Kinetics and mechanistic study on deoxygenation of pyridine oxide catalyzed by {MeReVO(pdt)} 2 dimer

Abstract

The oxorhenium(V) dimer {MeReO(pdt)}2 (where pdt = 1,2-propanedithiolate) catalyze the oxygen atom transfer (OAT) reaction from the pyridine oxide to triphenylarsine (Ph3As). The rate law is given by ν = k[Re-dimer][PyNO] and zero order dependence on Ph3As. The value of k at 25 °C in CHCl3 is 139 ± 3 L mol−1 s−1. The activation parameters are ΔH‡ = 12.2 ± 1.0 kcal mol−1 and ΔS‡ = −7.9 ± 3.24 cal K−1 mol−1. According to the proposed mechanism, the rate determining step is the oxidation of ReVO to ReVIIO2 and the pyridine release. The triphenylarsine enters the catalytic cycle after the rate determining step. The reaction constant ρ = −1.4 obtained from Hammett correlation with σ for different substituted pyridine N-oxide. The computational study indicates that the oxidation of ReV to ReVII and release of the pyridine step is insensitive to the nature of the substituent on the pyridine with the average estimated activation barrier ≈11.5 kcal/mol from six different substituted pyridine oxide. It is proposed that electron donor substituent enrich the equilibrium of the first step of the proposed mechanism which is the coordination of the pyridine oxide with one rhenium atom to form I1 (Scheme 2). The electron donor substituent on the pyridine increase the concentration of I1 which will increase the rate of the reaction as the ν = k2[I1].