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.