Cyclophane-Type Imidazolium Salts with Planar Chirality as a New Class of N-Heterocyclic Carbene Precursors

Cyclophane-type imidazolium salts with planar chirality were synthesized from enantiopure 2-amino alcohols, of which the N(1) and N(3) positions were connected with a bridge. The structural profiles of the imidazolium salts and their derivative N-heterocyclic carbenes (NHCs) were investigated by means of several analyses. The chiral NHCs derived from these imidazolium salts were found to catalyze the asymmetric cross-annulation of an a,p-unsaturated aldehyde with a ketone by means of the conjugated umpolung of the enal to give the target gamma-lactone with good to excellent enantioselectivity (up to 94% ee). Based on the expected structure of the NHCs and their intermediates, together with the absolute configuration of the products, a plausible mechanism for the stereocontrol was proposed.