Allenyl Enolates — A New Class of Chiral Ambident Nucleophiles, 1. Reaction with C 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 β‐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.