An efficient platinum-catalyzed oxidation process and mechanism for the facile conversion of benzoxazoles to aminophenols

Abstract An efficient platinum (IV)-catalyzed oxidation process has been developed for the facile conversion of benzoxazoles to aminophenols in good to excellent yields using sodium salt of N-haloarenesulfonamides (N-haloamines) at an alkaline pH. The general process involves the selective and preferential oxidation of a five membered oxazole ring of benzoxazoles using N-haloamines without affecting six membered ring. The detailed catalysis, mechanistic and kinetic investigations have been made for the oxidation reactions. Under similar experimental conditions, the reactions with different N-haloamines proceed with a common oxidation mechanism and follow an identical kinetics. As a result, the common oxidation mechanism which operates in all the reactions has been proposed and the related identical kinetic model was designed. To understand the detailed kinetics and mechanism of the reactions, the reactions have been subjected to changes in (i) dielectric permittivity, (ii) primary salt effect, (iii) effect of reduction products of N-haloamines and (IV) halide ion concentrations. Solvent isotope studies made in a mixture of H 2 O–D 2 O indicate the participation of OH − ion in the formation of transition states. The reactions have also been carried out in the absence of platinum catalysts and the studies imply that the catalyst accelerates the reaction rates with 10–12-fold faster. The reactions were carried out at different temperatures and the activation parameters have been computed for both catalyzed and uncatalyzed reactions. The calculated isokinetic temperature ( β ) of 370 K obtained from enthalpy–entropy relationships and Exner correlations (365 K) was much higher than the experimental temperature of 313 K, and thereby it was concluded that the reactions were preceded under enthalpy control. The catalytic constants ( K C ) were calculated for each N-haloamine at different temperatures and the corresponding activation parameters were deduced. Spectroscopic studies have been made for an intermediate complex formation between N-haloamine and Pt(IV). The catalytic method developed for the oxidation process was found to be very efficient and the involvement of cost-effective reagents makes the reaction simple and convenient for scaling the method for the industrial/technological operations with suitable modifications.