Diaryl sulfonium salts

Aryldiazonium salts as UV initiators were described as early as 1973 [14]. Cationic polymerization with these initiators is not oxygen sensitive. This first class of salts had poor pot life and had problems due to liberation. The next generations, diaryl iodonium and diaryl sulfonium salts, overcame most of these problems. They were developed about the same time by 3M and General Electric [15-18]. Cationic systems utilizing cycloaliphatic epoxies exhibit the interesting feature that once polymerization begins, it can continue in the dark. Compared to free radical methods, use of cationic UV cure remains relatively small, but has found certain niches, such as metal varnishes. 

While electron beams can produce cations, they are not effective at producing cationic cure in the absence of suitable photoinitiators. The same cationic photoinitiators used for UV cure are often also e-beam sensitive. Examples are triaryl sulfonium or diaryl iodonium salts [41]. 

Solladie-Cavallo s group used Eliel s oxathiane 1 (derived from pulegone) in asymmetric epoxidation (Scheme 1.3) [1]. This sulfide was initially benzylated to form a single diastereomer of the sulfonium salt 2. Epoxidation was then carried out at low temperature with the aid of sodium hydride to furnish diaryl epoxides 3 with high enantioselectivities, and with recovery of the chiral sulfide 1. 

Optically active sulfonium and selenonium salts are well known and the stereochemistry of the isomers has been studied.1 3 Optically active cyclic diaryl(alkoxy)-sulfonium salts 14, 15, and 16, stabilized by intramolecular sulfur-oxygen interaction, were synthesized in 2000 by reacting optically active spirosulfuranes with trimethyloxonium tetrafluoroborate.29 The absolute configurations were assigned on the basis of the reaction mechanism. The sulfonium salts were hydrolyzed in KHC03aq. to yield optically active sulfoxides in over 86% ee (Scheme 7). 

Arylmethyl(homobenzyl)ethylsulfonium salts are appropriate substrates for Suzuki-type coupling reactions. In this reaction, performed on a polymer-bound sulfonium tetrafluoroborate, the benzyl fragment on the sulfur atom was transferred to the boronic acid residue. The sulfonium salt was prepared from an al-kylthiol resin by alkylation with a substituted benzyl halide to give thioether 98 and subsequent alkylation with triethyloxonium tetrafluoroborate. Reaction with a boronic acid derivative yielded diaryl methanes 99 [94] (Scheme 6.1.22). 

Various dialkyl- and diaryl(vinyl)sulfonium salts react with l//-indole-2-carbaldehyde in the presence of NaH to form a tetracyclic oxirane, for example, 167 these upon treatment with sodium azide form tricyclic azido alcohols analogous to compound 168 in 72% yield (Scheme 35) <1999T10659>. This annulation was a key step in total synthesis of Mitomycin K <1996TL6049>. 

Both the H chemical shifts and NOE data suggest that chloro-A -sulfanes and sulfonium salts 24 and 25 having an o-MeO, o-Cl, or o-Me substituent on the aryl ring assume a skew conformation, whereas the aryl ring in compounds without an ortV o-substituent can rotate practically freely about the S-C(l ) axis. H and N NMR examinations of diaryl(acylamino)(chloro)-A -sulfanes 24a-e showed that they exist only in CDCI3 and dimethyl sulfoxide-t4 (DMSO- 6) solvents (Equation 3), whereas in MeOD complete ionic dissociation takes place, leading to the sulfonium chloride (see Table 5) <2001J(P2)339>. 

Akhtar et al. [47] described an improved triarylsulfonium salt synthesis by modifying previous P205 based reactions. Use of methanesulfonic acid solvent for the condensation of aromatic compounds with dialkyl or diaryl sulfoxides rapidly produced high yields of sulfonium salt under mild conditions. 

However, Trost et ah treated triaryl sulfonium salts with aryl lithiums to obtain only diaryl sulfides and diaryls, via sulfuranes [38]. These two contradictory results obtained by different groups have not been resolved. 

Organic sulfides can also be alkylated to afford sulfonium alkyl sulfates. Use of dialkyl sulfates is sometimes essential, as, for instance, diaryl sulfides308 and methyl p-tolyl sulfide are not converted into sulfonium salts by alkyl halides. 

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. 

Diaryl iodonium and triaryl sulfonium salts are among the most versatile photoinitiators. These compounds have been thoroughly investigated by Crivello... 

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 ... 

The reduction potentials for S,S-dialkyl-S-(hydroxyphenyl) sulfonium salts do not appear to have been measured. These sulfonium salts do not undergo electron transfer photosensitization. Rather, photosensitization has been achieved using typical diaryl ketones such as benzophenone, Michler s ketone, and thioxanthone as photosensitizers. Strongly acidic solutions are produced during photosensitized photolysis, suggesting the formation of a protonic add as one of the products. 

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