Synthesis of diaryliodonium salts

The synthesis of diaryliodonium salts was first described by Hartmann and Meyer in 1894 Since that time, the number of general preparative methods has been increased through the work of Beringer and his coworkers and by Mason and his research group Some of the more synthetically useful schemes are shown in 

These methods fall into two broad classifications those involving electrophilic aromatic substitution (methods 1-6) and those proceeding by means of and organo-metallic mediated displacement reactions (methods 7-8). The choice of a specific method depends on whether the desired diaryliodonium salt bears substituents and where they are situated in the molecule. A review of the merits and limitations of the above methods has recently been published by Olah Using such methods, a wide diversity of photoinitiators having a considerable breadth of ultraviolet absorption characteristics can be prepared. An extensive and useful compendium of diaryliodonium salts prepared up to 1956 has been published by Beringer and Gindler 

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. 

Ar2UHS04 -t- NH4CI Ar2UCri -H NH4HSO4 ArjUcr + AgX At2UX- -t- AgClj 

Diaryliodonium salts have been characterized by a number of analytical techniques. BeringerPetrosyan , and Nesmeyanovhave used NMR to examine the electronic and inductive effects of the positively charged iodonium moiety at the various aromatic ring positions and to observe its interaction with different anions. Their results correlate well with that obtained from conductivity measurements and 

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Reaction of some / -trifyloxy-vinyl(phenyl)iodonium triflates with aryllithi-ums can lead to the synthesis of diaryliodonium salts in a manner analogous to... 

This short survey of our investigations on onium compounds is incomplete if we do not mention a novel synthesis of diaryliodonium salts discovered in collaboration with Freidlina (43) ... 

Treatment of the linear p-halovinyliodine dichloride (51) with aryllithium is an indirect method for the synthesis of diaryliodonium salts, ligand exchange being the main process. 

Peacock and Fletcher have reported a simple, one-step procedure for the synthesis of diaryliodonium salts by the electrochemical oxidation of aryl iodide at a carbon felt anode in acetic acid in the presence of an arene [407,408]. This reaction gives good yields of diaryliodonium salts for aryl iodides and arenes with alkyl substituents in the benzene ring. 

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

Since the first synthesis of diaryliodonium salts in 1894 [56], a large variety of synthetic routes to diaryliodonium salts have been reported. Although the majority of these are stepwise, efficient one-pot methods have recently been developed. The... 

Scheme 4 Synthesis of diaryliodonium salts from iodine(in) precrasors...

A similar beneficial effect of 1,1-diphenylethylene on the yields of arylation products was later observed in the synthesis of benzonitrile derivatives by reaction of diaryliodonium salts with potassium cyanide and in the reaction of diaryliodonium salts with the sodium salt of nitroalkanes. 7 In the latter case, the reaction was therefore considered to result from intermediate inner-sphere radicals. Some years later. Barton et al. showed that 1,1-diphenylethylene acts as an efficient inhibitor of the radical chain process in the reaction of enolates with diaryliodonium salts. They concluded that the arylation products arose from a non-radical process. 

Iodonium Salts. Recent developments in the synthesis of iodonium salts useful in photoinitiated polymerization processes have been driven primarily by the desire for increased solubility of the salts in the polymerization media. Metathesis of diaryliodonium bisulfate to diaryliodonium tetrafluoroborate was patented by Klemm and Bartin [27] in 1986,... 

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). 

Olofsson and coworkers have developed several efficient one-pot syntheses of diaryliodonium salts [385-390], A general and universal procedure provides both symmetrical and unsymmetrical diaryliodonium triflates 272 from both electron-deficient and electron-rich arenes 271 and aryl iodides 270 using mCPBA as the oxidant and trifiic acid (Scheme 2.78) [385-387]. The electron-rich diaryliodonium tosylates are prepared similarly using toluenesulfonic acid instead of trifiic acid as the additive [387]. Symmetrical diaryliodonium triflates can be synthesized by a modified one-pot procedure from iodine, arenes, mCPBA and trifiic acid under similar conditions [374,375]. A similar procedure based on a one-pot reaction of arylboronic acids, aryl iodides, mCPBA and BF3 -Et20 has been used for regioselective synthesis of unsymmetrical diaryliodonium tetrafiuoroborates [388,389]. In a further improvement of this approach, a range of... 

Olofsson and coworkers have developed a general and high-yielding synthesis of various diaryl ethers 681 using the reaction of diaryliodonium salts with phenols under basic conditions (Scheme 3.273) [874]. The scope of products includes bulky orf/io-substituted diaryl ethers, which are difficult to obtain by metal-catalyzed protocols. A similar procedure has been used for the metal-free synthesis of aryl esters from carboxylic acids and diaryliodonium salts [875]. 

Carroll and coworkers have published a series of papers on the use of aryliodonium salts in the synthesis of fiuorine-containing aromatic and heterocyclic products [63, 74, 75]. It has been found that the addition of radical scavengers such as TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) during the fluoridation of diaryliodonium salts leads to a significant improvement of both the reproducibility of the process and the... 

Chen and collaborators developed an efficient, regioselective, Cu(II)-catalyzed one-pot synthesis of substituted quinazolines via a [2 + 2 + 2] cascade annulation of diaryliodonium salts with two nitriles. The authors proposed the following mechanism (Scheme 34) (13CC6752). The first nitrile reacts with the diaryliodonium salt to form A/-arylnitrilium intermediate 72, which is treated with the second nitrile to give an intermediate 73. Subsequent electrophilic aromatic substitution yielded the desired multi-substituted quinazolines 74 in moderate-to-good yields. 

The vast majority of fluoridations of diaryliodonium salts have been performed tmder metal-free conditions, but Sanford and coworkers recently developed copper-catalyzed conditions for efficient fluoridation with KF (Scheme 11a). The method has a wide scope, and imsymmetric mesityl salts could be chemoselectively employed, giving easy access to a range of fluoro(hetero)arenes. The methodology was subsequently modified to suit radiofluorination and applied to the synthesis of the radiotracers 4-[ F]fluorophenylalanine and 6-[ F]fluoroDOPA [152, 153]. 

Another successful application of diaryliodonium salts was demonstrated in synthesizing quinazolines 143 (Scheme 32). The developed one-pot strategy involved the reaction of aromatic nitriles 142 (2 mol) with diaryliodonium salts (Imol) in the presence of copper triflate as a catalyst. The proposed mechanistic pathway is believed to involve [2-f2-f2] cascade annulation. This novel synthesis of quinazolines further extended the scope and reactivity of diaryliodonium salts in the synthesis of azaheterocycles (Scheme 32) [48]. 

As with many cross-coupling reactions that result in the formation of C—P(0) bonds, the use of bulky electrophiles results in slow reactions. A potentially attractive solution to this problem has been reported by Tang (Schemes 4.194 and 4.195) [333]. He successfully coupled a range of diaryliodonium salts with secondary phosphine oxides using simple copper halides as catalysts. When unsymmetrical diaryliodonium salts were used, the bulkier group was preferentially transferred. Furthermore, the chemistry tolerated bulky groups on the secondary phosphine oxide. Another attractive aspect of this reaction was that it was rapid at room temperature. In addition to the synthesis of bulky phosphine oxides, the authors reported over two dozen additional examples (70-98% yields). 

The use of diaryliodonium salts in organic synthesis has grown significantly dne to their nnique reactivity profiles [159]. The ability to tune the electronic and steric components of the diaryliodonium salt to suit the needs of specific reaction is driving this chemistry. 

In recent years, a variety of hypervalent iodine reagents have been available. The versatility of these hypervalent organoiodine reagents in organic synthesis has been well recognized. Diaryliodonium salts constitute an important reagent class for the transfer of aryl groups. These iodonium ion salts have been used effectively in C-arylation of a variety of nucleopohiles.112 The arylation of the anion of nitroalkanes with diaryliodonium salts was already reported in 1963.113... 

Liu, Z. and Chen, Z., Studies on the application of hypervalent iodine in synthesis. 11. Synthesis of arylphosphonates by arylation of phosphite anions using diaryliodonium salts, Synthesis, 373, 1993. 

The synthetic potential of palladium-mediated cross-coupling reactions (Heck, Suzuki, Stille, Sonogashira, Buchwald-Hartwig) led to the search for a practical synthesis of p-[ F]fluoroiodo- and p-[ F]fluorobromobenzene. p-[ F]Fluoroio-dobenzene (G, X = iodine) can be obtained in poor yield from p F]fluoride and a trimethylammonium precursor (P7). p-p F]Fluorobromobenzene can be prepared in a more reproducible way from 5-bromo-2-nitrobenzaldehyde (radiochemical yields > 70%). The synthesis involves a two-step procedure radiofluorination (F for NO2 substitution), then a catalysed decarbonylation [190,191]. Also very efficient is the one-step reaction of p F]fluoride with a suitable diaryliodonium salt (P6) giving >70% radiochemical yield [192-194]. 

A. Shah, V.W. Pike, D.A. Widdowson, The synthesis of [F-18]fluoroarenes from the reaction of cyclotron-produced [F-18]fluoride ion with diaryliodonium salts, J. Chem. Soc. Perkin Trans. I Organic and Bio-Organic Chemistry 13 (1998) 2043-2046. 

Diaryliodonium salts (diaryl-A3-iodanes) are widely used as arylating agents. There are a number of methods available for their synthesis typically involving two or three steps.378,379 A recent one-pot approach, however, offers a simple and high-yielding access to unsymmetrical diaryliodonium triflates using meto-chloroperbenzoic acid (mCPBA) as the oxidant380 [Eq. (4.111)]. Moreover, symmetrical diaryliodonium salts can directly be prepared from iodine and arenes without the use of expensive aryl iodides [Eq. (4.112)]. 

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]. 

In a rare example of the use of iodonium salts for heteroatom-heteroatom bond formation, diaryliodonium halides were employed with sodium 0,0-diethyl phosphoroselenolate for a one-pot synthesis of diaryl diselenides (Scheme 9) [27]. These transformations probably occur via arylation of the phosphoroselenolate salt with the diaryliodonium ions, hydrolysis of the resulting aryl phosphoroselenolates with sodium hydroxide, and air oxidation of the arene-selenide ions thus produced. 

Total synthesis of a selective thyromimetic SK FL-94901 (76) was accomplished by diaryl ether formation from symmetrical or unsymmetrical diaryliodonium salts. 

Apart from copper(I)-mediated reactions, few studies of the treatment of vinyliodonium salts with carbanions have appeared. The vinylations of the 2-phenyl- and 2- -hexyl-l,3-indandionate ions shown in equations 222 and 223 are the only reported examples of vinyliodonium-enolate reactions known to this author26,126. ( ,)-l-Dichloroiodo-2-chloroethene has been employed with aryl- and heteroarvllithium reagents for the synthesis of symmetrical diaryliodonium salts (equation 224)149,150. These transformations are thought to occur via the sequential displacement of both chloride ions with ArLi to give diaryl (/ -chlorovinyl)iodanes which then decompose with loss of acetylene (equation 225). That aryl(/ -chlorovinyl)iodonium chlorides are viable intermediates in such reactions has been shown by the conversion of ( )-(/ chlorovinyl)phenyliodonium chloride to diaryliodonium salts with 2-naphthyl- and 2-thienyllithium (equation 226)149,150. 

Diaryliodonium salts are viable alternatives to aryl halides for palladium-catalyzed cross-coupling reactions (02CRV2523). In the area of heterocyclic synthesis, Pd(II)-catalyzed carbonylation of diaryliodonium... 

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