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12.2.1.1.4.1.3 Method 3: From 2-Aminoalkyl- or 2-Acyl-1-nitroarenes

DOI: 10.1055/sos-SD-012-00277

Stadlbauer, W.Science of Synthesis, (200212245.

In addition to the decomposition of azidoarenes, the generation of arylnitrenes by reduction of nitroarenes offers a widely used approach to indazoles. The reducing agents employed include: triethyl phosphite,[‌364‌‌369‌] iron(II) oxalate,[‌370‌] carbon monoxidepalladium complexes,[‌371‌,‌372‌] lithium aluminum hydride,[‌373‌] zinc,[‌67‌,‌68‌] or hydrazine.[‌374‌] Generally syntheses from azidoarenes give reasonable yields,[‌359‌] but the reduction of 2-[(alkylamino)methyl]-1-nitroarenes with metals is not a good approach to 2H-indazoles.[‌375‌‌377‌] Electrochemical reduction of 2-nitrosobenzylamines, however, gives 2H-indazoles in high yields,[‌378‌] and good results also follow the use of hydrazine as the reducing agent. Indeed, many 3-alkyl-1H-indazoles of type 75 with plant growth regulating activities have been synthesized in this manner.[‌374‌] For example, 2-amino-2-(5-chloro-2-nitrophenyl)propanonic acid (74) reacts with hydrazine hydrate to form (5-chloro-1H-indazol-3-yl)acetic acid (75) in 86% yield (Scheme 29).[‌374‌]

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