Synthesis of triarylsulfonium salts

A considerable number of synthetic routes have been developed for the preparation of symmetrical and asymmetrical triarylsulfonium salts. Several excellent recent reviews of the subject reflect the current state of activity in this field The most important synthetic routes to the synthesis of triarylsulfonium salts are briefly summarized in Eqs. (13)-(20). 

As in the case of the analogous diaryliodonium salts, only those triarylsulfonium salts bearing anions of low nucleophilic character are broadly applicable as photoinitiators for cationic polymerization. Triarylsulfonium halides and bisulfates undergo facile 

Although the above metathesis is an equilibrium process, nearly quantitative yields of triarylsulfonium BF, AsFg, PFg , SbFg, and CIO4 salts can be obtained since these salts precipitate from solution as they are formed due to their insolubility in water. 

In Table 2 are shown the structures of some representative triarylsulfonium salts. Unsubstituted triphenylsulfonium salts have their major absorption band near 230 nm. 

The above schemes amply demonstrate how the structure of triarylsulfonium salts can be manipulated to produce photoactive absorption bands in the mid region of the ultraviolet spectrum. Using similar synthetic techniques, it should be possible to purposefully design photoinitiators which respond to any given wavelength in the ultraviolet spectrum and into the visible region of the electromagnetic spectrum. 

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Ellwood [33] described the synthesis of triarylsulfonium salts via reaction of a diarylsulfide, Lewis acid catalyst, and a halogen in the presence of an inert solvent. Anion exchange afforded the desired salt. 

Wu et al. in 1988 [43], discussed the synthesis of triarylsulfonium salts by the photolysis of a diaryliodonium salt in the presence of diphenyl sulfide. They explained the reaction using an electron transfer mechanism (see below). 

The one pot synthesis of triarylsulfonium salts by reaction of an aromatic Grignard reagent with thionyl chloride was anonymously reported in 1988 [44]. 

Using the above synthesis, a wide variety of triarylsulfonium salt photoinitiators can be prepared. In Table 1 are shown some representative triarylsulfonium salt photoinitiators which were prepared during the course of this research. All the compounds in this table are well characterized crystalline compounds with well defined melting points, elemental analyses, ultraviolet, proton, and nmr spectra (T7). The ability of triarylsulfonium salts bearing non-nucleophilic counterions to serve as photoinitiators is completely general and includes all the symmetrical, unsymmetrical, substituted and unsubstituted as well as polynuclear and heterocyclic salts shown in Table 1. 

Crivello [34] patented the synthesis of triarylsulfonium polyhalogen metal salts by oxidation of a polyarylsulfide in the presence of a strong acid and under dehydrating conditions. 

Attention was given to the synthesis of bulky borate anions that seemed to display the required properties. In particular, the tetrakis(pentafluorophenyl)borate anion has focused our interest. This compound is a very stable, crystalline compound and is insensitive to air or moisture. We have found that certain of these salts, as will be described further in this paper, give excellent cationic photoinitiators when associated with a diaryliodonium cation, a triarylsulfonium cation or a ferrocenium cation... 

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. 

The major portion of the article will be devoted to a discussion of the synthesis and mechanistic aspects of photoinitiation by individual photoinitiator systems. Cationic polymerizations induced by these photoinitiators are dark, i.e., non-photo-chemical, processes which are governed by the same parameters which must be taken into account in polymerizations which occur in the presence of conventional initiators such as Lewis and Bronsted acids. Accordingly, cationic polymerizations induced by halogen and sulfur based onium salt photoinitiators will be discussed only from the context in which they are influenced by factors which have their origin in the photoinitiator. Since the photochemistry of diaryliodonium and triarylsulfonium salts is similar, these two types of photoinitiators will be discussed together. The photolysis of dialkylphenacylsulfonium and dialkyl-4-hydroxyphenylsulfonium salts proceeds by a different mechanism, and they will be discussed separately. 

Although somewhat less efficient than triarylsulfonium salts or diaryliodonium salts as photoinitiators, these compounds are nonetheless compensated for their reduced efficiency by their broader spectral response and by the facility of their synthesis. With respect to thermal stability, these compounds occupy a position intermediate between the very reactive trialkylsulfonium salts on one hand and the comparatively unreactive triarylsulfonium salts on the other. Clearly, this stability is derived from delocalization of the positive charge on sulfur by means of the conjugated enol form of the phenacyl group in III and by interaction with the electron rich phenolic ring in IV. 

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