Experimental Evidence for y-Agostic Assistance in beta-Methyl Elimination,the Microscopic Reverse of alpha-Agostic Assistance in the Chain Propagation Step of Olefin Polymerization

摘要

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 /_3-methyl elimination,forming the ion-paired species [(CpR_n)_2Zr(CH_3)] [CH_3B(C6F5)_3] along with isotopologs of isobutene.The relative amounts of d_3- and cfo-isobutene afford the isotope effect for beta-methyl elimination.For Cp_2Zr(CH_3)(CH_2C(CH_3)_2CD_3) (Cp = (eta~5-C_5H_5)),a kinetic deuterium isotope effect of 1.40(_2) has been measured at _2_3 deg C.Comparable deuterium kinetic isotope effects have been observed for four other zirconocene methyl neopentyl compounds:[Cp*(C5Me_4H)-Zr] (Cp* = (eta~5-C_5Me_5)),[Cp*_2Zr],[rac-(EBI)Zr] (EBI = ethylenebis(indenyl)),and [(THP)Zr] (THP = 1,2-(SiMe_2)_2(eta~5-_3,5-C_5H(CHMe_2)_2)(eta~5-C_5H_3)).The direction and magnitude of these effects are consistent with y-agostic assistance in the transition state for beta-methyl elimination,the microscopic reverse of a-agostic assistance in the transition state for olefin insertion into the Zr-methyl bond.