Lewis Acidity of Carbon in Activated Carbonyl Group vs. B(C6F5)3 for Metal-Free Catalysis of Hydrogenation of Carbonyl Compounds

In this work, using DFT calculations, we investigated Lewis acidities of carbon (in activated carbonyl group) in comparison to the B(C₆F₅)₃ in combination with dioxane as the Lewis base (LB) for metal-free catalysis of heterolytic H₂ splitting and hydrogenation of carbonyl compounds. We found that in case of carbon as the Lewis acid (LA) the reaction is controlled by frontier molecular orbital interactions between the H₂ and LA-LB fragments at shorter distances. The steric effects can be reduced by electrophilic substitutions on the carbonyl carbon. Synergic combination between stronger orbital interactions and reduced steric effects can lower the barrier of the H₂ splitting below 10 kcal/mol. With the B(C₆F₅)₃, the H₂ splitting is controlled by electrostatic interactions, which cause to form an early transition state. An advantage of employing Lewis acidity of the activated carbonyl carbon for hydrogenation is that the hydride-type attack and hydrogenation of the C=O bond occur in a single step throughout H₂ splitting. Hence, stronger Lewis acidity of the C(C=O) reinforces hydrogenation without prohibition of the hydride delivery.