Preparation of σ- and π-allylicopper(III) intermediates in SN2 and SN2′ reactions of organocuprate(I) reagents with allylic substrates

The first pi-allyl complexes of Cu(III) have been prepared and characterized by using rapid injection nuclear magnetic resonance spectroscopy (RI-NMR). The prototype, (eta(3)-allyl) dimethylcopper(III), was prepared by injection of allyl chloride into a THF-d(8) solution of iodo-Gilman reagent, Me(2)CuLi .Lil (A), spinning in the probe of an NMR spectrometer at -100 degrees C. A sigma-allyl ate complex, lithium (eta(1)-allyl)trimethylcuprate(III), was prepared in high yield by including 1 equiv of tributylphosphine in the reaction mixture or by using allyl acetate as the substrate, Cyano ate complex, lithium cis-(eta(1)-allyl)cyanodimethylcuprate(III) was obtained in high yield by injecting allyl chloride or allyl acetate into the cyano-Gilman reagent, Me(2)CuLi .LiCN (B), in THF-d(8) at -100 degrees C. Reactions of A with allylic substrates show a definite dependence on leaving group (chloride vs acetate), whereas those of B do not. Moreover. these reagents have different regioselectivities, which in the case of A vary with temperature. Finally, the exclusive formation of cis-cyano sigma-allyl Cu(III) intermediates in both the 1,4-addition of B to a-enones and its S(N)2a reaction with allylic substrates now makes sense in terms of pi-allyl intermediates in both cases, thus unifying the mechanisms of these two kinds of conjugate addition.