Chiral base mediated asymmetric synthesis of tricarbonyl(η6-arene)chromium complexes

Abstract

Enantiomerically enriched tricarbonyl(η⁶-arene)chromium complexes can be obtained in up to 94% ee from the enantioselective ortho-metallation reaction of certain symmetrically substituted complexes with Me₃SiCl, mediated by a chiral lithium amide base. The level of asymmetric induction depends upon the nature of the starting ring substituent(s) and, in the case of anisole-type complexes, a proton transfer reaction involving the chiral metallated intermediate and the neutral tricarbonyl(η⁶-arene)chromium complex results in rapid racemisation in the absence of an in situ electrophilic quench. Non-racemic derivatives of a tricarbonyl(η⁶-arene)chromium complex derived from 1,3-dihydroisobenzofuran are also prepared in up to 99% ee by chiral base deprotonation, and trends in the regioselectivity of metallation of this complex with a range of lithium amide bases are described.