Diaryliodonium salts preparation

Diaryliodonium Salts Preparative Methods. Carbon-Carbon Bond-Forming Reactions. Arylation of Heteroatoms. 

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... 

This procedure represents a novel, convenient, and fairly general method for preparing y-aryl-/3-diketones. By this method the submitters have phenylated the dianion of 1-phenyl-1,3-butanedione (61%), 2,4-heptanedione (98%), 2,4-nonanedione (78%), 2,4-tridecanedione (53%), and 3,5-heptanedione (50%).6 Substituted diaryliodonium salts have also been used to produce l-(4-chlorophenyl)-2,4-pentadione (44%), 4-(4-methyl-phenyl)-l-phenyl-l,3-butanedione (44%), and l-(4-methyl-phenyl)-2,4-nonanedione (21%).6 Under these conditions no more than a trace, if any, of arylation at the a-position of the /3-diketones was observed by gas chromatography analysis. 

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]. 

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],... 

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 first preparations of diaryliodonium salts have been reported in the 19th century, but refinements and improvements keep appearing to date. In most cases an iodoaryl species containing iodine(III) is coupled with an arene or a derivative of it in a typical electrophilic aromatic substitution. Lithiated, stannylated or silylated aryls and arylboronic acids or borates have been introduced recently in order to avoid harsh conditions and to improve yields. The iodoaryl species may be also formed in situ from arenes and iodine(III) reagents. 

Aryl and heteroaryl (furyl, thienyl) boronic acids are especially suitable for the preparation of their iodonium salts, having the added advantage of better yields and lack of toxicity [108]. Tetraarylborates (sodium or potassium) reacted with (diacetoxyiodo)arenes in acetic acid to afford diaryliodonium salts in excellent yield (Scheme 37). It appears that triarylboranes formed upon reaction of the borates with acetic acid serve actually as the real arylating agents [109]. 

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. 

It is well-known that nucleophilic substitution reactions of diaryliodonium salts (74), prepared by the reaction of substituted arenes (73) with iodine(III) reagents such as PIDA, PIFA, Phl=0, and I(OCOCF3)3, smoothly proceed to yield various functionalized arenes (75) [49] [Eq. (15)]. 

Diaryliodonium salts. lodosoarenes are useful for the preparation of diaryl-iodonium salts. One method involves Friedel-Crafts type condensation with an aromatic compound in the presence of acids. For example, a cold solution of 5 g. 

In the presence of a catalytic amount of PdCh, Ar>,BiX2 (X=C1, OAc) couples with arylstannanes to give biaryls (Scheme 14.155) or, under a CO atmosphere, diaryl ketones (Scheme 14.156) [316]. Unsymmetrical biaryls are also prepared by Pd-catalyzed reaction of diaryliodonium salts with Ar3BiX2 [317]. Terminal acetylenes are phenylated by Ph3BiF2 under CuCl catalysis to afford phenyl-substituted acetylenes [318]. 

Hydroxamic acids react with diaryliodonium salts to afford the O-phenyl derivatives. number of heterocyclic compounds containing A -hydroxyamino groups were selectively arylated on the oxygen atom. 0 (Table 5.4) In the case of oximate anions, ambident behaviour was observed, with predominant 0-arylation. With heterocyclic oximes, the 0-aryl ethers were mainly formed and served as precursors to prepare unstable aryl fulminates, 19 (Table 5.5) In the reaction of quinolone derivatives with diaryliodonium salts, the products of O- or C-arylation were obtained.(Table 5.6)... 

Crivello et al. [31], reported the synthesis of alkoxy substituted diaryliodonium salts, which offered the advantages of easy preparation, very good photoresponse, low toxicity for those having at least eight carbons in the alkoxy chain, and higher solubility in nonpolar monomers. 

Crivello et al. [32], described arylsulfonium salts with two or three photoactive groups in the same molecule. These were prepared via arylation of thiophenoxyaryl sulfides by the thermal decomposition of diaryliodonium salts. The extended conjugation of the chromophores introduced long wavelength absorption bands. 

Covalent I1" is known also in the compound triphenyliodine, (C6H5)3I, and a large number of diaryliodonium salts, such as (C6H5)2I+X, where X may be one of a number of common anions. Aryl compounds such as C6H5IC12 are also well known and can be prepared by direct interaction they can be regarded as trigonal-bipyramidal with axial chlorine atoms and equatorial phenyl group and lone-pairs. The compounds can be used for chlorination of alkenes. 

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. 

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... 

Polystyrene-supported diaryliodonium salts 33 can be prepared by the reaction of diacetate 4 with arenes in the presence of sulfuric acid (Scheme 5.19) [55,56]. Polymer-supported aryliodonium salts are useful aryl transfer reagents in Pd(II)-catalyzed cross-coupling reactions with salicylaldehydes [56] and aromatic hydrocarbons [55]. 

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. 

Numerous works on the optimization of [ F]-fluoridations and the preparation of specific [ F]-labeled radiotracers using diaryliodonium salts have been published. Wiist and coworkers have developed a convenient access to 4-[ F]fiuoroiodobenzene (28) employing 4,4 -diiododiaryliodonium salt 27 as a precursor (Scheme 7.8) [71-73]. 4-[ F]Fluoroiodobenzene (28) has been further utilized in Sonogashira or Stille cross-coupling reactions for the preparation of numerous radiotracers. For example, the Stille reaction with 4-[ F]fiuoroiodobenzene has been used for the synthesis of radiotracers for monitoring COX-2 expression by means of PET. By using optimized reaction conditions F-labeled COX-2 inhibitors 29 and 30 could be obtained in radiochemical yields of up to 94% and 68%, respectively, based upon 4-[ F]fiuoroiodobenzene (28) [72]. 

This reaction can be applied to the preparation of diaryliodonium salts. 

Borylated pyrrole 88 was prepared by Oestreich by treatment of the corresponding pyrrole with pinacolborane and a ruthenium(II) thiolate complex. The direct synthesis of 88 promises to find wide utility in medicinal chemistry and was applied to a variety of substituted indoles (13JA10978). N-Methylpyrrole was directly arylated at room temperature by photoredox catalysis with diaryliodonium salts to furnish 89 in 84% yield (13SL507). 

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