On the Mechanism of Addition of Lithium Pinacolone Enolate to Benzaldehyde:  Polar or Electron Transfer?

Abstract

The carbonyl-carbon kinetic isotope effect (KIE) and the substituent effect were measured for the reaction of lithium pinacolone enolate (CH2C(OLi)C(CH3)3) with benzaldehyde, and the results were compared with those for other lithium reagents such as MeLi, PhLi, and CH2CHCH2Li. Ab initio MO calculations (HF/6-31+G*) were carried out to estimate the equilibrium IE on the addition to benzaldehyde. A carbonyl addition reaction, in general, proceeds by way of either a polar direct nucleophilic attack (PL) in a one-step or a two-step process going through a radical ion intermediate (eq 1). The carbonyl-carbon KIE is of primary nature for the PL or the RC rate-determining ET mechanism, while it is considered to be secondary for the ET rate-determining mechanism. The reaction of lithium pinacolone enolate with benzaldehyde gave a small positive KIE (12k/13k = 1.019), which is larger than the theoretical equilibrium IE (12K/13K = 1.006) determined by the MO calculations. Thus, there is a reaction-coordinate contribution to the observed KIE. This is in sharp contrast to the absence of KIE (12k/14k = 1.000) measured previously for the MeLi addition. Dehalogenation and enone-isomerization probe experiments showed no evidence of a single electron transfer to occur during the course of the reaction. The primary carbonyl-carbon KIE together with the substituent effect and chemical probe experiments led to the conclusion that the reaction of lithium pinacolone enolate with benzaldehyde proceeds via the polar mechanism.