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

Abstract

Irradiation of 1-bromo-2-[(phenylmethyl)thio]benzene (1), 1-iodo-2-[(phenylmethyl)thio]benzene (2), and 1-iodo-4-[(phenylmethyl)thio]benzene (3), in DMSO as solvent in the presence of pinacolone enolate ion led to entirely different product distributions. Thus, irradiation of 1 afforded exclusively fragmentation of the C–S bond of the thiobenzyl moiety, yielding bibenzyl and 2-bromobenzenethiol, whereas irradiation of 2, under the same reaction conditions, 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 enolate ion, afforded p-iodobenzenethiol as the only product under controlled-irradiation 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 reactions from pinacolone enolate ion to the substrates. In this fashion, irradiation of 2 generates a radical anion which readily fragments to iodide ion and the respective aryl radical, which undergoes internal cyclization. Loss of 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 radicals and the respective sulfides, precluding the chain mechanism.

Product quantum yields were determined from irradiation of 1, 2, and 3 in the presence of pinacolone enolate ion, and are in accordance with the mechanisms proposed.

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