Sulfoxide, methyl p-tolyl

Cyclodextrins, toroidal molecules composed of 6, 7 and 8 D-glucose units, are now commercially available at reasonable cost. They form inclusion compounds with a variety of molecules and often differentially include sulfoxide enantiomers29,30. This property has been used to partially resolve some benzyl alkyl, phenyl alkyl and p-tolyl alkyl sulfoxides. The enantiomeric purities after one inclusion process ranged from 1.1 % for t-butyl p-tolyl sulfoxide to 14.5% for benzyl r-butyl sulfoxide. Repeating the process on methyl p-tolyl sulfoxide (10) increased its enantiomeric purity from 8.1% to 11.4% four recrystallizations raised the value to 71.5%. The use of cyclodextrins in asymmetric oxidations is discussed in Section II.C.l and in the resolution of sulfmate esters in Section II.B.l. 

Schneider and Simon82 prepared / -ketosulfoxides 47a and 47b by sulfinylation of the dianions of the methyl acetoacetates 48a and 48b with sulfinate ester 19 followed by decarboxylation of the intermediate products (Scheme 2). Apparently this avoids racemiz-ation experienced by others in the direct synthesis of these compounds9. /J-Ketosulfoxides are also available from the reaction of the anion derived from methyl p-tolyl sulfoxide with esters (see Section II.E). They can also be obtained, in some cases, through the hydrolysis of a-sulfinylhydrazones whose synthesis is described below. Mention has already been made of the synthesis of 2-p-tolylsulfinylcycloalkanones such as 32. 

An optically active sulfoxide may often be transformed into another optically active sulfoxide without racemization. This is often accomplished by formation of a new bond to the a-carbon atom, e.g. to the methyl carbon of methyl p-tolyl sulfoxide. To accomplish this, an a-metallated carbanion is first formed at low temperature after which this species may be treated with a large variety of electrophiles to give a structurally modified sulfoxide. Alternatively, nucleophilic reagents may be added to a homochiral vinylic sulfoxide. Structurally more complex compounds formed in these ways may be further modified in subsequent steps. Such transformations are the basis of many asymmetric syntheses and are discussed in the chapter by Posner and in earlier reviews7-11. 

Only relatively few nucleophilic substitution reactions at sulfur proceed with retention. Oae found that (R)-(+)-methyl p-tolyl sulfoxide exchanged 180 with dimethyl sulfoxide at 150 °C much faster than it racemized thus, the exchange took place with retention. A cyclic intermediate, 136, was proposed to account for this behavior12,147. The same sulfoxide was found to react with N, JV -ditosylsulfurdiimide, 137, with either retention or inversion depending on the reaction conditions. Christensen148 observed retention in benzene whereas Cram and coworkers149 found that inversion took place in pyridine. A four-membered ring intermediate, 138, was postulated to account for the retention, whereas a... 

Kagan and coworkers determined and studied in detail the crystal structure of a 1 1 molecular complex between (i )-methyl p-tolyl sulfoxide (10) and S)-N-(3,5-dinitrobenzoyl)-l-phenylethylamine (11). They suggested that amide 11, which had been used as a chiral solvating agent for sulfoxides, might find use as a resolving agent for these compounds. ... 

Carbanions derived from optically active sulfoxides react with esters, affording generally optically active )S-ketoesters ° . Kunieda and coworkers revealed that treatment of (-t-)-(R)-methyl p-tolyl sulfoxide 107 with n-butyllithium or dimethy-lamine afforded the corresponding carbanion, which upon further reaction with ethyl benzoate gave (-l-)-(R)-a-(p-tolylsulfinyl)acetophenone 108. They also found that the reaction between chiral esters of carboxylic acids (R COOR ) and a-lithio aryl methyl sulfoxides gave optically active 3-ketosulfoxides The stereoselectivity was found to be markedly influenced by the size of the R group of the esters and the optical purity reached to 70.3% when R was a t-butyl group. 

No stereoselectivity was observed in the formation of a 1 1 diastereomeric mixture of 2-hydroxy-2-phenylethyl p-tolyl sulfoxide 145 from treatment of (R)-methyl p-tolyl sulfoxide 144 with lithium diethylamide . However, a considerable stereoselectivity was observed in the reaction of this carbanion with unsymmetrical, especially aromatic, ketones The carbanion derived from (R)-144 was found to add to N-benzylideneaniline stereoselectivity, affording only one diastereomer, i.e. (Rs,SJ-( + )-iV-phenyl-2-amino-2-phenyl p-tolyl sulfoxide, which upon treatment with Raney Ni afforded the corresponding optically pure amine . The reaction of the lithio-derivative of (-t-)-(S)-p-tolyl p-tolylthiomethyl sulfoxide 146 with benzaldehyde gave a mixture of 3 out of 4 possible isomers, i.e. (IS, 2S, 3R)-, (IS, 2R, 3R)- and (IS, 2S, 3S)-147 in a ratio of 55 30 15. Methylation of the diastereomeric mixture, reduction of the sulfinyl group and further hydrolysis gave (—)-(R)-2-methoxy-2-phenylacetaldehyde 148 in 70% e.e. This addition is considered to proceed through a six-membered cyclic transition state, formed by chelation with lithium, as shown below . ... 

Esterification of p-toluenesulfinic acid with (- )-menthol gives a mixture of two diastereomers, which equilibrate to the pure (-)-menthyl (S)-p-toluenesulfinate diastereomer in the presence of hydrochloric acid (80% yield). The report includes an improved procedure for reaction of 1 with CH,MgI to give (R)-( + )-methyl p-tolyl sulfoxide.1... 

If for some special reason the entering and leaving groups are forced into a situation where one must occupy a radial position and the other an apical one, the reaction will proceed with retention of configuration at the sulfinyl group. The exchange of oxygen-18 between dimethyl sulfoxide and l80-labeled, optically active methyl p-tolyl sulfoxide (153) (Oae et al., 1968) would appear... 

The optically active sulfoxides 39 and 40 were formed in optical yields up to 20% and with opposite configurations at sulfur. The reaction of the carbanion derived from racemic methyl p-tolyl sulfoxide (41) with 0.5 molar equivalent of (-)-menthyl carboxylates 42 was found (70) to occur asymmetrically to give the corresponding optically active -ketosulfoxide 43 and methyl p-tolyl sulfoxide 41, having opposite configurations at sulfur. The degree of stereo-... 

Similar differentiation between enantiomers by means of NMR can also be achieved by the use of chiral lanthanide shift reagents (243). Tris-[3-(heptafluoropropylhydroxymethylene)-d-camphorato] -europium was used for the first time (244) for determining the enantiomeric content of benzyl methyl sulfoxide 34. The enantiomeric composition of the partially resolved methyl p-tolyl sulfoxide 41 was estimated using tris-[3-(r-butylhydroxymethylene)-c -camphorato]-europium (245). Another complex of europium, tris-[3-(trifluoro-methylhydroxymethylene)-c -camphorato] europium (TFMC), in contrast to those mentioned above, was effective in the differentiation of various enantiomeric mixtures of chiral sulfinates (107), thiosul-finates (35), and sulfinamides (246). 

All diaryl sulfoxides, some alkyl aryl sulfoxides (e.g., methyl p-tolyl sulfoxide, 41), and some dialkyl sulfoxides (e.g., methyl 1-adamantyl sulfoxide), were found to undergo racemization according to the pyramidal inversion mechanism. Mislow and co-workers (248) have found that for most of the compounds investigated, irrespective of the nature of the substituents attached to sulfur, the first-order rate constant for racemization in p-xylene at 210 C is about 3 X 10" sec", corresponding to a half-life of about 6 hr. Moreover, the activation parameters do not show significant differences and their values were contained in a narrow range 35 to 42 kcal/mol and... 

AS -8 to +4 e.u. Even though the racemization rate constants differ slightly, their distinct dependence on the steric and to a lesser extent on the electronic effects of the substituents bonded to the sulfinyl sulfur atom was noted. It deserves adding that the activation volume for racemization of methyl p-tolyl sulfoxide 41, A F 0 ml/mol, is also consistent with the pyramidal inversion mechanism (249). 

Addition of the lithium salt 299 obtained from chiral methyl p-tolyl sulfoxide 41 to carbonyl and imino systems as well as to epoxides was thoroughly studied by Tsuchihashi and co-workers. 

Both enantiomers of methyl p-tolyl sulfoxide are available from the above procedure by selection of the appropriate diethyl tartrate. This procedure describes the preparation of (S)-(-)-methyl p-tolyl sulfoxide which is not easy to prepare by the Andersen method " using (+)-raenthol. 

The enantiomeric purity of the crude (S)-methyl p-tolyl sulfoxide produced from the oxidation is close to 90% (measurement made on a sample of 200 mg of material purified by flash chromatography on silica gel, eluent ethyl acetate - ethanol = 96 4). However, when oxidation is performed on a... 

S. H. Zhao, 0. Samuel, and H. B. Kagan 49 ENANTIOSELECTIVE OXIDATION OF A SULFIDE (S)-(-)-METHYL p-TOLYL SULFOXIDE... 

Little work has been done in this area with Ru catalysts. CM-A-[Ru(0)(py)(bpy)3] V water-CH3CN was used for the stoicheiometric oxidation of methyl-p-tolyl sulfide to (R)-methyl-p-tolyl sulfoxide with an e.e. of ca 15% [121],... 

Enantioselective Oxidation of Sulfides Synthesis of (-)-(S(-Methyl p-Tolyl Sulfoxide... 

A quite general method of access to optically pure sulfoxides is due to Andersen [96-98] a menthyl sulfinate ester is reacted with a Grignard reagent. Both enantiomers of menthyl p-toluenesulfinate are commercially available. A large-scale preparation of (-)-menthyl (S)-p-toluenesulfinate as well as that of (7 )-(+)-methyl p-tolyl sulfoxide is described [99] by Solladi et al. Other related approaches are presented and discussed in [86]. 

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