Sulphoxides elimination reactions

The planar pericyclic mechanism which is accepted for the sulphoxide-elimination reaction is supported by the temperature-dependence of the relative reaction rates for n-C7HuSOPh and its / - H-analogue. ... 

Dimethyl sulphoxide has recently been used for tosylate elimination reactions [131 ], It is not clear whether this is a simple Ei process. The nucleophilic reactivity of dimethyl sulphoxide (p. 46) suggests the possibility of an indirect mechanism, with preliminary Sn2 substitution of the tosylate group by the oxygen atom of the reagent. This would permit elimination of an equatorial tosylate via the more favourably oriented axial oxysulphonium ion (c/. p. 47). 

The configurations, conformations, chiroptic properties, and pyrolytic elimination reactions of steroidal sulphoxides and sulphinates, at C-3, C-6, and C-7, have been studied in detail. In general, though not without exception, the direction of elimination is controlled by the configuration at the sulphur atom, whenever two possibilities exist for the essential syn mechanism. The favoured... 

Fig, 4. Variation of the primary kinetic isotope effect in the elimination reaction of 2-phenylethyl-dimethylsulphonium bromide with hydroxide ion as the solvent composition is changed from water to 84% dimethyl sulphoxide. (A similar profile has recently been obtained for the elimination reaction of 2-phenylethyltrimethylammonium ion under the above reaction conditions " ). (Reproduced with permission from Cockerin ".)... 

Elimination Reactions of Aliphatic Sulphoxides.— The general process R S(O)-CR R CR R H R R C=CR R + R SOH represents the area in which current activity is mostly concentrated, though an interesting variation ... 

A very interesting approach to optically active sulphoxides, based on a kinetic resolution in a Pummerer-type reaction with optically active a-phenylbutyric acid chloride 269 in the presence of /V,A -dimethyIaniline, was reported by Juge and Kagan332 (equation 149). In contrast to the asymmetric reductions discussed above, this procedure afforded the recovered sulphoxides in optical yields up to 70%. Chiral a, /1-unsaturated sulphoxides 270 were prepared via a kinetic resolution elaborated by Marchese and coworkers333. They found that elimination of HX from racemic /i-halogenosulphoxides 271 in the presence of chiral tertiary amines takes place in an asymmetric way leading to both sulphoxides 270 and 271, which are optically active (optical yields up to 20%) with opposite configurations at sulphur (equation 150). 

Since a great number of such transformations were described in the chemical literature, only selected examples of general importance will be presented here. This section will consist of the following parts reactions of the sulphoxide a-carbanions introduction, substitution, transformation and elimination of heteroatomic groups attached to organic substituents in sulphoxides additions to unsaturated sulphoxides other modifications of organic substituents in sulphoxides. 

On the other hand, in the case of a-halogenoethyl sulphoxides 503 an SN2-type displacement occurs with mercaptide anions and leads to a-alkylthioethyl sulphoxides 504, while the elimination-addition mechanism is operative with alkoxide anions, affording jS-alkoxyethyl sulphoxides577,596 505 (equation 306). Finally, the reaction of 1-halogeno-l-methylethyl derivatives with both nucleophiles mentioned above occurs via the elimination-addition mechanism596 (equation 307). The substitution reaction can also take place intramolecularly (equation 308) and it proceeds very easily (cf. Section IV.A.2.C)484,600. 

Hydroxyalkyl sulphoxides 515 can be dehydrated either by treatment with phosphoric acid (equation 315) or by the alkylation with Mel in the presence of an excess of sodium hydride611 (equation 316). For other dehydration reactions see References 475 and 505 (Section IV.A.2.d). For elimination of amines see References 164 and 529 (Section IV.A.2.e). 

Halogenovinyl sulphoxides 551 react with nucleophiles to give -substituted vinyl sulphoxides 552. The first step in the reaction is a Michael addition, followed by an elimination of a halide anion605,627 (equation 351). 

For a recent discussion on the stereochemical aspects of the Diels-Alder reaction with vinyl sulphoxides see References 662, 663. It should be pointed out that vinyl sulphoxides can be considered in [2 + 4]-cycloadditions as acetylene synthons since the sulphinyl moiety may be removed from the product by sulphenic acid elimination. Paquette and coworkers took advantage of this fact in the synthesis of properly substituted anthracenes 562664, (equation 360). 

Tertiary amine N-oxides may also be used to convert sulphoxides to sulphones16. The reaction proceeds by initial attack by the N-oxide oxygen atom on the sulphoxide moiety, followed by subsequent elimination of the amine. In order to obtain good yields, the reaction must be carried out at 190°Cfor 20 hours with a 20-fold excess of N-oxide in the presence of acid catalysts. The sulphone must then be separated by chromatography, thus making the method less attractive than other procedures and so it has not been employed synthetically. 

Heterocyclic sulphoxides 65 mass spectra of 130-132 Hexahydronaphthalenols, synthesis of 310 Hofmann elimination 953 HOMO energies 1048, 1049 Homolytic substitution 1109 intramolecular 846 Horner-Wittig reaction 333 Hot electrons 892, 893 HSAB theory 282, 549 Hydrides, as reducing agents 934-941, 959 Hydrogen abstraction, photochemical 874, 876, 877, 879, 880... 

Reaction of the carbanion of chloromethyl phenyl sulphoxide 409 with carbonyl compounds yields the corresponding 0-hydroxy adducts 410 in 68-79% yield. Each of these compounds appears to be a single isomer (equation 242). Treatment of adducts 410 with dilute potassium hydroxide in methanol at room temperature gives the epoxy sulphoxides 411 (equation 243). The ease of this intramolecular displacement of chloride ion contrasts with a great difficulty in displacing chloride ion from chloromethyl phenyl sulphoxide by external nucleophiles . When chloromethyl methyl sulphoxide 412 is reacted with unsymmetrical ketones in the presence of potassium tcrt-butoxide in tert-butanol oxiranes are directly formed as a mixture of diastereoisomers (equation 244). a-Sulphinyl epoxides 413 rearrange to a-sulphinyl aldehydes 414 or ketones, which can be transformed by elimination of sulphenic acid into a, 8-unsaturated aldehydes or ketones (equation 245). The lithium salts (410a) of a-chloro-/ -hydroxyalkyl... 

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