Enantioselective oxidations of sulfides to sulfoxides

Encapsulation in Y zeohte was also the method chosen to immobihze Mn complexes of C2-symmetric tetradentate hgands (Fig. 24) [75]. These materials were used as catalysts for the enantioselective oxidation of sulfides to sulfoxides with NaOCl. The lack of activity when the larger io-dosylbenzene was used as an oxidant was interpreted as an indication that the reaction took place inside the zeolite microporous system. Both the chemo- and enantioselectivity were dependent on the structure of the sulfide. (2-Ethylbutyl)phenylsulfide led to better results than methylphenylsulfide, although in all cases the enantioselectivity was low (up to 21% ee). 

Colonna, S., Gaggero, N., Pasta, P., Ottolina G., Enantioselective oxidation of sulfides to sulfoxides catalysed by bacterial cyclohexanone monooxygenases. Chem. Commun., 1996, 2303. 

Reactions where NLE have been discovered include Sharpless asymmetric epoxi-dation of allylic alcohols, enantioselective oxidation of sulfides to sulfoxides, Diels-Alder and hetero-Diels-Alder reactions, carbonyl-ene reactions, addition of MesSiCN or organometallics on aldehydes, conjugated additions of organometal-lics on enones, enantioselective hydrogenations, copolymerization, and the Henry reaction. Because of the diversity of the reactions, it is more convenient to classify the examples according to the types of catalyst involved. 

Uemura reported a highly enantioselective oxidation of sulfides to sulfoxides using a chiral titanium complex prepared from chiral BINOL and Ti(0-i-Pr)4, and this reaction exhibits a remarkable asymmetric amplification (Scheme 9.15) [33]. 

Several oxaziridines related to (14) (eq 8) have been used, most notably in the enantioselective oxidation of sulfides to sulfoxides, of selenides to selenoxides, and of alkenes to oxiranes, It is also the reagent of choice for the hydroxylation of lithium and Grignard reagents and for the asymmetric oxidation of enolates to give a-hydroxy carbonyl compounds, - A similar chiral fluorinating reagent has also been developed, ... 

Considerable advances have been made recently towards the enantioselective oxidation of sulfides to sulfoxides. The reactions can be technically challenging but high enantioselectivies can be achieved. 

The development of other efficient methods for the enantioselective oxidation of sulfides to sulfoxides has been pursued for many years. Various chiral additives and metal complexes have been investigated. However, most of these systems have disadvantages such as formation of sulfone, long reaction times, unsatisfactory yields and poor enantiomeric excesses. Diethyl tartrate remains the additive (combined with titanium alcoholates) which gives the best results in terms of enantioselectivity. Since 1989, it has been found that other chiral ligands can be efficient in asymmetric induction. 

More recently, Jansen and coworkers [90] used 4-hydroxyacetophenone monooxygenase (HAPMO) for aerobic oxidation of sulfides. Interestingly, both PhSMe and p-MeC6H4SMe gave the corresponding sulfoxides in >99% ee, which should be compared with the 99 and 37% ee, respectively, obtained with CHMO [86]. The flavoenzyme HAPMO, which has been cloned [90, 91], is a promising biocatalyst for enantioselective oxidation of sulfides to sulfoxides. 

HP as the oxygen source, have been reviewed, with major focus on the their synthetic potential. Recent experimental investigations of the nature of catalytically active species and mechanisms of their action are summarized.Asymmetric oxidation reactions, viz., sulfoxidation, epoxidation, dihydroxylation, and aminohydroxylation in water have been reviewed. The focus is on the development of catalytic oxidation in water, particularly the use of HP in the presence of metal catalysts. The enantioselective oxidation of sulfides to sulfoxides is also included. ... 

Figure 9 Enantioselective oxidation of sulfides to sulfoxides. (From Ref. 20.)...

Oxometalloporphyrins were taken as models of intermediates in the catalytic cycle of cytochrome P-450 and peroxidases. The oxygen transfer from iodosyl aromatics to sulfides with metalloporphyrins Fe(III) or Mn(III) as catalysts is very clean, giving sulfoxides, The first examples of asymmetric oxidation of sulfides to sulfoxides with significant enantioselectivity were published in 1990 by Naruta et al, who used chiral twin coronet iron porphyrin 27 as the catalyst (Figure 6C.2) [79], This C2 symmetric complex efficiently catalyzed the oxidation... 

Asymmetric oxidation of sulfides to sulfoxides is undoubtedly the most straightforward route to optically pure (o.p.) sulfoxides. It is thus not surprising that various groups have been trying to develop efficient methods toward this end. Several o.p. sulfoxides have been obtained, by either diastereoselective or enantioselective oxidation of sulfides. 

Kagan and Pitchen ° and Modena and coworkers independently reported the oxidation of sulfides to sulfoxides using modified Sharpless epoxidation catalyst (titanium/diethyl tartrate). By 1987, Kagan had already reported a catalytic variation of the reaction and an improved catalytic system allows for the use of lower (10 mol%) loading of catalyst. For example, sulfide (5.143) undergoes sulfoxidation with good enantioselectivity. An alternative catalyst based on Ti(0 Pr)4 and BINOL is also effective for sulfoxidation, providing up to 96% ee. ... 

Davis, F. A., Me Caulley, J. P. Jr and Harakal, M. E. (1984) Chiral sulfamides optically active 2-sulfamyloxaziridines. Hi enantioselectivity in the asymmetric oxidation of sulfides to sulfoxides,... 

The Coprinus cinerem peroxidase (Cip) was later used in ionic liquid [BMImJPFfi as a reaction medium in peroxidase-catalyzed oxidation of sulfides to sulfoxides in high enantioselectivity (92% ee) [84]. However the reaction was very slow (14% conversion after 16h). Glucose oxidase from Asper llus niger was employed to generate H2O2 from O2. 

The selective oxidation of sulfides to sulfoxides is an important reaction in biological systems, ft is a well-studied model reaction for oxo atom transfer and it is of importance for applications in pharmaceutical and preparative organic chemistry [1, 2, 60]. Iron- and ruthenium-based sulfoxidation catalysts have also been explored in the area of asymmetric synthesis and enantioselective catalysis... 

Chiral salen(Mn ") complexes have been used as catalysts in the oxidation of sulfides to sulfoxides by 30% aqueous H2O2 in acetonitrile [28]. The use of 2-3 mol% of catalyst led to an efficient reaction with enantioselectivities of up to 68% ee. 

Uemura and coworkers utilized (R)-binaphthol 85 as chiral ligand in place of DET in association with Ti(IV)/TBHP, which not only mediated the oxidation of sulfides to (R)-configurated sulfoxides, but also promoted the kinetic resolution of sulfoxides (equation 50). In this latter process the two enantiomers of the sulfoxide are oxidized to sulfone by the chiral reagent at different rates, with decrease of the chemical yield, but increase of the ee values. Interestingly, the presence of ortho-nilro groups on the binaphthol ligand lead to the reversal of enantioselectivity with formation of the (5 )-configurated sulfoxide. Non-racemic amino triols and simple 1,2-diols have been successfully used as chiral mediators. 

The Orsay system. A good example of serendipity is the discovery by Kagan and co-workers46 at Orsay that 1 mol of water was necessary to produce the active catalyst able to oxidize prochiral sulfides to sulfoxides with high ee. Optimization of the stoichiometry of the titanium complex permitted the determination of the combination Ti(0-i-Pr)4/(/ ,/ )-DET/H20 (1/ 2/ 1) at -20 °C in CH2C12 as the optimal conditions to achieve high enantioselectivity. Table 6 shows some representative results obtained for the oxidation of several thioethers with tert-butyl hydroperoxide (TBHP) under these conditions.50,51... 

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