Computational exploration of ligand effects in copper-catalyzed boracarboxylation of styrene with CO2

Xiangying Lv a, Yan-Bo Wu b and Gang Lu *c
aKey Laboratory for Yellow River and Huai River Water Environment and Pollution Control, Ministry of Education, Henan Key Laboratory for Environmental Pollution Control, School of Environment, Henan Normal University, Xinxiang, Henan 453007, P. R. China
bKey Lab for Materials of Energy Conversion and Storage of Shanxi Province and Key Lab of Chemical Biology and Molecular Engineering of Ministry of Education, Institute of Molecular Science, Shanxi University, Taiyuan, Shanxi 030006, P. R. China
cDepartment of Chemistry, University of Pittsburgh, Pittsburgh, PA, USA. E-mail: gal40@pitt.edu

Received 11th August 2017 , Accepted 28th September 2017

First published on 29th September 2017


The critical ligand effects in copper-catalyzed boracarboxylation of styrene were investigated using density functional theory (DFT) calculations. Based on the rate-determining CO2 insertion step, the computations reveal that the reactivity of the catalysts ligated by monophosphine ligands is controlled by the ligand's electronic properties. This is consistent with the nature of nucleophilic attack on CO2 by the benzylcopper intermediate. In contrast, the NHC ligands exert significant steric effects on the reactivity. The ineffectiveness of bidentate phosphine ligands originated from the large distortion of the catalyst and CO2 that is caused by the sterically congested transition state of CO2 insertion.


1. Introduction

Olefin carboxylation with CO2 can not only offer great opportunities for making value-added carboxylic acids,1 but also motivate the interest in utilizing CO2 as an appealing C1 building block in organic synthesis.2–23 In this connection, various transition metal-based catalytic systems have been developed for accessing the carboxylation of the unsaturated carbon–carbon bond using CO2.24–36 Recently, the Popp group reported a copper-catalyzed boracarboxylation of styrenes with CO2 and B2pin2.37 As shown in Scheme 1, the addition of CO2 and the Bpin group into styrene delivers the boracarboxylation product 2 with high regioselectivity (the minor regioisomer is not shown). The undesired borylation product 3 and protoboration product (not shown) can also be obtained under the experimental conditions. The most remarkable feature of this reaction is the significant ligand effect on the reactivity of boracarboxylation. Three types of ligands, monodentate phosphine ligands (e.g., PPh3, PMePh2, PEt3, PCy3), N-heterocyclic carbene (NHC) ligands (e.g., SIPr, IPr, SIMes, IMes, ICy) and bidentate phosphine ligands (BINAP, Me-BPE, Xantphos), have been experimentally explored in this reaction. Among these, PCy3, IMes and ICy supporting copper catalysts are the most active ones (entries 4, 8, and 9). In contrast, other monophosphine and NHC ligands with different P-bound substituents and N-bound substituents display relatively low yields of the desired product 2 (entries 1–3, 5–7). In addition, the bidentate phosphine-based catalysts show quite low reactivities in this reaction (entries 10–12). The origin of these striking ligand effects on reactivity is still unexplored with computations.
image file: c7cy01637a-s1.tif
Scheme 1 Cu-catalyzed boracarboxylation of styrene. Experimental conditions: styrene (0.25 mmol), THF (4 mL), B2pin2 (1.5 equiv.), NaOtBu (3.0 equiv.), CuCl (12 mol%), phosphine ligand (13 mol%) or (NHC)CuCl (12 mol%).

The general mechanism of this reaction is straightforward, including the addition of LCu–B to styrene (I and II, Scheme 2), CO2 insertion into the Cu–C bond (III) and transmetalation with copper carboxylate (IV). The products 2 and 3 are generated from the competing pathways diverged at benzylcopper intermediate I. Although previous studies have gained useful insights into the reaction mechanism,38–42 the origin of the ligand effect on the reactivity remains elusive because earlier computational studies often employ small model phosphine and NHC ligands, and do not have a systematic comparison among various different ligands. Herein, we performed DFT calculations to identify how the ligand's electronic and steric properties dominate the reactivity in this copper-catalyzed boracarboxylation reaction.


image file: c7cy01637a-s2.tif
Scheme 2 Proposed mechanism.

2. Computational methods

All calculations were performed with Gaussian 09.43 In geometry optimizations, the B3LYP density functional and a mixed basis set of SDD for Cu and 6-31G(d) for other atoms were used. All minima have zero imaginary frequency and all transition states have only one imaginary frequency. Single-point energies were calculated using the M06 functional44,45 and a mixed basis set of SDD for Cu and 6-311+G(d,p) for other atoms. Solvation energy corrections were calculated using the SMD model.46 THF was used as solvent in the calculations. The same level of theory was often used in recent computational studies of Cu-catalyzed reactions.47–49 The natural bond orbital (NBO) charge was calculated at the M06/SDD–6-311+G(d,p) level in the THF solvent based on the geometry optimized at the B3LYP/SDD–6-31G(d) level. The 3D structures of molecules were generated using CYLview.50

3. Results and discussion

We first studied the mechanism of Cu-catalyzed boracarboxylation of styrene using CO2 and B2pin2. LCu–Bpin (4 in Fig. 1), generated from the reaction of LCu–OtBu with B2pin2 (see Fig. S1 in the ESI for details), is considered as the active catalyst. The addition of 4 to styrene determines the regioselectivity of the reaction. The borocupration transition state with Cu attacking the α-carbon of styrene (5-TS, ΔG = 11.1 kcal mol−1) is significantly more favorable than that with Cu attacking the β-carbon of styrene (7-TS, ΔG = 22.1 kcal mol−1). This is mostly due to the formation of a stable benzylic copper intermediate (6) via5-TS.38
image file: c7cy01637a-f1.tif
Fig. 1 Energy profile of Cu-catalyzed boracarboxylation of styrene. Energies are with respect to the separated LCu–Bpin and styrene.

Two competing pathways, CO2 insertion and β-H elimination, are possible after the formation of intermediate 6. The β-H elimination pathway is disfavored both kinetically and thermodynamically (11-TS, ΔG611-TS = 26.2 kcal mol−1). This is in line with the quite low yield of the borylation product 3 when ICy is employed as the ligand (entry 9 in Scheme 1). In contrast, the CO2 insertion with a barrier of 20.1 kcal mol−1 (9-TS, with respect to 6) is an irreversible process. This represents the rate-determining step in the overall catalytic cycle. The resulting copper carboxylate 10 can be transformed to the major boracarboxylation product 2via transmetalation and acidification steps.

Based on the rate-determining CO2 insertion, we then investigated the ligand effect on the reactivity. The computed barriers of CO2 insertion with different ligands are shown in Table 1. Typically, the nucleophilicity of the negatively charged α-carbon in 6 is crucial for the reactivity of CO2 insertion.51 Thus, we calculated the NBO charges on the α-carbon in 6 with different ligands (Table 1). As expected, due to their stronger ability for electron donation, the NHC and bidentate phosphine ligands furnish more negative charges on the α-carbon. However, the trend of the barriers of CO2 insertion is not consistent with the change of the nucleophilicity of the α-carbon. Although having a more negative charge on the α-carbon, the benzylcopper intermediates with SIPr, IPr and bidentate phosphine ligands lead to higher barriers than those of monophosphine-supported benzylcopper with less negative charges (entries 5, 6, 10–12 vs. entries 1–4). We further plotted the relationship between the barrier of CO2 insertion and the charge of α-carbon with monophosphine and NHC ligands (Fig. 2). The crossing trend indicates that the reactivity of CO2 insertion is not only affected by the nucleophilicity of the benzylcopper intermediate. The monophosphine ligand with stronger donicity leads to a lower barrier (e.g., PCy3). On the contrary, the ICy ligand with the lowest barrier affords the least negative charge compared to other NHC ligands. Therefore, we hypothesize that both the ligand's electronic and steric properties would influence the reactivity of nucleophilic attack on CO2 by benzylcopper intermediates.

Table 1 Ligand effects on the reactivity of CO2 insertion
Entry Ligand Barrier (ΔG, kcal mol−1) Charge (e) on α-carbona ν(CO)b (cm−1) % Vbur
a The α-carbon of styrene attached to Cu in 6 (Fig. 1). b The values taken from ref. 52 and 54.
1 PPh3 25.5 −0.626 2068.9 26.8
2 PMePh2 24.5 −0.629 2067.0 25.1
3 PEt3 23.8 −0.635 2061.7 24.8
4 PCy3 20.7 −0.635 2056.4 29.0
5 SIPr 27.9 −0.645 2051.5 36.1
6 IPr 26.8 −0.643 2050.5 31.6
7 SIMes 22.7 −0.640 2051.2 32.5
8 IMes 22.7 −0.640 2050.5 31.5
9 ICy 20.1 −0.634 2049.7 26.0
10 BINAP 30.3 −0.655
11 Me-BPE 27.8 −0.649
12 Xantphos 26.4 −0.655



image file: c7cy01637a-f2.tif
Fig. 2 Relationship between the barrier of CO2 insertion (ΔG) and the charges on the α-carbon atom.

To verify our hypothesis, we use Tolman's electronic parameter (TEP, νco)52 and the buried volume parameter (% Vbur)53 to study the ligand's electronic and steric effects on the reactivity, respectively. For the monodentate phosphine ligands, a good linear correlation between ΔG and the ligand's TEP (νco) was observed (R2 = 0.92, Fig. 3). This indicates that the reactivity of CO2 insertion with the copper catalyst ligated by monophosphine ligands (e.g., PPh3, PMePh2, PEt3 and PCy3) is controlled by the ligand's electronic properties. In contrast, ΔG poorly correlates with the TEP of NHC ligands54 (see Fig. S2 for details).


image file: c7cy01637a-f3.tif
Fig. 3 Correlation between the barrier of CO2 insertion (ΔG) and the νco of monophosphine ligands.

Next, we studied the effect of the ligand's steric properties on the reactivity. Unfortunately, for both NHC ligands and monophosphine ligands, we did not observe good correlations between the ΔG of CO2 insertion and the % Vbur (R2 = 0.67 for NHC ligands in Fig. 4; see the correlation with monophosphine ligands in Fig. S3). Nevertheless, the general trend shown in Fig. 4 suggests that the reactivity is suppressed by the bulkiness of NHC ligands. The ICy ligand with the smallest % Vbur leads to the lowest barrier.


image file: c7cy01637a-f4.tif
Fig. 4 Correlation between the barrier of CO2 insertion (ΔG) and the % Vbur of NHC ligands.

Because the buried volume parameter may oversimplify the catalytic pocket formed by a given ligand,53 certain critical interactions between the catalyst and the substrate may be overlooked. Thus, after finding a general trend of the ligand's steric effect on reactivity shown in Fig. 4, we further compared the optimized geometries of CO2 insertion transition states with SIPr, IPr, SIMes, IMes and ICy ligands (13-TS16-TS and 9-TS, Fig. 5a). Clearly, there are repulsive interactions between CO2 and the NHC ligand in these transition states, as evidenced by the shortest O⋯H distance (highlighted in red, Fig. 5a). Moreover, the O⋯H distances in 13-TS/14-TS, 15-TS/16-TS, and 9-TS are significantly different. As can be seen in Fig. 5b, a very good linear correlation between the barrier of CO2 insertion and the O⋯H distance between CO2 and the NHC ligand was observed (R2 = 0.97). This result indicates that the effect of NHC ligands on the reactivity is mostly derived from the steric repulsion between CO2 and the N-bound substituents. The relatively long O⋯H distance in 9-TS with the ICy ligand exerts small steric hindrance when incorporating CO2, thus leading to high reactivity (ΔG69-TS = 20.1 kcal mol−1). In contrast, because the ortho-substituent in the NHC ligand points towards CO2 in the geometries of transition states (13-TS16-TS), bulkier substituents lead to closer proximities to CO2, resulting in more disfavored repulsions thus decreasing the reactivities (ortho-isopropyl substituents in 13-TS/14-TSvs. ortho-methyl substituents in 15-TS/16-TS).55


image file: c7cy01637a-f5.tif
Fig. 5 (a) Optimized geometries of the transition states of CO2 insertion with different NHC ligands. (b) Correlation between the barrier of CO2 insertion (ΔG) and the shortest O⋯H distance between CO2 and the ligand.

Compared to the abovementioned monophosphine ligands and NHC ligands, the catalysts supported by the bidentate phosphine ligands (e.g., BINAP, Me-BPE and Xantphos) show extremely high barriers of CO2 insertion, although these ligands can dramatically enhance the nucleophilicity of the benzylcopper intermediate (entries 10–12 in Table 1). This can be ascribed to the steric congestion around the four-coordinated copper center in the CO2 insertion transition states. To better understand the low reactivities with bisphosphine ligands, we used the distortion/interaction model56 to analyze the difference in reactivity of the catalysts with PCy3, BINAP, Me-BPE and Xantphos ligands (Fig. 6). In the distortion/interaction analysis, the geometry of the CO2 insertion transition state was dissected into two fragments, CO2 and LCu–Bpin. The distortion energies of each fragment (ΔEdist(CO2) and ΔEdist(LCu-B)) from the transition states with bisphosphine ligands are larger than those with PCy3, respectively. The computed activation energies (ΔE) in the gas phase are consistent with the total distortion energies (ΔEdist(CO2) + ΔEdist(LCu-B)) among these four phosphine ligands. This result suggests that the CO2 insertion is more sensitive to the steric crowdedness of bidentate phosphine ligands than their electronic donicities.


image file: c7cy01637a-f6.tif
Fig. 6 Computed distortion energies of the CO2 fragment (shown in blue) and the LCu–Bpin fragment (shown in orange) in the transition states of CO2 insertion.

Conclusions

In summary, we performed DFT calculations to investigate the origin of ligand effects in the Cu-catalyzed boracarboxylation of styrene in the presence of B2pin2 and CO2. The computed mechanism indicates that the CO2 insertion is the rate-determining step in the overall catalytic cycle. Although the nucleophilic attack in the CO2 insertion step is typically favored by electron-rich benzylcopper intermediates, the computations reveal that both the electronic and steric properties of ligands can affect the reactivity. For monophosphine ligands, the electron donation ability of the ligand is the dominant factor in promoting the reactivity. For NHC ligands and bisphosphine ligands, the bulkiness of the ligand leads to steric repulsions between the ligand and CO2, or increases the total distortion of the catalyst and CO2. Both these effects result in low reactivity of CO2 insertion. These theoretical insights into ligand effects may be useful for designing efficient transition metal catalysts in CO2 transformations.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21507025), the Key Research Project of Henan Province (15A610001), and the Scientific Research Starting Foundation of Henan Normal University (5101219170104).

References

  1. L. J. Gooßen, N. Rodríguez and K. Gooßen, Angew. Chem., Int. Ed., 2008, 47, 3100–3120 CrossRef PubMed .
  2. L. Janis, Curr. Org. Chem., 2005, 9, 605–623 CrossRef .
  3. T. Sakakura, J.-C. Choi and H. Yasuda, Chem. Rev., 2007, 107, 2365–2387 CrossRef CAS PubMed .
  4. D. J. Darensbourg, Chem. Rev., 2007, 107, 2388–2410 CrossRef CAS PubMed .
  5. M. Aresta and A. Dibenedetto, Dalton Trans., 2007, 2975–2992 RSC .
  6. C. M. Rayner, Org. Process Res. Dev., 2007, 11, 121–132 CrossRef CAS .
  7. S. N. Riduan and Y. Zhang, Dalton Trans., 2010, 39, 3347–3357 RSC .
  8. R. Martín and A. W. Kleij, ChemSusChem, 2011, 4, 1259–1263 CrossRef PubMed .
  9. K. Huang, C.-L. Sun and Z.-J. Shi, Chem. Soc. Rev., 2011, 40, 2435–2452 RSC .
  10. M. Cokoja, C. Bruckmeier, B. Rieger, W. A. Herrmann and F. E. Kühn, Angew. Chem., Int. Ed., 2011, 50, 8510–8537 CrossRef CAS PubMed .
  11. Y. Tsuji and T. Fujihara, Chem. Commun., 2012, 48, 9956–9964 RSC .
  12. L. Zhang and Z. Hou, Chem. Sci., 2013, 4, 3395–3403 RSC .
  13. F. J. Fernandez-Alvarez, A. M. Aitani and L. A. Oro, Catal. Sci. Technol., 2014, 4, 611–624 CAS .
  14. C. Maeda, Y. Miyazaki and T. Ema, Catal. Sci. Technol., 2014, 4, 1482–1497 CAS .
  15. C. Martín, G. Fiorani and A. W. Kleij, ACS Catal., 2015, 5, 1353–1370 CrossRef .
  16. A. Tlili, E. Blondiaux, X. Frogneux and T. Cantat, Green Chem., 2015, 17, 157–168 RSC .
  17. B. Yu and L.-N. He, ChemSusChem, 2015, 8, 52–62 CrossRef CAS PubMed .
  18. D. Yu, S. P. Teong and Y. Zhang, Coord. Chem. Rev., 2015, 293, 279–291 CrossRef .
  19. S. Wang, G. Du and C. Xi, Org. Biomol. Chem., 2016, 14, 3666–3676 CAS .
  20. M. Brill, F. Lazreg, C. S. J. Cazin and S. P. Nolan in Transition metal-catalyzed carboxylation of organic substrates with carbon dioxide, ed. X. B. Lu, 2016, pp. 225–278 Search PubMed .
  21. M. Ahamed, J. Verbeek, U. Funke, J. Lecina, A. Verbruggen and G. Bormans, ChemCatChem, 2016, 8, 3692–3700 CrossRef CAS .
  22. M. Börjesson, T. Moragas, D. Gallego and R. Martin, ACS Catal., 2016, 6, 6739–6749 CrossRef PubMed .
  23. Y. Li, X. Cui, K. Dong, K. Junge and M. Beller, ACS Catal., 2017, 7, 1077–1086 CrossRef CAS .
  24. L. Zhang, J. Cheng, B. Carry and Z. Hou, J. Am. Chem. Soc., 2012, 134, 14314–14317 CrossRef CAS PubMed .
  25. T. Fujihara, Y. Tani, K. Semba, J. Terao and Y. Tsuji, Angew. Chem., Int. Ed., 2012, 51, 11487–11490 CrossRef CAS PubMed .
  26. Y. Tani, T. Fujihara, J. Terao and Y. Tsuji, J. Am. Chem. Soc., 2014, 136, 17706–17709 CrossRef CAS PubMed .
  27. B. Miao, S. Li, G. Li and S. Ma, Org. Lett., 2016, 18, 2556–2559 CrossRef CAS PubMed .
  28. P. Shao, S. Wang, C. Chen and C. Xi, Org. Lett., 2016, 18, 2050–2053 CrossRef CAS PubMed .
  29. X. Wang, M. Nakajima and R. Martin, J. Am. Chem. Soc., 2015, 137, 8924–8927 CrossRef CAS PubMed .
  30. T. Fujihara, T. Xu, K. Semba, J. Terao and Y. Tsuji, Angew. Chem., Int. Ed., 2011, 50, 523–527 CrossRef CAS PubMed .
  31. M. D. Greenhalgh and S. P. Thomas, J. Am. Chem. Soc., 2012, 134, 11900–11903 CrossRef CAS PubMed .
  32. S. Li, W. Yuan and S. Ma, Angew. Chem., Int. Ed., 2011, 50, 2578–2582 CrossRef CAS PubMed .
  33. C. M. Williams, J. B. Johnson and T. Rovis, J. Am. Chem. Soc., 2008, 130, 14936–14937 CrossRef CAS PubMed .
  34. K. Murata, N. Numasawa, K. Shimomaki, J. Takaya and N. Iwasawa, Chem. Commun., 2017, 53, 3098–3101 RSC .
  35. M. Gaydou, T. Moragas, F. Juliá-Hernández and R. Martin, J. Am. Chem. Soc., 2017, 139, 12161–12164 CrossRef CAS PubMed .
  36. M. Gaydou, T. Moragas, F. Juliá-Hernández and R. Martin, J. Am. Chem. Soc., 2017, 139, 12161–12164 CrossRef CAS PubMed .
  37. T. W. Butcher, E. J. McClain, T. G. Hamilton, T. M. Perrone, K. M. Kroner, G. C. Donohoe, N. G. Akhmedov, J. L. Petersen and B. V. Popp, Org. Lett., 2016, 18, 6428–6431 CrossRef CAS PubMed .
  38. L. Dang, H. Zhao, Z. Lin and T. B. Marder, Organometallics, 2007, 26, 2824–2832 CrossRef CAS .
  39. H. Zhao, Z. Lin and T. B. Marder, J. Am. Chem. Soc., 2006, 128, 15637–15643 CrossRef CAS PubMed .
  40. L. Dang, Z. Lin and T. B. Marder, Organometallics, 2008, 27, 4443–4454 CrossRef CAS .
  41. L. Dang, H. Zhao, Z. Lin and T. B. Marder, Organometallics, 2008, 27, 1178–1186 CrossRef CAS .
  42. J. Won, D. Noh, J. Yun and J. Y. Lee, J. Phys. Chem. A, 2010, 114, 12112–12115 CrossRef CAS PubMed .
  43. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford, CT, 2009 Search PubMed .
  44. Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 2008, 120, 215–241 CrossRef CAS .
  45. Y. Zhao and D. G. Truhlar, Acc. Chem. Res., 2008, 41, 157–167 CrossRef CAS PubMed .
  46. A. V. Marenich, C. J. Cramer and D. G. Truhlar, J. Phys. Chem. B, 2009, 113, 6378–6396 CrossRef CAS PubMed .
  47. Y. Yang and P. Liu, ACS Catal., 2015, 5, 2944–2951 CrossRef CAS .
  48. Y. Yang, S.-L. Shi, D. Niu, P. Liu and S. L. Buchwald, Science, 2015, 349, 62–66 CrossRef CAS PubMed .
  49. Y. Yang, I. B. Perry, G. Lu, P. Liu and S. L. Buchwald, Science, 2016, 353, 144–150 CrossRef CAS PubMed .
  50. C. Y. Legault, CYLview, 1.0b, Université de Sherbrooke, 2009 Search PubMed .
  51. A Hammett study shows that more electron-rich styrene leads to increased reaction rate (see Fig. S4). Our previous study provides an example of promoting CO2 insertion via polarizing CO2 using a Lewis acid: X. Lv, L. Zhang, B. Sun, Z. Li, Y.-B. Wu and G. Lu, Catal. Sci. Technol., 2017, 7, 3539–3545 CAS .
  52. C. A. Tolman, Chem. Rev., 1977, 77, 313–348 CrossRef CAS .
  53. L. Falivene, R. Credendino, A. Poater, A. Petta, L. Serra, R. Oliva, V. Scarano and L. Cavallo, Organometallics, 2016, 35, 2286–2293 CrossRef CAS  , Based on the geometries of (PR3)Ir(CO)2Cl and (NHC)Ir(CO)2Cl optimized using the BP86 functional and a mixed basis set of SDD for Ir and TZVP for other atoms, the % Vbur of the monophosphine and NHC ligands were calculated using the web-based SambVca 2.0 program https://www.molnac.unisa.it/OMtools/sambvca2.0/index.html). The default parameters in SambVca 2.0 were used as suggested by Cavallo: sphere radius: 3.5 Å, distance from the center of the sphere: 2.1 Å, mesh spacing: 0.1 Å, H atoms omitted, atom radii: Bondi radii scaled by 1.17.
  54. D. G. Gusev, Organometallics, 2009, 28, 6458–6461 CrossRef CAS .
  55. In addition to the suppressing effect of bulkier NHC ligands on the reactivity, the computations show that the reactions with bulkier styrenes, such as α-methylstyrene, trans-β-methylstyrene, and mesitylethylene, are also slowed down (see detailed energy profiles in Fig. S5).
  56. F. M. Bickelhaupt and K. N. Houk, Angew. Chem., Int. Ed., 2017, 56, 10070–10086 CrossRef CAS PubMed .

Footnote

Electronic supplementary information (ESI) available: Additional discussions of computational results; Cartesian coordinates and energies of the optimized structures. See DOI: 10.1039/c7cy01637a

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