Reactivity of Alcohols in Chemoselective Transfer Hydrogenation of Acrolein over Magnesium Oxide as the Catalyst

This paper presents the first full thermodynamic description of the hydrogen transfer between acrolein, the simplest alpha,beta-unsaturated aldehyde, and a set of aliphatic alcohols, both primary and secondary. The vapour phase transfer hydrogenation of acrolein into allyl alcohol with various primary and secondary aliphatic alcohols used as hydrogen donors in the presence of MgO as the catalyst has been studied. Despite differentiated reactivity exhibited by these alcohols, a high chemoselectivity (>80%) to allyl alcohol has been observed for all of them. On the basis of thermodynamic calculations it has been found that secondary alcohols as hydrogen donors are more reactive than primary ones. However, ethanol or butan-1-ol have shown the highest reactivity. In their presence yields of allyl alcohol higher than 60% have been noted, which greatly exceed those predicted by thermodynamic calculations based on the following equation: acrolein ? ethanol (butan-1-ol) -> allyl alcohol + acetaldehyde (butyraldehyde). Although similar yields have been reported in literature, no subsequent nor side reactions have been discussed even though the attained yield cannot be accounted for by this reaction alone. As a possible explanation of the discrepancy the occurrence of a disregarded reaction, for which Delta G < 0, has been considered. It has been shown that aldol condensation fulfills these thermodynamic requirements, however, the products of this reaction are noted only at the beginning of the process and the decrease of their amount does not influence the yield of allyl alcohol.