Hydration of Aromatic Nitriles Catalyzed by Mn-OH Complexes: A Rationalization from Quantum Chemical Investigations

The nitrile hydration
reaction mechanism catalyzed by a MnI(NH)-OH complex has
been elucidated by means of DFT computations,
first exploring three mechanistic hypotheses experimentally suggested
in the literature and then proposing a novel catalytic cycle proceeding
along a combination of two pathways, suitable to describe the whole
process. The results show that the reaction proceeds through a nucleophilic
attack by the hydroxide to the Mn-coordinated nitrile substrate, leading
to amide formation, ultimately released as the iminol tautomer upon
a Mn–N to Mn–O switching step. A rapid tautomerization
to yield the amide product closes the cycle. An explanation of the
enhancement of the experimental turnover number (TON) with the electron-withdrawing
properties of the substituents was also provided by comparing the
free energy profiles obtained for the hydration of benzonitrile (PhCN), p-N-dimethylbenzonitrile (NMe2PhCN) and p-trifluoromethylbenzonitrile (CF3PhCN) substrates, which nicely combines experimental and theoretical
results. In addition, to introduce a further advance in this novel
field and to gain useful insights for the design of more efficient
MnI catalysts, a newly designed Mn­(NCH3)-OH
complex is investigated herein and proposed for the nitrile hydration
reaction, interestingly showing an improved catalytic activity in
comparison with the previous MnI(NH)-OH complex.