CATALYTIC ASYMMETRIC HYDROGENATION OF KETOPANTOLACTONE BY RHODIUM(I) AMINOPHOSPHINE-PHOSPHINITE COMPLEXES - A THEORETICAL-ANALYSIS BY MOLECULAR MECHANICS AND EXTENDED HUCKEL CALCULATIONS

A combination of molecular mechanics methods and extended Huckel calcu lations has been used as a tool in an attempt to rationalize experimen tal results obtained during catalytic asymmetric hydrogenation of dihy dro-4,4-dimethyl-2,3-furandione (ketopantolactone, KPL) using chiral a minophosphine-phosphinite (AMPP) chlororhodium complexes. This study i s based upon the hypothesis that the enantioselectivity of prochiral k etone hydrogenation is controlled by the relative energies of the diff erent diastereomeric intermediates obtained from oxidative addition of hydrogen and ketone coordination to the chiral chlororhodium complexe s. Thus, a series of possible six-coordinate structures of general for mula [RhCl(H)(2)(AMPP)(KPL)] has been generated for three AMPP ligands , (S)-Ph,Cp-isoAlaNOP (I), (R)-Ph,Cp-isoAlaNOP (II) and (S)-Cp,Cp-isoA laNOP (III), and each octahedral dihydrido complex has been optimized first by molecular mechanics (steric energy) and then by extended Huck el calculations (total energy). For the most stable complex (lowest to tal energy) in each series, the configuration of pantolactone arising from reductive elimination was determined assuming nucleophilic attack of the preferred H atom to the carbonyl C atom of KPL. This procedure has been shown to reproduce the enantioselectivities observed with li gands I-III. Namely, this methodology suggests the inversion in the co nfiguration of the prevailing enantiomer of pantolactone upon using sy stem I instead of system III, in perfect agreement with the experiment s. The most stable six-coordinate intermediates arising from the latte r systems are both cis-dihydrido structures differing from each other from the respective localization of hydride and chloride ligands at th e apical sites, and from the conformation adopted by the seven-membere d metallacycle. Computations conducted on systems I and II also indica te the inversion of configuration of the produced pantolactone, as exp ected from ligands of opposite absolute configuration. For systems I a nd II, the most stable cis-dihydrido octahedral complexes are shown to be almost antipodal structures, only differing from each other from t he conformation of the chiral backbone.