Sulphonium salt

Sulphonium Salts.—Alkoxysulphonium salts ROSRg X and aminosulphonium salts are reactive intermediates, and were mentioned in this context in the 

The absolute configuration of a chiral trialkylsulphonium ion has been established 75 years after the first preparation of such species. (+)-MethyIethyl-n-propylsulphonium 2,4,6-trinitrobenzenesulphonate has the (5)-configuration (35), which was established through correlation with lactic acid and confirmed by X-T y analysis. 

Mechanistic studies with sulphonium salts concern decomposition kinetics in hydroxylic solvents H- H exchange of the a-protons of methyl and allyl-sulphonium salts, the relative rates of which are strongly dependent upon both solvent and micellar effects radical chain reactions of triarylsulphonium halides and sodium alkoxides, which lead to arenes, anisoles, diaryl sulphides, and aldehydes or ketones and the catalysis of hydrocarbon autoxidation by 

Kobayashi, H. Minato, J. Fukui, and N. Kamigata, Bull. Chem. Soc. Japan, 1975,48, 729. H. Minato, T. Miura, F. Takagi, and M. Kobayashi, Chem. Letters, 1975, 211. 

This short section is located immediately before coverage of sulphonium salts, a group of compounds most importantly represented in naturally occurring compounds. Possible roles for S -adenosyl-L-methionine have stimulated studies with model compounds similar studies may follow from the fact that Bleomycin Aj is a sulphonium salt, carrying the grouping —CONH(CH2)3 Me2. 5-(Dimethylsulphonio)pentanoic acid has been identified as a constituent of Diplotaxis tenuifolia  

Block s book lists a number of simple organosulphur compounds which occur naturally, and further examples include 5-methylthiomethyl 2-methylbutanethioate, MeCH2CHMeC(0)SCH2SMe (a new addition to the 24 organosulphur compounds already isolated from the essential oil of hops), and methyl l-(l-methylthioethyl)-thioethyl disulphide, a constituent of roast pork (see p. 21). 

The opportunity has been taken to bring together recent papers on trialkyl-and triaryl-sulphonium salts with those covering 5-heteroatom-sulphonium salts, R R SX Y (X = halogen, NR R OR , or SR ), the latter class having been mostly covered in previous volumes of this series in the sections on sulphides and sulphoxides. 

Trialkyl- and Triaryl-sulphonium, -selenonium, and -teOuronium Salts.—Some modifications of known preparative methods are summarized, with references, at the end of this section. Interesting practical procedures have been described for the 

Reactions of Organosulfur Compounds , Academic Press, New York, 1978. 

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Thioethers usually yield sulphonium salts when warmed with ethyl iodide and allowed to cool. The physical properties (b.p., density and refractive index) are useful for identification purposes. 

A detailed study revealed that sulphides may react with nitric acid to give sulphoxides, sulphones and their nitro derivatives54. However, under suitable conditions the nitric acid oxidation of sulphides leads to a selective formation of sulphoxides. This is probably due to the formation of a sulphonium salt 30 which is resistant to further oxidation50 (equation 12). 

Few 1 -benzothiophene-S-oxides 218 were obtained in moderate yields by treatment of 1-arylacetylenes 219 with sulfur dioxide and benzene in the presence of antimony pentafluoride250 (equation 127). A series of cyclic sulphoxides have been prepared by hydrolysis of the corresponding alkoxy sulphonium salts 220251-254 (equation 128). Syn-sulphoxide 221 was obtained in a low yield (15-20%) in the reaction of the dianion of cyclooctatetraene 222 with thionyl chloride255 (equation 129). 

Optically active sulphoxides 311 and 312 have been prepared stereospecifically either by hydrolysis of the optically active sulphonium salt 313 or by the reaction of p-tolyl magnesium bromide with optically active sulphinate 314, respectively377 (equations 167 and 168). 

Massardier, V. and Vialle, J., Investigation of liquid chromatographic systems for the separation of sulphonium salts, Anal. Chim. Acta, 281, 391, 1993. 

By contrast, O in (43) is sufficiently electronegative not to donate an electron pair (unlike Oe in ROe and RCO20 above), and hydrolysis of EtOCH2CH2Cl thus proceeds via ordinary SN2 attack by an external nucleophile—which is likely to be very much slower than the internal nucleophilic attack in (42) — (44). That a cyclic sulphonium salt such as (44) is involved is demonstrated by the hydrolysis of the analogue (45), which yields two alcohols (the unexpected one in greater yield) indicating the participation of the unsymmetrical intermediate (46) ... 

Cotton treated with bis(2-isocyanatoethyl) disuphide in dimethylformamide at 80 °C, followed by reduction with tri-n-butylphosphine in methanol containing 10% water. This gives the 2-mercaptoethylcarbamyl ester, which is treated with methyl iodide to form sulphonium salts. (Scheme 10.66). 

Polyphosphates and phosphates have also been obtained under phase-transfer catalytic conditions by nucleophilic displacement reactions on haloalkanes, tosyl-oxyalkanes and sulphonium salts by polyphosphate or phosphate anions [e.g. 7, 11-15]. The procedure has been used with success for the phosphorylation of terpenes [11] and nucleosides [12, 13]. 

Reduction of sulphonium salts polarographic half-wave potentials, Ey. ref. [54], in water cyclic voltammetry peak potentials, Ep ref. [55], in acetonitrile at glassy carbon, scan rate 50 mV s. ... 

The reductive cleavage of sulphonium salts in aprotic solvents leads to the generation of radical and then carbanions in a further electron transfer step. Protonation of the carbanion by extraneous water leaves a hydroxide ion. Basic species formed in this way can abstract a proton from sulphonium ion to give the ylid, which is not reducible. A good example is the reduction of 9 in dimethylsulphox-ide, which consumes only one Faraday and follows the course shown [58]. 

The reported 2 e reductive cleavage of phenylsulphonylacetonitrile (PhSO CH CN) and the observation that in protic media the products were PhSO and CH CN, suggested that this reaction could be a useful source of CHjCN. However, careful re-examination showed that in acetonitrile solution the reaction is pseudo one-electron, analogous to the phosphonium and sulphonium salts (Scheme 8), and that phenylsulphonylacetonitrile is sufficiently acidic rapidly to protonate "CHjCN assuming additivity of substituent effects an estimate of pK 14-16 was made for PhSOjCHjCN, cf. pK 31 for CH,CN... 

Derivatives of the general formula (7) in Table 6 have been successfully used as probases and their properties in this context are being further explored. In common with the azobenzenes and ethenetetracarboxylate esters, the fluoren-9-ylidene derivatives usually display two reversible one-electron peaks in cyclic voltammetric experiments. Although disproportionation is possible (cf. Scheme 12) it is the dianions which are the effective bases. It was shown early on that the radical-anions of such derivatives are long-lived in relatively acidic conditions (e.g. in DMF solution the first reduction peak of Ph C -.QCN) remains reversible in the presence of a 570-fold molar excess of acetic acid, at 0.1 V s ). Even the dianions are relatively weak bases, useful mainly for ylid formation from phosphonium and sulphonium salts (pKj s 11-15) they are not sufficiently basic to effect the Wittig-Homer reaction which involves deprotonation of phosphonate esters... 

Table 1. Ion pair dissociation constants, Kd, of sulphonium salts at 20° C (52)...

Additional evidence for this mechanism is provided by the observation that whereas simple trialkylsulphonium salts R3S+X, are poor initiators for the polymerisation, 2-thia-alkyl sulphonium salts give rise to polymerisations having characteristics similar to the second stage described previously (53), i.e. 

The chirality exhibited by the pyramidal sulphur compounds (e.g. sulphonium salts, sulphoxides and sulphinic esters) should be noted, but cannot be considered within the scope of this book. 

Benzylideneaniline (18.1 g, 0.1 mol) and tetrabutylammonium hydrogen sulphate (0.5 g, 1.35 mol) are dissolved in dichloromethane (100 ml) and a layer of 50 per cent aqueous sodium hydroxide introduced under this solution. Trimethylsulphonium iodide (20.4 g, 0.1 mol) is then added and the whole warmed at 50 °C with vigorous stirring for 2 hours, whereupon the originally undissolved sulphonium salt disappears. The mixture is poured on to ice, the organic phase separated, washed with water and dried. The solvent is evaporated and the residue distilled under reduced pressure to afford 1,2-diphenylaziridine (94%), b.p. 120°C/0.05mmHg. 

The latter on treatment with thallium trifluoroacetate, under carefully defined conditions followed by aqueous work-up, led directly to narwedine (291) [74] in 51% yield. The remarkable transformation is believed to occur via the cationic intermediate 292, formed by an oxidative process, undergoing a 1,2 C-C shift to generate the carbenium ion 293. Aromatisation involving the cleavage of C-Pd bond, followed by hydrolysis of the resulting sulphonium salt 294 liberated the phenol, which smoothly underwent Michael addition to the proximate enone system, furnishing ( )-narwedine (291) in 44% overall yield. 

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