Highly regio- and stereocontrolled synthesis of vinyl sulfides via transition-metal-catalyzed hydrothiolation of alkynes with thiols

Regio- and stereoselectivity in the hydrothiolation of alkynes with thiols in the presence of a variety of transition-metal catalysts is investigated in detail. Among the catalysts employed, RhCl(PPh3)(3) exhibits excellent catalytic ability toward the anti-Markovnikov addition of thiols (ArSH) to alkynes (RC=CH), which affords the corresponding vinylic sulfides (trans-RCH=CHSAr) regio- and stereoselectively. The reaction may proceed by the formation of hydrorhodium sulfide species (H-[Rh]-SAr) and probably via the subsequent hydrorhodation of alkynes to provide vinylrhodium intermediates (RCH=CH-[Rh] -SAr). In contrast, PdCl2(PhCN)(2)(-)catalyzed hydrothiolation of aromatic alkynes (ArC=CH) takes place to give the corresponding Markovnikov adducts (R(ArS)C=CH2) with excellent regioselectivity, probably via thiopalladation of alkynes by palladium sulfide species (ArS-[Pd]-Cl), which may be formed by ligand-exchange reaction between PdCl2(PhCN)(2) and ArSH. Furthermore, in the case of alkynes bearing propargylic protons (R'CH2C=CH), a sequential addition/isomerization reaction occurs to provide the internal vinylic sulfides (R'CH=C(SAr)CH3) regioselectively. From the same starting materials (alkyne and thiol), therefore, the regioselectivity of hydrothiolation can be attained simply by changing the catalysts, i.e., RhCl(PPh3)(3) and PdCl2(PhCN)(2).