Chiral CO-emulating ligands: From arene chromium chemistry to enantioselective catalysis

Abstract

A one pot nucleophile/electrophile addition/hydrogenation sequence was applied to [(benzene)Cr(CO)3] to give predominantly the 4,5-trans-disubstituted cyclohexene. Several asymmetric modifications of the sequential addition of C-nucleophiles and C-electrophiles to (arene)Cr(CO)3 complexes are discussed. A mechanistically intriguing route involves the use of chiral phosphorous ligands to control the diastereoselectivity in the migratory CO insertion step and/or the reductive elimination step in the sequence. Ephedrine and norephedrine derived ligands gave product ee of up to 69 %. New C2-chiral bidentate ligands (L*) which emulate some of the bonding characteristics of CO were synthesized and briefly considered for this application. The main interest in these ligands concerns their potential in catalytic C-C bond forming reactions; a first application to the Lewis acid [CpFeL*]+ catalyzed Diels-Alder reaction between enals and dienes was successfully realized.