FREE-RADICAL HOMOLYTIC SUBSTITUTION AT SELENIUM - AN EFFICIENT METHODFOR 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 substitu tion by aryl and vinyl radicals 14 and 20 at the selenium atom with th e expulsion of benzyl radical followed by facile dehydration to afford the aromatic selenophene ring system in each case. Competitive rate s tudies on the ring closure of the 2-[2-(benzylseleno)ethyl]phenyl radi cal 25 in the presence of tri-n-butyltin hydride to give 2,3-dihydrobe nzo[b]selenophene (27) and 1-(benzylseleno)-2-phenylethane (28) provid e 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 hampere d by what we attribute to be the involvement of a slow, but competive nonradical process.