CATALYTIC HYDROGENATION OF ALPHA BETA-UNSATURATED KETONES .4. EFFECT OF MEDIUM ON PRODUCT STEREOCHEMISTRY

The hydrogenation of Δ1,9-octalone-2 has been run in a number of different solvents and under a variety of conditions. In neutral medium it has been found that the product stereochemistry was dependent not only on the polarity but also on the type of solvent used. In aprotic media it is proposed that a nonpolar solvent promotes a 1,2-adsorption process and a polar solvent favors one occurring through 1,4 adsorption. In hydroxy lie media hydrogen bonding to the carbonyl group or hemiketal formation is thought to occur in solvents of high polarity thus forcing the reaction to take place by way of 1,2 adsorption while in less polar hydroxy lie solvents such solvation is not important and the reaction proceeds through a 1,4-adsorption process. The product stereochemistry not only depends on the nature of the adsorption process but also on the “hydrogen availability” to the catalyst. In acidic media product stereochemistry is dependent on the strength of the acid, the mode of adsorption of the substrate and the “hydrogen availability” to the catalyst. These data are shown to be compatible with a protonation-hydride ion transfer mechanism or with a process involving hydrogenation of the enol but not with one in which the acid serves only to increase the polarity of the solvent. In basic solutions the product stereochemistry obtained was markedly dependent on the amount of base present, particularly in the very dilute region. These results are interpreted as indicating that the reaction proceeds by way of kinetically and thermodynamically controlled enolate ion formation. It is proposed that these enolates are very strongly adsorbed on the catalyst surface and that the product stereochemistry can best be explained by way of a hydride ion transfer from the catalyst followed by protonation of the adsorbed species from the solution. © 1969, American Chemical Society. All rights reserved.