Visible-Light- and PPh₃-Mediated Direct C–N Coupling of Nitroarenes and Boronic Acids at Ambient Temperature

Abstract

We present here a metal-free, visible-light-and triphenylphosphine-mediated intermolecular, reductive amination between nitroarenes and boronic acids at ambient temperature without any photocatalyst. Mechanistically, a slow reduction of nitroarenes to a nitroso and, finally, a nitrene intermediate occurs that leads to the amination product with concomitant 1,2-aryl/-alkyl migration from a boronate complex. A wide range of nitroarenes underwent C−N coupling with aryl-/alkylboronic acids providing high yields.

A Minireview on the Scope of Cadogan Cyclization Reactions Leading to Diverse Azaheterocycles

“…After this, a very important 1,2-phenyl/alkyl migration takes place from the nucleophilic boronate to the electrophilic nitrogen atom to afford 8 c, which finally undergoes hydrolysis to give product 10. [21]…”

“…In 2019, a series of carbazole alkaloids such as murrayafoline A (16 a), koenoline (16 b), mukoeic acid (16 c), 3-Methoxy-9Hcarbazol-1-yl)-methanol (16 d), 3-methoxy-9H-carbazole-1-carboxylic acid (16 e), carprofen and its analogues (19), glycozolicine (21), clauszoline K ( 22), 2-methyl-9H-carbazole (24), and glycoborine (26) were synthesized by Bhatthula and colleagues through Cadogan cyclization as one of the key steps for their synthesis. [26] Interestingly, most of these synthesized alkaloids are of natural origin.…”

“…[35] Manna and colleagues, developed a metal-free, visible-light-and PPh 3 -mediated intermolecular reductive amination reaction, for the first time, between nitroarenes (126) and boronic acids (127) without using any photocatalyst as shown in Scheme 29. [21] Here, the nitroarenes acted as photosensitizers. The mechanism of the reaction evidenced the involvement of slow reduction of nitroarenes firstly to nitroso and, then to a nitrene intermediate affording the amination product with the association of 1,2-aryl/-alkyl migration from a boronate complex.…”

“…The mechanism of the reaction evidenced the involvement of slow reduction of nitroarenes firstly to nitroso and, then to a nitrene intermediate affording the amination product with the association of 1,2-aryl/-alkyl migration from a boronate complex. [21] As an application of this methodology, the research group showed the scope of this reaction showing exceptional functional group tolerance with moderate to good yield products as shown in Scheme 29.…”

“…Furthermore, the formation of compound 10 via intermolecular Cadogan reaction can be explained initially by the formation of a nitrene intermediate 8 a by addition of Cadogan reagent to compound 8 which further reacts with 9 to generate boronate species 8 b via nucleophilic addition of lone pair of nitrogen to the vacant p orbital of boron from 9 . After this, a very important 1,2‐phenyl/alkyl migration takes place from the nucleophilic boronate to the electrophilic nitrogen atom to afford 8 c , which finally undergoes hydrolysis to give product 10 [21] …”