Iodonium salts, cyclic

A number of 1-hydroxybenziodoxoles and related heterocycles, more efficiently through their triflates, upon reaction with RC = CSiMe3 afforded either alkynyl iodonium salts or the corresponding cyclic iodanes, depending on the conditions. Some of these compounds reacted with NaN3 and were converted into /J-azidoalkenyl derivatives as illustrated in Scheme 55 [127,167,168]. 

Ethylene reacted with iodonium salts in the presence of a palladium catalyst and a base to afford directly 1,2-bis arylated products (stilbenes). Styrene underwent arylation under similar conditions [44], Allylic cyclic carbonates were efficiently phenylated by diphenyliodonium tetrafluoroborate because of the mild conditions, no ring opening occurred, as was the case when iodobenzene was used. 

To a mixture of the iodonium salt (172 mg, 0.46 mmol) and palladium acetate (4 mg, 5 mol%) was added sodium bicarbonate (77 mg, 0.92 mmol) followed by the cyclic carbonate (110 mg, 0.46 mmol) in DMF (5 ml), under nitrogen, at room temperature. After stirring for 2 h, the reaction mixture was quenched with saturated aqueous ammonium chloride solution and extracted with ether (2 x 20 ml). The extract was dried, the solvent removed and the residue chromatographed on silica (ethyl acetate-hexanes 1 4) to afford 4-phenylvinyl-5-benzyloxymethyl-1,3-dioxol-2-one (140 mg, 97%), m.p. not given. 

Iodonium salts with a hydroxyl or a carboxyl group are readily converted into their inner salts, i.e. zwitterionic compounds of various types. 2-(Phenyliodonio) benzoate, whose structure may be cyclic, belongs to this category it is prepared from o-iodobenzoic acid upon oxidation and coupling with benzene (or arenes) [51], On strong heating, iodobenzene and carbon dioxide are eliminated, with formation of benzyne ... 

Vinyl(aryl)iodonium salts with cyclic vinyl ligands... 

Finally, the participating alkyl chain for such insertion reactions need not be attached directly to vinyl carbon. Thus, exposure of the [/2-(f2-butylsulfonyl)vinyl]iodonium salt shown in equation 236 to triethylamine gives a cyclic sulfone generated via carbenic insertion into the -butyl group32. 

Cyclohexenyl and cyclopentenyl iodonium tetrafluoroborates were also photolyzed in methanol. Ring-strained five-membered cyclic vinyl cation could be generated photochemically as well as the six-membered cyclic vinyl cation. In conclusion, vinyl iodonium salts are excellent pregenitors for vinylic cations due to high nucleofiigality of the iodonio group (about 10 -fold of triflate), although they are in equilibrium with the less reactive hypervalent... 

A mild method for the A-arylation of both aromatic amines and cyclic secondary aliphatic amines using aryl iodonium salts was reported by Kang and coworkers359. 

Syntheses of cyclic iodonium salts and other heterocycles from alkenylio-donium salts 00UK118. 

Arylphosphonic acid esters are produced in the reaction of diaryl iodonium salts with trialkyl phosphites in the presence of copper salts, but despite the simplicity of the procedure yields are low owing to side reactions. The cyclic bisarylphosphinic acid (66) is conveniently prepared by fusion of the phosphine oxide (67) with sodium hydroxide. A successful... 

Reaction of trimethylsilylalkynes with 16 in dichloromethane in the presence of BF3 OEt2 at room temperature followed by heating in methanol at 60 °C results in the stable heterocyclic alkynyliodonium salts 17 [Eq. (9)] [31]. These species represent intramolecular iodonium salts where the counterion is a carboxylate. Unlike the acyclic alkynyl(phenyl)iodonium carbox-ylates 18, that are unstable to isolation and decompose to the corresponding alkynyl benzoates 19 [Eq. (10)] [32], the cyclic analogs 17 are readily isolable. 

Although no Diels-Alder cycloadditions have been reported for the simple alkylethynyl-iodonium salts, the p-substituted systems 11 readily react with diverse dienes [25], The full scope of this reaction is summarized in Scheme 3-9, from which it may be seen that cyclic, acyclic, and endocyclic dienes react with 11 under unusually mild conditions to form cycloadducts, 118-121, in good to excellent isolated yields. All the cycloadducts 118-121 are stable, microcrystalline solids. These adducts have the additional advantage of carrying two different functionalities, Y and IPh OTf, that may be employed for further synthetic elaboration [89]. 

The most important representative of cyclic iodonium salts, the dibenziodolium or diphenyleneiodonium (DPI) cation 238, known in the form of iodide, chloride, hydrosulfate, hexafluorophosphate, or tetrafluoroborate salts, can be obtained by three different procedures (A, B and C) summarized in Scheme 2.71. Method A, originally developed by Mascarelli and Benati in 1909 [355], uses 2,2 -diaminodiphenyl (235) as the starting material, which upon diazotization with sodium nitrite in a hydrochloric acid solution followed by potassium iodide addition, gives DPI 238 as iodide salt. A similar reaction starting from 2-amino-2 -iododiphenyl 236 affords DPI as hexafluorophosphate or tetrafluoroborate in excellent yields (Method B) [356]. The third method involves the peracetic oxidation of 2-iodobiphenyl (237) to an iodine(III) intermediate that then cyclizes in acidic solution (Method C) [357]. More recently, these methods were used to prepare the tritium labeled DPI and of its 4-nitro derivative [358]. 

The preparation and chemistry of cyclic iodonium salts has been summarized in a review of Grushin [361]. Several examples of known cyclic iodonium salts are shown in Figure 2.10 and include 4,5-phenanthryleneiodonium salts (239) [362], 10//-dibenz[fe,e]iodinonium salt (240) [363], 10,11-dihydrodibenz[fe,/]iodeponium salt (241) [363], phenoxiodonium salt (242) [363], 10-acetylphenaziodonium salt (243) [363], 10-oxidophenothiiodonium salt (244) [363], the bicyclic bis-iodonium salt 245 [364], ben-ziodolium chloride 246 [359] and iodolium salt 247 [365],... 

Figure 2.10 Examples of known cyclic iodonium salts.

Several fluoroalkyliodonium salts of other types have been reported [77,530,531], The reaction of tosylate 384 with cyclic enaminones 385 affords stable iodonium salts 386 (Scheme 2.111) [530], Mild thermolysis of salts 386 in boiling acetonitrile cleanly affords 2,2,2-trifluoroethyl tosylate and the respective iodoenaminone [530],... 

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],... 

More recently, iodonium salts have been widely used as photoinitiators in the polymerization studies of various monomeric precursors, such as copolymerization of butyl vinyl ether and methyl methacrylate by combination of radical and radical promoted cationic mechanisms [22], thermal and photopolymerization of divinyl ethers [23], photopolymerization of vinyl ether networks using an iodonium initiator [24,25], dual photo- and thermally-initiated cationic polymerization of epoxy monomers [26], preparation and properties of elastomers based on a cycloaliphatic diepoxide and poly(tetrahydrofuran) [27], photoinduced crosslinking of divinyl ethers [28], cationic photopolymerization of l,2-epoxy-6-(9-carbazolyl)-4-oxahexane [29], preparation of interpenetrating polymer network hydrogels based on 2-hydroxyethyl methacrylate and N-vinyl-2-pyrrolidone [30], photopolymerization of unsaturated cyclic ethers [31] and many other works. 

The focus of this chapter is to provide an overview of the reactivity of diaryliodonium salts, discuss common synthetic routes to them, and describe the recent developments in applications of these reagents after a brief historical perspective in each section. The chemistry of hypervalent iodine compounds has been summarized in a number of books and reviews [1-3], and the topics of diaryliodonium salts [4, 5], cyclic diaryliodonium salts [6], and iodonium salts [7] have been reviewed. Literature summaries of mechanistic aspects and selected transformations with diaryliodonium salts are referenced in the appropriate sections below. 

Scheme 13 Synthesis of 2,2 -diiodobiaryls via cyclic iodonium salts...

Oh and coworkers reported a room temperature arylation of a-substituted malonates with NaH in DMF in 1999 [183]. These conditions were recently applied in the chemoselective arylation of a cychc p-ketoester with an unsymmetric, boron-substituted diaryliodonium salt, leading to a product with a suitable handle for further functionalizations [82]. Olofsson and coworkers performed a detailed chemoselectivity investigation with unsymmetric iodonium salts in the a-arylation of malonates, which resulted in the discovery of an anti ort/io-effect and the conclusion that both mesityl and anisyl moieties are suitable dummy groups in these reactions [27]. This fact was utilized by the group of Shibata, who developed a pentafluorophenylation of cyclic p-ketoesters with unsymmetric TRIP salts under mild conditions (Scheme 14a) [81]. 

Stereoselective carbon-carbon bond formations with hypervalent iodine reagents are also prominently described in the literature. Direct asynunetric a-arylation reactions are not easy to perform. Ochiai et al. synthesized chiral diaryliodonium salts such as [l,l -binaphthalen]-2-yl(phenyl)iodonium tetrafluoroborate derivatives 21 via a BFs-catalyzed tin-X -iodane exchange reaction and developed the direct asymmetric a-phenylation of enolate anions derived from cyclic p-ketoesters (Scheme 7) [37]. A beautiful example of direct asymmetric a-arylation of cyclohexanones in the course of a natural product synthesis was presented through the desymmetrization of 4-substituted cyclohexanones using Simpkin s base, followed by coupling with diaryliodonium salts [38]. Other binaphthyl iodonium salts related to 21 have also been reported [39]. 

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