Oxazoles and oxadiazoles

Oxadiazoles are a class of heterocyclic molecules that contain a five-membered ring comprised of two N atoms, two C atoms, and one O atom in the sequence -0-C=N-N=C-. In the oxazoles, the same elements are arranged in the sequence -0-C=N-C=C-. 

Crisa (1983) measured hole. mobilities of a mixture of 2,5-bis(4-diethylaminophenyl)-l,3,4-oxadiazole and a polyester. The time, thickness, and field dependencies of the photocurrent transients agreed with predictions of the Scher-Montroll model (1975). According to the model, the transit time scales with thickness and field as (L/E)l/a. The experimental value of a was 0.80. The study of Crisa, and later work by Bos and Burland (1987), are the only literature references to polymers where the scaling relationships predicted by Scher and Montroll have been observed over a range of thicknesses and fields. 

Naito and Kanemitsu (1996) investigated the relationship between the prefactor mobilities, zero-field mobilities, and the glass transition temperatures of OX doped polyarylate (PA), PC, poly(methyl methacrylate) (PMMA), PS, poly(vinyl chloride) (PVC), polyethylene terephthalate) (PET), and poly(vinyl butyral) (PVB), DEH doped PC, 5(p-diethylaminophenyl)-l-phenyl-3-(/ -diethylaminostyryl)-2-pyrazoline (DEASP) doped PS, and DEASP doped PC. OX, DEH, and DEASP are highly polar molecules with similar dipole moments. By modifying the polymer, the glass transition temperature can be varied over 

As discussed in more detail in Chapter 6, the phthalocyanines are either metallized or metal free. The metal compounds can contain either two monovalent metal atoms, a divalent metal atom, or a tri- or tetravalent metal atom with one or two associated ligands. The phthalocyanines can exist in several ciystalline forms. Most studies have been made of vapor-deposited or dispersion layers. 

Valerian and Nespurek (1993) measured the hole range of vapor-deposited a-H2Pc. The values were between 4 x 10-11 and 2 x 10-9 cm2/V, compared to 

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The direct crosscoupling of unfunctionalised aromatic compounds with terminal allgmes affords the same products as in the Sonogashira coupling reaction and is veiy attractive. 1,3,4-Oxadiazoles and oxazoles are coupled with terminal allgmes in the presence of a stoichiometric amount of cop-per(i) chloride at 120 °C under oigrgen (Scheme 15.56). Here, aUgmes must... 

Cross-coupling. Heteroarenes, such as 1,3,4-oxadiazole and oxazole, efficiently undergo a copper(II) chloride-mediated oxidative coupling with terminal alkynes (eq 34). Electron-rich alkynes effectively coupled with 2-phenyl-1,3,4-oxadiazole, although electron-deficient substrates led to a decreased yield. The 1,3,4-oxadiazoles with various substituents at the 2-position were also suitable substrates, but the slow addition of terminal alkynes was essential to avoid undesired diyne formation. The desired 2-alkynyl-5-aryloxazoles were obtained in DMSO at 150 °C. 

A comparison of MP2/6-31G structural parameters of 1,2-oxazole 19 (isox-azole) and 1,3-oxazole 20 with microwave data is provided by Kassimi et al. (Scheme 16) [96JPC8752]. The general agreement is excellent. The same authors investigated dipole moments, quadrupole moments, octopole moments, and dipole polarizabilities of 19 and 20 together with several oxadiazoles and oxatriazoles [96JPC8752, 99JPC(A) 10009]. For the mean polarizability of these species, they found the approximative formula... 

Descriptor frequence values (cm )] are claimed to be helpful in predicting the direction of these rearrangements (96ZOK1742). These values were developed for oxazoles, 1,2,4-oxadiazoles, and furazanes. The calculated descriptor values [v sr(cm )] for the E- and Z-isomers of 4-aminofurazan 3-carboxamidoximes [E-I and Z-I, = (CH2)4, (013)2] and their rearranged 3-(substituted amino)furazan 4-carboxamidoximes [II,... 

Isoxazoles, 1,2,5-oxadiazoles and 1,3,4-oxadiazoles are apparently inert under dye-sensitized photo-oxygenation [74a], while a thiotriamide has been found in the photo-oxygenation of arylthiazole presumably via the undetected endoperoxide in a fashion similar to oxazoles [67,80]. 

The photoelectron spectra of oxazole, isoxazole, 1,2,5-oxadiazole and 1,3,4-oxadiazole in the He(I) and He(II) regions (77JST(40)l9l) and those of benzoxazole and other benzazoles in the He(I) region (78JST(43)203) have been measured. The orbital energies were assigned on the basis of ab initio calculations. The data for oxazole and benzoxazole are reported in Table 9. 

The current organic photoreceptors are triarylamines, triarylmethanes, hy-drazones, oxadiazoles, pyrazolines, oxazoles, and more recently, stilbene derivatives. The polymer matrix, on the other hand, is constituted by polyesters and polycarbonates (Fig. 5). The common presence of aromatic amines as substituents in all these materials contributes to efficient hole transport [44]. The nonbonding electron pair on the nitrogen atom, in fact, confers on these molecules a low oxidation potential, and consequently, the production of a chemically stable radical cation with the possibility of an effective overlap of nonbonding molecular orbitals between neighboring molecules. 

Lithiation of the methyl derivatives of such five-membered heteroaromatics as pyrrole , thiophene , l,3-thiazole ° , 1,3-oxazole , isoxazole , 1,3,4-thiadiazole , 1,3,4-oxadiazole and imidazole also occurs. For the sulfur heterocyclics, ring metallations and ring opening after lithiation are complications. 

TABLE 1.55 2,5-DIHYDRO-l,2,4-OXADIAZOLES FROM OXAZOLES AND ARYLNITROSO... 

Scheme 12 Structures of monomers containing oxazole, oxadiazole, and oxindole moieties...

Hyperbranched Poly(aryl ether)s with Oxazole, Oxadiazole, and Oxindole Moieties... 

Thus quaternized thiazoles (170) consume two equivalents of OH on titration because the pseudo bases (171) ring open to (172), which form anions (173). Quaternized oxazoles (174) are readily attacked by hydroxide to give open-chain products such as (175) (74AHC(17)99), and quaternized 1,3,4-oxadiazoles behave similarly. Quaternary isothiazoles (e.g. 176) are cleaved by hydroxide (72AHC(l4)l), as are 1,2,4-thiadiazolium salts (177 178). 

Oxygen-containing azoles are readily reduced, usually with ring scission. Only acyclic products have been reported from the reductions with complex metal hydrides of oxazoles (e.g. 209 210), isoxazoles (e.g. 211 212), benzoxazoles (e.g. 213 214) and benzoxazolinones (e.g. 215, 216->214). Reductions of 1,2,4-oxadiazoles always involve ring scission. Lithium aluminum hydride breaks the C—O bond in the ring Scheme 19) 76AHC(20)65>. 

Methyl groups on C-linked phenyl attached to oxazoles, isoxazoles and oxadiazoles react with benzylidineaniline to give stilbene derivatives (Scheme 51) <78AHC(23)l7l). 

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