Asymmetric catalytic synthesis using arene chromium carbonyls

Recently, many organic chemists have become interested in research to the synthesis of enantiomerically pure organics, useful for synthesis of biologically active agents. In many cases, enantioselective catalysis is the most suitable and efficient method into enantiopure systems.; The basis for most catalytic enantioselective reactions involves formation of a chiral metallocyclic Lewis acid from a chiral catalyst precursor and a suitable organometallic species. A traditional method to coordinate the substrate to the catalyst has been to employ a Lewis acidic metal center to chelate [σ] the substrate via a carbonyl group, into an asymmetric environment. Subsequent asymmetric additions to the substrate would then be influenced by local steric bulk of the catalyst. Such coordination locks the reactant into a chiral environment where steric and/or electronic effects prejudice approach of an incoming reactant to make formation of one enantiomer more favorable than the other.; In our case, the metallocyclic systems envisaged have been sourced from enantiopure building blocks using the η6 arene chromium tricarbonyl group as a key stereodirective element. Complexation with a Cr(CO)₃ group provides a temporary and powerful means of polarity inversion for aromatic rings and can lead to interesting synthetic applications. The tricarbonyl arene chromium moiety has a number of distinct advantages associated with it, is particularly well suited to incorporation into a chiral catalyst.; Based on the results of this research, it has been determined that the chromium tricarbonyl functionality has an influential positive effect on the enantioselectivity of all reactions studied. In the case of Diels-Alder cycloaddition reactions, complexation of the catalysts led to increases of up to 55–60% in enantiomeric excess. In the case of alkylation reactions, complexation of the catalysts increased the enantiomeric excess of the product up to 20–40% when compared with uncomplexed analogues.