ENANTIOSELECTIVE HYDROGENATION .3. METHYL PYRUVATE HYDROGENATION CATALYZED BY ALKALOID-MODIFIED IRIDIUM

Enantioselective hydrogenation of methyl pyruvate, MeCOCOOMe to methyl lactate, MeCH(OH)COOMe, is catalyzed in solution at room temperature by supported iridium catalysts modified with cinchona alkaloids. Modif ication with cinchonidine or quinine yields R-lactate in excess, where as modification with cinchonine or quinidine favors S-lactate formatio n. Ir/SiO2 catalysts (20%) calcined at 393 to 573 K and reduced at 523 to 593 K were highly active for racemic hydrogenation in the absence of a modifier (rates typically 1.8 mol h-1 g(cat)-1) and were comparab ly active when modified with cinchonidine but gave an enantiomeric exc ess of about 30%. Use of higher calcination or reduction temperatures led to substantially inferior activity and selectivity. The high rates recorded for both racemic and enantioselective reactions are dependen t on the catalysts being activated before use by a procedure involving exposure of the catalyst to air after the initial reduction. Use of a Cl-free precursor gave an Ir/SiO2 catalyst (20%) of superior activity but inferior enantioselectivity. Ir/CaCO3 (5%) was more active for ra cemic hydrogenation than for enantioselective hydrogenation, but provi ded the highest value of the enantiomeric excess 39%. Kinetics of reac tion are reported. Exchange of H for D in 10,11-dihydrocinchonidine at room temperature over Ir/CaCO3 occurred in the quinoline moiety but n ot in the quinuclidine ring system, indicating that the alkaloid was a dsorbed to the Ir surface via the interaction of its pi-electron syste m. For both silica-supported and calcium carbonate-supported Ir, the p resence of chloride ion in the catalyst was advantageous for the achie vement of enantioselectivity. (C) 1994 Academic Press, Inc.