Hammett correlations in the photosensitized oxidation of 4-substituted thioanisoles

Singlet oxygen is quenched by a series of 4-substituted thioanisoles (methoxy to nitro), with rate constant k(t) = 7 x 10(4) to 7 x 10(6) M-1 s(-1), close to the value observed for the myoglobin-catalyzed sulfoxidation of the same sulfides. Correlations with sigma (rho = -1.97) and with E-ox (slope -3.9 V-1) are evidence for an electrophilic mechanism. In methanol sulfoxides are formed (85%) via an intermediate quenched by diphenyl sulfoxide; competing minor paths lead to arylthiols, arylsulfenic acid, and aryl sulfoxides. In aprotic solvents, the sulfoxidation is quite sluggish, but carboxylic acids (mostly less than or equal to 0.1 M) enhance the rate by a factor of > 100. The protonated persulfoxide is formed in this case and acts as an electrophile with sulfides, again with a rate constant correlating with sigma (rho = -1.78).