Carbon-13 kinetic isotope effects in the reactions of Grignard reagents with benzophenone

Abstract

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 to a kinetic isotope effect (KIE), which for 14 C has been reported to be near unity for the reaction of tert-butylmagnesium chloride