Dual-Hydrogen-Bond Donor and Brønsted Acid Co-Catalysis Enables Highly Enantioselective Protio-Semipinacol Rearrangement Reactions

A catalytic protio-semipinacol ring-expansion reaction has been developed for the highly enantioselective conversion of tertiary vinylic cyclopropanols into cyclobutanone products bearing α-quaternary stereogenic centers. The method relies on the co-catalytic effect of a chiral dual-hydrogen-bond donor (HBD) with hydrogen chloride. Experimental evidence is provided for a stepwise mechanism where protonation of the alkene generates a short-lived, high-energy carbocation which is followed by C–C bond migration to deliver the enantioenriched product. This research applies strong acid/chiral HBD co-catalysis to weakly basic olefinic substrates and lays the foundation for further investigations of enantioselective reactions involving high-energy cationic intermediates.