Synthesis of N-Arylindazoles and Benz-imidazoles from ortho-Aminobenzoximes

Significance

Reported is the synthesis of both N-aryl-1H-indazoles 2 and benzimidazoles 3 via a common aryl oxime intermediate 1. Based on previous results (J. P. Stambuli and co-workers Org. Lett. 2008, 10, 1021), the N-aryl oxime 1 was converted into the N-arylindazole 2 via an interesting intramolecular N-N bond-forming reaction. However, the choice of base determines the selectivity of 2/3 product formation. Thus, the 2-aminopyridine affords N-arylindazoles 2, whereas the Et₃N gives preferentially the N-arylbenzimidazoles 3. High yields for the synthesis of products 2 are obtained for substrates 1 that contain arenes with EWGs and EDGs. However, low yields of indazoles were obtained with arenes bearing 4-CF₃ and 4-NO₂ groups. For the synthesis of benzimidazoles, high yields are obtained for EWG arenes. However, this method is not applicable to strongly EDG arenes. The formation of the benzimidazole is rationalized by a mechanism involving Beckmann rearrangement followed by an intramolecular trapping of the formed nitrilium ion 4 by the aryl-amine.