Arylating agents, iodonium salts

Arylaminoketones 18, 482 Arylating agents, iodonium salts as — 17, 269 C-Arylation by diazo coupling 16, 842... 

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]. 

Alkynones and indoles. Stitching of the aryl group of an iodonium salt with a 1 -alkyne by CO furnishes alkynones. On the other hand, CO serves as a reducing agent in the cyclization of 2-nitrostyrenes which results in the formation of indoles ... 

The best and most versatile contemporary method [22] of preparation of alkynyl(aryl)io-donium salts employs readily available alkynylstannanes [23], and the easily prepared [24] cyano(phenyl)iodonium triflate 7 as an iodonium transfer agent in dichloromethane at low temperatures [Eq. (3)]. This procedure provides excellent yields of iodonium triflates 10 and is applicable to a very broad range of alkynylstannanes, including those with strongly electron-withdrawing groups as summarized in Scheme 3-1. Particularly noteworthy and valuable are the P-keto- and P-amido-substituted species 11 and the cyano-functionalized molecule 11 (Y=CN) [25, 26]. 

Heck reaction with acrylic acid can be carried out at room temperature if diaryliodonium salts are taken as the arylating agents in water [95]. At room temperature, only one aryl group of the iodonium salt is transferred to the product ... 

Besides halides and triflates, other electrophiles can be applied to Heck reactions. The first classical alternative was diazonium salts. Reactions proceed in the absence of phosphine (partly due to the fact that phosphines result in uncontrolled decomposition of the diazonium salt). The Heck reaction using these species can be useful in cases when mild conditions are required. Alternatively, iodonium salts behave in a similar manner to diazonium salts and show better tolerance to bases. " The reactions take place at ambient temperature and so are once again most useful in situations when mild conditions are required. Some main group metallie eompounds such as lead(IV) and thallium(III) have also been shown to undergo Heck-type chemistry and can be useful in specific cases. Of particular interest is the fact that acid chlorides and anhydrides can be employed in Heck chemistry, the use which was pioneered by Blaser and Spencer in 1982. " The process involves oxidative addition of palladium into the C-X bond followed by decarbonylation to yield the intermediate ArPdX species, de Vries has exploited this reaction, demonstrating the use of benzoic anhydride (105) as an effective arylating agent. ... 

Both diazonium salts and iodonium salts can be effectively used as arylating agents. The aryl group is transferred either heterolyt-ically or homolytically, depending on the choice of salt and reaction conditions. In acid solution diazonium salts decompose relatively cleanly to give products consistent with a polar mechanism. In basic solution the product is a complex mixture resulting from free radical intermediates (6). When either benzenediazonium chloride or fluoborate is decomposed in acidic methanol, the major product, formed in 93% yield, is anisole, and less than 1% biphenyl is isolated. In the same solvent with an acetate buffer, the product contains 85-90% benzene, 4.5% biphenyl, 0.6% azobenzene, some anisole, and 80-90 mole % formaldehyde per mole of diazonium salt decomposed (7). 

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