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

Abstract

The nitrile hydration reaction mechanism catalyzed by a Mn I (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 (NMe 2 PhCN) and p-trifluoromethylbenzonitrile (CF 3 PhCN) 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 Mn I catalysts, a newly designed Mn(NCH 3 )-OH complex is investigated herein and proposed for the nitrile hydration reaction, interestingly showing an improved catalytic activity in comparison with the previous Mn I (NH)-OH complex.

Dehydrogenation of Formic Acid to CO2 and H2 by Manganese(I)–Complex: Theoretical Insights for Green and Sustainable Route

“…For this purpose, optimizations of 2 catalyst in different spin states (2S + 1 = 1, 3, 5) have been performed. The singlet state (2S + 1 = 1) resulted in being the most stable one in agreement with d 6 configuration of Mn(I) species, as analogously determined in a recent study concerning the Mn(I)-mediated catalysis of organic nitriles to amides [34], while the triplet and quintet lay at 12.1 kcal/mol and 18.7 kcal/mol, respectively, with respect to the singlet spin state.…”

“…In addition, the dispersions according to Grimme's scheme (D3-BJ) have been included in all the computations [42]. The strategy here presented has been successfully described and adopted in previous works on catalytic systems containing first row transition metals [34,[43][44][45][46][47]. Given the lack of information on the multiplicity of the catalytic system, preliminary investigation has been carried out considering different spin states, (2S + 1 = 1, 3, 5).…”

Reactivity of [(PNP)Mn(CO)2] with Organophosphates

“…Prior studies exploring nitrile hydration catalysis with 2 describe outer-sphere hydroxide as the lower-energy pathway for the nucleophilic attack on the nitrile as opposed to [Mn−OH]. 68 An outer-sphere mechanism could be expected to produce the Mn−phosphate product with a distribution of 17 O incorporation into the P�O double bond and Mn−O−P bridging atom (Figure S42), a product distribution which was not observed.…”

Recent Advances in Chemical Modifications of Nitriles

“…Well-known for its important applications in daily life, [14] amides, as a kind of important chemical raw materials, are suitable for producing lubricating oil, plastics, and detergent, etc. [15,16] Considering the vital application value of amides, general and efficient methods for the synthesis of amides have been developed from the hydration reaction of nitriles. In recent years, different catalysts including transition metal catalysts in homogeneous systems, metal nanoparticle catalysts and ionic liquid catalysts in heterogeneous systems have been designed to further enrich the synthetic approaches to amides.…”