ADDITION OF HETEROAROMATIC THIOLS TO ELECTRON-RICH ALKENES - A REVERSED HETERO ENE REACTION

Benzothiophenethiol la reacted with styrene to give thiol 2a, which re sulted from a ''reversed hetero ene reaction''. When the reaction was carried out in the presence of radical precursors, products deriving f rom radical addition to the styrene double bond of both thiols la and 2a were formed. The reaction of 1a with butyl vinyl ether (EVE) gave t he alpha,beta-unsaturated dithioester 23a, deriving from the ''ene rea ction product'' 22a through elimination of butanol. Similar behavior w as exhibited by benzofuranthiol Ib, which reacted with styrene to give thiol 2b and with EVE to give the thionoester 23b. Both dithio- and t hionoesters 23a,b were trapped as Diels-Alder adducts. In contrast, th iophenethiol Ic did not react with styrene and reacted with EVE to giv e an electrophilic 1:1 adduct. Ene reaction of thiols la,b occurred on ly with electron-rich olefins. In agreement, la did not react with hex -l-ene and methyl acrylate and reacted with dimethyl maleate to give a nucleophilic adduct.