Os(VIII)-catalyzed and uncatalyzed oxidation of biotin by chloramine-T in alkaline medium: Comparative mechanistic aspects and kinetic modeling

Sodium N-chloro-p-toluenesulfonamide (chloramine-T or CAT) is the prominent member of aromatic sulfonyl haloamines and has received considerable attention as a mild oxidant for several organic compounds. D-Biotin, a member of the B-vitamin family, is an essential nutrient in human nutrition as is commonly referred to as Vitamin H. Controlled oxidation of biotin to biotin sulfoxide forms a very important synthetic route in biochemical reactions. Optimum conditions have been developed for the oxidation of biotin to biotin sulfoxide. Literature survey has revealed that no attention has been paid towards the controlled oxidation of biotin to biotin sulfoxide by N-haloamines from the kinetic and mechanistic view points. This prompted us to undertake the title investigation. The kinetics of Os(VIII) catalyzed and uncatalyzed oxidation of biotin by CAT have been studied in NaOH medium at 303 K under identical experimental conditions. The stoichiometry (1: 1) and the oxidation product (biotin sulfoxide) are the same for Os(VIII) catalyzed and uncatalyzed reactions. Biotin sulfoxide was confirmed by GC-MS analysis. In Os(VIII) catalyzed oxidation, the rate law is -d[CAT]/dt=k[CATJ[Os(VIII)]/[NaOH] but it takes the form -d[CAT]/dt = k[CATJ[Biotin](x)/[NaOH](-y) for uncatalyzed reaction, where x and y are less than unity. The reaction was subjected to changes in: (a) concentration of added reduction product of CAT, p-toluenesulfonamide, (b) ionic strength, (c) dielectric permittivity and (d) halide ions effect in both the cases. Proton inventory studies made in a mixture of H2O-D2O indicated the participation of OH- ion in the formation of transition state. The reaction fails to initiate polymerization of acrylonitrile. Activation parameters for the composite reaction were deduced from Arrhenius plots. In case of uncatalyzed reactions, the reaction constants involved in the proposed scheme were deduced. Under the identical set of experimental conditions, the kinetics of Os(VIII) catalyzed oxidation of biotin by CAT in alkaline medium has been compared with uncatalyzed reactions, revealing that the catalyzed reactions are eight fold faster than the uncatalyzed reactions. Hence, Os(VIII) acts as an efficient catalyst for the oxidation of biotin by CAT in alkaline medium. The catalytic constant (K-C) has been calculated at different temperatures and the values of activation parameters with respect to catalyst have been evaluated from the plots of log K-C versus I IT. Some spectroscopic evidence for the formation of 1: 1 complex between oxidant and Os(VIII) has been obtained. CH3C6H4SO2NHCl has been postulated as the reactive oxidizing species in both the cases. The observed experimental results have been explained by plausible mechanisms and the related rate laws have been deduced for both catalyzed and uncatalyzed reactions. (c) 2006 Elsevier B.V. All rights reserved.