The mechanisms of the palladium-catalyzed hydrothiolation of aliphatic and aromatic alkynes with aliphatic and aromatic thiols have been investigated using density functional theory calculations done at the B3LYP/6-31G(d,p)(SDD for Pd)level of theory.Solvent effects on these reactions have been explored by calculations that included a polarizable continuum model(PCM)for the solvent(THF).Three pathways which lead to the formation of Markovnikov-type vinyl sulfides or cis-configured anti-Markovnikov-type products via the alkyne or the nucleophilic attack were performed.Our calculation results suggested the following.(1)The first step of the cycle is the proton-transfer process from thiols onto the palladium atom to form a palladium-thiolate intermediate through the three-membered ring transition structure.The palladium-thiolate species is attacked on alkynes and reductive elimination produce the observed products and liberate the catalyst.(2)The higher activation energies for the alkyne into the palladium-thiolate bond indicate that this step is the rate-determining step.The Markovnikov-type vinyl sulfide product of the attack of Pd atom on terminal carbon atom of aliphatic alkyne is favored.On the other hand,when the alkyne is substituted with an aromatic group,cis-configured anti-Markovnikov-type product of the attack onto the substituted carbon of the alkyne is favored.(3)The activation energy would reduce when the thiols are substituted with aromatic group.Our calculated results are consistent with the experimental observations of Frech et al.for the palladium-catalyzed hydrothiolation of alkynes to thiols.