Enantioselectivity sulfoxide formation

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 enantioselective sulfoxidation of thioanisole for the (R)-isomer by H64D/V68A and H64D/V68S Mbs suggest the sulfoxidation intermediate shown in Scheme 9 could be stabler for (R)-sulfoxide formation over the (S)-isomer. The chiral discrimination for (R)- and ( -intermediates is caused by steric interaction between the transition state and the heme cavity. However, it is... 

SIBX is a non-explosive formulation of the X -iodane 2-iodoxybenzoic acid (IBX) stabilized by benzoic acid. This reagent combination can be used as a suspension in various organic solvents to oxidize sulfides to sulfoxides. Most yields were comparable to those obtained using IBX or other iodanes such as PhlO and Phl02. The use of a chiral tartaric acid-based source in addition to SIBX gave asymmetric sulfoxide formation with moderate enantioselectivities [81]. 

Flavins are a group of natural enzyme cofactors with interesting redox and photochemical properties that can participate in a wide set of reactions [12]. The most common flavin cofactors are flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD). Enzymes harboring one of these cofactors are called flavoenzymes. A large number of flavoenzymes have been extensively studied for their structural and mechanistic properties, and they are gaining momentum in industrial biocata-lytic applications [13,14], Flavoenzymes have evolved to become powerful oxidative biocatalysts they can catalyze not only simple alcohol oxidations but they were also foimd to be efficient in catalyzing, for example, oxidative C—C bond formation and enantioselective sulfoxidations. 

An achiral reagent cannot distinguish between these two faces. In a complex with a chiral reagent, however, the two (phantom ligand) electron pairs are in different (enantiotopic) environments. The two complexes are therefore diastereomeric and are formed and react at different rates. Two reaction systems that have been used successfully for enantioselective formation of sulfoxides are illustrated below. In the first example, the Ti(0-i-Pr)4-f-BuOOH-diethyl tartrate reagent is chiral by virtue of the presence of the chiral tartrate ester in the reactive complex. With simple aryl methyl sulfides, up to 90% enantiomeric purity of the product is obtained. 

Ohta and coworkers used a bacterium, Corynebacterium equi IFO 3730, rather than a fungus, to oxidize eight alkyl phenyl and p-tolyl sulfides to their respective sulfoxides (119, 120) of configuration R. Virtually all of the sulfur compounds were accounted for as the sum of uncreacted sulfide, sulfoxide and sulfone. The enantiomeric purities of the sulfoxides obtained were quite good and are shown below in parentheses. The formation of the allyl sulfoxides in high optical purity is noteworthy. The authors believe that the sulfoxides were formed by enantioselective oxidation of the sulfides rather than by enantioselective oxidation of racemic sulfoxides, since the yield of sulfoxides was greater than 50% in five of the ten oxidations reported (see also Reference 34). 

Catalytic oxidations of sulfides were carried out in 1,2-dichloroethane with cumyl hydroperoxide by using 10 mol % of the catalyst. The best enantioselectivity was achieved with complex 6c. However, sulfone was always produced as byproduct of the reaction. Even with a limited amount of hydroperoxide, the sulfone formation could not be avoided. For example, the reaction of methyl p-tolyl sulfide using 0.5 mol equiv. of cumyl hydroperoxide with respect to sulfide gave a 62 38 mixture of the corresponding (.S j-sulfoxide and sulfone. The reaction of benzyl phenyl sulfide led to the formation of (5)-sulfoxide (84% ee) and sulfone ([sulfox-ide]/[sulfone] = 77 23). It was established that sulfone was produced from the early stages of the reaction. It was also demonstrated that some kinetic resolution of the sulfoxide cooperated with the enantioselective oxidation of the sulfide. A unique feature of this oxidation system, as compared to those using various Ti(IV)/(DET) complexes, is the insensitivity of the enantioselectivity (40-60% ee at 0°C) to the nature of the alkyl group of sulfides Ar-S-alkyl. 

Uemura et al. [49] found that (R)-1,1 -binaphthol could replace (7 ,7 )-diethyl tartrate in the water-modified catalyst, giving good results (up to 73% ee) in the oxidation of methyl p-tolyl sulfoxide with f-BuOOH (at -20°C in toluene). The chemical yield was close to 90% with the use of a catalytic amount (10 mol %) of the titanium complex (Ti(0-i-Pr)4/(/ )-binaphthol/H20 = 1 2 20). They studied the effect of added water and found that high enantioselectivity was obtained when using 0.5-3.0 equivalents of water with respect to the sulfide. In the absence of water, enantioselectivity was very low. The beneficial effect of water is clearly established here, but the amount of water needed is much higher than that in the case of the catalyst with diethyl tartrate. They assumed that a mononuclear titanium complex with two binaphthol ligands was involved, in which water affects the structure of the titanium complex and its rate of formation. 

Bolm and Bienewald discovered in 1995 that some chiral vanadium (IV)-Schiff base complexes were efficient catalysts (1 mol %) for sulfoxidation [71a]. The catalyst 20 was prepared in situ by reacting VO(acac)2 with the Schiff base of a fJ-aminoalcohol (Scheme 6C.8). Reactions were conveniently performed in air at room temperature by slow addition of 1.1 mol equiv. of aqueous hydrogen peroxide (30%). Under these experimental conditions the reaction of methyl phenyl sulfide gave the corresponding sulfoxide in 94% yield and 70% ee. The best enantioselectivity was obtained in the formation of sulfoxide 21 (85% ee). Many structural analogues of catalyst 20 were screened for their efficacy, but none of... 

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). 

Chiral (salen)Mn(III)Cl complexes are useful catalysts for the asymmetric epoxidation of isolated bonds. Jacobsen et al. used these catalysts for the asymmetric oxidation of aryl alkyl sulfides with unbuffered 30% hydrogen peroxide in acetonitrile [74]. The catalytic activity of these complexes was high (2-3 mol %), but the maximum enantioselectivity achieved was rather modest (68% ee for methyl o-bromophenyl sulfoxide). The chiral salen ligands used for the catalysts were based on 23 (Scheme 6C.9) bearing substituents at the ortho and meta positions of the phenol moiety. Because the structures of these ligands can easily be modified, substantia] improvements may well be made by changing the steric and electronic properties of the substituents. Katsuki et al. reported that cationic chiral (salen)Mn(III) complexes 24 and 25 were excellent catalysts (1 mol %) for the oxidation of sulfides with iodosylbenzene, which achieved excellent enantioselectivity [75,76]. The best result in this catalyst system was given by complex 24 in the formation of orthonitrophenyl methyl sulfoxide that was isolated in 94% yield and 94% ee [76]. 

Corynebacterium equi IF0 3730 gave high enantioselectivity in the oxidation of aryl alkyl sulfides [106], The results listed in Scheme 6C, 11 arise from experiments with no formation of sulfone, which occurs quite easily in several cases. Thio ketals and thio acetals were oxidized into mono S-oxides by various fungal species with enantioselectivity up to 70% ee [107]. Corynebacterium equi was very successfully used in the oxidation of formaldehyde dithioacetals to mono S-oxide or sulfone-sulfoxide, depending on the substrates. Thus n-Bu-S-CH-S-n-Bu was transformed to n-Bu-S02-CH2-S(0)-n-Bu with more than 95% ee in 70% yield. 

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