ja8b13481_si_002.cif (775.33 kB)

Accessing Multiple Classes of 2H‑Indazoles: Mechanistic Implications for the Cadogan and Davis–Beirut Reactions

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posted on 26.03.2019, 00:00 by Jie S. Zhu, Clarabella J. Li, Ka Yi Tsui, Niklas Kraemer, Jung-Ho Son, Makhluf J. Haddadin, Dean J. Tantillo, Mark J. Kurth
The Cadogan cyclization is a robust but harsh method for the synthesis of 2H-indazoles, a valuable class of nitrogen heterocycles. Although nitrene generation by exhaustive deoxygenation is widely accepted as the operating mechanism in the reductive cyclization of nitroaromatics, non-nitrene pathways have only been theorized previously. Here, 2H-indazole N-oxides were synthesized through an interrupted Cadogan/Davis–Beirut reaction and are presented as direct evidence of competent oxygenated intermediates; mechanistic implications for both reactions are discussed. Isolation and characterization of these N-oxides enabled a formal Cadogan cyclization at room temperature for 2H-indazole synthesis.