Enantioselective homocoupling of allenylic alcohols through divergent cooperative catalysis

An enantioselective homocoupling of branched allenylic alcohols is developed under cooperative iridium and Lewis acid catalysis. In this reaction, racemic allenylic alcohol is transformed, under Lewis acidic Sc(OTf)₃, into an α,β-unsaturated enol (cross dienol) through a Meyer-Schuster-type 1,3-hydroxy transposition. In an independent cycle, catalyzed by a combination of an Ir(i)/(phosphoramidite,olefin) complex and Sc(OTf)₃, allenylic alcohol is proposed to produce an η²-Ir(i)-bound allenylic carbocation intermediate, which is intercepted by the in situ generated cross dienol. Overall, starting from racemic branched allenylic alcohols, α'-allenylic α,β-unsaturated ketones are produced, without using any preformed carbon nucleophile, in moderate to good yields with excellent enantioselectivities. This strategy may be termed as divergent cooperative catalysis, where a single substrate is converted into two transient intermediates of complementary polarity under the influence of two different catalysts. The selective coupling of these two polarity-matched intermediates results in the desired product. Mechanistic details are unraveled through experimental studies and density functional theory (DFT) calculations.