Distinct fragmentation patterns of the radical anions derived from 1-halo-2-and-4-(phenylmethylthio)benzenes

Irradiation of 1-bromo-2-[(phenylmethyl)thio]benzene (1), 1-iodo-2-[(phenyl methyl)thio]benzene (2), and 1-iodo-4-[(phenylmethyl)thio]benzene (3), in D MSO as solvent in the presence of pinacolone enolate ion led to entirely di fferent product distributions. Thus, irradiation of 1 afforded exclusively fragmentation of the C-S bond of the thiobenzyl moiety, yielding bibenzyl a nd 2-bromobenzenethiol, whereas irradiation of 2, under the same reaction c onditions, afforded the intramolecularly-cyclized product benzothiochromene (9), which arises from a C-I bond scission. Irradiation of 3, in DMSO as solvent and in the presence of pinacolone enol ate ion, afforded p-iodobenzenethiol as the only product under controlled-i rradiation conditions. The differences in product distributions upon irradiation of compounds 1, 2 , and 3 are ascribed to the different fragmentation rates of the C-X and S- benzyl bonds in the radical anion intermediates which arise from ET reactio ns from pinacolone enolate ion to the substrates. In this fashion, irradiat ion of 2 generates a radical anion which readily fragments to iodide ion an d the respective aryl radical, which undergoes internal cyclization. Loss o f a proton regenerates the radical anion of 9, which in turn transfers the odd electron back to a substrate molecule 2 to continue the chain process. Conversely, the radical anions derived from 1 and 3 fragment into benzyl ra dicals and the respective sulfides, precluding the chain mechanism. Product quantum yields were determined from irradiation of 1, 2, and 3 in t he presence of pinacolone enolate ion, and are in accordance with the mecha nisms proposed.