Mechanism and Regioselectivity of C–N Bond Cleavage and Ring

Article pubs.acs.org/Organometallics

Mechanism and Regioselectivity of C−N Bond Cleavage and Ring Expansion of N‑Heterocyclic Carbenes Ran Fang,* Lizi Yang, and Qiang Wang Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province and College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China S Supporting Information *

ABSTRACT: We report here the theoretical analysis of the mechanism and regioselectivity of the direct insertion of silane moieties into the C−N bond of N-heterocyclic carbenes leading to eventual ring expansion and formation of diazasilinanes. Symmetrically and unsymmetrically substituted NHC have been considered to account for some experimental observations. These reaction steps include (1) Si−H bond activation of the silane at the NHC; (2) amide transfer to the silicon atom to yield a six-membered intermediate; (3) hydride or phenyl group transfer to the carbon atom to give the product. In addition, the computed results over a variety of silanes agree with experimental evidence and suggest that both steric and electronic effects play a crucial role in the regioselectivity outcome and the involvement of operative intermediates in the reaction pathway.

1. INTRODUCTION Carbenes (derivatives of divalent carbon) are usually short-lived reactive species.1 It is only among the N- and P-substituted carbenes that stable, isolable compounds are found at room temperature.2 The direct observation of singlet alkyl carbenes usually requires matrix isolation conditions,3 while indirect observation and kinetic measurements in solution can be performed by the pyridine ylide method developed by Platz and co-workers.4 Since the isolation of stable N-heterocyclic carbenes (NHCs) in 1991,5 the chemistry of NHCs and related molecules6 has grown rapidly, and numerous applications of NHCs in main group7 and transition-metal chemistry,8 as well as catalysis,9 have been reported. 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 equally capable of some of these tasks.10 For example, Bertrand and co-workers11 report that stable acyclic and cyclic (alkyl)-amino carbenes (CAACs) react with CO to afford amino ketenes, which are indefinitely stable at room temperature both in solution and in the solid state. The activation of H2 and NH3 by CAACs using experimental as well as theoretical methods has also been extensively studied.12 Furthermore, activation of P4 and Si−H bonds has also been reported by Bertrand and co-workers.13 A number of groups have observed that N-alkyl and N-aryl-substituted © 2013 American Chemical Society

NHCs are prone to both C−H and C−N activation of the peripheral organic substituents within the coordination sphere of transition-metal centers.14,15 Recently, the direct insertion of silylene moieties into the C−N bond of N-heterocyclic carbenes with ring expansion and formation of diazasilinanes has been achieved by Radius et al.16 According to the experimental results, the general mechanisms were postulated to explain the formation of a six-membered heterocycle (Scheme 1). By using the symmetrically substituted NHC, the first step of the reaction involves activation of one of the Si−H bonds of silanes at the NHC to give intermediate A, followed by amide transfer to the silicon atom to yield intermediate B. The final step of the reaction sequence involves group (H or Ph) transfer from the silicon atom to the carbon atom to give C. Alternatively, using the unsymmetrically substituted NHC, the first step of the reaction also involves activation of one of the Si−H bonds of silanes at the NHC to give intermediate A. The next step of amide transfer to the silicon atom yields intermediates D and F through regioselective C−N bond cleavage, respectively. The final step of the reaction sequence involves group (H or Ph) transfer from the silicon atom to the carbon atom to give E and G.16 Received: July 2, 2013 Published: December 18, 2013 53

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Scheme 1. Proposed Reaction Pathway for the Title Reactiona

a

Red structures are proposed transition states, and those in black are the key intermediates proposed by Radius et al.16

singly occupy two carbon orbitals.3,4 The selected optimized stationary structures (minima, saddle points) on the potential energy surfaces of the reactions are depicted schematically in Figure 1 with selected key geometry parameters (bond lengths and angles). The free energy changes relative to the starting materials (NHC + H4Si, NHC + H3SiPh, NHC + H2SiPh2, and NHC + HSiPh3) in the solvent for these reactions are presented in Table 1. Energy profiles for the reactions are shown in Figure 2. Unless otherwise noted, the relative energies discussed later refer to the value in toluene solvent. The detailed structural parameters and energies for the structures determined here are collected in the Supporting Information. Finally, since the excitation energies from the singlet ground state to the first triplet excited state are 86.3 and 81.7 kcal/mol for model symmetrically and unsymmetrically substituted NHCs, only the singlet ground state is considered throughout this work. 3.1. C−N Bond Cleavage and Ring Expansion of Symmetrically Substituted NHC (R4 = R5 = Me). The orbital interactions that occur between the Si−H unit and the carbene can reveal the extent of the insertion reaction. As a singlet carbene has a vacant orbital and a filled nonbonding orbital3,4 and in that respect resembles transition-metal centers, we reasoned that sufficient overlap between the Si−H unit and the carbene might still occur and thus enable activation of silanes. It is rational to expect that the first step in the insertion reaction of NHCs with silanes is the formation of a precursor complex (PC). However, no “Werner-like” adducts can be formed due to the strong nucleophilic character of these carbenes. According to our calculation results, the first step for the activation of one of the Si−H bonds involves a concerted hydrogen migration through a three-center transition structure (TS) to give intermediate A-1, B-1, C-1, and D-1, respectively. Vibrational analysis shows that all of these TS structures are a first-order saddle point with only one imaginary frequency of 814i, 1152i, 1104i, and 1208i cm−1 for A-TS1, B-TS1, C-TS1, and D-TS1, respectively. Moreover, the IRC calculations confirmed that these TSs connect the corresponding reactants and products. From the calculation

As we know, the insertion reaction of Si−H bonds is one of the fundamental and most widely studied reactions.17 Therefore, we have now undertaken a detailed investigation of the C−N bond cleavage and ring expansion of N-heterocyclic carbenes via hydrosilanes by using the density functional theory (DFT). With this theoretical study, we hope (i) to obtain a detailed understanding of the reactivities and regioselectivity of silanes toward NHCs, (ii) to investigate the influence of different silanes upon the geometries and energies of both the precursor complexes and the transition structures, (iii) to predict the trends in activation energies and reaction energies, and (iv) to probe electronic effects on the activities in a series of silanes and NHCs.

2. COMPUTATIONAL METHODS Geometries, energies, and first- and second-energy derivatives of all of the stationary points found here were fully optimized by DFT using the GAUSSIAN 09 program suite.18 For DFT calculations, the M06-2X hybrid functional,19 combined with the standard 6-31+G(d,p) basis set,20 was selected. The M06-2X hybrid functional gives reliable energies for a variety of chemical applications.21 The spin-unrestricted (UM062X) formalism was used for open-shell (triplet) species, and their ⟨S2⟩ values were nearly all equal to ideal value (2.00). Vibrational frequency calculations at the M06-2X/6-31+G(d,p) level were used to characterize all stationary points as minima or transition states. The relative energies are, thus, corrected for vibrational zero-point energies, and the transition structures in the reaction paths were examined by using the intrinsic reaction coordinate (IRC).22 The solvent effect was taken into account by M06-2X/6-311++G(d,p) single-point calculation with the integral equation formalism polarizable continuum model (IEFPCM) in toluene (ε = 2.37). The radii and nonelectrostatic terms were taken from Truhlar and co-workers’ universal solvation model (SMD).23

3. RESULTS AND DISCUSSIONS It is well established that carbene exists in two possible states. It is either in a singlet state (S0), in which one carbon orbital is empty and the second contains two unshared spin-paired electrons, or in a triplet state (T1), in which two electrons with parallel spins 54

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Figure 1. Selected optimized structures for symmetrically and unsymmetrically substituted NHCs, with selected structural parameters (bond lengths in Å; detailed optimized structures for symmetrically and unsymmetrically substituted NHCs are collected in the Supporting Information; see Figures S1 and S2).

results, the transition states of all the reaction pathways investigated have a common structure in which the angle of approach of the silane is roughly perpendicular to the carbene ring plane. The distances of C−Si for A-TS1, B-TS1, C-TS1, and D-TS1 are 3.206, 3.095, 3.117, and 3.067 Å, respectively. Very recently, Wilson and Dutton have presented in their paper calculations on the reaction of NHC with Ph2SiH2 (M06-2X/ B3LYP, 6-31G(d) for optimization; single-point energy with PBE1PBE and B3LYP, as well as SCS-MP2 and SOS-MP2 methods, with a TZVP basis set; TZVP, MP2 + M06-2X/TZVP thermochemical correction). A similar three-center transition structure has been found for the reaction. They have shown that the three-center transition structure is the rate-determining one (30.8 kcal/mol) of the overall process.24 As the reaction goes from the reactant to the transition state, the distances between the Si and H atoms change from 1.480, 1.483, 1.485, and 1.487 Å to 2.061,1.950, 1.960, and 1.935 Å for A-TS1, B-TS1, C-TS1, and D-TS1, respectively. Furthermore, the distances of the C1−H bond are 1.234, 1.297, 1.295, and 1.300 Å for A-TS1, B-TS1, C-TS1, and D-TS1, respectively. The N1−C1−Si angles are 113.5°, 110.3°, 111.7°, and 110.7° for A-TS1, B-TS1, C-TS1, and D-TS1, respectively. This indicates an onset of the sp2−sp3 rehybridization is required for product formation. These changes in the angles and bond distances show that the C1−H bond will be formed and the Si−H bond will be cleaved for the first step. That is to say, the insertion of Si−H of silanes by NHC involves the bond forming between carbon, silicon, and hydrogen in concert with one Si−H bond breaking in the silanes. It is generally accepted that a singlet carbene undergoes Si−H bond insertion in a concerted single-step process through a triangular transition state in which the empty p-orbital of the carbene interacts with the filled σ (Si−H)-orbital (Scheme 2).25 In order to obtain clear information about the mechanism and orbital interactions of potential intermediate structures along the reaction pathway, the intrinsic reaction coordinate calculations were performed for A-TS1 (results are shown in Figure 3). As a

Table 1. Thermodynamic Properties (Relative Free Energies and Activation Free Energies in the Gas Phase and in Solution) of the Structures in Figure 2a system

ΔGrelgas

ΔG⧧gas

ΔGrelsol

ΔG⧧sol

NHC + H4Si A-TS1 A-1 A-TS2 A-2 A-TS3 A-3 NHC + H3SiPh B-TS1 B-1 B-TS2 B-2 B-TS3 B-3 NHC + H2SiPh2 C-TS1 C-1 C-TS2 C-2 C-TS3 C-3 NHC + HSiPh3 D-TS1 D-1 D-TS2 D-2 D-TS3 D-3

0 31.9 7.0 34.0 16.1 23.5 −22.1 0 30.9 7.4 32.9 18.4 21.5 −24.7 0 33.2 8.7 33.5 19.9 20.8 −26.8 0 31.9 12.4 31.6 20.1 28.4 −25.8

0 31.9

0 30.1 13.1 40.6 20.7 29.3 −15.5 0 33.4 13.9 40.5 23.9 27.9 −17.7 0 36.2 15.6 41.0 25.8 27.3 −19.3 0 36.4 19.8 39.9 26.5 36.2 −17.5

0 30.1

26.9 7.4 0 30.9 25.5 3.1 0 33.2 24.8 1.0 0 31.9 19.3 8.3

27.5 8.6 0 33.4 26.6 4.0 0 36.2 25.4 1.5 0 36.4 20.1 9.7

a

These values, in kcal/mol, were calculated at the M06-2X/6-31+G(d,p) level of theory and included the zero-point energy correction, using singlepoint integral equation formalism polarizable continuum model (IEFPCM) calculations at the M06-2X/6-311++G(d,p) level of theory to model the effect of the solvent (toluene).

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Figure 2. Energy profiles for symmetrically substituted NHCs; the relative energies are given in kcal/mol. (A, B, C, and D are taken for calculations on the reaction with SiH4, PhSiH3, Ph2SiH2, and Ph3SiH, respectively).

Scheme 2. Schematic Representation of the Mechanism of Insertion of NHC into Silanes

choice, we selected three intermediate structures, I (before transition structures), A-TS1 (transition structures), and II (after transition structures), as the model for our calculation (Figure 3). The calculation results for orbital interactions of the HOMO and LUMO for I, A-TS1, and II are shown in Figure 4. As we can see from Figure 4, the HOMOs for structures I and TSa1 describe the interaction of the nonbonding orbital with the Si−H σ-bonding orbital, while the LUMO is the carbene carbon p-orbital and the Si−H σ-bonding orbital. As the reaction goes from I to II, the HOMO for structures II is the orbital overlap between the carbene carbon p-orbital and the Si−H σ-bonding orbital (newly formed Si−C σ bond), while the LUMO describes the newly formed Si−C σ* bond orbital of the carbene carbon porbital and the Si−H σ-bonding orbital. Overall the reaction mechanism involved a proton transfer (not radical or hydride) from R3SiH to the carbene and the empty p-orbital of the carbene interacting with the filled σ (Si−H)-orbital (Scheme 2). Thus, these bond changes can be mainly attributed to the interaction of the Si−H σ-bonding orbital and carbene carbon p-orbital of the NHC associated with an elongation of the Si−H bonds. Table 1 lists the calculated free energies of activation and reaction of the reactions studied. The barrier heights for the insertion reactions increase in the order A-TS1 (30.1 kcal/mol) < B-TS1 (33.4 kcal/mol) < C-TS1 (36.2 kcal/mol) < D-TS1 (36.4 kcal/mol). The energies of the reaction are 13.1, 13.9, 15.6, and 19.8 kcal/mol for

Figure 3. Intrinsic reaction coordinate (IRC) calculation results for A-TS1 and selected structures along the reaction pathway of the title reaction, calculated at the M06-2X/6-31+G(d,p) level of theory.

A-1, B-1, C-1, and D-1, respectively. It is important to point out that this step is also the rate-determining one. Our calculations indicate that the phenyl group of silanes has a large effect on the insertion barriers. In other words, the SiH4 species readily 56

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Figure 4. Orbital interactions for HOMO and LUMO for structures of I, TSa1, and II.

undergoes insertion reactions with NHC, while HSiPh3 has a higher insertion barrier. The lower barriers found for H4Si compared to those for H3SiPh, H2SiPh2, and HSiPh3 can mainly be attributed to the following reasons. First, a broad range of insertion reactions for carbene have already been examined, and it is evident that the insertion reactions are better related to the singlet−triplet gap of the reactants such as the carbenes and silylenes,25 i.e., ΔEst (=Etriplet − Esinglet for X2C:) + ΔEσσ* (=Etriplet − Esinglet for silanes). In other words, the smaller the ΔEst + ΔEσσ* value, the lower the activation barrier. The theoretical calculations suggest that the ΔEst + ΔEσσ* are 174.8, 183.0, 191.5, and 194.0 kcal/mol for NHC + H4Si, NHC + H3SiPh, NHC + H2SiPh2, and NHC + HSiPh3, respectively. These indicate that the former should have lower activation barriers, and the latter, higher. Second, the electron transfers of the H atom were 0.204, 0.209, 0.212, and 0.218 au for A-TS1, B-TS1, C-TS1, and D-TS1, respectively (Table 2 gives the

Table 3. Thermodynamic Properties (Relative Free Energies and Activation Free Energies in the Gas Phase and in Solution) of the Structures in Figure 5a

Table 2. Natural Charges at the Participating Atoms for Symmetrically Substituted NHC with SiH4a atom

NHC + SiH4

A-TS1

A-1

H Si N1 C1

−0.103 0.411 −0.365 0.028

0.101 −0.160 −0.214 0.061

0.178 0.546 −0.157 −0.615

a

These values, in au, were calculated at the M06-2X/6-31+G(d,p) level of theory.

natural charges at the participating atoms for symmetrically substituted NHC with SiH4; detailed natural charges at the participating atoms for symmetrically substituted NHC were collected in the Supporting Information; see Table S1). The smaller the value found for A-TS1, the lower the barrier to insertion of silane into NHC. As the reaction goes from the transition state to A-1, B-1, C-1, and D-1, the C1−H single bonds become completely formed and the Si−H bonds become completely broken. In order to accomplish C−N bond cleavage and ring expansion, in a subsequent step A-1, B-1, C-1, and D-1 undergo amide transfer to the silicon atom to yield intermediates A-2, B-2, C-2, and D-2 through A-TS2, B-TS2, C-TS2, and D-TS2, respectively. Figure 1 (Figure S1) shows that the distances of the C1−N1 bond are 2.236, 2.162, 2.185, and 2.203 Å in A-TS2, B-TS2, C-TS2, and D-TS2, respectively. The free activation energy of the second step is 27.5, 26.6, 25.4, and 20.1 kcal/mol for A-TS2, B-TS2, C-TS2, and D-TS2, respectively, and the energy of reaction for A-2, B-2, C-2, and D-2 is 7.6, 10.0, 10.2, and 6.7 kcal/mol with respect to A-1, B-1, C-1, and D-1, respectively. The higher barrier found for A-TS2 and B-TS2 than that of C-TS2, and D-TS2 can be mainly attributed to the electronic effect of the N1 atom. For example, the NBO charges for the N1 atom of A-1, B-1, C-1, and D-1 were −0.157, −0.068, 0.048, and 0.071 au, respectively.

system

ΔGrelgas

ΔG‡gas

ΔGrelsol

ΔG‡sol

NHC + H4Si E-TS1 E-1 E-TS2 E-2 E-TS3 E-3 E-TS4 E-4 E-TS5 E-5 NHC + HSiPh3 F-TS1 F-1 F-TS2 F-2 F-TS3 F-3 F-TS4 F-4 F-TS5 F-5 NHC + H2SiPh2 G-TS1 G-1 G-TS2 G-2 G-TS3 G-3 G-TS4 G-4 G-TS5 G-5 NHC + HSiPh3 H-TS1 H-1 H-TS2 H-2 H-TS3 H-3 H-TS4 H-4 H-TS5 H-5

0 31.6 3.5 29.2 20.5 25.3 −20.6 30.5 18.1 24.7 −21.7 0 29.1 1.7 28.3 23.6 24.2 −22.4 30.1 20.9 23.7 −24.9 0 30.5 2.1 31.3 19.9 25.9 −25.6 28.9 20.5 22.1 −26.5 0 30.3 1.4 35.0 30.0 34.6 −20.8 30.0 21.7 29.3 −22.8

0 31.6

0 30.6 10.1 34.6 24.3 30.1 −15.0 35.8 21.6 29.4 −16.2 0 31.8 7.8 35.0 29.6 29.8 −16.3 36.0 25.4 29.1 −19.0 0 33.7 8.8 37.6 25.6 32.1 −17.9 35.1 25.8 27.6 −20.0 0 34.8 8.9 42.2 36.7 42.2 −12.8 36.9 28.4 36.8 −15.2

0 30.6

25.7 4.8 27.0 6.6 0 29.1 26.7 0.6 28.5 2.7 0 30.5 29.2 6.0 26.8 1.5 0 30.3 33.7 4.6 28.7 7.6

24.5 5.8 25.7 7.8 0 31.8 27.1 0.2 28.2 3.7 0 33.7 28.8 6.5 26.3 1.8 0 34.8 33.3 5.5 27.9 8.4

a

These values, in kcal/mol, were calculated at the M06-2X/6-31+G(d,p) level of theory and included the zero-point energy correction, using single-point integral equation formalism polarizable continuum model (IEFPCM) calculations at the M06-2X/6-311++G(d,p) level of theory to model the effect of the solvent (toluene).

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Figure 5. Energy profiles for unsymmetrically substituted NHC; the relative energies are given in kcal/mol.

Positive charges found for the N1 atom of C-TS2 and D-TS2 make the cleavage of the C−N bond more feasible than that of A-TS2 and B-TS2. The subsequent step for migration of hydride or phenyl groups results in the formation of the final product A-3, B-3, C-3, and D-3 through A-TS3, B-TS3, C-TS3, and D-TS3, respectively. The final barriers of 8.6, 4.0, 1.5, and 9.7 kcal/mol are required to release the product. These final steps are exothermic by −36.2, −41.6, −45.1, and −43.0 kcal/mol, respectively, and the whole processes are exothermic, the products being 15.5, 17.7, 19.3, and 17.5 kcal/mol lower in energy than the reactants. Besides the migration of hydride, the phenyl migrations can also account for the formation of other products. However, the higher activation barriers found for phenyl migrations of H3SiPh (11.0 vs 4.0 kcal/mol) and H2SiPh2 (10.9 vs 1.5 kcal/mol) indicate that the pathway for phenyl migrations was unfavorable (detailed energy profiles of HSiPh3 and H2SiPh2 for phenyl migration are collected in the Supporting Information; see Figure S3). 3.2. C−N Bond Cleavage and Ring Expansion of Unsymmetrically Substituted NHC (R4 = Me, R5 = iPr). Next, we consider the reaction of silanes with unsymmetrically substituted NHC. The selected optimized stationary structures (minima, saddle points) on the potential energy surfaces of the reactions are also depicted schematically in Figure 1 with selected key geometry parameters. The energy changes relative to the starting materials (NHC + H4Si, NHC + H3SiPh, NHC + H2SiPh2, and NHC + HSiPh3) either in the gas phase or in solvent for these reactions are presented in Table 3. Energy profiles for the reactions are shown in Figures 5 and 6. Like the insertion reaction of symmetrically substituted NHC, the interaction of the Si−H σ-bonding orbital and carbene carbon p-orbital of the NHC leads to intermediates E-1, F-1, G-1, and H-1, through a three-center four-electron transition structure (E-TS1, F-TS1, G-TS1, and H-TS1). IRC calculations confirmed that these TSs connect the corresponding reactants and products. Examination of Figures 1 and S2 reveals that the distances of C−Si for E-TS1, F-TS1, G-TS1, and H-TS1 are 3.224, 3.122, 3.136, and 3.129 Å, respectively. As the reaction goes from the reactant to the transition state, the distances of C1−H and Si−H

are 1.272, 1.318, 1.315, 1.320 Å and 2.010, 1.931, 1.941, 1937 Å for E-TS1, F-TS1, G-TS1, and H-TS1, respectively. The N2−C1−Si angles are 110.6°, 106.3°, 106.3°, and 105.8° for E-TS1, F-TS1, G-TS1, and H-TS1, respectively. This indicates that an onset of the sp2−sp3 rehybridization is required for the formation of E-1, F-1, G-1, and H-1 formation. As the reaction goes from the transition state to E-1, F-1, G-1, and H-1, the C1−H single bonds become completely formed and the Si−H bonds become completely broken. Table 3 shows that the barrier heights for the insertion of an unsymmetrically substituted NHC with silanes are 30.6, 31.8, 33.7, and 34.8 kcal/mol for E-TS1, F-TS1, G-TS1, and H-TS1, respectively. The energies of the reaction are 10.1, 7.8, 8.8, and 8.9 kcal/mol for E-1, F-1, G-1, and H-1, respectively. Similar to the reaction of a symmetrically substituted NHC with silanes, the calculations indicate that the phenyl group of silanes also has a large effect on the insertion barriers. The lower barriers found for H4Si compared to those for H3SiPh, H2SiPh2, and HSiPh3 can also mainly be attributed to the singlet−triplet gap of the reactants (the ΔEst +ΔEσσ* are 170.2, 179.4, 186.9, and 189.4 kcal/mol for NHC + H4Si, NHC+ H3SiPh, NHC+ H2SiPh2, and NHC+ HSiPh3, respectively) and the electron transfer of a H atom (the electron transfers of a H atom are 0.189, 0.201, 0.207, and 0.208 au for E-TS1, F-TS1, G-TS1, and H-TS1, respectively). Detailed natural charges at the participating atoms for unsymmetrically substituted NHCs are collected in the Supporting Information; see Table S2). After formation of E-1, F-1, G-1, and H-1, the C1−N2 and C1−N1 bonds can be further activated to undergo amide transfer to the silicon atom. The attack of the silicon atom on the σ bond of C1−N1 or C1−N2 of E-1, F-1, G-1, and H-1 leads to two different pathways to yield E-2, F-2, G-2, H-2 and E-4, F-4, G-4, H-4. Figures 1 and S2 reveal that the distances of the C1−N1 bond are 2.172, 2.150, 2.249, and 2.294 Å for E-TS2, F-TS2, G-TS2, and H-TS2, respectively, and the distances of the C1−N2 bond are 2.088, 2.169, 2.162, and 2.128 Å for E-TS3, F-TS3, G-TS3, and H-TS3, respectively. The experiments show that the use of PhSiH3 leads to the isomers F-3 and F-5 in a ratio of 0.88:1.00 according to 1H NMR spectroscopy, affording the sterically 58

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Figure 6. Energy profiles for unsymmetrically substituted NHC; the relative energies are given in kcal/mol.

HSiPh3 and H2SiPh2 for phenyl migration are collected in the Supporting Information; see Figure S4).

unfavorable F-3 as the major product. The reaction of Ph2SiH2 gave a 1.00:0.53 mixture of G-3 and G-5 in favor of the sterically less crowded G-5. By using Ph3SiH, insertion into the C−N bond was observed exclusively with formation of isomer H-5.16 According to our calculation results, the free activation energies for E-TS2, F-TS2, G-TS2, H-TS2 and E-TS4, F-TS4, G-TS4, H-TS4 are 24.5, 27.1, 28.8, 33.3 and 25.7, 28.2, 26.3, 27.9 kcal/mol, respectively. The lower free activation energy found for E-TS2 and F-TS2 indicates that the cleavage of the C1−N1 bond leads to the major product (E-3 and F-3). However, the higher free activation energies found for G-TS2 and H-TS2 indicate that the cleavage of the C1−N2 bond would be the major product (E-5 and F-5). The results show that the electronic effect in H3SiPh and H4Si is more prominent than the steric effect of the phenyl groups. The steric effect increases with an increase in the number of phenyl groups at the Si atom. Therefore, the steric effect of the phenyl groups outweighs the electronic effect for H2SiPh2 and HSiPh3. Also, our calculation results were consistent with the experimental observations. The final step of migration of hydride or phenyl groups results in the formation of the final product E-3, F-3, G-3, H-3 and E-5, F-5, G-5, H-5 through E-TS3, F-TS3, G-TS3, H-TS3 and E-TS5, F-TS5, G-TS5, H-TS5, respectively. The final barriers of 5.8, 0.2, 6.5, 5.5 and 7.8, 3.7, 1.8, 8.4 kcal/mol are required to release the product. These final steps are exothermic by −39.3, −45.9, −43.5, −49.5 and −37.8, −44.4, −45.8, −43.6 kcal/mol, and the whole processes are exothermic by −15.0, −16.3, −17.9, −12.8 and −16.2, −19.0, −20.0, −15.2 kcal/mol less than the reactants. Apart from the migration of hydride for H3SiPh (5.8 vs 0.2 kcal/mol) and H2SiPh2, the phenyl migrations can also account for the formation of other product. Also, the higher activation barriers found for phenyl migrations of H3SiPh (5.8 vs 0.2 kcal/mol) and H2SiPh2 (8.1 vs 1.8 kcal/mol) indicate that the pathway for phenyl migrations was unfavorable (detailed energy profiles of

4. CONCLUSIONS In summary, this study has provided a detailed theoretical demonstration concerning the reaction trajectory and theoretical estimation of the activation energy and reaction enthalpy for the title reactions. The computational results based on the model complex (symmetrically and unsymmetrically substituted NHC) and substrate HnSiPhm showed that the reaction occurs via three major steps: (1) activation of one of the Si−H bonds of silanes assisted by NHC, (2) amide transfer to the silicon atom, yielding a six-membered intermediate, and finally (3) hydride or phenyl group transfer of this intermediate from the silicon atom to the carbon atom to give the product. Calculations indicate that for both symmetrically and unsymmetrically substituted NHC, activation of one of the Si−H bonds of silanes to give intermediates was the rate-determining step with an activation free energy of 30.1−36.4 kcal/mol in the absence of solvent. Furthermore, the reactivity of silanes decreases in the order: H4Si > H3CH3Ph > H2SiPh2 > HSiPh3. Another conclusion of the present work is that the cleavage of the C−N bond has high regioselectivity for a given unsymmetrically substituted NHC. The reaction of PhSiH3 and SiH4 affords the sterically unfavorable product. However, Ph2SiH2 and Ph3SiH give a product in favor of the sterically less crowded ones. These computational results are consistent with the experimental observations of Radius et al. for the activation of C−N bonds by stable singlet carbenes. The predictions may be useful as a guide to future synthetic efforts and to problems that merit further study by both theory and experiment.

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ASSOCIATED CONTENT

S Supporting Information *

The complete citation for ref 10, the Cartesian coordinates for the calculated stationary structures, and the sums of the 59

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Organometallics

Article

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electronic and zero-point energies for the transition states and intermediates and products and reactants obtained from the DFT calculations are given. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been supported by the National Natural Science Foundation of China (Nos. 21203080 and 21301080), Specialized Research Fund for the Doctoral Program of Higher Education (Grant Nos. 20110211120012 and 20120211120015), and the Fundamental Research Funds for the Central Universities (Grant Nos. lzujbky-2013-58, lzujbky-2013-62, and lzujbky2013-66). The high-performance computing facility at the Gansu Computing Center is also acknowledged.

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