Katsuya Ishiguro

Novel aspects of carbonyl oxide chemistry

The structural and chemical properties of 1,3-dipolar peroxidic species (X—O—O) such as ozone (X=O) and carbonyl oxides (X=R2C) depend significantly on the nature of X. Although cyclic O3 is thermodynamically unstable, dioxiranes, cyclic isomers of carbonyl oxides, are isolable and have been fully characterized. In contrast to the well known electrophilic nature of ozone, carbonyl oxides usually act as a nucleophilic oxygen transfer agent, but their reactivity could be altered by substituents. It is expected that the chemistry of R2C—O2 species will provide a good starting point for a better understanding of dioxygen complexes (X—O2) with various Xs. Interesting aspects of carbonyl oxide chemistry are discussed, including methods of generation, cyclization to dioxiranes and the nature of their oxygen transfer activities.

18O-tracer study on the rearrangement of carbonyl oxide intermediates to esters

Abstract A tracer study for the rearrangement of carbonyl oxides to esters using doubly-labeled oxygen lead to a conclusion that carbonyl oxides do not isomerize directly to dioxyranes and the rearrangement to esters proceeds via a cyclic tetroxide.

Formation and reactions of aryldiazomethane radical cations

Abstract Dicyanoanthracene-sensitized photolysis of aryldiazomethanes affords stilbenes with cis/trans ratios of ∼3 via free radical cations; anodic oxidation gives similar results but via radical cations adsorbed on electrode surface.

On the rearrangement in carbonyl oxide intermediates

Abstract Sensitized photooxidation of alkylphenyldiazomethanes under oxygen afforded, in addition to alkyl phenyl ketones, alkyl benzoates, which are probably formed by a rearrangement via dioxirane diradical.

Spin selectivity in the oxygenation of singlet phenylhalocarbenes with oxygen

Singlet phenylhalocarbenes are shown to react with triplet oxygen and the apparent spin-forbidden oxygenation rates are strongly dependent on substituents, i.e. in the decreasing order of Br>Cl≫F for halogen and of p-NO2>H≫p-MeO for p-substituents. These results suggest the oxygenation of triplet halocarbene equilibrated with ground-state singlet, resulting in the first estimation of energy difference between the singlet and triplet states.

ESR Study on Diradicals Generated by One-Electron Oxidation of Stable Organic Radicals

Abstract The electronic structure of cationic species generated by γ-radiolysis of stable organic radicals such as substituted iminoxyls, galvinoxyls, and nitronyl nitroxides, was studied in CFC13 matrix at 77 K. In most cases, the ESR signals of the radicals were decreased by the one-electron oxidation but the spectra of biradicals were not observed at all. However, p-hydroxyphenyl-substituted nitronyl nitroxide exhibited, on radiolysis, ESR signals ranging from 3180 to 3330 G, which was possibly assigned to that of the triplet biradical formed by the oxidation and the following deprolonation.

Turning the reactivity of carbonyl oxides from nucleophelic to electrophilic. A novel deoxygenation of sulfoxides byelectrophilic α,α,α-triflouroacetophenone O-oxide.

Abstract The well-known nucleophilic reactivity of carbonyl oxides could be turned into electrophilic by a potent electron-attracting group. Thus α,α,α-triflouroacetophenone O -oxide both sulfides and sulfides electrophically, and the most characteristic reaction is the deoxygenation of sulfoxides.

Effect of concentrations of radical-ion intermediates in the photochemical redox reaction of the diphenylmethyl radical

Abstract Effect of concentrations of radical ion intermediates in photoinduced electron transfer (PET) reactions was examined for the redox reaction of diphenylmethyl radical. The irradiation of sym -tetraphenylacetone in the presence of aromatic donor and acceptor in MeOH–MeCN (1:9) leads to (i) the simultaneous generation of diphenylmethyl radical and a pair of radical ions, (ii) the electron transfers between them affording diphenylmethyl cation and anion, and (iii) the formation of Ph 2 CHOMe and Ph 2 CH 2 by the trapping reactions with methanol. The product ratios of Ph 2 CH 2 /Ph 2 CHOMe are shown to be dominated by the steady-state concentrations of radical ions. Thus, pulsed irradiations resulted in quite different product ratios depending on the pulse intervals. The relative rates extrapolated to infinite, which are directly related to the intrinsic electron transfer rates with radical ions, are shown to follow the Rehm–Weller-type relationship. The importance of the persistent radical ion effect for PET reactions, just as the Ingold–Fischer persistent radical one, is discussed.

Preparation and property of 2-(3',5'-di-tert-butylphenyl-4'-oxyl)-4,4,5,5-tetramethyl-4,5,-dihydro-1H-imidazole-3-oxide-1-oxyl

Abstract Title compound, a cross-conjugated biradical with phenoxylnitronyl nitroxide structure, was prepared by the lead dioxide oxidation of a p-hydroxyphenyl nitronyl nitroxide, 2-(3,5-di-tert-butyl-4-hydroxyphenyl)-4,4,5,5-tetramethyl-4,5,-dihydro-1H-imidazole-3-oxide-1-oxyl (1b), and was characterized by ESR spectroscopy. On the other hand, the y-radiolysis in a freon matrix and the anodic or chemical oxidation of 1b did not generate 2b but lead to the formation of a phenol-substituted nitronyl nitroxide cation 3b. Thus, a stable acid/base pair with different spin multiplicities, 2b and 3b, could be prepared independently.

Spectroscopic studies of various carbene–radical-cations fromed by radiolysis of solutions on halogenocarbons

Cationic species produced in the γ-radiolysis of the halogenocarbon–aryldiazomethane mixtures have been studied at 77 K by electron spin resonance (e.s.r.) and optical absorption spectroscopy. A diphenyldiazomethane radical-cation ([(C6H5)2CN2]+˙) produced by the radiolysis of n-C4H9Cl–(C6H5)2CN2(0.1 mol %) mixtures is found to have an absorption band at 650 nm. When the cation is irradiated with visible light, a different cation is produced which has an absorption spectrum at 390 nm, interpretable in terms of a diphenyl carbene radical-cation ([(C6H5)2C]+˙). A phenyldiazomethane radical cation ([C6H5CHN2]+˙) and a phenylcarbene radical-cation ([C6H5CH]+˙) have absorption bands at 515 and 475 nm, respectively. The e.s.r. spectrum of [(C6H5)2CN2]+˙ cations, produced by the radiolysis of CCl3F–(C6H5)2CN2(0.2 mol %) mixtures at 77 K, shows a broad singlet spectrum. Upon irradiation with visible light the broad spectrum changes into a sharp singlet, which is ascribed to [(C6H5)2C]+˙ cations. Formation of [C6H5CHN2]+˙ and [C6H5CH]+˙ cations has also been studied by e.s.r. spectroscopy. The effect of deuteriation on the e.s.r. spectra has been stuided by use of deuteriated aryldiazomethanes, such as (C6D5)2CN2′ C6H5CDN2′ and C6D5CDN2.