Aryliodonium salts,

The reaction of benzotriazoles with aryl halides catalyzed by a mixture of Pd(dppe)Cl2 (DPPE = bis-(diphenylphosphino)ethane) or Pd(dppf)Cl2, copper(I)iodide or copper(II)carboxylates, and a phase-transfer catalyst has been shown to proceed in good yield in DMF solvent.104 Both copper and palladium were required for these reactions to occur at the N-l position in high yields. Similar results for the coupling of amines with aryliodonium salts in aqueous solvent were observed.105... 

Besides, iodoarenes aryliodonium salts were used as electrophilic coupling partners in the synthesis of ynones. The carbonylative cross-coupling can be run in aqueous DME in the presence of Pd(OAc)2 and Gul as co-catalyst and NaHG03 as a base at 30 °G. ... 

Aryliodonium salts have been found to be coinitiators for photooxidizable dye sensitization (105). Smith polymerized aerylamide-bis(acrylamide) mixtures using acridine, xanthene, or cyanine dyes and, for example, diphenyllodonium chloride as an electron acceptor. Reduction of the salt results in the formation of phenyl radicals. 

The self-condensation of iodosylbenzene was the first reported synthesis of a diaryliodonium salt back in 1892. The mechanism of the reaction was delineated only recently. This approach served for the synthesis of p-(phenylene)bis-(aryliodonium) salts [47], as well as some oligomers from (diacetoxyiodo)ben-zene and triflic acid [117], followed by coupling with an arene (Scheme 39). Under suitable conditions the same reaction can lead to simple phenyl(aryl)-iodonium triflates [118]. 

Aryl-, as well as heteroaryliodonium salts, belong to the most common, stable, and well investigated class of polyvalent iodine compounds. The preparation and chemistry of aryliodonium salts was extensively covered in several reviews [5,7,9,10]. Diaryliodonium salts have found synthetic application as arylating reagents in reactions with various organic substrates. 

Several reactions of aryliodonium salts leading to the formation of new C-C bond are known. The most important and synthetically useful reactions include the direct arylation of carbon nucleophiles, the transition metal mediated crosscoupling reactions, and the reactions involving the generation and trapping of the benzyne intermediates. 

Arylations with aryliodonium salts can be effectively catalyzed by transition metals. The arylation of the lithium enolates of cyclic ketones with di-phenyliodonium triflate in the presence of stoichiometric quantities of copper cyanide affords the corresponding a-phenylated ketones in moderate yields [73]. 

Similar to alkenyliodonium salts (see Sect. 6.3), aryliodonium salts are highly reactive substrates in Heck-type olefination and other palladium-catalyzed coupling reactions. Aryliodonium salts can serve as very efficient reagents in the palladium-catalyzed arylations of acrylic acid 101 [75], organotin compounds 102 [76], sodium tetraphenylborate 103 [77], and copper acetylide 104 [78] (Scheme 45). 

Alkyltrifluoroborates (RBF3K, see 12-28) react with aryl triflates or aryl halides,or aryliodonium salts with a palladium catalyst, to give the arene. The reaction is compatible with sensitive functionality, such as an epoxide unit. 

Carbonylation. Aryliodonium salts undergo carbonylation, which results in methyl aroates or alkynyl aryl ketones, depending on the presence of MeOH or 1-aIkynes. - ... 

Coupling of allylic alcohols CH2=CHCH(OH)R with aryliodonium salts to give ArCH=CHCH(OH)R using polymeric phosphine ligated PdClj and NaHCOj enjoys many advantages. The catalyst can be easily removed from the reaction mixtures and reused more than 10 times. In the absence of base or with a catalyst prepared from Pd(OAc)j the yields are slightly lower. 

Photochemistry of Triarylsulfonium Salts. A similar mechanism was proposed for the photolysis of triarylsulfonium salts having nonnucleophilic anions. The arylsulfonium and to a lesser extent, the aryliodonium salts have improved thermal stability as compared to the diazonium salts, however, their absorption maximum occurs at a much shorter wavelength since the aromatic rings are isolated by the heteroatom, and therefore not conjugated. 

Figure 8, Chain reaction photoinitiation. The efficiency of photoinitiation can be increased by chain reactions. Hydrogen abstraction by triplet excited benzophenone forms a THF free radical. Subsequent oxidation by the aryliodonium salt produces the THF cation capable of initiating polymerization, and a phenyl radical. Hydrogen abstraction by the phenyl radical produces the THF free radical completing the...

Ledwith and coworkers have studied the cationic polymerization of THF initiated by onium salts in combination with free radical proginators.(20-23) When 2-2-dimethoxy-2-phenylacetophenone (IRGACURE 651) was irradiated in THF, the experiment containing an aryliodonium salt had a seven fold greater polymerization rate than a comparable experiment containing an arylsulfonium salt (21). 

Figure 9. Photoreduction of onium salts by free radicals. Photoinitiators which directly form easily oxidized free radicals can efficiently sensitize aryliodonium salts, resulting in the formation of acid (HAsP ). Having higher reduction potentials, the arylsulfonium salts do not oxidize most free radicals.

Following this clear demonstration, a large series of nucleophilic reactions of alkynyl aryliodonium salts were subsequently discovered. 24 They have for the most part been explained by the... 

Polyfluoroalkyl)aryliodonium salts react with nucleophiles to afford the polyfluoroalkylated nucleophiles under mild conditions. The products are almost exclusively derived from the fluoroalkyl moiety. Two types of fluoroalkyl chain Rf have been involved in these reactions the perfluoroalkyl and the a,a-dihydroperfluoroalkyl compounds. 11,205,206... 

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