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

Herein, we report on the synthesis of four organosulfide-based covalent organic polymers prepared via click processes, consisting of either S(N)2 (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 C-13 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 m(2) 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 SMIDLINE HORIZONTAL ELLIPSISO (n(O) -> sigma(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.

Automated summary

In this study, four organosulfide-based covalent organic polymers (SOPs) were synthesized via click reactions, using S(N)2 or thiol-yne coupling reactions. The formation of SOPs in high yields was confirmed by solid-state C-13 NMR and FTIR spectroscopies, and the molar ratio of C : S in SOP-type materials confirmed the stoichiometric products. The SOPs exhibited high porosity, with specific surface areas up to 180 m(2) g(-1), which strongly depended on the ligands used. Additionally, the SOPs showed excellent catalytic activity in carboxylic acid esterification reactions, with high conversions and efficient recyclability. The proposed reaction mechanism involved the activation of carboxylic acids by hypervalent SMIDLINE HORIZONTAL ELLIPSISO (n(O) -> sigma(S)*) interactions with sulfur centers of the SOPs, enhancing the electrophilic character of the carboxylic carbon and facilitating alcohol addition. Therefore, SOPs represent a new class of metal-free heterogeneous Lewis acid (organo)catalysts.