FREE-RADICAL HOMOLYTIC SUBSTITUTION AT SELENIUM - AN EFFICIENT METHOD FOR THE PREPARATION OF SELENOPHENES

Substituted and unsubstituted 1-(benzylseleno)-4-iodobut-3-en-2-ols 12 and 2-(benzylseleno)-1-(2-iodophenyl)ethanols 18 react smoothly with tris(trimethylsilyl)silane in benzene at 80-degrees-C (AIBN initiator) to afford selenophenes 16 and benzoselenophenes 21 in excellent yield. These reactions presumably involve intramolecular homolytic substitution by aryl and vinyl radicals 14 and 20 at the selenium atom with the expulsion of benzyl radical followed by facile dehydration to afford the aromatic selenophene ring system in each case. Competitive rate studies on the ring closure of the 2-[2-(benzylseleno)ethyl]phenyl radical 25 in the presence of tri-n-butyltin hydride to give 2,3-dihydrobenzo[b]selenophene (27) and 1-(benzylseleno)-2-phenylethane (28) provide a rate constant for ring closure (k(c)) of approximately 3 X 10(7) s-1 at 80-degrees-C. The determination of more accurate data is hampered by what we attribute to be the involvement of a slow, but competive nonradical process.