Mechanistic study on MeReVO(edt)Im catalysis for the oxygen atom transfer from pyridine oxide to Ph3As: Kinetics and computational study

Abstract

The oxorhenium(V) monomer (CH3Re(V)O(edt)(Im) (where edt = 1,2 ethanedithiolate, Im= Imidazole) catalyzes the oxygen atom transfer reaction from pyridine N-oxide and substituted pyridine N-oxide to triphenylarsine (Ph3As). The kinetic results have shown that the reaction rate is first order with respect to the catalyst and pyridine N-oxide concentrations, and first order with respect to [Ph3As] at low concentration and zero order at high [Ph3As]. Also, the reaction is zero order with respect to the imidazole concentration. Therefore, rate law is R = k [Re-Im] [PyNO][Ph3As] at low Ph3As concentration and the rate law is R = k [Re-Im] [PyNO] at high Ph3As concentration. The electronic effect of the oxidant was investigated by studying the reaction rate constants of different substituted pyridine NOxide. The correlation of the rate constant values with σ is linear with reaction constant ρ = -2.4. A negative value indicates that the rate increases with electron-donating substituents. The effect of electron donor substituent on the reaction rate is mainly due to the enhancement of the binding of pyridine N-oxide to the ReVO in the first step of the proposed mechanism. The oxidation of ReVO to ReVIIO2 and release of the pyridine step is not highly sensitive to the nature of the substituent on the pyridine. The computational study indicates that the oxidation of ReVO to ReVIIO2 and release of the pyridine step is insensitive to the nature of the substituent on the pyridine with the average estimated activation barrier ≈ 9 kcal/mol and its less than the estimated activation barrier in the absence of imidazole (≈ 11.5 kcal/mol). This result support that the imidazole act as nucleophile to assist the oxidation of ReVO to ReVIIO2 step.