Hydrogenation of citral: a wide-spread model reaction for selective reduction of α,β-unsaturated aldehydes

Abstract

Citral (1) is an α,β-unsaturated aldehyde classified as a monoterpenoid compound from the 3,6-dimethyloctane series, characterized by a head-to-tail linkage of two isoprenoid subunits. The “citrus effect” in many consumer goods can be traced back to the odor of 1, which is also an important building block in the fine chemical industry for production of carotenoids, vitamins, and other flavors and fragrances. The presence of three unsaturated sites in the molecule, i.e. a carbonyl group, a conjugated and an isolated CC bond, makes 1 an interesting model compound for catalytic tests in the field of selective hydrogenation of α,β-unsaturated carbonyl compounds. Several types of catalysts have been described so far for this reaction. The choice of active metal as well as the manipulation of the electronic and steric conditions of the active metal sites in heterogeneous but also homogeneous catalysts allowed the discrimination of distinct reaction pathways affording partially hydrogenated products. Thus, industrially important products like citronellal (2), geraniol (E-3), nerol (Z-3), or citronellol (5) are available from 1 by addition of hydrogen to one of the unsaturated sites. The present review discusses the various possibilities of catalyst design leading to selective but also active catalyst systems for hydrogenation of 1 covering the literature from 1990–2012. Beside supported catalysts with one or two transition metals containing active sites for surface hydrogen splitting, the fields of enzymatic and catalytic transfer hydrogenation are highlighted. In addition scope and limitation of advanced reaction technologies with respect to hydrogenation of 1 are presented, i.e. continuous processing, reduction in supercritical CO₂, or the effect of ionic liquids as solvents or reaction modifiers.