Oxadiazole substituents

Similar PPV-based copolymers with carbazole and fluorene units in the backbone 101 and 102 (and also similar copolymers with oxadiazole substituents 103 and 104) have been synthesized by Ree and coworkers [148], Much lower PL efficiency in films was found in this case for carbazole-containing polymers 102 and 104 (1-4%), when compared to materials prepared by Shim (95 and 96, 59-64%) (Chart 2.21). 

Electron acceptor dicyanovinyl and oxadiazole substituents have been recently introduced into phenylene units of the PPV block copolymers (179,180) [211]. Blue and blue-greenish PL emission was observed for 179 and 180, respectively, but the PLQY was relatively low even in solution (13 and 24%) and no EL device has yet been reported. 

Scheme 29 Synthesis of a fluorene with oxadiazole substituents...

Recently, Shu, Jen and coworkers [252] prepared a fluorene 152 with two electron-accepting oxadiazole substituents by nudeophihc substitution... 

Substituents are expected to alter the electron density at the multiply-bonded nitrogen atom, and therefore the basicity, in a manner similar to that found in the pyridine series. The rather limited data available appear to bear out these assumptions. The additional ring nitrogen atoms in triazoles, oxadiazoles, etc. are quite strongly base-weakening this is as... 

Oxadiazoles are difficult to alkylate, unless the ring contains a strong electron donor group such as an amino substituent. 

In azole chemistry the total effect of the several heteroatoms in one ring approximates the superposition of their separate effects. It is found that pyrazole, imidazole and isoxazole undergo nitration and sulfonation about as readily as nitrobenzene thiazole and isothiazole react less readily ica. equal to m-dinitrobenzene), and oxadiazoles, thiadiazoles, triazoles, etc. with great difficulty. In each case, halogenation is easier than the corresponding nitration or sulfonation. Strong electron-donor substituents help the reaction. 

Oxazoles are easily cleaved. 2,5-Dialkyl-l,3,4-oxadiazoles (159) in aqueous solution with acid or base give hydrazides (if suitable substituents are present, further reaction can occur see Section 4.02.3.5.1). 3-Methyl-l,2,4-oxadiazole (160) is easily hydrolyzed to acetamidoxime (61CKL)292). 

Substituents cannot directly conjugate with /3-pyridine-like nitrogen atoms. Azole substituents which are not a or y to a pyridine-like nitrogen react as they would on a benzene ring. Conjugation with an a-pyridine-like nitrogen is much more effective across a formal double bond thus the 5-methyl group in 3,5-dimethyl-l,2,4-oxadiazole (323) is by far the more reactive. 

In the 1,2,4-thiadiazole ring the electron density at the 5-position is markedly lower than at the 3-position, and this affects substituent reactions. 5-Halogeno derivatives, for example, approach the reactivity of 4-halogenopyrimidines. The 1,2,4-oxadiazole ring shows a similar difference between the 3- and 5-positions. 

Reaction of a hydrazide (128) with phosgeneiminium chloride (115) led to the 2-dimethylamino-l,3,4-oxadiazole (129) in 90% yield (75AG(E)806). The 1,3,4-thiadiazole system was also obtained in an analogous reaction in which the dithioimidate (130) underwent reaction with the thiohydrazide (131). Depending on the nature of X in (131), the 2-substituent in the resultant 1,3,4-thiadiazole (132) may be varied (80ZC413). Although (130)... 

Other non-traditional preparations of 1,2,3-triazoles have been reported. The rearrangement in dioxane/water of (Z)-arylhydrazones of 5-amino-3-benzoyl-l,2,4-oxadiazole into (2-aryl-5-phenyl-27/-l,2,3-triazol-4-yl)ureas was investigated mechanistically in terms of substituents on different pathways <06JOC5616>. A general and efficient method for the preparation of 2,4-diary 1-1,2,3-triazoles 140 from a-hydroxyacetophenones 139 and arylhydrazines is reported <06SC2461>. 5-Alkylamino-] //-], 2,3-triazoles were obtained by base-mediated cleavage of cycloadducts of azides to cyclic ketene acetals <06S1943>. Oxidation of N-... 

Irradiation of 1,2,4-oxadiazoles 205 bearing fluorinated substituents in the 3- or 5-positions in the presence of an amine delivered the corresponding 1,2,4-triazoles 206a-e and 207a-e via a photochemical rearrangement. Several other competing reactions served to divert some of the reactive intermediates and, hence, yields of the fluorinated triazoles were modest (Equation 64 and Table 44) <2005H(65)387>. 

Oxadiazole 220 bearing a ferrocenyl substituent was reacted with ammonium ethanoate and hydrazine to yield the corresponding 1,2,4-triazoles 221 and 222 in reasonable yield (Scheme 19) <2003JOM(675)1>. 

Multinuclear NMR data have been recorded for molsidomine 12 and its metabolite SIN-1 13, confirming that both are closed ring structures and that the positive charge is accommodated at the N3 position <1996CHE1358>. The c C4-C5 coupling constants for selected 1,2,3-oxadiazoles lie between 69.4 and 89 Hz, but correlation of these values with measures of bond order and aromaticity is difficult due to substituent effects <2000MRC617, 2002JST269>. 

The dipole moments of both 3- and 4-phenylsydnones containing dimethylamino and nitro substituents were calculated (ah initio 3-21G basis set) the magnitude increases with the electron donor attached to the phenyl ring <1995JPC1923>. Quadrupole moments, octopole moments, and polarizability of 1,2,3-oxadiazole have been determined by ab initio calculations and simple models <1996JPC8752, 1999JPC10009>. 

There have been no reports of 1,2,3-oxadiazoles synthesized by transformation of another ring since the last survey <1996CHEC-II(4)165> other than those involving cyclization of substituents attached to benzene (Section 5.03.9). 

The role of substituents X on the mononuclear heterocyclic rearrangement (MHR) of 20 phenylhydrazones 54 of 3-benzoyl-5-phenyl-l,2,4-oxadiazole into the triazoles 55 (Equation 2) has been investigated, allowing the influence of X on the product distribution to be evaluated and first-order rate constants and Hammett correlations to be determined <1999T12885>. 

The same research group also showed that the r-butyldimethylsilyl (TBDMS)-protected 4-substitued 4,5-dihydro-l,2,4-oxadiazol-5-one 192 afforded the alcohol 193 on treatment with ethanolic HC1. Mesylation and treatment of the intermediate with sodium iodide gave the iodofluoroalkenyl-substituted 4,5-dihydro-l,2,4-oxadiazol-5-one 194 (Scheme 26) <2004T10907>. 

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