Consecutive Condensation, C-N and N-N Bond Formations: A Copper- Catalyzed One-Pot Three-Component Synthesis of 2H-Indazole

Indazole derivatives have received an increasingly significant attention in modern drug discovery since the indazole unit is a key structural motif in various drug substances exhibiting a wide spectrum of biological activities like anti-platelet, anti-inflammatory, anti-cancer, anti-tumour, anti-contraceptive, anti-depressant, anti-microbial and anti-HIV. In addition, indazoles act as efficient bioisosters of benzimidazoles and indoles [1]. Due to the unique biological activities of indazole derivatives, a number of synthetic methods were developed to access indazole scaffold. However, most of the existing approaches have led to the thermodynamically favoured 1H-idazoles or a mixture of 1H- and 2H-indazoles [2]. Accordingly, the difficulty in the synthesis of 2H-indazoles led to much less study on the applications of 2H-indazoles. Thus, the development of efficient and applicable methods for selective synthesis of 2H-indazoles is of great significance to synthetic chemists. In 2011, Rao and coworkers reported the efficient copper-catalyzed intramolecular cyclization of 2-azidoimines through N-N bond formation to afford the corresponding 2H-indazoles [3]. However, this method requires multi-steps process for preparation of the prerequisite 2-azidoimines as the key starting material. In the light of remarkable advantages of multi-component reactions in organic synthesis, the three component synthesis of 2H-indazoles from 2-bromobenzaldehydes, amines, and NaN3 would be a highly advantages and attractive strategy. In this regard, we now report a highly efficient and straightforward synthesis of 2H-indazoles via one-pot three-component reaction of readily available 2-bromobenzaldehyde, structurally diverse amines, and NaN3 in the presence of {methyl-2-{[1-methyl-2-(2-phenolate)methylidyne nitrilo] ethyl}-aminato(-1)-1-cyclopentenedithio carboxy-late Copper(II), [Cu(cdsalMeen)] as the novel and highly efficient heterogeneous catalyst (Scheme 1).