Photo-activated metal carbonyl complexes as stereoselective catalysts for the homogeneous hydrogenation of dienes

Significantly increased activity of Cr(CO)₆ was achieved for the stereoselective homogeneous hydrogenation of methyl sorbate andtrans,trans-conjugated fatty esters at ambient temperature and pressure by exposing the catalyst to UV irradiation (3500 Å) in a solvent mixture of cyclohexane-acetonitrile (20:1). In this solvent mixture, methyl sorbate was converted quantitatively at ambient conditions into methylcis-3-hexenoate, and methyltrans-9,trans-11-octadecadienoate into methylcis-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 Mo(CO)₆ were irradiated at 2537 Å 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 CH₃CN- and H₂-Cr(CO)₃ complexes as intermediates for the stereoselective 1,4-addition of hydrogen totrans,trans-conjugated dienes.