DFT Studies on the Carboxylation of Arylboronate Esters with CO2

Organometallics 2010, 29, 917–927 DOI: 10.1021/om901047e

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DFT Studies on the Carboxylation of Arylboronate Esters with CO2 Catalyzed by Copper(I) Complexes Li Dang 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

Todd B. Marder* Department of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K. Received December 7, 2009

DFT calculations have been carried out to study the carboxylation reactions of arylboronate esters with CO2 catalyzed by (NHC)Cu(I) complexes (NHC = N-heterocyclic carbene). The results affirm the basic mechanistic proposal that (1) the transmetalation of the aryl group of the boronic ester to the Cu(I) metal center occurs first; (2) CO2 then inserts into the resulting Cu-Ar bond to give a carboxylate complex; and (3) displacement of the carboxylate by the base adduct of the arylboronic ester substrate completes the catalytic cycle and affords the product ArCOOK. Transmetalation was shown to proceed via a B-O-Cu-bridged intermediate that is formed from the KOR adduct of the arylboronic acid, rather than by the CuOR complex, although both pathways leading to the bridged intermediate are extremely facile. Insertion of CO2 into the Cu-Ar bond is the rate-determining step, in which nucleophilic attack of the aryl ligand on the electron-deficient carbon atom of CO2 leads to the formation of the new C-C bond. Substituents on the aryl group modify its nucleophilicity, affecting the insertion barrier. We predict that alkylboronic esters, but not alkynylboronic esters, would be suitable substrates for the reaction. Introduction CO2 is a greenhouse gas and causes serious harm to the environment. Over the last several decades, much effort has been spent on its activation and utilization, and great progress

has been made in exploring its reactions with transition metal complexes.1 Despite its chemical inertness, CO2 can be activated by many transition metal complexes.2-4 For example, coupling reactions of CO2 with alkyl halides,5 alkenes,6 alkynes,7 allenes,7d,8 organoboranes,9 etc., catalyzed by transition metal complexes have been achieved. Recently, it was found that organoboranes, such as aryl- and alkenylboronate esters, can be carboxylated by reaction with CO2 in the presence of the catalyst [(IPr)CuCl] (IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) in refluxing THF (eq 1).10 In this reaction, wide functional-group compatibility has been achieved.

*Corresponding authors. E-mail: [email protected]; todd.marder@ durham.ac.uk. (1) (a) Braunstein, P.; Matt, D.; Nobel, D. Chem. Rev. 1988, 88, 747. (b) Gibson, D. H. Chem. Rev. 1996, 96, 2063. (c) Aresta, M. Carbon Dioxide Recovery and Utilization; Kluwer Academic: Dordrecht, The Netherlands, 2003. (d) Sakakura, T.; Choi, J.-C.; Yasuda, H. Chem. Rev. 2007, 107, 2365. (e) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388. (f) Aresta, M.; Dibenedettob, A. Dalton Trans. 2007, 2975. (2) (a) Leitner, W. Coord. Chem. Rev. 1996, 153, 257. (b) Yin, X.; Moss, J. R. Coord. Chem. Rev. 1999, 181, 27. (c) Jessop, P. G.; Leitner, W. Chemical Synthesis using Supercritical Fluids; VCH/Wiley: Weinheim, 1999. (d) Marks, T. J.; et al. Chem. Rev. 2001, 101, 953. (e) Takimoto, M.; Kawamura, M.; Mori, M.; Sato, Y. Synlett 2005, 2019. (f) Shimizu, K.; Takimoto, M.; Sato, Y.; Mori, M. Org. Lett. 2005, 7, 195. (3) (a) Rosenthal, U.; Ohff, A.; Baumann, W.; Kempe, R.; Tillack, A.; Burlakov, V. V. Angew. Chem., Int. Ed. Engl. 1994, 33, 1850. (b) Burlakov, V. V.; Arndt, P.; Baumann, W.; Spannenberg, A.; Rosenthal, U. Organometallics 2006, 25, 1317. (c) Beweries, T.; Burlakov, V. V.; Peitz, S.; Arndt, P.; Baumann, W.; Spannenberg, A.; Rosenthal, U. Organometallics 2008, 27, 3954. (4) (a) Dedieu, A.; Ingold, F. Angew. Chem., Int. Ed. Engl. 1989, 28, 1694. (b) Musashi, Y.; Sakaki, S. J. Am. Chem. Soc. 2000, 122, 3867. (c) Musashi, Y.; Sakaki, S. J. Am. Chem. Soc. 2002, 124, 7588. (d) Ohnishi, Y.; Matsunaga, T.; Nakao, Y.; Sato, H.; Sakaki, S. J. Am. Chem. Soc. 2005, 127, 4021. (e) Ohnishi, Y.; Nakao, Y.; Sato, H.; Sakaki, S. Organometallics 2006, 25, 3352. (5) (a) Arzoumanian, H.; Cochini, F.; Nuel, D.; Rosas, N. Organometallics 1993, 12, 1871. (b) Tokuda, M.; Kabuki, T.; Katoh, Y.; Suginome, H. Tetrahedron Lett. 1995, 36, 3345. (c) Kamekawa, H.; Senboku, H.; Tokuda, M. Electrochim. Acta 1996, 42, 13. (d) Shi, M.; Nicholas, K. M. J. Am. Chem. Soc. 1997, 119, 5057. (e) Franks, R. J.; Nicholas, K. M. Organometallics 2000, 19, 1458. (f) Johansson, R.; Wendt, O. F. Dalton Trans. 2007, 488.

(6) (a) Galindo, A.; Pastor, A.; Perez, P. J.; Carmona, E. Organometallics 1993, 12, 4443. (b) Takimoto, M.; Mori, M. J. Am. Chem. Soc. 2001, 123, 2895. (c) Takimoto, M.; Mori, M. J. Am. Chem. Soc. 2002, 124, 10008. (d) Schubert, G.; Papai, I. J. Am. Chem. Soc. 2003, 125, 14847. (e) Takimoto, M.; Nakamura, Y.; Kimura, K.; Mori, M. J. Am. Chem. Soc. 2004, 126, 5956. (f) Papai, I.; Schubert, G.; Mayer, I.; Besenyei, G.; Aresta, M. Organometallics 2004, 23, 5252. (g) Graham, D. C.; Mitchell, C.; Bruce, M. I.; Metha, G. F.; Bowie, J. H.; Buntine, M. A. Organometallics 2007, 26, 6784, and references therein. (7) (a) Walther, D.; Br€aunlich, G.; Kempe, R.; Sieler, J. J. Organomet. Chem. 1992, 436, 109. (b) Mashima, K.; Tanaka, Y.; Nakamura, A. Organometallics 1995, 14, 5642. (c) Bennett, M. A.; Johnson, J. A.; Willis, A. C. Organometallics 1996, 15, 68. (d) Saito, S.; Nakagawa, S.; Koizumi, T.; Hirayama, K.; Yamamoto, Y. J. Org. Chem. 1999, 64, 3975. (e) Aoki, M.; Kaneko, M.; Izumi, S.; Ukai, K.; Iwasawa, N. Chem. Commun. 2004, 2568. (8) (a) Tsuda, T.; Yamamoto, T.; Saegusa, T. J. Organomet. Chem. 1992, 429, C46. (b) Takimoto, M.; Kawamura, M.; Mori, M. Org. Lett. 2003, 5, 2599. (c) Takaya, J.; Iwasawa, N. J. Am. Chem. Soc. 2008, 130, 15254. (d) North, M. Angew. Chem., Int. Ed. 2009, 48, 4104. (9) (a) Ukai, K.; Aoki, M.; Takaya, J.; Iwasawa, N. J. Am. Chem. Soc. 2006, 128, 8706. (b) Takaya, J.; Tadami, S.; Ukai, K.; Iwasawa, N. Org. Lett. 2008, 10, 2697. (10) Ohishi, T.; Nishiura, M.; Hou, Z. Angew. Chem., Int. Ed. 2008, 47, 5792.

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Organometallics, Vol. 29, No. 4, 2010

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Figure 1. Free energy profiles calculated for the (NHC)Cu(OMe)-catalyzed carboxylation of phenylboronic ester based on the mechanism proposed in Scheme 1. The relative free energies and electronic energies (in parentheses) are given in kcal/mol.

Scheme 1

to examine the factors that influence the catalytic reactions. In addition, we examined the importance of base adduct of arylboronate esters in the catalytic reactions and explored whether or not alkynyl or alkyl boronate esters can be carboxylated. Understanding of the reaction mechanism is expected to lead to more efficient synthetic strategies and more efficient catalysts for carboxylation reactions utilizing CO2.

Computational Details

On the basis of a series of stoichiometric reactions, a reaction mechanism for the Cu-catalyzed reactions shown in eq 1 was proposed (Scheme 1).10 The proposed mechanism involves three important steps, i.e., metathesis of arylboronate ester with [(IPr)Cu(OtBu)] (1) to give the copper(I) aryl species 2, insertion of CO2 into the Cu-C(aryl, alkenyl) bond of 2 to give the carboxylate complex 3, and a ligand exchange with tBuO- followed by hydrolysis to give the final carboxylated product. In this paper, with the aid of density functional theory calculations, we explore the energetics associated with the reaction pathway proposed, from which the rate-determining step can be delineated. We also wanted

Molecular geometries of the model complexes were optimized without constraints via DFT calculations using the Becke3LYP (B3LYP)11 functional. Frequency calculations at the same level of theory have also been performed to identify all of the stationary points as minima (zero imaginary frequencies) or transition states (one imaginary frequency) and to provide free energies at 298.15 K, which include entropic contributions by taking into account the vibrational, rotational, and translational motions of the species under consideration. Transition states were located using the Berny algorithm. Intrinsic reaction coordinates (IRC)12 were calculated for the transition states to confirm that such structures indeed connect two relevant minima. The 6-311G* Pople basis set13 was used for the B and C atoms that interact with Cu during the reaction and all atoms of (11) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200. (c) Lee, C.; Yang, W.; Parr, G. Phys. Rev. B 1988, 37, 785. (d) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F. J. Phys. Chem. 1994, 98, 11623. (12) (a) Fukui, K. J. Phys. Chem. 1970, 74, 4161. (b) Fukui, K. Acc. Chem. Res. 1981, 14, 363. (13) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650.

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performing single-point self-consistent reaction field (SCRF) calculations based on the polarizable continuum model (PCM)17 for several selected gas-phase optimized species by adding one diffuse function to the basis sets of all atoms. THF was used as the solvent, corresponding to the experimental conditions, and the atomic radii used for the PCM calculations were specified using the UAKS keyword. The inclusion of solvation effects in our calculations did not change the conclusions. For example, the barrier in the gas phase for the ratedetermining step was calculated to be 19.6 kcal/mol, whereas in THF it was 22.5 kcal/mol. Details can be found in the Supporting Information. All of the DFT calculations were performed with the Gaussian 03 package.18

Results and Discussion

Figure 2. Optimized geometries with selected structural parameters (distances in A˚) for the species involved in the (NHC)Cu(OMe)-catalyzed carboxylation of phenylboronic ester with CO2. Selected calculated structural parameters for the model complexes 2A and 4A are compared to the experimental values (in parentheses) for (IPr)Cu(Ph) and (IPr)Cu[O(CO)Ph] where IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene. the CO2 molecule, while the 6-311G* Wachters-Hay basis set14 was used for Cu. For all other atoms including K, the 6-31G basis set was used.15 Molecular orbitals obtained from the B3LYP calculations were plotted using the Molden 3.7 program written by Schaftenaar.16 The solvent effect was examined by (14) (a) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. (b) Hay, P. J. J. Chem. Phys. 1977, 66, 4377. (15) (a) Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (c) Binning, R. C.Jr.; Curtiss, L. A. J. Comput. Chem. 1990, 11, 1206. (16) Schaftenaar, G. Molden v3.7; CAOS/CAMM Center Nijmegen: Toernooiveld, Nijmegen, The Netherlands, 2001. (17) (a) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999. (b) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117.

Carboxylation of a Phenylboronate Ester with CO2. We first calculated the energy profile for the carboxylation of PhBneop (neop = neopentaneglycolato = OCH2CMe2CH2O) with CO2 catalyzed by [(IPr)Cu(OtBu)] on the basis of the mechanism proposed in Scheme 1. In the calculations, we used [(NHC)Cu{OMe}] {NHC =1,3-dimethylimidazol2-ylidene} (1A) as the model catalyst. Our prior work has established the validity of using 1,3-dimethylimidazol2-ylidene as a model for IPr in catalytic reactions employing copper(I) systems.19 In Figure 1, and the following figures that contain potential energy profiles, calculated relative free energies (kcal/ mol) and relative electronic energies (kcal/mol, in parentheses) are presented. The electronic energies simply represent bond strengths, whereas the free energies take into account entropic effects. They are similar in cases where the number of reactant and product molecules is equal, for example, one-to-one or two-to-two transformations, but differ significantly for one-to-two or two-to-one transformations. In this paper, relative free energies are used to analyze the reaction mechanism because the entropic effects are important in reaction pathways. As shown in Figure 1, 1A and PhBneop form a B-O-Cubridged acid-base adduct, IntA. A similar adduct with diboron reagents has been postulated in our previous work,20 and an example was recently observed experimentally in the reaction of a NiOtBu complex with B2cat2 (cat = 1,2-O2C6H4).21 The adduct IntA undergoes metathesis between the Cu-O and Ph-B bonds with a barrier of 10.3 kcal/ mol to form MeOBneop and 2A. This metathesis step corresponds to the transmetalation of the aryl group of the boronic ester to the Cu(I) metal center. Insertion of CO2 into the Cu-Ph bond of 2A by transferring the phenyl ligand to the carbon atom of CO2 occurs via the transition state TSA(2-3) to give 3A, (NHC)Cu[O(CO)Ph]. This CO2 insertion step has a barrier of 19.6 kcal/mol and is rate-determining. The alternative CO2 insertion in which the phenyl ligand migrates to the oxygen atom of CO2 to give 5A containing a Cu-C(O)-O-Ph linkage was also studied. However, the barrier is inaccessibly high (56.2 kcal/mol). Intermediate 3A (18) Frisch, M. J.; et al. et al. Gaussian 03, revision B.05; Gaussian, Inc.: Pittsburgh, PA, 2003. (19) (a) Zhao, H. T.; Lin, Z.; Marder, T. B. J. Am. Chem. Soc. 2006, 128, 15637. (b) Dang, L.; Zhao, H. T.; Lin, Z.; Marder, T. B. Organometallics 2008, 27, 1178. (c) Zhao, H. T.; Dang, L.; Marder, T. B.; Lin, Z. J. Am. Chem. Soc. 2008, 130, 5586. (d) See also: Dang, L.; Lin, Z.; Marder, T. B. Chem. Commun. 2009, 3987. (20) Kleeberg, C.; Dang, L.; Lin, Z.; Marder, T. B. Angew. Chem., Int. Ed. 2009, 48, 5350. (21) Scott, J.; Mindiola, D. J. Dalton Trans. 2009, 8463.

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Figure 3. Free energy profiles calculated for the formation of IntA via the two paths shown in Scheme 2. The relative free energies and electronic energies (in parentheses) are given in kcal/mol.

undergoes isomerization through a rotation of the C(O)Ph group around the C-O single bond to give the more stable intermediate 4A, from which a ligand exchange with MeOoccurs to regenerate the catalyst 1A and give PhCOO- as the final product. Figure 2 shows the optimized structures with selected structural parameters for the species involved in Figure 1. In Figure 2, the calculated structures of the model complexes 2A and 4A are compared with their corresponding experimental ones. Calculated geometric parameters for 2A [CuC(NHC) = 1.88 A˚, Cu-C(Ph) = 1.91 A˚] and for 4A [Cu-C(NHC) = 1.84 A˚, Cu-O = 1.86 A˚] agree well with experimentally determined values10 for the (IPr)Cu(C6H4-4-OMe) complex [Cu-C(IPr) = 1.90 A˚, Cu-C(C6H4-4-OMe) = 1.91 A˚] and for the (IPr)CuO(CO)(C6H4-4-OMe) complex10 [Cu-C(IPr) = 1.87 A˚, Cu-O = 1.86 A˚], thus confirming that the basis sets are adequate for the present study. In 4A, the secondary Cu-O distance is quite long (Figure 2), and thus, this interaction must be very weak at best. To investigate whether Beg (eg = ethylene glycolato = OCH2CH2O) can be used as a resonable model to replace Bneop as well as Bpin, as examined in our previous study,19 DFT calculations were carried out on the metathesis of PhBeg with the Cu-OR bond in 1A. The metathesis can occur with a barrier of 11.5 kcal/mol to give MeOBeg and 2A, suggesting that Beg is a good model for Bneop and also that PhBpin should be a reasonable substrate for the carboxylation reaction, although this has not yet been reported experimentally. A slightly larger calculated barrier for Beg likely results from the increased rigidity of the Beg ring compared to that of the Bneop ring. We also note that the calculated metathesis barrier for reaction of Cu-OR with the B-B bond in B2eg2, examined in our previous study,19b is much lower than that for reaction of Cu-OR with Ph-Beg. This can be attributed to the fact that (1) the B-B bond in B2eg2 is weaker than the C-B bond in Ph-Beg, and (2) the Cu-Beg bond is stronger than the Cu-Ph bond. Transmetalation of Arylboronate with LCuX to Give LCuAr: The Role of the Base. Following the originally

proposed mechanism and the stoichiometric model studies,10 we have arrived at the critical species IntA for transmetalation (or metathesis) by reaction of preformed LCu-OR with ArBneop. However, the detailed mechanism of formation of intermediates similar to IntA is often debated because the role of the base is ambiguous.22 Thus, IntA could arise, as proposed (Scheme 1), via a ligand exchange of a copper halide complex with KOR to generate a copper alkoxy complex, which then reacts with arylboronate (shown as path a in Scheme 2). Alternatively, PhBneop and KOMe could initially form an adduct that subsequently reacts with the copper halide to give IntA and potassium halide (shown as path b in Scheme 2). The calculated energy profiles for these two processes (Figure 3) show that the alternative process (path b), proceeding via stable intermediate B 3 LCuCl, is actually favored. A similar conclusion was drawn by Maseras et al. for transmetalation in the Suzuki-Miyaura reaction.22d For the purposes of these particular calculations, Scheme 2

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Figure 4. Optimized geometries with selected structural parameters (distances in A˚) for the species shown in Figure 3.

given that we are not considering solvation effects, and in order to reference all species on the same energy scale, we note that we have maintained KCl coordination to the intermediates throughout, as shown in Figure 4. However, both processes leading to IntA 3 KCl (gas phase) or IntA (in solution) are very facile, and, critically, the base is required to assist the overall transmetalation process. While the details (22) (a) Smith, G. B.; Dezeny, G. C.; Hughes, D. L.; King, A. O.; Verhoeven, T. R. J. Org. Chem. 1994, 59, 8151. (b) Matos, K.; Soderquist, J. A. J. Org. Chem. 1998, 63, 461. (c) Miyaura, N. J. Organomet. Chem. 2002, 653, 54. (d) Braga, A. A. C.; Morgon, N. H.; Ujaque, G.; Maseras, F. J. Am. Chem. Soc. 2005, 127, 9298.

of the approach to IntA are a matter of intellectual curiosity, with regard to the current catalytic process, it does not matter which path to IntA is followed, because neither is rate-determining (vide supra). As path b is the one followed, Scheme 3 illustrates the overall catalytic cycle using the ArBneop 3 KOMe adduct as the substrate. Understanding CO2 Insertion into a Cu-Aryl Bond. As mentioned above, the CO2 insertion is the rate-determining step for the catalytic reaction. Comparing the CO2 insertion into Cu-Ph and Cu-Beg bonds, the barrier for CO2 insertion into the Cu-Ph bond is higher than that which we found previously for CO2 insertion into a Cu-Beg bond.19a The

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Figure 5. HOMO and HOMO-1 calculated for the transition state of the CO2 insertion with (NHC)CuPh (2A). The orbital energies are given in eV.

Scheme 3 Figure 6. Free energy profiles calculated for the insertion reactions of CO2 with the model complexes (NHC)Cu(p-nitrophenyl) (2A-NO2) and (NHC)Cu(p-methoxyphenyl) (2A-NMe2). The calculated relative free energies and electronic energies (in parentheses) are given in kcal/mol.

latter process proceeds via a CO2 π-coordinated intermediate, while no such interaction with NHC-Cu-Ph was found in the present work. In the CO2-coordinated intermediate in the former case, the back-bonding interaction between the Cu-B σ bond and the CO2 carbon is significant. Therefore,

we believe that the greater back-bonding interaction to the CO2 carbon of the Cu-boryl versus the Cu-Ph σ bond explains the smaller barrier found for CO2 insertion into the Cu-B bond. This reflects the exceptional σ-donor properties and high nucleophilicity of boryl ligands23 resulting from the electropositive nature of boron. We plotted the highest and the second highest occupied molecular orbitals (HOMO and HOMO-1) calculated for the insertion transition state (Figure 5). These two occupied orbitals are relevant to bond formation and cleavage during the insertion process. In the HOMO, it is clear that the migrating phenyl ligand uses its sp2-hybridized orbital at the metal-bonded carbon to interact in a σ bonding fashion with one of the π* orbitals of the CO2 moiety. The insertion process results from nucleophilic attack of the Cu-Ph σ bond on the CO2 carbon.19a Interaction between the Cu-R σ bonding orbital and one of the π* orbitals of the CO2 moiety was previously emphasized in theoretical studies of CO2 (23) Zhu, J.; Lin, Z.; Marder, T. B. Inorg. Chem. 2005, 44, 9384.

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Figure 7. Optimized structures with selected structural parameters (distances in A˚) for selected species involved in Figure 6.

insertion into a Cu-Me bond.24 In the HOMO-1, a π bonding MO of the phenyl ligand interacts with the copper center. These results point to the fact that both the σ and π orbitals at the metal-bonded carbon of the migrating phenyl ligand contribute significantly to the stability of the transition state, making the CO2 insertion barrier accessible. Here, even with the help of the π orbital at the metal-bonded carbon, CO2 insertion into a Cu-C bond has a higher barrier (24) (a) Sakaki, S.; Ohkubo, K. Organometallics 1989, 8, 2970. (b) Sakaki, S.; Musashi, Y. Inorg. Chem. 1995, 34, 1914.

than CO2 insertion into a Cu-Beg bond, in which the empty p orbital of boron is not involved in the insertion process, as shown in our previous study.19a To examine further the effect of the migrating group on the insertion barrier, we calculated the barriers for CO2 insertion into the Cu-C(C6H4-4-NO2) and Cu-C(C6H4-4-NMe2) bonds (Figure 6). The electron-withdrawing substituent (-NO2) indeed increases the insertion barrier significantly, while the electron-donating substituent (-NMe2) lowers the barrier. Among the three insertion transition states (Figures 2 and 7), TSA-NO2(2-3) has the longest Cu-C(aryl)

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Figure 8. Free energy profiles calculated for the insertion reactions of CO2 with the model complexes (NHC)Cu(vinyl) (2B), (NHC)Cu(ethyl) (2C), and (NHC)Cu(ethynyl) (2D). The calculated relative free energies and electronic energies (in parentheses) are given in kcal/mol.

and shortest C(aryl)---C(CO2) distances and TSA-NMe2(2-3) has the shortest Cu-C(aryl) and longest C(aryl)---C(CO2) distances, corresponding to a late and an early transition state, respectively, consistent with the relative barriers of the three insertion reactions. The order of the exothermicity of the three insertion reactions also parallels that of the barriers, a trend related to the strength of the C(aryl)-C(CO2) bond formed from the insertion reactions. The electron-donating substituent (-NMe2) gives a short and strong C-C bond in the insertion product (Figure 7).

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Reactions Involving Other Hydrocarbyl Boronate Esters. As mentioned in the Introduction, aryl- and alkenylboronate esters were used experimentally to react with CO2.10 It was also of interest to see whether alkynyl- and alkylboronates could react with CO2. We therefore examined the insertions of CO2 into Cu-C(vinyl, ethyl, ethynyl) bonds (Figure 8). Insertion of CO2 into the Cu-C(vinyl) (Figure 8a) bond has approximately the same barrier as that into the Cu-C(phenyl) bond (Figure 1). Insertion of CO2 into the Cu-C(ethyl) bond is thermodynamically more favorable but kinetically less favorable, with a higher barrier of 24.1 kcal/ mol (Figure 8b) versus 19.6 kcal/mol for the Cu-C(phenyl) complex, vide supra. These results are consistent with the experimental observations that reaction of (IPr)CuCH3 with CO2 at room temperature afforded (IPr)Cu[O(CO)CH3] in 95% yield after 2.5 h,25 while reaction of (IPr)CuC6H4-OMe with CO2 gave 92% yield of the carboxylate product at -78 °C to room temperature within 10 min.10 In a previous theoretical study on the reaction of CO2 with a phosphine-ligated copper(I) methyl complex reported by Sakaki et al., the barrier for insertion was calculated to be 23 kcal/mol using the MP2 method,26 a value similar to the one we calculated for the insertion of CO2 into the (NHC)Cu-C(ethyl) bond. For the insertion of CO2 into the Cu-C(ethynyl) bond (Figure 8c), the reaction is endothermic, with a barrier of 30.0 kcal/mol. One rare report suggesting the insertion of CO2 into a Cu-C(alkynyl) bond is the Cu(I)-catalyzed carboxylation of terminal alkynes in the presence of bromoalkanes, which gives the corresponding ester product at 100 °C in polar, aprotic solvents.27 The instability of copper phenylpropionate to decarboxylation at 35 °C was noted. The barrier difference presented in Figure 8 is related to the extent of structural distortion in the transition states. Figure 9 shows the optimized geometries with selected structural parameters for the species shown in Figure 8. The Cu-C(hydrocarbyl) bonds are lengthened by 0.09, 0.11, and 0.12 A˚ from 2B, 2C, and 2D to TSB(2-3), TSC(2-3), and TSD(2-3), respectively. The order of the barriers is closely related to the magnitude of the Cu-C(hydrocarbyl) bond lengthening. To understand better the differences among the CO2 insertions into different Cu-C bonds, we examined the frontier molecular orbitals of the four N-heterocyclic carbene-ligated copper(I) complexes studied. In Figure 10, the HOMOs of 2A, 2B, and 2C correspond to the respective Cu-C(hydrocarbyl) σ bonds. The Cu-C(hydrocarbyl) σ bonding orbital of 2D lies far below the HOMO and is HOMO-2 instead. It is clear that the σ bonding orbital of 2C has the highest orbital energy, while that of 2D has the lowest orbital energy, consistent with the notion that Cu-C(sp3) is the weakest bond and Cu-C(sp) is the strongest. In the discussion of CO2 insertion into Cu-C(aryl), we pointed out that both the σ and π orbitals at the Cu-bonded carbon of the migrating phenyl ligand contribute significantly to the stability of the CO2 insertion transition state. The Cu-C(aryl) σ bond acts as a nucleophile, attacking the CO2 carbon, while the π system of the aryl ligand is able to stabilize the transition-state structure through interaction (25) Mankad, N. P.; Gray, T. G.; Laitar, D. S.; Sadighi, J. P. Organometallics 2004, 23, 1191. (26) Sakakl, S.; Ohkubo, K. Organometallics 1989, 8, 2970. (27) Fukue, Y.; Oi, S.; Inoue, Y. J. Chem. Soc., Chem. Commun. 1994, 2091.

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Figure 9. Optimized structures with selected structural parameters (distances in A˚ and angles in deg) for selected species involved in Figure 8.

with the metal center during the Cu-C(aryl) σ bond breaking process. Although 2C has the highest-lying Cu-C(ethyl) σ orbital, making the Cu-C(ethyl) σ bond the most nucleophilic, it lacks a π bonding molecular orbital to interact with the copper center, resulting in the high insertion barrier. 2D has the lowest Cu-C(ethynyl) σ bonding orbital. In addition, the ethynyl ligand also has the lowest energy π bonding orbitals. All of these make the CO2 insertion into the Cu-C(ethynyl) bond difficult, and thus the insertion barrier is the highest (Figure 10). Figure 8 also shows that the three insertions have distinctly different exothermicity. Each insertion reaction breaks one Cu-C(hydrocarbyl) bond and one CO2 π bond and forms one Cu-O(CO2) bond and one (CO2)C-C(hydrocarbyl) bond. If we assume that the Cu-O bonds in the three insertion products have similar bond strengths, the different exothermicity should be closely related to the Cu-C(hydrocarbyl) and (CO2)C-C(hydrocarbyl) bond strengths of different hydrocarbyls. We compared the Cu-C(hydrocarbyl) and (CO2)C-C(hydrocarbyl) bond strengths of different hydrocarbyls by calculating the reaction energies of the isodesmic reactions shown in eqs 2-5, from which we see that the Cu-C(vinyl) bond is stronger by 16.9 kcal/mol than the Cu-C(ethyl) bond and the (CO2)C-C(vinyl) bond is stronger by 12.5 kcal/mol than the (CO2)C-C(ethyl) bond. The difference in exothermiciy between the reaction of 2B and that of 2C is 4.6 kcal/mol in favor of 2C (Figure 8), a value very close to 4.4 kcal/mol (= 16.9 - 12.5) obtained

simply by considering the Cu-C(hydrocarbyl) and (CO2)C-C(hydrocarbyl) bond strengths of different hydrocarbyls. Similarly, from the reaction energies calculated for eqs 4 and 5, we see that the Cu-C(ethynyl) bond is stronger by 44.8 kcal/mol than the Cu-C(vinyl) bond and the (CO2)C-C(ethynyl) bond is stronger by 27.1 kcal/mol than the (CO2)C-C(vinyl) bond. The difference in exothermiciy between the reaction of 2B and that of 2D is 16.4 kcal/mol in favor of 2B (Figure 8), a value again very close to 17.7 kcal/ mol (= 44.8 - 27.1) obtained simply by considering the Cu-C(hydrocarbyl) and (CO2)C-C(hydrocarbyl) bond strengths of different hydrocarbyls. The calculations based on the isodesmic reactions show that the bond energy difference between Cu-C(sp) and Cu-C(sp2) is much greater than that between Cu-C(sp2) and Cu-C(sp3). Similarly, the bond energy difference between LCuOC(O)-C(sp) and LCuOC(O)-C(sp2) is larger than that between LCuOC(O)-C(sp2) and LCuOC(O)C(sp3). These results are consistent with the DFT results (28) (a) Rosini, G. P.; Liu, F.; Krogh-Jespersen, K.; Goldman, A. S.; Li, C.; Nolan, S. P. J. Am. Chem. Soc. 1998, 120, 9256. (b) Fernandez, I.; Frenking, G. Chem.-Eur. J. 2006, 12, 3617. (29) (a) Powell, C. E.; Cifuentes, M. P.; McDonagh, A. M.; Hockless, D. C. R. Inorg. Chim. Acta 2003, 352, 9. (b) Kuo, C. K.; Chang, J. C.; Yeh, C. Y.; Lee, G. H.; Wangc, C. C.; Peng, S. M. Dalton Trans. 2005, 3696. (c) Tam, A. Y.Y.; Lam, W. H.; Wong, K. M. C.; Zhu, N.; Yam, V. W.W. Chem. -Eur. J. 2008, 14, 4562. (d) Zhang, L.; Xi, B.; Liu, I. P.; Choudhuri, M. M. R.; Crutchley, R. J.; Updegraff, J. B.; Protasiewicz, J. D.; Ren, T. Inorg. Chem. 2009, 48, 5187.

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Figure 10. Frontier molecular orbitals calculated for the model complexes (NHC)Cu(phenyl) (2A), (NHC)Cu(vinyl) (2B), (NHC)Cu(ethyl) (2C), and (NHC)Cu(ethynyl) (2D). The orbital energies are given in eV.

reported earlier for Ir-CH3, Ir-Ph, Ir-CtCH, C(sp3)-C(sp3), C(sp2)-C(sp3), and C(sp)-C(sp3) bond energies.28,29

Conclusions The detailed mechanism for carboxylation of arylboronic esters with CO2 catalyzed by copper(I) alkoxy complexes was

studied with the aid of DFT by calculating the geometries and energies of the relevant intermediates and transition states. The computational results based on the model complex (NHC)Cu(OMe) and substrate PhBneop showed that the catalyzed carboxylation occurs via three major steps: (1) transmetalation of PhBneop to LCuX assisted by base; (2) CO2 insertion into the Cu-Ph bond to give the Cu-OC(O)Ph carboxylate intermediate; and finally, (3) reaction of this intermediate with the base adduct of the PhBneop substrate, which regenerates the catalyst and affords the benzoate product. We have shown that transmetalations of PhBeg, PhBneop, and PhBpin to Cu have similar barriers, and therefore, ArBpin esters should also be suitable substrates for the carboxylation reaction. Interestingly, we found that the transmetalation likely proceeds via formation of the ArBneop 3 KOR adduct rather than by reaction of KOR with (NHC)CuCl. However, regardless of the sequence of steps leading to the B-O-Cu-bridged intermediate, IntA, the process is extremely facile. Insertion of CO2 into the Cu-C(Ph) bond was found to be the rate-determining step in the catalytic cycle. Molecular orbital analysis of the transition state for the insertion process shows that the Cu-C(Ph) σ bond nucleophilically attacks the electron-deficient CO2 carbon. The π bonding molecular orbital of the phenyl ligand also contributes to the

Article

stability of the transition state by interacting with the metal center. The calculated barrier for CO2 insertion into a Cu-C(sp3) bond is greater by ca. 4 kcal/mol, and that into a Cu-C(sp) bond by ca. 10 kcal/mol, compared with insertion into a Cu-C(sp2) bond. These results point to the possibility of employing alkylboronate esters to react with CO2 under somewhat harsher reaction conditions, whereas the related carboxylation using alkynylboronate esters seems less likely, although it may be possible to devise ways to trap the products. Higher barriers for the insertions into Cu-C(sp3) and Cu-C(sp) bonds are related to the lack of a π bonding molecular orbital in the alkyl ligand and to the very strong Cu-C σ bond for the alkynyl ligand.


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Acknowledgment. This work was supported by the Research Grants Council of Hong Kong (HKUST 601507 and HKU1/CRF/08). T.B.M. thanks the Royal Society (UK) for support via an International Outgoing Short Visit Grant, the Royal Society of Chemistry for a Journals Grant for International Authors, and the EPSRC for support via an Overseas Travel Grant. Supporting Information Available: Complete refs 2d and 18, results of calculations that include solvation effects, and tables giving Cartesian coordinates and electronic energies for all of the calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org.

DFT Studies on the Carboxylation of Arylboronate Esters with CO2

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