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

Isotopically labeled zirconocene methyl neopentyl complexes of the formula (CpRn)(2)Zr-(CH3)(CH2C(CH3)(2)CD3) are obtained via reaction of LiCH2C(CH3)(2)CD3 with (CpRn)(2)Zr(CH3)(CI). Addition of B(C6F5)(3) to (CPRn)(2)Zr(CH3)(CH2C(CH3)(2)CD3) results in beta-methyl elimination, forming the ion-paired species [(CpRn)(2)Zr(CH3)][CH3B(C6F5)(3)] along with isotopologs of isobutene. The relative amounts of d(3)- and do-isobutene afford the isotope effect for beta-methyl elimination. For CP2Zr(CH3)(CH2C(CH3)(2)CD3) (Cp = (eta(5)-C5H5)), a kinetic deuterium isotope effect of 1.40(2) has been measured at 23 degrees C. Comparable deuterium kinetic isotope effects have been observed for four other zirconocene methyl neopentyl compounds: [Cp*(C5Me4H)-Zr] (Cp* = (eta(5)-C5Me5)), [CP*Zr-2], [rac-(EBI)Zr] (EBI = ethylenebis(indenyl)), and [(THP)Zr] (THP = 1,2-(SiMe2)(2)(eta(5)-3,5-C5H(CHMe2)(2))(eta(5)-C5H3)), The direction and magnitude of these effects are consistent with gamma-agostic assistance in the transition state for beta-methyl elimination, the microscopic reverse of alpha-agostic assistance in the transition state for olefin insertion into the Zr-methyl bond.