Diaryliodonium salt photolysis

Diarylide yellow pigments, 74 317 79 433 Diaryliodonium salt photoinitiators, 74 270 Diaryliodonium salts, 79 108 photolysis of, 70 414... 

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. 

Diaryliodonium salts [53c, 57-59] form iodobenzene, biphenyl, benzene, iodobi-phenyl, and several other products during irradiation. The photolysis of butyl-triphenylborate salts in the presence of electron acceptors generates biphenyl and the butyl radical [24, 54a, 60, 61], while the photolysis of the analogous tetraphe-nylborates also produces biphenyl as the major product. 

Polymerizations also have been effected with heat ° and Photolysis of thietane in the presence of catalytic amounts of diaryliodonium salts gives a 14% yield of polymer. Poly(hexafluorothietane) is obtained on photolysis of the monomer for four days. 6-Thiabicyclo[3.1.1]heptane is said to give positive indications for free-radical formation in acrylonitrile. ... 

The mechanism by which diaryliodonium salts initiate polymerization of epoxides has been explained as involving acids produced from photolysis of the diaryliodonium salt ( , 4) ... 

Figure 3. Photolysis of diaryliodonium salts having nucleophilic anions results...

Figure 4, Photolysis of diaryliodonium salts having nonnucleophilic anions results in the formation of strong acids (HPFe). Nonnucleophilic acids such as HPFe can initiate cationic polymerization of vinyl ethers and cyclic ethers.

Typical photosensitizers for diaryliodonium salts are condensed ring aromatic hydrocarbons, diaryl ketones, and acridinium dyes. Condensed ring aromatic hydrocarbons are particularly effective photosensitizers for triarylsulfonium salts. The use of photosensitizers in onium salt photolysis permits the photoimaging processes induced by these compounds to be optimally fitted to the specific irradiation source used for their exposure. 

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 product mixture obtained on photolysis of diaryliodonium salts is, in fact, much more complex than these data suggest. DeVoe et al [22] have identified, in addition to iodobenzene, acetanilide, biphenyl, two iodo-biphenyl isomers in the product mixture from photolysis of Ph2I+PFg in AN or HzO at 254 nm. With the chloride counterion, this list expanded to include chlorobenzene and hydroxybiphenyl (presumably the 2-isomer) Ph2I+I, on the other hand, photolyzed cleanly at 313 nm to iodobenzene. A similar mix of products, including benzene and a third iodobiphenyl isomer was observed by Dektar and Hacker [70] on photolysis of the triflate salt under a variety of conditions (see Table 4, 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... 

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). 

Photolysis of diaryliodonium salts take place either through homolytic or through heterolytic cleavage of the halogen-aryl bond to form species which react with a hydrogen donor compound to yield a Brpnsted acid that initiates polymerization (Scheme 11.3). 

Notably, the electron-donating subtituents on the aromatic structures not only shifts absorption bands to longer wavelengths but also favors photolysis of diaryliodonium salts to afford higher polymerization rates. 

The photolysis mechanism is similar with the diaryliodonium salts. When irradiated in appropriate wavelengths, TPSs undergo either a homolytic or a hetero-lytic cleavage followed by a proton release after some additional steps, which are summarized in Scheme 11.4. 

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). 

SCHEME 12.5 Proposed photochemistry of diaryliodonium salts in the case of direct photolysis. 

SCHEME 12.6 Photolysis of diaryliodonium salts—sensitized energy transfer. 

SCHEME 12.7 Photolysis of diaryliodonium salts—electron transfer. 

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. 

In the same groundbreaking paper [21], CriveUo and coworkers also demonstrated that the anion plays no role in determining the photosensitivity of the iodonium salt and the photolysis rates of diaryliodonium salts having the same cations but different non-nucleophilic counterions (BF4, PFs, AsFs", or SbFe ) are identical. Likewise, the cation structure has little effect on the photodecomposition of diaryliodonium salts. The utility of iodonium salts as photoinitiators has been demonstrated in several cationic polymerizations using olefins, epoxides, cycUe ethers, lactones and cyclic sulfides as the monomers [21],... 

Over the past several years, there have been developed several new classes of onium salt photoinitiators capable of initiating cationic polymerization. The most significant of these are aryldiazonium salts, diaryliodonium salts, triarylsulfonium salts, and dialkylphenacyl-sulfonium salts. The mechanisms involved in the photolysis of these compounds have been elucidated and will be discussed. In general, on irradiation acidic species are generated which interact with the monomer to initiate polymerization. Using photosensitive onium salts, it is possible to carryout the polymerization of virtually all known cationically polymerizable monomers. A discussion of the various structurally related and experimental parameters will be presented and illustrated with several monomer systems. Lastly, some new developments which make possible the combined radical and cationic polymerization to generate interpenetrating networks will be described. 

Attempts to quench the photolysis of diaryliodonium and triarylsulfonium salts using various triplet quenchers have failed suggesting that cleavage of these compounds occurs from the excited singlet state. The photolysis rate of diaryliodonium salts has been shown to be solvent dependent (17rl8). In alcohols and ethersr higher quantum... 

In the case of photo-initiator types (a) and (b), it has been clearly shown that mode of initiation involves the formation on photolysis of a Bronsted acid, HX, which corresponds to the anion associated with the starting salt. Equations (22) and (23) give the mechanism of the photolysis proposed for diaryliodonium salts. 

In most respects, diaryliodonium salts are nearly ideal as cationic photoinitiators for the ring-opening polymerizations of heterocyclic monomers. The photolysis of diaryliodonium salts is highly efficient with quantum yields of the order of 0.7 based on the amount of acid generated. Not only are they... 

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. 

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