Unprecedented SnCl₂-Mediated Cyclization of Nitro Arenes via N−N Bond Formation

Abstract

[reaction: see text] A mild, efficient, one-pot protocol for the cyclization of nitro-aryl substrates using SnCl(2) has been described. The mechanistic course of the reaction suggests the involvement of a hydroxylamine intermediate leading to an intramolecular cyclization via N-N bond formation. The versatility of the methodology has been demonstrated by using two nitro-aryl substrates derived from dihydroisoquinolines and dihydro-beta-carbolines. The intramolecular cyclization led to the formation of indazoles in high yields and purities.

Transition Metal Catalyzed Reductive Cyclization Reactions of Nitroarenes and Nitroalkenes

“…The latter is isolobal to a phosphine and acts as a ligand. However, in the present case it should be noted that SnCl 2 reduces nitroarenes under much milder conditions than those employed for the catalytic reaction and that an over‐stoichiometric amount of SnCl 2 has even been employed to promote the reductive cyclization of N‐ (2‐nitroarylidene)amines to 2 H‐ indazoles in the absence of any metal catalyst . Since the amount of SnCl 2 is not negligible (0.5 equiv.…”

“…However, in the present case it should be noted that SnCl 2 reduces nitroarenes under much milder conditions than those employed for the catalytic reaction [130][131][132] and that an over-stoichiometric amount of SnCl 2 has even been employed to promote the reductive cyclization of N-(2-nitroarylidene)amines to 2H-indazoles in the absence of any metal catalyst. [133] Since the amount of SnCl 2 is not negligible (0.5 equiv. with respect to the nitroarene) in the cited system, its role as a direct reductant cannot be dismissed.…”

Recent Advances in Nitrogen–Nitrogen Bond Formation

“…Another example of N-O bond cleavage, in this case involving nitro group reduction, was disclosed by Kundu and co-workers in 2006 (Scheme 5). 19 In the presence of TsCl, the hydroxylamine M2 formed by reduction of the nitro group undergoes intramolecular cyclization to afford pyrazoles 6 in 89-91% yield.…”

The efficient in‐situ reduction and cyclization reaction of aromatic aldehyde, 1,3‐cyclopentanedione (tetronic acid), and nitro‐compound under SnCl2·2H2O‐THF medium

“…Under review of literatures, it was found that nitrogen‐containing heterocyclic compounds could be prepared by direct reduction of nitro compounds . Inspired by this approach, we have reported an efficient and simple method for the synthesis of pyrazolo[3,4‐ a ]acridin‐10‐one and pyrazolo[4,3‐ a ]acridin‐10‐one derivatives form 5‐nitro‐1 H ‐indazole and 6‐nitro‐1 H ‐indazole .…”