Stereoselective Catalysts for the Homogeneous Hydrogenation of Dienes

Significantly increased aCtlVlfY of Cr(CO)6 was achieved for the stereoselective homogeneous hydrogenation of methyl sorbate and tmns,trans-conjugated fatty esters at ambient temperature and pressure by exposing the catalyst to UV irradiation (3500.'1.) in a solvent mixture of cyclohexane-acetonitrile (20:1). In this solvent mixture, methyl sorb ate was converted quantitatively at ambient conditions into methyl cis-3-hexenoate, and methyl tmns-9,trans-ll-octadecadienoate into methyl cis-10-octadecenoate (99.9%). These products are expected by 1,4-addition of hydrogen. Under these conditions no hydrogenation of methyl linoleate occurred. Under the same conditions, cycloheptatriene-Cr(CO)3 showed lower activity than Cr(CO)6' and MO(CO)6 and mesitylene-Mo(CO)3 showed no significant activity toward conjugated substrates. When Cr(CO)6 and ""10(CO)6 were irradiated at 2537.'1. they caused the geometric isomerization of methyl sorbate without hydrogenation, but had no effect on methyl linoleate. A hydrogenation mechanism is proposed for Cr(CO)6 that involves CH 3 CNand Hz-Cr(CO)3 complexes as intermediates for the stereoselective l,4-addition of hydrogen to trans,trans-conjugated dienes.