Oxidation of sulfur compounds by oxo-Re(V) complexes containing salophen ligands through radical pathway

Abstract

Oxidation, which converts sulfides into sulfoxides, is a widely used method for producing synthetic intermediates in various industries such as chemical, medicinal chemistry, and biology. In this study, we synthesized six oxo-rhenium(V) complexes, ReOClL1-6, using salophen ligands, which are different groups on the N-aryl backbone and phenolate arms, to develop selective and stable oxidation catalysts. Their structures were confirmed using 1H-, 13C- NMR, MALDI-TOF MS, and FTIR. In particular, optimum conditions have been investigated to overcome the synthetic challenge of the direct and selective production of oxidation products such as sulfoxides and/or sulfones. This was achieved with a short reaction time, low catalyst loading, and excellent yields. ReOClL1-6 complexes are active homogeneous catalysts for the oxidation of sulfur compounds (withtBuOOH to sulfoxide). ReOClL4 complex demonstrated superior catalytic performance compared to other complexes, owing to the electronic properties of the ligand scaffold. A general mechanism was proposed for the oxorhenium-catalyzed oxidation reaction of sulfur compounds with tBuOOH as the oxidant. The mechanism includes (i) coordination of tBuOOH to the catalyst, (ii) a homolytictBuO–OH bond cleavage by oxo-Re(V) complexleads totBuO and OH radicals, (iii) coordination of OOtBu radicals to the oxo-Re center, formed by OtBu radicals transferring a peroxy H atom in the presence of excess tBuOOH and (iv) a second homolytic bond cleavage of tBuOOORe species by oxo-Re(V) complexleads totBuO and O2− radicals.