Diaryliodonium salts nucleophilic anions

Many different photoinitiators based on onium -type compounds with anions of low nucleophilicity also have been described in the literature as effective catalysts for the polymerization of epoxides Thus, diaryliodonium salts diaryliodosyl salts triarylsulfonium salts and related compoundstri-phenylsulfoxonium saltsdialkylphenacylsulfonium salts and dialkyl-4-hydroxyphenylsulfonium salts seem to be most suitable as photoinitiators for epoxy curing. Some of the principles of the reaction mechanism involving these initiators are discussed in detail in the following Sections. Various other onium photoinitiators such as diarylchloronium and diarylbromonium salts , thiopyrylium salts 3), triarylselenonium salts and onium salts of group Va elements >... 

Photochemistry of Diaryliodonium Salts. The photochemistry of diaryliodonium salts having nucleophilic anions is illustrated... 

Among the reactions of diaryliodonium salts with inorganic compounds, their hydrolysis has been studied in detail.40.41,43,51-53 jhe hydrolysis reaction proceeds quite slowly in boiling water. However, in the presence of aqueous alkali, the hydrolysis takes place in a few days for diphenyl-iodonium bromide to afford a mixture of iodobenzene, bromobenzene and phenol. With weakly nucleophilic anions, the reaction affords iodobenzene, phenol and a small amount of diphenylether. The reaction was considered to involve an unstable hydroxydiaryliodane, which is likely to be in equilibrium with diaryliodonium hydroxide. Decomposition of the hydroxyiodane leads to the... 

Several inorganic anions have been arylated by diaryliodonium salts, possessing a non-nucleophilic counteranion. A number of examples are reported in table 5.2. 

Diaryliodonium salts have found synthetic application as arylating reagents in reactions with various organic substrates under polar, catalytic, or photochemical conditions. Typical examples of arylations of nucleophiles under polar, non-catalytic conditions are shown in Scheme 3.271 and include the reactions of diaryliodonium salts with thiosulfonate anions 675 [861], fluoride anion [862,863], malonates 676 [864] and silyl enol ethers 677 [865]. 

Additional examples include the electrophilic arylations of sodium arenesulflnates [866], potassium car-bonotrithioates [867] and benzazoles [868] using diaryliodonium salts in ionic liquids and the arylations of anilines [869], sodium tetraphenylborate [870] and vinyUndiums [871]. Particularly important are the reactions of diaryliodonium salts with fluoride anion as nucleophile. This reaction is widely applied for introduction of the radioactive F isotope into different organic substrates to obtain labeled agents for positron emission tomography. This topic is covered separately in Chapter 7 due to its importance and wide usage. 

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],... 

Photoinitiators used for epoxy curing include aryldiazonium salts (ArN2+X ), diaryliodonium salts (Ar2l+X ), and onium salts of Group Via elements, especially salts of positively charged sulfur (ArsS+X ). The anions must be of low nucleophilicity, such as tetrafluoroborates or hexafluorophosphates, to promote pol5uner chain growth rather than chain termination. Upon UV irradiation. 

This section describes selected arylations of phosphorus, sulfur, and halide nucleophiles under metal-free and metal-catalyzed conditions. Arylations of other nucleophiles, e.g., selenium and tellurium, have been reviewed previously [4]. Aryl phosphonates [ArPO(OR)2] can be synthesized by arylation of phosphite anions with diaryliodonium salts and NaH in DMF at 70-80 °C [158]. A copper-catalyzed arylation of various phosphorous nucleophiles, e.g., diarylphosphine oxides and //-phosphonates, was recently reported to proceed at room temperature. The observed chemoselectivity with unsymmetric salts was opposite to the general trend in metal-catalyzed reactions (see Sect. 2.1), which was explained by a radical mechanism [159]. 

The arylation of nitrogen-containing heterocycles, such as benzotriazole, with aryl iodides can be done only in anhydrous media under rather harsh conditions (DMSO, DBU, 120°C, Pd2(DBA)3-CHCl3) No reaction occurred in the presence of the water-soluble TPPMS ligand. However, the arylation can be accomplished with diaryliodonium salts in neat water under phosphine-less conditions and in the absence of copper salt promoter [88]. Both products of substitution are formed, as the anion of benzotriazole is an ambident nucleophile ... 

Many attempts have been made to find alternatives to the most reactive onium salt photo initiators bearing the AsFe" and SbFg" anions. These efforts have mainly been in response to the perception of toxicity and/or the possibility heavy metal contamination due to arsenic- and antimony-containing residues that are of concern in certain applications. In recent years, there has been considerable progress in the development of novel anions with weakly coordinating (i.e., low nucleophilic) character. For example, Castellanos et al7 have introduced diaryliodonium salt photoinitiators with the tetrakis(pentafluor-ophenyl)borate ((C6F5)4B ) anion. Following closely on this... 

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... 

Arylations of weak organic nucleophiles are best achieved with iodonium salts possessing nucleofugic anions and, in some cases, can be facilitated with transition metal catalysts. Recent examples include Cu(II)-catalyzed S-phenyla-tions of 1-benzothiophenes with diphenyliodonium triflate [118], and Co(II)-catalyzed N-arylations of imidazoles with diaryliodonium tetrafluoroborates (Scheme 42) [119]. 

For reasons which will be detailed later, the simple diaryliodonium bisulfate and halide salts produced in the above synthetic procedures have only very limited use as photoinitiators in cationic polymerization. More generally applicable photoinitiators are obtained by converting the bisulfate and halide salts to those possessing anions of low nucleophilic character such as CF3SO3, BF, PF, AsF, or CIO4. This may be achieved by using one of the three metathetical methods shown in Eqs. 

As in the case of the diaryliodonium and triarylsulfonium salts, photoinitiated cationic polymerizations employing dialkylphenacylsulfonium and dialkyl-4-hydroxy-phenylsulfonium salts exhibit a marked dependency on the structures of both the cation and anion portions of the photoinitiator. Here also, the most efficient photoinitiators are those bearing anions having the poorest nucleophilic character possible i.e. BF4, PF, AsFg, and SbF. ... 

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