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 PF6) (6) has been studied as catalyst medium for biphasic homogeneous hydrogenations of sorbic acid (1). As catalyst we used the Cp*-ruthenium-complex [Cp*Ru(η4-CH3—CHCH—CHCH—COOH) (CF3SO3)] which efficiently enables the stereoselective hydrogenation of sorbic acid leading to the formation of cis-3-hexenoic acid (3) in selectivities of up to 90% with turnover frequencies of up to 1100 h—1. Compared to other biphasic systems the hydrogenation in bmim PF6 proceeds with enhanced activity. The kinetics can be described with a Michaelis Menten-equation, and the activation energy for the whole process was determined to be EA = 78 ± 5 kJ/mol.