Enantioselective N-Heterocyclic Carbene Catalysis by the Umpolung of α,β-Unsaturated Ketones

Abstract

N-Heterocyclic carbene-catalyzed formation of β-anionic intermediates from enones has been employed in the enantioselective synthesis of 2-aryl propionates. The reaction was achievable using a homochiral 4-MeOC6H4 morpholinone catalyst allowing the first example of enantioselective catalysis by umpolung of α,β-unsaturated ketones. The reaction is high yielding, and shows robustness with reasonable generality. A mechanism is proposed in which the enantiodetermining protonation is achieved using either hexafluoroisopropanol or the formed naphthol product.

Keywords: 1,4-addition; 2-aryl propionates; N-heterocyclic carbenes; enantioselective catalysis; β-azolium ylides.