ARTICLE
This journal isThe peroxometalate-based polymer immobilized ionic liquid phase catalyst [PO 4 {WO(O 2 ) 2 } 4 ]@PIILP has been prepared by an anion exchange of ring opening metathesisderived pyrrolidinium-decorated norbornene/cyclooctene copolymer and shown to be a remarkably efficient system for the selective oxidation of sulfides under mild conditions. A cartridge packed with a mixture of [PO 4 {WO(O 2 ) 2 } 4 ]@PIILP and silica operated as a segmented or continuous flow process and gave good conversions and high selectivity for either sulfoxide (92% in methanol at 96% conversion for a residence time of 4 min) or sulfone (96% in acetonitrile at 96% conversion for a residence time of 15 min). The immobilized catalyst remained active for 8 h under continuous flow operation with a stable activity/selectivity profile that allowed 6.5 g of reactant to be processed (TON = 46,428) while a single catalyst cartridge could be used for the consecutive oxidation of multiple substrates giving activity-selectivity profiles that matched those obtained with fresh catalyst .
IntroductionThe selective oxidation of sulfides is a challenging and technologically important process as sulfoxides and sulfones are versatile intermediates used in the synthesis of fine chemicals, bioactive compounds, agrochemicals, 1 as chiral auxiliaries 2 and most recently as ligands for transition metal asymmetric catalysis. 3 Moreover, sulfide oxidation is also the basis for the catalytic oxidative desulfurisation of crude oil in which sulfur compounds are removed by selective extraction of their sulfones into a polar solvent under much milder conditions than typically required for classical industrial catalytic hydrodesulfurization. 4 Sulfoxidations have traditionally employed either strong oxidants, (e.g. nitric acid or KMnO 4 ) which suffer numerous drawbacks such as low yields, extreme reaction conditions and poor E-factors, 5 or reagents such as mchloroperbenzoic acid, 6 UHP, 7 NaClO, 8 NaIO 4 , 9 oxone 10 and dimethyldioxirane 11 which are expensive, must be used in excess, generate stoichiometric amounts of by-product and involve protocols that require long reaction times, high temperatures and complicated handling procedures. Since hydrogen peroxide is economical, more environmentally benign and relatively atom efficient it is considered the oxidant of choice and as such numerous organocatalysts, 12 enzymes 13 as well as transition metal catalysts based on iron, 14 manganese, 15 vanadium, 16 titanium, 17 ruthenium, 18 molybdenum, 19 tungsten, 20 and zinc 21 have been developed for this process. In this regard, an efficient catalyst must be highly selective for either sulfone or sulfoxide, inexpensive, easy to prepare and handle, operate under mild conditions across a wide range of substrates and functionality, have long term stability and be easy to recover and recycle. While several of these criteria have been successfully realized there is still a demand to identify and develop alternative systems to address remaining drawbacks ...
