Diaryliodonium salts, structures

The exceptional case where no activating group is required is the use of diaryliodonium salts as precursors for labelling, permitting the fluorine-18-labelling of relatively electron-rich structures [192-194], A recent example of successful application is the preparation of 5-[ F]fluorouracil in 40% radiochemical yield (Scheme 44) [202], However, this methodology appears to be relatively difficult to use with complex structures [203-205],... 

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. 

Photoinitiators for Cationic Polymerization. Recently a class of photoinitiators for cationic polymerization was discovered by Crivello et al. (55). This class includes diaryliodonium (Structure I) (56. 57). triaryIsulfonium (Structure II) (58-62). dialkyIphenacylsulfonium (Structure III) (63). dialkyl-4-hydroxy-phenylsulfonium salts (Structure IV) (64). and triaryIselenonium... 

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 ... 

Figure 2.11 Primary and secondary bonding pattern in a single-crystal X-ray structure of a typical diaryliodonium salt PhilX (X = Cl, Br, I, or BF4).

The structure of iodonium triflate 395 was unambiguously established by a single-crystal X-ray analysis [138], The structural data revealed the expected geometry for iodonium salts with a C—I—C bond angle of 91.53°. The I—C bond distances of 2.131 and 2.209 A are longer than the typical bond length in diaryliodonium salts (2.0-2.1 A). The I - O distance between the iodine atom and the nearest oxygen of the triflate anion is 2.797 A. 

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],... 

Several problems, mainly due to the low selectivity of reactions, are associated with the use of diaryliodonium salts for [ F]-fluoridation. In the case of the reactions of symmetrical diaryliodonium salts, At2I+X , there is no problem with the regioselectivity of fluoridation however, only half of a molecule of substrate is converted into the target product and the second half gives aryl iodide as a by-product (Scheme 7.5), which results in a low atom economy. In addition, in this case the separation of aryl iodide and aryl fluoride may be complicated due to their similar structure and a chromatographic purification procedure (usually HPLC) is required for separation and purification of the target fluorinated product. 

Later, in 1995, Pike and Aigbirhio applied for the first time diaryliodonium salts for the preparation of F-labeled aryl fluorides using potassium [ F]-fluoride in the presence of the diaza-crown ether Kryptofix (K2.2.2 structure 24 in Scheme 7.7) in acetonitrile at 85 °C or 110 "C [67]. Under these conditions, the reaction of diphenyliodonium chloride provided [ F]-fluorobenzene in 31-78% radiochemical yield. The use of Kryptofix is required for phase transfer of the [ F]-fluoride ion obtained by the nuclear reaction in the cyclotron as a solution in water enriched with oxygen-18. 

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. 

Based on the above, an initiating composition for cationic photopolymerization, with visible and long-wavelength UV light was described by Crivello et al. The structure of the monomers plays a key role in these photosensitization processes. Useful aromatic ketones are camphoquinone, benzyl, 2-isopropylthioxanthone, or 2-ethylanthraquinone. The monomer-bound radicals reduce diaryliodonium salts or dialkyl phenacylsulfonium salts rapidly to form monomer-centered cations. These cations then initiate the polymerization of epoxides, vinyl ethers, and heterocyclic compounds. Onium salts with high reduction potential, however, such as triarylsulfonium salts, do not undergo this reaction. 

Properties and Synthesis of Diaryliodonium Salts 2.1 Structure, Reactivity, and Chemoselectivity... 

The diaryliodonium salts are highly stable in the absence of light, but decompose when irradiated at 313 or 365 nm with quantum yields of 0.2-0 3 (3). The rate of photodecomposition is independent of the structure of the counter anion, and is insensitive to temperature and to atmospheric oxygen (3). No thorough investigation of solvent effects has been reported, but decomposition rates appear to be identical in acetone and acetonitrile when quartz tubes are used and the irradiation source is a water-cooled Hanovia 450 W medium pressure mercury lamp (2). Decomposition in nitromethane is slower, probably as a result of the increased light absorption by this solvent. 

Nevertheless, syntheses of several diaryliodonium and triarylsulfonium salts that incorporate long wavelength absorbing chromophores have been successfully accomplished and their structures 7-9 are presented below. The replacement of one of the phenyl rings in a diphenyliodonium salt with a fluorenone moiety as shown in structure 7 results in the generation of a diaryliodonium salt with two absorption bands at 294 and 378 nm [FOU 94, HAR 01]. Both the absorption and the fluorescence spectra of 7 closely resemble those of fluorenone. Unfortunately, despite the long wavelength absorption of 7, photopolymerization studies showed that it was not more efficient as a photoinitiator than simple diphenyliodonium salts that do not possess the fluorenone chromophore. 

Pyrroloindolines 33 are associated with a diverse family of structurally complex polyindoline alkaloids endowed with interesting biological activities. Macmillan and Zhu reported an enantioselective tandem arylation-cyclization of indole acetamides 32 and diaryliodonium salts in the presence of Cu(I) to obtain pyrroloindolines 33. This rapid protocol provides a new strategy for the enantioselective synthesis of various pyrroloindolines (Scheme 3) [19]. 

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 bulky anion then stabilizes the intermediate adduct from protonation of the epoxy group and then facilitates insertion of epoxide at the cationic propagation site. Rapid polymerization can then occur. Cationic photopolymerization of epoxides often involves the photo-generation of acid from an initiator such as diaryliodonium or triaryl sulfonium salts (Crivello, 1999). The anions are important in controlling the addition at the cationic site and are typically BF4 and PFg. The reactivity of the system depends also on the structure of the epoxide. 

Of these, the diaryliodonium, triarylsulfonium, and ferrocenium salts are most practical and most often employed. Shown in (21a) to (21c) are the structures of typical members of these classes of compounds. 

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