281020 Theoretical Insights Into the Effects of Hydrocarbon Structure On the Catalytic Hydrogenation of Unsaturated Ketones
The catalytic hydrogenation of unsaturated ketones to unsaturated alcohols is an important step in the synthesis of different fine chemical and pharmaceutical as well as in biomass conversion strategies . The ability to selectively hydrogenation the carbonyl group over that of the C=C bond presents a significant challenge as the hydrogenation of C=C is thermodynamically favored over that of the C=O bond [2,3]. Recent experimental observations reveal that presence of substituents on the C=C bond may prevent its adsorption and hydrogenation and potentially increase the probability of hydrogenating the C=O bond and the formation of desired unsaturated alcohols [4,5]. Very little, however, is known about the specific role that substitution plays in the elementary hydrogenation mechanisms of unsaturated ketones. To probe this effect, we carried out a systematic density functional theoretical analysis to explore the reactivity trends for a series of model ketones over transition metal surfaces, beginning with the simplest unsaturated ketone, methyl vinyl ketone. Methyl substituents were added one by one to the C=C bond to establish their influence on the reaction kinetics.
Increasing degree of substitutions at the C=C bond was found to weaken the adsorption of the reactants near-linearly, and alter the energetics of hydrogenation steps by increasing the hydrogenation barrier at the substituted carbon site, while lowering the barrier to add the first hydrogen to carbon center of the C=O bond. The addition of methyl substituents to the C=C bond was found to enhance the addition of hydrogen to the carbonyl and inhibit the hydrogen addition to the C=C bond. The hydrogenation of the C=C group, however was still found to be more favorable than the hydrogenation of the C=O bond, which is consistent with experimental observations .
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