Unsaturated Sulphoxides

Unsaturated Sulphoxides.—Variations of known methods for the synthesis of vinyl sulphoxides are described in reports of asymmetric selectivity in the elimination of HCl from jS-halogenoethyl aryl sulphoxides using an optically active base, leading to partially resolved vinyl sulphoxides, and in reports of the condensation of carbonyl compounds with (Et0)2P(0)CHaS(0)R in the presence of a phase-transfer catalyst. Vinyl sulphoxides are formed when sulphenic acids are trapped after generation in the presence of an acetylenic ester, and a full account is available of the trapping of the sulphenic acid of 

Oikawa, T. Kurosawa, and O. Yonemitsu, Ghent. andPharm. Bull. Japan ), 1975, 23, 2466. 

Apart from one citation, all the papers collected here to illustrate the reactions of vinyl sulphoxides deal with addition reactions. As would be expected from the nature of the sulphoxide functional group, there are several novel features in these addition reactions compared with those of other vinyl compounds. Pyrolysis of alkyl styryl sulphoxides gives benzothiophens. The proposed mechanism, which involves formation and homolysis of a sulphenic acid followed by cyclization, (54) (55), is supported by the formation of benzo- 

Miyamoto, D. Fukuoka, K. Utimoto, and H. Nozaki, Bull. Chem. Soc. Japan, 1974, 47, 

Asymmetric induction is a factor in nucleophilic and electrophilic additions to a chiral vinyl sulphoxide. A full account is available of asymmetric syntheses involving the addition of piperidine and of bromine to chiral propenyl and vinyl p-tolyl sulphoxides, respectively (see Vol. 2, p. 50). An observation enhancing the value of a vinyl sulphoxide enantiomer in organic synthesis concerns the control exerted by the metal counterion in Michael addition of the diethyl malonate carbanion to (/ )-(+)-/ra/w-)S-styryl p-tolyl sulphoxide (58). The K-i- and Na salts in EtOH lead to a diastereoisomer mixture in which (59) predominates, while the Li+ salt in THF as the reaction medium leads to a preponderance of the epimeric adduct.  

Unsaturated Sulphoxides.—Addition of an alkynyl-lithium to an organo-borane R3B gives (RJBC CR )Li, which with methanesulphinyl chloride gives an unstable vinyl sulphoxide RlBCR =CR SOMe, which gives the alkyne R C CR the net result being the anti-Markownikov reductive alkynylation of an alkene, since RsB is prepared by hydroboration of an alkene.  

Substantial applications in synthesis have been established for (jR)-(+)-trans-styryl p-tolyl sulphoxide (63), which gives (64) on Michael reaction with diethyl malonate on the basis that the most stable conformation of the a-sulphinyl carbanion has the lone pair on carbon trans to sulphinyl oxygen in a polar solvent. The addition of a proton to (64) gives as the major (51%) 

Irradiation of methyl styryl sulphoxide or the corresponding sulphide in alcohols or AcOH gives addition products PhCH(0R)CH2S0Me.  

A detailed study of the mechanism of the allyl sulphoxide-sulphenate rearrangement and its uses in synthesis has been reported. Rates of steroidal allyl sulphoxide rearrangements are influenced by chirality at sulphur, and configurational aspects established for compounds of this 

The fact that selenoxides are converted into alkenes under much milder conditions than the corresponding sulphoxides has stimulated several workers to study methods for the introduction of a suitable selenium grouping into various aliphatic substrates. Reduction of diphenyl diselenide 

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TABLE 12. Synthesis of E-a, / -unsaturated sulphoxides, RISfO)CH=CHR2, by addition of sulphenic adds, R OH, to alkynes, R2C=CH... 

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). 

Further utility of the Andersen sulphoxides synthesis is demonstrated by the preparation of optically active unsaturated sulphoxides which were first prepared by Stirling and coworkers359 from sulphinate 276 and the appropriate vinylic Grignard reagents. Later on, Posner and Tang360 prepared in a similar way a series of ( )-l-alkenyl p-tolyl sulphoxides. Posner s group accomplished also the synthesis of (+)-(S)-2-(p-tolylsulphinyl)-2-cyclopentenone 287, which is a key compound in the chiral synthesis of various natural products361 (equation 159). 

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. 

Reaction of thiobenzophenone with chloromethyl methyl sulphoxide 412 does not give the expected 2,2-diphenyl-3-methylsulphinylthiirane 417, but the a, /1-unsaturated sulphoxide 418 in a 38% yield485 (equation 248). 

Sulphoxides also undergo Mannich-type condensation when reacted with aldehydes and secondary amines or their salts. In some cases, stable Mannich bases 462 can be isolated. They undergo amine elimination upon heating to give the corresponding a,/ -unsaturated sulphoxides 463164,529 (equation 276). 

Conjugate addition of enolate anions to a, jS-unsaturated sulphoxides followed by a sulphoxide- ketone transformation were used for the preparation of 1,4-dicarbonyl compounds and cyclopentenone derivatives (equation 355)648. 

Sulphines may react as dienophiles with 1,3-dienes with the formation of cyclic sulphoxides. Unstable 2,2-dichloro-5,6-dihydro-2ff-thiin-l-oxide 191 was formed in an exothermic reaction between 173aandcyclopentadieneat — 40 (equation 101). The simplest, parent sulphine, CH2 = S = O, prepared in situ by treatment of a-trimethylsilylmethanesulphinyl chloride with cesium fluoride, reacts with cyclopentadiene to give bicyclic, unsaturated sulphoxide 192 as a mixture of two diastereoisomers in a 9 1 ratio (equation 102). On the other hand, a,j8-unsaturated sulphine 193 (generated by thermolysis of 2-benzylidene-l-thiotetralone dimer S-oxide) in boiling toluene behaves as a 1,3-diene and was trapped by norborene forming sulphoxide 194 in 78% yield ° (equation 103). 

The Horner-Wittig reaction of a-phosphoryl sulphoxides 442, which are chemically stable, results in the formation of a, -unsaturated sulphoxides 443 in high yields (equation 264). The reaction has been found to be non-stereoselective, mixtures of E and Z isomers being formed from aldehydes and unsymmetrical ketones . In the case of aromatic aldehydes this reaction can also be advantageously performed in a two-phase catalytic system even without the usual PTC catalysts (Table 24). Intramolecular Horner-Wittig reaction of a-phosphoryl-5-oxosulphoxides 444 leads to a, -unsaturated cyclic sulphoxides 445 (equation 265). Starting from optically active 0,0-... 

The stereospecific base-cleavage of the trimethylsilyl group in 1,3-dithiane 1-oxides 499 enables to obtain the specifically deuteriated products 500 (equation 303), A nitro group in y-nitroalkyl sulphoxides 501 (obtained by the Michael addition of nitroalkanes to a, j8-unsaturated sulphoxides) is replaced by hydrogen by means of tributyltin hydride (equation 304). This reagent does not affect the sulphinyl function. The overall procedure provides an efficient method for the conjugate addition of alkyl groups to a, -unsaturated sulphoxides . ... 

Organic hydroperoxides have also been used for the oxidation of sulphoxides to sulphones. The reaction in neutral solution occurs at a reasonable rate in the presence of transition metal ion catalysts such as vanadium, molybdenum and titanium - , but does not occur in aqueous media . The usual reaction conditions involve dissolution of the sulphoxide in alcohols, ethers or benzene followed by dropwise addition of the hydroperoxide at temperatures of 50-80 °C. By this method dimethyl sulphoxide and methyl phenyl sulphoxide have been oxidized to the corresponding sulphone in greater than 90% yields . A similar method for the oxidation of sulphoxides has been patented . Unsaturated sulphoxides are oxidized to the sulphone without affecting the carbon-carbon double bonds. A further patent has also been obtained for the reaction of dimethyl sulphoxide with an organic hydroperoxide as shown in equation (19). 

Both inorganic and organic hypochlorites may be used for the oxidation of sulphoxides. The cheapest method involves the use of a commercial bleach, such as Chlorox . Such a method is indeed successful for unsaturated sulphoxides such as allyl methyl sulphoxide although the yields are generally low. Other sulphoxides may also be oxidized by this method, for example, dimethyl sulphoxide gave bis(trichloromethyl) sulphone in low yield . In some cases bis(dichloromethyl) sulphone was also isolated in very low yield. This oxidation procedure is also commonly used by organosulphur chemists for the removal of unwanted odours, caused by sulphoxides (and sulphides), from dirty glassware. 

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