Alkyl Radical Substitution on Disulfide in the Presence of Cobalt(II) Complexes.

Photolysis of methyl(L)cobaloxime (L = trans ligand) (1) or methyl(L)-O,O′-bis(difluoroboryl)cobaloxime (2) in the presence of bis(4-chlorophenyl) disulfide gave 4-chlorophenyl methyl sulfide together with 4-chlorophenylthio(L)cobaloxime (5) or 4-chlorophenylthio(L)-O,O′-bis(difluoroboryl)cobaloxime (6). Similar photolysis in the presence of 2,2,6,6-tetramethyl-1-piperidyloxyl (TEMPO) gives 1-methoxy-2,2,6,6-tetramethylpiperidine. The reactivity of the cobaloximes 1 and 2 to the disulfide and TEMPO increases when the cobaloximes have a trans ligand (L) of larger steric requirement and low pka, or when the equatorial ligand is an electronegative O-(difluoroboryl)dimethylglyoxime. Two probable mechanisms are proposed for the reaction. The first is a direct radical substitution on the disulfide with a methyl radical, and the second involves the initial interaction between the disulfide and the (L)cobaloxime radical to afford arylthio(L)cobaloxime (5 or 6) and an arylthio radical. The latter radical reacts wi...