Heterocycles from arylhydrazones

Mild oxidation of o-nitrobenzaldehyde arylhydrazones with either bromine and sodium acetate or lead tetraacetate results in the overall loss of two hydrogen atoms and production of a class of iV-aryl heterocycles, tiie structure of which was the subject of uncertainty and some controversy for more than fifty years. These oxidation products were first prepared and investigated by Chattawayj " who described them as isodiazomethanes and formulated them as the triaziridine derivates 72. Structural assignment was based entirely on evidence from degradation studies in particular, Chattaway showed that reduction of... 

The Buchwald-Hartwig amination reaction can facilitate the synthesis of nitrogen-containing heterocycles. For instance, Buchwald demonstrated that the amination reaction could be used to prepare N-arylhydrazones from hydrazones and aryl halides. Acid-catalyzed condensation of the hydrazone with a ketone then yielded the desired indoles.Hartwig described a similar arylhydrazone formation with DPPF as a ligand and CS2CO3 as a base."... 

Routes involving intramolecular C—N bond formation have proved to dominate this area. A number of researchers have reported the cycUzation of appropriately substituted arylhydrazones, as illustrated by Scheme 24.16. Intramolecular coupling of bromo-substituted arylhydrazone 35 using a palladium catalyst derived from biden-tate ligand 36 delivers l//-indazole 37 in good yield [76]. The scope of such a transformation was expanded when it was shown that arylhydrazines such as 38 could also be used. In this case, in situ aromatization occurs following intramolecular N-arylation to yield the desired heterocycle [77]. 

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