Triphenyl sulfonium salts

Tetraphenylsulfurane is the simplest but most unstable among the tetraaryl-sulfurane species. Although only tetraphenyltellurane has been isolated, and its crystal structure has been determined, tetraphenylsulfurane and -selenurane have neither been detected directly nor isolated in the earlier studies. Furukawa and Sato reinvestigated the reactions of triphenyl sulfonium salt 99 and diphenyl sulfoxide 100 with phenyllithium (PhLi) in tetrahydrofuran (THF) in order to attempt detection of tetraphenylsulfurane [10—S—4(C4)] 101 directly by 13C, and CH-COSY NMR experiments at low temperature, as shown in Scheme 40 [91]. As results, the detection of tetraphenyl sulfurane has been achieved by low temperature NMR studies while tetraphenyl tellurane has been isolated [92]. 

For illustrative purposes, the photolysis of triphenyl sulfonium salts, one of the most popular acid generators, is presented in Scheme 7.26. In a viscous medium, a cage effect dominates the distribution of the photolysis products. ... 

In addition to iodonium, sulfonium and selenonium compounds, onium salts of bromine, chlorine, arsenic, and phosphoras are also stable and can act as sources of cation radicals as well as Bronsted acids, when irradiated with light. Performance of diaryl chloronium and diaryl bromonium salts was studied by Nickers and Abu. Also, aryl ammonium and aryl phosphonium, and an alkyl aryl sulfonium salt were investigated. It appears that the general behavior of these materials is similar to diphenyl iodonium and triphenyl sulfonium salts. These are formations of singlet and triplet states followed by cleavages of the carbon-onium atom bonds and in-cage and out of cage-escape reactivity. The anions of choice appear to be boron tetrafluoride, phosphorus hexafluoride, arsenic hexafluoride, and antimony hexafluoride. 

A number of structural modifications were carried out to shift the absorption to longer wave lengths.One example is formation of a 4-thiophenoxy derivative of triphenyl sulfonium salt, that aets as a chromophore. Variations of that and other modifications are shown below ... 

Few photoinitiators are available for cationic systems. The most widely used are diaryl iodonium salts such as diaryliodonium hexafluoro-antimonate, triaryl salts such as triphenyl sulfonium hexafluoro-phosphate, and mixed triphenyl sulfonium salts. These photoinitiators are decomposed by UV light by a homolytic cleavage to produce a radical anion and a radical cation. The latter abstracts hydrogen from surrounding molecules and generates a proton, which is the initiating species ... 

Scheme3.46 Generalized mechanism of proton generation by photolysis of diphenyliodonium and triphenyl sulfonium salts. An aromatic group.

As expected on the basis of the higher stability of sulfonium salts as compared with oxonium salts, thiopyrylium cations are more stable and less reactive than pyrylium cations hydride-accepting ability decreases in the order pyrylium> selenopyrylium> thiopyrylium [95], 2,4,6-Triphenyl-thiopyrylium salts react with ammonia and primary alkylamines forming the corresponding pyridine and pyridinium salts, respectively, but they do not react with aniline or its derivatives [96-99], As described below, the Se- or Te-analogs are less stable than the thiopyrylium salts. 

The reactions of sulfonium salts with organometallic reagents probably involve five-coordinate sulfur species. For example, dephenylation of triphenyl-sulfonium tetrafluoroborate (37) with vinyllithium and the stereospecific... 

A photochemical study- of the electron-transfer reactions of a sulfonium salt, 4-cyanobenzylmethylphenylsulfonium tetrafluoroborate, has been reported to give phenyl methyl sulfide quantitatively. 9,10-Dimethylanthracene and naphthalene have both been used in the sensitised reactions of triphenyl-sulfonium hexafluoroantimonate and the reactions encountered involve the singlet states and produce the radical-cation of the sensitiser and phenyl radicals. The phenyl radicals are formed by bond fission within the neutral triphenylmethyl radical. The reactions of some aromatic sulfonium compounds have been patented for use in resin compositions. ... 

Quantum yields of acid, and iodobenzene or phenyl sulfide were measured by irradiating. 03 molar acetonitrile solutions of diphenyl iodonium hexafluoroarsenate and triphenyl sulfonium hexa-fluoroarsenate. Photosensitizers were included at identical optical density (1.0) at the wavelength of irradiation (366 nm). Relative polymerization rates were calculated as a function of the gelation time of a solution of 50% acetone, and 50% cycloaliphatic diepoxide resin ARALDITE CY-179 (v/v) containing the photosensitizer and onium salt as above. Experimental details were published previously.(8,15) IRGACURE and ARALDITE are registered trademarks of the Ciba-Geigy Corporation. 

Benzylic-type sulfonium salts derived from thiophene-2- and 3- and furan-2-methanols, where halides are not stable, have been nsed as snbstrates for Suzuki, Stille and Negishi couplings. The 1-Boc-pyrrole-2-methanol analogne was only snccessful in StiUe couplings. An important feature of this method is that triphenyl phosphite was reqnired as the ligand to overcome the problem of reaction of the usual phosphine ligands with the snlfonium salts. ... 

Abstraction of labile hydrogen atoms, for example, from tetrahydrofuran (THF), by photoexcited ketones also yields easily oxidized radicals [60a]. Quantum yields for benzophenone sensitized photolysis of diphenyliodo-nium hexafluoroarsenate in acetonitrile with hydrogen atom donors are shown in Table 9. Diaryliodonium salts are capable of oxidizing electron rich radicals from isopropanol and THF but triarylsulfonium salts are not [96]. The oxidation potential of tetrahydrofuranyl radical is —0.35 V versus SCE [109], apparently sufficient for irreversible reduction of diphenyliodonium cation ( red = approx. —0.7 V versus SCE, see above) but not of triphenyl-sulfonium cation ( red = —1.2V versus SCE). 

A typical acid photogenerator is triphenyl sulfonium triflate (Figure 6.13), which generates trifluoromethane sulfonic acid. Onium salt photoinitiators of this type can induce cationic photopolymerisation under photoirradiation. The reaction is complex and several products of this type are used with Lewis acids, such as AsPF ", AsF " and BF4". See, for example, [44]. 

Some inhibitors are reduced on the surface to yield secondary products that are themselves the active inhibitors. In strong mineral acids, elements from Groups VI and VII tend to become protonated, a necessary prerequisite for many reduction reactions. Such is the case for triphenyl benzyl phosphonium chloride, which forms triphenyl phosphine, and triphenyl arsenic oxide, which undergoes protonation (permitting it to dissolve) and forms triphenyl arsine on the surface. Some sulfonium salts, e.g., tribenzylsulfonium hydrogen sulfate, and dibenzylsulfoxide also can be reduced by iron in HCI. 

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