Department of Chemistry , University of California Davis , 1 Shields Avenue , Davis , California 95616 , United States.
Department of Chemistry , American University of Beirut , Beirut 1107 2020 , Lebanon.
J Am Chem Soc. 2019 Apr 17;141(15):6247-6253. doi: 10.1021/jacs.8b13481. Epub 2019 Apr 4.
The Cadogan cyclization is a robust but harsh method for the synthesis of 2 H-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, 2 H-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 2 H-indazole synthesis.
卡多根环化反应是一种合成 2H-吲唑的有效但苛刻的方法，2H-吲唑是一类有价值的含氮杂环化合物。尽管通过彻底脱氧生成氮烯被广泛认为是硝基芳烃还原环化反应的作用机制，但之前仅提出过非氮烯途径。在这里，通过中断的卡多根/戴维斯-贝鲁特反应合成了 2H-吲唑 N-氧化物，并将其作为含氧中间体的直接证据；讨论了这两个反应的机理意义。这些 N-氧化物的分离和表征使 2H-吲唑的室温下的形式卡多根环化反应成为可能。
