Diaryliodonium salt photolysis reaction

The caged species may escape geminate recombination and produce various species that can initiate cationic polymerization. Solvent (RH) often participates in these reactions producing protonic acids. As shown in Eq. (44), protonic acids are also formed by reaction of radical cations with aryl radicals or by Friedel-Crafts arylation. Up to 70% of the protonic acid is formed upon photolysis of diaryliodonium salts [205]. In addition to initiation by protons, arenium cations and haloarene radical cations can react directly with monomer. The efficiency of these salts as cationic initiators depends strongly on the counterions. Those with complex anions such as hexafluoroantimonate, hexafluorophosphate, and triflate are the most efficient. 

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). 

Photochemical Sensitization. Photolysis of diaryliodonium salts in the presence of benzoin ethers results in efficient reaction of the iodonium salt [96,97]. Scheme 5 illustrates the mechanism of photolysis according to Ledwith [96] and Timpe [92], Accordingly, photocleavage of benzoin ethers yields easily oxidized ketyl radicals (and acyl radicals which can also initiate radical polymerization). That only ketyl radicals participate in photochemical sensitization of onium salt decomposition was confirmed by ESR spin trapping with benzylidene-tcrt-butylamine-AT-oxide [10b]. As the chemistry... 

Dektar and Hacker report on the photochemistry of diaryliodonium salts in the case of direct photolysis and in the case of energy transfer from a photosensitizer [26], The photoproducts and the reaction mechanisms are different in the two situations (Scheme 12.5). 

Triplet sensitization of sulfonium salts proceeds exclusively by the homolytic pathway, and that the only arene escape product is benzene, not biphenyl or acetanilide. However, it is difficult to differentiate between the homolytic or heterolytic pathways for the cage reaction, formation of the isomeric halobiaryls. Our recent studies on photoinduced electron transfer reactions between naphthalene and sulfonium salts, have shown that no meta- rearrangement product product is obtained from the reaction of phenyl radical with diphenylsulfinyl radical cation. Similarly, it is expected that the 2- and 4-halobiaryl should be the preferred products from the homolytic fragments, the arene radical-haloarene radical cation pair. The heterolytic pathway generates the arene cation-haloarene pair, which should react less selectively and form the 3-halobiaryl, in addition to the other two isomers. The increased selectivity of 2-halobiaryl over 3-halobiaryl formation from photolysis of the diaryliodonium salts versus the bromonium or chloronium salts, suggests that homolytic cleavage is more favored for iodonium salts than bromonium or chloronium salts. This is also consistent with the observation that more of the escape aryl fragment is radical derived for diaryliodonium salts than for the other diarylhalonium salts. 

Many early workers reported that diaryliodonium salts slowly decomposed on standing More recent investigations, however, have shown that these compounds are completely stable when stored in the dark It would seem likely that the previous observations were the result of inadvertent photolysis caused by their exposure to light during storage. In the past few years, a number of workers have investigated the photolysis of diaryliodoniiun salts and have elucidated the structure of their primary photoproducts Crivello and Lam have proposed the following mechanistic pathway which accoimts for the photolysis reaction products which have been observed. 

The presence of aromatic free radicals as the other primary photoproducts of the photolysis of diaryliodonium and triarylsulfonium salts has been conflrmed by the isolation of their reaction products, namely, aromatic hydrocarbons and biaryls from the photolysis mixtures. Cross coupling experiments in which the simultaneous photolysis of two diaryliodonium salts bearing different subtituents on their aromatic rings are carried out, yield a mixture of biaryl products including those in which the aromatic rings are derived from both starting materials. Clearly, the aryl radicals which are produced on photolysis have sufficient lifetimes to diffuse from the reaction site and to couple with one another. 

The aryl radicals produced from the photolysis of the diaryliodonium salt abstract a hydrogen atom from THF producing the aryl hydrocarbon and the THF radical. The THF radical is further oxidized by the diaryliodonium salt, resulting in the formation of a stabilized THF cation which may initiate cationic polymerization. Simultaneously, the aryl radical which is the principal chain carrier is regenerated. Because such free radical chain reactions are inhibited by oxygen, this process is most efficiently carried out in the absence of air. 

The evolution of nitrogen on photolysis of the aryIdiazonium salts appears to have limited the use of these systems to thin film applications such as container coatings and photoresists (23). Other efficient photoinitiators that do not produce highly volatile products have been disclosed (24-27). These systems are based on the photolysis of diaryliodonium and triarylsulfonium salts. Structures I and II, respectively. These salts are highly thermally stable salts that upon irradiation liberate strong Bronsted acids of the HX type (Reactions 43 and 44) that subsequently initiate cationic polymerization of the oxirane rings ... 

It should be pointed out that the photolysis of the diaryliodonium and triaryl-sulfonium salts proceeds differently from the photolysis of the diazonium systems. The reaction mechanism of the photolysis of iodonium salts is described by Reactions [6.46] and [6.47]. 

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