Reversal of enantioselectivity in cobalt(ii)-catalyzed asymmetric Michael–alkylation reactions: synthesis of spiro-cyclopropane-oxindoles

Abstract

Herein, an asymmetric Michael–alkylation reaction with a reversal of enantioselectivity catalyzed by chiral cobalt complexes is reported. Using 2.5 mol% of Co(acac)₂-imidazolidine-pyrroloimidazolone pyridine as catalyst, a Michael–alkylation reaction between 3-chloro-oxindoles and β,γ-unsaturated-α-ketoesters was achieved, yielding chiral spiro-cyclopropane-oxindoles with a complete and controlled switch in stereoselectivity (up to 99% and −98% ee). The hydrogen bonding between the N–H group in the ligand and the oxyanion of the enolate derived from 3-chloro-oxindole is proposed to direct the reversal of enantioselectivity.