.alpha. Agostic assistance in Ziegler-Natta polymerization of olefins. Deuterium isotopic perturbation of stereochemistry indicating coordination of an .alpha. carbon-hydrogen bond in chain propagation

Abstract

The well-defined, homogeneous Ziegler-Natta olefin polymerization systems that have been reported recently provide an unprecedented opportunity to investigate the mechanism of this important process. While a consensus appears to be developing that in all these systems the active catalysts are the 14-electron, d{sup 0} (or d{sup 0}f{sup n}) metallocene alkyls, Cp{sub 2}MR (M = lanthanide or group 3 transition metal) or (Cp{sub 2}MR){sup +} (M = group 4 transition metal), the mechanism for chain propagation and the geometry of the transition state for olefin insertion into the metal-carbon bond have not yet been unequivocally established. In a cleverly conceived experiment, Grubbs et al. probed for an {alpha} agostic interaction in the transition state for olefin insertion. Racemic 1-d{sub 1}-5-hexenylchlorotitanocene was prepared and found to undergo AlCl{sub 2}(CH{sub 2}CH{sub 3})-induced cyclization to a mixture of cis- and trans-2-d{sub 1}-cyclopentylmethyl stereoisomers. Any {alpha} agostic assistance in the insertion step is expected to favor the trans product (vide infra). Hydrolysis and {sup 2}H NMR analysis of the resultant mixture of deuteriomethylcyclopentanes revealed a 1.00 {plus minus} 0.05 ratio of trans:cis products, arguing against an {alpha} agostic assisted insertion in their system, however. The scandium hydride, {l brace}({eta}{sup 5}-C{sub 5}Me{sub 4}){sub 2}SiMe{sub 2}{r brace}Sc(PMe{submore » 3})H ( OpSc(PMe{sub 3})H'), cleanly catalyzes the hydrocyclization of 1,5-hexadiene to methylcyclopentane. The authors have adapted this catalytic hydrocyclization reaction along the lines of the Grubbs experiment to probe for {alpha} agostic assistance with the scandium system.« less