Mechanistic Origin of Regioselectivity in Nickel-Catalyzed Olefin

May 22, 2012 - Mechanistic Origin of Regioselectivity in Nickel-Catalyzed Olefin .... Lung Wa Chung. Asian Journal of Organic Chemistry 2018 7 (3), 52...
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Mechanistic Origin of Regioselectivity in Nickel-Catalyzed Olefin Hydroheteroarylation through C−H Activation Yuan-Ye Jiang, Zhe Li, and Jing Shi* Department of Chemistry, University of Science and Technology of China, Hefei 230026, People's Republic of China S Supporting Information *

ABSTRACT: Ni-catalyzed addition of electron-deficient arenes and heteroarenes to olefin substrates through C−H activation provides an important method for the synthesis of diarylalkanes. This reaction usually generates Markovnikov adducts for aryl olefins, whereas anti-Markovnikov adducts are obtained for alkyl-substituted alkenes. To understand the mechanistic origin of this interesting regioselectivity, we conducted density functional theory calculations using the reactions of benzoxazole with styrene and 1-hexene as models. The calculation results are consistent with experimental observations, showing that the reaction proceeds through a mechanism involving Ar−H oxidative addition, hydronickelation, and C−C reductive elimination. Further calculations indicate that a better antiMarkovnikov regioselectivity can be obtained for olefins substituted with more bulky alkyl groups, whereas a better Markovnikov regioselectivity can be achieved for more electron-deficient para-substituted styrenes. Further analysis shows that a secondary orbital overlap exists between Ni and the aryl group of styrene in the C−C reductive elimination transition state. This secondary orbital interaction is the key factor causing Markovnikov regioselectivity for vinylarenes. On the other hand, in the absence of this secondary orbital overlap the steric effect dominates the selectivity and, therefore, leads to the anti-Markovnikov products for alkylsubstituted alkenes.

1. INTRODUCTION Metal-catalyzed hydroarylation or hydroheteroarylation of olefins provides a highly atom-economical approach for the synthesis of 1,1- or 1,2-diarylalkanes.1 This category of reactions can proceed through two fundamentally different mechanisms. The first mechanism corresponds to Friedel− Crafts-type alkylation of arenes, where the metal functions as a Lewis acid. Through the Friedel−Crafts-type alkylation mechanism, carboncationic species are formed and their relative stabilities determine the regioselectivity for the electrophilic attack of the olefin. As a result, formation of branched (Markovnikov) products is favored and the arylation substrates are limited to electron-rich arenes (Scheme 1).2 To overcome these limitations, increasing attention has been paid in recent years to the addition of electron-deficient arenes to olefins via arene C−H activation.1e,3 For instance, Matsumoto et al. reported in 2002 an Ir-catalyzed hydrophenylation of olefins,3a whereas Darses et al. reported in 2008 a Ru-catalyzed hydroarylation of styrenes.3b Different from the Friedel−Crafts alkylation, these C−H activation methods mostly lead to the formation of antiMarkovnikov products. A possible explanation is that the steric effect of the substituents on the olefin hinders the C−C bond formation step and, therefore, leads to anti-Markovnikov products. This explanation stems from Murai’s experimental observation for Ru-catalyzed addition of aromatic C−H bonds © 2012 American Chemical Society

Scheme 1. Two Different Mechanisms for Metal-Catalyzed Hydroarylation of Vinylarenes

to olefins:4 i.e., the reaction of acetophenone with styrene gave a mixture of anti-Markovnikov and Markovnikov products in a 6:1 ratio, whereas the coupling of acetophenone with 2-methylstyrene gave only the anti-Markovnikov adduct (Scheme 2). Such improvement of anti-Markovnikov selectivity using 2-methylstyrene instead of styrene was also observed by Darses et al. in a similar catalytic system.3b Nonetheless, these experiments also raise an interesting and important challenge: how can we generate Markovnikov adducts in transition-metalcatalyzed olefin hydroarylation via C−H activation? Received: April 20, 2012 Published: May 22, 2012 4356

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Scheme 2. Ru-Catalyzed Hydroarylation of Styrene with Acetophenone Reported by Murai et al.

In this context, the recently developed first-row-transitionmetal-catalyzed hydro(hetero)arylation of vinylarenes provides a promising method to overcome the above problem. As an excellent example, Yoshikai et al. reported Co-catalyzed hydroarylation of styrenes with 2-phenylpyridine,5 which can produce either Markovnikov or anti-Markovnikov adducts in good selectivity by variation of the ligand. More importantly, as a more generally applicable method, the Hiyama and Miura groups reported Ni-catalyzed alkylation of aromatic or heteroaryl C−H bonds.6 Both electron-deficient fluoroaryl and heteroaryl C−H bonds can add to aryl olefins, affording Markovnikov adducts in moderate to excellent yields (eq 1). On the other hand, it is interesting to note that antiMarkovnikov adducts were obtained when alkyl-substituted alkenes were used as substrates (eq 2). The interesting regioselectivity of the Ni-catalyzed olefin hydroheteroarylation reaction prompts an understanding of its mechanism. In this direction, Hiyama et al.7 prepared compound 1a and examined its reaction with 2-vinylnaphthalene

(2b; see eq 3). They found that deuteration occurred at the terminal methyl group (Cβ-D), at the benzylic position (Cα-D), and even in the recovered substrate. Accordingly, Hiyama et al. proposed a plausible catalytic cycle which starts with reversible Ni-catalyzed oxidative addition of the Ar−H bond. This step is followed by coordination of the vinylarene, reversible hydronickelation, and irreversible C−C reductive elimination (Scheme 3). Note that a similar mechanism has also been proposed for Ni-catalyzed hydrocyanation8 and hydroalkynylation9 and Pd-catalyzed hydrosilylation10 of olefin derivatives. The proposed catalyzed cycle not only can explain the deuterium experiments but also indicates which step determines the regioselectivity. Because oxidative addition and hydronickelation are both reversible, intermediates D and E should reversibly convert into each other (Scheme 3). As a result, C−C reductive elimination is the regioselectivity-determining step. On the basis of this argument, a question then arises as to why C−C bond formation of benzoxazole with the substituted olefinic carbon is more favorable than that with the 4357

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energy corrections. For each saddle point, intrinsic reaction coordinate (IRC) analysis17 was performed to confirm that the saddle point connected the correct reactant and product on the potential energy surface. Single-point energy calculations were performed on the stationary points by using a larger basis set: i.e., SDD for Ni and 6-311+G(2d,p) for the other elements. The solvent effect (solvent n-hexane) was calculated by using the self-consistent reaction field method with the CPCM solvation model18 and UAHF radii. Natural bond orbital (NBO) calculations were performed using NBO version 3.1 in Gaussian 09.19 The reported energies are the gas-phase energies ΔGgas (single-point energies corrected by Gibbs free energy corrections). All the gas-phase free energies reported in this paper correspond to the reference state of 1 mol/L and 298 K. Note that by employing a similar computational method, Houk et al. recently studied the Ni-catalyzed Suzuki−Miyaura cross-coupling of aryl carbamates and sulfamates,20a Nakamura et al. studied the Ni-catalyzed cross-coupling of aryl halide with Grignard reagents,20b and Jones et al. studied the activation of the C−S and C−CN bonds of 2-cyanothiophene with [Ni(dippe)H]2.20c

Scheme 3. Proposed Mechanism of Ni-Catalyzed Olefin Hydroheteroarylation

3. RESULTS AND DISCUSSION 3.1. Model Chemistry. Before studying the mechanistic origin of regioselectivity, we first calculated the energy profile of Hiyama’s proposed mechanism to ensure that our computation can reproduce the experimental observations. The crosscouplings of styrene, 1-hexene, and benzoxazole, catalyzed by Ni[COD]2/IMes, are chosen as our model reactions to study the details of the proposed mechanism (Scheme 4). In the beginning we tried to optimize the intermediates and transition states in the solution (solvent n-hexane). However, many structures were not able to converge when optimized in solution. Furthermore, we compared ΔGgas (the free energy in the gas phase), ΔGsol (the free energy in solution for the geometry optimized in gas phase), and ΔG′ sol (the free energy in solution for the geometry optimized in solution) for transition states TS4 and TS5 (which are the most critical ones in the proposed catalytic cycle). Though ΔGsol and ΔG′sol are 2−4 kcal/mol higher than ΔGgas of the corresponding transition states, TS4 is always more stable than TS5 (for more details please see the Supporting Information, Table S1). Therefore, for practical reasons we decided to use the gas-phase optimized geometry and the gas-phase free energies in the present study. This choice is not only computationally less expensive but also legitimate in theory because the solvent of the present reaction (i.e., n-hexane) has a very low dipolar constant. 3.2. Mechanism for Styrene. 3.2.1. Oxidative Addition/ Hydronickelation/Reductive Elimination Mechanism. Before C−H bond activation, a number of Ni(0) complexes may exist in equilibrium through ligand exchange in solution.

unsubstituted olefinic carbon. This question is difficult to answer by employing the knowledge obtained from the studies on Ru- and Ir-catalyzed hydroarylation of olefins.11 As a result, in the present study we use computational methods to examine the regioselectivity of the Ni-catalyzed hydroheteroarylation reaction. Our results indicate that the secondary orbital overlap between the aryl group and Ni is a key factor in determining the regioselectivity in this system. Markovnikov adducts are obtained for aryl olefins because the corresponding reductive elimination step is accelerated due to the secondary orbital interaction. On the other hand, without this orbital interaction, anti-Markovnikov adducts are obtained for alkyl alkenes due to the steric effect of substituents on the olefin. Note that the Jones and Cundari12 groups previously reported computational studies on Ni-catalyzed Csp3−H activation processes, whereas the present study provides a rare example for the theoretical analysis of the Ni-catalyzed Csp2−H activation process.13

2. COMPUTATIONAL DETAILS All calculations were performed using the Gaussian09 suite of programs.14 Geometry optimizations were conducted without any constraints using the B3LYP method15 in the gas phase. The standard Pople all-electron basis set 6-31G(d) was used for all the atoms except for Ni atoms, which were described by the LANL2DZ basis set and the effective core potential implemented.16 Frequency analysis was conducted at the same level of theory to verify the stationary points to be real minima or saddle points and to get the thermodynamic

Scheme 4. Model Reactions

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Figure 1. Energy profile of hydroheteroarylation of styrene.

other hand, CP3 was not located successfully. An incomplete C−H bond between the hydrogen atom and Cter is automatically formed during the optimization of CP3, indicating that hydronickelation is facile. Following the coordination of styrene to Ni, hydronickelation occurs through the four-centered transition states TS2 and TS3, respectively. The free energies of TS2 and TS3 are +18.1 and +17.5 kcal/mol, which are close to each other.22 After hydronickelation, the alkylnickel intermediate CP6 is formed directly, whereas CP5 is formed with styrene rotating along the Ni−Cben bond to approach a more stable η3 binding mode. Benzylnickel intermediate CP5 with the coordination of the phenyl ring is +8.0 kcal/mol more stable than the σ-alkylcoordinated CP6. CP5 and CP6 undergo C−C reductive elimination via transition states TS4 and TS5, respectively. TS4 is more favored than TS5 by +1.1 kcal/mol in free energy.23,24 Therefore, the branched Markovnokov adduct is the main product, which is consistent with experiments. Note that the C−C reductive elimination is the rate-determining step and the total energy barrier is +23.4 kcal/mol.25 Through TS4 or TS5, branched or more stable linear products are formed, respectively. After reductive elimination, NiL combines a styrene and the catalytic cycle is completed. The reverse energy barrier of C−C reductive elimination is over +29 kcal/mol. Considering the low concentration of the activated catalyst, we must conclude that the reverse process of C−C reductive elimination should be very slow (i.e., “irreversible”). On the other hand, both the forward and reverse energy barriers of Ar−H oxidative addition and hydronickelation steps are much lower than that of C−C reductive elimination. Therefore, the Ar−H oxidative addition and hydronickelation steps are both reversible. This theoretical calculation explains the deuteration pattern observed by Hiyama et al. Note that in a previous DFT study on hydrocyanation of

By comparison of the energy of various Ni(0) complexes, we found that CP0, in which nickel is coordinated by an IMes and a styrene, was the most stable species (for more details please see the Supporting Information, Table S2). Thus, we set CP0 with benzoxazole as the zero point of free energy for the coupling of styrene with benzoxazole. The energies of the intermediates and transition states of the proposed mechanism (Scheme 3) are shown in Figure 1. From CP0, CP1 can be formed by coordination of benzoxazole to Ni to replace styrene. Then benzoxazole undergoes oxidative addition through the three-centered transition state TS1,21 which has an energy barrier of +14.9 kcal/mol (Figure 2). Subsequently, the three-coordinated nickel hydride CP2 is formed, in which benzoxazole is trans to IMes. Another oxidative addition transition state, TS1b, in which a styrene is still coordinated to Ni, was investigated. Because TS1b is 4.0 kcal/mol less stable than TS1, we excluded it for the mechanism of oxidative addition. Furthermore, we failed to locate the transition state of oxidative addition in which the hydrogen is cis to styrene. When conducting geometry optimization, we found that the target structure isomerizes to the transition state of concerted oxidative addition and hydronickelation: i.e., TS1c. IRC analysis indicates that the hydrogen atom transfers from benzoxazole to the CC bond of styrene while keeping bonded to the Ni atom during the whole process. TS1c is +7.6 kcal/mol less stable than TS1, and therefore, we also excluded TS1c for the mechanism of oxidative addition. After oxidative addition, the CC bond of styrene coordinates to Ni, forming the 16e complexes CP3 and CP4. The terminal carbon atom (marked as Cter) in CC is close to the hydrogen atom in CP3. The carbon atom at the benzylic position (marked as Cben) is close to the hydrogen atom in CP4. CP4 is less stable than CP2 by +5.7 kcal/mol. On the 4359

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Figure 2. Critical structures in the hydroheteroarylation of styrene. Note that some atoms far from the metal center are not shown or are blurred for clarity.

are found to be highly endothermic, and we excluded them for Ni-catalyzed hydroheteroarylation of olefins accordingly. 3.3. Mechanism for Alkyl-Substituted Olefins. With regard to alkyl-substituted alkenes, we use 1-hexene as the model substrate to study whether or not their regioselectivity is also determined by the C−C reductive elimination step. As shown in Figure 3, we found that CP1 is +0.2 kcal/mol more stable than the complex in which the Ni atom is coordinated by an IMes and a hexene. Thus, we set CP1 with hexene as the zero point of free energy for the coupling of hexene with benzoxazole. According to our results, the catalytic cycle also consists of reversible oxidative addition, hydronickelation, and irreversible C−C reductive elimination. Similar to the geometry optimization of CP3, an incomplete C−H bond between the hydrogen atom and the double bond of styrene is automatically formed when we tried to locate complexes CP8 and CP9. This phenomenon may indicate that hydronickelation is facile. The C−C reductive elimination is the rate-determining step, and the overall energy barrier is +21.1 kcal/mol. Anti-Markovnikov adducts are kinetically favored, which is consistent with the experimental observations. Therefore, the mechanism proposed by Hiyama et al. is also valid for Nicatalyzed hydroheteroarylation of alkyl-substituted olefins. 3.4. Mechanistic Origin of the Regioselectivity. The above computational results indicate that the Ar−H oxidative

styrene, Vogt et al. also indicated that hydrocyanation of styrene involves reversible hydronickelation and irreversible C−C reductive elimination.8h It should be pointed out that the triplet Ni0−NiII cycle for the oxidative addition/hydronickelation/reductive elimination mechanism was also examined. Computational results show that the triplet Ni0−NiII cycle is more energetically disfavored than the singlet Ni0−NiII cycle and no spin crossover property exists (for more details please see the Supporting Information, Figure S1). Accordingly we exclude the triplet Ni0−NiII cycle for the oxidative addition/hydronickelation/reductive elimination mechanism. 3.2.2. Radical Mechanism. Many Ni-catalyzed cross couplings are proposed to involve Ni(I) intermediates and alkyl radical if halogenated alkane is used as the coupling partner.26 Three radical pathways27 are examined for comparison with the oxidative addition/hydronickelation/reductive elimination pathway, and related results are shown in Scheme 5. One radical pathway involves single-electron transfer (SET) from the Ni0 complex to benzoxazole, resulting in the cleavage of the C−H bond of benzoxazole. An alternative radical pathway involves hydrogen atom transfer (HAT) from benzoxazole to the Ni0 complex, forming a Ni(H) complex. In contrast, the aryl group transfer (AGT) pathway involves the transfer of a benzoxazole radical to the Ni0 complex. All three pathways 4360

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Scheme 5. Free Energies of Intermediates in Radical Pathways for the Coupling of Benzoxazole with Styrenea

a

Gas-phase free energies and solvent-corrected free energies (in parentheses, solvent n-hexane) are given in kcal/mol.

Figure 3. Energy profile of hydroheteroarylation of 1-hexene.

this speculation, we conducted a calculation to examine the steric effect of alkyl substituents with different sizes (Table 1). When the steric hindrance of the alkyl group increases, there is nearly no difference in the energy of TS9. This may be explained by the fact that the alkyl group is located far from the Ni center in TS9. Thus, the steric effects of the alkyl group cannot affect the reaction center. This argument is supported by the fact that the bond lengths Ni−C1, Ni−C2, and C1−C2 in TS9 remain almost unchanged when the steric hindrance of the alkyl group increases (see Figure 4). On the other hand, the energy of TS8 increases dramatically with an increase in the steric hindrance of the alkyl group. In TS8, the alkyl group is located near the Ni center and causes a

addition/hydronickelation/C−C reductive elimination mechanism explains the Ni-catalyzed hydroheteroarylation of both aryl- and alkyl-substituted olefins. In the present section, we use the tools of computational chemistry to analyze what factors cause the different regioselectivities for aryl and alkyl substrates. 3.4.1. Mechanistic Origin of Anti-Markovnikov Regioselectivity for Alkyl Alkene. First, in the transition state of C−C reductive elimination, C−C bond formation will be sterically hindered if the approaching carbon atom is substituted with a bulky group. As a result, TS8 is less stable than TS9 because of the repulsion effect of the alkyl group on olefins, causing antiMarkovnikov adducts to become the major products. To confirm 4361

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formation of Markovnikov adducts through C−C reductive elimination. To further validate our speculation, we added groups with different electron-withdrawing abilities at the para position in styrene to see how the free energy barrier gap of TS4 and TS5 (i.e., ΔΔGgas = ΔGgas(TS5) − ΔGgas(TS4)) changes.29 As shown in Table 2, when the electron-withdrawing ability of the para-substituted group increases, the energy of TS5 does not change much. The reason is that the aryl group does not coordinate to the Ni atom in TS5. In addition, there is a methine carbon between the aryl group and Cter. Thus, the aryl group only slightly affects the electron density of Cter by an electron-withdrawing effect. Different from the case for TS5, the free energy of TS4 is significantly lowered when the electron-withdrawing ability of the para-substituted group is increased. This is understandable, in that the aryl group directly coordinates to Ni in TS4. Thus, the electronic properties of the aryl group have a more significant influence on the reactive center in TS4 in comparison to that in TS5. Note that the deuterium labeling study on the hydrocyanation of vinylarene also indicated that electron-withdrawing para aryl substituents increase the rate of C−C reductive elimination.8a Furthermore, we also analyzed the relationship between ΔΔGgas, the LUMO orbital energy of different para-substituted styrenes, and the charge on the Ni center in TS4. We found that when the LUMO orbital energy in TS4 decreases, ΔΔGgas will increase, corresponding to a higher Markovnikov regioselectivity (Figure 6). According to this result, we speculate that the secondary orbital overlap in TS4 acts as a bridge connecting the aryl group and Ni. The aryl group also acts as a π acceptor to promote the electron transfer from Ni to itself. In such a case, Ni receives more electrons transferred from benzoxazole and styrene. As a result, the C−C reductive elimination in TS4 is accelerated. Note that recent mechanistic studies on the reductive elimination of [Csp3−Pd−Csp3]30 and [Ar−Pd−Csp3]31 also indicate that reductive elimination can be accelerated by the coordination of a π acceptor (e.g., an electron-deficient olefin) to the metal center.32 When we examine whether or not such a secondary orbital overlap exists in the transition states of C−C reductive elimination for 1-hexane, no such orbital overlap exists between the butyl group and the Ni center in either TS8 or TS9 (see Figure 7).

Table 1. Free Energies (kcal/mol) of TS8/TS9 for Different Alkenes (ΔΔGgas = ΔGgas(TS8) − ΔGgas(TS9)) alkene

ΔGgas(TS8)

ΔGgas(TS9)

ΔΔGgas

MeCHCH2 (nBu)CHCH2 (iPr)CHCH2 (tBu)CHCH2

+25.3 +27.6 +28.9 +34.7

+21.3 +21.1 +19.7 +21.0

+4.0 +6.5 +9.2 +13.7

repulsion effect. This argument is supported by the fact that the Ni−C1 bond in TS8 becomes significantly longer with the increase of the steric hindrance of the alkyl group (2.090 Å in TS8-Me, 2.095 Å in TS8-nBu, 2.109 Å in TS8-iPr, and 2.187 Å in TS8-tBu). Due to the different orientations of the alkyl groups in TS8 and in TS9, steric effects barely affect the free energy of TS9 but significantly increase the free energy of TS8. Therefore, when the substituent group is more bulky, a greater anti-Markovnikov regioselectivity will be obtained for alkyl-substituted olefins. 3.4.2. Mechanistic Origin of Markovnikov Regioselectivity for Aryl Alkene. Like the alkyl group, an aryl group can also have steric effects. According to the A values that are derived from energy measurements of a monosubstituted cyclohexane ring, the Ph group is expected to be more bulky than iPr but less bulky than tBu (the A values for iPr, Ph, and tBu groups are 2.2, 2.8, and 4.7−4.9, respectively).28 If steric effects dominate the regioselectivity, anti-Markovnikov addition will be favored than Markovnikov addition by at least 9.2 kcal/mol for styrene according to the results in Table 1. In this context, it is surprising to note that for a styrene substrate TS4 actually has a lower energy than TS5, so that the Markovnikov adduct is kinetically favored. Therefore, there must be another factor promoting the formation of the Markovnikov adduct in TS4. Through analysis of the structures of TS4 and TS5, we find that the aryl group of styrene shows dramatically different orientations. In TS4, the aryl group together with Cben coordinates to the Ni atom in a η3 binding mode, whereas in TS5 the aryl group locates far from Ni. The HOMO and LUMO of TS4 have an overlap between Ni and the aryl group, whereas no such orbital overlap exists in TS5 (Figure 5). Thus, the coordination of the aryl group to Ni in TS4 causes a secondary orbital overlap, promoting the

Figure 4. Structures of TS8/TS9 for different alkenes. Note that some atoms far from the metal center are not shown or are blurred for clarity. 4362

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Figure 5. HOMO/LUMO profiles of TS4 and TS5.

expected to have a great influence on the regioselectivity. The acceleration effect resulting from a secondary orbital overlap between the aryl group and the metal center would be crippled if the aryl group coordinates to metal poorly. Then it is understandable that the use of 2-methylstyrene instead of styrene would show a worse Markovnikov regioselectivity for the Ru catalyst system.3b,4,33 Because the repulsion between 2-methylphenyl and the metal center is expected to be more significant than that between phenyl and the metal center, the interaction of 2-methylphenyl with metal is weaker than that of phenyl. Accordingly, the acceleration effect resulting from the secondary orbital overlap between the aryl group and metal center is crippled. At the same time, the greater steric effect of the 2-methylphenyl group further hinders the C−C bond formation between acetophenone and the substituted carbon of the olefin. Because of these two reasons, Ru-catalyzed

Table 2. Free Energies (kcal/mol) of TS4/TS5 for Different Para-Substituted Styrenes (ΔΔGgas = ΔGgas(TS5) − ΔGgas(TS4)) vinylarene

ΔGgas(TS4)

ΔGgas(TS5)

ΔΔGgas

PhCHCH2 (p-F-C6H4)CHCH2 (p-CF3-C6H4)CHCH2 (p-NO2-C6H4)CHCH2

+23.4 +23.1 +22.0 +19.7

+24.5 +24.6 +23.7 +22.9

+1.1 +1.4 +1.7 +3.1

Thus, no acceleration effect resulting from the secondary orbital overlap exists in TS8 or TS9. In such a case, the less sterically crowded TS9 becomes more energetically favorable than TS8, causing anti-Markovnikov adducts to become the major products. 3.5. Effects of Substrates and Coordination Environment of Metal on Regioselectivity. According to our proposition, the coordination of an aryl group to metal is

Figure 6. Relationship of the free energy gap of TS4 and TS5, ΔΔGgas: (black squares) Mulliken charges of Ni in TS4; (red circles) LUMO energies of TS4. 4363

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Figure 7. HOMO/LUMO profiles of TS8 and TS9.

hydroarylation of 2-methylstyrene only afforded anti-Markovnikov products. On the other hand, it is interesting that Ni-catalyzed hydroheteroarylation of 2-methoxystyrene still affords a Markovnikov adduct as the major product.6c This phenomenon may stem from the unique coordination environment of the Ni catalyst. In the reductive elimination transition state, Ni is coordinated by only one IMes ligand. The substrates coordinate to the metal center in a direction perpendicular to the aryl arm of the IMes ligand (see Figure 2). Vinylarenes are far from the IMes ligand; thus, no significant repulsion of them will be induced when 2-methoxystyrene coordinates to the metal center in an η3 binding mode. It is interesting to note that, in the presence of the more bulky ligand IPr (1,3-(2,6-diisopropylphenyl)imidazol-2-yl-idene), the yield for coupling of 1a with styrene sharply decreases to 2%. To test the tolerance capacity of the Ni/IMes catalyst for bulky substrates in the aspect of regioselectivity, the energy barriers of reductive elimination of some alkyl-substituted styrene have been computed (Table 3). In comparison with styrene, o-methyl and o-isopropyl substitutions of styrene only slightly decrease the energy gap of the two reductive elimination transition states, leading to different regioselective products. This is difficult to achieve using Ru catalysts, as we have mentioned earlier. The excellent Markovnikov selectivity is also maintained for both m- and p-tert-butyl-substituted styrenes. Only when the volume of ortho substitution (o-tert-butyl and o-dimethyl substitutions) of styrene is further increased is there a significant decrease of Markovnikov selectivity for the Ni/IMes catalyst. The above computational results show that the Ni/IMes catalyst system has enough coordination space for a wide scope of vinylarenes to give Markovnikov products. We may interpret the difference in regioselectivity between Ru- and Ni-catalyzed systems by the following explanation: a less coordinated catalyst is expected to be less sensitive to the steric effects of substituents on the olefin for Markovnikov addition. Such a system

Table 3. Free Energy Barriers of Reductive Elimination (from Benzylnickel(II) Complexes to the C−C Reductive Elimination Transition State, in kcal/mol) for Different Alkyl-Substituted Styrenesa vinylarene

ΔGMar

ΔGanti‑Mar

ΔGselectivity

PhCHCH2 (o-Me-C6H4)CHCH2 (o-iPr-C6H4)CHCH2 (o-tBu-C6H4)CHCH2 (m-tBu-C6H4)CHCH2 (p-tBu-C6H4)CHCH2 (2,6-Me2C6H3)CHCH2

+12.6 +13.2 +13.6 +16.3 +10.4 +13.1 +11.4

+21.4 +20.9 +21.6 +21.2 +20.5 +22.0 +16.3

+8.7 +7.7 +8.0 +4.9 +10.1 +8.9 +5.0

a

The energy barriers of Markovnikov and anti-Markovnikov selectivity are denoted ΔGMar and ΔGanti‑Mar, respectively. ΔGselectivity = ΔGanti‑Mar − ΔGMar. Note that the M06 method was employed for single-point energy calculations because the B3LYP method greatly underestimated the energetic difference between the reductive elimination transition states for different regioselectivity.

has the potential to be used to transform more bulky substrates into Markovnikov adducts. In contrast, a more coordinated catalyst is expected to be more sensitive to the steric effects of substrates for Markovnikov addition.

4. CONCLUSION To understand the mechanistic origin of regioselectivity in Nicatalyzed olefin hydroheteroarylation, we conducted density functional theory calculations on the reactions of benzoxazole with styrenes and alkyl olefins. We found that a secondary orbital overlap between the aryl group of vinylarenes and Ni exists in the C−C reductive elimination transition state. This secondary orbital interaction can accelerate reductive elimination and is proposed to be the key factor leading to Markovnikov regioselectivity for vinylarenes. On the other hand, there is no such secondary orbital overlap between the alkyl group and Ni in the C−C reductive elimination transition 4364

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2006, 128, 11374. (e) Flanagan, S. P.; Guiry, P. T. J. Organomet. Chem. 2006, 691, 2125. (f) Göthlich, A. P. V.; Tensfeldt, M.; Rothfuss, H.; Tauchert, M. E.; Haap, D.; Rominger, F.; Hofmann, P. Organometallics 2008, 27, 2189. (g) RajanBabu, T. V.; Casalnuovo, A. L. Pure App. Chem. 1994, 66, 1535. (h) Bini, L.; Pidko, E. A.; Müller, C.; van Santen, R. A.; Vogt, D. Chem. Eur. J. 2009, 15, 8768. (9) Shirakura, M.; Suginime, M. Org. Lett. 2009, 11, 523. (10) (a) Gibson, S. E.; Rudd, M. Adv. Synth. Catal. 2007, 349, 781. (b) Hayashi, T.; Hirate, S.; Kitayama, K.; Tsuji, H.; Torii, A.; Uozumi, Y. J. Org. Chem. 2001, 66, 1441. (11) Some studies on the mechanism of olefin hydroarylation catalyzed by different Ir and Ru catalytic systems indicate that insertion of olefins is the regioselecivity-determining step. For more details please see: (a) Oxgaard, J.; Muller, R. P.; Goddard, W. A., III; Periana, R. A. J. Am. Chem. Soc. 2004, 126, 352. (b) Lail, M.; Bell, C. M.; Conner, D.; Cundari, T. R.; Gunnoe, T. B.; Petersen, J. L. Organometallics 2004, 23, 5007. (12) (a) Atesin, T. A.; Li, T.; Lachaize, S.; Brennessel, W. W.; Garcia, J. J.; Jones, W. D. J. Am. Chem. Soc. 2007, 129, 7562. (b) Li, T.; Jones, W. D. Organometallics 2011, 30, 547. (c) Cundari, T. R.; Pierpont, A. W.; Vaddadi, S. J. Organomet. Chem. 2007, 692, 4551. (d) Cundari, T. R.; Jimenez-Halla, J. O. C.; Morello, G. R.; Vaddadi, S. J. Am. Chem. Soc. 2008, 130, 13051. (e) Pierpont, A. W.; Cundari, T. R. Inorg. Chem. 2010, 49, 2038. (13) Recently Perutz et al.'s study supported a different C−H activation mechanism called ligand-to-ligand hydrogen transfer (LLHT) for Ni-catalyzed Csp2-H activation process; see Guihaume, J.; Halbert, S.; Eisenstein, O.; Perutz, R. N. Organometallics 2012, 31, 1300. (14) Frisch, M. J., et al. Gaussian 09, revision B01; Gaussian, Inc., Wallingford, CT, 2010. (15) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (16) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (17) (a) Fukui, K. J. Phys. Chem. 1970, 74, 4161. (b) Fukui, K. Acc. Chem. Res. 1981, 14, 363. (18) (a) Cramer, C. J.; Truhlar, D. G. Chem. Rev. 1999, 99, 2161. (b) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999. (c) Fu, Y.; Liu, L.; Li, R.-Q.; Liu, R.; Guo, Q.-X. J. Am. Chem. Soc. 2004, 126, 814. (d) Qi, X.-J.; Fu, Y.; Liu, L.; Guo, Q.-X. Organometallics 2007, 26, 4197. (19) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO Version 3.1. (20) (a) Quasdorf, K. W.; Antoft-Finch, A.; Liu, P.; Silberstein, A. L.; Komaromi, A.; Blackburn, T.; Ramgren, S. D.; Houk, K. N.; Snieckus, V.; Garg, N. K. J. Am. Chem. Soc. 2011, 133, 6352. (b) Yoshikai, N.; Matsuda, H.; Nakamura, E. J. Am. Chem. Soc. 2008, 130, 15258. (c) Grochowski, M. R.; Li, T.; Brennessel, W. W.; Jones, W. D. J. Am. Chem. Soc. 2010, 132, 12412. (21) Similar transition state structures of oxidative addition were reported: (a) Butschke, B.; Schwarz, H. Organometallics 2011, 30, 1588. (b) Phapale, V. B.; Guisan-Geinos, M.; Bunuel, E.; Cardenas, D. J. Chem. Eur. J. 2009, 15, 12681. (c) Li, Z.; Fu, Y.; Guo, Q.-X.; Liu, L. Organometallics 2008, 27, 4043. (d) Li, Z.; Fu, Y.; Zhang, S.-L.; Guo, Q.-X.; Liu, L. Chem. Asian J. 2010, 6, 1475. (22) The geometry optimization of TS2, TS6, and TS7 failed to converge. Their free energies fluctuate in a range of ±0.2 kcal/mol. We directly conducted frequency analysis and other related energy computations on the energetically fluctuating structures as a last resort. Their energies are perhaps overestimated. (23) The presented computational method underestimated the energetic difference between TS4 and TS5 because the Markovnikov addition is exclusive. Despite this deficiency, our computation result is consistent with the mechanism (Scheme 3) that is supported by experiments. Thus, we believe that our computational method is sufficiently accurate to qualitatively study the mechanistic origin of regioselectivity that we focus on in this study.

state for alkyl-substituted olefins. As a result, sterically less hindered anti-Markovnikov addition becomes kinetically favored for the alkyl olefins. Our study presents a rare example for the theoretical analysis of the Ni-catalyzed Csp2−H activation process. Our results reveal the unique features of the Ni/IMes catalyst system and may provide useful insights into the regioselectivity of olefin hydroarylation catalyzed by other transition metals.

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ASSOCIATED CONTENT

* Supporting Information S

Text giving the complete ref 14 and tables giving additional details of the energy profiles and Cartesian coordinates. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (Grant Nos. 20932006, 20902088) and the Fundamental Research Funds for the Central Universities (Grant No. WK2060190008). We also thank the USTC and Shanghai Supercomputer Center for technical support.

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REFERENCES

(1) (a) Dyker, G. Angew. Chem., Int. Ed. 1999, 38, 1698. (b) Kakiuchi, F.; Chatani, N. Adv. Synth. Catal. 2003, 345, 1077. (c) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. Rev. 2002, 102, 1731. (d) Murai, S.; Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Kamatani, A.; Sonoda, M.; Chatani, N. Nature 1993, 366, 529. (e) Jia, C. G.; Piao, D. G.; Oyamada, J.; Lu, W. J.; Kitamura, T.; Fujiwara, Y. Science 2000, 287, 1992. (2) (a) Zhang, Z. B.; Wang, X.; Widenhoefer, R. A. Chem. Commun. 2006, 3717. (b) Wang, M. Z.; Wong, M. K.; Che, C. M. Chem. Eur. J. 2008, 14, 8353. (c) Sun, H. B.; Li, B.; Hua, R. M.; Yin, Y. M. Eur. J. Org. Chem. 2006, 4231. (d) Kischel, J.; Jovel, I.; Mertins, K.; Zapf, A.; Beller, M. Org. Lett. 2006, 8, 19. (e) Rueping, M.; Nachtsheim, B. J.; Scheidt, T. Org. Lett. 2006, 8, 3717. (3) (a) Matsumoto, T.; Periana, R. A.; Taube, D. J.; Yoshida, H. J. Mol. Catal. A 2002, 180, 1. (b) Martinez, R.; Genet, J. P.; Darses, S. Chem. Commun. 2008, 3855. (c) Periana, R. A.; Liu, X. Y.; Bhalla, G. Chem. Commun. 2002, 3000. (4) Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Kamatani, A.; Sonoda, M.; Chatani, N.; Murai., S. Bull. Chem. Soc. Jpn. 1995, 68, 62. (5) Gao, K.; Yoshikai, N. J. Am. Chem. Soc. 2011, 133, 400. (6) (a) Mukai, T.; Hirano, K.; Satoch, T.; Miura, M. J. Org. Chem. 2009, 74, 6410. (b) Nakao, Y.; Kashihara, N.; Kanyiva, K. S.; Hiyama, T. J. Am. Chem. Soc. 2008, 130, 16170. (c) Nakao, Y.; Kashihara, N.; Kanyiva, K. S.; Hiyama, T. Angew. Chem., Int. Ed. 2010, 49, 4451. (7) Yoshikai et al. also reported a deuteration experiment to study the mechanism of Co-catalyzed hydroarylation of styrenes. This reaction is also proposed to proceed through oxidative addition, insertion of styrene into the Co−H bond, and reductive elimination. C−C reductive elimination is also proposed to be the rate-determining step. For more details please see ref 5. (8) (a) Casalnuovo, A. L.; RajanBabu, T. V.; Ayers, T. A.; Warren, T. H. J. Am. Chem. Soc. 1994, 116, 9869. (b) Bini, L.; Müller, C.; Vogt, D. ChemCatChem 2010, 2, 590. (c) Burello, E.; Marion, P.; Galland, J.; Chamard, A.; Rothenberg, G. Adv. Synth. Catal. 2005, 347, 803. (d) Wilting, J.; Janssen, M.; Müller, C.; Vogt, D. J. Am. Chem. Soc. 4365

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Organometallics

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(24) To compare the feasibility of different pathways, we used the energetic span model developed by Kozuch and Shaik, which considers the effect of both the energy barrier of the elementary reaction and the concentration of the substrates. For more details please see: (a) Kozuch, S.; Shaik, S. Acc. Chem. Res. 2011, 44, 101. (b) Zhang, S.-L.; Liu, L.; Fu, Y.; Guo, Q.-X. Organometallics 2007, 26, 4546. (25) Note that our energies reported in this paper correspond to the reference state of 1 mol/L. Considering that the concentration of the activated catalyst is significantly lower than 1 mol/L, the computed energy barrier is normally lower than the actual energy barrier. (26) (a) Yao, T.; Hirano, K.; Satoh, T.; Miura, M. Chem. Eur. J. 2010, 16, 12307. (b) Zhou, J.; Fu, G. C. J. Am. Chem. Soc. 2004, 126, 1340. (c) Powell, D. A.; Maki, T.; Fu, G. C. J. Am. Chem. Soc. 2005, 127, 510. (d) González-Bobes, F.; Fu, G. C. J. Am. Chem. Soc. 2006, 128, 5360. (e) Saito, B.; Fu, G. C. J. Am. Chem. Soc. 2007, 129, 9602. (f) Lin, X.; Phillips, D. L. J. Org. Chem. 2008, 73, 3680. (g) Anderson, T. J.; Jones, G. D.; Vicic, D. A. J. Am. Chem. Soc. 2004, 126, 8100. (h) Yu, X.; Yang, T.; Wang, S.; Xu, H.; Gong, H. Org. Lett. 2011, 13, 2138. (i) Li, Z.; Zhang, S.-L.; Fu, Y.; Guo, Q.-X.; Liu, L. J. Am. Chem. Soc. 2009, 131, 8815. (j) Lin, B.-L.; Liu, L.; Fu, Y.; Luo, S.-W.; Chen, Q.; Guo, Q.-X. Organometallics 2004, 23, 2114. (27) Jones, G. O.; Liu, P.; Houk, K. N.; Buchwald, S. L. J. Am. Chem. Soc. 2010, 132, 6205. (28) Eliel, E. L.; Wilen, S. H.; Mander, L. N. In Stereochemistry of Organic Compounds; Wiley: New York, 1994. (29) Note that CP0 with benzoxazole is less stable than the Ni(0) complexes coordinated by more electron-deficient styrene with benzoxazole but more stable than η3-benzyl Ni(II) complexes. We set CP0 with benzoxazole as the reference point to examine the effects of para substitutents in styrene on regioselectivity, which we focus on in this study. (30) Perez-Rodriguez, M.; Braga, A. A. C.; Garcia-Melchor, M.; Perez-Temprano, M. H.; Casares, J. A.; Ujaque, G.; de Lera, A. R.; Alvarez, R.; Maseras, F.; Espinet, P. J. Am. Chem. Soc. 2009, 131, 3650. (31) Zhang, H.; Luo, X.; Wongkhan, K.; Duan, H.; Li, Q.; Zhu, L.; Wang, J.; Batsanov, A. S.; Howard, J. A. K.; Marder, T. B.; Lei, A. Chem. Eur. J. 2009, 15, 3823. (32) In addition to the π system of an aryl group or vinyl group, the coordination of the oxygen of an acyl group to metal in the transition state is also expected to facilitate reductive elimination; see: Hartwig, J. F. In Organotransition Metal Chemistry: From Bonding to Catalysis; Wiley-VCH: Weinheim, Germany, 2010. (33) Murai et al. proposed that the Ru-catalyzed olefin hydroarylation proceeds via the same mechanism: i.e., reversible Ar−H oxidative addition, hydrometalation, and irreversible C−C reductive elimination, which is supported by deuterium labeling experiment (please see ref 4 for more details).

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