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Dual-Hydrogen-Bond Donor and Brønsted Acid Cocatalysis Enables Highly Enantioselective Protio-Semipinacol
Melanie A S Blackburn1, Corin C Wagen1, M Raul Bodrogean1
1Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States.
Abstract:
A catalytic protio-semipinacol ring-expansion reaction has been developed for the highly enantioselective conversion of tertiary vinylic cyclopropyl alcohols into cyclobutanone products bearing α-quaternary stereogenic centers. The method relies on the cocatalytic 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 cocatalysis to weakly basic olefinic substrates and lays the foundation for further investigations of enantioselective reactions involving high-energy cationic intermediates.
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