Homolytic Substitution at Selenium: Ring Closure of ω-(Benzylseleno)alkyl Radicals

Article abstract of DOI:10.1016/S0040-4020(01)86334-1

The ring closure of a series of ω-(benzylseleno)alkyl radicals (1) has been studied.Thiohydroxamic esters derived from ω-(benzylseleno)alkanoic acids decompose smoothly, upon irradiation, with the loss of carbon dioxide to afford 5- and 6- membered selenium-containing rings in 78-95 percent yield.The thiohydroxamic ester derived from 7-(benzylseleno)heptanoic acid affords the 7-membered heterocycle, selenopane in approximately 50 percent yield.These reactions presumably involve intramolecular free radical homolytic substitution at selenium and appear to proceed readily for both primary and secondary carbon-centered radicals.The 5-(benzylseleno)hex-2-yl radical (1f) appears to ring close without stereoselectivity, to give a 1:1 mixture of cis- and trans-2,4-dimethyltetrahydroselenophene, a finding in keeping with molecular mechanics (MM2) calculations. Key Words: Homolytic Substitution; Selenium; Radical Ring Closure; Stereoselectivity; Thiohydroxamic Ester