Bifunctional Iminophosphorane Superbases Enable the Highly Enantioselective Sulfa-Michael Addition to Fully Substituted Cyclopropene Carboxylic Acid Derivatives

The metal-free catalytic enantioselective intermolecular conjugate addition of alkyl thiols to unactivated β-substituted cyclopropene carboxylic acid derivatives has been developed. High enantiomeric excesses and yields were consistently achieved across a broad range of thiol pronucleophiles and fully substituted cyclopropene electrophiles under mild reaction conditions, enabled by a novel <i>tert</i>-leucine-derived amide-containing bifunctional iminophosphorane (BIMP) catalyst. Additionally, lowering the catalyst loading to 5.0 mol % was possible and allowed the reaction to be carried out on a gram scale.