The Meerwein–Ponndorf–Verley type reaction in a mixture of supercritical isopropanol/CO2 in a continuous flow reactor in the presence of alumina

A system for efficient reduction of aromatic and aliphatic aldehydes to respective alcohols by the Meerwein–Ponndorf–Verley reaction at a contact time of less than 4 min and temperature below 473 K has been developed. The system comprises a continuous flow reactor, mixed supercritical isopropanol/CO2 solvent and alumina as a catalyst. Reduction of p-nitobenzealdehyde at 425 K yields p-nitrobenzyl alcohol; at 465 K the main reaction product is p-toluidine. α,β-Unsaturated monoterpenic aldehyde myrtenal was reduced into corresponding α,β-unsaturated alcohol myrtenol. At higher temperatures, reduction of myrtenol to saturated alcohol myrtanol becomes possible.