The Two Faces of Carbenium Ions: Intermolecular Trapping of Prochiral Carbocations with Stereocontrol

Abstract

AbstractCarbenium ions had been suspected and subsequently shown to be key intermediates of a large family of cationic transformations. An enormous body of work is dedicated to their characterization and various roles in reaction mechanisms, e. g. the SN1 reaction. Far less attention has been paid to their engagement in asymmetric synthesis and indeed, the differentiation of the diastereo‐ or even enantiotopic faces of structurally unbiased, non‐heteroatom‐stabilized carbenium ions is a daunting challenge. This Review summarizes the approaches taken to achieve the stereoselective addition of nucleophiles to these fleeting intermediates, including recent advances where non‐covalent interactions and chiral counterions are employed to discriminate either side of a trigonal‐planar carbenium ion bearing three different substituents.