Acceleration and Retardation by Mercaptans of Photoreduction by Amines

Abstract

AbstractAromatic mercaptans and disulfides retard photoreduction of benzophenone by amines and are equilibrated to ∼62% thiyl as mercaptan in the process. Racemization of sec‐butylamine and introduction of deuterium into triethylamine indicate that ∼55% of the retardation arises from a hydrogen‐transfer repair mechanism, after correction for light absorption and quenching. The limiting step in the cyclic repair mechanism is reduction by mercaptan of the amine derived radical k10, in competition with oxidation of it by ground state ketone, k9. Ratios of rate constants for these reactions are, with 2‐mercaptomesitylene as retarder, for sec‐butylamine, k10a/k9a = 30 and for triethylamine, k10b/k9b = 310. The latter high value is attributed to low k9b, slow oxidation of this amine‐derived radical by ketone. The similar high value in reduction by 2‐propanol is attributed to rapid reduction of alcohol‐derived radical by the mercaptan. Aliphatic mercaptan, 0.02 M, accelerates photoreduction of 0.4 M benzophenone by 2 M sec‐butylamine in benzene by 50%, Φ = 1.82, and is not oxidized or equilibrated in the process. Acceleration is less efficient in acetonitrile and tert‐butyl alcohol and is not observed in water. Rate of the less efficient reduction by secondary amine is more than doubled by mercaptan. Inefficient retardation is observed in photoreduction by triethylamine in acetonitrile and in all cases at high concentration of aliphatic mercaptan. In retardation by n‐pentyl mercaptan, k10c/k9a = 0.24 in reduction by sec‐butylamine, and k10d/k9b = 0.88 in reduction by triethylamine. Ratios of reactivity toward aromatic and aliphatic mercaptans are k10a/k10c = 125 with sec‐butylamine, and k10b/k10d = 350 with triethylamine, both much higher than with 2‐propanol, suggesting lower reactivity of α‐aminoalkyl than α‐hydroxyalkyl radical toward reduction by mercaptan. Acceleration by aliphatic mercaptan arises from decrease in the quenching component normal in photoreduction by amines, and is attributed to catalysis by mercaptan of proton transfer from amine‐derived radical‐cation to ketyl radical‐anion in a ternary complex. Similar ternary complexes with aromatic mercaptan or thioether may lead to increased quenching and to retardation not accounted for by hydrogen‐transfer repair.