Arynes pericyclic reactions

In 1953, Robert s experiments on the conversion of C-labeled chlorobenzene with KNH2 into aniline gave strong support to the intermediacy of ortho-benzyne in this and related reactions. Additional direct evidence for the existence of ortho-benzyne was provided by the observation of its IR spectrum, sohd-state dipolar NMR spectrum, and NMR in a molecular container, and by UV photoelectron spectroscopy. Even at low temperatures, arynes are extraordinary reactive. The reactions of arynes can be divided into three groups (i) pericyclic reactions, (ii) nucleophilic additions, and (iii) transition-metal catalyzed reactions. The pericyclic reactions can be divided into several categories such as Diels-Alder reactions, [2-f2] cycloadditions, 1,3- and l,4-dipolar cycloadditions, and the ene reactions. Arynes react with practically aU kinds of nucleophiles. More recently, the transition-metal catalyzed reactions of arynes have been studied, in particular those involving palladium. 

The reactions of arynes can be divided into three groups (i) pericyclic reactions, (ii) nucleophilic additions, and (iii) transition-metal catalyzed reactions. 

An intramolecular pericyclization reaction provides either di- or trisub-stituted pyridines in good yields (Scheme 14) (13EJ02537). Arynes were generated from 2-(trimethylsilyl)phenyltriflate and combined with isocyanides and 3-bromopropyne in one-pot. CsF was the best source of fluoride to provide the benzyne, but yields of the pyridine were low imtil CSCO3 was added. The rate of benzyne formation was modulated with a cosolvent of MeCN and toluene and the ratio of precursors was tweaked. With the optimized conditions, the reaction was shown to be fairly robust in the face... 

The strained nature of the aryne triple bond results in an easily accessible LUMO and so, arynes are excellent electrophiles that react with different anionic or neutral nucleophilic species. This low-lying LUMO also makes arynes prone to participate in a variety of pericyclic reactions. In addition, arynes are also able to undergo interesting transition metal-catalyzed processes. The increased availability of functionalized aryne precursors as well as the possibility of generating arynes under reagent-free conditions is allowing the development of new aryne-mediated synthetic strategies and the discovery of new reactivity patterns, which will expand the repertoire of chemistry that is actually possible with arynes. 

An aryne-ene reaction has been reported and evidence supports it as a pericyclic reaction [59], Write a mechanism for the reaction below. When the allylic methyl groups are substituted with deuterium, greater than 95% deuterium transfer is observed. How does this support the conclusion that the reaction is conceited ... 

Enamides 34 have been also described as useful partners for [2+2] cycloaddition reactions with benzyne (1) to afford amido-functionalized benzocyclobutane derivatives 35. By engaging this process with a pericyclic ring-opening intramolecular [4+2] cycloaddition, the total syntheses of cheli-donine 37a and norchelidonine 37b from the corresponding aryne precnrsor and enamide 36 have been reported (Scheme 12.21) [33]. 

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