Rearrangements involving arynes

The amination of 2-chloropyridine-A-oxide (53) with potassium amide in liquid ammonia yielded a mixture of 2-(55) and 3-amino-pyridine-A-oxide (56) in 5-10% total yield.This rearrangement might be explained by an aryne mechanism involving 2,3-pyridyne-A-oxide (54). Since the structure of 56, with its quaternary nitrogen atom, is more analogous to that of 3-methoxybenzyne (39) than to that of 2,3-pyridyne (26), an orientation effect directing the amide ion to C-3 can be expected here. 

Many rearrangements occur via reactive intermediates, but arynes must be among those least prone to rearrange. The generation and reaction of an aryne does involve some measure of rearrangement, which provided strong evidence for the existence of arynes. 

SCHEME 73 Thia-Fries rearrangement involving arynes to form phenoxathiin-dioxide [100]. 

Benzoxazoles. - Both o- and m-halogenobenzanilides (413) are converted into the amidines (415) by potassium amide in liquid ammonia these are formed by aryne cyclization to 2-phenylbenzoxazole (414) and subsequent aminoly-sis." The conversion of acetophenone oxime into 2-methylbenzoxazole by the action of phosphorus oxychloride involves a Beckmann rearrangement. Pyrolysis of aryl azidoformates, ArOaCNa, gives benzoxazol-2-ones. The azide (416) is converted into the benzoxazole (417) on heating. The... 

Despite their high electrophilicity, arynes are discriminatory toward nucleophiles (33). An interesting reaction involving an additive rearrangement of benzyne with alkylidenetriphenylphosphoranes (34) is depicted as the following. 

As the intermolecular multicomponent reactions, three-component cycloaddition reactions (21.2 [2+2-1-2] cycloaddition and 21.3 [3+2+1] cycloaddition) and two-component cycloaddition reactions (21.4 [4+2] cycloaddition) are described. As the intramolecnlar single-component reactions, cycloaromatization reactions (21.5 intramolecular hydroarylation of alkynes and cychzation via transition metal vinybdenes) are described. Aromatic ring constrnction reactions involving aryne reactions (Chapter 12), rearrangement reactions (Chapters 16 and 18), metathesis reactions (Chapter 17), and coupling reactions (Chapters 19 and 20) are described in these different chapters. 

In 2006, Larock and coworkers developed a [3 -f 2] cycloaddition strategy involving pyridine N-oxides and arynes to generate hydroxyphenylpyridines [103]. This reaction involves the initial formation of [3 + 2] cycloaddition product followed by its rearrangement to afford 3-(2-hydroxyaryl) pyridines in good yields (42-92%). This reaction was carried out in the presence of CsF in MeCN at room temperature. Under these reaction conditions various substituted ar5mes and pyridine N-oxide derivatives are well tolerated (Scheme 76). 

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