Addition reactions of O-bound cyclic nickel enolates to (alpha),(beta)-unsaturated ketones

The addition of the O-bound nickel enolates Ni(C6H4-o-C((omicron)CHR)O)(dippe)(R = H, 1; R = Me, 2) to (alpha), (beta)-unsaturated ketones proceeds with complete stereoselectivity giving rise to [2 + 4] cycloadducts that can further evolve to generate open-chain, Michael-like products. The stereoselectivity of these reactions suggests a concerted mechanism, through an exo transition state.