Diaryliodonium halides

A Michaelis-Becker approach using the sodium salt of dialkyl phosphites has been successful for the formation of C-P bonds to aromatic rings.184 Reaction of the dialkyl phosphite in DMF with the appropriate diaryliodonium halide provides the corresponding dialkyl arylphosphonate in good yield (81 to 93%). This approach to the arylphosphonate esters proceeds in superior yield under relatively mild conditions and therefore is one to be accorded serious consideration for the preparation of such materials. 

The use of diaryliodonium salts for direct arylations of nucleophilic species is a well-established practice. Examples of C-heteroatom bond formation by this approach, including uncatalyzed arylations of dialkyl phosphite, thiocarboxy-late, arylthiosulfonate, dialkyl phosphorothiolate, arylselenolate, and aryltel-lurolate salts with symmetrical diaryliodonium halides, are shown in (Scheme 40) [110-115]. 

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. 

TABLE 10., 27I NQR coupling constants and asymmetry parameters for diaryliodonium halides [Ar1 Ar2IX]2... 

The arylation of nucleophiles by reaction with diaryliodonium salts can be greatly facilitated by copper catalysis. This effect was observed by Beringer et al in the thermal decomposition of diaryliodonium halides as well as by Caserio et al in the hydrolysis of diaryliodonium salts. 2 jhe thermal decomposition of diphenyliodonium chloride shows a reduced activation energy upon copper catalysis Ea = 19 kcal/mole in Methylene glycol in the presence of CuCl instead of 31 kcal/mole in the absence of catalyst.From the synthetic point of view, the copper-catalysed arylation with diaryliodonium salts has been particularly useful in the case of a number of reactions involving heteroatomic nucleophiles, in particular for 5-, 5e-, O- and N-arylation reactions. 

Reduction of diaryliodonium cations in dimethylformamide exhibits four waves. Contrary to observations in aqueous solution, under these conditions the potential of the first wave is counterion dependent for diaryliodonium halides [54], The first wave (la, —0.39 to —0.49 V versus SCE) is assigned to the reduction of phenylmercuric halide, formed by reaction of diphenyliodo-nium halide with mercury electrode. In agreement with Beringer s results in water, the second wave (lb, —0.57 V versus SCE) is associated with reduction of solution-soluble diphenyliodonium cation at an electrode surface saturated with phenylmercury radical from wave la. The third wave (II, —1.70 V versus SCE) corresponds to two-electron reduction of iodobenzene to benzene, while the fourth wave (III, —1.95 V versus SCE) is due to one-electron reduction of phenylmercury radical [54], Unsymmetrical diaryliodonium salts cleave the aryl-iodine bond corresponding to the more electron deficient aryl group [58]. 

Neckers and coworkers have prepared diaryliodonium butyltriphenylborate salts 15-18 (Figure 7.2) by anion exchange of the respective diaryliodonium halides with tetramethylammonium butyltriphenylborate... 

Alcock, N.W. Countryman. R.M. Secondary bonding. Part 14. Structural isomerism in diaryliodonium halides and the structure of di(/ -tolyl)iodonium bromides. J. Chem. Soc., Dalton Trans. 1987. 193. 

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