Shigeya Niizuma

Synthesis, structure, and one-electron redox reactions of 4,7-disubstituted benzotrichalcogenoles containing sulfur and/or selenium atoms

Abstract Stable 4,7-disubstituted benzotrichalcogenoles containing sulfur and/or selenium atoms in the five-membered ring were systematically and selectively prepared in good yields by reaction of the corresponding benzodichalcogenastannoles, a synthetic equivalent of benzenedichalcogenols, with an S 1 or Se 1 source. Characterization of these new trichalcogenole frameworks was performed by multi-nuclear NMR studies and X-ray crystallographic analyses. The cyclic voltammograms of the trichalcogenoles showed well-defined reversible electrochemical redox couples with low oxidation potential. Novel radical ions were isolated in quantitative yields in the one-electron oxidation of the trichalcogenoles with one equivalent of NOPF 6 as a one-electron oxidant. The structures of the radical cation salts were analyzed by 31 P-NMR and EPR spectroscopies, and elemental analyses. The salts underwent one-electron reduction on treatment with one equivalent of samarium(II) iodide to give the neutral starting trichalcogenoles quantitatively.

Novel photochemical ring contraction of 1,4,7,10-tetraisopropyldibenzo[c,g][1,2,5,6]tetrachalcogenocins: efficient synthesis, structure, and redox reactions of 1,4,6,9-tetraisopropylchalcogenanthrenes

Abstract 1,4,7,10-Tetraisopropyldibenzo[c,g][1,2,5,6]tetrachalcogenocins, which were readily synthesized from 4,7-diisopropyl-2,2-dimethyl-1,3,2-benzodichalcogenastannoles, underwent selective ring contraction under irradiation with a high-pressure mercury lamp to give the corresponding 1,4,6,9-tetraisopropylchalcogenanthrenes in moderate yields. Facile one-electron redox reactions were observed between the chalcogenanthrenes and the corresponding stable chalcogenanthrenium radical cations by chemical or electrochemical treatment.

Photodimerization of 2,6-diphenylpyrylium salt in the solution

Abstract By the photoillumination of 2,6-diphenylpyrylium tetrafluoroborate (DPP) in tetrahydrofuran (THF), its dimer is confirmed to be formed through the pyranyl radical which is produced by one electron reduction of DPP.

Preparation and novel one-electron redox reactions of a new stable 4,7-diisopropylbenzo[1,2-d][1,2,3]triselenole and its radical cation salt

Stable benzene-fused triselenide compounds, 4,7-diisopropylbenzo[1,2-d][1,2,3]triselenole 2 and its radical cation salt, 4,7-diisopropylbenzo[1,2-d][1,2,3]triselenolium hexafluorophosphate 3, are prepared and undergo novel one-electron redox reactions.

A new type of photodimerization reaction for coumarin derivatives

Irradiation (λ >300 nm) of coumarin-3-carboxylic acid gives the decarboxylated 4,4′-dimer of chroman-2-one, the structure of which is quite different from that of [π2s+π2s] cycloaddition dimer as is observed in the past in the photoreaction of coumarin or some of its derivatives.

Photoreduction of flavone: identification of the photoproducts and reaction mechanism

Abstract The photoreduction of flavone was investigated in 2-propanol or in benzene containing N,N-dimethylaniline by a continuous light irradiation ( λ = 300 nm), a flash photolysis and CIDEP methods. The photoreaction proceeds via the triplet state of flavone and gives the dimers of flavanon in 2-propanol, the diastereoisomers, which are different from the pinacol-type dimer.

Photodimerization reaction of 10-methylacridone

Abstract The photoreaction of 10-methylacridone with N,N-dimethylaniline proceeds via the triplet state of the former and results in electron transfer followed

Photochemical production of semiquinone radicals of o-methylfluorescein methyl ester in ethanol

Abstract The mechanism of the formation and the decay of radicals photochemically produced from o-methylfluorescein methyl ester (MF) was investigated by continuous light and flash photolysis. Two kinds of radicals were found. One is attributed to the semiquinone radical (MFH·) of MF and the other to the anion radical (MF · − ). The pKa value for MFH· was determined to be 10.5 in ethanol-water (1:1 by volume) at 25 °C. Electron spin resonance and electron nuclear double resonance spectra of MFH· were measured in neutral ethanol and a comparison of the hyperfine splitting constants for MFH· with those for the fluorescein dianion radical (FH·2−) supports the previously proposed mechanism of the rapid proton exchange between the solvent (water) and FH·2−.

Photodecarboxylation of chromone-2-carboxylic acid in aerated and deaerated ethanol solution

Abstract The typical photodecarboxylation of chromone-2-carboxylic acid occurs in aerated and deaerated ethanol solutions to afford 4-hydroxycoumarin and 2-(1′-hydroxyethyl)-chromone, respectively. The former proceeds through an addition of the ground state oxygen molecule, while the latter through the recombination between the ketyl radical and the 1-hydroxyethyl radical followed by the release of CO2. Chemiluminescence technique was successfully applied to detect ketohydroperoxide intermediates assumed in the photoreaction of the aerated system.

Studies of photoinduced tautomerism of 2-hydroxyphenazine using flash photolysis

Abstract Flash photolysis of 2-hydroxyphenazine in aprotic and protic solvents gives the corresponding unstable ketonic form which reverts to the parent molecule in the dark. The activation energy and the frequency factor of the dark reaction were determined to be 6.1 ± 0.2 kcal mol −1 and 10 8 s −1 respectively. The rather small value of the frequency factor may be attributed to the weakness of the NH bond of the ketonic form. For 1-hydroxyphenazine, no transient is observed. It is suggested that a rapid deactivation of S 1 is induced by intramolecular hydrogen bond formation between the hydroxyl group and the nitrogen atom.