Mechanism of remote conjugate addition of lithium organocuprates to polyconjugated carbonyl compounds.

Regioselective reaction of a lithium organocuprate (R2CuLi) and a polyconjugated carbonyl compound affords a remote-conjugate-addition product. This reaction proceeds particularly cleanly when the conjugation is terminated by a C-C triple bond. The reaction pathways and the origin of the regioselectivity of this class of transformations are explored with the aid of density functional calculations. The outline of the reaction pathway is as follows. An initially formed beta-cuprio(III) enolate intermediate undergoes smooth copper migration along the conjugated system. This process takes place faster than reductive elimination of intermediary sigma/pi-allylcopper(III) species, since the latter reaction disrupts the conjugation in the substrate and hence is not preferred. The copper migration to the acetylenic terminal affords a sigma/pi-allenylcopper(III) intermediate, which undergoes facile and selective C-C bond forming reductive elimination at the terminal carbon atom. The present mechanistic framework shows good agreement with some pertinent experimental data, including 13C NMR chemical shifts and kinetic isotope effects.