Asymmetric Catalytic Formal 1,4-Allylation of β,γ-Unsaturated α-Ketoesters via Allylboration/Oxy-Cope Rearrangement.

Abstract

A highly enantioselective formal conjugate allyl addition of allylboronic acids to β,γ-Unsaturated α-Ketoesters has been realized by employing a chiral NiII/N,N -dioxide complex as the catalyst. This transformation proceeded via allylboration/oxy-Cope rearrangement tandem process, providing a facile and rapid route to γ-allyl-α-ketoesters with moderate to good yields (66-92%) and excellent ee values (90-99% ee). The isolation of 1,2-allylboration products enabled us to get insight into the mechanism of subsequent oxy-Cope rearrangement reaction: substrate-induced chiral transfer and chiral Lewis acid accelerated process. Based on the experimental investigations and DFT calculations, a rare boat-like transition-state model was proposed to elucidate the origin of high chirality transfer during the oxy-Cope rearrangement.