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 proceeds by an allylboration/oxy-Cope rearrangement sequence, providing a facile and rapid route to γ-allyl-α-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.
Keywords: allylic compounds; asymmetric catalysis; nickel; reaction mechanisms; rearrangements.
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