HOMOLYTIC SUBSTITUTION AT SELENIUM - RING-CLOSURE OF OMEGA-(BENZYLSELENO)ALKYL RADICALS

The ring closure of a series of omega-(benzylseleno)alkyl radicals (1) has been studied. Thiohydroxamic esters derived from omega-(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% yield. The thiohydroxamic ester derived from 7-(benzylseleno)heptanoic acid affords the 7-membered heterocycle, selenopane in approximately 50% yield. These reactions presumably involve intramolecular free radical homolytic substitution at selenium and appear to proceed readily for both primary and secondary carbon-centred 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.