Mechanistic studies of the tellurium(II)/tellurium(IV) redox cycle in thiol peroxidase-like reactions of diorganotellurides in methanol

Di-n-hexyl telluride (2), di-p-methoxyphenyl telluride (3), and (S)-2-(1-N,N-dimethylaminoethyl)phenyl phenyl telluride (4) catalyzed the oxidation of PhSH to PhSSPh with H₂O₂ in MeOH. Telluride 2 displayed greater rate acceleration than the diaryltellurides 3 and 4 as determined by the initial velocities, v₀, for the rate of appearance of PhSSPh determined at 305 nm by stopped-flow spectroscopy. Rate constants for the oxidation of tellurides 2-4 (k_ox), rate constants for the introduction of PhSH as a ligand to the Te(IV) center (k_PhsH) of oxidized tellurides 5-7, and thiol-independent (k₁) and thiol-dependent (k₂) rate constants for reductive elimination at Te(_IV) in oxidized tellurides 5-7 were determined using stopped-flow spectroscopy. Oxidation of the Te atom of the electron-rich dialkyl telluride 2 was more rapid than oxidation of diaryl tellurides 3 and 4. The dimethylaminoethyl substituent of 4, which acts as a chelating ligand to Te(IV), did not affect k_ox. Values of kphSH for the introduction of PhSH to oxidized dialkyl tellurane 5 and oxidized diaryl tellurane 6 were comparable in magnitude, while the chelating dimethylaminoethyl ligand of oxidized telluride 7 diminished k_PhSH by a fator of 10³. Reductive elimination by both first-order, thiol-independent (k₁) and second-order, thiol-dependent (k₂) pathways was slower from dialkyl Te(IV) species derived from 2 than from diaryl Te(IV) species derived from 3. The chelating dimethylaminoethyl ligand of Te(IV) species derived from 4 diminished k₁ by a factor of 50 and k₂ by a factor of 3 (relative to the 3-derived species).