Reaction of Hydroxysulfuranyl Radical with Molecular Oxygen: Electron Transfer vs. Addition

The reaction of the hydroxyl radical with 2-(methylthio)methyl acetate (2-MTMA) leads to the formation of a hydroxysulfuranyl radical >S[sup [center dot]][minus]OH which exists in a protonation/deprotonation equilibrium with pK[sub a] = 10.85. Deprotonation most probably occurs via the equilibrium >S[sup [center dot]][minus]OH [rightleftharpoons] H[sup +] + >S[sup [center dot]][minus]O[sup [minus]] but may be followed by an intramolecular proton transfer from the C[sub 2]-carbon of 2-MTMA. The hydroxysulfuranyl radical reacts with molecular oxygen with k[sub 8] = (1.1 [+-] 0.2) x 10[sup 8] M[sup [minus]1] s[sup [minus]1] whereas the deprotonated species reacts with k[sub 24] = (5.7 [+-] 1.0) x 10[sup 8] M[sup [minus]1] s[sup [minus]1]. Mechanistically, the reaction of the nondeprotonated species with oxygen involves the intermediary formation of an oxygen adduct as concluded from time-resolved conductivity experiments and attempts to scavenge reducing intermediates with tetranitromethane. The sulfur-oxygen adduct is proposed to be a peroxyl radical bound to a tetravalent sulfur, as in the general structure R[sub 2](OH)SOO, which may exist in equilibrium with a pentavalent sulfur-[open quotes]side on[close quotes] oxygen complex. R[sub 2](OH)[sup [center dot]]S-O-O. 46 refs., 7 figs.