Oxidation of Thioanisole

Reprinted from reference [7], with permission from Elsevier. 

A solution of benzyl-4,6-0-isopropylidene-a-D-glucopyranoside, 2-deoxy-2-[[(2-hydroxy-3,5-di-tert-butylphenyl)methylene]imine] (0.03 mmol) in dichloromethane (1.5 mL) was added to a solution of [VO(acac)2] (0.02 mmol) in the same solvent (1.5 mL). After 10 min of stirring, thioanisole (Immol) was added followed by hydrogen peroxide (1.1 mmol). After 1 h, the addition of a saturated solution of sodium sulfite quenched the reaction. The organic phase was filtered through a bed of Celite contained in a Pasteur pipette, dried over sodium sulfate and evaporated under vacuum to afford the reaction products (Table 9.6). 

This paper validates the assumption that ligands for asymmetric catalysis can be obtained by simple functionalization of common carbohydrates. Condensation of 2-aminoglucose with substituted-2-hydroxybenzaldehydes affords 0,N,0 -tridentate ligands whose activity in the V-catalyzed asymmetric oxidation of thioanisole is reported in Table 9.6. 

Transition Metals for Organic Synthesis, Beller, M. and Bolm, C. (Eds) Wiley-VCH Weinheim, 1998. 

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Figure 7.32 Biocatalytic oxidation of thioanisole to (5)-methy 1-phenyl-sulfoxide...

Pezzotti, F. and Therisod, M. (2007) Enantioselective oxidation of thioanisole with an alcohol oxidase/ peroxidase bienzymatic system. Tetrahedron Asymmetry, 18 (6), 701-704. 

Perfiuoroolefins, 0-substituted, from a-trifluoromethyl ketones, 48,119 Periodate oxidation of thioanisole, 46, 78... 

Ionic liquids can be used as co-solvents for CPO-catalysed sulfoxidation. Table 11.1 gives details about different ionic liquids. The procedure is very easy to reproduce and the oxidation of thioanisole proceeds with high chemo- and stereo-selectivity. 

Table 9.6 Asymmetric oxidation of thioanisole using some iminosugar complexes as catalysts...

They showed that despite the fact that lower activities were generally observed, significant improvements of enantioselectivity in the oxidation of thioanisole by PAMO and EtaA could be induced by the addition of short-chain alcohols such as methanol and ethanol. Remarkably, methanol was able to cause a reversal of PAMO enantiopreference in the case of several substrates. Reversal of enantio-preference was also observed with EtaA when using t-BuOMe. The authors hypothesize that in these enzymes solvents exert their influence on enantioselectivity by binding in or near the enzyme active site and, depending on their structure, interfere with the association of the substrate. 

The Bolm protocol was recently used by Ellman et al. for the enantioselective oxidation of -butyl disulfide 22 [72], Excellent result was achieved in the formation of thiosulfinate 22 (91% ee, 93% yield) by using catalyst 20 (0.25 mol %) in a 0,5 mmol scale. In spite of extensive screening of chiral Schiff bases related to catalyst 20, better enantioselectivity was not realized. Chiral thiosulfinate 22 is a convenient starting material for the preparation of r-butyl sulfi-namides and t-butyl sulfoxides. Vetter and Berkessel modified the structure of the Schiff base moiety of catalyst 20 by replacing the aryl ring with a 1,l -binaphthyl system [73]. The corresponding vanadium catalyst realized 78% ee in the oxidation of thioanisol, which was better than that attained by the Bolm catalyst (59% ee). 

Figure 3.103 Oxidation of thioanisole with cyclohexanone monooxygenase in the presence of chloroperoxidase.

Sulfur-centered radical cations derived from aromatic thioethers (Ar-S-Ar) have been investigated much less extensively. An important feature of one-electron oxidation of aromatic thioethers is the lack of dimeric radical sulfur radical cations (ArS.. S-Ar) because of the spin delocalization onto the aromatic ring. Oxidation of thioanisole (Ar-S-CHj) by OH radicals was studied using pulse radiolysis. At neutral pH, OH addition led to the prompt formation of monomeric sulfur radical cations and hydroxycyclohexadienyl radicals (see Scheme 8). 

The redox reactivity of divanadium salen615 and other complexes555 have been investigated. Disproportionation of [ V(salen) 2(//-0)] was observed under electrochemical conditions.6 The reduced, [Vin(salen)]+ complex was found to be the essential species in the catalysis of the electroreduction of 02 by four electrons in CH2C12.616 While [ V(salcn) 2(/x-0)] was proposed to be the active species in the redox reaction, more recently [Vin(salen)]+ was identified as a reservoir from which the active species forms.616 Schiff base complexes encapsulated in zeolite Y are catalytically active in the oxidation of thioanisole with H202.14 Acid-promoted disproportionation of a Vlv phenolate under anaerobic conditions was proposed as a model reaction for the vanadium uptake in tunicates.139... 

Sulfides — sulfoxides [1, 810J. The oxidation of thioanisole to methyl phenyl sulfoxide by the method of Leonard and Johnson5 has been described in detail.58 The procedure has been applied to the preparation of ten other sulfoxides with yields... 

The possible intermediacy of sulfur radical cations on enzymatic oxidation of phenylthioethers has attracted considerable interest. A number of enzymes are known to catalyze the oxidation of sulfides to sulfoxides [204]. Such oxidations may proceed by a one-step, two-electron oxidation concomitant with atom transfer or by initial one-electron transfer forming an intermediary sulfur radical cation. An electron-transfer mechanism has been suggested [205] for cytochrome P-450 oxidation of thioanisole derivatives ArSMe. The evidence for this... 

Bakker et al. [25] replaced the zinc in thermolysin with anions snch as molybdate, selenate, or tungstate to give an enzyme that catalyzed the nonenantioselective oxidation of thioanisoles with hydrogen peroxide. Van de Velde et al. [26] added a vanadate ion to the active site of phytase to create a catalyst for enantioselective oxididation of thioanisole to the sulfoxide in 66% ee. Selenosubtihsin (protease subtilisin where a selenoserine replaces the active-site serine) catalyzes the enantioselective reduction of hydroperoxides [27] with enantioselectivity >100 for one substrate. 

Enantioselective sulfoxidation has been also studied using polymeric aminoalcohol-derived Schiff bases. Polymers 354-357 have been prepared by copolymeristation of the corresponding monomer with either MMA and EGDMA or styrene and DVB (Scheme 150) [219], These four polymers were then stirred with VO(acac)2 for 4h at rt to lead to the vanadium complexes. For the enantioselective oxidation of thioanisole, although the yields were similar to those obtained with the homogeneous analogs, the enantioselectivity were lower. The best selectivity was obtained with the catalyst derived from 355. 

Impressive enantioselectivities (up to >99.9% enantiomeric excess) were observed with a large range of thioethers. However, moderate yields were obtained [ca. 30-40%), which was attributed to a kinetic resolution in the oxidation of sulfoxide to sulfone, thus reducing the yield in sulfoxide. The heterogeneous nature of the catalyst was confirmed by inductively coupled plasma (ICP) spectroscopic analysis of the liquid phase (<1 ppm of titanium). The catalyst was recycled by simple filtration, and was reused at least 8 times in oxidation of thioanisole without any loss of enantioselectivity. 

N,N-diall lamines and the oxidation of thioanisoles (SR2) to generate sulfoxides (R2SO)7 ... 

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