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Transfer Hydrocyanation by Nickel(0)/Lewis Acid Cooperative Catalysis, Mechanism Investigation, and Computational Prediction of Shuttle Catalysts Shao-Fei Ni,† Ti-Long Yang,† and Li Dang*,†,‡ †

Department of Chemistry, Southern University of Science and Technology, Shenzhen, 518055, People’s Republic of China Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Guangdong 515063, People’s Republic of China

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S Supporting Information *

ABSTRACT: A theoretical investigation on transfer hydrocyanation of simple olefins catalyzed by shuttle catalysts is presented in this work, which uncovers the reaction mechanism together with the important role of the Lewis acid. The calculated results show that Ni(0)/LA (Lewis acid)-catalyzed transfer hydrocyanation consists of five key steps: oxidative addition of the nitrile, β-H elimination, ligand exchange, alkene insertion, and reductive elimination. The computational results reveal that the effect of the Lewis acid is mainly reflected in the interaction with the N atom of the nitrile group, which weakens the C(sp3)−C(sp) σ bond, thus lowering the barrier of the oxidative addition step, which is the rate-determining step of the catalyzed reaction. These results are consistent with the experimental observations. Furthermore, our calculation results with several newly designed ligands show that the introduction of an electron-donating group to the phosphine ligand promotes transfer hydrocyanation, while an electron-withdrawing group blocks this reaction. The present work will provide great support for the understanding of transfer hydrocyanation and give a valuable guide for the further design of transition-metal/Lewis acid cooperative shuttle catalysts.

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chemists from working with it in academic laboratories.7 An alternative strategy for this direct dangerous hydrocyanation reaction is that inspired by a group transfer process using shuttle catalysis,8,9 such as H2-free transfer hydrogenation,10−12 transfer hydroformylation,13 transfer hydroacylation,14−17 transfer hydromagnesiation,18 and transfer silacyclopropanation.19,20 During the process catalyzed by shuttle catalysis, the transfer of small molecules between two substrates is an isodesmic process.8,9 In 2016, Fang et al. reported their unique strategy of transfer hydrocyanation by using RCN instead of the dangerous HCN.21 As shown in eq 1, the nickel catalyst

INTRODUCTION The nitrile group is very important in the synthesis of natural products, medical molecules, dyes, cosmetics, and materials.1,2 In addition, compounds including a −CN group can be used as potential precursors for the generation of many other chemicals, such as amine, amide, amidine, aldehyde, carboxylic acid, triazole, etc., which has resulted in the ceaseless investigation of organonitriles.3 The most common and useful strategy to obtain these organonitriles is hydrocyanation of alkenes or alkynes catalyzed by transition-metal complexes.4,5 The first example of this strategy dates back to 1954, when Arthur et al. first reported the dicobalt octacarbonyl (Co2(CO)8) catalyzed addition of HCN to nonfunctionalized alkenes to produce the corresponding organonitriles. In addition, the most successful and outstanding example for the application of hydrocyanation is the DuPont adiponitrile process, which has been commercialized for the production of the precursor of nylon-6,6 and polyurethanes from 1,3butadiene.6 Although great success has been achieved, it is still an important challenge to achieve the goals of finding a lowcost catalyst with high efficiency and selectivity. Moreover, one of the serious drawbacks that blocks this process is the inherent danger of hydrogen cyanide, which is usually used as the reactant. The extreme toxicity, low boiling point (27 °C), and corrosive and explosive nature of HCN has discouraged © XXXX American Chemical Society

Ni(COD)2 is used as the precatalyst and the DPEphos ligand was added. Excess Lewis acid, AlMe2Cl, is necessary to finish this reaction. However, the detailed mechanism for this nickelcatalyzed transfer hydrocyanation reaction has not been thoroughly studied. Although the role of the Lewis acid in the hydrocyanation process has been reported as accelerating the cleavage of the C−C bond22 or helping to control the regioselectivity,23 the role of AlMe2Cl in this transfer Received: March 23, 2017

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DOI: 10.1021/acs.organomet.7b00218 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics hydrocyanation process catalyzed by shuttle catalysis needs further theoretical investigation. In this work, from a mechanistic point of view, a deeper insight into the reaction mechanism and the structure−performance relationships, as well as the precise role of the Lewis acid, has been achieved by using DFT calculations. Furthermore, better ligands for the nickel(0)/Lewis acid cooperative catalysis are also predicted on the basis of this theoretical study.

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Scheme 1. Proposed Reaction Mechanism

COMPUTATIONAL DETAILS

In this work, the ω-B97XD24 functional is used for all of the geometry optimization calculations and all calculations were carried out by using the Gaussian 09 program.25 The effective core potentials (ECPs) of Hay and Wadt with a double-ζ valence basis set (LanL2DZ) were exployed for Ni, P, and Cl26−29 and polarization functions were also added for Ni ( f = 3.130), P (f = 0.387), and Cl ( f = 0.640);30,31 the allelectron 6-31G* basis set was used in describing all other main-group atoms.32−34The choice of the basis set was tested, and the details are shown in Figure S3 in the Supporting Information. Geometric structures of all species in this work were optimized in the gas phase at T = 298.15 K and 1 atm pressure. The harmonic vibrational frequencies and the number of imaginary frequencies determine the nature of all intermediates (no imaginary frequency) and transition state structures (only one imaginary frequency). The latter were also confirmed to connect appropriate intermediates, reactants, or products by intrinsic reaction coordinate (IRC) calculations.35,36 The gas-phase Gibbs free energies, G, were calculated within the harmonic potential approximation at optimized structures. On the basis of the gas-phase optimized geometries, the solvation effect of tetrahydrofuran was incorporated with the SMD solvent model at the level of M11-L/6311+G(d,p) theory.37 The solution-phase Gibbs free energy is calculated by adding the correction of the Gibbs free energy in the gas phase to the electronic energy in the solvent. The same methodology has been widely used in many recent theoretical works.38 The chosen hybrid functional ω-B97XD is reasonable on the basis of comparing different results calculated by functionals with different empirical dispersion models, such as B3LYP-D339 and M062X.40 The results will be discussed in the Results and Discussion. The charge decomposition analysis was carried out by the Multiwfn program.41,42 The 3D molecular structures of all species shown in the Supporting Information were drawn by using the CYL view program.43

structural parameters for all the transition states and intermediates involved in the potential energy surfaces are shown in the Supporting Information. Mechanism of Transfer Hydrocyanation without Lewis Acid. As shown in Figure 1, biolefin ligand CODs are replaced by the DPEphos ligand and substrate R1 quickly to afford the Ni(0) intermediate 1, in which the CN triple bond in R1 coordinates with the Ni center. From the intermediate 1, the oxidative addition of R1 to the Ni(0) center occurs by the cleavage of the unstrained σ-C(sp3)−C(sp) bond to give the intermediate 2 via a barrier of 37.9 kcal mol−1 in free energy. This high reaction barrier corresponds to the high thermodynamic stability of the C(sp3)−C(sp) bond (the bond dissociation energy of C−CN is reported to be over 100.0 kcal mol−1).44 Intermediate 2 has a planar four-coordinate structure, which is more unstable than the reactant by 16.3 kcal mol−1 in energy. From this unstable Ni(II) complex 2, β-H elimination from the C atom to the Ni(II) center occurs via TS2-3 with a barrier of 30.0 kcal mol−1, followed by the release of gas-phase 2-methylpropene (P1) and the formation of the planar intermediate 4. As shown in Figure 2, the coordination of α-methylstyrene (R2) to intermediate 4 leads to two different products, the linear anti-Markovnikov product P2-L and the branched Markovnikov product P2-B. By the coordination of the C C double bond of R2 to the Ni(II) center in different directions, intermediates 5 and 9 are formed. From intermediate 5, the insertion of the CC double bond of the alkene into the Ni−H bond leads to the intermediate 6. The barrier for this migratory insertion is 29.3 kcal mol−1, which is lower than that of the C−C bond cleavage step. Subsequently, a rapid structure rearrangement and isomer-

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RESULTS AND DISCUSSION In this theoretical work, isovaleronitrile (R1) and αmethylstyrene (R2) (Scheme 1) are used as reactants, since they yield the best results in the experiment. Transfer hydrocyanation from R1 to R2 catalyzed by the Ni(0)/LA catalysis system may produce either linear anti-Markovnikov product P2-L or branched Markovnikov product P2-B, leaving 2-methylpropene (P1) as the byproduct. Our calculations are based on the proposed mechanism of this transfer hydrocyanation reaction as shown in Scheme 1. The mechanism begins with the abstraction of CN and H from oxidative addition of organonitrile (isovaleronitrile R1) to the Ni(0) center and β-hydride elimination to form a molecular shuttle Ni(II) HCN. Then alkene (α-methylstyrene (R2)) inserts into the Ni−H bond and reductive elimination occurs to give a new nitrile product. Unless otherwise noted, the calculated solvation-corrected relative Gibbs free energies ΔGsol (kcal/mol) are presented in the following figures that contain the potential energy profiles and are discussed in this paper. The relative Gibbs free energies ΔGgas (kcal/mol), relative electronic energies ΔEgas in the gas phase (kcal/mol), and relative electronic energies ΔEsol in solution phase (kcal/mol) are given in the Supporting Information. In addition, the optimized structures with selected B


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Organometallics

Figure 1. Free energy profile for C−C cleavage and β-H elimination of the mechanism of transfer hydrocyanation without the participation of a Lewis acid.

Figure 2. Free energy profiles leading to the linear anti-Markovnikov product and the branched Markovnikov product from intermediate 4.

introduced by Kozuch and Shaik45 will be used in the following discussion of the reaction mechanism. At the start of this process, the original reactants Ni(COD)2 and substrate R1 can be considered as the TOF-determining intermediate (TDI) and the transition state TS1-2 for the C(sp3)−C(sp) bond cleavage is the TOF-determining transition state (TDTS). The energetic span (δE) of the TDI and TDTS is calculated to be 37.9 kcal mol−1. This high energy span makes the transfer hydrocyanation reaction unfavorable at room temperature and atmospheric pressure. Consistently, in experiments, this transformation is difficult and no products are observed even at a temperature of 100 °C for 16 h.21

ization of intermediate 6 by the barrierless rotation of the Ni− C bond lead to the intermediate 7, which undergoes the reductive elimination process to form the intermediate 8 by overcoming a barrier of 34.8 kcal/mol. Then the substrate exchange of intermediate 8 with R1 conducts this reaction into the second catalytic cycle, accompanied by the release of the linear anti-Markovnikov product P2-L. Similar to this antiMarkovnikov route, the Markovnikov route also undergoes the migratory insertion and reductive elimination process to form the branched Markovnikov product P2-B. Figures 1 and 2 give us a whole picture of the reaction mechanism of this transfer hydrocyanation reaction. Here the energetic span concept C


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Organometallics

on the diphosphine ligand, and (c) the cyano nitrogen atom and the oxygen atom of the diphosphine ligand interact with the Lewis acid at the same time. In a previous investigation, it was also reported that the Lewis acid could associate with the P atom of the ligand.23 However, we failed to find this structure in our current investigation. This agrees with a previous report that the Lewis acid prefers to interact with the “harder” O atom rather than the “softer” P atom. In the following section, the interaction of a Lewis acid with intermediate 1 in these three ways will be discussed one by one. Figure 4 shows the Gibbs free energy profiles for the first interaction mode of a Lewis acid (Figure 3). With this interaction of the Lewis acid with an N atom of the CN group, intermediate 1 is stabilized by 19.6 kcal mol−1. From the intermediate LAN-1, the barrier of the oxidative addition of C− CN bond on a Ni center is reduced to 27.9 kcal/mol, which is lower than the barrier for a pathway without Lewis acid. After the β-H elimination and the release of one molecular gas-phase product P1 (2-methylpropene), the planar four-coordinated intermediate LAN-4 is formed. This step needs to overcome an activation barrier of 27.3 kcal mol−1, which is slightly lower than that of the C−CN cleavage. Another alkene, α-methylstyrene ,will coordinate to the Ni(II) center to form the intermediate LAN-5, which undergoes the migratory insertion and reductive elimination process to form the final main product, linear antiMarkovnikov species P2-L. As shown in Figure S2 in the Supporting Information, the formation of the branched product P2-B as well as P2-L would be possible at the beginning of the reaction. However, the reverse reaction would take this branched product back to the intermediate LAN-4, finally leading to the linear product P2-L, because P2-L is thermodynamically more stable than P2-B. This theoretical investigation is in line with the experimental observation that the P2-L is the main product.21According the energetic span concept, the TDI and TDTS are LAN-1 and LAN-TS1-2. The energy span (δE) is calculated to be 27.9 kcal mol−1. To further justify the validity of the use of a ω-B97XD functional, two other functionals with empirical dispersion correction, B3LYPD3 and M06-2X, are also employed in our calculations. The results calculated by these different functionals are shown in Figure S3 in the Supporting Information, together with the results from different basis sets (Supporting Information), which agree well with each other, confirming the reliability of the employment of the ω-B97XD functional and basis set. In Figure 5, the interaction of one molecule of Lewis acid with the O atom of the ligand is considered. Unfortunately, the interaction of Lewis acid with the O atom of the phosphine ligand makes the CN coordinating intermediate (LAO-1) very unstable, which is 10.7 kcal mol−1 higher in energy than intermediate 1. Furthermore, oxidative addition of C−CN to the Ni center is also very difficult due to very high reaction barrier. Moreover, the β-H elimination, olefin insertion, and reductive elimination to release anti-Markovnikov product P2L require an even higher energy cost to achieve (49.7, 48.7, and 47.8 kcal/mol, respectively, in Figure 5). Obviously, the pathway in Figure 5 cannot compete kinetically with that in Figure 4. The phenomenon that coordinations of the Lewis acid to different positions (N atom and O atom) have opposite effects on the C−CN bond cleavage can be well explained by a fragment molecular orbital analysis and a charge decomposition analysis41,42 for transition states TS1-2, LAN-TS1-2, and LAOTS1-2. This analysis has also been used in the charge transfer of

Other pathways to produce P2-L and P2-B have also been studied, and the results are shown in Figure S1 in the Supporting Information. In these pathways, R2 coordinates to the Ni center of the intermediate 2 rather than intermediate 4. Then H transfer occurs from the β carbon of the alkyl ligand to either the terminal carbon or the middle carbon of R2, leading to the formation of intermediates 14 and 16, respectively. From intermediates 14 and 16, reductive elimination occurs to release the linear anti-Markovnikov product P2-L and the branched Markovnikov product P2-B. However, the direct H transfers have barriers of 62.6 and 73.1 kcal mol−1, making these routes dynamically unfavorable. Up to now, we have not been able to find a pathway to generate benzyl nitrile quickly under moderate conditions without the help of a Lewis acid. With these calculation results, we are inspired to think about the crucial role of the Lewis acid in the experiment. What Is the Role of the Lewis Acid? Lewis acid is often used as a cocatalyst in most of the transition-metal-catalyzed reactions, such as the carbocyanation reaction of alkynes,46,47 which also undergoes a C−C bond cleavage step. A thorough investigation and understanding of the role that the Lewis acid plays in transfer hydrocyanation would be helpful for the further development of more efficient transition-metal/Lewis acid catalytic systems. One may notice that the stability of intermediate 1 is related to the barrier of the rate-determining step in Figure 2. A Lewis acid can increase the stability of this intermediate and reduce the following barrier. Figure 3 gives us

Figure 3. Three interaction modes of a Lewis acid with intermediate 1 (ESP isovalue 0.0004).

the molecular electrostatic potential of the intermediate 1. The red areas represent the regions of negative electrostatic potentials. There are two sites (one is the cyano nitrogen atom and the other is the oxygen atom on the diphosphine ligand) which could have interaction with the electron-deficient Lewis acid, as shown in Figure 3. That is to say, the Lewis acid can bind to the intermediate 1 in three ways: (a) one molecule of the Lewis acid interacts with the cyano nitrogen atom, (b) one molecule of the Lewis acid interacts with the oxygen atom D


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Organometallics

Figure 4. Free energy profile of the mechanism of transfer hydrocyanation with one Lewis acid binding to the N atom of a nitrile.

Figure 5. Free energy profile of the mechanism of transfer hydrocyanation with one Lewis acid binding to the O atom of the diphosphine ligand.

the C−CN bond cleavage in a previous report.48 As shown in Figure 6, the transition state TS1-2 could be separated into two fragments, denoted as the Ni center and the C−CN part. During the cleavage of the C−CN σ bond, charge is transferred from the highest occupied molecular orbital (HOMO) of the Ni center to the lowest unoccupied molecular orbital (LUMO) of the C−CN part. The HOMO of the Ni center is mainly composed of the dπ orbital of the transition metal, while the C−CN part is mainly composed of the σ* orbital of the C−CN bond and the π* orbital of the −CN substituent. The charge transferred from the Ni center to the C−CN part in TS1-2 is 0.156 e. Similar to the case of TS1-2, transition state LAN-TS12 corresponding to the C−CN bond cleavage with the help of the Lewis acid coordinated to the N atom can be also divided into two parts, the charge transfer from the HOMO of the Ni center (composed of dπ) to the CN-LAN part (composed of σ* and π*). The charge transfer of this oxidative addition process is 0.409 e for LAN-TS1-2, which is much more than that (0.156 e) in TS1-2. With the coordination of a Lewis acid to the N

atom, charge transfer occurs strongly from the HOMO of the Ni center to the antibonding orbital of the C−CN σ bond, which will obviously weaken the C−CN bond and accelerate the C−CN bond cleavage. This is also consistent with previous investigations that the Lewis acid coordinated on the N atom of the −CN group could accelerate the charge transfer and then make the C−CN bond weaker.22,48−50 In contrast, the interaction between the Lewis acid and the O atom of the diphosphine ligand in the transition state LAO-TS1-2 in Figure 6 makes the charge transfer (0.130 e) more difficult from the HOMO of the Ni-LAO to the LUMO of the CN part in comparison to that in TS1-2. As a result, the cleavage of the C− CN bond in this case is more difficult. Finally, two molecular Lewis acids interact with two nucleophilic sites in intermediate 1 separately and the results are shown in Figure 7. The coordination of another Lewis acid makes the intermediate 2LA-1 more unstable than intermediate LAN-1, but it is still more stable than the intermediate 1 and LAO-1. With the participation of two molecular Lewis acids, the E


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Organometallics

Figure 6. Fragment molecular orbital analysis and charge decomposition analysis of the transition states corresponding to the C−C bond cleavage (isovalue 0.02).

Figure 7. Free energy profile of the mechanism of transfer hydrocyanation with one Lewis acid binding with the N atom of nitrile and another binding with the O atom of the diphosphine ligand.

nitrile group. In fact, the Lewis acid can bind with the nucleophilic sites alternately, suggesting that we can combine Figures 4 and 7 together. As shown in Figure 8, intermediate 2LA-1 is not as stable as intermediate LAN-1. In addition, the reaction barrier for C−CN cleavage from 2LA-1 is lower than that from LAN-1. From 2LA-2, one Lewis acid can be released to form LAN-2, a more stable intermediate. From this stable

reaction follows routes similar to those in Figures 4 and 5, including oxidative addition, β-H elimination, ligand exchange, migratory insertion, and reductive elimination. The TDI and TDTS for pathway in Figure 7 are 2LA-1 and 2LA-TS2-3, respectively. The total barrier for this route is 31.2 kcal mol−1, which is 3.3 kcal mol−1 higher than that of the route in Figure 4, where only one molecule of the Lewis acid coordinates to the F


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Organometallics

Figure 8. Possible reaction route with the association/dissociation of excess Lewis acid.

Figure 9. Free energy profiles for the computational prediction of the catalytic efficiency of different ligands.

intermediate, the reaction steps leading to the final product can readily occur, which have been discussed in the previous section. By this way, the total barrier of the reaction is 24.7 kcal/mol, which is consistent with the experimental results that the reaction can obtain high yields at 75 °C.21 At this point, the dissociation of the Lewis acid could be possible by the dilution of the concentration of the Lewis acid in the solution experimentally. Therefore, the reaction mechanism can be concluded as consisting of the following processes: association of two molecules of Lewis acid at the beginning to give the intermediate 2LA-1, C−CN bond cleavage, dissociation of one molecule of the Lewis acid on the O atom of the ligand to form the intermediate LAN-2, β-H elimination, ligand exchange, migratory insertion, and reductive elimination to complete the catalytic cycle. Another possible role of the Lewis acid in this reaction is to generate the cationic aluminum species, R2Al+, through a dissociation reaction. This cationic species could be stabilized by coordinating to the substrate, the product, or the solvent in

the reaction system.51 It has been confirmed by theoretical calculations that the promoting species in the Petasis−Ferrier rearrangement is the cationic aluminum species R2Al+, instead of the usually considered neutral Lewis acid R3Al or R2AlCl.52 In this work, the acidity of the cationic species Me2Al+ is stronger than that of the neutral species AlMe2Cl and these two Lewis acid species will play similar roles in the catalytic cycle. In addition, the strong Lewis acidity Me2Al+ may make the C−CN bond cleavage more favorable. Therefore, the role of Me2Al+ will not be investigated in this work. While the mechanism and the role of the Lewis acid in this reaction are clear, it is still necessary to elucidate another experimental observation about the linear anti-Markovnikov selectivity (the linear anti-Markovnikov product P2-L is the main product and the branched Markovnikov product P2-B is the byproduct).21 In regard to the branched Markovnikov product P2-B, a possible reaction mechanism with the anticipation of a Lewis acid has also been investigated and the results are shown in Figure S2 in the Supporting G


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Organometallics Information. However, computational results indicate that all of the possible routes with or without the participation of the Lewis acid are thermodynamically unfavorable in comparison with the routes leading to the linear anti-Markovnikov product P2-L. Our calculated results show that, at the beginning of the reaction, the branched product P2-B would be formed together with P2-L, and later the branched product P2-B will be transferred to the thermodynamically more stable product P2L. This agrees well with the experimental observation that the linear product P2-L is the main product and the ratio of the linear to branched products is over 95:5.21 Here, we need to mention that people may argue that the very stable intermediate LAN-1 in Figure 8 can be easily formed, and once this intermediate is formed, C−CN cleavage will be very difficult. We have considered changing the substituent on the diphosphine ligand to make the C−CN cleavage easier. Therefore, we have calculated the catalyzed reaction pathway with several newly designed ligands, and the results are given in the following section. Computational Prediction of Nickel(0)/Lewis Acid Cooperative Shuttle Catalysts. As discussed in the previous section, the coordination of a Lewis acid on the N atom of the nitrile group will accelerate the reaction by transferring more electrons from the HOMO of the Ni center to the C−C σ* bond. However, the coordination of a Lewis acid on the O atom of the diphosphine ligand makes this charge transfer less favorable and makes the C−C bond cleavage more difficult. Inspired by the effect of the Lewis acid on the catalytic efficiency of this reaction, we introduced the strongly electron withdrawing group CF2 and the electron donating group CH2 to the diphosphine ligand to replace the O atom to form the new ligands L1 and L2. The catalytic mechanisms with/without a Lewis acid of these two new ligands were investigated. Here, we studied only the routes with one molecule of Lewis acid coordinated to the N atom of the nitrile group, and the computational results are shown in Figure 9. The mechanism without the anticipation of Lewis acid is shown in Figure S4 in the Supporting Information, which is obviously unfavorable in comparison with the routes in Figure 9. Both of these catalysts could catalyze the reaction with steps similar to those of the previously discussed catalyst. However, by introduction of the electron-withdrawing group CF2 on the ligand, the barrier for the cleavage of the C−CN bond is 33.7 kcal/mol, which is obviously higher than that of the DPEphos ligand. In contrast, the use of the electron-donating group CH2 lowers this barrier to 25.9 kcal mol−1. This means that increasing the electrondonating ability of the diphosphine ligand will make the cleavage of the C−CN bond more favorable. In order to make the ligand more electron rich, four −CH3 groups are introduced to the DPEphos ligand to replace the four phenyl groups on the P atoms. As shown in Figure 9, with the introduction of the electron donating group −CH3, the barrier for the transition state L3-LAN-TS1-2 of C−CN cleavage decreases to 24.1 kcal/mol. The strongly electron donating groups make the transfer hydrocyanation reaction more favorable.

intermediate, followed by the oxidative addition of C−CN to the Ni center, β-H elimination, ligand exchange, and migratory insertion, the catalytic cycle being completed by reductive elimination to release the linear anti-Markovnikov product. The oxidative addition of the C−CN bond is the rate-determining step, and the reaction cannot occur at room temperature without the help of a Lewis acid due to very high barrier of oxidative addition of the C−CN bond. Therefore, the role of excess Lewis acid has also been studied in this work. The binding of a Lewis acid with the N atom of the nitrile group accelerates the reaction, while the binding of a Lewis acid to the O atom of the diphosphine ligand prohibits the reaction. Charge transfer from the catalyst to the cyanide substrate can explain different barriers of C−CN cleavage in three cases. At the same time, three new ligands for the Ni(0)/Lewis acid cooperative catalysis have been designed by introducing different groups to the diphosphine ligand. The computational results show that electron donating groups on the diphosphine ligand accelerate the reaction. It is suggested that ligand L3, which introduces −CH3 on P atoms, will be a promising ligand in catalyzing transfer hydrocyanation. Given the current interest in TM/LA catalysis and transfer hydrocyanation, this work will be helpful in the design of transition-metal/Lewis acid cooperative shuttle catalysis systems.

CONCLUSIONS In this work, the mechanism of Ni(0)-catalyzed transfer hydrocyanation reaction has been investigated. The results show that the major pathway proceeds through a mechanism that starts with initial ligand exchange between the Ni(COD)2 and the DPEphos ligand to form the η2-side-on-coordinated

(1) Wang, R.; Falck, J. R. RSC Adv. 2014, 4, 1062−1066. (2) Wen, Q.; Lu, P.; Wang, Y. RSC Adv. 2014, 4, 47806−47826. (3) Bini, L.; Müller, C.; Vogt, D. ChemCatChem 2010, 2, 590−608. (4) Huthmacher, K.; Krill, S. In Applied Homogeneous Catalysis with Organometallic Compounds, 2nd ed.; Cornils, B., Hermann, W. A., Eds.; Wiley-VCH: Weinheim, Germany, 2002; Vol. 1, p 465. (5) Tolman, C. A. Chem. Rev. 1977, 77, 313−348.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00218. Coordinates of the optimized structures (XYZ) Details of the potential energy surface (PDF)

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AUTHOR INFORMATION

Corresponding Author

* E-mail for L. D.: [email protected]. ORCID

Ti-Long Yang: 0000-0002-9126-8840 Li Dang: 0000-0003-2666-7607 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (No. 21573102) is appreciated. We are thankful to the Department of Education of Guangdong Province. We thank Prof. Michael B. Hall (Texas A&M University), Prof. Wei Guan (Northeast Normal University), and Prof. Shigeyoshi Sakaki (Kyoto University Fukui Institute for Fundamental Chemistry) for giving suggestions and sharing experiences in fragment molecular orbital analysis and charge decomposition analysis.

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