Description of the structure-chemoselectivity relationship in the transfer hydrogenation of α,β-unsaturated aldehydes and ketones with alcohols in the presence of magnesium oxide

Abstract

The vapour-phase catalytic transfer hydrogenation (CTH) of fifteen acyclic α,β-unsaturated carbonyl compounds, i.e. ten aldehydes and five ketones, with ethanol or 2-propanol as hydrogen donors has been studied over pure MgO as the catalyst. The obtained values of the yields of unsaturated alcohols (UOLs) have been used to calculate a normalized value called the relative directional reactivity (RDR). Among substituted α,β-unsaturated aldehydes transfer hydrogenated with ethanol only two of them have shown a lower reactivity (RDRE < 1) than that noted for acrolein (for which RDRE = 1), namely 3,3-dimethyl- and 2-phenylacrolein. It has been found that the introduction of an alkyl substituent into position 2 in an acrolein molecule as well as a phenyl group into position 3 leads to a distinct increase of an RDRE value for such aldehydes. In contrast, very pronounced differences in relative directional reactivity values (RDRP) for both α,β-unsaturated aldehydes and ketones have been observed when 2-propanol was used as the hydrogen donor. The ranges of the RDRP values are: 0.45–2.52 and 0.04–1.71 for aldehydes and ketones, respectively. The proposed normalization procedure allowed us to obtain values which could be used to make meaningful conclusions as to the influence of the structure of α,β-unsaturated carbonyl compounds on their reactivity. Moreover, it was used to determine which of these compounds react in accordance with the thermodynamic description, and whose reactivity is kinetically controlled.