Experimental Evidence for γ-Agostic Assistance in β-Methyl Elimination, the Microscopic Reverse of α-Agostic Assistance in the Chain Propagation Step of Olefin Polymerization

Abstract

Isotopically labeled zirconocene methyl neopentyl complexes of the formula (CpR_n)_2Zr(CH_3)(CH_2C(CH_3)2CD_3) are obtained via reaction of LiCH_2C(CH_3)_2CD_3 with (CpR_n)_2Zr(CH_3)(Cl). Addition of B(C_6F_5)_3 to (CpR_n)_2Zr(CH_3)(CH_2C(CH_3)_2CD_3) results in β-methyl elimination, forming the ion-paired species [(CpR_n)_2Zr(CH_3)][CH_3B(C_6F_5)_3] along with isotopologs of isobutene. The relative amounts of d_3- and d_0-isobutene afford the isotope effect for β-methyl elimination. For Cp_2Zr(CH_3)(CH_2C(CH_3)_2CD_3) (Cp = (η^5-C_5H_5)), a kinetic deuterium isotope effect of 1.40(2) has been measured at 23 °C. Comparable deuterium kinetic isotope effects have been observed for four other zirconocene methyl neopentyl compounds:  [Cp^*(C_5Me_4H)Zr] (Cp^* = (η^5-C_5Me_5)), [Cp^*_2Zr], [rac-(EBI)Zr] (EBI = ethylenebis(indenyl)), and [(THP)Zr] (THP = 1,2-(SiMe_2)_2(η^5-3,5-C_5H(CHMe_2)_2)(η^5-C_5H_3)). The direction and magnitude of these effects are consistent with γ-agostic assistance in the transition state for β-methyl elimination, the microscopic reverse of α-agostic assistance in the transition state for olefin insertion into the Zr−methyl bond.