Conversion of imidazoles to imidazolones with chloramine-B: kinetic and mechanistic study

Abstract

Oxidative conversion of 1H-imidazole (IzlH) and 2-substituted imidazoles viz., 2-ethyl-1H-imidazole (2-EtIzlH), 2-methyl-1H-imidazole (2-MeIzlH), 1H-imidazole-2-ester (2-EsIzlH), and 1H-imidazole-2-carbaldehyde (2-CaIzlH) to the corresponding imidazolones with sodium N-chlorobenzenesulfonamide or chloramine-B (CAB) were established. This reaction was kinetically investigated at 303 K in HClO₄ medium. Under comparable experimental conditions, the oxidation reactions followed identical kinetics for all the five imidazoles with a first-order dependence of rate on [CAB]_o and fractional-order dependence each on [imidazole]_o and [H⁺]. Effects of added benzenesulfonamide, halide ions, NaClO₄, and MeOH on the rate of reaction have been studied. Reaction products were identified and characterized by GC–MS and NMR spectral analysis. The rates are measured at different temperatures and the composite activation parameters have been computed from the Arrhenius plots. Isokinetic temperature (β) was found to be 449 K which is higher than the experimental temperature (303 K), indicating that the rate has been under enthalpy control. Relative reactivity of these imidazoles is in the order: 2-EtIzlH > 2-MeIzlH > IzlH > 2-EsIzlH > 2-CaIzlH. H₂O⁺Cl have been postulated as the reactive oxidizing species. The reaction mechanism and the derived rate law are in good agreement with the observed experimental results. Spectroscopic studies have been made for an intermediate complex formation between CAB and imidazole. Advantages of the present method include: mild reaction conditions, cost-effectiveness, short reaction time, ease of isolation of products, and the use of eco-friendly reagents. Hence, the methodology developed could be adopted for the facile oxidative conversion of imidazoles to imidazolones and can be scaled up to industrial operation with suitable modification.

Graphical abstract