DIASTEREOSELECTIVE HYDROGENATION OF SUBSTITUTED AROMATICS ON SUPPORTED METAL-CATALYSTS

The purpose of this work was to hydrogenate disubstituted aromatic com pounds on supported metal catalysts to produce enantiomers of the corr esponding cyclohexane derivatives using a diastereoselective approach, i.e., associating the organic substrates with chiral auxiliaries. The diastereoselective catalytic hydrogenation of N-(2-methylbenzoyl)-(S) -proline methyl ester was performed on carbon-supported metal catalyst s, optionally modified by the adsorption of EDCA (ethyldicyclohexylami ne). The formation of cis-diastereoisomers predominated and a cyclohex enic compound was formed transiently After cleavage of the proline est er auxiliary, optically active 2-methylcyclohexane carboxylic acids we re obtained. The influence on reaction kinetics and diastereoselectivi ty of various factors were investigated on Rh/C catalysts; these facto rs were pressure, substrate concentration, particle size, and thermal pretreatments of the rhodium catalysts modified with an amine. The rea ction rate and the diastereoisomeric excess were found to be very sens itive to these parameters as well as to the presence of water on the c atalyst. The importance of the nature of the metal (Rh, Ru, Pt, Pd) wa s clearly demonstrated. Pt and Pd showed very low activity. Without ad dition of amine, unmodified rhodium catalysts were nonselective as lon g as the aromatic substrate was present, whereas the ruthenium catalys ts produced moderate selectivity. In the latter case, the carbonyl and carboxyl groups of the molecule could interact with the surface, thus determining which face of the aromatic ring would be preferentially a dsorbed. Then, because of the consecutive hydrogenation of the cyclohe xenic compound, diastereoselective excesses (d.e.) of 17 and 32%, resp ectively, were observed in favor of the (1R,2S,2'S) isomer. In the pre sence of an achiral amine (e.g., EDCA) on rhodium catalysts, d.e. atta ining ca. 50% were measured in favor of the (1S,2R,2'S) isomer. No inv ersion of configuration was observed with modified ruthenium, probably because the aromatic substrate is more strongly bonded to the surface than in the case of rhodium and thus displaces the amine. In contrast , the aromatic substrate is weakly bonded to platinum and palladium co mpared to the EDCA molecules which block the surface, In the absence o f amine, the weaker adsorption of the aromatic substrate is responsibl e for the formation of irans products. (C) 1997 Academic Press.