Diastereoselective Transfer Hydrogenation of Cyclic and Bicyclic Ketones over Selected Metal Oxides as Catalysts

The diastereoselectivity of the liquid- and vapor-phase Catalytic Transfer Hydrogenation (CTH) of cyclic ketones: x-methylcyclohexanones (x = 2, 3 or 4), 4-<i>t</i>-butylcyclohexanone, and bicyclic ketones: 2-norbornanone, camphor, fenchone, and a tricyclic ketone (2-adamantanone) with secondary alkanols (2-propanol, 2-butanol, 2-pentanol, or 2-octanol) as hydrogen donors in the presence of ten metal oxides as the catalysts was studied. Among the oxides, only four, namely, MgO, ZrO₂·nH₂O, ZrO₂, and Al₂O₃, exhibited good or high activity. The reaction products are diastereoisomeric alcohols, the relative ratio of which depends on the structure of the ketone, mode of reaction, temperature, and, in the liquid-phase mode, reaction time. The results of vapor-phase CTH revealed that, in this mode of reaction, the diastereoselectivity to the <i>trans</i> isomer is lower than in the liquid phase. For the three x-methylcyclohexanones, the most pronounced difference between the experimental values and reference values was noted for x = 3. For bicyclic ketones, the implementation of heterogeneous catalysts allowed us to obtain a substantial excess of the less favorable diastereomer. In the case of 2-norbornanone, the thermodynamic equilibrium mixture contains 21% <i>endo</i> and 79% <i>exo</i> alcohols, whereas our product mixtures contained up to 79% of the <i>endo</i> isomer.