Bi oxadiazoles

Pioneering work on this subject [8] reported the preparation of various perfluoro-alkylamidoxiines 11 (Rf=CF3, C2F5, C3F7, C7F15) and their acylation with perfluoro-acylchlorides (Rf COCI) followed by cyclodehydration to produce 3,5-bis(perfluoroalkyl)-l,2,4-oxadiazoles 12 either symmetrically (Rf=Rf ) or unsymmetrically substituted. By following the same methodology, bis-oxadiazoles 14 (n=3) could be obtained (Scheme 3) [8, 9],... 

Scheme 3 Synthesis of perfluoroalkylated bis-oxadiazoles 14 by the anudoxime route...

Scheme 4 Bis-oxadiazoles 15 and 18 obtained by the amidoxime route followed by dehydration step with P2O5...

An interesting application of the cyclodehydration approach is the synthesis of bis-oxadiazoles 65 by dehydration of bis-diacylhydrazines 64 [36,37], Similarly, reaction of perfluoroanhydride 66 leading to 67 is also reported [38]. Bis-oxadiazoles 69, which have a good thermal stability, are prepared by cyclodehydration of the corresponding tetrafluoroisophthaloylbis(perfluoroacyl-hydrazines) 68 (Scheme 20) [39]. 

Bifunctional reagents have been considered for the construction of polymeric structures. The reaction of a,(o-bis(tetrazol-5-yl)perfluoroalkane 75 with (o-cyanoperfluoroanhydrides 79 (at 150 °C) produces bis-oxadiazoles 80 from which further functionalization may be added on the two terminal nitriles (Scheme 23) [42,43]. 

By the use of the same methodology, the N,N-difluoroaminodifluoromethyl-tetrazole 82 reacts with perfluoroacyl chlorides or oxalyl chloride leading to the corresponding oxadiazoles 81 or bis-oxadiazole 83, respectively (Scheme 24) [13]. 

Hydrocarbon oxadiazoles do not readily undergo Diels-Alder reactions, but 2,5-bis(trifluoromethyl)-l, 3,4-oxadiazole reacts with a number of strained or elec-... 

Ligand abbreviations (6-Mepy)(py)2tren and (6-Mepy)2(py)tren = tris[4-[(6-/ )-2-pyridyl]-3-aza-6-butenyl]amme, R = H or CH3 HB(pz)3 = hydro-tris(pyrazolyl)borate paptH = 2-(2-pyridylamino)-4-(2-pyridyl)thiazole phenmethoxa = 3-[2-(l,10-phenanthrolyl)]-5-methyl-l,2,4-oxadiazole pyimH = 2-(2 -pyridyl)itnidazole pybimH = 2-(2 -pyridyl)benzimidazole ppa = JV -(2-pyridylmethyl)picolineamidine tpmbn = tetrakis(2-pyridylmcthyl)-meso-2,3-butane-diamine tppn = tetrakis-(2-pyridylmethyl)-l-methyl-l,2-propanediainine tpchxn = tetrakis(2-pyridylmethyl)-tra s-l,2-cyclohexanediamine biz = 2,2 -bi-1,4,5,6-tetrahydropyrimidine. 

Figure 6-11. 1 -H-Benzotriazole, 2,5-bis(N-pyridyl)-1,3,4-oxadiazole, cyclohexylammonium benzoate, benzylsulfonylacetic acid, 2,4-diamino-6-mercapto pyrimidine, hydroxamic acid, 3-phenyl-2-propyn-1-ol, dicyclopentadiene dicarboxylic acid.

During the final coupling reaction between free amine 36 and oxadiazole 2 unse-lective acylation of the phenolic OH occurs to give the bis-acylated compound 43. 

Fig. 9 (a) Molecular structures of novel ESIPT dyes, 2,5,-bis[5-(4-t-butylphenyl)-[l,3,4]oxadia-zol-2-yl]-phenol (SOX), and 2,5-bis[5-(4-t-butylphenyl)-[l,3,4]oxadiazol-2-yl]-benzene-l,4,-diol (DOX). (b) Emission colors in the Commission Internationale de L Eclariage (CEE) chromaticity diagram. The inner oval and the filled circle at coordinate (x,y) of (0.33, 0.33) indicate the white region and the ideal color, respectively. Note that PS and PVK denote polystyrene and poly (N-vinylcarbazole) film (reprint from ref. [91], Copyright 2005 Wiley-VCH)... 

The first synthesis of Cu(n) bis-complexes 89 of substituted 1,2,4-oxadiazoles 88 has been reported, with complexa-tion occurring selectively on the 4-nitrogen <1999ICA1> (Equation 7). 

In a similar process involving the stable fully conjugated 1,2,4-oxadiazole N4 palladium(rv) complexes 186, liberation from palladium was achieved either with l,2-bis(diphenylphosphino)ethane (DPPE) or with an excess of sodium sulfide (Equation 29) <2005EJI845>. 

The reaction of malonates 213 with 2equiv of an amidoxime in the presence of potassium carbonate results in the formation of the bis-l,2,4-oxadiazoles 214 (Equation 35), a process that also gives excellent yields of the mono-1,2,4-oxadiazoles when a 1 1 ratio is employed (see Table 5) <2006TL3629>. 

The oxidation of aromatic aldoximes with ceric ammonium nitrate produces nitrile oxides which undergo subsequent cycloaddition to nitriles to produce 1,2,4-oxadiazoles (Equation 47) <1997PJC1093>. The anodic oxidation of aromatic aldoximes in the presence of acetonitrile has been reported to give low yields of either 3-aryl-5-methyl-1,2,4-oxadiazoles (2-25%) or 3,5-bis-aryl-l,2,4-oxadiazoles (6-28%), although the synthetic utility of this route is limited by competitive deoximation to the carbonyl being the major reaction pathway <1997MI3509>. 

The cyclic nitrone 365 reacts with the metal-coordinated nitrile 366 to give the complex 367 from which the bicyclic 2,3-dihydro-l,2,4-oxadiazole 368 was liberated by the use of l,2-bis(diphenylphosphanyl)ethane (dppe) (Scheme 61) <2003JCD2540>. 

Two molecules with comparable geometry in an asymmetric unit were found for 3,4-bis(4-fluorophenyl)-l,2,5-oxadiazole 2-oxide. The bond length of the dipolar N-O bond is 1.107 (7) A <2006AXEo4827>. In the molecule of 5-(6,7-dimethoxy-l,2,3,4-tetrahydroisoquinolin-2-yl)-4-phenyl-l,2,5-oxadiazole Ar-oxide, the six-membered heterocyclic ring has a flattened boat form. Intermolecular C-H- O hydrogen bonds link the molecules into dimers, which may be effective in the stabilization of the crystal structure <2006AXEo3130>. 

Halogenalkyl-substituted furazanes were also used for functionalization of the compounds. Thus, 3,4-bis(chloro-methyl)-l,2,5-oxadiazole 92 and 2(R)- fS>[ 3,5-bis(trifluoromethyl)pheny I ethoxy -3f.V,)-pheny I morpholine 93 gave 2W-[lW-[3,5-bis(trifluoromethyl)phenyl]ethoxy]-4-(4-dimethylaminomethyl-l,2,5-oxadiazol-3-yl)methyl-3( 3 Tphenyl-morpholine 94 (Equation 21) <1996W09629328>. 

The bis(l,2,5-oxadiazole A-oxidc) derivatives 101 with oxadiazole tings linked to piperazine residue were synthesized via modification of chloromethyl or formyl group of furoxans (Scheme 25) <2000AP387>. 

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