Oxadiazoles pyrolysis

Although formally it could be classified with the ring transformations (Section 4.04.3.2.2), conversion of 2,4-diphenyl-l,3,4-oxadiazol-2-one (593) by flash vacuum pyrolysis at 500 °C into 3-phenylindazole (595) involves a C(3)—C(3a) ring closure of the diphenylnitrilimine (594) (79AG(E)721). 

Kinetic studies of pyrolysis of azides, giving oxadiazole A-oxides in near-quantitative yields, showed that the 5-azido-6-nitroquinoline pyrolyzed in acetic acid 27.6 times faster than did 5-azidoquinolines, because of the -M effect of the group adjacent to the azide group (85AJC1045). 

The above strategy was tested [27] with a 3-layer LED consisting of a poly(2,5-thienylene vinylene) (PTV) layer, known to have particularly low oxidation potential [28], followed by a layer of l,4-fcrs-(4 -diphenylaminostyryl)-2,5-di-methoxy-benzene (DASMB) [29] and a layer of 2-(4-biphenyl)-5-(4-tcrt-butyl-pheenyl)-1,3,4-oxadiazol (PBD) dispersed in polystyrene (PS) in a 20 80 ratio. Films of poly-(2,5-thienylene-a-bromoethylcne) were obtained by vapor phase pyrolysis of 2,5-W.v-(bromomethyl)lhiophcne and subsequent vapor deposition of the quinoid monomers onto a cold substrate following a previously published procedure [30]. They were converted to PTV by temperature-induced elimination of HBr. 

Online coupling of flash-vacuum pyrolysis and mass spectrometry was applied to 3,4-dicyano-l,2,5-oxadiazole-2-oxide (dicyanofuroxan). The 1,2,5-oxadiazole is almost quantitatively pyrolyzed at 500-600 °C. Using collisional activation, the main pyrolysis products were identified as NCCNO and (CN)2 <1997BSB545>. 

Thermolysis of tetrazole oximes (478) in a suitable hydrocarbon solvent, or pyrolysis without it, produces 3,5-disubstituted 1,2,4-oxadiazoles (480) (Scheme 75) (81BSB193 87BSB675 91MIP137367). in the case of 5-unsubstituted tetrazoles (478 R = H), the reaction goes by two concomitant pathways, which imply elimination of hydrazoic acid or nitrogen. 

Vinyl oxadiazoles of the formula LIX in which R = H or lower alkyls of from 1 to 4 carbon atoms have been recently patented 90a). They have been prepared by pyrolysis of the corresponding acetates. 

Oxadiazoles undergo thermal and photochemical ring cleavage at the 0(1)-N(2) and C(3)-C(4) bonds to yield nitrile and nitrile oxide fragments, and products derived therefrom. Thus, diphenylfurazan (30, X = 0) decomposes under flash vacuum pyrolysis conditions (600°C, 10-3 mm... 

Muchall et al. (98CC238) have recently investigated the gas-phase thermolysis of 2,5-dihydro-2,2-dimethoxy-2,5,5-trimethyl-l//-l,2,4-oxadiazole (75) by PE spectroscopy. Decomposition of 75 was induced by means of a continuous wave (CW) C02 laser as directed heat source at 26 W, which corresponds to a temperature of 500 50°C. When the PE spectra of acetone, tetramethoxyethene, and dimethyl oxalate were subtracted from the pyrolysis spectrum, a sim-ple spectrum remained that could be identified as that of dimethoxycarbene. Thermolysis in solution (94JA1161) had shown formation of tetramethoxyethene, and FVP experiments (92JA8751) gave dimethyl oxalate, both of which arise from the common precursor, dimethoxycarbene. Thermolysis of oxadiazolines similar to 75 in solution affords dialkoxycarbenes via an intermediate carbonyl ylide (94JOC5071). 

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... 

Oxadiazole is thermally stable and this stability is increased on substitution, particularly by aryl and perfluoroalkyl groups. Oxadiazolin-5-ones and -5-thiones are somewhat less stable and undergo selective pyrolysis at high temperatures. 

BSF1149) and a plot of the tt densities against the log rates of pyrolysis of l-(thiazolyl)ethyl acetates is linear. The reactivity order for 1,2,5-diazoles from ir-density measurements is oxadiazole > selenadia-zole > thiadiazole (73CHE1331) the effects of substituents in 1,2,4-tri-azole have also been calculated (71CHE377). 

On being heated, the oxadiazoles (39, 40) both yielded phenyl isocyanate and benzonitrile, but only the mass spectrum of (39) showed strong peaks corresponding to these pyrolysis products at m/e 103, 119 (J. L. Cotter and Knight, 1966). 

Pyrolysis and Graphitization of Polyoxadiazole. Table 1 shows electrical conductiyity at room temperature for yarious condensation polymers heat-treated at 1000 and 2500 C. For a HTT near 1000 C, poly(p-phenylene-1,3,4-oxadiazole) (POD) has the highest conductivity of 340 S/cm, twice as high as that of polyimide (KAP)... 

Experimental attempts to synthesize any of the polymers described above have not been successful except a preliminary preparation of poly-perylene (Murakami and Yoshimura, 1984). Alternative efforts are currently being made to obtain polyacenic material through pyrolysis of various organic polymers such as phenol-formaldehyde resin (Yamabe et al., 1983 Tanaka et al., 1984b), polyacrylonitrile (Teoh et al., 1982, 1983), and poly(p-phenylene-1,3,4-oxadiazole) (Murakami et al., 1983). The group of these pyrolytic polymers behaves rather like amorphous semiconductors and is considered to be composed of fragments of polyacene to graphite, that is, a sort of coke, coal, and so on. 

Ring contraction in pyridylnitrenes formed by pyrolysis of pyrido[2,3-a]-[l,2,4]oxadiazol-2-ones has also been reported.362... 

Table 12. Pyrolysis of tetrazolo[l,5-a]pyridine 124) i) and pyrido[2,3-o][l,2,4]-oxadiazol-2-one (f25) 2)...

Oxadiazoles. The ojadiazoline (490) decomposes in solution to yield the carbonyl ylide Me2C-0=CMe0Me as the initial product.Flash vacuum pyrolysis of the 7V-butenyloxadiazolinone (491) affords a mixture containing the dienes (492) and (493). These are formed via a nitrile imine and a diazocompound as the key intermediates (see Scheme 9). °... 

Aminotetrazole behaves like a normal primary amine in reactions with chloroform in base,328 phenyl isocyanate,328 p-toluenesulfonyl chloride,328 and aromatic aldehydes,43 and the expected derivatives of the amino moiety are obtained without interference at the tetrazole ring. With acid chlorides and anhydrides acylation of the amino group is accompanied by ring cleavage and yields 2-acylamino-1,3,4-oxadiazoles.43,328 Flash vacuum pyrolysis of 1 -phenyltetrazole at 500° has recently been reported to give phenylcyanamide as the only detectable product.333... 

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