Ionic liquid-phase asymmetric catalytic hydrogenation: hydrogen concentration effects on enantioselectivity

Molecular hydrogen is almost four times more soluble in the ionic liquid 1-n-butyl-3-methylimidazolium tetrafluoroborate ((BMIBF4)-B-.) than in its hexafluorophosphate ((BMIPF6)-P-.) analogue at the same pressure. The Henry coefficient solubility constant for the solution BMI-BF4/H-2, is K=3.0 x 10(-3) mol L-1 atm(-1) and 8.8 x 10(-4) mol L-1 atm(-1) for (BMIPF6)-P-./H-2, at room temperature. The asymmetric hydrogenation of (Z)-alpha -acetamido cinnamic acid and kinetic resolution of (+/-)-methyl-3-hydroxy-2-methylenebutanoate by (-)-1,2-bis((2R,5R)-2,5-diethylphospholano)benzene(cyclooctadiene)rhodium(I) trifluoro-methanesulfonate and dichloro[(S)-(-)-2,2'-bis(di-p-tolylphosphino)-1,1'-binaphthyl]ruthenium(II) complexes immobilised in BMI-PF, and (BMIBF4)-B-. were investigated. Remarkable effects in the conversion and enantioselectivity of these reactions were observed as a function of molecular hydrogen concentration in the ionic phase rather than pressure in the gas phase. (C) 2001 Elsevier Science Ltd. All rights reserved.