Mechanism of the Grignard Addition Reaction. VIII. Reaction Rates and Product Distribution for the Reactions of t-Butylmagnesium Chloride and Methylmagnesium Bromide with Substituted Benzophenones.

Abstract

Ph2CO (I) and substituted benzophenones react with tert-BuMgCl to give 1,2-, 1,4-, or 1,6-addn. products and benzopinacols. The product distribution is sensitive to the steric effects of the substituents. The overall rate is independent of steric factors, and a linear Hammett plot is obtained. This suggests an initial rate limiting step, common to all 4 reactions. This step is a 1-electron transfer from the Grignard reagent to the benzophenone with the formation of tert-Bu • and the benzophenone ketyl radical anion. A heterolytic mechanism is proposed for the reactions of MeMgBr with I. A competing radical mechanism is responsible for the conjugate addn. obsd. with sterically hindered benzophenones under forcing conditions. Generally, a radical mechanism is implied when (i) conjugate addn. to aromatic systems occurs, or (ii) Grignard addn. reactions occur with rates which are much higher for reagents branched in the α-position than for the unbranched ones. [on SciFinder(R)]