3.5- Diphenyl-1,2,4-oxadiazoles

Dimethyl-1,2,4-oxadiazole has no UV absorption above 200 nm. For 3-phenyl-1,2,4-oxa-diazole = 238 nm (a = 14000), for the 5-phenyl isomer = 250 nm (a = 16100), and for 3,5-diphenyl-1,2,4-oxadiazole 2maj = 245 nm (a = 37200) (ether) have been reported <64HCA942>. 3-Ethyl-5-phenyl-1,2,4-oxadiazole has 2n,ax(CHCl3 or 2-propanol) = 284 nm, while for the fluorescence maxfCHCh) = 310 nm has been observed <77JOCi555>. Hypochromism with increase in con-... 

Benzofurazans are thermally more stable but can be cleaved photolytically. For example, benzo-furazan itself in benzene affords cyanoisocyanate (17) and azepine (18), the latter being formed by reaction of the solvent with the putative intermediate acylnitrene (19) (Scheme 5) further supporting evidence for the proposed pathway is provided by trapping the nitrile oxide precursor with dimethyl acetylenedicarboxylate and isolation of the methylurethane derivative of the isocyanate <75JOC2880>. Photolysis of diphenylfurazan yields benzonitrile, diphenylfuroxan and 3,5-diphenyl-1,2,4-oxadiazole. 

Erste Untersuchungen zur Reaktion von Benzoesaure-amid-hydroximiden mit Benzoesaure beschreiben die Bildung von 3,5-Diphenyl-1,2,4-oxadiazol (Schmp. 107°)6. Ameisensaure, Essigsaure, Propansiiure oder Butansaure liefern ebenfalls 3,5-Diphenyl-l,2,4-oxadiazol und keine 5-Alkyl-3-phenyl-l,2,3-oxadiazole70. 

Figure 13 3,5-diphenyl-1,2,4-oxadiazole-based bent-core mesogens. 

A mixture of a-benzil dioxime and polyphosphoric acid heated 12 min. at 120° 3,5-diphenyl-1,2,4-oxadiazole. Y 99%. 

An external magnetic field was observed to have practically no effect on the intensity of the thianthrene (430 nm) emission, indicating that no triplet states are involved. The shorter-wavelength emission of the oxadiazole 102, however, is probably due to a triplet-triplet annihilation reaction of diphenyloxadiazole triplets. These are produced in the radical-ion reaction between 101 and 102, yielding thianthrene excited-singlet molecules and diphenyl-oxadiazole excited-triplet molecules ... 

The oxidation of benzaldoxime with perchloryl fluoride (FClOj) has been reported [29 a) to give a complex mixture in which benzaldoxime benzoate and diphenyl oxadiazole are the main products. Sodium nitrohydroxamate [Na2(02NN0)] has been reported [99b) to oxidize benzyl chloride to a mixture of compounds from which benzyl alcohol, benzaldehyde, benzoic acid, 3,4,5-triphenylisoxazole, benzyl-ethyl-ether, phenylnitromethane and diphenyloxadiazole have been isolated. 

It must also be noted that the wave lengths of the absorption bands of diphenyl oxadiazole (245 mp) and the two monophenyl oxadiazoles... 

Moreover, the UV adsorption band of diphenyl oxadiazole and (3-phenyloxadiazolyl) 5-phenyl oxadiazolyl methane have the same wave length (246 mfi). Such a similarity shows the additive character of the conjugated systems present in both molecules. 

In the first paper about oxadiazoles, published in 1884, Tiemann [114) mentioned that benzonitrile is formed when diphenyl oxadiazole is heated with hydrochloric acid and zinc dust. Until recently, no further information concerning the reduction of the oxadiazole ring was available. 

The action of bromine, ferricyanide,89 and other mild oxidizing agents90 directly on benzamidoxime has been reported to give diphenyl-oxadiazole (16), While it is clear that one nitrogen has been oxidized, no intermediate was isolated. 

DPA-thianthrene (3) or 2,5-diphenyl oxadiazole (4)-thianthrene are energy-sufficient systems ... 

N N 3-( p-Bromophenyl)-4-methyl- 1,2,5-oxadiazole 2-oxide 3-(p-bromo-phenyl)-4-methyl-l,2,5-oxadiazole 5-oxide 3,4-diphenyl-l,2,5-oxadiazole... 

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). 

On irradiation in ether at 254 nm 2,5-diphenyl-l,2,4-oxadiazole (102) did not give any interesting result. On the contrary, the irradiation in an immersion apparatus gave, with low conversion (33%), a mixture of three products (103-105) (Scheme 42) [68TL2417 95H(41)2095]. 

The results described above represent the first example of the FR mechanism (Scheme 1). Semiempirical calculations on this molecule showed that the intersystem crossing to the excited triplet state is favored The reaction cannot be sensitized by xanthone because the triplet state of 3,4-diphenyl-1,2,5-oxadiazole is lower than that of xanthone. The cleavage of the triplet state to the biradical is favored, considering the relative energy of this intermediate (Fig. 23) (OOOUPl). 

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> ... 

The 170 NMR spectrum of 4,5-diphenyl-l,3,4-oxadiazolium-2-olate was reported earlier <1996CHEC-II(4)268>. Since then, no report on 170 NMR spectra of 1,3,4-oxadiazoles has been published. 

Diphenyl-l,3,4-oxadiazole crystallization revealed two polymorphic forms (centrosymmetric and non-centrosymmetric) of the substance. Raman spectra of both phases recorded between 15 and 1700 cm-1 showed well-resolved internal modes and the external lattice vibrations below 200 cm-1, offering a fast tool for discrimination between different polymorphs. The internal modes were dominated by two groups, one around 1000 cm-1 and the second one between ca. 1500 and 1600 cm-1 <2003JST219>. 

More recently, the El and electrospray ionization (ESI) mass spectra of 2,5-diaryl- and 2-arylamino-5-aryl-l,3,4-oxadiazoles, as well as their complexes with copper cations, were studied. Under ESI conditions, loss of NH3 and HNCO, from complexes of 2,5-diphenyl-l,3,4-oxadiazole, 2,5-bis(2-pyridyl)-l,3,4-oxadiazole, or 2,5-bis(4-pyridyl)-1,3,4-oxadiazole with copper cation, was observed <2004JMP272>. An unusual elimination of isocyanic acid was found in fragmentation of some protonated 2,5-diaryl derivatives <2002RCM390>. 

Diphenyl-l,3,4-oxadiazole is polymorphic (centrosymmetric monoclinic structure with space group P21/c and monoclinic non-centrosymmetric structure with space group P21/c). DSC investigations showed an irreversible transition from the first to the second form at 97 °C <2003JST219>. 

Oxadiazoles are weak Hammett bases. The basicity constants of 2,5-diphenyl-1,3,4-oxadiazole (pK -2.49) and of 2-(4-methylphenyl)-5-phenyl-l,3,4-oxadiazole (pAa -1.15) were measured by the method of Yates and MacClelland in an aqueous solution of sulfuric acid in the range from pH 7 to H0 -10. Both compounds exhibited luminescence properties depending on the acid concentration <1996SAA1875>. 

Epoxide derivatives of 2,5-bis-oxyphenyl-l,3,4-oxadiazoles and fluorescein, which are luminescent epoxide monomers, were synthesized and their luminescence properties were studied <1999CHE358>. The excited state intramolecular proton transfer reactions and luminescent properties of the ort j-hydroxy derivatives of 2,5-diphenyl-1,3,4-oxadiazole were elucidated to conclude that the proton phototransfer reaction is very efficient in the studied... 

Acyl nitroso compounds react with 1, 3-dienes as N-O heterodienophiles to produce cycloadducts, which have found use in the total synthesis of a number of nitrogen-containing natural products [21]. The cycloadducts of acyl nitroso compounds and 9,10-dimethylanthracene (4, Scheme 7.3) undergo thermal decomposition through retro-Diels-Alder reactions to produce acyl nitroso compounds under non-oxidative conditions and at relatively mild temperatures (40-100°C) [11-14]. Decomposition of these compounds provides a particularly clean method for the formation of acyl nitroso compounds. Photolysis or thermolysis of 3, 5-diphenyl-l, 2, 4-oxadiazole-4-oxide (5) generates the aromatic acyl nitroso compound (6) and ben-zonitrile (Scheme 7.3) [22, 23]. Other reactions that generate acyl nitroso compounds include the treatment of 5 with a nitrile oxide [24], the addition of N-methyl morpholine N-oxide to nitrile oxides and the decomposition of N, O-diacylated or alkylated N-hydroxyarylsulfonamides [25-29]. 

The hydrogen bonding acceptor ability (log K ) for 3,5-diphenyl-l,2,4-oxadiazole has been determined <89JCS(P2)1355>. An aromaticity index indicates low aromaticity of the parent 1,2,4-oxadiazole <85T1409>. 

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