Asymmetric oxidation, of sulfides

Contrary to the asymmetric epoxidation of allylic alcohols, the presence of water in this reaction is crucial for the enantioselection. Nevertheless, the main drawback of this method was the use of a stoichiometric amount of titanium and the use of tert-butyl hydroperoxide (TBHP) as oxidant. Efforts have been devoted to the development of more sustainable processes focusing on catalytic reactions and on the nature of oxidising agent. In 1987, Kagan and coworkers reported a catalytic version using as low as 10 mol% of titanium chiral complex in the presence of activated molecular sieves (MS). It is noteworthy that they also improved the process by using less-explosive and less-reactive cumene hydroperoxide as oxidant. A few years later, the group of Uemura developed a complementary catalytic system based on the 

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. 

The use of ionic liquids to perform asymmetric sulfoxidation reactions was proposed by Halligudi and coworkers. A chiral titanium-BINOL complex was immobilised onto ionic liquid-modified mesoporous silica (SBA-15) support 14 and the resulting heterogeneous catalyst was successfully employed in asymmetric sulfoxidation of thioanisole using TBHP as 

Correia et al. used titanium-salan eatalysts in ionie liquids as solvents. Unfortunately, despite good catalytie aetivity, only modest enantioselec-tivities were detected (around 20% enantiomerie exeess). Oxidation of sulfides was also performed with silica-immobilised eatalysts. Titanium-tartaric acid catalysts were grafted onto amorphous and MCM-41 silica by reaction of the metal with the silanol groups on the surface, and used in the sulfide oxidation. In both cases, sulfoxides with low enantiomeric excesses were obtained. 

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Davis and coworkers40 have developed use of diastereomerically pure 2-sulfonyl and 2-sulfamyloxaziridines for asymmetric oxidation of sulfides into sulfoxides (equation 7). The best results (using the sulfamyloxaziridines) range from 38 to 68% enantiomeric purity of the resultant sulfoxides. The structural diversity of such substituted oxaziridines, their... 

Okrasa and co-workers reported an interesting combination reaction of glucose oxidase and peroxidase in a mixed solvent of [bmimJpFg] with water (Fig. 18). Asymmetric oxidation of sulfide was accomplished successfully in the reaction system. ... 

Figure 18 Glucose oxidase-mediated asymmetric oxidation of sulfide in IL...

In 1960, Montanari and Balenovic and their coworkers described independently the first asymmetric oxidation of sulfides with optically active peracids. However, the sulphoxides were formed in this asymmetric reaction (equation 130) with low optical purities, generally not higher than 10%. The extensive studies of Montanari and his group on peracid oxidation indicated that the chirality of the predominantly formed sulphoxide enantiomer depends on the absolute configuration of the peracid used. According to Montanari the stereoselectivity of the sulphide oxidation is determined by the balance between one transition state (a) and a more hindered transition state (b) in which the groups and at sulphur face the moderately and least hindered regions of the peracid,... 

Kagan oxidations (a) Kagan, H.B. (2000) Asymmetric oxidation of sulfides, in Catalytic Asymmetric Synthesis, 2nd edn (ed. I. Ojima), John Wiley Sons, Inc., New York, pp. 327 (b) Pitchen, P.,... 

As an extension, this type of reagent can also be used to oxidize sulfides. Scheme 4-43 depicts the asymmetric oxidation of sulfides catalyzed by salen Mn(III) complexes. 

In 1986, Puchot et al.104 studied the nonlinear correlation between the enantiomeric excess of a chiral auxiliary and the optical yield in an asymmetric synthesis, either stoichiometric or catalytic. Negative NLEs [(—)-NLEs] were observed in the asymmetric oxidation of sulfide and in [.S ]-proline-mediated asymmetric Robinson annulation reactions, while a positive NLE [(+)-NLEs]... 

In addition, a recent report details a very efficient nonenzymatic method for the asymmetric oxidation of sulfides this employs an organo-vanadium species featuring the imine (38) (Scheme 25)[111]. A second, complementary strategy for the preparation of optically active sulfoxides involves the enantioselective oxidation of racemic sulfoxides. ... 

Asymmetric oxidation of sulfides and kinetic resolution of sulfoxides... 

ASYMMETRIC OXIDATION OF SULFIDES AND KINETIC RESOLUTION OF SULFOXIDES... 

An attractive route to chiral sulfoxides is based on asymmetric oxidation of unsymmetrical sulfides by means of chiral oxidizing reagents. The first asymmetric oxidation of sulfides with optically active pera-cids (eq. [1]) has been independently described in 1960 by two groups headed by Montanan (36) in Italy and by Balenovic (37) in... 

According to this correlation model, in which the principles of steric control of asymmetric induction at carbon (40) are applied, the stereoselectivity of oxidation should depend on the balance between one transition state [Scheme 1(a)] and a more hindered transition state [Scheme 1(6)] in which the groups and R at sulfur face the moderately and least hindered regions of the peroxy acid, respectively. Based on this model and on the known absolute configuration of (+)-percamphoric acid and (+)-l-phenylperpropionic acid, the correct chirality at sulfur (+)-/ and (-)-5 was predicted for alkyl aryl sulfoxides, provided asymmetric oxidation is performed in chloroform or carbon tetrachloride solution. Although the correlation model for asymmetric oxidation of sulfides to sulfoxides is oversimplified and has been questioned by Mislow (41), it may be used in a tentative way for predicting the chirality at sulfur in simple sulfoxides. 

Asymmetric oxidation of sulfides to sulfoxides occurs in the presence of chiral catalysts. It was found (53) that oxidation of benzyl methyl sulfide with iodine suspended in (i )-2-methyl-2-phenylsuccinate 33 buffer gives optically active benzyl methyl sulfoxide 34 having 6.35% optical purity. Much higher asymmetric... 

Optically active iV-p-toluenesulfinyl piperidine 75 prepared in low optical purity by oxidation of iV-p-toluenesulfenyl piperidine with (+)-percamphoric acid represents the first example of a chiral sul-flnamide (105). As in the case of asymmetric oxidation of sulfides and sulfenates, the synthetic utility of this method is strongly limited by its low stereoselectivity. 

Kagan HB (1993) Asymmetric oxidation of sulfides. In Ojima I (ed) Catalytic asymmetric synthesis. VCH, Weinheim, p 203... 

Cumene hydroperoxide was selected because it was recently observed that it gives in many cases better ee s in asymmetric oxidation of sulfides than the original procedure with t-butyl hydroperoxide. " ... 

Oxidation of chiral sulfonimines (R"S02N=CHAr)and chiral sulfamyl-imines (R RNS02N=CHAr)affords optically active 2-sulfonyloxaziridines and 2-sulfamyloxaziridines, respectively. These chiral, oxidizing reagents have been used in the asymmetric oxidation of sulfides to sulfoxides (15-68% ee), 11-13 selenides to selenoxides (8-9% ee] enolates to a-hydroxycarbonyl compounds (8-37% ee) and in the asymmetric epoxidation of alkenes (15-40% ee)... 

For an excellent review on asymmetric oxidation of sulfides see Reference 191. 

In combination with H2O2 (salen)Mn(III) complexes 173a, b, i-n have also been employed by Jacobsen and coworkers as catalysts for the asymmetric oxidation of sulfides to sulfoxides, without a need for additives. From the structurally and electronically different Mn-salen catalysts screened, 173i turned out to be the most active and selective one (equation 58) . While dialkyl sulfides underwenf uncafalyzed oxidation with H2O2, aryl alkyl sulfides were oxidized only slowly compared wifh fhe cafalyzed pathway. Using... 

SCHEME 111. Titanium-catalyzed asymmetric oxidation of sulfides with carbohydrate hydroperoxides... 

A solid-phase sulfur oxidation catalyst has been described in which the chiral ligand is structurally related to Schiff-base type compounds (see also below). A 72% ee was found using Ti(OPr-i)4, aqueous H2O2 and solid-supported hgand 91 . More recently, a heterogeneous catalytic system based on WO3, 30% H2O2 and cinchona alkaloids has been reported for the asymmetric oxidation of sulfides to sulfoxides and kinetic resolution of racemic sulfoxides. In this latter case 90% ee was obtained in the presence of 92 as chiral mediator. ... 

A different approach to the metal peroxo-mediated asymmetric oxidations of sulfides was proposed by Chmielewski and coworkers and more recently by Korb, Adam and coworkers based on the use of optically active hydroperoxides. First attempts to use... 

Optically active Schiff-base oxovana-dium(IV) complexes catalyze the asymmetric oxidation of sulfides to sulfoxides by peroxides [86]. The catalytically active species is VO(V) rather than VO(IV) and is formed in situ under the reaction conditions. A series of related complexes based on the optically active ligand shown in Eig. 15 shows linear dependence of their oxidation Ef values on the Hammett parameters of functional group X. These values ranged from 0.18 V versus Cp2Ee/DMSO for X = NO2 to —0.18 V for X = OCH3 [87]. A few complexes of planar tetradentate non-Schiff base ligands have also been investigated [88]. 

WO3-3O % H2O2-CINCHONA ALKALOIDS A NEW HETEROGENEOUS CATALYTIC SYSTEM FOR ASYMMETRIC OXIDATION OF SULFIDES AND KINETIC RESOLUTION... 

BENZYL-4,6-O-ISOPROPYLIDENE-a-D-GLUCOPYRANOSIDE, 2-DEOXY-2-[[(2-HYDROXY-3,5-di-lerl-BUTYLPHENYL)METHYLENE]IMINE] AS A LIGAND FOR VANADIUM-CATALYZED ASYMMETRIC OXIDATION OF SULFIDES... 

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