Sulfoxides, kinetic resolution

A one-pot titanium-catalyzed tandem sulfoxidation-kinetic resolution process was developed by Chan using TBHP as the oxidant This process combines asymmetric sulfoxidation (at 0°C) and kinetic resolution (at room temperature). Excellent enantiomeric excesses (up to >99.9%) and moderate to high chemical yields of sulfoxides were obtained [270] (Scheme 14.113). The effect of fluorine substitution at the backbone of BINOL on the catalytic activity in titanium-catalyzed sulfide oxidation with TBHP or cumyl hydrc en peroxide (CHP) was studied by Yudin [271]. Introduction of fluorines into the BINOL scaffold was found to increase the electrophilic character of the Lewis acidic titanium center of the catalyst The most intriguing difference between the FsBINOL and BINOL systems is the reversal in the sense of chiral induction upon fluorine substitution. A steroid-derived BINOL ligand has also been used for the same reaction [272]. 

The reaction of the enantiomerically pure sulfoxide anion (0.5 equiv) with a racemic bicyclic enone allows for the kinetic resolution of the enone15b. 

Numerous reactions of racemic sulfoxides with chiral reagents have been accomplished2,12. These examples of kinetic resolution usually lead to sulfoxides of low enantiomeric purity, but there are some exceptions. 

The hydrolysis of seven alkyl arenesulfinylalkanoates by the bacterium Corynebacterium equi IFO 3730 studied by Ohta and coworkers34 are recent examples of kinetic resolutions which give sulfoxides of high enantiomeric purity and in reasonable yield. Compounds 16a, 16b and 16c were recovered in 30 to 43% yield and in 90 to 97% e.e. The S enantiomers underwent hydrolysis more rapidly than the R isomers. Sulfoxide 17 was isolated in 22% yield and 96% e.e., but sulfoxide 18 was completely metabolized. Esters other than methyl gave inferior results. The acids formed upon hydrolysis, although detected, were for the most part further metabolized by the bacterium. 

In principle, it should be possible to selectively reduce one of the enantiomers in a racemic sulfoxide mixture that is, an asymmetric kinetic resolution via reduction should... 

Enzyme-mediated hydrolysis of some racemic co-arenesulfinylalkanoic methyl esters, ArSO(CH2) COOMe, using Corynebacterium equi has led to a kinetic resolution in which the unreacted sulfinyl esters are enriched in one enantiomer at the sulfoxide center49. The enantiomeric purity of unreacted sulfinyl acetates and propionate ranges from 90 to 97%. 

This type of asymmetric conjugate addition of allylic sulfinyl carbanions to cyclopen-tenones has been applied successfully to total synthesis of some natural products. For example, enantiomerically pure (+ )-hirsutene (29) is prepared (via 28) using as a key step conjugate addition of an allylic sulfinyl carbanion to 2-methyl-2-cyclopentenone (equation 28)65, and (+ )-pentalene (31) is prepared using as a key step kinetically controlled conjugate addition of racemic crotyl sulfinyl carbanion to enantiomerically pure cyclopentenone 30 (equation 29) this kinetic resolution of the crotyl sulfoxide is followed by several chemical transformations leading to (+ )-pentalene (31)68. 

Mikolajczyk and coworkers have summarized other methods which lead to the desired sulfmate esters These are asymmetric oxidation of sulfenamides, kinetic resolution of racemic sulfmates in transesterification with chiral alcohols, kinetic resolution of racemic sulfinates upon treatment with chiral Grignard reagents, optical resolution via cyclodextrin complexes, and esterification of sulfinyl chlorides with chiral alcohols in the presence of optically active amines. None of these methods is very satisfactory since the esters produced are of low enantiomeric purity. However, the reaction of dialkyl sulfites (33) with t-butylmagnesium chloride in the presence of quinine gave the corresponding methyl, ethyl, n-propyl, isopropyl and n-butyl 2,2-dimethylpropane-l-yl sulfinates (34) of 43 to 73% enantiomeric purity in 50 to 84% yield. This made available sulfinate esters for the synthesis of t-butyl sulfoxides (35). 

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

Kinetic resolution of racemic 4-bromophenyl methyl sulfoxide. Ill... 

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

KINETIC RESOLUTION OF RACEMIC 4-BROMOPHENYL METHYL SULFOXIDE... 

Table 8.2 Kinetic resolution of racemic sulfoxides (R-SO-Me) with 1.

The above procedure can be exploited for the asymmetric oxidation of racemic sulfoxide1 1, and high stereoselection can be frequently observed. Moreover unreacted / -sulfoxides were always recovered as the most abundant enantiomers, kinetic resolution and asymmetric oxidation being two enantioconvergent processes. Thus, by the combined routes, higher enantioselectivity can be observed with dialkyl sulfoxides, usually obtained with poor to moderate e.e.s. 

Since in principle the reactions of enantiomeric sulfoxides with a chiral reagent are expected to proceed at unequal rates, a possibility exists for obtaining chiral sulfoxides, especially when the reacting racemic sulfoxide is used in excess in relation to the chiral reagent. A typical example of such a kinetic resolution of a racemic sulfoxide is its reaction with a deficiency of chiral peracid, affording a mixture of optically active sulfoxide and achiral sulfone (62,63). However,... 

Recently, Juge and Kagan (68) reported that a more efficient kinetic resolution of racemic sulfoxides takes place in the Pummerer-type reaction with optically active a-phenylbutyric acid chloride 38 in the presence of N,A-dimethylaniline. In contrast to the asym-... 

A very interesting approach to chiral a unsaturated sulfoxides 39 based on a kinetic resolution was elaborated by Marchese (69), who found that asymmetric elimination of racemic j3-halogenosul-foxides 40 takes place in the presence of chiral tertiary amines. 

The kinetic resolution of /3-hydroxy sulfides mediated by CHMO provides an excellent result in the case of sulfide ( )-2 and moderate results with ( )-l and ( )-3. Indeed, the enzyme-catalysed oxidation to sulfoxide 2a showed remarkable enantio- and diastereo-selectivity with an enantiomeric ratio E of 299 and with an ee > 98 % (C = 47 %). 

Keywords Peroxidase, Biocatalysis, Asymmetric synthesis. Kinetic resolution. Hydroperoxide, Epoxidation, Sulfoxidation, Halogenation, Hydroxylation, Phenol coupling. 

As shown in Table 12,H202 and fBuOOH have been used frequently as oxygen donors in peroxidase-catalyzed sulfoxidations. Other achiral oxidants, e.g. iodo-sobenzene and peracids, are not accepted by enzymes and, therefore, only racemic sulfoxides were found (c.f. entries 34-36). Interestingly, racemic hydroperoxides oxidize sulfides to sulfoxides enantioselectively under CPO catalysis [68]. In this reaction, not only the sulfoxides but also the hydroperoxide and the corresponding alcohol were produced in optically active form by enzyme-catalyzed kinetic resolution (cf. Eq. 3 and Table 3 in Sect. 3.1). 

In the sulfoxidation, small to appreciable amounts of over oxidation with formation of undesired sulfone were observed, a result that implies that kinetic resolution may be involved in influencing the overall stereochemical result 105). This was shown to be the case. Indeed, some of the mutants are also excellent catalysts in the kinetic resolution of racemic sulfoxides such as 25 105). Directed evolution was then applied successfully to eliminate undesired sulfone formation, specifically, by going through a second cycle of epPCR 105). This is significant because it shows for the first time that an undesired side reaction can be eliminated by directed evolution. 

Hoft reported about the kinetic resolution of THPO (16b) by acylation catalyzed by different lipases (equation 12) °. Using lipases from Pseudomonas fluorescens, only low ee values were obtained even at high conversions of the hydroperoxide (best result after 96 hours with lipase PS conversion of 83% and ee of 37%). Better results were achieved by the same authors using pancreatin as a catalyst. With this lipase an ee of 96% could be obtained but only at high conversions (85%), so that the enantiomerically enriched (5 )-16b was isolated in poor yields (<20%). Unfortunately, this procedure was limited to secondary hydroperoxides. With tertiary 1-methyl-1-phenylpropyl hydroperoxide (17a) or 1-cyclohexyl-1-phenylethyl hydroperoxide (17b) no reaction was observed. The kinetic resolution of racemic hydroperoxides can also be achieved by chloroperoxidase (CPO) or Coprinus peroxidase (CiP) catalyzed enantioselective sulfoxidation of prochiral sulfides 22 with a racemic mixmre of chiral hydroperoxides. In 1992, Wong and coworkers and later Hoft and coworkers in 1995 ° investigated the CPO-catalyzed sulfoxidation with several chiral racemic hydroperoxides while the CiP-catalyzed kinetic resolution of phenylethyl hydroperoxide 16a was reported by Adam and coworkers (equation 13). The results are summarized in Table 4. 

TABLE 4. Kinetic resolution of racemic hydroperoxides via CPO and CiP catalyzed enantioselec-tive sulfoxidation... 

With substrates 16b and 17a, Hoft and coworkers observed only low ee values of up to 4% for the hydroperoxides. On the other hand, phenyl ethyl hydroperoxide (16a) could be isolated in high enantiomeric excess of >99% from the CPO-catalyzed reaction. The observed enantioselectivities of the sulfoxides varied, depending on the conversion of the sulfide and the hydroperoxide used, being highest with 16a (92% ee). Unfortunately, the CPO-catalyzed resolution of chiral hydroperoxides is difficult on a preparative scale because of the high dilution necessary (0.5ttmolmL ). In the CiP-catalyzed kinetic resolution of 16a better results were obtained compared to the CPO-catalyzed reaction (see Table 5). 

Sulfides are generally oxidized much faster than alkenes, and in the presence of excess oxidant further oxidation to the sulfone occurs. In the cases where the reaction is conducted in an asymmetric way, the chiral catalytic system may react faster with one enantiomeric sulfoxide to form the sulfone than with the other, so that kinetic resolution of the primarily formed sulfoxide may occur. In general, the reaction is carried out with alkyl hydroperoxides like TBHP in the presence of a metal catalyst like Mo, W, Ti or V complexes. In some cases the sulfoxidation with hydroperoxides can take place without the need of a metal catalyst. Both examples will be discussed in the following. 

As already reported in Section II.A.2, the enzymes chloroperoxidase (CPO) and Copri-nus peroxidase (CiP) catalyze the enantioselective oxidation of aryl alkyl sulfides. If a racemic mixture of a chiral secondary hydroperoxide is used as oxidant, kinetic resolution takes place and enantiomerically enriched hydroperoxides and the corresponding alcohols can be obtained together with the enantiomerically enriched sulfoxides. An overview of the results obtained in this reaction published by Wong and coworkers, Hoft and... 

The catalytic sulfoxidation system developed by Uemura and coworkers in 1993381,386 consisted of Ti(OPr-i)4, (f )-BINOL, H2O and TBHP in different compositions. The best results (highest ee) were obtained with low amounts of catalyst 0.025 eq. Ti(OPr-i)4, 0.05 eq. (f )-BINOL, 0.5 eq. H2O and 2 eq. TBHP. With this method sulfoxides could be obtained with excellent enantioselectivities (ee 96%), although yields were low (28-44%) due to kinetic resolution of the formed sulfoxides to give the corresponding sulfones. More detailed investigations by Uemura and coworkers showed that an enantiomeric excess of 50% of the sulfoxides is obtained at the initial stage of the reaction and that an increase in ee results the longer the reaction takes place . So the Ti-(i )-BINOL complex catalyzes... 

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