Asymmetric Catalytic Formal 1,4-Allylation of beta,gamma-Unsaturated alpha-Ketoesters: Allylboration/Oxy-Cope Rearrangement

摘要

A highly enantioselective formal conjugate allyl addition of allylboronic acids to beta,gamma-unsaturated alpha-ketoesters has been realized by employing a chiral Ni-II/N,N '-dioxide complex as the catalyst. This transformation proceeds by an allylboration/oxy-Cope rearrangement sequence, providing a facile and rapid route to gamma-allyl-alpha-ketoesters with moderate to good yields (65-92 %) and excellent ee values (90-99 % ee). The isolation of 1,2-allylboration products provided insight into the mechanism of the subsequent oxy-Cope rearrangement reaction: substrate-induced chiral transfer and a chiral Lewis acid accelerated process. Based on the experimental investigations and DFT calculations, a rare boatlike transition-state model is proposed as the origin of high chirality transfer during the oxy-Cope rearrangement.