydrosilylation of Phenylacetylenea

a

Energies are tabulated relative to 1 (kcal/mol). D


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Scheme 3. Computed Free Energies Relevant to the Isomerization of Ruthenium Vinyl Complexesa

a

Energies are tabulated relative to (Z)-9 (kcal/mol).

(Figure 4)with a barrier of 35.1 kcal/mol relative to (Z)-9. This large energy barrier would be inconsistent with the room-

Insertion of PhCCH into this complex is calculated to be 28.7 kcal/mol downhill to give the Ru-vinyl complex 6. The recoordination of PCy3 to the vinyl complex would lead to the inactive complex (PCy3)2(CO)(Cl)RuCHCHPh (4). The catalytically relevant step involves the addition of silane to form the silyl complex (PCy3)(CO)Ru(SiMe3)Cl (7) along with the formation of PhCHCH2. Binding of PCy3 to the vinyl complex 6 is 13.5 kcal/mol downhill, while binding of HSiMe3 is uphill by 5.1 kcal/mol. This 18.6 kcal/mol preference for binding of phosphine over silane is consistent with the observed experimental kinetics, in that the phosphine dissociation is the rate-limiting step of the catalytic reaction. The silyl complex (PCy3)(CO)Ru(SiMe3)Cl (7) can then bind alkyne to give the alkyne-bound complex (PCy3)(CO)(Cl)(SiMe3)Ru(PhCCH) (8), which is slightly downhill by 1.7 kcal/mol. Insertion of the silyl group into the alkyne gives the silylated (Z)-Ru-vinyl complex 9, which is calculated to be 15.6 kcal/mol downhill from 8. As the catalysis progresses, the coordination of vinylsilane product 2 to the silyl complex 7 is expected to become more significant, while the inhibition effect due to the recoordination of PCy3 or styrene would be minimal due to their relatively low concentration. Since understanding the time and nature of the isomerization is crucial for further development of this family of catalysts, the possible product formation pathways from 9 were probed (Scheme 3). The (Z)-Ru-vinyl complex 9 could first bind the silane substrate to form (PCy 3 )(CO)(Cl)Ru(CPh CHSiMe3)(HSiMe3) ((Z)-10), which could then undergo hydrogen transfer to release the trans-vinylsilane product (E)2a. Alternatively, (Z)-Ru-vinyl complex 9 or the silane-bound complex 10 could undergo isomerization. Since the deuterium labeling study strongly suggested that Ru-vinylidene intermediate species are not likely to be involved, they were only probed with smaller model systems (using PMe3 instead of full PCy3). Indeed, the DFT/MM calculations suggested that the free energies of Ru-vinylidene complexes are much higher than those of vinyl complexes and thus the former complexes are not likely to be involved in the catalysis. Binding of the HSiMe3 substrate to (Z)-9 is calculated to be uphill by 9.2 kcal/mol. Hydrogen transfer then occurs via an oxidative hydrogen migration mechanismas evidenced by the short Ru−H bond distance (1.62 Å) in the transition state

Figure 4. Transition state on the hydrogen migration step for the formation of (E)-2. Cyclohexyl groups and methyl groups on one of the silicon atoms are omitted for clarity. Color code: gray, carbon; white, hydrogen; pink, ruthenium; purple, silicon; green, phosphorus; blue, chlorine; red, oxygen. Bond lengths are given in Å.

temperature reactions observed experimentally and also consistent with the lack of formation of the trans-vinylsilane product (E)-2a. Isomerization from the (Z)-9 complex to (E)-9 occurs with a barrier of only 11.8 kcal/mol, unusually low for what is ostensibly rotation about a carbon−carbon double bond. The transition state for CC rotation, however, resembles the metallocyclopropene structures reported by Trost and coworkers (Figure 5).21 Interestingly, for the current system, a stable metallocyclopropene intermediate (i.e., local minimum) could not be located. The cis-vinylsilanyl complex (E)-9 is calculated to be 6.3 kcal/mol more stable than (Z)-9. Binding of silane to (E)-9 is uphill by 10.8 kcal/mol. Subsequent hydrogen transfer to give cis-vinylsilane product (Z)-2a is calculated to occur with a barrier of 19.0 kcal/mol relative to E


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cently.22 In the Ir-catalyzed reaction, the alkyne coordination and Cp-ring slippage step has been determined to be the turnover-limiting step, while the silyl migration step has been shown to be the most energetically demanding step for the Rucatalyzed reaction. A similar low-energy cis−trans isomerization process via η2-vinyl species has been computed in the Rucatalyzed study.22b From the aggregate of the experimental and computational studies, a reasonable mechanism for the hydrosilylation reaction is presented in Scheme 4. The experimental phosphine inhibition kinetics implicated that the phosphine dissociation leads to the catalytically active species and that this step is a likely rate-limiting step. The computational study confirmed this notion in that the most energetically demanding step is the formation of the 14-electron Ru-silyl species 7. The Ru-silyl species 7 would be initially generated from the insertion of alkyne followed by the addition of silane substrate and the elimination of styrene. The regioselective migratory insertion of the silyl group to the terminal carbon of the alkyne substrate would form the Ru-vinyl species 9. Normally, the formation of the cis-vinylsilane product (Z)-2 is favored. The computational study revealed a low-energy barrier between cis- and trans-vinyl species, which is favored for sterically nondemanding alkyne substrates. Hence, we hypothesize that the magnitude of ΔΔG⧧ for cis−trans isomerization of the Ru-vinyl species (Z)-/(E)-9 appears to be the main factor for preventing the formation of trans-vinylsilane product. This mechanistic rationale also explains the apparent insensitivity of electronic influence on dictating the rate and the E/Z stereoselectivity of the product formation. In case of more sterically demanding alkynes such as tert-butylacetylene, a less accessible transition state may be required to rationalize the formation of (E)-vinylsilanes (E)-2.

Figure 5. Transition state structure for the E/Z isomerization of the Ru-vinyl species 9. Cyclohexyl groups are omitted for clarity. Color code: gray, carbon; white, hydrogen; pink, ruthenium; purple, silicon; green, phosphorus; blue, chlorine; red, oxygen. Bond lengths are given in Å.

the (E)-10 complex, which is far lower than the 35.1 kcal/mol calculated for the formation of trans-vinylsilane product (E)-2a. The calculations thus suggest that isomerization of (Z)-9 to (E)-9 is incredibly facile and occurs before binding of the silane substrate. The low energy for isomerization by rotation about the CC bond can be understood in terms of the η2-vinyl/ metallocyclopropene structure of the transition state, as previously suggested by Trost and co-workers.21 With such a low barrier for double-bond isomerization, the stereochemistrydetermining step must therefore be the hydrogen transfer step from the silane substrate to the vinyl complex intermediate. There is a large difference in the calculated barrier for release of the two products, which is consistent with the observed selectivity. Two computational studies on the Ir- and Rucatalyzed hydrosilylation reaction have been reported re-

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CONCLUSIONS The ruthenium hydride complex 1 has been found to be an effective catalyst for promoting regio- and stereoselective hydrosilylation of alkynes to form cis-vinylsilane products (Z)-2. The detailed kinetic experiments led to the empirical rate law of kobs[1] and uncompetitive phosphine inhibition kinetics. The computational studies revealed electronic factors on guiding Z-selective vinylsilane product formation. The

Scheme 4. Proposed Mechanism of the Hydrosilylation of Phenylacetylene

F


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ACKNOWLEDGMENTS Financial support from the National Science Foundation (CHE-1358439) is gratefully acknowledged. T.R.C. and D.R.P. thank the Department of Energy (DE-FG0203ER15387) for partial support of this research at UNT.

combined experimental and computational studies provided a detailed mechanism that features the initial phosphine dissociation as the turnover-limiting step of the catalytic cycle and a facile cis−trans isomerization of the Ru-vinyl species in mediating stereoselective formation of the vinylsilane products.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

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General Information. All operations were carried out in an inertatmosphere glovebox or by using standard high-vacuum and Schlenk techniques unless otherwise noted. Toluene, hexanes, and Et2O were distilled from purple solutions of sodium and benzophenone immediately prior to use. The NMR solvents were dried from activated molecular sieves (4 Å). All organic substrates were received from commercial sources and used without further purification. The 1 H, 2H, 13C, and 31P NMR spectra were recorded on a 300 or 400 MHz FT-NMR spectrometer. GC and GC-MS spectra were recorded from the department spectrometers, while elemental analyses were performed at a commercial service laboratory. Representative Procedure of the Catalytic Reaction. In a glovebox, an alkyne (1.0 mmol), HSiEt3 (2.0 mmol), and the catalyst 1 (7 mg, 1 mol %) were dissolved in CH2Cl2 (3−5 mL) in a 25 mL Schlenk tube equipped with a Teflon stopcock and a magnetic stirring bar. The reaction tube was brought out of the glovebox and was stirred at room temperature for 4−8 h. The tube was opened to the air at room temperature, and the crude product mixture was analyzed by GC-MS. Analytically pure organic product was isolated by column chromatography on silica gel (n-hexane/EtOAc). Deuterium Labeling Study. In a glovebox, HSiEt3 (2.0 mmol) and DCCPh (1.0 mmol) were added via a syringe to a 25 mL Schlenk tube equipped with a magnetic stirring bar and Teflon stopcock. The catalyst 1 (1 mol %) predissolved in CH2Cl2 (3 mL) was added to the reaction tube. The reaction tubes were brought out of the box, and their contents were stirred at room temperature for 4 h. The solvent was removed by a rotary evaporator, and the organic product was isolated by column chromatography on silica gel (nhexane/EtOAc). The deuterium content of the product was measured by both 1H NMR (CDCl3, cyclohexane (10 mg, internal standard)) and 2H NMR (CH2Cl2 with 50 μL of acetone-d6). Phosphine Inhibition Study. In a glovebox, HSiEt3 (0.20 mmol), HCCPh (0.10 mmol), 1 (3 mg, 2 mol %), and C6Me6 (2 mg, internal standard) were dissolved in 0.5 mL of CDCl3 solution in a J. Young NMR tube with a Teflon screw cap. A predissolved PCy3 in CDCl3 solution (0.4−1.4 mM) was added to the tube via syringe. The tube was brought out of the glovebox. The reaction was monitored by 1 H NMR in 30 min intervals. The rate was measured by the 1H integration of the product peak at δ 5.81 (CHSi) and was normalized against the internal standard peak. The kobs value was estimated from the first-order plot of ln[product] vs reaction time.

REFERENCES

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S Supporting Information *

Text, figures, and xyz files giving experimental procedures and spectroscopic data of organic products and Cartesian coordinates and energies of the key reaction intermediates and transition structures obtained in the computational study. This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail for T.R.C.: [email protected]. *E-mail for C.S.Y.: [email protected]. Notes

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