A density functional study on olefin insertion and hydrogen transfer in the reaction between Cl2Ti+–ethyl and ethylene. Possible implications for the stereochemistry and chain termination in olefin polymerization

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Abstract
Calculations based on density functional theory have been carried out on the reaction between Cl2Ti+–ethyl (1) and ethylene. In this study 1 was taken as a model for the cationic metallocenes of group-4 elements, which have been developed by Kaminsky and Brintzinger as efficient catalysts for the polymerization of olefins. The ground state structure of 1 has a β-agostic conformation in which a single Cβ—H bond is directed towards the metal center. It was assumed that this conformation also serves as a model for the resting state of the growing chain attached to the cationic group-4 metallocenes between insertions. Two paths were considered for the reaction between 1 and ethylene. The first (2) has ethylene approaching the agostic Cβ—H bond, whereas ethylene in the second approach (3) attacks the Ti—Cα link from the side opposite to the Cβ—H bond. The front-side attack (2) results in a transfer of hydrogen from the β-carbon of ethyl to ethylene and represents a chain-terminating step with an activation energy of 5.3 kcal/mol. It was not possible to locate a path leading to olefin insertion from the front-side attack (2). The back-side attack (3) resulted readily in insertion with an activation energy of 3.9 kcal/mol. The study made use of full transition state optimization as well as a tracing of the reaction paths by the intrinsic reaction coordinate (IRC) method of Fukui. Previous investigations have all assumed that olefin insertion takes place via a front-side approach (2) based on the known stereochemistry of α-olefin polymerization. The present study suggests that insertion takes place by a back-side approach (3), and this suggestion is discussed in connection with the known stereochemistry of olefin polymerization. Keywords: Ziegler–Natta, olefin polymerization, density functional.