1 iodonium salts

Diaryliodonium salts are the most stable and well-investigated class of iodonium salts. The first example of these compounds, (4-iodophenyl)phenyliodonium bisulfate, was prepared by Hartmann and Meyer in 1894 from iodosylbenzene and sulfuric acid [368]. Diaryliodonium salts Ar2l+ X are air- and moisture-stable compounds, whose physical properties are strongly affected by the nature of the anionic part of the molecule. In particular, diaryliodonium salts with halide anions are generally sparingly soluble in many organic solvents, whereas triflate and tetrafluoroborate salts have a better solubility. The chemistry of aryl-and heteroaryliodonium salts has been extensively covered in several reviews [361,369,370]. 

Older methods for the preparation of symmetrical diaryliodonium salts are based on the reaction of arenes with potassium iodate or KIO3/I2 in the presence of concentrated sulfuric acid [371, 372]. It is assumed that the mechanisms of these reactions involve initial formation of the electrophilic iodyl or iodosyl species, I02+HS04 or IO+HS04, which further react with arenes, finally forming diaryliodonium salts. 

Ar = Ph, 4-MeC6H4,4-Bu CeH4, 2,4-Me2CsH3,4-FC6H4,4-CIC6H4,4-BrCsH4 

Several modified procedures for the preparation of diaryliodonium triflates have been reported in a more recent literature [382-384], Kitamura and Hossain have developed a one-pot reaction to obtain symmetrical diaryliodonium triflates in good yields directly from iodoarenes and aromatic substrates using K2S2O8 as an 

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Other pseudo-halides are used for carbonylation. Phenyl tluorosulfonate (484) can be carbonylated to give benzoate[337]. Aryl(aryl)iodonium salts[338], aryl(alkenyl)iodonium salts (485)[339], and arylialkynyl)iodonium salts (486)[340] are reactive compounds and undergo carbonylation under mild conditions (room temperature, 1 atm) to give aryl, alkenyl, and alkynyl esters. lodoxybenzene (487) is carbonylated under mild conditions in... 

The alkenyl(phenyl) iodonium salt 725 undergoes the facile cross-coupling with vinylstannane to form the conjugated diene 726[594]. 

Efficient homocoupling of the aryi iodonium salt 827 using Zn is catalyzed by Pd(acac)2[708], Homocoupling of the aryisuifonyl chloride 828 as a pseudohalide takes place in the presence of 2 equiv. of Ti tctraisopropoxide[7Q9]. 

Triarylbismuthines have been synthesized by means of the Nesmeyanov reaction that employs an arenediazonium salt such as the tetrafluoroborate, a bismuth trihahde, and a reduciag agent (51). The decomposition of iodonium salts ia the preseace of bismuth trichloride and metallic bismuth also leads to the formation of triarylbismuthines, Ar Bi (52) ... 

Experiments with unsymmetrical iodonium salts iadicate that the bismuth atom is preferentially arylated by the more electron-attracting aryl group. 

While electron beams can produce cations, they are not effective at producing cationic cure in the absence of suitable photoinitiators. The same cationic photoinitiators used for UV cure are often also e-beam sensitive. Examples are triaryl sulfonium or diaryl iodonium salts [41]. 

Oxo-bridged diiodonium triflate [Zefirov s reagent), a useful reagent for the synthesis of triflate esters and iodonium salts, can be prepared by the treatment of lodosobenzene with triflic anhydride [92] or by the reaction of lodobenzene diacetate with tnflic acid [93] (equation 44). 

Another useful reagent for the preparation of alkynyl lodonium Inflates is [cyano(trifluoromethylsulfonyloxy)(phenyl)]iodine [/i7, 138, 139, 140] prepared from iodosobenzene, trimethylsilyl tnflate, and trimethylsilyl cyanide (equation 71). This reagent reacts with various stannylacetylenes under very mild conditions to form the corresponding alkynyl iodonium salts in high yields [139] (equation 72)... 

Low yields of C-arylated ketones have also been obtained by reaction of pyrrolidine enamines with diaryl iodonium salts (68). 

The initiating radicals are assumed to be SCN, ONO or N3 free radicals. Tris oxalate-ferrate-amine anion salt complexes have been studied as photoinitiators (A = 436 nm) of acrylamide polymer [48]. In this initiating system it is proposed that the CO2 radical anion found in the primary photolytic process reacts with iodonium salt (usually diphenyl iodonium chloride salt) by an electron transfer mechanism to give photoactive initiating phenyl radicals by the following reaction machanism ... 

Iodosoarenes are useful in the preparation of iodonium salts, Ar2I+X-.6... 

Iodosobenzene diacetate is used as a reagent for the preparation of glycol diacetates from olefins,9 for the oxidation of aromatic amines to corresponding azo compounds,10 for the ring acetylation of N-arylacetamides,11 for oxidation of some phenols to phenyl ethers,12 and as a coupling agent in the preparation of iodonium salts.13 Its hydrolysis to iodosobenzene constitutes the best synthesis of that compound.14... 

See Glaser, R. Horan, C.J. Nelson, E.D. Hall, M.K. J. Org. Chem., 1992,57, 215 for the influence of neighboring group interactions on the electronic stmcture of diazonium ions. Aryl iodonium salts Ar2l also undergo substitutions by this mechanism (and by a free-radical mechanism). 

Crivello, J. V., Lockhart, T. P., and Lee, J. L., Diaryl-iodonium salts as thermal initiators of cationic polymerization,... 

The iodosobenzene HBF4 complex 2022 adds to the enol silyl ether 653 of acetophenone to give the labile iodonium salt 2023, which reacts with cyclohexene or tetramethylethylene to give the adducts 2024 and 2025 [188] (Scheme 12.55). 

The benzoiodoxol 2035 is converted by Me3SiN3 19 into 2036, which oxidizes cyclohexene to a-azidocyclohexanone 2037 [192], whereas 2035 reacts with bis(trimethyl-silyl)acetylene 2038 to give the iodonium salt 2039 [193] (Scheme 12.57). 

Iodination of moderately reactive aromatics can be effected by mixtures of iodine and silver or mercuric salts.31 Hypoiodites are presumably the active iodinating species. Bis-(pyridi nc)iodonium salts can iodinate benzene and activated derivatives in the presence of strong acids such as HBF4 or CF3S03H.32... 

The functionalized [4]radialene 86 offers opportunities for further transformations by hydrolytic cleavage of the O-silylenol moieties and by oxidative desilylation (Scheme 16). Base- and acid-catalyzed hydrolyses lead to different products (130 and 131, respectively)60. By analogy with the formation of 1,4-diketones by oxidative coupling of two siloxyalkene molecules, treatment of 86 with the iodonium salt Phl+—O—+I—Ph BF4 in dichloromethane leads to 132 which is immediately... 

Arylethynyl(phenyl)iodonium salts, RC=CI+Ph 4-MeC6H4S03-, react as 1,3-dipolarophiles with nitrile oxides R CNO to afford phenyl(substituted isox-azolyl)iodonium salts 210, which give iodoisoxazoles on reaction with nucleophiles. The crystal structure of 210 (R = Ph, R1 = mesityl) has been determined (369). 

Palladium-catalyzed directed intramolecular activations of aryl C-H bonds have been reported, as in the phenyla-tion of heterocycle analogs. Palladacycles are proposed intermediates, acting as effective catalysts, and the mechanism is likely to proceed via oxidation of Pd(ll) to Pd(iv) by the iodonium salt, as for the Equation (57), which described the activation of benzylic i/-CH bonds (Equations (121)—(123).109... 

Nucleophilic vinylic substitution and vinyl cation intermediates in the reactions of vinyl iodonium salts. 37, 1... 

Scheme 4.36. Hydrozirconation of alkynyl(phenyl)iodonium salts.

Furthermore, arylthiophenes have been prepared using the Stille coupling of hypervalent iodonium salts [96] or organolead compounds [97, 98] as electrophiles in place of aryl or vinyl halides and triflates. Hypervalent iodonium salts are sufficiently reactive to undergo coupling at room temperature. 

Two approaches have been applied for the synthesis of imidazo[2,l + thia/,ole ring systems. Reaction of 2-mercapto-benzimidazole with perfluoro-2-methylpent-2-ene in the presence of triethylamine gave compound 419 (Equation 192), and cyclocondensation of 2-imidazolidinethione with the alkynyl(phenyl)iodonium salt 420 afforded product 421 (Equation 193) <2001RCB1446, 2003JCM715>. 

Iodometric ozone analysis, 17 811 Iodometric titration, 13 588 Iodonium salts, 15 166... 

The focus of Chapters 3 and 4 is on vinyl cations. In Chapter 3, T. Muller et al. discuss the preparation, isolation, and characterization of unusually stable vinyl cations whereas Chapter 4 by T. Okuyama and M. Fujita describes the generation and reactions of vinyl cations formed via solvolysis of vinyl iodonium salts. 

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