THE MECHANISM OF THE ZIRCONIUM CATALYZED ETHYL-MAGNESIATION AND 2-MAGNESIOETHYL-MAGNESIATION OF UNACTIVATED ALKENES

Deuterium labelling experiments prove that the zirconocene dichloride catalysed ethylmagnesiation of alkenes occurs via a zirconocene eta(2)-ethylene complex and allow a deuterium isotope effect for a key beta-hydride transfer to be estimated (k(H)/k(D) = 2.5). Transmetallation from zirconium to magnesium to form 1,4-dimagnesiated reagents is shown to be an intramolecular process. Kinetic studies show that the reaction between eta(2)-ethylene zirconocene and the alkene is rate limiting and that Lewis bases inhibit the reaction by decreasing the amount of eta(2)-ethylene zirconocene in equilibrium with the 'ate' complex [Cp(2)Zr(CH2=CH2)Et].[MgX.Base](+).