Carbonyl Activation by Borane Lewis Acid Complexation: Transition States of H Splitting at the Activated Carbonyl Carbon Atom in a Lewis Basic Solvent and the Proton-Transfer Dynamics of the Boroalkoxide Intermediate.

By using transition-state (TS) calculations, we examined how Lewis acid (LA) complexation activates carbonyl compounds in the context of hydrogenation of carbonyl compounds by H in Lewis basic (ethereal) solvents containing borane LAs of the type (C F ) B. According to our calculations, LA complexation does not activate a ketone sufficiently enough for the direct addition of H to the O=C unsaturated bond; but, calculations indicate a possibly facile heterolytic cleavage of H at the activated and thus sufficiently Lewis acidic carbonyl carbon atom with the assistance of the Lewis basic solvent (i.e., 1,4-dioxane or THF). For the solvent-assisted H splitting at the carbonyl carbon atom of (C F ) B adducts with different ketones, a number of TSs are computed and the obtained results are related to insights from experiment. By using the Born-Oppenheimer molecular dynamics with the DFT for electronic structure calculations, the evolution of the (C F ) B-alkoxide ionic intermediate and the proton transfer to the alkoxide oxygen atom were investigated. The results indicate a plausible hydrogenation mechanism with a LA, that is, (C F ) B, as a catalyst, namely, 1) the step of H cleavage that involves a Lewis basic solvent molecule plus the carbonyl carbon atom of thermodynamically stable and experimentally identifiable (C F ) B-ketone adducts in which (C F ) B is the "Lewis acid promoter", 2) the transfer of the solvent-bound proton to the oxygen atom of the (C F ) B-alkoxide intermediate giving the (C F ) B-alcohol adduct, and 3) the S 2-style displacement of the alcohol by a ketone or a Lewis basic solvent molecule.