Selective Hydrogenation of α,β-Unsaturated Aldehydes Over Intermetallic CompoundsA Critical Review

The selective hydrogenation of α,β-unsaturated aldehydes is a complex reaction. Of industrial interest is the thermodynamically unfavored hydrogenation of the carbonyl group, which is a prerequisite for the synthesis of pharmaceuticals and fragrances. Several strategies to enhance the selectivity using intermetallic compounds have been proposed, which are summarized and critically assessed concerning discrepancies between the reports in literature. Analysis shows that strategies enabling vertical adsorption of α,β-unsaturated aldehydes via the carbonyl group yield high selectivities to unsaturated alcohols. The frequently reported influence of charge transfer, which involves the presence of an intermetallic compound with high electronegativity difference of the constituent elements, does not correlate reliably with selectivity. Evaluation of the literature reveals that often a number of different influence factors might be responsible for enhanced selectivity, which are frequently hard to disentangle.