Sulphonium salts formation

Takano s approach involves sulphonium salt formation on the... 

Ester and sulphonium salt formation in the active-site labelling of jS-D-galacto-sidase from E. coli by 2,6-anhydro-l-deoxy-l-diazo-D- /ycera-L-manno-heptitol has been examined.2,6-Anhydro-l-deoxy-l-diazo-D- /yccro-L-wflnno-heptitol... 

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). 

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... 

To the first category belongs the photochemical formation of carbenhim ions or protonic acids directly in the polymerization medium this field, discussed in Sect. 3.1 and 3.2 has recently been reviewed by Smets at the lUPAC Macronmlecular Sympo-sium2 ) When cyclohexene oxide is used as a monomer the order of reactivities for iodonium or sulphonium salts, giving photochemically protonic acid, depend on the structure of anion MtX in the following way ... 

Although sulphonium salts (244) are known to react with a,)9-unsaturated ketones with formation of the corresponding oxiranes several a,)9-unsaturated ketones , esters and lactoneshave been converted into cyclopropanes by treatment with these reagents (244) (equation 74). Also diphenyl- and benzylidenesulphuranes (245, 246)... 

Most recently a rather similar study has been carried out on the polymerization of propene sulphide [137]. This system, however, is kinetically more complicated because of the formation of a relatively stable twelve membered ring sulphonium ion, which can apparently slowly re-initiate a second stage polymerization. A comparative study of various substituted thietans has been reported and confirms that the rates and the equilibria of reactions between sulphides and sulphonium salts depend upon the difference in basicity of the reacting and product sulphides. 

Irradiation of sulphonium salts (308) in acetonitrile solution has been shown to lead to lateral-nuclear rearrangement and the gg formation of l-(methylthio)-2-substituted alkylnaphthalenes (309). The quantum yield of product formation varies between 0.24 and 0.1 but bond cleavage to yield 1-naphthyl methyl sulphide also occurs and MeCONH-CHg-R is a by-product of the migrating group and the solvent. The steric crowding in r.tris(trimethylsilyl )methyl] benzene (310) is considered to be the origin of its photolability and on irradiation it efficiently yields (311), (312), and five... 

The photolysis of triarylsulphonium salts yields diarylsulphides and products of lateral nuclear shift reactions which are ortho, meta, and para aryl substituted diaryldisulphides such as (342). A by-product in these reactions is a proton because of this these reactions have been applied to photoinitiation of cationic polymerisations. A full paper describing a detailed study of the reaction mechanism has been published.In addition, the product distribution obtained by photolysis of triphenylsulphonium salts in films of the polymer of 4-(tert -butoxycarbonyloxy)styrene has been compared with that obtained in solution.The synthesis of some new triarylsulphonium salts and their application for photoinitiation of cationic polymerisation has also been reported.The formation of the products arising from lateral nuclear shifts in sulphonium salts occurs under direct photolysis but not under triplet sensitisation. 

The magnitude of the effect for the elimination reactions indicates a high degree of bond formation between the base and the proton in the transition state. Proton transfer is extensive in the ammonium compound and the proton is just more than half transferred to the base for the sulphonium salt. These... 

Synthesis of Monosaccharide Glycosides.- A very useful review has appeared on newer methods for glycoside formation, particularly those based on 0-alkylation and on the use of glycosyl trichloro-acetimidates, sulphonium salts and fluorides. A shorter paper has... 

The sulphonium salts used in the Stevens rearrangement need not be prepared initially from a thiol however, this is feasible and is often the case. This method, therefore, allows the conversion of a thiol to a sulphonium salt, followed by rearrangement with concomitant carbon—carbon bond formation. Removal of the sulphur moiety following rearrangement permits, in efiect, a thiol to function in a reaction that leads to bond formation between two R groups that originally were attached to sulphur (equation 92). 

Bu OK in THF. Further studies of the formation of sulphides from rearrangements of sulphonium salts provide more details of the repression of the rearrangement by adjacent substituents capable of causing delocalization of charge from the intermediate ylide. 

Further studies of carbene insertion into the C S bond are reported, the authors earlier work on alkyl allyl sulphides, using bis(methoxy-carbonyl)carbene, now being supplemented by a study of the relative tendencies towards C—S insertion and C=C addition for the same substrates with carbethoxycarbene (from ethyl diazoacetate). 1,2-Diphenyl-ethyl phenyl sulphide, PhCH(CHaPh)SPh, is formed by the action of benzyne on dibenzyl sulphide, via the sulphonium ylide. Alkylation at sulphur is one of the best-known reactions of sulphides, and novel syntheses of this type have been reported in which the formation of hydro-quinone sulphonium salts, from quinone and a sulphide in 70% H2SO4 at — 5 to -f 5 and Markownikov addition of a sulphide to an alkene under similar conditions are discussed. 

The thio-Claisen rearrangements of allylic phenyl sulphides, allyl thienyl sulphides, addition products of acetylene derivatives and allyl mercaptan, and dimethylallyl 2-indolyl sulphonium salts have been described. Of particular interest is a recent paper reporting the quantitative formation of (62) by the thermal rearrangement of the sulphoxide (61). The second step in the mechanism (Scheme 3) proposed for this reaction... 

The epoxide-forming reaction of sulphonium ylides with carbonyl compounds continues to be widely applied in a routine sense at the same time as new and unique applications are studied. Bravo et al. have reported that, whereas indole and dimethylsulphonium-methylide afford iV-methylindole (i.e. protonation of the ylide followed by sulphonium salt alkylation of the indole), the keto-amide (28) gives an epoxide. Similarly, the amide function in (29) did not interfere with epoxide formation. In... 

The formation of S-halogeno-sulphonium salts from the halogen and a sulphide is in competition with an a-halogenation-oxidation reaction which leads to sulphonyl chlorides when water is present. The initial attack at sulphur may be followed by a Pummerer rearrangement to the adjacent carbon atom, and in the case of unsymmetrical sulphides the regioselectivity is determined by the relative electron density at the a-carbon atoms. ... 

The standard method for the conversion of a-thio-esters into a,/3-unsaturated esters involves oxidation to the corresponding sulphoxide followed by pyrolysis. An alternative approach, which avoids the use of an oxidant, is by formation of a sulphonium salt using CFjSOaCHzSiMea followed by ylide formation using caesium fluoride and rearrangement (Scheme 44). This method can run into difficulties if the starting thio-ester already contains an olefinic bond. 

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