Organometallics 2010, 29, 6343–6349 DOI: 10.1021/om100746r
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Abnormal N-Heterocyclic Carbene Promoted Suzuki-Miyaura Coupling Reaction: A Comparative Study Xiangya Xu, Baochang Xu, Yongxin Li, and Soon Hyeok Hong* Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore Received July 29, 2010
A C2-protected imidazolium salt that generates an abnormal N-heterocyclic carbene (NHC) was applied for the palladium-catalyzed Suzuki-Miyaura coupling reactions. The abnormal NHC precursor promoted the reactions while suppressing homocoupling of aryl boronic acid, which was observed with the corresponding normal NHC precursor. The related well-defined abnormal NHC-based palladium complexes also showed better activity than the normal NHC-based analogues, exhibiting faster reaction rates especially at the initial stage of the reaction.
Introduction N-Heterocyclic carbenes (NHCs) have been intensively studied in organometallic chemistry and homogeneous catalysis for the past decade.1 The NHCs are stronger neutral electron donors and less oxidation sensitive than tertiary phosphines, so as to be excellent ligands for transition metals. Most of the NHCs are derived from imidazolium or 4,5-dihydroimidazolium salts and bind a metal at the C2 position. However, it has been reported that metalation of imidazolium salts can take place at the C4 or the C5 carbon since Crabtree and co-workers discovered it for the first time.2 These “abnormal” N-heterocyclic carbenes (aNHCs) are reported as even stronger donors than C2-binding “normal” N-heterocyclic carbenes (nNHCs), which may provide new opportunities in catalysis.1,3 It has been reported that the binding mode of NHC to a metal such as Pd could have a substantial effect on the catalytic behavior.4 When normally bound [Pd(IMes)2Cl2] complex 1 and mixed bound complex 2 (Figure 1) were screened for the Suzuki-Miyaura and the Heck cross-coupling reactions, interestingly, complex 1 was inactive for the cross-coupling reactions, while complex 2 was active. This study suggested that a catalytically active NHC-Pd precursor
cannot be complex 1 in the Suzuki and the Heck coupling reactions under in situ catalytic conditions using IMes 3 HCl and a Pd precursor, and abnormal binding of carbene might play a role during the catalysis. Albrecht and co-workers also reported that the catalytic activity of cationic C4-bound Pd(dicarbene) complex 3 is significantly higher in alkene hydrogenation than its C2-bound analogue 4 (Figure 1).5 However, most examples including ones shown above have different steric effects near the Pd center, making it difficult to conclude whether the observed activity is from steric differences or from the different electronic environment of aNHC.6,7 In addition, in contrast to extensive use of imidazolium salt for in situ generation of NHC-based catalytic species in Pd catalysis,8 to the best of our knowledge,
*To whom correspondence should be addressed. E-mail: hongsh@ ntu.edu.sg. (1) (a) de Fremont, P.; Marion, N.; Nolan, S. P. Coord. Chem. Rev. 2009, 253, 862. (b) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122. (c) Nolan, S. P., Ed. N-Heterocyclic Carbenes in Synthesis; WileyVCH: Weinheim, Germany, 2006. (d) Sommer, W. J.; Weck, M. Coord. Chem. Rev. 2007, 251, 860. (e) Dr€oge, T.; Glorius, F. Angew. Chem., Int. Ed. 2010, 49, 6940. (2) Gr€ undemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.; Crabtree, R. H. Chem. Commun. 2001, 2274. (3) (a) Schuster, O.; Yang, L. R.; Raubenheimer, H. G.; Albrecht, M. Chem. Rev. 2009, 109, 3445. (b) Arnold, P. L.; Pearson, S. Coord. Chem. Rev. 2007, 251, 596. (c) Albrecht, M. Chem. Commun. 2008, 3601. (d) Crabtree, R. H. Pure Appl. Chem. 2003, 75, 435. (e) Guisado-Barrios, G.; Bouffard, J.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2010, 49, 4759. (4) Lebel, H.; Janes, M. K.; Charette, A. B.; Nolan, S. P. J. Am. Chem. Soc. 2004, 126, 5046.
(5) (a) Heckenroth, M.; Kluser, E.; Neels, A.; Albrecht, M. Angew. Chem., Int. Ed. 2007, 46, 6293. (b) Heckenroth, M.; Neels, A.; Garnier, M. G.; Aebi, P.; Ehlers, A. W.; Albrecht, M. Chem.;Eur. J. 2009, 15, 9375. (6) (a) Campeau, L. C.; Thansandote, P.; Fagnou, K. Org. Lett. 2005, 7, 1857. (b) Sch€utz, J.; Herdtweck, E.; Herrmann, W. A. Organometallics 2004, 23, 6084. (7) (a) W€ urtz, S.; Glorius, F. Acc. Chem. Res. 2008, 41, 1523. (b) Martin, R.; Buchwald, S. L. Acc. Chem. Res. 2008, 41, 1461. (c) Singh, R.; Viciu, M. S.; Kramareva, N.; Navarro, O.; Nolan, S. P. Org. Lett. 2005, 7, 1829. (d) Singh, R.; Nolan, S. P. J. Organomet. Chem. 2005, 690, 5832. (8) (a) Marion, N.; Nolan, S. P. Acc. Chem. Res. 2008, 41, 1440. (b) Kantchev, E. A. B.; O'Brien, C. J.; Organ, M. G. Angew. Chem., Int. Ed. 2007, 46, 2768. (c) Kantchev, E. A. B.; O'Brien, C. J.; Organ, M. G. Aldrichim. Acta 2006, 39, 97. (d) Díez-Gonzalez, S.; Marion, N.; Nolan, € S. P. Chem. Rev. 2009, 109, 3612. (e) Herrmann, W. A.; Ofele, K.; von Preysing, D.; Schneider, S. K. J. Organomet. Chem. 2003, 687, 229. (f) Hillier, A. C.; Grasa, G. A.; Viciu, M. S.; Lee, H. M.; Yang, C. L.; Nolan, S. P. J. Organomet. Chem. 2002, 653, 69.
r 2010 American Chemical Society
Published on Web 11/09/2010
Figure 1. Normal and abnormal NHC-based Pd complexes.
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Organometallics, Vol. 29, No. 23, 2010
Xu et al. Table 1. NHC-Promoted Suzuki-Miyaura Cross-Coupling Reactions (NHC:Pd = 1:1)a
Figure 2. Normal and abnormal NHC precursors.
there is no example of using abnormal binding carbene precursors for similar in situ generation of catalyst species. During our recent research focus on the NHC-based transition metal complexes for efficient organic transformations,9 we became interested in abnormal NHC as a new ligand scaffold in homogeneous catalysis. Herein, we report the first example of using a C2-protected imidazolium salt as an aNHC precursor in a Pd-catalyzed cross-coupling reaction. To confirm the observation from in situ catalysis, we also screened the activity of the related well-defined aNHC and nNHC Pd complexes.
Results and Discussion NHC Precursor. Two imidazolium salts, 5 and 6, were chosen as NHC precursors to investigate the activity between the normal and the abnormal carbenes (Figure 2). Both precursors have isopropyl and phenyl groups as wingtip groups, providing similar steric effects of the NHC ligands. Percent buried volumes (%Vbur) of the chosen nNHC and aNHC, calculated on the basis of X-ray crystal structures of 7 and 9 using SambVca software, are 29.5 for the nNHC of 5 and 30.0 for the aNHC of 6 (radius of the sphere, R, is 3.5 A˚; M-NHC length is 2.1 A˚; Bondi radii scaled by 1.17).10 For compound 6, the C2 and C4 carbons are functionalized by phenyl groups, leaving the C5 position available only for the binding of Pd.11 The phenyl group was chosen to avoid possible C-C cleavage observed when a methyl or an isopropyl group was used as the C2 protecting group.12 The precursor 5 is a stereotype of NHC precursors used in in situgenerated transition metal catalysis. While the most acidic C2 position is the most favorable binding site, there is a possibility for palladium to bind to the C4 or C5 position, as in the case of 2.4 Electronic properties of the corresponding carbenes of 5 and 6 were examined by Crabtree and co-workers by measuring carbonyl stretching frequencies of Ir(CO)2Cl(NHC) complexes.13 While the nNHC from 5 showed slightly higher electron-donating property (ν(CO) 2061, 1976 cm-1) than PCy3 (ν(CO) 2072, 1984 cm-1), aNHC from 6 exhibited (9) (a) Zhang, J.; Muthaiah, S.; Ghosh, S. C.; Hong, S. H. Angew. Chem., Int. Ed. 2010, 49, 6391. (b) Zhang, Y.; Chen, C.; Ghosh, S. C.; Li, Y.; Hong, S. H. Organometallics 2010, 29, 1374. (c) Ghosh, S. C.; Muthaiah, S.; Zhang, Y.; Xu, X.; Hong, S. H. Adv. Synth. Catal. 2009, 351, 2643. (d) Muthaiah, S.; Ghosh, S. C.; Jee, J.-E.; Chen, C.; Zhang, J.; Hong, S. H. J. Org. Chem. 2010, 75, 3002. (e) Ghosh, S. C.; Hong, S. H. Eur. J. Org. Chem. 2010, 4266. (10) (a) Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.; Cavallo, L. Eur. J. Inorg. Chem. 2009, 1759. (b) http://www. molnac.unisa.it/OMtools/sambvca.php. (c) Clavier, H.; Nolan, S. P. Chem. Commun. 2010, 46, 841. (11) (a) Gr€ undemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.; Crabtree, R. H. J. Am. Chem. Soc. 2002, 124, 10473. (b) Bacciu, D.; Cavell, K. J.; Fallis, I. A.; Ooi, L. L. Angew. Chem., Int. Ed. 2005, 44, 5282. (12) (a) Chianese, A. R.; Zeglis, B. M.; Crabtree, R. H. Chem. Commun. 2004, 2176. (b) McLachlan, F.; Mathews, C. J.; Smith, P. J.; Welton, T. Organometallics 2003, 22, 5350. (c) Cavell, K. J.; McGuinness, D. S. Coord. Chem. Rev. 2004, 248, 671. (13) Chianese, A. R.; Kovacevic, A.; Zeglis, B. M.; Faller, J. W.; Crabtree, R. H. Organometallics 2004, 23, 2461.
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