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-RCHCHSAr) 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 (RCHCH−Rh−SAr). In contrast, PdCl2(PhCN)2-catalyzed hydrothiolation of aromatic alkynes (ArC⋮CH) takes place to give the corresponding Markovnikov adducts (R(ArS)CCH2) 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‘CHC(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.