Enantioselective Hydrogenation: V. Hydrogenation of Butane-2,3-dione and of 3-Hydroxybutan-2-one Catalysed by Cinchona-Modified Platinum

Abstract Pt/silica modified by cinchonidine and cinchonine is active for the enantioselective hydrogenation of butane-2,3-dione to butane-2,3-diol in dichloromethane at 268–298 K and 10 bar pressure. Reaction proceeds in three stages. In the first, about 85% of the butane-2,3-dione is converted to 3-hydroxybutan-2-one and 15% to three higher molecular weight products by hydrodimerisation. The initial enantiomeric excess in the hydroxybutanone is modest (20 to 40%(R) with cinchonidine as modifier, 10%(S) with cinchonine as modifier) and dependent on the amount of alkaloid used in catalyst preparation. In the second stage, 3-hydroxybutan-2-one is converted to butane-2,3-diol; a marked kinetic effect is observed whereby the minority enantiomer is converted preferentially to butanediol and the enantiomeric excess in the remaining hydroxybutanone increases dramatically to values in the range 62 to 89%(R) and to 30%(S). Under all conditions, the most abundant stereochemical form of the final product is meso-butane-2,3-dione. In the third stage the three dimers are slowly converted by hydrogenation, dissociation, and further hydrogenation to butane-2,3-diol. In the absence of alkaloid, butane-2,3-dione hydrogenation to racemic products in dichloromethane solution proceeds in two distinct stages with no dimer formation. Butane-2,3-dione hydrogenation has also been studied over Pt/silica modified anaerobically by exposure to cinchonidine in ethanol under propyne at 2 bar. This catalyst is remarkably active for the conversion of diketone to diol in ethanol at 293 K and 10 bar and kinetic selection in the second stage of reaction is again observed. The hydrogenation of racemic 3-hydroxybutan-2-one in dichloromethane over cinchonine-modified Pt/silica at 273 K and 10 to 40 bar pressure also showed kinetic selection, an enantiomeric excess of up to 70%(S) appearing in the reactant as it was consumed. Mechanisms which account for these hydrogenations and dimerisations and for the enantioselectivities observed and their variation are presented. This diketone hydrogenation provides an example of consecutive thermodynamic and kinetic control of enantioselectivity in a multistage catalytic reaction.