Generation of Arynes

Probably the most important of all precursors to ortho-benzyne is benzenediazonium-2-carboxylate. The compound is easily prepared by 

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Reactions of lithium derivatives and Grignard reagents from polyfluoroalkenes or polyfluoroaromatic compounds are often complicated by eliminations but are generally much more useful in synthesis than polyfluoroalkane derivatives. Generation of arynes is discussed later (see Chapter 9). 

Another major approach to the generation of arynes and hetarynes involves the use of precursors which can be induced by thermal and photochemical means to eliminate small molecules such as CO, C02, S02 and N2 along with the desired intermediate. Precursors, for example, include 34,86 55,87) and 56.88 ... 

A recent collection of references for the generation of arynes is given in Sato H, Isono N, Miyoshi I,Mori M (1996) Tetrahedron 52 8143... 

Aryne generation. The desilylative route (e.g., by CsF) is the most expedient method for generation of arynes. 1,2-Functionahzation of arenes from 2-trimethylsilylaryl triflates is readily achieved as long as noninterfering co-reactants are used. Thus trapping by organoazides leads to benzotriazoles, by phenyhodonium diacyhnethyhdes leads to... 

Particularly useful have been the o-dihalobenzenes (32b), which permit the generation of arynes without the use of organometallic or strongly basic reagents. A recent variant traps the initially formed anion (33) as its TMS derivative (32f), which then can be converted to the aryne 1 with the nonbasic and nonnucleophilic KF. ... 

Table 1. Generation of Arynes via Aryl Anion Intermediates...

Five-Membered Hetarynes 381 TABLE 2. Generation of Arynes by Loss of Nitrogen from 0-C5H4XY (41) ... 

The same intermediate (43) was proposed more seriously, apparently substantiated, and then finally rejected as being involved in the generation of arynes from A -nitrosoacetanilides (4 If). The precursors, which can be... 

TABLE 4, Generation of Arynes from Cyclic Systems without Nj Loss... 

By using the fluoride-induced generation of arynes from o-(silyl)aryl triflates, Larock and Liu have developed the aryne-mediated A-arylation of amines, sulfonamides, and carbamates as well as the O-arylation of phenols and carboxylic acids. Remarkably, a wide variety of functional... 

SCHEME 28.17 Sulfoxide/metal exchange for the generation of arynes. 

The Kobayashi s method for the generation of arynes depends on factors such as the fluoride source, solvent, and temperature. Three commonly used conditions for aryne generation from 1 are CsF in CH3CN, KF (along with 18-crown-6... 

A transition-metal-free approach for the efficient synthesis of benzofurans was developed by Liang, Li, and coworkers by the addition of arynes to iodonium ylides (Scheme 34) [57]. The reaction proceeds via the addition of iodonium ylide 79 to arynes generating the alkoxy ylide 80, which undergoes intramolecular cyclization followed by release of iodobenzene affords the product. CsF was used for the generation of arynes and the reaction proceeds under mild conditions, and using this method, the synthesis of various of functionalized benzofurans can be achieved. 

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