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R-Agostic Interactions and Olefin Insertion in Metallocene Polymerization Catalysts

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R-Agostic Interactions and Olefin Insertion in Metallocene Polymerization Catalysts Acc. Chem. Res. 1996, 29, 85-93 85 r-Agostic Interactions and Olefin Insertion in Metallocene Polymerization Catalysts ROBERT H. GRUBBS* AND GEOFFREY W. COATES† Contribution No. 9144 from The Arnold and Mabel Beckman Laboratory of Chemical Synthesis, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 Received September 25, 1995 Introduction step.23-27 Brintzinger has proposed that an R-agostic interaction rigidifies the transition state of a C2- In the 40 years since the discovery of transition symmetric catalyst undergoing propylene insertion, metal catalysts for olefin polymerization by Ziegler 1,2 thereby increasing the isotacticity of the resulting and Natta, a substantial amount of research has polypropylene (Figure 1).23,24 Specifically, the R-ago- been directed toward understanding the basic mecha- stic interaction firmly orients the polymer chain into nistic steps of this important industrial process. At- the open sector of the catalyst structure to minimize tention has focused in particular on the process of interactions between the alkyl substituent of the monomer enchainment, which is generally agreed to monomer and the ligand/polymer array during olefin 3-5 occur through olefin coordination followed by inser- insertion. In addition, Guerra has proposed that the tion into a metal-carbon bond. Four general mecha- 28 high syndiospecificity of Cs-symmetric catalysts for nistic proposals for the nature of this insertion have propylene polymerization results from a γ-agostic emerged (Scheme 1). The first is alkyl migration to structure which persists in the catalytic species after 6-10 the coordinated olefin and is known as the Cossee- olefin insertion.29 Several theoretical studies suggest Arlman mechanism. The second model, proposed by 11,12 Rooney and Green, involves an oxidative 1,2- * E-mail: [email protected]. hydrogen shift from the R-carbon of the polymer chain, † E-mail: [email protected] generating a metal-alkylidene hydride. This species (1) Natta, G.; Pino, P.; Corradini, P.; Danusso, F.; Mantica, E.; Mazzanti, G.; Moraglio, G. J. Am. Chem. Soc. 1955, 77, 1708-1710. then reacts with an olefin to generate a metallacy- (2) Ziegler, K.; Holzkamp, E.; Breil, H.; Martin, H. Angew. Chem. clobutane, and reductive elimination completes the 1955, 67, 426, 541. (3) Brookhart, M.; Volpe, A. F., Jr.; Lincoln, D. M.; Horva´th, I. T.; propagation sequence. Somewhat intermediate to the Millar, J. M. J. Am. Chem. Soc. 1990, 112, 5634-5636. first two proposals is a mechanism proposed by Green, (4) Wu, Z.; Jordan, R. F.; Petersen, J. L. J. Am. Chem. Soc. 1995, 117, Rooney, and Brookhart13-15 where a hydrogen atom 5867-5868. (5) Casey, C. P.; Hallenbeck, S. L.; Pollock, D. W.; Landis, C. R. J. on the R-carbon of the growing polymer chain interacts Am. Chem. Soc. 1995, 117, 9770-9771. with the metal center throughout the catalytic cycle. (6) Breslow, D. S.; Newburg, N. R. J. Am. Chem. Soc. 1959, 81,81- 86. This three-center, two-electron covalent bond, termed (7) Cossee, P. Tetrahedron Lett. 1960, No. 17,12-16. 15,16 an “agostic interaction”, occurs when the hydrogen (8) Cossee, P. Tetrahedron Lett. 1960, No. 17,17-21. atom is simultaneously bonded to both a carbon and (9) Cossee, P. J. Catal. 1964, 3,80-88. (10) Arlman, E. J.; Cossee, P. J. Catal. 1964, 3,99-104. a metal atom. The fourth mechanistic possibility is (11) Green, M. L. H. Pure Appl. Chem. 1978, 50,27-35. olefin insertion where an R-hydrogen interacts with (12) Ivin, K. J.; Rooney, J. J.; Stewart, C. D.; Green, M. L. H.; Mahtab, the metal center only during the transition state of R. J. Chem. Soc., Chem. Commun. 1978, 604-606. (13) Dawoodi, Z.; Green, M. L. H.; Mtetwa, V. S. B.; Prout, K. J. Chem. the C-C bond forming step. This mechanism is a Soc., Chem. Commun. 1982, 1410-1411. hybrid of the Cossee-Arlman and modified Green- (14) Laverty, D. T.; Rooney, J. J. J. Chem. Soc., Faraday Trans. 1 1983, 79, 869-878. Rooney mechanisms. (15) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250, Agostic interactions are of general interest in orga- 395-408. nometallic chemistry since they often lead to C-H (16) Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog. Inorg. Chem. 17,18 1988, 36,1-124. activation. Concerning polymerization catalysts, (17) Ginzburg, A. G. Russ. Chem. Rev. 1988, 57, 1175-1193. it has been proposed that an understanding of the (18) Gleiter, R.; Hyla-Kryspin, I.; Niu, S. Q.; Erker, G. Organometallics 1993, 12, 3828-3836. factors which control â-agostic interactions will yield (19) Schmidt, G. F.; Brookhart, M. J. Am. Chem. Soc. 1985, 107, 19 better control of catalyst activity and polymer mo- 1443-1444. lecular weights.20-22 The R-agostic interaction is of (20) Burger, B. J.; Thompson, M. E.; Cotter, W. D.; Bercaw, J. E. J. Am. Chem. Soc. 1990, 112, 1566-1577. particular interest since it might dramatically lower (21) Resconi, L.; Piemontesi, F.; Franciscono, G.; Abis, L.; Fiorani, T. the activation barrier to olefin insertion23 and influ- J. Am. Chem. Soc. 1992, 114, 1025-1032. ence the stereochemical outcome of the olefin insertion (22) Guo, Z.; Swenson, D. C.; Jordan, R. F. Organometallics 1994, 13, 1424-1432. (23) Prosenc, M. H.; Janiak, C.; Brintzinger, H. H. Organometallics Robert H. Grubbs was born near Possum Trot, KY, in 1942. He received his 1992, 11, 4036-4041. B.A. and M.S. degrees from the University of Florida working with M. Battiste and (24) Ro¨ll, W.; Brintzinger, H. H.; Rieger, B.; Zolk, R. Angew. Chem., his Ph.D. from Columbia University for work with Ronald Breslow. After a postdoctoral Int. Ed. Engl. 1990, 29, 279-280. year with James P. Collman at Stanford he joined the faculty at Michigan State (25) Lee, I.-M.; Gauthier, W. J.; Ball, J. M.; Iyengar, B.; Collins, S. University in 1969. In 1978 he moved to the California Institute of Technology, Organometallics 1992, 11, 2115-2122. where he is now the Victor and Elizabeth Atkins Professor of Chemistry. (26) Burger, B. J.; Cotter, W. D.; Coughlin, E. B.; Chacon, S. T.; Hajela, Geoffrey W. Coates was born in 1966 in Evansville, IN. He obtained a B.A. S.; Herzog, T. A.; Ko¨hn, R.; Mitchell, J.; Piers, W. E.; Shapiro, P. J.; degree in chemistry from Wabash College in 1989 working with Roy G. Miller and Bercaw, J. E. In Ziegler Catalysts; Fink, G., Mu¨ lhaupt, R., Brintzinger, a Ph.D. in organic chemistry from Stanford University in 1994. His thesis work, H. H., Eds.; Springer-Verlag: Berlin, 1995; pp 317-331. under the direction of Robert M. Waymouth, investigated the stereoselectivity of (27) Yu, Z.; Chien, J. C. W. J. Polym. Sci., Part A 1995, 33, 125-135. metallocene-based Ziegler−Natta catalysts. He is currently a NSF postdoctoral fellow (28) Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D. J. Am. Chem. with Robert Grubbs at the California Institute of Technology. Soc. 1988, 110, 6255-6256. 0001-4842/96/0129-0085$12.00/0 © 1996 American Chemical Society 86 Acc. Chem. Res., Vol. 29, No. 2, 1996 Grubbs and Coates Scheme 1a a “P” denotes a polymer chain. Although significant advances in understanding classical Ziegler-Natta catalysts at the molecular level have been made,36 the heterogeneous nature of these systems impedes detailed investigations. More recently, the development of well-defined, homoge- neous metallocene catalysts has created new op- portunities for mechanistic studies. In order to dis- tinguish among possible mechanisms, a homogeneous system was designed in our laboratory that probed the presence of an R-agostic interaction through the isotopic perturbation of stereochemistry (vide infra).37 In this study it was concluded that, for the system examined, there was not a significant effect due to an Figure 1. Proposed species preceding insertion for isospecific catalysts with and without R-agostic interactions. R-agostic interaction on either the rate or stereochem- istry of olefin insertion, thus ruling out the modified Green-Rooney mechanism. Also, a substantial amount that γ-agostic interactions are present following 15,16 30-32 of evidence presently available (X-ray, IR, NMR) insertion and that these contacts can be relatively 3,20,22,26,38,39 40 long lasting.33-35 This interaction might serve to indicates that â-agostic (not R-agostic) stabilize the catalytic species (both geometrically and structures characterize ground states for this class of electronically) between the insertion and monomer catalysts. coordination steps and result in a regular alternation Recently, neutral group III and cationic group IV between si- and re-preferring sites (Figure 2). metallocene alkyls have been established to be the active catalysts in olefin polymerization.41,42 Since (29) Cavallo, L.; Guerra, G.; Vacatello, M.; Corradini, P. Macromol- ecules 1991, 24, 1784-1790. (36) For three recent reviews, see: Fink, G., Mu¨ lhaupt, R., Brintz- (30) Kawamura-Kuribayashi, H.; Koga, N.; Morokuma, K. J. Am. inger, H. H., Eds. Ziegler Catalysts; Springer-Verlag; Berlin, 1995. Chem. Soc. 1992, 114, 8687-8694. Macromol. Symp. 1995, 89, 1-586. Brintzinger, H. H.; Fischer, D.; (31) Kawamura-Kuribayashi, H.; Koga, N.; Morokuma, K. J. Am. Mu¨ lhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. Chem. Soc. 1992, 114, 2359-2366. 1995, 34, 1143-1170. (32) Meier, R. J.; van Doremaele, G. H. J.; Iarlori, S.; Buda, F. J. Am. (37) Clawson, L.; Soto, J.; Buchwald, S. L.; Steigerwald, M. L.; Grubbs, Chem. Soc. 1994, 116, 7274-7281. R. H. J. Am. Chem. Soc. 1985, 107, 3377-3378.
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