Arynes fragmentation reactions

Fragmentation Reactions. Oxidation of 1-aminobenzotriazoles with Pb(OAc) or MIS is used to generate arynes through the intermediacy of an aminonitrene 8. The subsequent fragmentation with loss of two molecules of leads to the corresponding aryne derivative (Scheme 12.8) [15]. 

Complex 115 reacts analogously with ethene. At ambient temperatures, trimethylgallium is rapidly liberated. At the same time, the remaining (r 2-aryne)zirconocene fragment takes up 1 equivalent of ethene to form the metallaindane system 122. Compound 122 was identified by comparison with an authentic sample [194], Prolonged exposure of the reaction mixture to ethene eventually led to the formation of 123. 

In 669 a 3-atom chain links the aryne and furan moieties. The reaction was also successful with a 4-atom link (672 674) but, with an analogous 2-atom link, fragmentation to diazoquinone 676 occurred". The analogous chemistry, but with an all-carbon link (that would be less prone to fragmentation), has not been explored. 

Both 1,2,4 or better 1,2,3-benzotriazines, 49d and 49c, respectively, loose Nj and nitriles on thermolysis to give arynes. The triazinones 49f undergo a similar minor fragmentation with expulsion of nitrogen and isocyanates. The major reaction leads to azetinones 54 which were shown not to be precursors of arynes. 

One of the best routes to arynes is the oxidation of aminotriazoles (49j) with lead tetraacetate. Variations involve generating the intermediate nitrene 57 or nitrenoid with other oxidizing agents or from other precursors (49k-49n). By analogy the nitrene 58 or nitrenoid obtained by oxidation of 1-aminoindazole (59) might be expected to fragment to arynes by loss of Nj and a nitrile. In fact this reaction constitutes an excellent synthesis of the previously discussed aryne precursors, the 1,2,3-benztriazines (49e). A vibrationally excited form of 49c is presumably an intermediate in the pyrolysis of sulfoximides of 2-aminoindazoles (60) to arynes. 

When the nucleophilic and the electrophilic positions of the reagent confronted to the aryne are not c-bonded, a cascade intermolecular nucleophilic addition-intramolecular electrophilic cycUzation of arynes can take place. The fragmentation step, which is cmcial for the insertion reaction of arynes into a-bonds, is not involved in annulation processes because the intermediate obtained from the cyclization is usually a stable five- or six-membered ring system. 

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