A Prediction of Proton‐Catalyzed Hydrogenation of Ketones in Lewis Basic Solvent through Facile Splitting of Hydrogen Molecules

Abstract

A ketone's carbonyl carbon is electrophilic and harbors a part of the lowest unoccupied molecular orbital of the carbonyl group, resembling a Lewis acidic center; under the right circumstances it exhibits very useful chemical reactivity, although the natural electrophilicity of the ketone's carbonyl carbon is often not strong enough on its own to produce such reactivity. Quantum chemical calculations predict that a proton shared between a ketone and the Lewis basic solvent molecule (dioxane or THF) activates carbonyl carbon to the point of enabling a facile heterolytic splitting of H . Proton-catalyzed hydrogenation of a ketone in Lewis basic solvent is the result. The mechanism involves the interaction of H with the enhanced Lewis acidity of a carbonyl carbon and the free Lewis basic solvent molecule polarizes H and enables the hydride-type attack on carbonyl carbon, which is very strongly influenced by the proton shared between a ketone and solvent. The hydride-type attack on carbon is reminiscent of the splitting of H by singlet carbenes except that, in this case, a Lewis base from the surrounding environment (solvent) is necessary for polarization of H and acceptance of the proton resulting from the heterolytic splitting of H .

Carbonyl Activation by Borane Lewis Acid Complexation: Transition States of H2 Splitting at the Activated Carbonyl Carbon Atom in a Lewis Basic Solvent and the Proton‐Transfer Dynamics of the Boroalkoxide Intermediate

“…With the data presented in Table and the results from our recently published evaluation of Brønsted acid activation/catalysis at the electronic structure level, one can qualitatively compare the effect of Lewis acid complexation with the effect induced by a strong Brønsted acid, having a form of the solvent‐bound proton. According to the data presented in Table , the Lewis acid complexation gives approximately 2 eV stabilization to the MO energy of the LUMO(ketone).…”

“…According to the data presented in Table , the Lewis acid complexation gives approximately 2 eV stabilization to the MO energy of the LUMO(ketone). As recently reported by us, the LUMO(ketone) is stabilized by more than 6 eV in a situation when a proton is shared between a ketone and a Lewis basic solvent molecule (i.e., dioxane or THF) through a tight hydrogen bond …”

“…With all of the above‐given information adding up into a sizable rationale, in this article we primarily focus on understanding of the transition states of the solvent‐assisted heterolytic cleavage of H 2 at the carbonyl carbon atom as the enhanced Lewis acidic center in (C 6 F 5 ) 3 B–ketone adducts; (C 6 F 5 ) 3 B acts as “Lewis acid promoter”, as stipulated in reference . For the solvent‐assisted H 2 cleavage at a carbonyl carbon atom, we will compare the herein reported the effect of Lewis acid complexation to the recently reported effect of the Brønsted acid activation by the solvated proton. The significance of this article is the indication that the reaction pathway of reaction (1) does not necessarily involves the borohydride intermediate with the Brønsted‐activated (protonated) carbonyl compound—in principle, calculations indicate that the boron atom of (C 6 F 5 ) 3 B and the carbonyl carbon atom of the (C 6 F 5 ) 3 B–ketone adducts could be the competing Lewis acidic centers in the process of the solvent‐assisted heterolytic H 2 splitting in Lewis basic solvent.…”

Computational Elucidation of a Role That Brønsted Acidification of the Lewis Acid‐Bound Water Might Play in the Hydrogenation of Carbonyl Compounds with H2 in Lewis Basic Solvents

“…[14] Therefore, the barrierh eights reported in Ta bles 1a nd 2b enefita ppreciably from the Brønsted acidification of the LA-boundw ater. [14] Therefore, the barrierh eights reported in Ta bles 1a nd 2b enefita ppreciably from the Brønsted acidification of the LA-boundw ater.…”

“…[17] Motivated by all of the above and in the light of our recently published results, [14] in this Communication we computationally check the effect that the Brønsted acidification of the LAbound water might have on the mechanism of reaction( 1). Either ar elatively strong Brønsted acid or solvated protons can promote the electrophilic (Lewis acidic) character of the carbonyl carbon, C(carbonyl), and this can facilitate nucleophilic attack on the C(carbonyl) by even aw eak nucleophile (water, alcohols etc.).…”