Mechanism of the Manganese-Pincer-Catalyzed Acceptorless Dehydrogenative Coupling of Nitriles and Alcohols

A recent study showed that a Mn-pincer could catalyze the acceptorless dehydrogenative coupling of nitriles and alcohols to yield acrylonitriles. The reaction mechanism proposed in that work contained some intermediates that, in most of the cases, were not characterized. Moreover, one of the intermediates involved a charged separation, which is unlikely in apolar solvents. To clarify the reaction mechanism of this critical reaction, we decided to perform a DFT study. Our results prove the existence of a cooperative effect of the metal and the ligand in several steps of the catalytic cycle. We also find the presence of several equilibria between isomeric intermediates where water, or the same alcohol reagent, takes part in assisting the proton transfer. Furthermore, we have analyzed the charge-separated structure proposed experimentally and have found a nearly pure covalent bond between the two expected charged moieties. Finally, the Knoevenagel condensation step that generates the acrylonitrile is found to be the rate determining step.