DFT Studies on the Carboxylation of the C–H Bond of Heteroarenes

Article pubs.acs.org/Organometallics

DFT Studies on the Carboxylation of the C−H Bond of Heteroarenes by Copper(I) Complexes Alireza Ariafard,*,†,‡ Fatemeh Zarkoob,† Hossein Batebi,† Robert Stranger,§ and Brian F. Yates*,‡ †

Department of Chemistry, Faculty of Science, Central Tehran Branch, Islamic Azad University, Shahrak Gharb, Tehran, Iran School of Chemistry, University of Tasmania, Private Bag 75, Hobart TAS 7001, Australia § Research School of Chemistry, Australian National University, Canberra ACT 0200, Australia ‡

S Supporting Information *

ABSTRACT: In this study, we have used density functional theory to identify a new mechanism for the formation of carboxylate compounds from heteroarenes, such as benzoxazole, in the presence of copper catalysts. This new mechanism involves the formation of a carbene intermediate that is indirectly stabilized by the electron-releasing copper. This intermediate carbene can isomerize to the experimentally observed resting state of the catalytic cycle, but it is the intermediate carbene itself that has the greater reactivity toward CO 2 and that leads to the final carboxylate product via a lower-energy pathway. Our findings demonstrate the importance of considering metal-stabilized carbenes in such reactions. Our findings also suggest that this carbene intermediate can act as a nucleophile in other organometallic reactions.

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Scheme 1

INTRODUCTION

During the past decade, CO2 activation by organometallic compounds has emerged as a powerful method in organic synthesis and has attracted experimental1 and theoretical2 interest. It is well established that many transition-metal complexes are able to catalyze carboxylation of allylstannanes, 3 organoboronicesters,4 alkenes,5 alkynes,6 bis-dienes,7 allenes,8 organozincs,9 and aromatic C−H bonds with CO2.10 Recently, Hou and co-workers11 and Cazin, Nolan, and co-workers12 independently demonstrated a methodology for the carboxylation of the C−H bond of heteroarenes using [(IPr)CuOR] catalysts, where IPr = 1,3-bis(diisopropyl)phenylimidazol-2ylidene and R = H, tBu (eq 1). The crucial importance of this catalytic process has been highlighted by two recent papers. 13 The investigation of several stoichiometric reactions for the carboxylation of benzoxazole led Hou and co-workers to propose the reaction mechanism shown in Scheme 1.11 The proposed mechanism starts with protonolysis of [(IPr)CuOR] (I) by heteroarenes to give the Cu complex II. The insertion of CO2 into the Cu−C bond through nucleophilic attack of the coordinated heteroaryl to the carbon atom of CO2 gives the carboxylate complex III. The conversion of II to III was found to be reversible for the benzoxazole substrate. Finally, the catalyst I can be regenerated through the reaction of III with a base, such as KOR. Given that computational chemistry is of great use in understanding reaction mechanisms, we here present a theoretical study in order to investigate energetically each step of the proposed catalytic cycle. We hope to provide new insights into the mechanism of the carboxylation reaction catalyzed by the Cu(I) complex14 for the rational design of a more efficient synthetic approach. © 2011 American Chemical Society

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COMPUTATIONAL DETAILS

Gaussian 0915 was used to fully optimize all the structures reported in this paper at the B3LYP level of density functional theory (DFT). 16 The effective core potential of Hay and Wadt with a double-ξ valence basis set (LANL2DZ)17 was chosen to describe Cu. The 6-31G(d) basis set was used for other atoms.18 A polarization function of ξ f = 3.525 was also added to Cu.19 This basis set combination will be referred to as BS1. Frequency calculations were carried out at the same level of theory as for structural optimization. To further refine the energies obtained from the B3LYP/BS1 calculations, we carried out Received: August 9, 2011 Published: October 31, 2011 6218

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single-point energy calculations for all the structures with a larger basis set (BS2) at the B3LYP and M0620 levels. BS2 utilizes the quadruple-ζ valence def2-QZVP21 basis set on Cu and the 6-311+G(2d,p) basis set on other atoms. The solvation energies were calculated using BS2 on gas-phase optimized geometries with the CPCM solvation model 22 using THF as a solvent. To estimate the corresponding Gibbs free energies, ΔG, the entropy corrections were calculated at the B3LYP/ BS1 level, adjusted by the method proposed by Okuno,23 and finally added to the B3LYP/BS2 and M06/BS2 total energies. Because recent studies have established that M06 predicts the activation energies more accurately than B3LYP,24 we have used the potential and Gibbs free energies obtained from the M06/BS2//B3LYP/BS1 calculations in THF throughout the paper unless otherwise stated. The results related to the B3LYP/BS2//B3LYP/BS1 calculations are contained in the Supporting Information. The partial atomic charges were calculated on the basis of natural bond orbital (NBO) analyses.25

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RESULTS AND DISCUSSION

We started our theoretical study by investigating the mechanistic possibilities of the carboxylation of heteroarenes using the Nolan catalyst, [(IPr)CuOH], where various substrates are considered: benzoxazole, oxazole, and thiazole. A similar mechanism was observed for all the substrates, and therefore, below we only discuss the results related to the carboxylation of benzoxazole and include other results in the Supporting Information. As proposed by Hou, the carboxylation process is separated into three steps. Herein, we will explore the mechanism of each step in detail. Protonolysis of [(IPr)CuOH] by Benzoxazole. The first step of this reaction is suggested to be C−H activation of the aromatic substrates. The mechanisms of C−H activation by organometallic complexes have been extensively studied.26 The proton transfer from a C−H bond to a strongly polarized M−X bond (X = OR, NRR′) is referred to as “1-2 addition”.27 Our calculations show that benzoxazole can bind to the hydroxyl ligand through a hydrogen bond that produces 2_a (Figure 1).28 The outer-sphere complex 2_a that is 5.5 kcal/mol less stable than 1 + a at the M06/BS2 level shows a slight C−H activation; both the C−H and the Cu−OH bonds are stretched by 0.018 Å, while the H···O distance is 1.896 Å (Figure 2). The NBO charge on benzoxazole in 2_a is calculated to be −0.026. These results suggest that the charge transfer occurs from the OH ligand to benzoxazole by a slight weakening of the Cu− OH bond. Two pathways29 for C−H activation from 2_a were found (Figure 1). Pathway I proceeds through the four-centered transition structure30 1TS_a (ΔG ‡ = 29.0 kcal/mol), which corresponds to the migration of the benzoxazolyl fragment to the Cu atom concomitant with the proton transfer from benzoxazole to the OH ligand (Figure 1). The IRC and geometry optimization calculations show that this transition structure directly gives the C−H activation product 4_a and leads to the dissociation of H2O.31 The C−H activation in pathway II takes place through the five-membered transition structure 2TS_a (ΔG ‡ = 23.6 kcal/mol) in which the nitrogen atom on the heteroarene starts to interact with the Cu metal center while the proton is being transferred from the heteroarene to the hydroxyl oxygen atom. This transition structure releases one H2O and yields the N-bound Cu

Figure 1. Computed energy profile for C−H activation of benzoxazole mediated by (IPr)Cu−OH. The relative free energies and potential energies (in parentheses) obtained from the M06/BS2//B3LYP/BS1 calculations in THF are given in kcal/mol.

intermediate 3_a. This intermediate, having a free carbene on the heteroaryl ring, isomerizes to the C−H activation product 4_a with a small activation barrier of 4.5 kcal/mol through a 1,2 shift process. The argument related to the formation of carbene can be supported by a significant C1−N1 bond lengthening during the course of the C−H activation from 2_a (1.296 Å) to 2TS_a (1.324 Å) and then to 3_a (1.348 Å) (Figure 2). The kinetic selectivity calculated for producing 3_a suggests that pathway II is kinetically more favorable; 2TS_a is calculated to be 5.4 kcal/mol more stable than 1TS_a (Figure 1). In the transition structure of the C−H activation process, one of the lone pairs of the OH ligand interacts strongly with the vacant σ*C−H orbital, polarizing the C−H σ bond toward C1, enhancing the nucleophilicity of benzoxazolyl toward the Cu metal center with an increase in its electron population (Scheme 2); the NBO charge on benzoxazolyl in 2_a, 1TS_a, and 2TS_a is calculated to be −0.294, −0.626, and −0.513, respectively. Our NBO calculations show that both the 4s and the 4p orbitals of Cu are involved in the Cu−benzoxazole interactions in the transition structures. According to the calculated second-order perturbation interaction energies (E(2)),32 benzoxazole interacts with Cu in 2TS_a more strongly than that in 1TS_a (Scheme 2). The lpOH → σ*C−H interaction in the transition structure causes the Cu−OH and C−H bonds to be extremely weakened. The Cu−OH bond is elongated from 1.816 Å in 2_a to 2.031 Å in 1TS_a and to 2.153 Å in 2TS_a. The C−H distance, which is 1.099 Å in 2_a, is lengthened to 1.569 Å in 1TS_a and to 1.381 Å in 2TS_a (Figure 2). Conversely, the relevant interaction results in a shortening of the HO···H distance in the transition structures (from 1.895 Å in 2_a to 1.105 Å in 1TS_a and to 1.215 Å in 2TS_a) (Figure 2). However, less C−H bond breaking and less 6219

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Figure 2. Selected structural parameters (Å) calculated for various species involved in the Cu-catalyzed carboxylation of the C−H bond of heteroarenes.

increases strongly (the negative charge increases from −0.523 to −0.681). These results support the electron flow mechanisms shown in Figure 1; the nucleophilic attack on Cu in pathway I occurs through the C1 atom and in pathway II through the N1 atom. We believe that the interaction of N1 with Cu in pathway II increases the acidity of C1−H benzoxazole as a result of moving the electron density away from C1 during the C−H activation process.33 The higher the acidity, the more favorable the C−H activation process. This argument can find further support from the analysis of the activation strain model34 in which the transition structure energies are dissected into the deformation energy (ΔEdef) and the interaction energy (ΔEint) (Scheme 3). ΔEdef is defined as the required energy for deforming benzoxazole and (IPr)CuOH to their geometries in the transition structure, and ΔEint represents the interaction energy between the deformed benzoxazole and (IPr)CuOH fragments to form the transition structure. The results of the calculations (Table 1) show that the interaction of the N1 atom with Cu in the five-membered transition structure 2TS_a causes ΔEint for pathway II (−47.9 kcal/mol) to be larger than that in pathway I (−40.8 kcal/mol), although this interaction deforms the (IPr)CuOH fragment in 2TS_a more severely; ΔEdef(2) for pathway II (26.4 kcal/mol) is much greater than that for pathway I (10.7 kcal/mol). 35 A less negative value of ΔEint + ΔEdef(2) for pathway II than for pathway I suggests that the interaction of N1 with Cu in 2TS_a has overall a less stabilizing effect on the transition structure. However, as mentioned above, the relevant Cu−N1 interaction moves the electron density away from C1, increasing the acidity of benzoxazole during the C−H activation process, giving an earlier transition structure for 2TS_a. This feature, which is reflected in the much lower deformation energy of benzoxazole

Scheme 2

O−H bond forming are observed for 2TS_a than for 1TS_a. The electron population on benzoxazolyl in 2TS_a is also smaller than in 1TS_a (vide supra). The second-order perturbation interaction between lpOH and σ*C−H in 2TS_a is much smaller than that in 1TS_a. All these results confirm an earlier transition structure for 2TS_a, a result that is consistent with the higher stability of 2TS_a as compared with 1TS_a. An NBO charge analysis shows that, for the C−H activation process from 2_a to 1TS_a, a dramatic increase in the population of C1 (the positive charge decreases from 0.384 to 0.145) in conjunction with a moderate increase in the population of N1 (the negative charge increases from −0.523 to −0.547) is observed. A reverse trend in the electron population is found for the conversion of 2_a to 2TS_a; the population of C1 increases moderately (the positive charge decreases from 0.384 to 0.312), while the population of N 1 6220

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calculations show that this reaction is thermoneutral, a result that is consistent with the experimental findings (see Scheme 1). Our calculations also confirm that, in good agreement with the experimental evidence, the Cu metal center in the CO2 activation product 5_a is chelated by the heteroaryl− carboxylate ligand. The nonchelate structure is calculated to be 2.3 kcal/mol less stable than 5_a. The mechanism of the CO2 insertion into the Cu−R bond has been investigated previously, and it has been found that the insertion process occurs through the nucleophilic attack of the Cu−R σ bond on the electron-deficient carbon atom of CO2.2h Our calculations show that the activation barrier for the CO2 insertion (4_a → 5_a) through the most accepted mechanism (pathway III in Figure 3) is as high as 30.7 and 32.4 kcal/mol at the M06/BS2 and B3LYP/BS2 levels, respectively, an activation barrier that is much higher than what we calculated previously2j for CO2 insertion into the Cu−Ph bond (19.7 kcal/mol at the B3LYP/ BS2 level). A high activation barrier is also calculated for other heteroaryl systems, 30.9/29.3, 28.2/24.3, and 34.9/33.1 kcal/ mol for CO2 insertion into the Cu−R bond, where R = oxazolyl, thiazolyl, and 1,3,4-oxadiazolyl, respectively (the calculated free energy changes at the B3LYP and M06 levels are listed before and after the slash, respectively). The higher activation barrier for CO2 insertion into the Cu−heteroaryl bonds is likely related to the stronger Cu−heteroaryl bonds as compared with the Cu−Ph bond; the homolytic bond dissociation energy of (IPr)Cu−R is calculated to be 105.7/ 110.1, 105.2/110.6, 95.4/101.5, 113.8/118.3, and 83.5/90.9 kcal/mol for R = benzoxazolyl, oxazolyl, thiazolyl, 1,3,4oxadiazolyl, and phenyl, respectively, where the calculated bond energies at the B3LYP and M06 levels are listed before and after the slash, respectively. Indeed, heteroaryl ligands having electronegative atoms on their ring are capable of polarizing the Cu−R σ bond toward the R group and increase the ionic character of the Cu−R bond, thereby enhancing the Cu−R bond strength.36 The greater negative NBO charge on the heteroaryl rings supports the higher ionicity of the M− C(heteroaryl) bonds: −0.733, −0.646, −0.633, −0.655, and −0.614 on R = benzoxazolyl, oxazolyl, thiazolyl, 1,3,4oxadiazolyl, and phenyl groups in (IPr)Cu−R, respectively. These results demonstrate that the CO2 insertion into Cu−R (R = heteroaryl) via pathway III (the most accepted mechanism) requires overcoming a high barrier. However, we could establish an alternative pathway through which the CO 2 insertion reaction becomes more facile (pathway IV). We believe that the C−H activation product 4_a can convert back to intermediate 3_a, and now, the free carbene on the heteroaryl of 3_a is capable of acting as a nucleophile. The very high nucleophilicity of the free carbene renders the nucleophilic attack on CO2 barrierless, as evidenced by a relaxed potential energy surface scan for R = oxazolyl (Figure 4). A comparison of pathways III and IV (Figure 3) shows that the CO2 insertion in such systems should happen via pathway IV, which is energetically more favorable than pathway III. Reaction of 5_a with OH−. The last step of the catalytic cycle was proposed to be the loss of the carboxylate species through the coordination of KOH. We assume that KOH is largely dissociated into K+ + OH− in solution. Our calculations show that a barrierless addition of the free OH− to the Cu metal center of 5_a affords the three coordinate complex 6_a. This intermediate is 10.4 kcal/mol below 5_a. The adduct 5_a without any barrier then undergoes the elimination of the

Scheme 3

Table 1. Analysis of the Activation Strain Model for Transition Structures 1TS_a and 2TS_a Using the M06/ BS2//B3LYP/BS1 Calculations pathway

ΔE ‡

ΔEdef(1) (benzoxazole)

ΔEdef(2) (LCuOH)

ΔEint

ΔEint + ΔEdef(2)

I II

21.0 14.5

51.1 35.9

10.7 26.4

−40.8 −47.9

−30.1 −21.4

in 2TS_a (35.9 kcal/mol) than in 1TS_a (51.1 kcal/mol), renders pathway II more favorable than pathway I. Insertion of CO2 into the Cu−C Bond. The next step in the catalytic cycle is the conversion of 4_a to 5_a through CO2 insertion into the Cu−benzoxazole bond (Figure 3). Our

Figure 3. Computed energy profile for CO2 activation mediated by (IPr)Cu−benzoxazolyl. The relative free energies and potential energies (in parentheses) obtained from the M06/BS2//B3LYP/BS1 calculations in THF are given in kcal/mol. 6221

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Scheme 4

Figure 4. Potential energy surface for C1−C2 bond forming computed at the B3LYP/BSI level for R = oxazolyl.

carboxylate species a_CO2 to regenerate the active catalyst 1 (Figure 5).

affording intermediate IV in which the nitrogen on the heteroaryl group is bound to Cu. This intermediate with a free carbene on its heteroaryl ring can convert to C−H activation product II with a low activation barrier. The reactivity of II toward the CO2 insertion reaction via nucleophilic attack of the coordinated heteroaryl to CO 2 is negligible. However, this step (CO2 activation) can proceed via the isomerization of II to IV, followed by the nucleophilic attack of the free carbene on CO2 (see V in Scheme 4), forming intermediate III. In the final step, III reacts with KOR, giving both the carboxylate species and the active catalyst [(IPr)CuOR].

Figure 5. Computed energy profile for reaction of 5_a with OH−. The relative free energies and potential energies (in parentheses) obtained from the M06/BS2//B3LYP/BS1 calculations in THF are given in kcal/mol.

We found that the presence of the base OH− is necessary to release the carboxylate group. Our calculations confirm that the heterolytic Cu−chelate bond breaking of 5_a with an energy cost of 47.7 kcal/mol is unlikely to occur. The coordination of OH− weakens the Cu−O bond and breaks the Cu−N bond in the chelate Cu complex 5_a, as evidenced by the lengthening of the corresponding bonds in 6_a (Figure 2). The coordination of OH− makes the carboxylate dissociation much more facile. A comparison of the molecular orbitals of the three coordinate d 10 Cu(I) species 5_a and 6_a shows that, in both cases, the Cu dxy orbital has a σ*-antibonding character with respect to the carboxyl group (see Figure S1 in the Supporting Information). The more nucleophilic nature of the hydroxyl group causes the dxy orbital in 6_a to be polarized more along the Cu− O(carboxylate) bond, thereby weakening this bond more strongly. The higher nucleophilicity of the OH group is reflected in the stronger Cu−OH bond in 1_a as compared with the Cu−chelate interaction in 5_a; the heterolytic bond dissociation of Cu−OH is calculated to be 68.6 kcal/mol. New Proposed Mechanism. On the basis of the aforementioned results, we propose a new reaction mechanism for the carboxylation of the C−H bond of heteroarenes by (IPr)Cu−OR, as illustrated in Scheme 4.37 The first step of this reaction is C−H activation through transition structure TSI−II,

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CONCLUDING REMARKS

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ASSOCIATED CONTENT

In hindsight, the new reaction mechanism that we have outlined in this paper should not have been unexpected. Carbenes are known to react readily with CO 2,38 and heterocyclic carbenes are well-known species, although the metal-stabilized carbene 3_a is perhaps not an obvious intermediate at first sight. Nevertheless, once formed, this carbene can react in a straightforward manner with CO2 and avoid the difficult pathways via other intermediates. We believe that the electron-releasing nature of Cu is important for the stabilization of the intermediate carbene via an increase in the population of the nitrogen atom adjacent to the carbene center. Finally, our prediction of a carbene intermediate in the catalytic cycle has broader ramifications for experimental design and substrate suitability. Our findings suggest that this carbene intermediate could act as a nucleophile in other organometallic reactions.

S Supporting Information *

Complete ref 15, additional relative energy surfaces, and Cartesian coordinates of all optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org. 6222

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(22) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995. (23) It is a method through which one can estimate the entropy difference between the gas and liquid phases. Okuno, Y. Chem.Eur. J. 1997, 3, 212. (24) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215. (25) Glendening, E. D.; Read, A. E.; Carpenter, J. E.; Weinhold, F. NBO, version 3.1; Gaussian, Inc.: Pittsburgh, PA, 2003. (26) (a) Boutadla, Y.; Davies, D. L.; Macgregor, S. A.; PobladorBahamondeb, A. I. Dalton Trans. 2009, 5820 and references therein. (b) Balcells, D.; Clot, E.; Eisenstein, O. Chem. Rev. 2010, 110, 749 and references therein. (27) (a) Fulton, J. R.; Holland, A. W.; Fox, D. J.; Bergman, R. G. Acc. Chem. Res. 2002, 35, 44. (b) Feng, Y.; Lail, M.; Barakat, K. A.; Cundari, T. R.; Gunnoe, T. B.; Petersen, J. L. J. Am. Chem. Soc. 2005, 127, 14174. (c) Tenn, W. J.; Young, K. J. H.; Bhalla, G.; Oxgaard, J.; Goddard, W. A.; Periana, R. A. J. Am. Chem. Soc. 2005, 127, 14172. (d) Feng, Y.; Lail, M.; Foley, N. A.; Gunnoe, T. B.; Barakat, K. A.; Cundari, T. R.; Petersen, J. L. J. Am. Chem. Soc. 2006, 128, 7982. (e) Feng, Y.; Gunnoe, T. B.; Grimes, T. V.; Cundari, T. R. Organometallics 2006, 25, 5456. (f) Tenn, W. J.; Young, K. J. H.; Oxgaard, J.; Nielsen, R. J.; Goddard, W. A.; Periana, R. A. Organometallics 2006, 25, 5173. (g) Oxgaard, J.; Tenn, W. J.; Nielsen, R. J.; Periana, R. A.; Goddard, W. A. Organometallics 2007, 26, 1565. (h) Cundari, T. R.; Grimes, T. V.; Gunnoe, T. B. J. Am. Chem. Soc. 2007, 129, 13172. (28) One reviewer suggested that we examine the possibility of formation of a Cu−N adduct. Our calculations showed that such an adduct does not correspond to a local minimum on the potential energy surface and the optimization of this adduct led to the outersphere complex 2_a. (29) An alternative pathway leading to the C−H activation is the oxidative addition mechanism in which a transient Cu(III) hydride species is formed. Our preliminary calculations show that this mechanism represents an unfeasible pathway; since the oxidative addition product is very unstable (approximately 60 kcal/mol less stable than 2_a), the Cu(III) species is not a local minimum on the PES and spontaneously converts to 2_a. (30) The six-membered transition structure 1′TS_a in which a water molecule is bridged between the hydroxyl ligand and the benzoxazole substrate is calculated to be ΔG ‡ = 8.0 kcal/mol higher in energy than 1TS_a. This is because this transition structure suffers from a greater entropy penalty upon simultaneous interaction of three species, H 2O, benzoxazole, and LCuOH. (31) The reaction of benzoxazole + (IPr)CuOH → H2O + (IPr) Cu(benzoxazolyl) is calculated to be thermoneutral. An analogous result is also established for the experimentally studied reaction of benzoxazole + (IPr)CuOtBu → tBuOH + (IPr)Cu(benzoxazolyl). It appears that the results calculated for the reaction energies are not consistent with the high yield observed experimentally for the C−H activation product (93%). We believe that hydrogen-bonding interactions among the water molecules formed in situ provide an additional stability to the C−H activation products, making the C−H activation reaction thermodynamically more feasible. (32) The larger the number, the stronger the orbital interaction. (33) Our calculations show that the C−H bond is not activated via the interaction of the oxygen atom on the heteroarene with the Cu metal center, a result that can be attributed to the absence of a formal π bond between C1 and O. Attempts to locate the corresponding transition structure were not successful and led to transition structure 1TS_a. (34) (a) Bickelhaupt, F. M. J. Comput. Chem. 1999, 20, 114. (b) Diefenbach, A.; Bickelhaupt, F. M. J. Phys. Chem. A 2004, 108, 8640. (c) Diefenbach, A.; de Jong, G. T.; Bickelhaupt, F. M. J. Chem. Theory Comput. 2005, 1, 286. (35) The greater deformation for pathway II than pathway I is evidenced by a longer Cu−OH distance and a smaller L−Cu−OH angle in 2TS_a. The Cu−OH bond in 2TS_a is 0.122 Å longer than that in 1TS_a (Figure 2), and the L−Cu−OH angle in 2TS_a (122.6°) is much smaller than that in 1TS_a (155.1°).

ACKNOWLEDGMENTS We thank the Australian Research Council for financial support and the Australian National Computational Infrastructure and the University of Tasmania for computing resources.

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REFERENCES

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dx.doi.org/10.1021/om200744a | Organometallics 2011, 30, 6218−6224

Organometallics

Article

(36) A similar explanation has been provided by Eisenstein, Perutz, and co-workers to account for the Pd−C bond energy correlations. Clot, E.; Megret, C.; Eisenstein, O.; Perutz, R. N. J. Am. Chem. Soc. 2009, 131, 7817. (37) Nolan and co-workers also showed that [(IPr)CuOR] is capable of catalyzing the carboxylation of C−H bonds of polyfluorinated arenes (see ref 12). Our preliminary calculations for 1,3difluorobenzene showed that the transition structure of fluorideassisted C−H activation is not a stationary point on the potential energy surface. Thus, we predict a different mechanism for the carboxylation of polyfluorinated arenes. (38) (a) Riduan, S. N.; Zhang, Y. G.; Ying, J. Y. Angew. Chem., Int. Ed. 2009, 48, 3322. (b) Huang, F.; Lu, G.; Zhao, L.; Li, H.; Wang, Z.-X. J. Am. Chem. Soc. 2010, 132, 12388. (c) Huang, F.; Zhang, C.; Jiang, J.; Wang, Z.-X.; Guan, H. Inorg. Chem. 2011, 50, 3816.

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dx.doi.org/10.1021/om200744a | Organometallics 2011, 30, 6218−6224