ENOLATES OF ALPHA-ALLENYL KETONES - FORMATION AND ALDOL REACTIONS OF CUMULENOLATES

Enolization of alpha-allenyl ketones under kinetic conditions, followed by reactions with aldehydes and ketones, affords aldol products that suggest the intermediacy of cumulenolates, formed via the abstraction of a vinylic alpha-hydrogen. The origin for this marked difference with alpha-alkenyl ketones is attributed to the enhanced acidity of allenic hydrogens, to the predominance of the s-trans conformation, and to lithium complexation with both the carbonyl and the allene moieties. The relative stability of these enolates was examined with the use of semiempirical (MNDO) calculations which indicated that the lowest energy isomers are the (Z)-alkynolates. Both experimental and computational evidence suggest that the kinetic intermediate is the cumulenolate, while the thermodynamic enolate is the (Z)-alkynenolate.