INTRAMOLECULAR HOMOLYTIC SUBSTITUTION WITH AMIDYL RADICALS - A FREE-RADICAL SYNTHESIS OF EBSELEN AND RELATED ANALOGS

Irradiation of a water-cooled benzene solution of pyridine-2-thioneoxy carbonyl (PTOC) imidate esters 9 derived from N-butyl-2-(benzylseleno) benzamide (6, R = Bu), 2-(benzylseleno)-N-hexylbenzamide (6, R = Hex), N-benzyl-2-(benzylseleno)benzamide (6, R = Bn), and 2-(benzylseleno)- N-cyclohexyl-benzamide (6, R = c-Hex) with a 250-W low-pressure mercur y lamp affords the corresponding 1,2-benzisoselenazol-3(2H)-ones (1) i n yields of 81-91% (R = primary alkyl) and 45% (R = c-Hex). Presumably , these transformations involve formation of amidyl radicals 2 which u ndergo subsequent intramolecular homolytic substitution at the seleniu m atom with expulsion of a benzyl radical. PTOC imidate esters derived from 2-(benzylseleno)benzanilide (6, R = Ph) and 2-(benzylseleno)N-te rt-butylbenzamide (6, R = t-Bu) were unable to be prepared in this man ner. 1,2-Benzisoselenazol-3(2H)-ones (1, R = Ph, Hex, i-Pr, t-Bu) coul d also be prepared in 76-85% yield by reaction of the corresponding 2, 2'-diselenobis(benzamide) (15) with benzoyl or tert-butyl peroxide. Th e mechanisms of these transformations are discussed.