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

Benzothiophenethiol la reacted with styrene to give thiol 2a, which resulted from a ''reversed hetero ene reaction''. When the reaction was carried out in the presence of radical precursors, products deriving from 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 the alpha,beta-unsaturated dithioester 23a, deriving from the ''ene reaction product'' 22a through elimination of butanol. Similar behavior was exhibited by benzofuranthiol Ib, which reacted with styrene to give thiol 2b and with EVE to give the thionoester 23b. Both dithio- and thionoesters 23a,b were trapped as Diels-Alder adducts. In contrast, thiophenethiol Ic did not react with styrene and reacted with EVE to give an electrophilic 1:1 adduct. Ene reaction of thiols la,b occurred only 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.