Aryl phenyl sulfides, oxidation

The oxo(salen)Mn(V) complex, (8) is the effective oxidant in the Mn(III)salen-catalysed oxidation of aryl phenyl sulfides by hypochlorite in MeCN/H20. First-order dependence on oxidant and sulfide is observed and the rate is reduced by electron-attracting substituents in the sulfides and electron-releasing substituents in salen. The Hammett p-value (-2.16.) determined for substituted sulfides indicates an electron-deficient transition state Hammett correlations for substituted salen complexes give positive /O-values for each sulfide. The substituent-, acid-, and solvent effects indicated direct transfer of the 0-atom from the oxidant to the substrate in the rate-determining step. ... 

Preparation of various enantiomerically pure sulfoxides by oxidation of sulfides seems feasible in the cases where asymmetric synthesis occurs with ee s in the range of 90% giving crystalline products which can usually be recrystallized up to 100% ee. Aryl methyl sulfides usually give excellent enantioselectivity during oxidation and are good candidates for the present procedure. For example, we have shown on a 10-mmol scale that optically pure (S)-(-)-methyl phenyl sulfoxide [a]p -146 (acetone, o 1) could be obtained in 76% yield after oxidation with cumene hydroperoxide followed by flash chromatographic purification on silica gel and recrystallizations at low temperature in a mixed solvent (ether-pentane). Similarly (S)-(-)-methyl o-methoxyphenyl sulfoxide, [a]p -339 (acetone, o 1.5 100% ee measured by HPLC), was obtained in 80% yield by recrystallizations from hexane. 

A further catalytic method for asymmetric sulfoxidation of aryl alkyl sulfides was reported by Adam s group, who utilized secondary hydroperoxides 16a, 161 and 191b as oxidants and asymmetric inductors (Scheme 114) . This titanium-catalyzed oxidation reaction by (S)-l-phenylethyl hydroperoxide 16a at —20°C in CCI4 afforded good to high enantiomeric excesses for methyl phenyl and p-tolyl alkyl sulfides ee up to 80%). Detailed mechanistic studies showed that the enantioselectivity of the sulfide oxidation results from a combination of a rather low asymmetric induction in the sulfoxidation ee <20%) followed by a kinetic resolution of the sulfoxide by further oxidation to the sulfone... 

The Schiff base-oxovanadium(IV) complex formulated as 19 was found to catalyze the asymmetric oxidation of sulfides with cumene hydroperoxide (Scheme 6C.8) [70]. Various aryl methyl sulfides were used for this process (room temperature in dichloromethane and 0,1 mol equiv. of the catalyst). Chemical yields were excellent, but enantioselectivities were not higher than 40% for the resulting methyl phenyl sulfoxide, Complex 16a, where [Ti] was replaced by VO, was also examined in the oxidation of sulfides, but the reactions gave only racemic sulfoxides [68],... 

It is known that cyclodextrins have a hydrophobic cavity (a binding site for aromatics) and a hydrophilic external surface. A template-directed asymmetric sulfoxidation has been attempted with various aryl alky] sulfides [91]. Oxidations were performed by using metachlo-roperbenzoic acid in water in the presence of an excess of (3-cyclodextrin. The best ee (33%) was attained for meta- r-butyI)phenyl ethyl sulfoxide. The decrease in the amount of P-cyclodex-trin below 1 mol equiv. causes a sharp decrease in enantioselectivity because of competition with oxidation of free substrate by the oxidant. Similarly, DrabowiczandMikolajczyk observed modest asymmetric induction (27% ee) in the oxidation of Ph-S-n-Bu with H202 in the presence of P-cyclodextrin [92],... 

Oxidation of alkyl phenyl sulfides by pyridinium bromochromate (PBC) is accelerated by electron-donating alkyl groups or aryl substituents, indicating an electron-deficient sulfur centre in the transition state this is accounted for in terms of rate-determining electrophilic oxygen attack from PBC to the sulfide in an. S -like process.7... 

In the same year, Fujita s group63 reported the asymmetric oxidation of aryl methyl sulfide by hydroperoxides (TBHP, CHP) and an optically active catalyst formed by a Schiff base-oxovanadium(IV) complex 32, giving (S)-sulfoxides in low ee (up to 40%) (Fig. 4). Later, they developed64 a more promising approach using 33, a binuclear Schiff base-titanium(IV) complex (4 mol% equiv) to catalyze the asymmetric oxidation of methyl phenyl sulfide by trityl hydroperoxide in methanol at 0 °C. The (ft)-methyl phenyl sulfoxide was obtained with 60% ee. 

Alkyl aryl sulfides electrochemically oxidized on electrodes whose surfaces were modified by coating them with optically active compounds like camphoric acid [89] and poly(amino acids) [90, 91] afford mixtures of sulfoxides with variable enantiomeric excess. An optical yield of 93% is claimed when f-butyl phenyl sulfide is oxidized at a Pt electrode doubly coated with polypyrrole and poly(L-valine) [91]. 

Neighbouring sulfur atom participation in the electrochemical oxidation of alkyl aryl sulfides was also shown to be occurring, such as in naphtho[l,8-, c]-l,5-dithiocin (XXI) [101] and 2,6-bis[(methylthio)methyl]phenyl phenyl sulfide (XXII) [101]. 

The first example of FB oxidation of sulfides dates back to 1995 dibenzothiophene and diphenyl stdfide gave the corresponding sulfones in low yields (1.4% and 10%, respectively) upon treatment with O2 at 100 °C in the presence of a not fully characterized perfluorocarbon-soluble iron—phthalocyanine [19]. Following this earlier report, Co(ll)—tetraarylporphyrin Co-5 and Co(I I [—phthalocyanine Co-12 (cf Stmcture) were tested as catalysts for the FB oxidation of methyl phenyl sulfide and para-substituted aryl methyl sulfides with O2 and a sacrificial aldehyde (Table 3) [20]. 

A mild and chemoselective oxidation of sulfides to sulfoxides by o-iodooxybenzoic acid (IBX) catalyzed by tetraethylammonium bromide (TEAB) has been reported [73]. The reaction is highly selective, and no overoxidation to sulfone was observed. Simple aryl alkyl sulfides are oxidized in 93-98% yield in 0.3-2 h at room temperature with the use of 5 mol% of TEAB. Diphenyl sulfide and phenyl benzyl sulfide took 30 and 36 h, respectively, to go to completion under these conditions. 

With (salalen)Al complex 33 as the chiral catalyst, the reactions of aryl methyl sulfides proceeded smoothly to give the sulfoxides in high yields with excellent enantioselectivity, irrespective of the electronic nature and the location of the substituent on the aromatic ring (Scheme 19.38). Ethyl phenyl sulfide is also a good substrate for the oxidation which gave the sulfoxide in 80% yield with 91% enantiomeric excess. 

The complex (70) is the most efficient catalyst in the titaniumsalan-catalysed asymmetric oxidations of bulky aryl benzyl sulfides and small alkyl phenyl sulfides by H2O2 in CH2CI2 to corresponding sulfoxides with 77% ee. The kinetics suggest that a direct attack of the sulfide on the electrophilic active oxygen species occurs... 

A new catalytic procedure for the asymmetric oxidation of aryl alkyl and aryl benzyl sulfides to optically active sulfoxides by TBHP is mediated by a chiral titanium complex formed in situ by reacting Ti( -PrO)4, (R, / )-diphenylethane-l,2-diol, and water. The results were largely unaffected by the nature of the phenyl substituents, suggesting that the same mechanism operates in all cases. Only the / -N02 substituent on the aryl ring caused a considerable loss of enantioselectivity and this is attributed to the electron-withdrawing power of this group or, more likely, its coordinating ability. ... 

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