Mechanism of the palladium-catalyzed hydrothiolation of alkynes to thioethers: a DFT study

The mechanisms of the palladium-catalyzed hydrothiolation of alkynes with thiols were investigated using density functional theory at the B3LYP/6-31G(d, p) (SDD for Pd) level. Solvent effects on these reactions were explored using the polarizable continuum model (PCM) for the solvent tetrahydrofuran (THF). Markovnikov-type vinyl sulfides or cis-configured anti-Markovnikov-type products were formed by three possible pathways. Our calculation results suggested the following: (1) the first step of the cycle is a proton-transfer process from thiols onto the palladium atom to form a palladium-thiolate intermediate. The palladium-thiolate species is attacked on alkynes to obtain an elimination product, liberating 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 is favored. However, for the aromatic alkyne, the cis-configured anti-Markovnikov-type product is favored. (3) The activation energy would reduce when thiols are substituted with an aromatic group. Our calculated results are consistent with the experimental observations of Frech and colleagues for the palladium-catalyzed hydrothiolation of alkynes to thiols.