Sulfide organic polymers as novel and efficient metal-free heterogeneous Lewis acid catalysts for esterification reactions

Abstract

Herein, we report on the synthesis of four organosulfide-based covalent organic polymers prepared via click processes, consisting of either S_N2 (SOP-1 and SOP-3) or thiol–yne (SOP-2 and SOP-4) coupling reactions. Formation of the SOPs in high yields is confirmed by solid-state ¹³C NMR and FTIR spectroscopies, while the sulfur contents of SOP-type materials confirm the expected C : S molar ratio for the formation of stoichiometric products. CO₂ adsorption isotherms reveal the porosity of SOPs, with specific surface areas of up to 180 m² g⁻¹ and strongly dependent on the ligands used. The catalytic activity of SOPs is evaluated for carboxylic acid esterification reactions, obtaining high conversions and efficient recyclability. The proposed reaction mechanism consists of the activation of carboxylic acid by hypervalent S⋯O (n_O → σ_S*) interactions with sulfur centres of the SOPs, which increase the electrophilic character of the carboxylic carbon and facilitate the addition of the alcohol. Thus, SOPs constitute a novel class of metal-free heterogeneous Lewis acid (organo)catalysts.