Diaryliodonium salts hypervalent

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. 

Recent progress on the use of hypervalent iodine reagents for the construction of heteroatom-heteroatom bonds is reviewed. Reactions of aryl-A3-iodanes with heteroatom substrates derived from third-row elements and beyond are considered first, and an unusual example of heteroatom-heteroatom bond formation with diaryliodonium salts is then discussed. Finally, the use of sulfonylimino(aryl)iodanes for imidations of phosphorus, sulfur, selenium, and arsenic compounds, including enantioselective transformations (S,Se), and alternate hypervalent iodine approaches to N-sulfonylsulfilimines and N-sulfonylarsinimines are summarized. 

Diaryliodonium salts, with few exceptions, are stable compounds towards heat, oxygen and humidity they are mildly light-sensitive and should be stored in the dark, without refrigeration. Generally, their reactivity is less pronounced than that of other hypervalent iodine compounds. Indeed, in several of their reactions relatively drastic conditions may be necessary, especially for the least reactive heterocyclic iodonium salts. The search for optimum conditions is often desirable even for well-established reactions, by applying new findings concerning the use of specific... 

The remainder of the mechanistic information presented in this review draws from the investigations and proposals by Lockhardt391 and Barton and coworkers343 on the intermediacy of hypervalent copper(III) species during the reactions of diaryliodonium salts and pentavalent bismuth reagents. Most of the reactions utilize Cu(I) or Cu(II) salts as the starting copper source. Radical mechanisms are ruled out because the reactions are not inhibited when radical scavengers (such as 1,1-diphenylethylene) are added. It appears that Cu(H) is not the major catalytic species in the reaction. 

Notorious toxicity of heavy metal compounds can be avoided using hypervalent iodine(III) reagents [131]. Diaryliodonium salts yield biaryls in the reaction with methylmagnesium bromide in the... 

Several mechanistic studies on the reactions of diaryliodonium salts with nucleophiles have been published. Ochiai and coworkers performed a mechanistic study on the phenylation of 3-keto ester enolates with diaryliodonium salts. An aryl radical trap was added to the reaction without affecting the outcome, which indicates that radical pathways are unlikely and the reaction occurs by direct coupling of the ligands on the hypervalent iodine center [889]. 

Diaryliodonium salts and other hypervalent iodine compounds in the synthesis of biaryls... 

Aqueous media are often used for reactions with hypervalent iodine compounds. A number of mild and selective methods for arylation of acetylenes by diaryliodonium salts have been published. In neat water the reaction is run at room temperature in a heterogeneous system (PdCl2(PPh3)2, Cul, K2CO3, 10 mol % BU3N) and must be assisted by amine base, which possibly additionally serves as PTC agent. ... 

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. 

With the development of efficient synthetic routes to diaryliodonium salts, these hypervalent iodine compounds have become easily available and interesting alternatives to other arylation reagents. Their low toxicity, high stability, and high reactivity are attractive features which enable difficult transformations without the need for excess reagents or high temperatures. This chapter has summarized recent developments in metal-free and metal-catalyzed arylations of heteroatom nucleophiles and carbon-centered nucleophiles. The boundaries of the field have moved forward considerably in the last decade, and many more applications are foreseen based on the deepened mechanistic understanding of arylations under both metal-free and metal-catalyzed conditions. 

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]. 

Hypervalent diaryliodonium salts are important arylating agents in organic synthesis [375, 376]. The arylation of thienyUodonium bromides 160 with methoxybenzenes as aromatic nucleophiles 161 is possible under mild reaction conditions in hexafluoroisopropanol (HFIP) and without the necessity for a metal catalyst (Scheme 63, Table 40). TMSOTf is a suitable additive for the direct ipso substitution. 

A study, including computational results, of the role of copper enolate complexes in the a-arylation of enolates has shown that the reaction is likely to occur through oxidative addition of the iodoarene to form an aryl-copper(III) intermediate, (46), followed by reductive elimination to give the product. Copper(III) species are also likely to be involved in the meta-selective C-H arylation of acetanilides by diaryliodonium salts. However, radical species may be involved in the rhenium-catalysed electrophilic triflu-oromethylation of arenes using hypervalent iodine reagents. 

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