Oxadiazoles, literature reviews

The benzofuroxtin [benzofurazan oxide, 3,4-benzo-l,2,6-oxa-diazole-2-oxide, or 2,1,3-benzoxadiazole-l-oxide (1)] ring system has been reviewed briefly on several occasions, notably by Kaufman and Picard,Boyer, and Behr. The mqst recent of these covers the literature until 1959, and since that date there have been many advances in the subject. This, we feel, justifies the field being covered once more, and its separation from the monocyclic 1,2,5-oxadiazole oxides—the furoxans. We consider also other furoxano-fused compounds in this chapter, subject to the limitation that the ring adjacent to the furoxan is aromatic and six-membered. 

The preparation and reactions of 1,2,3-oxadiazoles have been reviewed by Meier and Hanold <94HOU(E8c)397). 1,2,3-Oxadiazoles were described by Clapp in the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) <84CHEC-I(6)365). The chemistry of sydnones is described in two other reviews <82X2965,91MI403-01). For this chapter the literature published between 1982 and mid-1995 has been surveyed. 

Furazans are formed via the Boulton-Katritzky rearrangement of the oximes of several classes of 3-acyl-l-oxa-2-azoles, including 1,2,4-oxadiazoles, isoxazoles, and furazans. These reactions have been intensively investigated over many years, and the literature comprehensively reviewed <81 AHC(29)i4i, 90KGS1443> the latter survey deals specifically with the exploitation of such rearrangements for furazan synthesis. 

Oxadiazoles have a wide variety of uses, in particular as biologically active compounds in medicine and in agriculture, as dyestuffs, UV absorbing and fluorescent materials, heat-resistant polymers and scintillators. Reviews of the relevant literature prior to 1965 are available <66AHC(7)183,62HC(17)263>. 

The seventh volume of Advances in Heterocyclic Chemistry includes surveys of four groups of heterocyclic compounds furans (P. Bosshard and C. H. Eugster), dithiolium salts (H. Prinzbach and E. Futterer), 1,3,4-oxadiazoles (A. Hetzheim and K. Mockel), and diquinolylme-thanes (G. Scheibe and E. Daltrozzo). Further chapters deal with applications of mass spectrometry to heterocycles (G. Spiteller), an area which has expanded very rapidly of late, and the halogenation of heterocycles (J. J. Eisch). Finally, a summary is given of reviews in the heterocyclic field, classified by subject (A. R. Katritzky and S. M. Weeds), which it is hoped may be of assistance in literature surveys. 

Hole transport in polymers occurs by charge transfer between adjacent donor functionalities. The functionalities can be associated with a dopant molecule, pendant groups of a polymer, or the polymer main chain. Most literature references are of doped polymers. The more common donor molecules include various arylalkane, arylamine, enamine, hydrazone, oxadiazole, oxazole, and pyrazoline derivatives. Commonly used polymers are polycarbonates, polyesters, and poly(styrene)s. Transport processes in these materials are unipolar. The mobilities are very low, strongly field and temperature dependent, as well as dependent on the dopant molecule, dopant concentration, and the polymer host This chapter reviews hole transport in polymers and doped polymers of potential relevance to xerography. The organization is by chemical classification. The discussion mainly includes molecularly doped, pendant, and... 

Owing to their tendency to dimerize to furoxans (1,2,5-oxadiazole 2-oxides), nitrile oxides 5 are usually generated in situ, i.e., in the presence of suitable dipolarophiles such as alkenes, alkynes, etc., from stable precursors such as aldoximes 12 (X = H) or from primary nitroalkanes 13 (Scheme 2) [5,57-67]. Generation of nitrile oxides 5 from aldoximes 12 (X = H) involves either direct oxidation or halogenation of aldoximes 12 (X = H) to hydroximoyl halides 12 (X = Cl or Br) followed by dehydrohalogenation [5,57-67,79,80]. Alternatively, nitrile oxides 5 are conveniently generated via dehydration of primary nitroalkanes 13 [ 17,38,39,65,66,81-95]. This review covers the literature in the last 10-15 years pertaining to the chemistry of isoxazoHnes synthesized from primary nitroalkanes 13. 

Alternatively, oxadiazole-containing dendrimers and starburst compounds have been synthesized that form stable glasses [73]. The general methods of how to synthesize this type of polymers are reviewed in the literature [74]. 

This review is an overview on the recent literature on 1,2,4-oxadiazole s chemistry and its major advances since the last reviews on this topic (2008MI243, 2009OBC4337). After a brief historical introduction and a general presentation of the main features of this heterocycle, modem synthetic methodologies will be presented together with recent mechanistic interpretations of 1,2,4-oxadiazoles thermal and photochemical reactivity. As for appHcations, the main fields of Hfe sciences and materials chemistry have been considered. A commentary on future prospects adds suggestions for further developments of 1,2,4-oxadiazole heterocycles. 

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