Spectral and Kinetic Study of 3-Methylquinoxalin-2-ones Photoreduced by Amino Acids: N-Phenylglycine Radical Chain Reactions and N-Acetyltryptophan Decarboxylation

Spectral Probe for Electron Transfer and Addition Reactions of Azide Radicals with Substituted Quinoxalin-2-Ones in Aqueous Solutions

The azide radical (N₃^●) is one of the most important one-electron oxidants used extensively in radiation chemistry studies involving molecules of biological significance. Generally, it was assumed that N₃^● reacts in aqueous solutions only by electron transfer. However, there were several reports indicating the possibility of N₃^● addition in aqueous solutions to organic compounds containing double bonds. The main purpose of this study was to find an experimental approach that allows a clear assignment of the nature of obtained products either to its one-electron oxidation or its addition products. Radiolysis of water provides a convenient source of one-electron oxidizing radicals characterized by a very broad range of reduction potentials. Two inorganic radicals (SO₄^●-, CO₃^●-) and Tl²⁺ ions with the reduction potentials higher, and one radical (SCN)₂^●- with the reduction potential slightly lower than the reduction potential of N₃^● were selected as dominant electron-acceptors. Transient absorption spectra formed in their reactions with a series of quinoxalin-2-one derivatives were confronted with absorption spectra formed from reactions of N₃^● with the same series of compounds. Cases, in which the absorption spectra formed in reactions involving N₃^● differ from the absorption spectra formed in the reactions involving other one-electron oxidants, strongly indicate that N₃^● is involved in the other reaction channel such as addition to double bonds. Moreover, it was shown that high-rate constants of reactions of N₃^● with quinoxalin-2-ones do not ultimately prove that they are electron transfer reactions. The optimized structures of the radical cations (7-R-3-MeQ)^●+, radicals (7-R-3-MeQ)^● and N₃^● adducts at the C2 carbon atom in pyrazine moiety and their absorption spectra are reasonably well reproduced by density functional theory quantum mechanics calculations employing the ωB97XD functional combined with the Dunning's aug-cc-pVTZ correlation-consistent polarized basis sets augmented with diffuse functions.

Enantioselective, Visible Light Mediated Aza Paternò-Büchi Reactions of Quinoxalinones

3-Substituted quinoxalin-2(1H)-ones and various aryl-substituted or tethered olefins underwent an enantioselective, inter- or intramolecular aza Paternò-Büchi reaction upon irradiation at λ=420 nm in the presence of a chiral sensitizer (10 mol %). For the intermolecular reaction with 1-arylethenes as olefin components, the scope of the reaction was studied (14 examples, 50-99 % yield, 86-98 % ee). The absolute and relative configuration of the products were elucidated by single-crystal X-ray crystallography. The reaction is suggested to occur by triplet energy transfer in a hydrogen-bonded 1:1 complex between the imine substrate and the catalyst. The intramolecular cycloaddition, consecutive reactions of the product azetidines, and an alternative reaction mode of quinoxalinones were investigated in preliminary experiments.

Spectral and Kinetic Study of 3-Styrylquinoxalin-2(1 H)-ones Photoreduced by N-Phenylglycine and Amines

The photoreduction by amines and N-phenylglycine, NPG, of six styrylquinoxalin-2(1 H)-ones derivatives substituted in the styryl moiety, R-SQ, was studied by using flash photolysis. The photoreaction is initiated via a single electron transfer from the electron donor (amines or NPG) to R-SQ excited triplet state, ³R-SQ*, with the formation of a triplet state radical ion pair or a charge transfer exciplex, ³[CRIP/CTE]. These species live longer than the respective ³R-SQ* and have very similar transient spectra. In the presence of NPG, these ³[CRIP/CTE] evolve on μs time scale to the respective hydrogenated radicals, R-SQH^•, whose transient spectra and reaction rate constants with NPG are reported. The identity of these hydrogenated radicals was supported by the spectra obtained with the α-H donor triethylamine and previous pulse radiolysis studies in 2-propanol. Our findings allow proposing a radical chain reaction mechanism that explains the observed spectral behavior and rationalizes formation of the main product formed by binding of four PhNHCH₂^• derived from NPG decarboxylation.

Visible light initiated release of calcium ions through photochemical electron transfer reactions

Photolysis of anthraquinone or flavin photosensitizers in the presence of calcium EDTA complexes results in decomposition of the EDTA complex, releasing free Ca²⁺. In the case of the flavin sensitizers, it is shown that millimolar concentrations of Ca²⁺ can be released using visible light (>440 nm) and with quantum yields as high as 0.31. The utility of this system is further demonstrated by in situ photogelation of an alginate solution.