Diaryliodonium tetrafluoroborates

Arylations of weak organic nucleophiles are best achieved with iodonium salts possessing nucleofugic anions and, in some cases, can be facilitated with transition metal catalysts. Recent examples include Cu(II)-catalyzed S-phenyla-tions of 1-benzothiophenes with diphenyliodonium triflate [118], and Co(II)-catalyzed N-arylations of imidazoles with diaryliodonium tetrafluoroborates (Scheme 42) [119]. 

The thermolysis of some diaryliodonium tetrafluoroborates in the presence of potassium fluoride led to the formation of fluoroarenes (38-85%) [60]. 

TABLE 9. 27I NQR coupling constants and asymmetry parameters for diaryliodonium tetrafluoroborates [Ar1 ArI]+BF4 ... 

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,... 

The mechanism of solvolysis of methoxy-substituted diaryliodonium tetrafluoroborates, Arl+Ph Bp4, in methanoi and 2,2,2-trifluoroethanol has been investigated [893]. The solvolysis products include alkox-ide substitution products (ArOR and PhOR) as well as iodoarenes (Phi and Arl). The ratios of products, ArOR/PhOR, range from 8 2 to 4 6. The results of this study provide experimental evidence against the formation of aryl cation under these conditions and support the pathways via ligand coupling or 5nAt2 mechanisms involving a solvent molecule as a nucleophile in the transition state [893]. If the reaction is performed in inert (not nucleophilic) solvent, various nucleophiles may be involved in the reaction with iodonium salt. 

Example 7.27 Synthesis of an Estrone-Derived Diaryliodonium Tetrafluoroborate [96]. 

The estrone-derived arylboronic acid (1.0 equiv) and CH Clj (0.075 M) were combined in an oven-dried round-bottom flask equipped with a stir bar. The mixture was cooled to 0 °C, BFj-OEtj (1.10 equiv) was added, and the mixture was stirred for 10 min. 2-(Diacetoxyiodo) mesitylene (1.05 equiv) was then added as a solution in CH Cl (0.33 M), and the mixture was warmed to room temperature and stirred for 2h. The reaction was quenched by the addition of saturated aqueous NaBF. After 30 min of vigorous stirring, the aqueous layer was extracted with CH Clj (2x). The combined organic layers were dried over MgSO, filtered, and concentrated under vacuum. Et O was added to the residual solid, and the diaryliodonium tetrafluoroborate (73%) was collected via filtration, washed with Et O, dried under vacuum overnight, and stored in a dry box under until use. 

Aryldiazonium (ArNjZ-), diaryliodonium (Ar2I+Z ), and triarylsulfonium (Ar3S+Z ) salts, where Z is a nonnucleophilic and photostable anion such as tetrafluoroborate (BF4), hexafluoroantimonate (SbFg), and tetraperfluorophenylborate [(CgFsXB-], and... 

The catalytic influence of 18-crown-6 on the production of fluorobenzene from diphenyliodonium tetrafluoroborate and KF in dichloroethane was documented some years ago [123]. More recently, diaryliodonium salts have been used for direct syntheses of [18F]-fluoroarenes [124,125]. After an initial study in which various counterions were surveyed, this was finally accomplished by the treatment of diaryliodonium triflates and trifluoroacetates with 18F K+-APE 2.2.2 (i.e., the aminopolyether 4,7,13,16,21,24,27-hexaoxa-l,10-diazabicyclo-[8.8.8]hexacosane) or Cs 18F in acetonitrile (Scheme 44). An added feature of these studies is rather extensive confirmation that nucleophiles are preferentially directed to the more deactivated ring of unsymmetrical diaryliodonium ions, unless one of the rings possesses ortho-substituents [125]. 

The outcome of the reaction of pyridine with diaryliodonium salts was dependent upon the nature of the counterion. With a non nucleophilic counterion as the tetrafluoroborate, the iV-phenylpyridinium salt (31) was obtained in high yield. 23 with bromide, no reaction occurred and with chloride, ring arylation was observed as a result of the formation of phenyl free radicals. 25... 

Biaryls. The coupling of diaryliodonium salts with aryllead triacetates to afford unsymmetrical biaryls is effected by (dba)3Pd2.CHCl3. For example,4-MeOCsH4Ph is obtained from the reaction of 4-anisyl(phenyl)iodonium tetrafluoroborate with PhPb(OAc>3... 

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]. 

Cationic iodonium and sulfonium salts of strong acids are well known as UV-activatable strong acid sources, but they have also found use as heat-activatable acid sources in epoxy resins. Diaryliodonium salts, such as diphenyliodonium tetrafluoroborate (67), have found use with activators, e.g., benzopinacol, in epoxy adhesive and coating formulations. Cure times of several minutes at 160 C were claimed, with good room temperature latency. In a series of patents issued to Asahi Denka Kogyo K.K., several strong acid salts of aliphatic sulfonium compounds have been described as catalysts in heat curing epoxy compositions (68). " The counteranions... 

Photoinitiators used for epoxy curing include aryldiazonium salts (ArN2+X ), diaryliodonium salts (Ar2l+X ), and onium salts of Group Via elements, especially salts of positively charged sulfur (ArsS+X ). The anions must be of low nucleophilicity, such as tetrafluoroborates or hexafluorophosphates, to promote pol5uner chain growth rather than chain termination. Upon UV irradiation. 

The first synthesis of biaryls with diaryliodonium salts was reported by Yan and coworkers in 2006, who treated various Ar2lX with excess sodium tetraphe-nylborate and tosic acid in water at 50 °C to obtain Ar-Ph in good yields [196]. Diaryliodonium bromides were more efficient than tetrafluoroborates, and... 

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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