Sulfonium salts decomposition

Shimizu and co-workers reported that thermal decomposition of A4-thiabenzenes ylides afforded both thienofuran and thiophene derivatives in addition to the expected alkyl-rearranged products. A plausible mechanism was proposed with a [3.1.0] bicyclic sulfonium salt 9 as the key reactive intermediate <2001J(P1)2269>. Warren and co-workers, in their study of stereospecific phenysulfanyl migrations, found that [l,4]-sulfanyl participation could compete with the usual [l,2]-sulfanyl participation <1999SL1211>. Rearrangement of alcohol 18 with TsCl in pyridine gave an inseparable mixture of isomeric chlorides, 19 and 20, in a ratio of 52 48, as shown in Equation (3). 

Dialkyl peroxydicarbonates, 18 477 decomposition of, 18 474 475 as free-radical initiators, 14 289-290 properties of, 18 470-471t Dialkylphenacyl sulfonium salts, 10 414-415... 

Nitrogen tetroxide also causes rapid racemization of sulfoxides however, the reaction takes place without decomposition (279,280). This process is initiated by the formation of a sulfonium salt, which then undergoes partial ionization to form a dication intermediate. 

Interaction with diethyl sulfide occurs with simultaneous elimination of nitrogen and formation of a sulfonium salt of type 306 its hydrolytic decomposition yields 5-hydroxy- (307) and 5-ethylthio-3-phenyl-l,2,4-thiadiazole (308), in proportions dependent upon the hydrogen ion concentration of the medium.160... 

By treating the azasulfenylation products with trimethyloxonium tetrafluoroborate, sulfonium salts 5 were obtained, which in acetonitrile at 80 °C underwent decomposition to give the corresponding 4,5-dihydro-l,3-oxazole 6 in good yield. Successive hydrolysis afforded the amino alcohol 7, so that the overall sequence consists of a cis oxyamination of a double bond. 

Sulfonium salts react in several ways. They may behave as a leaving group, undergoing substitution by a nucleophile or fragmenting with the formation of an alkene. However, the most important reaction of sulfonium salts involves the formation of an ylide in the presence of a base. The carbanion of this sulfur ylide is stabilized by the adjacent positively charged sulfonium ion. The reaction of the carbanion with a carbonyl group parallels that of a phosphonium ylide in the Wittig reaction. However, the decomposition of the intermediate dipolar species is different and leads to the formation of an epoxide (oxirane) rather than an alkene. 

Condensation with carbonyl compounds. Formation of epoxides from aldehydes by reaction with sulfonium ylides is subject to asymmetric induction. The latter species have been generated from 91, 92, and 93, and also those derived from monoterpenes, e.g., 94 " and 95.- Of course the ylides can be obtained in situ by deprotonation of sulfonium salts or copper-catalyzed decomposition of diazoalkanes (with the carbenoids trapped by the sulfides). 

Hofmann rule The principal alkene formed in the decomposition of quaternary ammonium hydroxides that contain different primary alkyl groups is always ethylene, if an ethyl group is present. Originally given in this limited form by A.W. Hofmann, the rule has since been extended and modified as follows When two or more alkenes can be produced in a P-elimination reaction, the alkene having the smallest number of alkyl groups attached to the double bond carbon atoms will be the predominant product. This orientation described by the Hofmann rule is observed in elimination reactions of quaternary ammonium salts and tertiary sulfonium salts, and in certain other cases. 

Depending on the nature of the counterion, triarylsulfonium salts are more or less stable. The most stable compounds have a non-nucleophilic counterion. On the other hand, the presence of a nucleophilic counterion such as a halide (Cl, Br or I) frequently results in the relatively facile decomposition by thermolysis to afford quantitatively the diarylsulfide and the corresponding aryl halide. The observation of the preferential relief of the steric strain in the case of phenyl-p-tolyl-(2,5-dimethylphenyl)sulfonium salts (23) was explained by the intramolecular decomposition of a first formed tetracoordinate intermediate. 35.36... 

Cationic polymerization of diethyleneglycol divinyl ether and butanediol divinyl ether in the presence of oniiim salts was induced by y-irradiation. The mechanism for the initiation process involves the reduction of onium salts either by organic free radicals or solvated electrons depending on the reduction potentials of the onium salts. The reduction potentials of sulfonium salts was determined by polarography at the dropping mercury electrode. Only solvated electrons were capable of reducing the salts with reduction potentials lower than approximately -100 kJ/mol. The redox process liberates the non-nucleophilic anion from the reduced onium salt and leads to the formation of a Bronsted acid or a stabilized carbenium ion. These species are the true initiators of cationic polymerization in this system. The y-induced decomposition of onium salts in 2-ethoxyethyl ether was also followed by measuring the formation of protons. An ESR study of the structure of the radicals formed in the y-radiolysis of butanediol divinyl ether showed that only a-ether radicals were formed. 

Sulfur ylides are useful reagents in organic synthesis. The ylide is formally a zwitterion in which a carbanion is stabilized by interaction with an adjacent sulfonium centre. They are usually prepared by proton abstraction from a sulfonium salt with a suitable base or by reaction of a sulfide with an alkylating agent such as Me30+BF4 or a carbene formed, for example, by metal-catalysed or photolytic decomposition of a diazo compound (1.103). 

The decomposition mechanism of the triaryl sulfonium salt is believed to take place somewhat similarly... 

The ratio of terminal to internal alkene from decomposition of some sulfonium salts under alkaline conditions is as indicated ... 

For some applications, for example curing of epoxy resins, the in situ generation of cationic initiators is required. This may be achieved by photochemical decomposition of suitable promotors to radicals, which in the presence of diazonium, sulfonium, or halonium salts (not able to initiate the polymerization by itself) are converted into corresponding cationic initiators [45-48]. 

Scheme 4 [2,3]Sigmatropic rearrangement of acetylenic and allenic sulfonium ylides via copper salt catalyzed thermal decomposition of diazomalonates...

The photochemical decomposition of sulfonium or diarylhalonium salts can also lead to the formation of protonic acids subsequently initiating polymerization ) ... 

Free carbene 429 generated by photolysis or thermolysis of the diazo compound cyclizes into the sulfonium ylide, which undergoes a thia-Stevens rearrangement to 430 in 19% yield. The same result was obtained using the corresponding p-tosylhydrazone sodium salt as a carbene precursor (83JA6096). Rh(II)-catalyzed decomposition of sulfur-containing diazo... 

Cathodic reduction of graphite in dipolar aprotic solutions of alkali and [NRJ salts (e.g., in ethers, esters, amides) usually yields solvated cation intercalation compounds " however, at high cation density the solvent may be squeezed out . The corresponding phosphonium and sulfonium graphite intercalation compounds are unstable their formation is accompanied by reductive decomposition of these cations . 

The ylide, of general structure (4.132), is formed by reaction of the corresponding sulphide (4.133) with a metallocarbenoid resulting from transition metal salt-mediated decomposition of a diazo compound (4.134) or N-tosyl hydrazone salt (4.135). Reaction of the sulfonium ylide (4.132) with aldehyde (4.136) yields the trans-epoxide (4.137) as the major product and sulfide (4.133) which is then returned to the catalytic cycle (Figure 4.10). 

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