Methanol as a clean and efficient H-transfer reactant for carbonyl reduction: Scope, limitations and reaction mechanism

The previously unexplored use of methanol as a H-transfer agent for the Meerwein-Ponndorf-Verley reduction of aromatic aldehydes and aryl ketones is described. Furfural, 5-hydroxymethylfurfural, benzaldehyde, and acetophenone were selectively reduced to the corresponding alcohols in mild conditions. The reaction mechanism was elucidated by means of reactivity tests and DFT calculations. It was found to include the highly efficient H-transfer with the formation of formaldehyde, which further reacted with excess methanol to generate the adsorbed hemiacetal. In turn, the latter reduced carbonyl, with formation of methylformate, which further decomposed into CO, CH4, and CO2. Compared to the alcohols typically used for carbonyl reductions, methanol showed the advantage of producing gaseous components as the only co-products, which are easily separated from the reaction medium. In the case of furfural a 100% yield to furfuryl alcohol was obtained, using the high-surface area MgO as the easily recoverable and reusable catalyst