Visible Light‐Induced Radical Cyclization of Ethyl 2‐(N‐Arylcarbamoyl)‐2‐Chloroiminoacetates: Synthesis of Quinoxalin‐2(1H)‐ones

Abstract

This paperr eveals that visible light irradiation with Ru(phen) 3 Cl 2 as photocatalyst can induce ethyl 2-(N-arylcarbamoyl)-2-chloroiminoacetates to undergo N À Cl cleavage to give a-(aminocarbonyl)iminyl radicalsu nder an argon atmosphere.T he subsequent cyclization of the thus formed iminyl radicals affords quinoxalin-2(1H)-one productsi ng ood yield in the presence of an inorganic base such as Na 2 CO 3 .T he reactions proceeded very well even without using ap hotocatalyst when DMF was used as the solvent. Thesep rotocols provide an ew,s imple method for the generation of iminyl radicals,a nd the reactions described herein constitute an efficient method for the synthesis of quinoxalin-2(1H)-onederivatives.

Synthesis of Quinoxaline Derivatives via Tandem Oxidative Azidation/Cyclization Reaction of N-Arylenamines

“…It is interesting to see that compounds 1l , 1n , 1o , and 1p , which incorporate an ortho -substituent at the N -phenyl ring, also reacted to give quinoxalines. This result is in contrast to that of our previous studies on the cyclization of α-(aminocarbonyl)­iminyl radicals, where an ortho -substituent at the N -phenyl ring would prevent the formation of a quinoxalin-2-one ring . This discrepancy can be attributed to the relief of steric hindrance in 1l – 1p , in which it is the small hydrogen atom other than an alkyl group that is attached to the nitrogen atom.…”

“…Recently, Spagnolo et al, Zhang et al., and our group found that the cyclization of iminyl radicals provides an effective means for the synthesis of a quinoxalin-2-one ring system, a sister structure of quinoxaline. We envisioned that the quinoxaline motif might also be prepared through a similar process.…”

A new era for homolytic aromatic substitution: replacing Bu3SnH with efficient light-induced chain reactions

“…Iminyl radical intramolecular HAS using acyl oximes, 67 and chloroimines 68 allowed the respective formation of a variety of phenanthridines and quinoxalin-2-ones (Scheme 22). The generation of iminyl radicals from acyl oximes is reminiscent of earlier approaches by Walton using UV-light (Scheme 11), 41,42 and allow aldehydes to act as precursors for in situ generated acyl oximes.…”

Photochemical radical cyclization reactions with imines, hydrazones, oximes and related compounds

“…For example, chloroimino derivatives like 178 undergo photochemically induced radical cyclization leading to quinoxalinones 179 (Scheme 32). 75 The reaction was carried out with visible light in the presence or the absence of a ruthenium base photoredox catalyst. Depending on the substitution pattern either the first or the second variant was more efficient.…”