“TYC” reaction between alkynes and catechol-thiol derivatives promoted by copper nanoparticles: Mechanism study by DFT calculation

In a recently published article by our research group, it was demonstrated that the hydrothiolation of activated alkynes (with electron-withdrawing groups), “TYC” reaction, is a successful way to functionalize thiols bearing catechols. The reaction was promoted by a heterogeneous catalyst composed of copper nanoparticles supported on TiO₂, showing regio- and stereoselective towards the anti-Markovnikov Z-vinyl sulfide in most of the cases studied, in good to excellent conversions (47-97%). However, no product was obtained when a deactivated alkyne such as propargylamine was tested, nor when FeNPs/TiO₂ was employed as catalyst.

Through DFT studies, the different chemical-interactions between the reagents, the metal and the support were modeled, employing propiolamide and CuNPs/TiO₂ as model alkyne and catalyst, respectively. Calculations were made using PBE functional with D3BJ dispersion correction, Def2-TZVP basis set and the CPCM model for the solvent (DCM). DFT studies and experimental data were consistent with a reaction mechanism based on a copper-catalyzed anti-Markovnikov hydrothiolation process leading to the formation of the Z-vinyl sulfide isomer. On the other hand, when FeNPs/TiO₂ was used as catalyst for the hydrothiolation reaction, no vinyl sulfide was obtained. Theoretical calculation revealed a decrease in charge distribution mainly at the internal Csp-atom of the acetylenic system. In contrast, for the CuNPs/TiO₂ the decrease in charge was mainly observed at the terminal Csp- atom, consequently leading to the anti-Markovnikov product. Besides, the system with propargylamine revealed that the formation of intermediates in the nucleophilic attack was thermodynamically unfavorable, unlike propiolamide, where the formation of intermediates was mainly exothermic.