Mechanistic Aspects of Samarium-Mediated σ-Bond Activations of Arene C-H and Arylsilane Si-C Bonds Ivan Castillo and T. Don Tilley* Contribution from the Department of Chemistry, UniVersity of California at Berkeley, Berkeley, California 94720-1460 ReceiVed June 15, 2001 Abstract: To investigate the potential role of Sm-Ph species as intermediates in the samarium-catalyzed redistribution of PhSiH 3 to Ph 2 SiH 2 and SiH 4 , the samarium phenyl complex [Cp* 2 SmPh] 2 (1) was prepared by oxidation of Cp* 2 Sm (2) with HgPh 2 . Compound 1 thermally decomposes to yield benzene and the phenylene- bridged disamarium complex Cp* 2 Sm(µ-1,4-C 6 H 4 )SmCp* 2 (3). This decomposition reaction appears to proceed through dissociation of 1 into monomeric Cp* 2 SmPh species which then react via unimolecular and bimolecular pathways, involving rate-limiting Cp* metalation and direct C-H activation, respectively. The observed rate law for this process is of the form: rate ) k 1 [1] + k 2 [1] 2 . Complex 1 efficiently transfers its phenyl group to PhSiH 3 , with formation of Ph 2 SiH 2 and [Cp* 2 Sm(µ-H)] 2 (4). Quantitative Si-C bond cleavage of C 6 F 5 SiH 3 is effected by the samarium hydride complex 4, yielding silane and [Cp* 2 Sm(µ-C 6 F 5 )] 2 (5). In contrast, Si-H activation takes place upon reaction of 4 with o-MeOC 6 H 4 SiH 3 , affording the samarium silyl species Cp* 2 SmSiH 2 (o-MeOC 6 H 4 )(7). Complex 7 rapidly decomposes to [Cp* 2 Sm(µ-o-MeOC 6 H 4 )] 2 (6) and other samarium-containing products. Compounds 5 and 6 were prepared independently by oxidation of 2 with Hg(C 6 F 5 ) 2 and Hg(o-MeOC 6 H 4 ) 2 , respectively. The mechanism of samarium-mediated redistribution at silicon, and chemoselectivity in σ-bond metathesis reactions, are discussed. Introduction The activation of C-H 1,2 and Si-H 3 σ-bonds by f-element complexes is well established. On the other hand, related activations of C-C 2a,4 and Si-C 3f,5 bonds appear to be more difficult and have been observed far less often, with the former being limited to -alkyl-transfer reactions. Several kinetic factors seem to favor C-H over C-C activation, including the inherently more hindered nature of C-C bonds, the statistical abundance of C-H bonds in most hydrocarbons, and the higher barrier for C-C activation due to its more directional bonding. 6 Nevertheless, remarkable examples of metal-mediated C-C bond activation by early transition metal centers have been reported by the group of Basset. 7 Such heterogeneous systems, which involve highly electrophilic, silica-supported early- transition metal centers, allow chemical transformations of hydrocarbons under mild conditions. In metal-silicon chemistry, the analogous preference for Si-H over Si-C activation may be explained by the same factors recognized to account for chemoselectivity in C-H versus C-C activation. Thus, in reactions of d 0 f n metal hydrides with hydrosilanes, the usual reaction pathway involves de- hydrocoupling via four-center transition state A and selective formation of a metal silyl derivative (Scheme 1). Such species have been invoked as intermediates in the dehydrocoupling of silanes, by way of reaction with more hydrosilane via transition state B to produce a Si-Si bond and regenerate the metal hydride. Thus, the first two reactions of Scheme 1 can account for the coordination-polymerization of organosilanes to poly- silanes by early-transition metal and f-element complexes. 8 The activation of Si-C bonds is potentially important in the development of new processes in organosilicon chemistry. In addition, studies of Si-C bond activations may provide important insights into designing comparable chemistry for C-C bonds, which is expected to be more difficult. Within this context, we have observed Si-C bond activation in samarium- (1) For example: (a) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491. (b) Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8111. (c) Bruno, J. W.; Smith, G. M.; Marks, T. J.; Fair, C. K.; Schultz, A. J.; Williams, J. M. J. Am. Chem. Soc. 1986, 108, 40. (d) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203. (2) For reviews see: (a) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51. (b) Bercaw, J. E. Pure Appl. Chem. 1990, 62, 1151. (c) Davis, J. A., Watson, P. L., Liebman, J. F., Greenberg, A., Eds.; SelectiVe Hydrocarbon ActiVation; VCH Publishers: New York, 1990. (d) Marks, T. J. Acc. Chem. Res. 1992, 25, 57. (3) For example: (a) Forsyth, C. M.; Nolan, S. P.; Marks, T. J. Organometallics 1991, 10, 2543. (b) Sakakura, T.; Lautenschlager, H. J.; Nakajima, M.; Tanaka, M. Chem. Lett. 1991, 913. (c) Tilley, T. D.; Radu, N. S.; Walzer, J. F.; Woo, H.-G. Polym. Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 1992, 33(1), 1237. (d) Molander, G. A.; Julius, M. J. Org. Chem. 1992, 57, 3266. (e) Radu, N. S.; Tilley, T. D. J. Am. Chem. Soc. 1995, 117, 5863. (f) Radu, N. S.; Hollander, F. J.; Tilley, T. D.; Rheingold, A. L. J. Chem. Soc., Chem. Commun. 1996, 2459. (4) (a) Watson, P. L.; Roe, D. C. J. Am. Chem. Soc. 1982, 104, 6471. (b) Bunel, E.; Burger, B. J.; Bercaw, J. E. J. Am. Chem. Soc. 1988, 110, 976. (5) (a) Radu, N. S.; Tilley, T. D.; Rheingold, A. L. J. Organomet. Chem. 1996, 516, 41. (b) Castillo, I.; Tilley, T. D. Organometallics 2000, 19, 4733. (6) (a) Crabtree, R. H. Chem. ReV. 1985, 85, 245 and references therein. (b) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870. (7) For example: (a) Lecuyer, C.; Quignard, F.; Choplin, A.; Olivier, D.; Basset, J.-M. Angew. Chem., Int. Ed. 1991, 30, 1660. (b) Rosier, C.; Niccolai, G. P.; Basset, J.-M. J. Am. Chem. Soc. 1997, 119, 12408. (c) Maury, O.; Lefort, L.; Vidal, V.; Thivolle-Cazat, J.; Basset, J.-M. Angew. Chem., Intl. Ed. 1999, 38, 1952. (d) Chabanas, M.; Vidal, V.; Cope ´ret, C.; Thivolle-Cazat, J.; Basset, J.-M. Angew. Chem., Int. Ed. 2000, 39, 1962. (8) (a) Woo, H.-G.; Tilley, T. D. J. Am. Chem. Soc. 1989, 111, 8043. (b) Woo, H.-G.; Heyn, R. H.; Tilley, T. D. J. Am. Chem. Soc. 1992, 114, 5698. (c) Woo, H.-G.; Walzer, J. F.; Tilley, T. D. J. Am. Chem. Soc. 1992, 114, 7047. (d) Tilley, T. D. Acc. Chem. Res. 1993, 26, 22. 10526 J. Am. Chem. Soc. 2001, 123, 10526-10534 10.1021/ja011472w CCC: $20.00 © 2001 American Chemical Society Published on Web 10/05/2001