ALLENYL ENOLATES - A NEW CLASS OF CHIRAL AMBIDENT NUCLEOPHILES .1. REACTION WITH C-ELECTROPHILES AND SI-ELECTROPHILES

The regio- and stereoselectivity of the reaction of allenyl enolates obtained by 1,6-addition of lithium dimethylcuprate to acceptor-substituted enynes 1 with C and Si electrophiles is examined. It is found that allyl bromide reacts at C-4 to give 4-substituted 2,4-dienoates 2; in contrast, methyl triflate and carbonyl compounds react at C-2 to yield 2-substituted beta-allenic esters 3 - 12. The hard electrophile chlorotrimethylsilane reacts at the enolate oxygen atom to provide allenyl enol ether 14. The regioselectivity of these trapping reactions depends only on the nature of the electrophile; the stereoselectivities, on the other hand, are a function of both the steric properties of the allenyl enolate and the type of electrophile. The regio- and stereoselectivities are rationalized in terms of charge control and frontier-orbital control, of kinetic and thermodynamic control, and of steric interactions in intermediates and transition states.