Lewis Acid Catalyzed

Nov 22, 2017 - Faculty of Chemistry, National & Local United Engineering Lab for Power Battery, Key Laboratory of Polyoxometalate Science of Ministry ...
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Synergistic Mechanistic Study of Nickel(0)/Lewis Acid Catalyzed Cyanoesterification: Effect of Lewis Acid Bo Zhu, Gui-Fang Du, Hang Ren, Li-Kai Yan,* Wei Guan,* and Zhong-Min Su Faculty of Chemistry, National & Local United Engineering Lab for Power Battery, Key Laboratory of Polyoxometalate Science of Ministry of Education, Key Laboratory for UV-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, People’s Republic of China S Supporting Information *

ABSTRACT: The highly selective nickel(0)-catalyzed cyanoesterification of cyanoformates with alkynes is greatly facilitated by Lewis acid (LA). We report density functional theory (DFT) investigations on the mechanistic details and provide a rational explanation of the “LA effect”. Our calculations disclose that an unusual biphosphine cycle including rate-determining oxidative addition of a C−CN bond, alkyne migratory insertion, and reductive elimination steps is more favorable than the generally accepted monophosphine cycle. The LA has a non-negligible impact on the mechanistic origin of selective C−CN bond activation. Furthermore, the presence of LA dramatically accelerates alkyne migratory insertion because the strong electron-withdrawing property of LA strengthens the coordination ability of the Ni center with alkyne to form the thermodynamically favorable five-coordinate nickel(II) cyanide carboxylate species, which then avoids the large deformation of the transition state of alkyne migration insertion to give a moderate activation energy. Thus, the proposed biphosphine cycle successfully rationalizes the experimental observation that the presence of LA is comparatively crucial to improve this reaction with high regioselectivity.

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produced β-cyano-substituted acrylate esters with both unsaturated CC and CN functional groups are usually used as a series of useful building blocks to construct various nitrile derivatives, thus making this reaction surely of great synthetic value.11 Palladium-catalyzed cyanoesterifications of cyanoformates with norbornenes and norbornadiene with exclusive exo selectivity have been reported by Nishihara and co-workers.12 Although they successfully isolated the key intermediate transPd(CN)(CO2Et)(PPh3)2 generated via the oxidative addition of Pd(PPh3)4 to ethyl cyanoformate, no complete mechanism or reasonable interpretation of high exo selectivity could be ascertained. Notably, a subsequent theoretical mechanistic investigation on this reaction performed by Mori and coworkers clarified the origin of the exo selectivity and provided theoretical guidance on adjusting the experimental conditions to control product selectivity for experimental chemists.13 Obviously, modern computational chemistry is of great significance to understand the reaction mechanism and solve specific problems in this field. Low cost and high efficiency are some of the top challenges in the catalytic field. In this regard, zerovalent nickel has been proven to be a powerful catalyst for various cyanofunctionalizations because it is viewed solely as a low-cost replacement of

INTRODUCTION The carbon−carbon (C−C) bond is the most widespread and fundamental bond existing in organic compounds.1 The selective C−C bond activation catalyzed by transition-metal (TM) complexes has received considerable interest due to the great advantage of constructing new chemical bonds.2 In particular, the development of TM-catalyzed C−CN bond cleavage is crucial for organic synthesis3 because nitriles (R− CN) can be transformed into other important compounds such as natural products, pharmaceuticals, agrochemicals, and materials by various CN group functionalization.4 The activation of a C−CN bond catalyzed by a TM can be simply categorized into cyanofunctionalization, decyanative crosscoupling, and cyanation according to the utility degree of nitriles.5 Among them, cyanofunctionalization referring to the adjacent difunctionalization of alkynes with carbonaceous groups has attracted increasing attention in organic synthesis, mainly because two different C−C bonds can be installed simultaneously onto an unsaturated bond with great atom efficiency.6−8 On the basis of the carbon types of C−CN bond activation, cyanofunctionalizations are usually classified as various types, including cyanoesterification, cyanoacylation, cyanoamidation, cyanoarenylation, cyanoalkenylation, and so on. Up to now, cyanoesterification occurring on alkynes, alkenes, and 1,2-dienes exclusively in a syn-addition fashion with high regioselectivities has achieved outstanding results in this field.9,10 It is worth mentioning that the subsequently © XXXX American Chemical Society

Received: July 27, 2017

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DOI: 10.1021/acs.organomet.7b00573 Organometallics XXXX, XXX, XXX−XXX

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role of LA in such reactions is crucial to discover a better TM/ LA synergistic catalytic systems. To this end, correct knowledge of such a synergistic mechanism involving the key intermediates and transition states is necessary. Regrettably, it is not easy to observe and investigate the reactive intermediates experimentally. In contrast, as mentioned above, theoretical and computational studies can provide meaningful information to understand the catalytic reaction even though it occurs on a model system.21 In this regard, we theoretically investigated the catalytic cycle of Ni(0)/LA-catalyzed cyanoesterification reaction of ethyl cyanoformate and alkynes. We addressed the following questions through computations. (1) What is the specific mechanism of this reaction? (2) How does the LA promote the reaction? As shown in the top half of Scheme 2, the monophosphine cycle is generally accepted for Ni(0)/LA-catalyzed cyanoester-

zerovalent palladium and provides ready access to available multiple oxidation states commonly involved in catalysis. For example, Nakao and co-workers have successfully achieved the C−CN bond cleavage of cyanoformates and their sequential cyanoesterifications of cumulative 1,2-dienes catalyzed by zerovalent nickel.11a However, as is well known, the C−CN bond is relatively strong and stable with a high bond dissociation energy (BDE). The selective activation of a C− CN bond by a TM catalyst is still a challenging target, and the general scope of these transformations remains unexplored, limiting the further development of this field to a great extent. Thus, it is urgent to explore a new synthetic strategy for improving selective C−CN bond activation. More recently, Lewis acids have emerged as highly powerful tools to assist TM in the activation of a C−C or C−heteroatom bond in organic synthesis.14 For instance, a pioneering investigation describing a Ni(dippe)/BPh3 (dppe = diphenylphosphinoethane) cooperatively catalyzed activation of C−CN bonds was reported by Jones and co-workers in 2004, where the addition of BPh3 could obviously facilitate the cleavage of the C−CN σ bond of aryl cyanide.15 Nakao and co-workers have made outstanding contributions in developing the addition reactions of various organic nitriles through cleavage of C−CN bonds by Ni(0)/LA cooperative catalysis.16 It is noteworthy that a regio- and stereoselective cyanoesterification reaction using Ni(0)/LA catalytic systems has been reported by Nakao and Hiyama, where cyanoformates react with alkynes to give cyano-substituted acrylate esters (Scheme 1).17 Interest-

Scheme 2. Catalytic Cycles of the Ni0L2/LA-Catalyzed Cyanoesterification of Cyanoformates with Alkynes

Scheme 1. Cyanoesterification of Cyanoformates with Alkynes Catalyzed by Ni0L2/LA

ification.17 The first key step is oxidative addition of a C−CN bond to a nickel(0) center that affords the cis-nickel(II) cyanide carboxylate complex b. The subsequent ligand (L) dissociation and alkyne (AL) coordination generate the monophosphine species c. Then, migratory insertion of unsaturated AL into the Ni−C(O) bond followed by reductive elimination affords the cyanoesterification product P and Ni(0) catalyst, completing the catalytic cycle. Alternatively, a biphosphine catalytic cycle involving the five-coordinate nickel(II) cyanide carboxylate species f is shown in the bottom half of Scheme 2. After oxidative addition, the coordination of AL to the Ni center of complex b without L dissociation gives the biphosphine complex f. Afterward, f undergoes migratory insertion and reductive elimination to afford P and regenerate Ni0L2 catalyst for the next catalytic cycle. The present calculations show that an unusual biphosphine cycle is more favorable than the generally accepted monophosphine cycle because the process of ligand dissociation from the Ni center is considerably endergonic. In addition, we have elucidated a novel possible rationalization of the “effect of Lewis acid” on the title reaction by comparing the full cycles with and without the direct involvement of LA. This study not

ingly, the experimental results indicate that the reaction can occur at 35 °C in the presence of LA while it is completely ineffective without LA even at 100 °C. In the above series of studies, the reaction rate is significantly accelerated, the yield of the product can be observably improved, the scope of nitriles applied in the reaction is greatly expanded, and/or a highly difficult reaction can occur under relatively mild conditions by employing LA cocatalyst. Currently, several mechanistic studies on the TM-catalyzed C−CN σ-bond activation have been performed theoretically.18 However, the mechanistic details referring to the synergistic effect of the TM/LA catalytic systems, especially the natural role of LA, are relatively rare. Previously, Sakaki and co-workers have theoretically investigated the synergistic mechanism of decyanative [4 + 2] cycloaddition of isatoic anhydrides with alkynes catalyzed by Ni(0)/LA systems.19 Consequently, they found that the charge transfer from the metal to the C−C σ* + π* antibonding molecular orbital could be enhanced by LA, promoting the C− C σ-bond cleavage. We believe that the synergistic function of TMs with LAs would be a powerful protocol to perform such C−C σ-bond activation reactions.20 Thus, understanding the B

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Figure 1. Energy profile (ΔG308.15) of the most favorable catalytic cycle in the presence of Lewis acid.

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RESULTS AND DISCUSSION The Most Favorable Catalytic Cycle. Figure 1 shows the most favorable catalytic cycle in the presence of Lewis acid, where precomplex C-LA formed by the coordination of Ni(PMe3)2 to the CN bond of the substrate S is viewed as a standard (energy 0). Actually, C-LA is more stable than the adduct of alkyne with Ni(PMe3)2 (CAL; Scheme S1 in the Supporting Information). The results are discussed on the basis of the singlet potential energy surface because the singlet species are more stable than the corresponding triplet species in the oxidative addition step (Table S1 in the Supporting Information). The present calculations propose an unexpected biphosphine catalytic cycle including three important elementary steps: (1) oxidative addition of the C−CN σ bond to the nickel(0) center producing the nickel(II) cyanide carboxylate intermediate 2a-LA, (2) migratory insertion of AL into the nickel−carboxyl (NiII−C(−OOEt)) bond to form intermediate 4a-LA, and (3) reductive elimination of the C−C bond to afford cyanoacrylate product P and regenerate the nickel(0) catalyst. The rate-determining step is the oxidative addition with an acceptable Gibbs activation energy (ΔG⧧) of 25.4 kcal/ mol. Specifically, prior to the oxidative addition, the Ni center migrates toward the C(−OOEt) atom of C-LA, leading to intermediate 1a-LA, which lies 21.8 kcal/mol higher than CLA. As presented in Figure 2, the C1−C2 bond is slightly elongated to 1.52 Å in 1a-LA from 1.48 Å in C-LA. Subsequently, the oxidative addition of the C−CN bond occurs via the classical three-membered-ring transition state TS1a-LA to give complex 2a-LA. In TS1a-LA, the C1−C2 distance is elongated to 1.57 Å, while simultaneously the Ni− C2 distance is shortened to 2.27 Å from 3.22 Å in C-LA, respectively. The geometry changes indicate that the cleavage of the C1−C2 bond and the formation of the Ni−C2 bond are in progress. 2a-LA is a pseudo-square-planar cis isomer of the four-coordinate Ni(II) cyanide carboxylate complex, where the Ni−C1 and Ni−C2 bonds are 1.85 and 1.90 Å, respectively. This step requires ΔG⧧ and the Gibbs free energy change (ΔG) of 25.4 and −6.3 kcal/mol in comparison to C-LA, respectively.

only adds an additional dimension to the current mechanism of the cyanoesterification but also will inspire chemists to develop synergistic catalytic systems with higher performance for various C−CN bond activation reactions.

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COMPUTATIONAL DETAILS AND MODELS

All the DFT calculations were performed with the Gaussian 09 package.22 All geometries were optimized without symmetry constraints at the level of the M06 hybrid functional,23 which was chosen according to the previous benchmark calculations in nickel(0)catalyzed reactions.18,19 The solvent effect was considered by employing the conductor-like polarizable continuum model (CPCM) and toluene solvent. The LanL2DZ and corresponding Hay−Wadt effective core potential (ECP)24 were applied for the core electrons of Ni, and the (541/541/311/1) basis set was used for its valence electrons, while the standard 6-31G(d) basis set was used for the other main-group elements (BSI). The vibrational frequency was calculated to check the nature of each stationary structure at the same theoretical level with optimization. Only one imaginary frequency was characterized for transition states, while no imaginary frequency was found for reactants, intermediates, and products. The intrinsic reaction coordinate (IRC) was calculated to ensure the correct connection between the transition state and the corresponding reactant/product. In addition, thermodynamic corrections to the Gibbs energy were obtained from frequency calculations at 308.15 K and standard atmospheric pressure. In addition, these values were corrected by applying the method developed by Whitesides et al. to consider a correct treatment of the standard state;25 see page S3 in the Supporting Information. Furthermore, single-point calculations were performed to refine the potential energies using a better basis set system (BSII). In BSII, the Stuttgart/Dresden ECP basis set (SDD) was used for the core electrons of Ni and the (311111/22111/411/11) basis set26,27 for its valence electrons and the 6-311+G(2d,p) basis sets were employed for other elements. As shown in Scheme 1, the cyanoesterification reaction catalyzed by Ni(cod)2/BPh3(LA) was chosen as the model reaction in our calculations; ethyl cyanoformate (S) was employed as the substrate, while 4-octyne and P[3,5-(CF3)2-C6H3]3 were simplified to but-2-yne (AL) as an alkyne substrate and PMe3 as a phosphine ligand, respectively. C

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Figure 3. Energy profiles (ΔG308.15) of oxidative addition.

for C−CN bond activation (ΔG⧧ = 25.4 kcal/mol), TS1b-LA for the C−O(Et) bond activation (ΔG⧧ = 32.0 kcal/mol), and TS1c-LA for the O−C(H2CH3) bond activation (ΔG⧧ = 64.4 kcal/mol). On the basis of the calculation results, C−CN bond activation requires the smallest energy barrier and produces the correspondingly most stable intermediate 2a-LA. This conclusion is consistent with the experimental observation17 and theoretical study19 that the C−CN bond is activated with high selectivity by Ni(0)/LA synergistic catalytic systems. In order to reveal the origin of this selectivity, we evaluated the bond energies of σ bonds (a−c) in S and S-LA. The results show that the bond energy28 (in kcal/mol) changes are in the order a (116.0) > b (111.1) > c (106.6) for S and b (113.5) > c (98.6) > a (90.8) for S-LA. It seems that the C−CN σ bond (a) is significantly activated by the interaction of LA with the CN group. This could be attributed to the strong electronwithdrawing property of LA which leads to a decrease in the electron density of the C−CN σ bond and accordingly weakens the chemical bond. Furthermore, we evaluated the bond energies29 (in kcal/mol) of the Ni−CN (−110.3), Ni−COOEt (−54.9), Ni−OEt (−63.4), and Ni−CO(CN) (−55.6) bonds in 2a-LA and 2b-LA, respectively. It can be seen that the strong Ni−CN bond in 2a-LA leads to 2a-LA being more stable than 2b-LA. Thus, the weak C−CN bond in S-LA and stability of 2a-LA are critical factors for the highly selective activation of the C−CN bond. In addition, previous theoretical studies have indicated that the charge transfer (CT) from the dπ orbital of Ni(0) to the low-lying unoccupied σ* + π* antibonding orbital of S-LA in the oxidative addition process could play an important role in promoting the bond cleavage.19 Thus, we performed charge decomposition analysis (CDA) for the three TSs mentioned above (TS1a-LA, TS1b-LA, and TS1c-LA) with the Multiwfn3.3.8 package.30 However, the calculations present the unexpected results that the CT amount is in the order TS1b-LA (0.520e) > TS1a-LA (0.342e) > TS1c-LA (0.119e), which is different from the decreasing order of the activation energy. The large CT amount of TS1b-LA can be understood because the TS1b-LA is productlike and the C−O π* orbitals of S and S-LA are distributed in the lowest unoccupied molecular orbitals (LUMOs), which is favorable to such CT (Figure S2 in the Supporting Information). Never-

Figure 2. Optimized structures of the most favorable catalytic cycle in the presence of Lewis acid.

The second process is migratory insertion. First, coordination of AL to 2a-LA can afford the five-coordinate nickel(II) cyanide carboxylate species 3a-LA, which is thermodynamically favorable. Then, via transition state TS2a-LA with a fourmembered Ni−C2−C3−C4 ring, AL inserts into the Ni− C(−OOEt) bond to give the 4a-LA intermediate. As shown in Figure 2, the related Ni−C2, C2−C3, and Ni−C4 bonds in TS2a-LA are 1.99, 1.96, and 1.95 Å, respectively. The ΔG⧧ and ΔG values relative to 3a-LA are 16.7 and −25.8 kcal/mol, respectively. Starting from 4a-LA, the C−C bond reductive elimination occurs via the transition state TS3a-LA to produce the 5a-LA intermediate with the Ni(0) center coordinating to the C3C4 bond of P. TS3a-LA has a Ni−C1−C4 threemembered-ring geometry where the C1−C4 distance is shortened to 1.51 Å from 2.51 Å in 4a-LA, suggesting that the formation of the C1−C4 bond is in progress. This step needs a moderate ΔG⧧ value of 21.4 kcal/mol relative to 4aLA. Finally, the desired product P is released through ligand exchange to regenerate C-LA. The Gibbs energy change of the overall cycle is −35.5 kcal/mol. Important Elementary Steps. Oxidative Addition. The first critical step is the oxidative addition of the substrate S to the nickel(0) center. In this section, one important task is to investigate which of the σ bonds of S (a−c) would be activated, which determines the selectivity of product; see the inset in Figure 3 for bonds a−c. Two cases have been investigated: one is where the LA interacts with the N atom of the cyano group (case A), and the other is where the LA interacts with the O atom of the carbonyl group (case B). In both cases A and B, the catalyst Ni(PMe3)2 can coordinate to either the CN or C O bond to form the four kinds of adducts C-LA, CO-LA, CLAO, and CO-LAO, respectively. Among them, the η2-side-on coordinated complex C-LA with an LA interaction with the N atom of cyano group is the most stable (Figure S1 in the Supporting Information). Then, from the above initial precomplex C-LA, three kinds of transition states (TSs) for oxidative addition were located, as shown in Figure 3: TS1a-LA D

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(29.5 kcal/mol). Notably, the formation of the 3a-LA intermediate is a key process in the presence of LA, avoiding the high energy demanding ligand dissociation step. Hence, it is concluded that the migratory insertion occurs more easily via the biphosphine path in comparison to the monophosphine path. In addition, to further verify the reliability of the biphosphine mechanism, we employed the exact phosphine ligand (P[3,5(CF3)2-C6H3]3)17 to evaluate the migratory insertion and ligand dissociation, which is in accordance with our present results (Figure S3 in the Supporting Information). At the end of this section, we should mention the regioselectivity in the migratory insertion, because AL may insert into the Ni−C(−OOEt) or the Ni−C(N) bonds. On the basis of the proposed biphosphine path, AL insertion into the Ni−C(N) bond from 3a-LA occurs through the transition state TS2a-LAN to afford the corresponding intermediate 4a-LAN. This step needs a very large ΔG⧧ value of 29.6 kcal/mol and has a small ΔG value of −10.0 kcal/mol relative to 3a-LA (Figure S4 in the Supporting Information). In a word, the former migratory insertion (3a-LA → TS2a-LA → 4a-LA, ΔG⧧ = 16.7 kcal/mol) occurs more easily than the latter (3a-LA → TS2a-LAN → 4a-LAN, ΔG⧧ = 29.6 kcal/mol). Thus, we can conclude that AL favors insertion into the Ni− C(−OOEt) bond rather than the Ni−C(N) bond. This is also consistent with the bond energy calculation that the Ni− C(N) bond (E = 104.8 kcal/mol) is much stronger than the Ni−C(−OOEt) bond (E = 59.0 kcal/mol).28 Reductive Elimination. The next process is reductive elimination, where the monophosphine and biphosphine paths were investigated. In the monophosphine path, AL first coordinates with the Ni center of the three-coordinate complex 4b-LA to form the four-coordinate species 4b-LAAL, which is exergonic by 5.9 kcal/mol (Figure 5, highlighted in blue).

theless, the C−O(−Et) bond cleavage still has a relatively large energy barrier due to large bond energy (113.5 kcal/mol). To sum up, the C−CN σ bond has not only the smallest bond energy but also a moderate CT amount among the three σ bonds, which leads to the smallest energy barrier of the C−CN σ-bond activation. Therefore, it can be concluded that the regioselectivity is determined by both the bond strength and the charge transfer process. More importantly, the LA takes on a non-negligible role in the selective activation of the C−CN σ bond. Migratory Insertion. After oxidative addition, the two reaction courses of migratory insertion shown in Figure 4

Figure 4. Energy profiles (ΔG308.15) of two pathways for migratory insertion.

have been considered: one is defined as the monophosphine path, and the other is the biphosphine path. In the generally accepted monophosphine path, the first step is the dissociation of one PMe3 to generate a three-coordinate monophosphine species. It is worth mentioning that the isolated phosphine ligand as a Lewis base may interact with a Lewis acid to provide energy stabilization. However, monitoring of the reaction by 31 P NMR spectroscopy indicates that there is no Lewis acid− Lewis base interaction between the sterically bulky P[3,5(CF3)2-C6H3]3 and B(C6F5).17 In order to reproduce the experimental results, the present calculations reasonably ignore such effects in the model reaction. Thus, the PMe3 dissociation from 2a-LA to afford intermediate 2am-LA is considerably endergonic at 29.5 kcal/mol (Figure 4, highlighted in blue). Then, the three-coordinate 2am-LA will bind with the AL to form the η2-alkyne intermediate 3b-LA. The following migratory insertion occurs via the four-membered transition state TS2b-LA, in which AL inserts into the Ni−C(−OOEt) bond to give the intermediate 4b-LA. This step needs a moderate ΔG⧧ value of 23.5 kcal/mol relative to 2a-LA. Alternatively, another biphosphine path has been explored (Figure 4, highlighted in black). Starting from 2a-LA, AL coordinates to the Ni center to generate the five-coordinate nickel(II) cyanide carboxylate species 3a-LA. Subsequently, AL insertion into the Ni−C(−OOEt) bond occurs through the transition state TS2a-LA to afford intermediate 4a-LA. The energy barrier of this step is only 16.7 kcal/mol relative to 3aLA, which is much lower than that of the monophosphine path

Figure 5. Energy profiles (ΔG308.15) of two pathways for reductive elimination.

Starting from 4b-LAAL, reductive elimination occurs easily with a small ΔG⧧ value of 13.3 kcal/mol. In contrast, the biphosphine path (4a-LA → TS3a-LA → 5a-LA) needs a larger ΔG⧧ value of 21.4 kcal/mol (Figure 5, highlighted in black). It seems that the Ni(phosphine)(alkyne) active species can promote the reductive elimination.19 Even so, the free energy profile of the biphosphine path lies lower than that of the monophosphine path, indicating that the biphosphine E

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Figure 6. Energy profile (ΔG308.15) of the catalytic cycle in the absence of Lewis acid.

of LA (Figure 1), which is larger than that in the absence of LA (ΔG⧧ = 21.2 kcal/mol, Figure 6). To explore the origin of this implausible result, energy decomposition analysis (EDA)31 has been carried out, in which C, C-LA, TS1a, and TS1a-LA are divided into two corresponding fragments as substrate (S or S-LA) and Ni catalyst (Nicat) moieties, respectively. As shown in Table 1, the

mechanism is more favorable under the experimental conditions. Effect of Lewis Acid. As mentioned in the Introduction, one of the important purposes of this study is to elucidate the Lewis acid (BPh3) effect on this catalytic reaction and explore new Ni(0)/LA synergistic catalytic strategies. Thus, in addition to the biphosphine catalytic cycle in the presence of LA, we studied a catalytic cycle in the absence of LA. Unexpectedly, there is no analogous five-coordinate nickel(II) cyanide carboxylate species such as 3a-LA located in the path without LA. As shown in Figure 6, the catalytic cycle begins at the precomplex C with η2-side-on coordination of the cyano group to the Ni because the complexes C1 (η1-end-on coordination of the cyano group to the Ni) and CO (η2-side-on coordination of the carbonyl CO bond to the Ni) are less stable than C (Figure S5 in the Supporting Information). The important elementary steps consist of oxidative addition, isomerization, migratory insertion, and reductive elimination. It is worth mentioning that migratory insertion is the rate-determining step, which requires a large ΔG⧧ value of 31.0 kcal/mol. It is obviously larger than that for the path with LA (ΔG⧧ = 25.4 kcal/mol), indicating that LA dramatically accelerates this reaction, in line with the experimental observation. Interestingly, the cis isomer 2a-LA is more favorable to bind AL to generate the stable five-coordinate 3a-LA rather than undergoes high energy demanding cis−trans isomerization, accordingly reducing the Gibbs activation energy of migratory insertion. This may be attributed to the strong electron-withdrawing property of LA, which can significantly increase the coordination interaction between the Ni center of 2a-LA and AL. However, no such five-coordinate intermediate can be located through AL coordination to cis isomer 2ac without LA; thus, AL insertion from cis isomer 2ac may not occur. Previous studies indicate that the strong electron-withdrawing property of LA should promote the CT from the 3dπ orbital of Ni(0) to the σ* + π* antibonding orbital of C−CN to accelerate the oxidative addition.19,20 However, an unexpected result is given in the present investigation. The ΔG⧧ value of the C−CN σ-bond activation is 25.4 kcal/mol in the presence

Table 1. Energy Decomposition Analysis for C, TS1a, C-LA, and TS1a-LA (kcal/mol) complex C TS1a ΔΔE C-LA TS1a-LA ΔΔE

ΔEint

ΔEPauli

ΔEelstat

ΔEorb

−65.6 −33.4 32.2 −94.8 −50.0 44.8

169.6 163.0 −6.6 186.0 187.4 1.4

−112.1 −95.2 16.9 −119.4 −112.5 6.9

−123.1 −101.2 21.9 −161.4 −124.9 36.5

energy changes of Pauli repulsion and electrostatic and orbital interaction between C and TS1a are −6.6, 16.9, and 21.9 kcal/ mol, respectively, and those between C-LA and TS1a-LA are 1.4, 6.9, and 36.5 kcal/mol, respectively. These EDA results indicate that oxidative addition with LA is more difficult than that without LA mainly because the decrease in orbital term is large in the LA in addition to the increase in the Pauli term. In the migratory insertion, the ΔG⧧ values are 16.7 kcal/mol relative to 3a-LA with LA and 31.0 kcal/mol relative to 3a without LA, respectively. This clearly shows that the LA coordination with the cyano nitrogen atom dramatically accelerates the AL insertion into the Ni−C(−OOEt) bond. Generally, the migratory insertion consists of two CT processes: one is the π orbital of AL to the Ni 3dσ orbital and another is π back-donation from the Ni 3dπ orbital to the π* orbital of AL.18b In migratory insertion without LA (3a → TS2a), the geometries of AL and P−Ni−P gradually distort from their almost linear structures to accelerate the CT from the Ni center to the π* orbital of AL. However, the large distortion of TS2a from 3a leads to a large ΔG⧧ value for migratory insertion. It is worth noting that there is no such distortion in migratory insertion with LA (3a-LA → TS2a-LA), F

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because the reaction course after oxidative addition has undergone significant changes in the presence of LA. A direct coordination of AL to the nickel center of cis isomer 2a-LA favors taking place in the presence of LA owing to the thermodynamic driving force of the formation of 3a-LA with a P−Ni−P angle of 111.8°. The geometry of 3a-LA avoids the large deformation of the following transition state to give a small activation energy of migration insertion. Furthermore, the electron population of the Ni center with LA decreases more than that without LA (Figure S6 in the Supporting Information), which indicates that the existence of LA accelerates the CT from the Ni center to the π* orbital of AL. This process effectively reduces the alkyne insertion activation energy, thus promoting the reaction to smoothly proceed. In addition, the ΔG⧧ values of the reductive elimination are 21.4 kcal/mol relative to 4a-LA in the presence of LA and 25.6 kcal/mol relative to 4a in the absence of LA. Thus, it can be seen that the presence of LA interacting with the cyano nitrogen atom also plays a positive role in promoting this step.

AUTHOR INFORMATION

Corresponding Authors

*E-mail for L.-K,Y.: [email protected]. *E-mail for W.G.: [email protected]. ORCID

Li-Kai Yan: 0000-0002-1352-4095 Zhong-Min Su: 0000-0002-3342-1966 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by the NSFC (21403033, 21773025, 21571031), the Fundamental Research Funds for the Central Universities, Jilin Provincial Education Department, and Open Project of Key Laboratory for UVEmitting Materials and Technology of Ministry of Education (130028697). We acknowledge the National Supercomputing Center in Shenzhen for providing the computational resources.

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REFERENCES

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CONCLUSION The Ni(0)/LA-catalyzed cyanoesterification reaction of ethyl cyanoformate and but-2-yne was theoretically investigated with the DFT method. We have disclosed the mechanistic details and provided a rational explanation of the “effect of Lewis acid”. The most favorable biphosphine catalytic cycle has been proposed. The important elementary steps consist of oxidative addition, alkyne migratory insertion, and reductive elimination. The rate-determining step is oxidative addition of the C−CN σ bond to the nickel(0) center, the ΔG⧧ value of which is acceptable (25.4 kcal/mol). The bond strength and charge transfer process determine the selective activation of the C− CN σ bond, and the LA has a non-negligible effect on it. The alkyne migratory insertion in the absence of LA is the most energy demanding with a ΔG⧧ value of 31.0 kcal/mol, which is much higher than that with LA (ΔG⧧ = 16.7 kcal/mol). The present result is in good agreement with the experimental observation that the presence of LA accelerates the reactions. This is because the strong electron-withdrawing property of LA not only enhances the Ni−PMe3 coordinate bond to avoid the high energy demanding dissociation of PMe3 but also makes the coordination ability of the Ni center with alkyne stronger to form the thermodynamically favorable five-coordinate nickel(II) cyanide carboxylate species to accelerate alkyne insertion. We believe that the present detailed computational studies provide a good understanding of the observed experimental facts and provide theoretical guidance on how to further develop novel Ni(0)/LA synergistic catalytic systems.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00573. Correction of translational entropy, computational details of energy decomposition analysis, and Scheme S1, Figures S1−S6, and Tables S1 and S2 as described in the text (PDF) Cartesian coordinates of all computed molecules (MOL) G

DOI: 10.1021/acs.organomet.7b00573 Organometallics XXXX, XXX, XXX−XXX

Article

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(24) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283. (25) Mammen, M.; Shakhnovich, E. I.; Deutch, J. M.; Whitesides, G. M. J. Org. Chem. 1998, 63, 3821−3830. (26) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987, 86, 866−872. (27) Martin, J. M. L.; Sundermann, A. J. Chem. Phys. 2001, 114, 3408−3420. (28) The bond energies (E) were calculated at the M06/BSI level with the following formula: E = EC − ER1 − ER2 − EBSSE, where E is the bond energy, EC is defined as the energy of the complex, ER1 and ER2 denote the energies of the corresponding radicals, and EBSSE is the energy of the basis set superposition error. (29) In view of the non-negligible weak interaction between bulk groups and LA in 2a-LA and 2b-LA, the bond energies of Ni−CN, Ni−COOEt, Ni−OEt, and Ni−CO(CN) were evaluated by employing the models with the same geometries as 2a-LA and 2b-LA excluding their LA moieties, respectively. The computational method of bond energy is given in ref 28. (30) Lu, T.; Chen, F. J. Comput. Chem. 2012, 33, 580−592. (31) The Supporting Information provides a more detailed description of the computational methods.

H

DOI: 10.1021/acs.organomet.7b00573 Organometallics XXXX, XXX, XXX−XXX