Base-free transfer hydrogenation of menthone by sub- and supercritical alcohols.

Graphical abstract Highlights • Base-free transfer hydrogenation of menthone by primary and secondary alcohols as a hydrogen donor under sub- and sc conditions. • The similar H -donor activity of all alcohols under non-catalytic sc conditions. • Only secondary alcohols demonstrate a high H -donor activity in the presence of Raney® Ni under subcritical conditions. • A quasi-equilibrium of isomeric menthone/isomenthone mixture reached via enolization under sc conditions. Abstract The base-free transfer hydrogenation (THR) of menthone with six aliphatic alcohols as hydrogen source has been developed under sub- and supercritical conditions. Under sc conditions (at 350 °C and 78–210 atm) without any catalyst, all alcohols demonstrated the similar H -donor activity, which was high enough. The menthone conversion was about 64 ± 5% for 5 h of the reaction, whereas the selectivity on menthol reached 92–99% for all alcohols except MeOH (73%). The non-catalytic THR at 250 °C was very slow and the conversion in the most active 2-PrOH was only 7.0–7.5% for 9 h of the reaction. Under subcritical conditions at 150 °C, the non-catalytic menthone reduction was negligible in all alcohols. But in secondary 2-PrOH and 2-BuOH, the base-free THR catalyzed by Raney® nickel results in excellent conversion and selectivity of menthol at 150 °C, whereas no conversion of menthone was observed in primary alcohols under the same reaction conditions. A quasi-equilibrium of isomeric menthone/isomenthone mixture reached via enolization was evaluated under sub- and supercritical conditions and the Gibbs energies of the isomerization in each alcohol were calculated. [ABSTRACT FROM AUTHOR]