RHODIUM DIHYDRIDE COMPLEXES AS MODELS FOR THE THEORETICAL-ANALYSIS OFENANTIOSELECTIVE HYDROGENATION REACTIONS

A combination of molecular mechanics methods and extended Huckel calcu lations has been applied in order to have access to the more stable co mplexes expected to be involved as catalytic intermediates in the enan tioselective hydrogenation of ketopantolactone (KPL) using chiral amin ophosphine-phosphinite (AMPP) chlororhodium complexes, The product sel ectivity has been deduced from correlations between the prevailing con figuration of the hydrogenated derivatives and the energetics of compe ting diastereomeric dihydride complexes of formula [RhCl(H)(2)(AMPP) ( KPL)] with the assumption that the enantioselectivity is controlled by the relative energies of such intermediates. The calculations have be en obtained from the application of sequential and exhaustive search m ethodologies. The procedure has been applied to complexes bearing the aminophosphine-phosphinites (S)-Cp,Cp-ProNOP (IV) and (S)-Ph,Cp-ProNOP (V) and bis(aminophosphanes) derived from 2(anilinomethyl)pyrrolidine (VI-M). The latter induce a reversal of configuration of the major en antiomer of the hydrogenation product when varying specific substituen ts at the phosphorus atoms. Computations were carried out also for com plexes bearing the two enantiomers (S)- and (R)-Ph,Cp-isoAlaNOP. The l owest energy complexes present enantiomeric structures. A novel insigh t into the local reactivity of the intermediates has been gained from determining the first migrating hydride according to the superdelocali zability parameter calculated for all isomers, Thus, the configuration s of pantolactone arising from the alkoxyrhodium species obtained when assuming a nucleophilic attack of one of the hydrides to the carbonyl group of the ketone has been defined and are in total agreement with the experimental data. (C) 1998 Elsevier Science Ltd. All rights reser ved.