An ionic liquid as catalyst medium for stereoselective hydrogenations of sorbic acid with ruthenium complexes

The ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate (bmim PF 6) (6) has been studied as catalyst medium for biphasic homogeneous hydroge nations of sorbic acid (1). As catalyst we used the Cp*-ruthenium-complex [ Cp*Ru(eta(4)-CH3-CH=CH-CH=CH-COOH) (CF3SO3)] which efficiently enables the stereoselective hydrogenation of sorbic acid leading to the formation of ci s-3-hexenoic acid (3) in selectivities of up to 90% with turnover frequenci es of up to 1100 h(-1). Compared to ether biphasic systems the hydrogenatio n in bmim PF6 proceeds with enhanced activity. The kinetics can be describe d with a Michaelis-Menten-equation, and the activation energy for the whole process was determined to be E-A = 78 +/- 5 kJ/mol.