THIOL PEROXIDASE-ACTIVITY OF DIORGANYL TELLURIDES

A number of diorganyl tellurides, including diaryl tellurides, diheteroaryl tellurides, and alkyl aryl and dialkyl tellurides, were found to catalyze the reaction of hydrogen peroxide with thiols. The thiol peroxidase- activity of the compounds was assessed by using a H-1 NMR method previously developed in our laboratories. In this assay, thiols (N-acetylcysteine, tert-butyl mercaptan, and 1-octyl mercaptan) were oxidized in the presence of hydrogen peroxide and catalyst (0.3 mol %) and the time required to reduce the thiol concentration with 50 %, t(50), determined, In a series of 4,4'-disubstituted (R = H,Me, OH, OMe, NH2, NMe(2), NHPh, CF3) diphenyl tellurides 5, the catalytic activity increased when mesomerically electron-donating substituents were present. Attempts to correlate the catalytic efficiency, expressed as log t(50)(-1), With Hammett sigma p(+)-values were successful in the 1-octyl mercaptan (r = 0.97; n = 8) and tert-butyl mercaptan (r = 0.92; n = 8) systems. In order to study the effect of coordinating, basic, acidic, or neutral substituents on catalyst efficiency, a series of 2,2'-disubstituted (R = CH2OH, CH(2)NMe(2), COOH, COOMe, OH, OMe, NH2) diphenyl tellurides: 6 were prepared and evaluated in the three thiol systems. As compared with their 4,4'-disubstituted counterparts, the 2,2'-disubstituted compounds were generally less active. The poor catalytic activity of bis(2,6-dimethylphenyl) telluride indicates the importance of steric factors. A two-step mechanism, involving H2O2-oxidation of the diorganyl telluride to a tellurium(IV) dihydroxide and reduction by thiol with disulfide formation, is proposed to account for the observed catalysis. The similar tao values obtained in the tert-butyl mercaptan and 1-octyl mercaptan systems seem to indicate that oxidation is rate-determining in the catalytic process. This view was also corroborated by the structure/ activity results obtained with the 4,4'-disubstituted diphenyl tellurides 5, 1H NMR experiments and results obtained using spoorer oxidant (tert-butyl hydroperoxide) in the standard 1-octyl mercaptan assay.