Efficient and selective oxidation of sulfides in batch and continuous flow using styrene-based polymer immobilised ionic liquid phase supported peroxotungstates

Abstract

Styrene-based peroxotungstate-modified polymer immobilized ionic liquid phase catalysts [PO₄{WO(O₂)₂}₄]@ImPIILP (Im = imidazolium) are remarkably efficient systems for the selective oxidation of sulfides under mild conditions both in batch and as a segmented or continuous flow process using ethanol as the solvent and mobile phase, respectively. The performance of these styrene-based systems has been compared against their ring opening metathesis polymerisation derived counterparts to assess their relative merits. A comparative survey revealed the catalyst supported on N-benzyl imidazolium decorated polymer immobilised ionic liquid to be the most efficient and a cartridge packed with a mixture of [PO₄{WO(O₂)₂}₄]@ImPIILP and silica operated as a segmented or continuous flow system giving good conversions and high selectivity for sulfoxide. The immobilised catalyst remained highly active for the sulfoxidation of thioanisole in ethanol with a stable conversion–selectivity profile for up to 8 h under continuous flow operation; for comparison conversions with a mixture of [NBu₄]₃[PO₄{WO(O₂)₂}₄] and silica dropped dramatically after only 15 min as a result of rapid leaching while [PO₄{WO(O₂)₂}₄]@ImPIILP prepared from commercially available Merrifield resin also gave consistently lower conversions; these benchmark comparisons serve to underpin the potential benefits of preparing the polymer immobilized ionic liquid supports.