C-13 KINETIC ISOTOPE EFFECTS IN THE REACTIONS OF GRIGNARD-REAGENTS WITH BENZOPHENONE

The rate-determining step for the reaction of benzophenone with Grignard reagents is an inner sphere electron transfer (ET) with a simultaneous transfer of magnesium to produce the magnesium benzophenone ketyl and the alkyl radical. For tertiary and secondary reagents the alkyl radical is free to escape from the cage, while for primary reagents and methyl partial bond formation between the radicals stabilizes the transition state and prevents escape. Substitution in the carbonyl group with isotopic carbon gives rise to a kinetic isotope effect (KIE), which for C-14 has been reported to be near unity for the reaction of tert-butylmagnesium chloride. In the present investigation, the KIE when C-13 was used was found to be high and of the same magnitude when either tert-butylmagnesium chloride or methylmagnesium bromide was used. The reported and confirmed very low KIE for allylmagnesium bromide is not evidence for, but rather evidence against, the formation of ketyl during the reaction.