Mechanism for Activation of the C–CN Bond of Nitriles by a Cationic

Jun 10, 2014 - The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan ...
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Mechanism for Activation of the C−CN Bond of Nitriles by a Cationic CpRhIII−Silyl Complex: A Systematic DFT Study Song-Lin Zhang,* Lu Huang, and Wen-Feng Bie The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, Jiangsu Province, People’s Republic of China S Supporting Information *

ABSTRACT: A theoretical study of reaction mechanisms for C−CN bond activation of nitriles by a RhIII−silyl complex is reported. Various mechanisms including direct oxidative addition, insertion of the cyanide CN triple bond into the RhIII−silyl bond followed by β-carbon elimination, insertion of the cyanide CN triple bond into the RhIII−silyl bond followed by α-carbon elimination (deisocyanide), radical mechanisms, and other possible alternatives have been evaluated. Our results provide strong evidence for the sequential mechanism of cyano insertion/α-C elimination (deisocyanide). The cyanide insertion step should be the rate-limiting step, while the deisocyanide step is facile. The intermediate from cyanide insertion, i.e., the Rh(III) η2-iminoacyl complex, has been identified and is in good agreement with the experimentally characterized X-ray crystal structure. The oxidative addition and cyanide insertion/β-C elimination mechanisms are kinetically inhibited due to extremely high activation barriers. Radical mechanisms are also kinetically unfavorable due to the electrophilic nature of the cationic Rh(III) complex. These findings distinguish the cationic RhIII−silyl complex from the electron-rich Ni(0) systems frequently exploited for the activation of cyanide C−CN bonds, where an oxidative addition mechanism should be operative. Furthermore, the rate-limiting step of cyano insertion into the Rh−silyl bond has been examined for various nitriles. The reactivity trend for these nitriles is also in good agreement with experimental observations, which show significant steric effects but small electronic effects for the RCN R group. The origin of the favorable insertion to give a Rh(III) η2-iminoacyl complex versus the formation of a Rh(III) η1-imino complex has been elucidated by using natural charge population analyses. It is attributed to the presence of two pairs of favorable stabilizing Rh···C and Si···N interactions in the transition state TS1 for cyano insertion in the insertion/deisocyanide mechanism. However, this effect is replaced by detrimental repulsive Rh···N and Si···C interactions in the isomeric transition state TS1′ with the reverse orientation of Rh−Si versus CN bonds. cleavage.2 Aromatization or allylation stabilization has also been used to bring about the C−C single bond activation in some particular pro-aromatized structures.3 Chelation assistance (or directing group assistance) is another effective way of promoting C−C single bond activation by stabilizing the intermediates from the C−C cleavage process.4 Additionally, the introduction of an activating group adjacent to the C−C bond to be cleaved is a practical way of realizing mild C−C activation reactions. Typical activating groups include carbonyls,5 imines,4d−g,i−k and cyanide.6−17 Furthermore, transitionmetal-catalyzed decarboxylative cross-coupling reactions have been an active field of C−C activation reactions in recent years.18,19 These reactions involve a critical β-C elimination step to extrude CO2. The above strategies for C−C activation, together with other alternatives,20 are often synergistically combined to achieve a particular C−C activation reaction. The activation of C−CN bonds in nitriles by transition-metal complexes has been studied intensely.6 The practical transitionmetal catalysts of choice are often Ni(0) and Rh(I) complexes with ancillary electron-rich σ-donating ligands such as

1. INTRODUCTION Among various kinds of fundamental and basic bond types, the C−C single bond is arguably the most important one for organic compounds. The development of C−C bond formation reactions has witnessed great progress over the last few decades. However, the reverse process, i.e., C−C cleavage reactions, have thus far been less developed in large part due to the inertness of the C−C single bond and unfavorable thermodynamic and kinetic factors.1 The cleavage of C−C single bonds is often highly endothermic and thus requires harsh reaction conditions or other driving forces to promote the reaction. Additionally, in organic compounds C−C single bonds are often accompanied by more reactive and abundant C−H bonds, and this renders C−C single bond activation less competitive in comparison to C−H activation reactions in many cases. In recent years, transition-metal-promoted C−C single-bond activation reactions have received great attention.1 A few strategies have been exploited to address the unfavorable thermodynamic and kinetic issues and thus promote the C−C single bond activation. A well-known strategy is to activate C− C single bonds in strained three- or four-membered-ring molecules that can release the ring strain after the C−C bond © XXXX American Chemical Society

Received: March 19, 2014

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cyanides with varied steric bulk and aryl cyanides with both electron-rich and electron-withdrawing substituents were reported to undergo facile R−CN bond cleavage to yield the RhIII−R type of products along with silyl isocyanide byproducts. Given the strongly electrophilic character of the Rh(III) silyl complex, intrinsic differences in mechanism are implied from the Ni(0)-mediated reactions. Bergman, Brookhart, et al. originally proposed an insertion/deisocyanide mechanism on the basis of NMR characterizations, X-ray diffraction analysis of a key Rh(III) η2-iminoacyl intermediate, and kinetic studies.10b This mechanism involves an initial insertion of the cyanide CN bond into the Rh−silyl bond to yield the Rh(III) η2-iminoacyl intermediate A (Scheme 2). A subsequent deisocyanide step, which is essentially an α-C elimination process, would yield the final products. The deisocyanide step is formally similar to the well-known decarbonylation process in which the iminoacyl group in intermediate A is replaced by an acyl group. However, this mechanism has not been supported by any computational studies, unlike those done for related Ni(0), Rh(I), and Fe(II) systems.11a,22,23 Given the novel mechanism proposed and the fundamental importance of C−C bond cleavage, the mechanism for Bergman and Brookhart’s reaction deserves a detailed and indepth theoretical study. Herein we present a systematic theoretical study on the evaluation of mechanisms for this particular reaction. The reactivity patterns for the electrophilic Rh(III) silyl complex with cyanides have been systematically evaluated under various possible reaction mechanisms, with the detailed structures and energetics of relevant intermediates and transition states identified. These results should therefore be of significance for the understanding of the activating modes for electrophilic Rh(III) silyl complexes toward nitriles and the development of catalytic C−C bond cleavage reactions under Rh catalysis.24

phosphines, bis-phosphines, or N-heterocyclic carbenes (NHCs). For example, Jones and Hiyama independently reported a series of studies on the Ni(0)-promoted activation of nitriles in the presence of various phosphine or bisphosphine ancillary ligands.7,8 Chatani and Jones and their coworkers have also described some Rh(I)-promoted C−CN bond activation/subsequent functionalization reactions.9 Additionally, C−CN bond cleavage by complexes of some other transition metals,12−17 such as Fe,11 Mo,12 Co,13 and Pd,14 has also been reported recently, some involving a photochemical activation strategy. A typical mechanism for the cleavage of the C−CN bond in nitriles is the direct oxidative addition mechanism (Scheme 1a).21 This mechanism for the activation of the C−CN bond is Scheme 1. Direct Oxidative Addition and Insertion/β-C Elimination Mechanisms

akin to the classical mechanism for aryl halide activation at transition-metal centers, often requiring electron-rich lowvalent transition-metal complexes (Scheme 1a). The Ni(0) systems described above by Jones and Hiyama are proposed to proceed by this mechanism. Evidence supporting the oxidative addition mechanism has also been presented by Jones, Sakaki, et al. through combined experimental and DFT studies.22 Another mechanism involving sequential cyano insertion/β-C elimination was originally proposed by Chatani et al. during their study of Rh(I)-catalyzed borylation of nitriles.9g An intermediate imino complex was crucial for the subsequent β-C elimination to cleave the C−CN bond accompanied by the release of boryl cyanide (Scheme 1b). In stark contrast, in 2002 Bergman, Brookhart, et al. reported a highly electrophilic complex, i.e., a cationic Cp*RhIII silyl complex, for the mild activation of nitriles even at room temperature (Scheme 2).10a A series of nitriles including alkyl

2. METHODS All calculations were performed with Gaussian 03.25 Density functional theory with the B3LYP functional was used, which incorporates Becke’s three-parameter hybrid exchange functional (B3) and the nonlocal correlation functional of Lee, Yang, and Parr (LYP).26 This functional has been shown to be reliable for treating similar transitionmetal-mediated C−CN bond cleavage reactions.27 Geometry optimizations were conducted without any constraint using a combined basis set in which Rh is described by the LANL2DZ basis set and the inherent effective core potentials,28 Si and P atoms are described with the 6-311+G(d,p) basis set with polarization and diffuse functions, and the Pople all-electron basis set 6-31G(d) is used for all the other atoms.29 Frequency analyses were conducted at the same level of theory for geometry optimization calculations to verify the optimized stationary points to be real minima or saddle points and to get relevant thermodynamic energy corrections. For each saddle point, an intrinsic reaction coordinate (IRC) analysis30 was carried out to confirm that it connected the correct reactant and product on the potential energy surface. Natural population analyses (NPA) were performed also at the same level of theory.31 Single-point electronic energy calculations were performed on the gas-phase stationary points by using a larger basis set: i.e., SDD32 for Rh and 6-311+G(d,p) for the other elements. Solvent effects were evaluated by estimating solvation energies on the gas-phase stationary points by using the self-consistent reaction field (SCRF) method33 with the CPCM-UAHF solvation model.34 Dichloromethane was used as the solvent during SCRF calculations, which was also the real solvent used in experiments. Single-point electronic energies plus corrections to Gibbs free energies (or enthalpies in some cases) and solvation energies were used to

Scheme 2. Cationic Rh(III) Silyl Complex Promoted RCN Activation Reaction

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Scheme 3. Model Reaction and Possible Mechanisms Evaluated in This Study

describe the reaction energetics throughout the study. All of the energies are at 298.15 K under 1 atm.

Scheme 4. Reaction Pathway of Oxidative Addition Mechanisma

3. RESULTS AND DISCUSSION 3.1. Model Reaction and Overview of Reaction Mechanisms. Bergman and Brookhart’s reaction systems have been studied in this paper. The only difference in our calculation model reaction from the real reaction is the use of a Cp ligand in place of a Cp* ligand (Scheme 3a). This simplification is believed to be reasonable, and the conclusions obtained should be essentially similar to those for the Cp* ligand.35 The possible mechanisms demonstrated in Scheme 3b have been evaluated, including direct oxidative addition, cyano insertion/deisocyanide, cyano insertion/β-C elimination, and radical mechanisms. The detailed results are described in the following sections. 3.2. Direct Oxidative Addition Mechanism. This mechanism involves the direct cleavage of the Ph−CN bond at the Rh(III) center via the typical three-membered-ring transition state TSoxd (Scheme 4), which has also been frequently invoked for aryl halide activations at transition-metal centers (please refer to the Supporting Information for detailed structures of TSoxd). A Rh(V) intermediate, featuring phenyl, cyanide, and silyl ligands, should be generated in principle. However, calculation results show that the free energy level of

a

All the values in parentheses are Gibbs free energies in kcal/mol.

TSoxd is extremely high, 69.2 kcal/mol, rendering this mechanism kinetically inhibited. The reason for this high activation barrier should probably be attributed to the unfavorable dissociation of the cyano group from the Rh center and the breaking of the π conjugation between phenyl and cyanide in TSoxd as well as the electrophilic character of the Rh(III) complex. This proposition is consistent with the accepted opinion that the oxidative addition mechanism is generally applicable to C−C activation only for electron-rich low-valent metal complexes, especially the Ni(0) bis-phosphine system. Additionally, the oxidative addition mechanism for nitrile activation by an Fe silyl complex has also been shown to be kinetically unfavorable by Nakazawa, Koga, et al.11a,23c C

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Scheme 5. Reaction Pathway for Insertion/α-C Elimination Mechanisma

a

All of the values in parentheses are Gibbs free energies in kcal/mol.

Figure 1. Optimized structures for intermediates and transition states involved in the cyano insertion/deisocyanide mechanism. Hydrogen atoms are omitted for clarity. All bond lengths are in Å and bond angles in deg.

3.3. Insertion/α-C Elimination (Deisocyanide) Mechanism. This mechanism involves an initial step of cyano insertion into the Rh−silyl bond in CP0 via the fourmembered-ring transition state TS1, resulting in the formation of the rhodium η2-iminoacyl complex CP1 (Scheme 5 and Figure 1). Isomerization of CP1 to the η1-iminoacyl complex CP2 takes place, in which coordination of the phenyl ring to the Rh center occurs. Subsequent α-C elimination (deisocyanide) via the three-membered-ring transition state TS2 leads to the extrusion of isocyanide and the formation of a Rh−Ph bond (Scheme 5 and Figure 1). The deisocyanide process is similar to well-known decarbonylation reactions. Calculation results show that the cyano insertion step has an activation free energy of 25.8 kcal/mol (the activation enthalpy is 23.5 kcal/mol), while it is slightly exothermic by 4.8 kcal/

mol. It is worth noting that, in the cyano insertion step, the C− N bond elongates significantly from 1.16 Å in CP0 to 1.22 Å in TS1 and 1.25 Å in CP1. Another significant change during the cyano insertion process is the significant bending of the Ph− C−N bond of nitrile, from nearly linear in CP0 to 141.8° in TS1 and 139.1° in CP1. These structural changes reflect the inherent change in the rehybridization of the carbon atom from the sp to sp2 state and the concurrent conversion of a triple bond to a double bond for the C−N linkage. In the optimized structure of CP1, the iminoacyl group is η2 bound to the Rh center with additional coordination of the nitrogen atom. The structural features of the optimized structure CP1 are in good agreement with the X-ray crystal structure reported by Bergman, Brookhart, et al.10b D

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Scheme 6. Reaction Pathway for the Insertion/β-C Elimination Mechanisma

a

All of the values in parentheses are Gibbs free energies in kcal/mol.

Figure 2. Relevant intermediates and transition states involved in the cyano insertion/β-C elimination mechanism. Hydrogen atoms are omitted for clarity. All bond lengths are in Å and bond angles in deg.

The deisocyanide step is very facile, with a low-lying transition state TS2 at 11.8 kcal/mol in free energy. Prior to the deisocyanide step, isomerization of CP1 to the η1-iminoacyl complex CP2 is required in which coordination of the phenyl ring to the Rh center occurs. This structural feature renders the complex ready for the subsequent Ph−C bond cleavage. In deisocyanide transition state TS2, the C−Cipso bond is significantly elongated to 1.79 Å, which is 0.3 A longer than that in precomplex CP2. Meanwhile, the Rh−C and Rh−Cipso bonds are substantially shortened to 1.92 and 2.22 Å, respectively, in TS2, in contrast to the corresponding values of 2.05 and 2.37 Å in CP2. Another noteworthy structural change is the decrease of C−Rh−Cipso bond angle from 50.7° in TS2 to 92.0o in Prod, which is accompanied by further shortening of the Rh−Cipso bond length. Obviously, this cyano insertion/deisocyanide mechanism is kinetically much more favorable in comparison to direct oxidative addition of the C−CN bond to Rh center. The

magnitude of the activation free energy of 25.8 kcal/mol (the activation enthalpy of 23.5 kcal/mol) is reasonable, given the rate constant of [3.1(2)] × 10−4 s−1 determined for phenyl cyanide (which corresponds to an activation barrier of ca. 23.3 kcal/mol).10b It is worth noting that the low-lying intermediate CP1 and the subsequent facile deisocyanide step results in irreversibility of the cyano insertion step, which is consistent with X-ray diffraction characterization of the crystal structure of the stable intermediate CP1 and consumption presented by Bergman, Brookhart, et al. Generally, this mechanism is consistent with the mechanistic proposal by Nakazawa, Koga, et al. during their study on acetonitrile activation by an Fe silyl complex where the cyano insertion/deisocyanide step is also shown to be the most favorable.11a,23c In summary, under the cyano insertion/deisocyanide mechanism, the strong C−CN bond is cleaved via two sequential irreversible steps: i.e., cyano insertion and deisocyanide. The cyano insertion step leads to the partial E

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greatly altered for the activation of the C−CN bond in the context of these two mechanisms. Finally, the possibility of initial isomerization of nitrile to isonitrile followed by the activation of isonitrile has also been considered. However, the isomerization of nitrile in the presence of the Rh(III) complex feature a high-lying Y-shaped transition state with a free energy of 61.6 kcal/mol (please refer to Supporting Information for more details). Therefore, the isomerization seems unlikely under the reaction conditions of Bergman and Brookhart’s reaction. Subsequent transformations were therefore not studied further. 3.7. Origins of Chemoselectivity for Cyano Insertion Step. Natural charge population analyses indicate that, in complex CP0, there are natural charges of −0.489, +1.672, +0.469, and −0.290 for Rh, Si, and cyano C and N atoms, respectively. It is interesting to note that, although the cationic complex CP0 is electrophilic as a whole, the natural charge at the Rh center is negative, which is possibly attributed to the strong σ-donating ability of the phosphine and Cp ligands. Additionally, the natural charges vary from −0.400, +1.767, +0.345, and −0.433 in TS1 to −0.172, +1.591, +0.148, and −0.330 in TS1′ for Rh, Si, and cyano C and N atoms, respectively. Therefore, interaction of the Rh−Si bond with the cyano bond in the manner shown in TS1 should be beneficial due to two pairs of favorable δ+/δ− stabilization interactions (Scheme 8), leading to the formation of Rh η2-iminoacyl

activation of the cyano group to the imino group, which is beneficial to the subsequent facile C−CN bond cleavage. The feasibility of the sequential partial activations lies in the low-lying stable intermediate of the rhodium η2-iminoacyl intermediate, which features σ bonding of the carbon atom of the cyano group and additional coordination of the nitrogen atom to the Rh(III) center. 3.4. Insertion/β-C Elimination Mechanism. This mechanism was originally proposed by Chatani et al. during their study of rhodium-catalyzed borylation of nitriles. It involves initial cyano insertion into the Rh−silyl bond with a reverse orientation of the Rh−silyl against the CN bond in comparison to that in the insertion/deisocyanide mechanism (please refer to TS1′ in Scheme 6 and TS1 in Scheme 5). Intermediate CP3 obtained from the cyano step is a rhodium imino complex. Isomerization to CP4 with the rhodium and the phenyl cis to each other is required for the subsequent β-C elimination step, which finally cleaves the C−CN bond to give the Rh−phenyl intermediate (Scheme 6 and Figure 2). Calculation results show that both the cyano insertion step and the β-C elimination step are kinetically inhibited, due to high activation barriers of 56.4 and 46.6 kcal/mol, respectively (Scheme 6). Therefore, this mechanism seems unlikely for the cationic Rh(III) silyl complex promoted nitrile activation. 3.5. Radical Mechanisms. Radical mechanisms involving the presence of radical species during the reactions have also been proposed for C−C single bond activation reactions.36 A typical radical mechanism, i.e., a single electron transfer (SET) mechanism which is initiated by intramolecular SET from Rh(III) to nitrile, has been evaluated (CP5; Scheme 3b).37 Calculation results show that the Gibbs free energy increases by 30.5 kcal/mol relative to CP0 for the SET process, thereby excluding such a SET mechanism given the mild reaction conditions (room temperature). Additionally, another mechanism involving the transfer of cyanide from nitrile to the Rh(III) center has also been considered, resulting in the formation of a Rh(IV) radical cation (CP6; Scheme 3b) and a phenyl radical.38 This mechanism is also excluded due to the increase of 62.6 kcal/mol in free energy relative to CP0 for this cyanide transfer process. 3.6. Other Mechanistic Alternatives. σ-bond metathesis of the C−CN bond with the Rh−Si bond (Scheme 7a) and a

Scheme 8. Chemical Selectivity Origins for the Cyano Insertion Step

complex CP1. The interaction with the reverse orientation shown in TS1′ should suffer from great electrostatic repulsion effects and therefore render TS1′ extremely high in free energy. It is worth noting that the silyl group is essentially electrophilic and ready to attack the electron-rich nitrogen atom of nitriles. The approach of the silyl group to the nitrogen of nitriles renders the carbon atom of the cyano group more electrophilic toward nucleophilic attack of the rhodium center. A similar observation of the electrophilic nature of the silyl group has been reported by Nakazawa, Koga, et al. during their study of the insertion of cyanide into Fe−silyl complexes.11a,23c Consequently, the selective formation of the Rh η2-iminoacyl intermediate is preferred over that of the Rh η1-imino complex, finally making the insertion/deisocyanide mechanism kinetically favored. As can be seen, the silyl group intrinsically acts as an activator for the partial activation of the cyano group. 3.8. Rate-Limiting Insertion Step for Various Nitriles. Various nitriles are examined for the rate-limiting step of insertion of the cyano group into the Rh−silyl bond. As can be seen, for either electron-rich or electron-withdrawing substituents on the phenyl ring, the activation enthalpies and the reaction exothermicity vary very slightly (refer to rows 2−4, Table 1). This indicates that the electronic effect of the substituents on the phenyl ring is small. This conclusion is consistent with kinetic observations by Bergman, Brookhart, et al. for this cyano insertion for PhCN, p-CF3-PhCN, and p-

Scheme 7. Other Mechanistic Alternativesa

a

Note: M denotes the CpRh(PMe3) fragment.

cyclic concerted mechanism with a five-membered-ring transition state (Scheme 7b) have also been studied. Unfortunately, attempts to locate the expected transition states were unsuccessful. Optimizations always go back to the left side of reactants, which should probably be due to the preferred linear geometry for the Ar−CN part, which is required to be F

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Table 1. Activation Barriers and Thermodynamics for the Rate-Limiting Insertion of a Cyano Group into the Rh−Silyl Bonda

a

RCN

ΔH⧧CP0→TS1

ΔHCP0→CP1

ΔG⧧CP0→TS1

ΔGCP0→CP1

PhCN p-CF3-PhCN p-OMe-PhCN MeCN iPrCN tBuCN

23.5 23.6 23.2 23.0 24.3 26.2

−3.3 −2.9 −3.6 −4.5 −0.4 +0.7

25.8 23.3 26.2 24.9 27.5 29.2

−4.8 −5.4 −4.7 −5.3 −0.2 +0.3

All of the values in the table are in kcal/mol.

Figure 3. Optimized transition state structures for MeCN, iPrCN, and tBuCN. Phosphorus, silicon, nitrogen, and rhodium atoms are depicted in red, purple, blue, and black, respectively. Values demonstrated are H···H contact distances in Å.

4. CONCLUSIONS In this study, we have performed a detailed theoretical study on the mechanism of Bergman and Brookhart’s reaction in which the nitrile C−CN bond is cleaved by a cationic Rh(III) silyl complex. The following conclusions and implications are drawn. (1) Through the systematic evaluation of various possible mechanisms for the C−CN bond activation in phenyl cyanide by the cationic Rh(III) silyl complex, the sequential cyano insertion/deisocyanide mechanism is supported by our results. This mechanism involves an initial and rate-limiting cyano insertion into the RhIII−silyl bond to give a stable Rh(III) η2iminoacyl intermediate, followed by rapid extrusion of isocyanide from the η2-iminoacyl intermediate to reach the desired product. The structural features of the Rh(III) η2iminoacyl intermediate are consistent with the experimentally characterized X-ray crystal structure. The magnitude of the activation barrier for the rate-limiting step of cyano insertion into the RhIII−silyl bond is in excellent agreement with the rate constant determined from kinetic studies and the reaction conditions. (2) The oxidative addition mechanism, which is frequently invoked for cyanide C−CN bond activation for electron-rich Ni(0) complex mediated reactions, has an extremely high activation barrier which is kinetically inhibited. This should be caused by the electrophilic character of the cationic Rh silyl complex and unfavorable energy-demanding geometry changes in the transition state for the oxidative addition mechanism. Another possible mechanism involving cyano insertion into the

OMe-PhCN. This can be rationalized by the fact that the phenyl group in these aryl cyanides is far from the reactive centers involved in the step of cyano insertion into the Rh−silyl bond. In stark contrast, the steric effect of the R group of RCN is very significant, as shown by the activation enthalpies of 23.0, 24.3, and 26.2 kcal/mol for MeCN, iPrCN, and tBuCN, respectively (refer to rows 4−6 in Table 1). This steric effect should arise from the repulsion effect of the R group with the phosphine, triphenylsilyl, and Cp ligands on the Rh(III) center, as reflected by the H···H distances among these ligands in the optimized transition state structures (Figure 3). There is only some weak repulsion effect between methyl and phosphine ligands in TS1MeCN. This steric effect increases substantially in TSiPrCN, as can be deduced from the closer contact of hydrogen atoms on these two ligands and the presence of more pairs of interactions. In TS1tBuCN, not only does the sterically demanding tert-butyl group feel the largest repulsion effect with the phosphine ligand but also there are also significant repulsion effects between the tert-butyl group and Cp and triphenylsilyl ligands (refer to TS1tBuCN, Figure 3). These results are also consistent with Bergman’s experimental conditions that tBuCN needs to be activated at an elevated temperature of 50 °C versus 25 °C for the other cyanides. In summary, the substrate effects are completely consistent with Bergman and Brookhart’s experimental observations and kinetic studies and thus provide strong support for the validity of the cyano insertion/deisocyanide mechanism for nitrile activation by the cationic RhIII−silyl complex. G

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Rh−silyl bond followed by β-C elimination has also been shown to be kinetically inhibited due to high activation free energies. Additionally, radical mechanisms and other mechanistic alternatives have also been evaluated and are less competitive in comparison to the cyano insertion/deisocyanide mechanism. (3) The origins of the favorable cyano insertion into the Rh− silyl bond to give the Rh(III) η2-iminoacyl intermediate vs the formation of the Rh(III) η1-imino intermediate have been elucidated by natural charge population analyses. The negatively charged Rh center and positively charged silyl center preferentially interact with electrophilic carbon and anionic nitrogen atoms, respectively, which leads to the selective formation of the Rh(III) η2-iminoacyl intermediate. In contrast, the interactions with reverse orientation of the Rh−silyl bond with CN should consequently suffer great repulsive effects and cause the cyano insertion to give the Rh(III) η1-imino intermediate to be kinetically inhibited. (4) Various nitriles have been examined for the rate-limiting cyano insertion step to get a knowledge of the substrate effects. The results clearly show a small electronic effect of the substituents on the phenyl ring of aryl cyanides but a significant steric effect for the R group of RCN in the cyano insertion step. These results are consistent with experimental observations and lend further support to the validity of the cyano insertion/ deisocyanide mechanism for the activation of nitriles by the cationic Rh(III) silyl complex. The reasons for the electronic and steric effects are also elucidated. Our results in this study present a detailed reaction pathway and energetics for Bergman and Brookhart’s reactions involving a novel Rh(III) silyl complex for the activation of nitriles and give new insight into the mechanism of C−CN bond activation and should therefore be valuable for the understanding and development of catalytic C−CN bond activation reactions.

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AUTHOR INFORMATION

Jones, W. D. J. Mol. Catal. A 2002, 189, 157. (d) Jun, C.-H. Chem. Soc. Rev. 2004, 33, 610. (e) Park, Y. J.; Park, J.-W.; Jun, C.-H. Acc. Chem. Res. 2008, 41, 222. (f) Najera, C.; Sansano, J. M. Angew. Chem., Int. Ed. 2009, 48, 2452. (g) Murakami, M.; Matsuda, T. Chem. Commun. 2011, 47, 1100. (h) Ruhland, K. Eur. J. Org. Chem. 2012, 2683. (2) For release of ring strain, see: (a) Murakami, M.; Amii, H.; Ito, Y. Nature 1994, 370, 540. (b) Murakami, M.; Amii, H.; Shigeto, K.; Ito, Y. J. Am. Chem. Soc. 1996, 118, 8285. (c) Murakami, M.; Takahashi, K.; Amii, H.; Ito, Y. J. Am. Chem. Soc. 1997, 119, 9307. (d) Murakami, M.; Itahashi, T.; Amii, H.; Takahashi, K.; Ito, Y. J. Am. Chem. Soc. 1998, 120, 9949. (e) Murakami, M.; Tsuruta, T.; Ito, Y. Angew. Chem., Int. Ed. 2000, 39, 2484. (f) Periana, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1984, 106, 7272. (g) Bishop, K. C. Chem. Rev. 1976, 76, 461. (h) Seiser, T.; Cramer, N. Org. Biomol. Chem. 2009, 7, 2835. (3) For aromatization and allylation, see: (a) Halcrow, M. A.; Urbanos, F.; Chaudret, B. Organometallics 1993, 12, 955. (b) King, R. B.; Efraty, A. J. Am. Chem. Soc. 1972, 94, 3773. (c) Crabtree, R. H.; Dion, R. P.; Gibboni, D. J.; McGrath, D. V.; Holt, E. M. J. Am. Chem. Soc. 1986, 108, 7222. (4) For chelation assistance strategy, see: (a) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759. (b) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870. (c) Gandelman, M.; Vigalok, A.; Konstantinovski, L.; Milstein, D. J. Am. Chem. Soc. 2000, 122, 9848. (d) Jun, C. H.; et al. J. Mol. Catal. A 2002, 189, 145. (e) Suggs, J. W.; Jun, C.-H. J. Am. Chem. Soc. 1984, 106, 3054. (f) Suggs, J. W.; Jun, C.H. J. Am. Chem. Soc. 1986, 108, 4679. (g) Suggs, J. W.; Jun, C.-H. J. Chem. Soc., Chem. Commun. 1985, 92. (h) Chatani, N.; Ie, Y.; Kakiuchi, F.; Murai, S. J. Am. Chem. Soc. 1999, 121, 8645. (i) Lei, Z.-Q.; Li, H.; Li, Y.; Zhang, X.-S.; Chen, K.; Wang, X.; Sun, J.; Shi, Z.-J. Angew. Chem., Int. Ed. 2012, 51, 2690. (j) Chen, K.; Li, H.; Li, Y.; Zhang, X.S.; Lei, Z.-Q.; Shi, Z.-J. Chem. Sci. 2012, 3, 1645. (k) Li, H.; Li, Y.; Zhang, X.-S.; Chen, K.; Wang, X.; Shi, Z.-J. J. Am. Chem. Soc. 2011, 133, 15244. (5) For adjacent carbonyls, see: (a) References 2a−e.. (b) Daugulis, O.; Brookhart, M. Organometallics 2004, 23, 527. (c) Jun, C.-H.; Lee, H. J. Am. Chem. Soc. 1999, 121, 880. (d) Jun, C.-H.; Lee, H.; Lim, S.-G. J. Am. Chem. Soc. 2001, 123, 751. (e) Dreis, A. M.; Douglas, C. J. J. Am. Chem. Soc. 2009, 131, 412. (f) Rathbun, C. M.; Johnson, J. B. J. Am. Chem. Soc. 2011, 133, 2031. (g) Wentzel, M. T.; Reddy, V. J.; Hyster, T. K.; Douglas, C. J. Angew. Chem., Int. Ed. 2009, 48, 6121. (h) Huffman, M. A.; Liebeskind, L. S. J. Am. Chem. Soc. 1991, 113, 2771. (6) For a review on C−CN bond cleavage, see: Tobisu, M.; Chatani, N. Chem. Soc. Rev. 2008, 37, 300. (7) (a) Atesin, T. A.; Li, T.; Lachaize, S.; Garcia, J. J.; Jones, W. D. Organometallics 2008, 27, 3811. (b) Swartz, B. D.; Reinartz, N. M.; Garcia, J. J.; Jones, W. D. J. Am. Chem. Soc. 2008, 130, 8548. (c) Acosta-Ramirez, A.; Munoz-Hernandez, M.; Jones, W. D.; Garcia, J. J. Organometallics 2007, 26, 5766. (d) Brunkan, N. M.; Brestensky, D. M.; Jones, W. D. J. Am. Chem. Soc. 2004, 126, 3627. (e) Garcia, J. J.; Arevalo, A.; Brunkan, N. M.; Jones, W. D. Organometallics 2004, 23, 3997. (f) Garcia, J. J.; Brunkan, N. M.; Jones, W. D. J. Am. Chem. Soc. 2002, 124, 9547. (g) Garcia, J. J.; Jones, W. D. Organometallics 2000, 19, 5544. (8) (a) Nakao, Y.; Yada, A.; Hiyama, T. J. Am. Chem. Soc. 2010, 132, 10024. (b) Hirata, Y.; Yukawa, T.; Kashihara, N.; Nakao, Y.; Hiyama, T. J. Am. Chem. Soc. 2009, 131, 10964. (c) Yada, A.; Yukawa, T.; Nakao, Y.; Hiyama, T. Chem. Commun. 2009, 3931. (d) Nakao, Y.; Ebata, S.; Yada, A.; Hiyama, T.; Ikawa, M.; Ogoshi, S. J. Am. Chem. Soc. 2008, 130, 12874. (e) Nakao, Y.; Hirata, Y.; Tanaka, M.; Hiyama, T. Angew. Chem., Int. Ed. 2008, 47, 385. (f) Nakao, Y.; Yukawa, T.; Hirata, Y.; Oda, S.; Satoh, J.; Hiyama, T. J. Am. Chem. Soc. 2006, 128, 7116. (g) Nakao, Y.; Oda, S.; Hiyama, T. J. Am. Chem. Soc. 2004, 126, 13904. (9) (a) Tobisu, M.; Kita, Y.; Chatani, N. J. Am. Chem. Soc. 2006, 128, 8152. (b) Tobisu, M.; Kita, Y.; Ano, Y.; Chatani, N. J. Am. Chem. Soc. 2008, 130, 15982. (c) Tobisu, M.; Nakamura, R.; Kita, Y.; Chatani, N. J. Am. Chem. Soc. 2009, 131, 3174. (d) Tobisu, M.; Nakamura, R.; Kita, Y.; Chatani, N. Bull. Korean Chem. Soc. 2010, 31, 582. (e) Kita, Y.; Tobisu, M.; Chatani, N. Org. Lett. 2010, 12, 1864. (f) Kita, Y.; Tobisu,

S Supporting Information *

Figures, tables, and xyz files giving additional computational results, electronic energies, thermal corrections, solvation energies, a full citation of Gaussian03 program, and all computed molecule Cartesian coordinates in a format for convenient visualization. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail for S.-L.Z.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21202062) and the Natural Science Foundation of Jiangsu Province (No. BK2012108). Financial support from MOE & SAFEA for the 111 Project (B13025), is gratefully acknowledged.

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REFERENCES

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