1.3.4- Oxadiazole anion radicals

An energy-sufficient mixed chemiluminescent radical-ion reaction is that of thianthrene (TH) radical cation 101 and 2.5-diphenyl-1.3.4-oxadiazole (DPO) radical anion 102 156> ... 

As described, other nucleophilic reactions in the anthraquinone series also involve the production of anion-radicals. These reactions are as follows Hydroxylation of 9,10-anthraquinone-2-sulfonic acid (Fomin and Gurdzhiyan 1978) hydroxylation, alkoxylation, and cyanation in the homoaromatic ring of 9,10-anthraquinone condensed with 2,1,5-oxadiazole ring at positions 1 and 2 (Gorelik and Puchkova 1969). These studies suggest that one-electron reduction of quinone proceeds in parallel to the main nucleophilic reaction. The concentration of anthraquinone-2-sulfonate anion-radicals, for example, becomes independent of the duration time of the reaction with an alkali hydroxide, and the total yield of the anion-radicals does not exceed 10%. Inhibitors (oxygen, potassium ferricyanide) prevent formation of anion-radicals, and the yield of 2-hydroxyanthraquinone even increases somewhat. In this case, the anion-radical pathway is not the main one. The same conclusion is made in the case of oxadiazoloanthraquinone. 

The ESR spectrum of the anion radical derived from 2,5-diphenyl-l,3,4-oxadiazole has been determined and the hyperfine splittings are based on MO calculations (80AHC(27)3i). In the same review, hyperfine splittings are given for the radicals produced on oxidation by lead oxide of 3-substituted oxadiazolidine-2,5-diones. ESR spectra of radical cations derived from 3,4-disubstituted 1,3,4-oxadiazolidines have been described (74JA2916). 

Attempts to reduce furoxans (e.g., 3,4-dimethyl-1,2-5-oxadiazole 2-oxide) to anion-radicals led to paramagnetic products of indeterminate structure or multielectron reduction products. ... 

The synthesis and properties of heat-resistant polyazomethines containing 2,5-disubstituted oxadiazole fragments, being insulators convertible into semiconductors by doping with iodine, have been described. The radical copolymerization of alkenes with the fluorescent co-monomer 2-/-butyl-5-(4 -vinyl-4-biphenylyl)-l,3,4-oxadiazole has resulted in useful macromolecular scintillators. Anionic polymerization of 2-phenyl-l,3,4-oxadiazolin-5-one has produced a nylon-type product <1996CHEC-II(4)268>. 

A photo-induced electron transfer (from either the sensitizer in its excited state to the oxadiazole in its ground state or from the electron-donor reagent such as triethylamine to the excited oxadiazole) has been suggested as an explanation for the breaking of the O—N bond of 5-aryl-3-methoxy-(or 5-aryl-3-phenyl-)-l,2,4-oxadiazoles (71) upon irradiation. The resulting oxadiazole radical anion underwent either a heterocycliza-tion to give quinazolin-4-ones or reduction to give open-chain products. 

Scheme 42 Fragmentation and heterocyctization of oxadiazole radical anions.

Diphenyl-l,3,4-oxadiazole reacts with sodium to yield a radical anion which dimerizes (70KGS(S2)303). 2-Substituted A2-l,3,4-oxadiazoline-5-thiones yield the corresponding 2-substituted 1,3,4-oxadiazoles on heating with Raney nickel. With an excess of nickel, ring cleavage to an amide results (B-61MI42300). 

The emission spectrum matches the fluorescence of both thianthrene (3) and 2,5-diphenyl oxadiazole (4). The radical anion and cation of both compounds are involved. 

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