Formal Hydride Transfer Mechanism for Photoreduction of 3-Phenylquinoxalin-2-ones by Amines. Association of 3-Phenylquinoxalin-2-one with Aliphatic Amines

Abstract

The photophysical and photochemical behavior of 1-methyl-3-phenylquinoxalin-2-one (MeNQ) and 3-phenylquinoxalin-2-one (HNQ) in the presence of amines is reported. While HNQ fluorescence shows an auxochromic effect and a bathochromic shift with added amines, explained by association of HNQ with amine in the ground state and emission from both excited species HNQ and [HNQ-amine], both MeNQ and HNQ are photoreduced efficiently on irradiation in the presence of amines, leading to the semireduced quinoxalin-2-ones, MeNQH(-) and HNQH(-), respectively, via an electron-proton-electron transfer, with unit quantum yields at high amine concentrations. The semireduced quinoxalin-2-ones XNQH(-) (X = H, Me) revert almost quantitatively to the parent XNQ in a dark thermal reaction with an activation free energy for MeNQH(-) of 17.4 and 25.9 kcal/mol in acetonitrile and benzene, respectively. Kinetic and spectroscopic (UV and NMR) evidence supports the proposed reaction mechanism for the reversible photoreduction.

Unexpected Imidazoquinoxalinone Annulation Products in the Photoinitiated Reaction of Substituted‐3‐Methyl‐Quinoxalin‐2‐Ones with N‐Phenylglycine

“…(2). The quinoxalin-2-one radical anion 1 •À is similar to those reported previously as intermediaries by us for the amine photoreduction of 3-phenyl-quinoxalin-2-ones (19,20).…”

“…Positive q values, similar to those obtained in this work, have been related to radical addition reactions of alkyl radicals to the aromatic nuclei of substituted toluenes (31), to aminoxyl radical addition to arylketenes (32), to the ratio between addition and electron abstraction on aromatic rings (33)(34)(35) and to the addition of thiyl radicals to vinyl monomers (36,37). Taking into account the stepwise photoreduction reported for 3phenyl-quinoxalin-2-ones (19,20), the lack of any photoreaction for 1a-f derivatives in aerated samples, and some laser flash photolysis results (vide infra) that showed that the first step of photoreaction is mainly a one electron transfer from the NPG, as PhNHCH 2 COO À , to the excited triplet state of the substituted 3-methyl-quinoxalin-2-one, 3 1, it can be proposed that the formation of a ion radical pair, Eq. (1) is the first step of the photoreaction.…”

“…(1) and the reaction of (H) 1d• and surplus NPG, Eq. (7), leading to metastable (H)1 À , which eventually will regenerate product 1 in a similar way to that which we reported for 3-phenyl-quinoxalin-2-ones and amines (19,20).…”

“…Ten years ago we studied two 3-phenyl-quinoxalin-2-one derivatives and found a nearly quantitative reversible photoreduction by amines (19). This photoreduction proceeded through a stepwise transference of electron-proton-electron from the amine to the excited triplet of the 3-phenyl-quinoxalin-2-ones, leading to a metastable semireduced anion capable of reverting almost quantitatively to the original quinoxalin-2-ones.…”

Radiation-induced reduction of quinoxalin-2-one derivatives in aqueous solutions

“…The photophysical and photochemical behavior of 1-methyl-3-phenyl-quinoxalin-2-one and 3-phenyl-quinoxalin-2-one in the presence of amines has been reported in some selected organic solvents (acetonitrile, methanol and hexane) (De la Fuente et al, 2002;De la Fuente et al, 2000). Spectral and kinetic characteristics of the intermediate species: triplet ion-radical pairs (Q  /amine  ) and hydrogenated neutral radicals (QH  ) have been obtained by laser flash photolysis (De la Fuente et al, 2002).…”