Unexpected Leaving Ability of (Phenyltelluro)formates in the Presence of Internal Nucleophiles:  Complications during Alkyl and Oxyacyl Radical Generation in the Preparation of Sulfur- and Selenium-Containing Heterocycles

Irradiation of a benzene solution of 1-(benzylseleno)-3-undecyl (phenyltelluro)formate (13) with a 250-W low-pressure mercury lamp leads to the formation of 4-octyl-3-oxaselenan-2-one (18) in good yield. This transformation represents the first reported example of intramolecular homolytic substitution by oxyacyl radicals at selenium. The analogous reaction involving 1-(benzylseleno)-4-dodecyl (phenyltelluro)formate (14) provides a complex mixture of products, while thermolysis of a benzene solution of 14, 1-(benzylseleno)-5-tridecyl (phenyltelluro)formate (15) or 1-(benzylseleno)-6-tetradecyl (phenyltelluro)formate (16) at 160° affords 2-octyltetrahydroselenophene (21), 2-octylselenane (22), and 2-octylselenopane (23) in 43−94% yield, respectively. The formation of the saturated selenium-containing heterocycles 21−23 is most likely to involve nucleophilic attack by the benzylseleno moiety with (decarboxylative) expulsion of phenyltelluride. The mechanisms of these transformations are discussed.