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

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 degrees 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 degrees 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 degrees 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 (R) nickel results in excellent conversion and selectivity of menthol at 150 degrees 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.