Computational Insight into the Mechanism of Nickel-Catalyzed

Article pubs.acs.org/Organometallics

Computational Insight into the Mechanism of Nickel-Catalyzed Reductive Carboxylation of Styrenes using CO2 Ruming Yuan†,‡ and Zhenyang Lin*,† †

Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China ‡ Department of Chemistry, Xiamen University, Xiamen 361005, People’s Republic of China S Supporting Information *

ABSTRACT: DFT calculations have been carried out to study the detailed mechanisms for the nickel-catalyzed reductive carboxylation of estersubstituted styrenes H2CCHAr using CO2 to form α-carboxylated products. Two possible mechanisms, the oxidative coupling mechanism and the nickel hydride mechanism, were calculated and compared. Our calculations show that, for the oxidative coupling mechanism, a metallacycle thermodynamic sink is generated from oxidative coupling between CO2 and a styrene substrate molecule on the nickel(0) metal center, which should be avoided in order for smooth reductive carboxylation of styrenes. For the nickel hydride mechanism, a nickel hydride species is the active species, from which styrene insertion into the Ni−H bond followed by reductive elimination produces the α-carboxylated product. Calculations show that either of these two steps (insertion and reductive elimination) can be the rate-determining step, and both transition states are only slightly more stable than the oxidative coupling transition state leading to the thermodynamic sink. Because of the competitive nature between the two mechanisms, the reaction conditions and other factors (substituent, pressure, and ligand) significantly affect the reaction outcome, all of which have been discussed in detail.

■

INTRODUCTION As an abundant renewable carbon source, carbon dioxide is an attractive C1 building block in synthetic organic chemistry.1−3 However, its high stability and low reactivity are the biggest challenge to the development of useful chemical reactions involving carbon dioxide. Among the reported reactions, carboxylation of unsaturated organic compounds catalyzed by low-valent nickel complexes has been well investigated. Despite this, the Ni-catalyzed carboxylation reactions investigated have been largely limited to the reactions of relatively active substrates, such as dienes,4 diynes,5 and alkynes.6 Common alkenes usually exhibit low reactivity in carboxylation reactions, and studies of their carboxylation reactions are rather limited.7,8 Recently, Rovis and co-workers reported nickel-catalyzed carboxylation of styrenes under an atmospheric pressure of CO2 and at room temperature (23 °C) (eq 1).8 Remarkably,

reactions are highly regioselective. All of these experimental results are interesting and require an in-depth understanding. In this paper, we report our detailed DFT calculations on the reaction mechanisms in order to gain insight into the interesting experimental observations (substituent effect, ligand effect, and regioselectivity).

■

styrenes bearing electron-withdrawing substituent(s) (such as −COOMe) on the phenyl ring can be catalyzed by the nickel(0) complex Ni(COD)2 to give α-carboxylated products using Et2Zn as the reducing agent or the hydride source in the presence of additives such as DBU and pyridine. It was also found that when the substituent R in eq 1 is a hydrogen atom, no desired product was observed. Furthermore, in the reactions, only a single regioisomer, the α-carboxylated product, was experimentally observed, indicating that the carboxylation © XXXX American Chemical Society

COMPUTATIONAL DETAILS

The well-established three-parameter hybrid functional B3LYP, which uses the Becke88 exchange functional in combination with the Lee− Yang−Parr correlation functional, was used in all of our calculations.9 In the B3LYP calculations, the nickel atom was treated with Hay’s effective core potential and the corresponding valence basis functions (Lanl2DZ).10 For oxygen atoms, the 6-311+G (d) basis set was used, while the other atoms (Zn, C, H, N) were described by the 631G(d,p) basis set.11 In addition, the polarizable continuum model (PCM) was chosen to account for the solvent effect. 12 In correspondence with the experimental conditions, tetrahydrofuran (THF) was adopted as the solvent. All of the structures were fully optimized in THF solution and visualized using the xyzviewer software developed by de Marothy.13 Vibrational frequency calculations at the same level of theory were performed to verify that a local minimum has no imaginary frequency and each transition state has only one single imaginary frequency. Intrinsic reaction coordinate (IRC) calculations were also performed to make sure the transition states indeed connect two relevant minima.14 To reduce the overestimation of the entropy contribution of the results, we employed a correction of −2.6 (or 2.6) kcal/mol for 2:1 (or 1:2) transformations, as many earlier theoretical studies did.15 Unless specifically mentioned, energies Received: September 23, 2014

A

dx.doi.org/10.1021/om500962a | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

reported here are entropy-corrected free energies at 298 K. Natural bond orbital (NBO) analyses were performed using the NBO program as implemented in the Gaussian software packages.16 All of the DFT calculations were carried out with the Gaussian 09 program.17 We also performed single-point energy calculations for some key intermediates and transition states using a larger basis set (6-311+ +G(d,p)) and optimized these structures using the dispersioncorrected DFT methods B3LYP-D3 and M06. The results of these additional calculations are presented in Table S1 in the Supporting Information. These results show that when the large basis set 6-311+ +G(d,p) was used, the differences in the barriers among different reaction pathways did not change significantly. The results from the B3LYP-D3 and M06 calculations show that the dispersion-corrected DFT methods somehow significantly overestimate the stability of the oxidative coupling transition states. According to the M06 and B3LYPD3 results, the metallacycle thermodynamic sink (generated from alkyne−CO2 oxidative coupling) would be very favorably formed, and the catalytic reactions would not be possible. In one of our earlier works,18 we also found that the dispersion-corrected DFT methods significantly overestimate the stabilities of C−C coupling transition states. Therefore, the dispersion-corrected DFT methods would not seem to be applicable to the system currently studied.

Scheme 2. Proposed Oxidative Coupling Mechanism

■

RESULTS AND DISCUSSION It is well-known that alkenes can readily insert into a metal− hydride bond to form a metal−alkyl species, which may offer a chance to facilitate the CO2 fixation by CO insertion to the metal−alkyl bond to achieve formation of a new C−C bond.19,20 On the basis of this notion, Rovis and co-workers proposed a reaction mechanism by considering the nickel hydride species B as the active species (Scheme 1), which can

Combining the possible mechanisms illustrated in Schemes 1 and 2, we present here Scheme 3, summarizing the various mechanistic possibilities discussed above, for our theoretical study. Scheme 3 considers an equilibrium between a CO2coordinated species and a styrene-coordinated species, which links the nickel hydride mechanism (the catalytic cycle on the left-hand side) and the oxidative coupling mechanism (the catalytic cycle on the right-hand side). For convenience of discussion, we will first examine the oxidative coupling mechanism. Oxidative Coupling Mechanism. We first used the experimentally employed ester-substituted styrene H2C CHAr (Ar = C6H4-p-COOMe) as the styrene substrate molecule in our theoretical calculations. Figure 1a shows the free energy profiles calculated for the nickel-mediated oxidative coupling of styrene with CO2 as well as the energetic relationship between the CO2-coordinated species 1 and the styrene-coordinated species 3. Figure 1b shows the free energy profiles calculated for D → E on the basis of the catalytic cycle shown on the right-hand side of Scheme 3. Figure 2 gives the structures calculated for the relevant transition states along the energy profiles shown in Figure 1. The steric crowdedness would prevent side-on η3 type binding on DBU. Calculations on hypothetical η3 species of Ni containing CO2 have been carried out. It is found that (η3-DBU)2Ni(CO2), (η3-DBU)Ni(CO2) + BDU, and (η3-DBU)(DBU)Ni(CO2) are higher in free energy by 36.0, 28.9, and 21.6 kcal/mol, respectively, than (DBU)2Ni(CO2) containing monodentate DBU ligands. Thus, only monodentate DBU is considered. From Figure 1a, we can see that the CO2-coordinated species 1 is slightly more stable by 3.0 kcal/mol than the styrenecoordinated species 3, consistent with an early report that CO2 binds Ni(mDBU)2 preferentially by 1.7 kcal/mol in comparison to ethylene.22 The stability of the Ni0−CO2 coordination is also manifested by the X-ray crystal structure of [Ni(PCy3)2(η2CO2)].24 Here, one may ask if it is possible to have an adduct between Et2Zn and Ni(DBU)2 to initiate the reaction. We calculated the binding free energies of Ni(DBU)2 with CO2 (−15.9 kcal/mol) and Et2Zn (−0.1 kcal/mol) and found that CO2 binds much more strongly with Ni(DBU)2 than does Et2Zn. In addition, a reaction free energy of −9.7 kcal/mol was calculated for Ni(COD)2 + CO2 + 2 DBU → (DBU)2Ni(CO2)

Scheme 1. Proposed Nickel Hydride Mechanism

be formed from the nickel ethyl species A by β-hydride elimination. Then this active species B reacts with a styrene molecule and CO2 sequentially to give the nickel carboxylate intermediate C. Finally, transmetalation between the nickel carboxylate intermediate C and Et2Zn occurs to give an αcarboxylated product and regenerate the nickel ethyl species A.8 It is also well-known that Ni(0) complexes can mediate oxidative coupling of alkenes with CO2.7,21,22 Therefore, an alternative mechanism involving oxidative coupling is possible. Scheme 2 shows an alternative possible mechanism in which the five-membered-ring metallacycle D is first formed, from which transmetalation occurs to give the species E, containing two alkyl ligands. This bis(alkyl) species E could undergo either β-hydride elimination and then reductive elimination to give the α-carboxylated product (experimentally observed) or direct reductive elimination to give the alkylative carboxylated product (not observed experimentally).8,23 B


Organometallics

Article

Scheme 3. Detailed Catalytic Cycles Considering Various Mechanistic Possibilities

cannot account for the nickel-catalyzed reductive carboxylation of styrenes, in view of the mild reaction conditions used in the experiment (an atmospheric pressure of CO2 and 23 °C). Furthermore, the regioselectivity is also wrongly predicted if the oxidative coupling mechanism is operative. From Figure 1, we can also conclude that the metallacycles 4 and 5 are thermodynamic sinks, and once they are formed, the reactions would likely stop because the reverse steps (decoupling) would not occur easily in view of the fact that the reverse free energy barriers (via TS2‑4 and TS2‑5) are greater than 30.0 kcal/mol. In other words, oxidative coupling should be avoided in order for smooth reductive carboxylation of styrenes. Nickel Hydride Mechanism. Now, let us examine the energetic aspect associated with the nickel hydride mechanism. Figure 3 shows the free energy profile for the nickel hydride mechanism on the basis of the catalytic cycle shown on the lefthand side of Scheme 3. Figure 4 gives the structures calculated for the relevant transition states along the energy profiles shown in Figure 3. In the nickel hydride mechanism, the first step in Scheme 3 is transmetalation between the CO2-coordinated species and Et2Zn(THF) to give a square-planar nickel(II) ethyl species. Similarly to what we have seen in the oxidative coupling mechanism, Et2Zn is more reactive than Et2Zn(THF) in the transmetalation process. As shown in Figure 3, the free energy barrier for the transmetalation process (1 → 16) was calculated to be 16.3 kcal/mol, which is noticeably lower than the barrier calculated for the oxidative coupling step presented in Figure 1a. Furthermore, we also see that transmetalation between the CO2-coordinated species 1 and Et2Zn(THF) is energetically less demanding than that between the metallacycle 4 or 5 and Et2Zn(THF). This is because the Ni−O bonding interaction in the CO2-coordinated species 1 is weaker than that in the metallacycle 4 or 5.

+ 2 COD. All of these results support the notion that (DBU)2Ni(CO2) is the starting species to initiate the reaction. As shown in Figure 1a, the CO2-coordinated species 1 and the styrene-coordinated species 3 are interconnected by the tetracoordinated intermediate 2, in which both CO2 and styrene are coordinated to the Ni(0) metal center. Our calculation results show that oxidative coupling occurs via the tetracoordinated intermediate 2. Since styrene is an asymmetric alkene, two pathways for the oxidative coupling are possible. Our calculation results (Figure 1a) show that CO2 preferentially couples with the styrene β-carbon over the styrene αcarbon. The pathway coupling with the styrene β-carbon (via TS2‑5) has an overall free energy barrier of 21.0 kcal/mol, which is 8.0 kcal/mol lower than the pathway coupling with the styrene α-carbon (via TS2‑4). Figure 1b shows the energy profiles calculated for the immediately following transmetalation between the metallacycle intermediates (4 and 5, derived from the oxidative coupling) and Et2Zn(THF). We used Et2Zn(THF) in our calculations because we found that Et2Zn(THF) is thermodynamically more stable (by 2.0 kcal/mol in free energy) than Et2Zn + THF. In order for the metallacycle intermediates 4 and 5 to interact with the transmetalating reagent Et2Zn(THF), it is reasonable to assume that dissociation of one DBU ligand occurs first. Furthermore, we also found that Et2Zn is more reactive than Et2Zn(THF) in the transmetalation process. Therefore, dissociation of THF from Et2Zn(THF) is also considered in our calculations (Figure 1b). The energy profiles for those less favorable transmetalation pathways, which do not consider dissociation of DBU and/or THF, are given in the Supporting Information. The results shown in Figure 1b indicate that the barriers calculated for the transmetalation processes are particularly high (32.8 kcal/mol via TS10‑12 and 33.4 kcal/mol via TS11‑13). These results imply that the oxidative coupling mechanism C


Organometallics

Article

Figure 1. (a) Free energy profiles calculated for the nickel-mediated oxidative coupling of styrene with CO2 as well as the energetic relationship between the CO2-coordinated species 1 and the styrene-coordinated species 3. (b) Free energy profiles calculated for D → E on the basis of the catalytic cycle shown on the right-hand side of Scheme 3. The relative free energies are given in kcal/mol.

resulting in hydride migration to the styrene β-carbon via the four-membered-ring transition state TS18‑20 to give the benzyl nickel species 20 is more favorable than the insertion resulting in hydride migration to the styrene α-carbon via TS18‑19 to give 19. The regioselectivity in the insertion seems closely related to the relative stability between 20 and 19. An NBO analysis for the benzyl nickel species 20 shows that the second-order perturbation energy accounting for the hyperconjugation interaction between the Ni−C σ bond and one π* orbital of the phenyl ring in 20 is indeed very large, 33.7 kcal/mol, explaining the higher stability of 20 versus 19.

On the basis of the nickel hydride mechanism, the next step is β-hydride elimination (Scheme 3). In order for β-hydride elimination to occur, dissociation of one DBU ligand is necessary. Via TS16‑17, dissociation of one DBU ligand from 16 forms the three-coordinated, T-shaped nickel(II) ethyl species 17 (Figure 3). From 17, β-hydride elimination occurs via the four-membered-ring transition state TS17‑18 to give the nickel hydride species 18. From 18, insertion of the styrene substrate into the Ni−H bond occurs. Due to the asymmetry of styrene, two insertion modes are possible. As shown in Figure 3, the insertion D


Organometallics

Article

reaction outcome can be easily manipulated/affected by varying the substituent groups on the styrene substrate. In the next section, we will focus our study on how different substituents affect the relative stabilities of the styrene insertion transition state TS18‑20 and the reductive elimination transition state TS20‑1 versus the oxidative-coupling transition state TS2−5. Reactions of Different Styrene Substrates Bearing Different Substituents. Experimentally, reaction of the nonsubstituted styrene substrate H2CCHPh results in no carboxylated product. It was also found that styrenes bearing electron-withdrawing substituents with positive Hammett σ values undergo reductive carboxylation reactions very efficiently, while styrenes bearing electron-donating substituents with negative Hammett σ values generally fail to produce the desired products. To understand the effect of substituents on the reaction outcomes, we performed calculations and compared relative stabilities of the styrene insertion transition states TS18‑20 and the reductive elimination transition states TS20‑1 versus the oxidative-coupling transition states TS2−‑5 for various styrene substrates bearing different substituents: R = H, NH2, OCH3, CF3, CN. Table 1 shows the results of our calculations. In the table, the relative free energies of the metallacycle intermediates 5 and the transmetalation transition states TS11‑13 (Figure 1) in the oxidative coupling mechanism for different styrene substrates are also given for readers to better understand the depths of the thermodynamic sinks. The calculation results clearly show that electron-donating substituents, including R = H, will make the oxidative coupling possible because the relevant oxidative coupling transition states TS2‑5 lie lower than the respective rate-determining transition states TS20‑1 (corresponding to reductive elimination) in the hydride mechanism. In other words, for the reactions of styrenes bearing electron-donating substituents including R = H, the thermodynamic sinks cannot be avoided, and therefore no catalysis can be achieved. The calculation results show that the electron-withdrawing substituents (CF3, CN, and COOMe) lower the relative free energies of TS2‑5, TS18‑20, and TS20‑1 significantly, especially for TS20‑1, which makes the nickel hydride mechanism favorable, leading to the carboxylated products, and avoids the occurrence of oxidative coupling. A plausible explanation can be offered here to understand why electron-withdrawing substituents lower the relative free energies of TS18‑20 and TS20‑1 significantly. TS18‑20 is the transition state for styrene insertion into Ni−H (Figure 3). An electron-poor styrene promotes the nucleophilic attack of the hydride ligand on the styrene substrate, facilitating the hydride migration. TS20‑1 is the transition state for the reductive elimination which couples the CHArMe ligand with the COOZnEt(THF) ligand in 20 (Figure 3). In view of the difference in the electronic properties between the two carbon ligands, we can conveniently assume that this C−C coupling involves nucleophilic attack of the former on the latter. Therefore, an electron-poor Ar increases the charge density of the CHArMe ligand and facilitates the C−C coupling process. Indeed, our NBO charge analysis supports this hypothesis. The NBO charges calculated for the CHArMe ligand in 20 are −0.238, −0.188, −0.222, −0.299, −0.317, and −0.306 for Ar = C6H5, C6H4-p-NH2, C6H4-p-OMe, C6H4-p-CF3, C6H4-p-CN, and C6H4-p-COOMe, respectively. The NBO charges correlate well with the reactivity of the CHArMe ligand. Another plausible explanation is that the stabilities of TS18‑20 and TS20‑1 are closely related to the stabilities of the intermediates 20. The

Figure 2. Calculated structures for the relevant transition states along the energy profiles shown in Figure 1. Bond lengths are given in Å.

The final step is reductive elimination to produce the carboxylated product and regenerate the CO2-coordinated species. From Figure 3, we can see that the pathway via TS20‑1 giving the α-carboxylated product is more favorable than that via TS19‑1 giving the β-carboxylated product, consistent with the experimental observations. Figure 3 shows that the rate-determining transition state for the nickel hydride mechanism is either TS18‑20, corresponding to the styrene insertion into the Ni−H bond, or TS20‑1, corresponding to the reductive elimination. Both transition states show similar stabilities (19.8 kcal/mol with respect to the energy reference point) and lie 1.2 kcal/mol lower than the transition state TS2‑5 calculated for the oxidative coupling discussed in the oxidative coupling mechanism. Clearly, the pathway leading to the carboxylated (here α-carboxylated) product in the hydride mechanism is more favorable than the pathway leading to the thermodynamic sinks resulting from the oxidative coupling step. While this result is consistent with the experimental observation that the carboxylated (here αcarboxylated) product is obtained, the very limited preference for the favorable pathway in the hydride mechanism leading to the carboxylated product over the oxidative coupling leading to the formation of a thermodynamic sink suggests that the E


Organometallics

Article

Figure 3. Free energy profiles calculated for the nickel hydride mechanism on the basis of the catalytic cycle shown on the left-hand side of Scheme 3. The free energies are given in kcal/mol.

versus the energy reference point (DBU)2Ni(η2-OCO) (1) + styrene + Et2Zn(THF) were considered. The transformation from the energy reference point to TS11‑13 is a transformation of three molecules to three molecules (Figure 1b). Similarly, the transformation from the energy reference point to 20, TS18‑20, or TS20‑1 is also a transformation of three molecules to three molecules (Figure 3). In the approximate method under assumption of ideal gases to estimate the dependence of relative free energies on pressure, a pressure change only affects those transformations which result in a change in the number of molecules. Effect of Different Ligands. Experimentally, when no additive was present or bipyridine, instead of DBU or pyridine, was used as the additive, the reactions resulted in no carboxylated products. In order to understand the effect of different ligands on the reaction outcomes, we performed calculations and compared relative stabilities of the key transition states and intermediates using COD, bipyridine, and pyridine as the ligands. Here, the reaction free energies for Ni(COD)2 + 2 pyridine + CO2 → (pyridine)2Ni(CO2) + 2 COD and Ni(COD)2 + bipyridine + CO2 → (bipyridine)Ni(CO2) + 2 COD were calculated to be −5.5 and −15.6 kcal/ mol, respectively. As shown in Table 3, we can clearly see that the effect of different ligands on the free relative energies of TS2‑5 for the oxidative coupling step is relatively small. However, the effect on the relative free energies of the key transition states in the hydride mechanism is significant. For the styrene insertion transition state TS18‑20, the relative free energies (28.9 kcal/mol for COD and 28.7 kcal/mol for bipyridine) increase by 9.0 kcal/mol in comparison with that (19.8 kcal/mol) for DBU. In

intermediates 20, having electron-withdrawing substituents, have a stronger hyperconjugation interaction between the Ni− C σ bond and one π* orbital of the phenyl ring and is therefore more stable than those with electron-donating substituents. Effect of Pressure. Experimentally, the carboxylation reactions were carried out under an atmospheric pressure of carbon dioxide. Therefore, it is important to understand how the pressure affects the reaction outcome. To achieve this, we estimated the relative stabilities of the key transition states and intermediates at different pressures. Table 2 shows that increasing the pressure obviously increases the stability of the oxidative coupling transition state TS2‑5 and intermediate 5 but has no effect on the stability of 20, TS11‑13, TS18‑20, and TS20‑1, corresponding to the transition states of transmetalation in the oxidative coupling mechanism, styrene insertion, and reductive elimination in the hydride mechanism, respectively. These results suggest that high pressure would favor the formation of the thermodynamic sinks and result in no carboxylated products. From these results, we can understand why an atmospheric pressure of carbon dioxide was adopted in the experiment. Here, we wish to comment on the observation that different atmospheric pressures show effects on TS2‑5 and 5 but not on 20, TS11‑13, TS18‑20, and TS20‑1. The relative free energies for the oxidative coupling transition state TS2‑5 and the metallacycle intermediate 5 were estimated with respect to the energy reference point (DBU)2Ni(η2-OCO) (1) + styrene (Figure 1a). In other words, transformation from the energy reference point to TS2‑5 or 5 corresponds to a transformation of two molecules to one molecule. When the relative stability of TS11‑13 was estimated, the relative free energies of TS11‑13 + DBU + THF F


Organometallics

Article

one DBU ligand is present. In other words, a bidentate ligand such as COD or bipyridine does not provide the convenience of ligand dissociation/association, resulting in instability of the transition state TS18‑20. Indeed, when the monodentate ligand pyridine was used in the calculations (the third entry in Table 3), the hydride mechanism is preferred over the oxidative coupling, consistent with the experimental observation that, when pyridine was used as the additive, the reactions leading to the α-carboxylated products occurred. Experimentally, reaction of the nonsubstituted styrene substrate H2CCHPh also resulted in the carboxylated product when DBU was replaced by Cs2CO3 or KHMDS. In order to understand how Cs2CO3 or KHMDS promotes the reaction, we also performed calculations and compared relative stabilities of the key transition states and intermediates. In our calculations, (THF)(Cs2CO3)Ni(CO2) or (THF)(KHMDS)Ni(CO2) was used as the energy reference point, considering that the reaction free energies for Ni(COD)2 + THF + Cs2CO3 + CO2 → (THF)(Cs2CO3)Ni(CO2) + 2 COD and Ni(COD)2 + THF + KHMDS + CO2 → (THF)(KHMDS)Ni(CO2) + 2 COD were calculated to be −7.0 and −9.8 kcal/mol, respectively. The results (Table 4) show that, when Cs2CO3 and KHMDS were used as ligands, the oxidative coupling between alkyne and CO2 can be effectively suppressed as a result of higher oxidative coupling barriers, because both Cs2CO3 and KHMDS are considered to be weak ligands. The effective suppression allows the reductive carboxylation to occur. However, neither the Cs2CO3 or the KHMDS ligand is a very effective ligand to promote the reductive carboxylation because of the higher relative energy for either TS18‑20 or TS20‑1 (Table 4 versus Table 1). These findings explain that the additives Cs2CO3 and KHMDS gave modest yields of the desired products.

■

CONCLUSIONS The detailed mechanisms for the reductive carboxylation of styrenes using CO2 catalyzed by nickel complexes have been studied with the aid of DFT calculations. Two possible mechanisms, the oxidative coupling mechanism and the nickel hydride mechanism, were calculated and compared. In the oxidative coupling mechanism, our calculation results show that the metallacycle intermediates generated from oxidative coupling between CO2 and a styrene substrate molecule on the nickel(0) metal center correspond to a thermodynamic sink: i.e., the barriers for both the reverse and forward steps from the intermediates are particularly high. In

Figure 4. Calculated structures for the relevant transition states in the nickel hydride mechanism. Bond lengths are given in Å.

other words, when a bidentate ligand such as COD or bipyridine is present, the oxidative coupling becomes favorable and the thermodynamic sinks cannot be avoided. The calculation results discussed above indicate that the favorable pathway in the hydride mechanism (Figure 3) involves ligand dissociation/association. In the key transition state TS18‑20, only

Table 1. Relative Free Energies of the Oxidative Coupling Transition States TS2‑5, the Metallacycle Intermediates 5, and the Transmetalation Transition States TS11‑13, Which Are Relevant in the Oxidative Coupling Mechanism, and the Styrene Insertion Transition States TS18‑20, the Reductive Elimination Transition States TS20‑1, and the Benzyl Nickel Species 20, Which Are Relevant in the Nickel Hydride Mechanism, Calculated for Reactions of Various Styrenes Bearing Different Substituentsa oxidative coupling mechanism

a

nickel hydride mechanism

substituent on aromatic ring of styrene substrate

TS2‑5 (oxidative coupling)

5 (metallacycle intermediate)

TS11‑13 (transmetalation)

TS18‑20 (styrene insertion)

20

TS20‑1 (reductive elimination)

−H −NH2 −OMe −CF3 −CN −COOMe

22.8 23.5 22.9 21.0 20.8 21.0

−8.2 −6.6 −8.0 −10.8 −11.0 −9.3

24.0 23.6 23.3 24.9 24.0 24.1

20.9 21.9 20.6 20.0 19.2 19.8

3.5 3.1 3.3 2.4 1.4 1.6

23.9 25.7 24.5 20.2 19.0 19.8

The relative free energies are given in kcal/mol. G


Organometallics

Article

Table 2. Relative Free Energies of the Oxidative Coupling Transition States TS2‑5, the Metallacycle Intermediates 5, and the Transmetalation Transition States TS11‑13, Which Are Relevant in the Oxidative Coupling Mechanism, and the Styrene Insertion Transition States TS18‑20, the Reductive Elimination Transition States TS20‑1, and the Benzyl Nickel Species 20, Which Are Relevant in the Nickel Hydride Mechanism, Calculated for the Reactions under Different Atmospheric Pressuresa oxidative coupling mechanism

a


pressure (atm)





20


1 10 20

21.0 19.6 19.2

−9.3 −10.7 −11.1

24.1 24.1 24.1

19.8 19.8 19.8

1.6 1.6 1.6

19.8 19.8 19.8

The free energies are given in kcal/mol. The styrene substrate molecule is the ester-substituted styrene H2CCHAr (Ar = C6H4-p-COOMe).

Table 3. Relative Free Energies of the Oxidative Coupling Transition States TS2‑5, the Metallacycle Intermediates 5, and the Transmetalation Transition States TS11‑13, Which Are Relevant in the Oxidative Coupling Mechanism, and the Styrene Insertion Transition States TS18‑20, the Reductive Elimination Transition States TS20‑1, and the Benzyl Nickel Species 20, Which Are Relevant in the Nickel Hydride Mechanism, Calculated for Reactions When Different Ligands Were Useda oxidative coupling mechanism


ligand coordinated to nickel center





20


COD bipyridine pyridine DBU

23.5 21.4 21.7 21.0

−3.5 −9.3 −9.0 −9.3

27.7 26.4 25.8 24.1

28.9 28.7 16.9 19.8

2.9 2.9 0.7 1.6

19.1 25.4 19.7 19.8

a

The free energies are given in kcal/mol. The styrene substrate molecule is the ester-substituted styrene H2CCHAr (Ar = C6H4-p-COOMe).

Table 4. Relative Free Energies of the Oxidative Coupling Transition States TS2‑5, the Metallacycle Intermediates 5, and the Transmetalation Transition States TS11‑13, Which Are Relevant in the Oxidative Coupling Mechanism, and the Styrene Insertion Transition States TS18‑20, the Reductive Elimination Transition States TS20‑1, and the Benzyl Nickel Species 20, Which Are Relevant in the Nickel Hydride Mechanism, Calculated for Reactions When Different Ligands Were Useda oxidative coupling mechanism


ligand coordinated to nickel center





20


Cs2CO3 + THF KHMDS + THF

24.3 34.6

−4.2 −0.3

15.2 27.9

20.5 25.0

−0.6 0.1

22.5 20.7

a

The free energies are given in kcal/mol. The styrene substrate molecule is H2CCHPh.

other words, CO2−styrene oxidative coupling should be avoided in order for the smooth reductive carboxylation of styrenes. In the nickel hydride mechanism, the first step is transmetalation between the CO2-coordinated species (DBU)2Ni(η2-CO2) and Et2Zn(THF) to give a square-planar nickel(II) ethyl species. In the nickel(II) ethyl species, β-hydride elimination occurs to give a nickel(II) hydride species, from which styrene insertion into the Ni−H bond (migration of hydride to the styrene terminal carbon) occurs followed by reductive elimination to produce the carboxylated product. The calculation results show that when the ester-substituted styrene H2CCHAr (Ar = C6H4-p-COOMe) is used as the substrate, the transition states for styrene insertion and reductive elimination have comparable stabilities and one of them can be the rate-determining transition state. Furthermore, these two transition states are only slightly more stable than the oxidative coupling transition state, which leads to the thermodynamic sink. The very limited preference for the nickel hydride pathway over the oxidative coupling pathway suggests that the reaction outcome can be easily manipulated/affected by substituents on the aryl ring of a styrene substrate as well as by other factors such as the pressure used in the reactions. Consistent with the experimental observations, the calculation

results show that electron-withdrawing substituents such as NO 2 , CN, and COOMe promote the nickel hydride mechanism and facilitate the reductive carboxylation reactions. It was also found that high CO2 pressure promotes the formation of the thermodynamic sink, explaining why 1 atm of CO2 was used in the experimentally reported reactions. In the most favorable pathway of the nickel hydride mechanism, it was also found that ligand (here DBU) dissociation and association are often involved, explaining why bidentate ligands such as COD and bipyridine are ineffective for the catalytic reactions. When Cs2CO3 and KHMDS were used as ligands, it was found that the oxidative coupling between alkyne and CO2 could be effectively suppressed. However, neither the Cs2CO3 nor the KHMDS ligand is a very effective ligand to promote the reductive carboxylation.

■

ASSOCIATED CONTENT

S Supporting Information *

A figure, tables, and an xyz file giving relative free energies of the key transition states and intermediates calculated for the reactions using different methods and basis sets, energy profiles for less favorable transmetalation pathways in the oxidativecoupling mechanism, and Cartesian coordinates and electronic H


Organometallics

Article

(6) (a) Burkhart, G.; Hoberg, H. Angew. Chem., Int. Ed. Engl. 1982, 21, 76−76. (b) Hoberg, H.; Schaefer, D.; Burkhart, G.; Kruger, C.; Romao, M. J. J. Organomet. Chem. 1984, 266, 203−224. (c) Tsuda, T.; Sumiya, R.; Saegusa, T. Synth. Commun. 1987, 17, 147−154. (d) Saito, S.; Nakagawa, S.; Koizumi, T.; Hirayama, K.; Yamamoto, Y. J. Org. Chem. 1999, 64, 3975−3978. (e) Takimoto, M.; Shimizu, K.; Mori, M. Org. Lett. 2001, 3, 3345−3347. (f) Shimizu, K.; Takimoto, M.; Mori, M. Org. Lett. 2003, 5, 2323−2325. (g) Shimizu, K.; Takimoto, M.; Sato, Y.; Mori, M. Org. Lett. 2005, 7, 195−197. (7) (a) Hoberg, H.; Peres, Y.; Kruger, C.; Tsay, Y.-H. Angew. Chem., Int. Ed. Engl. 1987, 26, 771−773. (b) Hoberg, H.; Schaefer, D. J. Organomet. Chem. 1982, 236, C28−C30. (c) Hoberg, H.; Schaefer, D. J. Organomet. Chem. 1983, 251, C51−C53. (d) Hoberg, H.; Peres, Y.; Milchereit, A. J. Organomet. Chem. 1986, 307, C38−C40. (e) Hoberg, H.; Ballesteros, A.; Sigan, A.; Jegat, C.; Milchereit, A. Synthesis 1991, 395−398. (f) Hoberg, H.; Ballesteros, A.; Sigan, A. J. Organomet. Chem. 1991, 403, C19−C22. (g) Hoberg, H.; Ballesteros, A.; Sigan, A.; Jegat, C.; Barhausen, D.; Milchereit, A. J. Organomet. Chem. 1991, 407, C23−C29. (h) Hoberg, H.; Ballesteros, A. J. Organomet. Chem. 1991, 411, C11−C18. (8) Williams, C. M.; Johnson, J. B.; Rovis, T. J. Am. Chem. Soc. 2008, 130, 14936−14937. (9) (a) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200−1211. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (c) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785− 787. (10) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−310. (11) (a) Hehre, W. J.; Ditchfie, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257−2261. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213−222. (12) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999−3093. (13) De Marothy, S. A. XYZViewer version 0.97; Stockholm, 2010. (14) (a) Fukui, K. J. Phys. Chem. 1970, 74, 4161−4163. (b) Fukui, K. Acc. Chem. Res. 1981, 14, 363−368. (15) (a) Benson, S. W. The Foundations of Chemical Kinetics; Krieger: Malabar, FL, 1982. (b) Okuno, Y. Chem. Eur. J. 1997, 3, 212−218. (c) Ardura, D.; López, R.; Sordo, T. L. J. Phys. Chem. B 2005, 109, 23618−23623. (d) Schoenebeck, F.; Houk, K. N. J. Am. Chem. Soc. 2010, 132, 2496−2497. (e) Liu, Q.; Lan, Y.; Liu, J.; Li, G.; Wu, Y. D.; Lei, A. J. Am. Chem. Soc. 2009, 131, 10201−10210. (f) Ariafard, A.; Brookes, N. J.; Stanger, R.; Yates, B. F. Organometallics 2011, 30, 1340−1349. (g) Yu, H.; Lu, Q.; Dang, Z.; Fu, Y. Chem. Asian J. 2013, 8, 8−18. (h) Ariafard, A.; Ghohe, N. M.; Abbasi, K. K.; Canty, A. J.; Yates, B. F. Inorg. Chem. 2013, 52, 707−717. (i) Fan, T.; Sheong, F. K.; Lin, Z. Organometallics 2013, 32, 5224−5230. (j) Xie, H.; Lin, Z. Organometallics 2014, 33, 892−897. (16) (a) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066− 4073. (b) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735−746. (c) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1985, 83, 1736−1740. (d) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899−926. (e) Carpenter, J. E.; Weinhold, F. J. Mol. Struct. (THEOCHEM) 1988, 46, 41−62. (f) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211−7218. (17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .;

energies for all of the calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Research Grants Council of Hong Kong (GRF 16303614) and the National Natural Science Foundation of China (21203156).

■

REFERENCES

(1) (a) Marks, T. J. Chem. Rev. 2001, 101, 953−996. (b) Behr, A. Angew. Chem., Int. Ed. Engl. 1988, 27, 661−678. (c) Louie, J. Curr. Org. Chem. 2005, 9, 605−623. (d) Aresta, M.; Dibenedotto, A. Dalton Trans. 2007, 28, 2975−2992. (e) Baiker, A. Appl. Organomet. Chem. 2000, 14, 751−762. (2) (a) Sakakura, T.; Choi, J. C.; Yasuda, H. Chem. Rev. 2007, 107, 2365−2387. (b) Sakakura, T.; Kohon, K. Chem. Commun. 2009, 1312−1330. (c) Decortes, A.; Castilla, A. M.; Kleij, A. W. Angew. Chem., Int. Ed. 2010, 49, 9822−9837. (d) Cokoja, M.; Bruckmeier, C.; Rieger, B.; Herrmann, W. A.; Kühn, F. E. Angew. Chem., Int. Ed. 2011, 50, 8510−8537. (3) (a) Yin, X.; Moss, J. R. Coord. Chem. Rev. 1999, 181, 27−59. (b) Riduan, S. N.; Zhang, Y. Dalton Trans. 2010, 39, 3347−3357. (c) Darensbourg, D. J. Inorg. Chem. 2010, 49, 10765−10780. (d) Zhang, L.; Cheng, J.; Ohishi, T.; Hou, Z. Angew. Chem., Int. Ed. 2010, 49, 8670−8673. (e) Yang, Z. Z.; Zhao, Y. N.; He, L. N. RSC Adv. 2011, 1, 545−567. (f) Ohishi, T.; Zhang, L.; Nishiura, M.; Hou, Z. Angew. Chem., Int. Ed. 2011, 50, 8114−8117. (g) Mansell, S. M.; Kaltsoyannis, N.; Arnold, P. L. J. Am. Chem. Soc. 2011, 133, 9036− 9051. (h) Kumar, S.; Kumar, P.; Jain, S. L. RSC Adv. 2013, 3, 24013− 24016. (i) Bontemps, S.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2013, 52, 10253−10255. (j) Zhang, Y.; Hanna, B. S.; Dineeen, A.; Williard, P. G.; Bernskoetter, W. H. Organometallics 2013, 32, 3969−3979. (k) Horn, B.; Limberg, C.; Herwig, C.; Braun, B. Chem. Commun. 2013, 49, 10923−10925. (4) (a) Hoberg, H.; Gross, S.; Milchereit, A. Angew. Chem., Int. Ed. Engl. 1987, 26, 571−572. (b) Hoberg, H.; Schafer, D.; Oster, B. W. J. Organomet. Chem. 1984, 266, 313−320. (c) Hoberg, H.; Apotecher, B. J. Organomet. Chem. 1984, 270, C15−C17. (d) Walther, D.; Dinjus, E.; Gorls, H. J. Organomet. Chem. 1985, 286, 103−114. (e) Hoberg, H.; Oster, B. W. J. Organomet. Chem. 1984, 266, 321−326. (f) Aoki, M.; Kaneko, M.; Izumi, S.; Ukai, K.; Iwasawa, N. Chem. Commun. 2004, 2568−2569. (g) Takimoto, M.; Kawamura, M.; Mori, M. Org. Lett. 2003, 5, 2599−2601. (h) Takimoto, M.; Kawamura, M.; Mori, M. Synthesis 2004, 791−795. (i) Aoki, M.; Izumi, S.; Kaneko, M.; Ukai, K.; Takaya, J.; Iwasawa, N. Org. Lett. 2007, 9, 1251−1253. (j) Takimoto, M.; Mori, M. J. Am. Chem. Soc. 2001, 123, 2895−2896. (k) Takimoto, M.; Mori, M. J. Am. Chem. Soc. 2002, 124, 10008−10009. (l) Takimoto, M.; Nakamura, Y.; Kimura, K.; Mori, M. J. Am. Chem. Soc. 2004, 126, 5956−5957. (5) (a) Tsuda, T.; Morikawa, S.; Sumiya, R.; Saegusa, T. J. Org. Chem. 1988, 53, 3140−3145. (b) Tsuda, T.; Morikawa, S.; Hasegawa, N.; Saegusa, T. J. Org. Chem. 1990, 55, 2978−2981. (c) Louie, J.; Gibby, J.; Farnworth, M. V.; Tekavec, T. N. J. Am. Chem. Soc. 2002, 124, 15188− 15189. (d) Duong, H. A.; Tekavec, T. N.; Arif, A. M.; Louie, J. Chem. Commun. 2004, 112−113. (e) Takavec, T. N.; Arif, A. M.; Louie, J. Tetrahedron 2004, 60, 7431−7437. (f) Dérien, S.; Clinet, J.-C.; Duñach, E.; Perichon, J. J. Am. Chem. Soc. 1991, 113, 8447−8454. (g) Duñach, E.; Perichon, J. J. Organomet. Chem. 1988, 352, 239−245. (h) Dérien, S.; Clinet, J.-C.; Duñach, E.; Perichon, J. J. Chem. Soc. Chem. Commun. 1991, 549−550. (i) Dérien, S.; Clinet, J.-C.; Duñach, E.; Perichon, J. J. Org. Chem. 1993, 58, 2578−2588. I


Organometallics

Article

Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian, Inc., Wallingford, CT, 2009. (18) Yuan, R. M.; Lin, Z. ACS Catal. 2014, 4, 4466−4473. (19) (a) Sakaki, S.; Musashi, Y. Inog. Chem. 1995, 34, 1914−1923. (b) Sakaki, S.; Ohkubo, K. Organometallics 1989, 8, 2970−2973. (c) Dang, L.; Lin, Z.; Marder, T. B. Organometallics 2010, 29, 917− 927. (d) Fan, T.; Chen, X.; Lin, Z. Chem. Commun. 2012, 48, 10808− 10828. (20) (a) Shi, M.; Nicholas, K. M. J. Am. Chem. Soc. 1997, 119, 5057− 5058. (b) Johnson, M. T.; Johansson, R.; Kondrashov, M. V.; Steyl, G.; Ahlquist, M. S. G.; Roodt, A.; Wendt, O. F. Organometallics 2010, 29, 3521−3529. (c) Wu, J.; Green, J. C.; Hazari, N.; Hruszkewycz, D. P.; Incarvito, C. D.; Schmeier, T. J. Orgnanometallics 2010, 29, 6369− 6376. (d) Wang, M.; Fan, T.; Lin, Z. Polyhedron 2012, 32, 35−40. (e) Hruszkewycz, D. P.; Wu, J.; Hazari, N.; Incarvito, C. D. J. Am. Chem. Soc. 2011, 133, 3280−3283. (f) Hruszkewycz, D. P.; Wu, J.; Green, J. C.; Hazari, N.; Schmeier, T. J. Organometallics 2012, 31, 470−485. (21) (a) Papai, I.; Schubert, G.; Mayer, I.; Besenyei, G.; Aresta, M. Organometallics 2004, 23, 5252−5259. (b) Fischer, R.; Langer, J.; Malassa, A.; Walther, D.; Gorls, H.; Vaughan, G. Chem. Commun. 2006, 2510−2512. (c) Kégl, T.; Ponec, R.; Kollár, L. J. Phys. Chem. A 2011, 115, 12463−12473. (d) Dedieu, A.; Ingold, F. Angew. Chem., Int. Ed. Engl. 1989, 28, 1694−1695. (e) Sakaki, S.; Mine, K.; Hamada, T.; Arai, T. Bull. Chem. Soc. Jpn. 1995, 68, 1873−1882. (f) Sakaki, S.; Mine, K.; Taguchi, D.; Arai, T. Bull. Chem. Soc. Jpn. 1993, 66, 3289− 3299. (g) Galan, F.; Fouassier, M.; Tranquille, M.; Mascetti, J.; Papai, I. J. Phys. Chem. A 1997, 101, 2626−2633. (h) Sakaki, S.; Kitaura, K.; Morokuma, K. Inorg. Chem. 1982, 21, 760−765. (22) (a) Graham, D. C.; Bruce, M. I.; Metha, G. F.; Bowie, J. H.; Buntine, M. A. J. Organomet. Chem. 2008, 693, 2703−2710. (b) Graham, D. C.; Mitchell, C.; Bruce, M. I.; Metha, G. F.; Bowie, J. H.; Buntine, M. A. Organometallics 2007, 26, 6784−6792. (23) (a) O’Brien, E. M.; Bercot, E. A.; Rovis, T. J. Am. Chem. Soc. 2003, 125, 10498−10499. (b) Johnson, J. B.; Rovis, T. Acc. Chem. Res. 2008, 41, 327−328. (24) Aresta, M.; Nobile, F. J. Chem. Soc. Chem. Commun. 1975, 636− 637.

J


Computational Insight into the Mechanism of Nickel-Catalyzed

Recommend Documents