Rhodium Complex Catalyzed Hydrogenation of α,β-Unsaturated Aldehydes to Unsaturated Alcohols

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Abstract
Rhodium complexes were found to be effective in catalyzing selective hydrogenation of cinnamaldehyde to cinnamyl alcohol in the presence of strongly basic amine such as triethylamine or N-methylpyrrolidine under oxo reaction conditions. The selectivity of analogous hydrogenation of aliphatic unsaturated aldehyde was poorer than that of aromatic aldehyde. When a catalytic amount of triphenylphosphine was added, the selectivity was drastically changed to exclusive formation of hydrocinnamaldehyde. In order to elucidate the nature of rhodiumamine interaction, Rh2Cl2(CO4) was immobilized on a cross-linked chloromethylated polystyrene which was functionalized with pyrrolidine, the polymer amine acting as a tertiary amine. The polymer–rhodium complex was found to be stable and to effect the selective hydrogenation of cinnamaldehyde, while addition of triphenylphosphine seemed to liberate the rhodium complex from the polymer. It is concluded that a rhodium–amine–carbonyl complex is responsible for the selective hydrogenation of α,β-unsaturated aldehyde to the unsaturated alcohol.