ENANTIOSELECTIVE HYDROGENATION - V - HYDROGENATION OF BUTANE-2,3-DIONE AND OF 3-HYDROXYBUTAN-2-ONE CATALYZED BY CINCHONA-MODIFIED PLATINUM

Pt/silica modified by cinchonidine and cinchonine is active for the en antioselective hydrogenation of butane-2,3-dione to butane-2,3-diol in dichloromethane at 268-298 K and 10 bar pressure. Reaction proceeds i n three stages. In the first, about 85% of the butane-2,3-dione is con verted to 3-hydroxybutan-2-one and 15% to three higher molecular weigh t products by hydrodimerisation. The initial enantiomeric excess in th e hydroxybutanone is modest (20 to 40%(R) with cinchonidine as modifie r, 10%(S) with cinchonine as modifier) and dependent on the amount of alkaloid used in catalyst preparation. In the second stage, 3-hydroxyb utan-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 hydroxybuta none increases dramatically to values in the range 62 to 89%(R) and to 30%(S). Under all conditions, the most abundant stereochemical form o f the final product is meso-butane-2,3-dione. In the third stage the t hree dimers are slowly converted by hydrogenation, dissociation, and f urther hydrogenation to butane-2,3-diol. In the absence of alkaloid, b utane-2,3-dione hydrogenation to racemic products in dichloromethane s olution proceeds in two distinct stages with no dimer formation. Butan e-2,3-dione hydrogenation has also been studied over Pt/silica modifie d anaerobically by exposure to cinchonidine in ethanol under propyne a t 2 bar. This catalyst is remarkably active for the conversion of dike tone to diol in ethanol at 293 K and 10 bar and kinetic selection in t he second stage of reaction is again observed. The hydrogenation of ra cemic 3-hydroxybutan-2-one in dichloromethane over cinchonine-modified Pt/silica at 273 K and 10 to 40 bar pressure also showed kinetic sele ction, an enantiomeric excess of up to 70%(S) appearing in the reactan t as it was consumed. Mechanisms which account for these hydrogenation s and dimerisations and for the enantioselectivities observed and thei r variation are presented. This diketone hydrogenation provides an exa mple of consecutive thermodynamic and kinetic control of enantioselect ivity in a multistage catalytic reaction. (C) 1998 Academic Press.