Chemo- and Diastereoselectivity in the Heterogeneous Catalytic Hydrogenation of 2,2′-Pyridoin and Its Derivatives

The heterogeneous catalytic hydrogenations of 2,2′-pyridoin and related compounds, such as 2,2′-pyridil and O-acetyl-2,2′-pyridoin, were investigated over noble metal catalysts. The influence of catalytic metals, catalyst supports, solvents, acid additives, prehydrogenation, and hydrogen pressure on the chemo- and diastereoselectivity is discussed in the hydrogenation of 2,2′-pyridoin. Although hydrogenolysis and ring saturation may occur as side reactions, high chemo- (90–100%) and moderate diastereoselectivity values were achieved. Over palladium black in an acetonitrile–water solvent mixture, the hydrogenation resulted in a meso/dl ratio of 72/28, while in the hydrogenation over rhodium on carbon the meso/dl ratio was 29/71. The phenomenon of diastereoselection in the hydrogenation is explained by the stereochemistry of the hydrogen addition, considering the cis–trans isomerization on the catalyst surface, the possible enolization, and the competing CC and CO reductions.