Oxadiazoles radicals from

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. 

Isobutane was lost from 49 (R = /-butyl) to give an oxadiazole (51) under the same conditions. The /-butyl free radical was shown to be a... 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

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). 

Cotter and Knight have reported the ionic and thermal fragmentation of 2,5-diphenyl-l,3,4-oxadiazole 24. The major fragmentation pathways for the molecule ion 8) and neutral 27) are shown in Eq. (13) and (14) respectively. The benzoyl cation mje 105) is certain to be relatively more stable than its neutral analog the reverse is true of the benzonitrile molecule. It seems likely that products which result from loss of H2, including benzoyl radicals will be obtained from 24 on photolysis. 

Oxadiazole derivatives (77) are produced by simple dehydration of hydrazine derivatives169-173 (Eq. 47) or by pyrolytic elimination of fluoro-acyl radicals or nitrogen from appropriate derivatives (Eqs. 48174 and 4917S). 

An attempt to carry out the Gomberg-Bachman-Hey arylation of (278) in toluene gave little of the desired 4-aryl product but the major products were the three 1,2,4-oxadiazoles (279, 280, 281). These products seem to arise from the radical intermediate,which fragments and subsequently reacts with toluene and phenyl isocyanate derived from the reaction between toluene and the nitrile (Scheme 66). 

The question remains will it be possible to isolate the cycloadduct between 1,3,4-oxadiazole and the dienophiles from the reaction mixture Let us examine the case that is likely to be very easy experimentally the addition of cyclopropene to 3,5-bis(trifluoromethyl)-l,3,4-oxadiazole. This cycloadduct actually represents a cyclic azo compound (Scheme 10). Azo compounds are known to decompose very easily producing nitrogen molecules and the corresponding radicals. The energy for the decomposition can be either thermal or photochemical [60]. In... 

Bifunctional systems form heterocyclic rings for example (194) yields (195). Photolysis of N-(triphenylmethyl)anilines results exclusively in C-N bond homolysis to give the trityl (< ) = 0.6-0.8) and anilino radicals." Reduction of the N-O bond cleaved intermediate (196) (formed on irradiation of 5-amino or 5-phenyl-1,2,4-oxadiazoles) by sodium hydrogen sulfide, thiols or thioamides yields the corresponding benzamidines (197). In contrast, in the presence of nucleophilic thioureas (199) or thiocarbamates (200), N-S bond formation occurs and subsequent elimination from the possible intermediate (201) yields the thiadia-zoles (198) or (202) respectively. ... 

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