Asymmetric Lewis base catalysis: I. Lewis base catalyzed halogenation and development of an enantioselective bromocycloetherification reaction; II. enantioselective addition of silyl ketene imines to carbonyl electrophiles

Abstract

The stereochemical stability of bromonium and chloronium ions has been investigated in the context of asymmetric catalysis. Bromonium ions have been found to racemize in the presence of olefins by an olefin to olefin transfer process. Chloronium ions do not racemize under the same conditions. The applicability of Lewis base catalysis to bromo- and iodo- cyclization has been investigated. A variety of Lewis bases with sulfur and selenium donors atoms were found to be effective catalysts. Some of these Lewis bases altered the observed ratio of product isomers, implying that they were present in the stereodetermining transition state and therefore had the capacity to produce asymmetric induction despite racemization of the halonium intermediate. Efforts to develop an enantioselective catalyst system based on this principle were unsuccessful, however the combination of an achiral Lewis base and a chiral phosphoric acid enabled a catalytic enantioselective bromoetherification of aryl-pentenols to be developed. Highly enantioselective additions of silyl ketene imines to aromatic aldehydes under SiCl4/chiral Lewis base catalysis were demonstrated. Additions of silyl ketene imines to aliphatic aldehydes were demonstrated with moderate to high enantioselectivity.