2-Phenyl-5- -l,3,4-oxadiazole

Isoxazoles unsubstituted in the 3-position react with hydroxide or ethoxide ions to give )3-keto nitriles (243) -> (244). This reaction involves nucleophilic attack at the 3-CH group. 1,2-Benzisoxazoles unsubstituted in the 3-position similarly readily give salicylyl nitriles (67AHC(8)277), and 5-phenyl-l,3,4-oxadiazole (245) is rapidly converted in alkaline solution into benzoylcyanamide (246) (61CI(L)292). A similar cleavage is known for 3-unsubstituted pyrazoles and indazoles the latter yield o-cyanoanilines. 

Many other examples have been prepared (2) from the corresponding aminomethyl heterocycles using the very versatile reaction between HHTs and diaryl phosphites, as demonstrated specifically above for the 5-phenyl-l,3,4-oxadiazole system. Conversion of the 2-aminomethyl-l,3,4-oxadiazole 68 to the required HHT intermediate 69 was accomplished... 

Quantum-chemical calculations were carried out and correlated with experimental observations concerning the electronic absorption, emission, and excitation spectra of (5-phenyl-l,3,4-oxadiazol-2-yl)-7-hydroxycoumarin 15 <2000SAA1773>. 

The fluorescence and laser properties of symmetrically and unsymmetrically substituted 2,5-diaryl-l,3,4-oxadiazoles were experimentally studied. It has been found that symmetrically substituted molecules (e.g., 2,5-di(2-naphthyl)-l, 3,4-oxadiazole) give laser oscillation at room temperatures, while unsymmetrical 2-(2-naphthyl)-5-phenyl-l,3,4-oxadiazole does not give laser action under any conditions, even at low temperatures <2000SAA2157>. 

In CHEC(1984) and CHEC-II(1996), only a few papers on crystal studies of 1,3,4-oxadiazole derivatives had been reported. The studies have shown that the oxadiazole and benzene rings in the crystal of 5-phenyl-l,3,4-oxadiazole derivative are nearly coplanar. 

Apart from the parent compound 1 and its very simple alkyl derivatives, 1,3,4-oxadiazoles are solids. Solid oxadiazoles containing biphenyl or triphenyl substituents exhibit interesting properties upon heating. The symmetric 2,5-bisbiphenyl-4-yl-l,3,4-oxadiazole 38 melts into an isotropic phase showing small monotropic mesophase. By contrast, the asymmetric (hockey stick-shaped) mesogen 2-terphenyl-4-yl-5-phenyl-l,3,4-oxadiazole 39 exhibits a more stable enantiotropic liquid crystalline phase (a smectic phase as well as a nematic phase) <2001PCB8845>. 

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>. 

Electrophilic substitution of the ring hydrogen atom in 1,3,4-oxadiazoles is uncommon. In contrast, several reactions of electrophiles with C-linked substituents of 1,3,4-oxadiazole have been reported. 2,5-Diaryl-l,3,4-oxadiazoles are bromi-nated and nitrated on aryl substituents. Oxidation of 2,5-ditolyl-l,3,4-oxadiazole afforded the corresponding dialdehydes or dicarboxylic acids. 2-Methyl-5-phenyl-l,3,4-oxadiazole treated with butyllithium and then with isoamyl nitrite yielded the oxime of 5-phenyl-l,3,4-oxadiazol-2-carbaldehyde. 2-Chloromethyl-5-phenyl-l,3,4-oxadiazole under the action of sulfur and methyl iodide followed by amines affords the respective thioamides. 2-Chloromethyl-5-methyl-l,3,4-oxadia-zole and triethyl phosphite gave a product, which underwent a Wittig reation with aromatic aldehydes to form alkenes. Alkyl l,3,4-oxadiazole-2-carboxylates undergo typical reactions with ammonia, amines, and hydrazines to afford amides or hydrazides. It has been shown that 5-amino-l,3,4-oxadiazole-2-carboxylic acids and their esters decarboxylate. 

C. Wang, M. Kilitziraki, L.-O. Palsson, M.R. Bryce, A.P. Monkman, and I.D.W. Samuel, Polymeric alkoxy PBD r2-(4-biphenyl)-5-phenyl-l,3,4-oxadiazole] for light-emitting diodes, Adv. Fund. Mater., 11 47-50, 2001. 

The crystal structures of 5-methyl-5-(2-methylprop-l-enyl)-2-phenyl-4-(4-phenylthiazol-2-yl)-A -1,3,4-oxadiazoline <90ZC26> and the rubidium and silver salts of 2-phenyl-A -l,3,4-oxadiazolin-5-one <85JOC4461> have been analyzed. A crystal study of bis-[2-(5-phenyl-l,3,4-oxadiazol-2-yl)-methyl] ether produced bond lengths and angles and showed the rings to be nearly coplanar <87AX(C)2166>. 

When part of an extended conjugated system, 2,5-disubstituted-l,3,4-oxadiazoles often fluoresce. This makes them potentially useful as laser dyes, optical brighteners, and scintillators. For example, oxadiazole (121a) <84GEP(O)3245202> and l,4-bis-(5-phenyl-l,3,4-oxadiazol-2-yl)naphthalene... 

Mercapto-5-phenyl-l,3,4-oxadiazol Eine Losung von 1,25 g (5,5 mmol) Kalium-C-(2-benzoyl-hydrazi-no)-dithiocarbonat erhitzt man in 10 m/ DMF 15 min unter RiickfluB, wobei sich eine Festsubstanz ab-scheidet. Die Reaktionsmischung wird mit Wasser verdiinnt und mit verd. Salzsaure versetzt. Die so er-haltene Substanz kristallisiert man aus 50%igem Ethanol um Ausbeute 0,65 g (73%) Schmp. 219-220°. 

Nitro-phenyl)-5-phenyl-l,3,4-oxadiazol (R = 4- 02-C6H4)407 0,19 g (1 mmol) 4-Nitro-benzoylchlorid gibt man zu einer Losung von 0,40 g (1 mmol) N-Triphenylphosphoranyliden-bcnzhydrazid und 0,20 g (2 mmol) Triethylamin in 30 ml Benzol. Man laBt die Reaktionsmischung 24 h bei 20° stehen und entfernt das Benzol. Der Riickstand wird mit Wasser und Ethanol gewaschen Ausbeute 0,25 g (94%) Schmp. 210-211°. 

Die Reaktion des Natrium-Salzes von 2-Acetylamino-5-phenyl-l,3,4-oxadiazol mit co-Brom-acetophenon verlauft dagegen ohne Abspaltung der N-Acetyl-Gruppe zu 5-[Acetyl-(2-oxo-2-phenyl-ethyl)-amino]-2-phenyl-l,3,4-oxadiazol (43% Schmp. 124 -125°)575. 

Eine andere Methode ist die Reaktion von 2-Amino-5-phenyl-l,3,4-oxadiazol mit Brom zum Perbromid, das bei Diazotierung zu 2-Brom-5-phenyl-l,3,4-oxadiazol (31% Schmp. 106 107°)62Onach dem Craig-Verfahren621 reagiert. Als Nebenprodukt erhalt man 2-Hydr-oxy-5-phenyl-l,3,4-oxadiazol. 

Amino-l,3,4-oxadiazole konnen fur die Einfuhrung heterocyclischer Substituenten iiber eine Ringumwandlung eines anderen labilen Heterocyclus genutzt werden. So reagiert 2-Amino-5-phenyl-l,3,4-oxadiazol mit 4-Benzyliden-2-methyl-5-oxo-4,5-dihydro-l,3-oxazol zu 2-(4-Benzyliden-2-methyl-5-oxo-4,5-dihydro-lH-imidazolo)-5-phenyl-1,3,4-oxadiazol649 (70% Schmp. 258°). 

Bei 2-Mercapto-5-phenyl-l,3,4-oxadiazol entsteht nicht nur unter Ringoffnung Benzamid, son-dern gleichzeitig in 20%iger Ausbeute 2-Phenyl-1,3,4-oxadiazol (s.a. S. 599). 

The photolysis of acyl azides has also been studied, and in some respects these appear to behave analogously. Pivaloyl azide, for example, adds to cyclohexene to give a 26% yield of an aziridine [Eq. (85)],321 and the assumption is that this addition again occurs via a nitrene. The photodecomposition of acetyl azide (295) in benzo-nitrile and phenylacetylene, on the other hand, affords322 2-methyl-5-phenyl-l,3,4-oxadiazole (296) and 2-methyl-5-phenyloxazole (297),... 

Finally, dianils of aromatic dialdehydes may react with 2 moles of a p-tolyl-substituted heterocycle. For example, 5-phenyl-2-(p-tolyl)-1,3,4-oxadiazole (71) reacts with Schiff s base from 2 moles of p-chloroaniline and 4,4 -diformylbiphenyl (72) to give 4,4 -bis[4-(5-phenyl-l,3,4-oxadiazol-2-yl)styryl]biphenyl (73).43 Along with further p-tolyl-substituted heteroaromatics, a number of carbocyclic aromatics have... 

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