ALPHA-DEUTERIUM AND ALPHA-C-13 KIES IN THE REACTION OF ALKYLLITHIUM AND ALKYLMAGNESIUM REAGENTS WITH BENZOPHENONE AND 2-OCTANONE

The addition reactions of methylmagnesium iodide and of ethylmagnesium bromide to benzophenone and 2-octanone proceed via concerted reaction mechanisms, while the addition of the corresponding alkyllithium reag ents have electron transfer mechanisms. This is concluded from the obs ervation of large normal alpha-deuterium KIEs for the reaction of RLi and small and inverse alpha-deuterium KIEs for the reaction of RMgX. A n ET mechanism for the reaction of ethyllithium with benzophenone is s upported by the finding of a 1,6-addition product (5%) and of an 0-alk yl product (traces) and by the observation that suppression of the red uction process by beta-deuteriation of the reagent causes an increase in the rate of addition. With ethyllithium and benzophenone addition a nd reduction therefore have a common precursor (radicals), while with ethylmagnesium bromide addition and reduction are independent reaction s. C-13 KIEs for substitution at the alpha-carbon of the reagents are of little diagnostic value.