(211f) A First Principles Analysis of the Selective Hydrogenation of Unsaturated Ketones: Influence of Hydrocarbon Structure
The selective hydrogenation of carbonyl groups in unsaturated ketones (UK) is important in the catalytic synthesis of various pharmaceutical and fine chemical intermediates as well as in biomass conversion strategies . The ability to selectively hydrogenation the C=O group over that of the C=C bond presents a significant challenge as the hydrogenation of the C=C is thermodynamically favored over that of the C=O bond [2,3]. Recent experimental observations reveal that the adsorption and selectivity for the hydrogenation of unsaturated ketones depends strongly on the substituents on the C=C bond [4,5]. One successful example is the selective hydrogenation of the sterically-hindered C=O group in ketoisophorone (KIP) over Pd/Al2O3 . Very little, however, is known about the specific role that substitutions play in the elementary hydrogenation mechanisms of unsaturated ketones. To probe this effect, density functional theory (DFT) calculations were carried out to explore the elementary hydrogenation steps for a series of model ketones over different metal surfaces, beginning with the simplest unsaturated ketone, methyl vinyl ketone. Methyl substituents were then added one by one to the C=C bond to establish their influence on the kinetics.
Increasing degree of substitution at the C=C bond was found to weaken the adsorption strength of UK reactants and alter the energetics of hydrogenation steps by increasing the hydrogenation barrier at the substituted carbon, while lowering the barrier to hydrogenate at its nearby carbon. Increasing the degree of substitution notably decreases the activation barrier for the first hydrogenation step of UK thus increasing the likelihood of selectively hydrogenating the C=O bond over the C=C bond.
We have also examined the influence of substituents on the hydrogenation of KIP over Pd. KIP is a much more complex ketone as it is comprised of three reducible groups and as such offers a number of different products that can form. Increasing the degree of substitution on the C=C bond was again found to increase the hydrogenation barrier for C=C while inducing a decrease in barrier to hydrogenate the sterically hindered C=O bond. This is in agreement with the experimental results which show enhanced selectivity to 4-HIP.
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