Alkyl aryl sulfoxides, chiral, reaction with

Oae and co-workers (288) were the first to show that nucleophilic displacement at sulfur is accompanied by retention of configuration. They found that chiral 0-labeled alkyl aryl sulfoxides exchange oxygen with dimethylsulfoxide at about 150°C, almost without racemization. To explain the steric course (retention) of this reaction, the formation of a trigonal-bipyramidal intermediate 246 was postulated in which the entering and departing oxygen atoms occupy apical and equatorial positions, respectively. 

Optically active dialkyl sulfoxides of high optical purity (>95%), which could not be prepared from menthyl alkanesulfmates, have been synthesized by Johnson et al. (93) via reaction of chiral alkyl aryl sulfoxides with alkyllithium reagents. The exchange of the... 

The second approach to chiral aminosulfonium salts consists of the conversion of chiral sulfoxides into aminosulfonium salts by means of MA -diethylaminosulfinyl tetrafluoroborate (165). The reaction occurs with predominant retention of configuration at sulfur with dialkyl and alkyl aryl sulfoxides. However, its stereospecificity is strongly dependent on the nature of substituents in the starting sulfoxide. In the case of diaryl sulfoxides this method failed to give chiral aminodiarylsulfonium salts. 

The main methodologies developed until now for enantioselective oxidation of sulfides are effective only in the oxidation of alkyl aryl sulfoxides. Dialkyl sulfoxides on the other hand are generally oxidized with only poor selectivity. In an attempt to solve this problem, Schenk s group69 recently reported a stereoselective oxidation of metal-coordinated thioethers with DMD. The prochiral thioether is first coordinated to a chiral ruthenium complex by reaction with the chloride complexes [CpRu[(S,S)-chiraphos]Cl], 36. Diastereoselective oxygen transfer from DMD produces the corresponding sulfoxides in high yield and selectivity. The chiral sulfoxides 37 are liberated from the complexes by treatment with sodium iodide. Several o.p. aryl methyl sulfoxides have been obtained by this method in moderate to high ee (Scheme 12). 

The typical S-oxidation with BVMOs allows the formation of chiral sulfoxides from organic sulfides. This oxidation has received much interest in organic chemistry due to its use in the synthesis of enantiomerically enriched materials as chiral auxiliaries or directly as biologically active ingredients. This reaction has been studied extensively with CHMO from Adnetohacter showing high enantioselectivi-ties in the sulfoxidation of alkyl aryl sulfides, disulfides, dialkyl sulfides, and cychc and acyclic 1,3-dithioacetals [90]. CHMO also catalyzes the enantioselective oxida-hon of organic cyclic sulfites to sulfates [91]. 

Uemura described use of a Ti(OiPr)4/(i )-BINOL complex for the oxidation of alkyl aryl sulfides with aqueous ferf-butyl hydroperoxide as stoichiometric oxidant [22]. At room temperature p-tolyl methyl sulfide was converted into the corresponding sulfoxide with 96% ee in 44% yield with as little as 5 mol % of the chiral ligand. The reaction is insensitive to air, while the presence of water seems to be essential for the formation of the catalytically active species, long catalyst lifetime, and high asymmetric induction. The authors observed a large positive non-linear effect which indicates that the actual catalyst consists of a titanium species with more than one (K)-BINOL ligand (11) coordinated to the metal. 

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... 

Asymmetric ene reaction of N-sulfinylcarbamatesf The ability of Lewis acids to promote ene reactions (11,413,414 12,389) is useful for asymmetric reactions. Thus the SnCU-promoted reaction of chiral N-sulfinylcarbamates (1) with alkenes results in thermally unstable adducts (2) in 65-91% yield. Use of trans-2-phenylcyclohexanol (13,244) or 8-phenylmenthol as the source of chirality results in high diastereoselective induction in generation of the new carbon to sulfur bond (usually >95 5). This reaction is applicable to both (E)- and (Z)-alkenes, but the former react more readily. These ene adducts can be transformed into optically active allylic alcohols (4) by N-alkylation and conversion to an aryl allylic sulfoxide (3), which undergoes rearrangement in the presence of a thiophile (piperidine) to 4, with retention of configuration at carbon imparted in the ene reaction. The overall process effects enantioselective allylic oxidation of an alkene with retention of the original position of the double bond. 

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