1.3.4- Oxadiazoles, perfluoroalkyl

The rearrangement mechanisms of 5-perfluoroalkyl-l,2,4-oxadiazoles such as the five-to-six membered ring-rearrangements by hydrazinolysis and the photoinduced competitive rearrangements have been investigated <06JOC8106 06JOC2740>. 

The photochemistry of 3-amino- <2002JOC6253> and some 3-iV-alkylamino-5-perfluoroalkyl-l,2,4-oxadiazoles <2004JOC4108> was investigated. The latter reaction led to 5-perfluoroalkyl-l,3,4-oxadiazoles followed eventually by their conversion into 5-perfluoroalkyl-l,2,4-triazoles. 1,3,4-Oxadiazoles were also obtained by photochemical interconversion of 3-acylamino-l,2,5-oxadiazole derivatives. Mechanisms of these reactions were proposed <2004JOC4108>. Examples of these interconversions are shown in Scheme 37. 

Perfluoroalkyl substituted oxadiazoles are known to react readily with ammonia at room temperature to produce triazoles, (a) Brown, H.C., and Cheng, M.T. (1962) J. Org. Chem., TJ, 3240-3243. 

Bis(perfluoroalkyl)-l,3,4-oxadiazoles (285) are attacked readily by hydrazine with ring opening and formation of the perfluoroacylated hydrazidines (286). Concurrent dehydration and oxidation of these intermediates with iron(III) chloride produces the corresponding 3,6-bis(perfluoroalkyl)-l,2,4,5-tetrazines (287). Ring closure of (286) without oxidation produces the corresponding 3,5-bis(perfluoroalkyl)-4-amino-l,2,4-triazoles (66JOC781). 

An interesting synthesis of targeted 5-perfluoroalkyl-l,2,4-triazoles takes advantage of the aforementioned MNAC [70] photorearrangement of 3-methylamino-l,2,4-oxadiazoles 109 (R = Me) which, when irradiated at 313 nm in methanol and in the presence of various nitrogen nucleophiles (ZH in Scheme 12.32), produces triazoles 119 together with the 1,3,4-oxadiazoles 112 expected from the base-catalyzed RCRE route (Scheme 12.32) [71]. 

Buscemi, S., Pace, A. and Vivona, N. (2000) Fluoro heterocycles. A photochemical methodology for the synthesis of 3-amino- and 3-(N-alkylamino)-5-perfluoroalkyl-l,2,4-oxadiazoles. Tetrahedron Letters, 41 (41), 7977-7981. 

The mass spectra of 1,3,4-oxadiazole, 2,5-diphenyl- and 2-perfluoroalkyl-l,3,4-oxadiazoles, including suggested fragmentation pathways, have been discussed (B-71MS528). The molecular ion is the base peak in the spectra of 1,3,4-oxadiazole and 2-amino-5-phenyl-... 

Oxadiazole is thermally stable and this stability is increased on substitution, particularly by aryl and perfluoroalkyl groups. Oxadiazolin-5-ones and -5-thiones are somewhat less stable and undergo selective pyrolysis at high temperatures. 

Bis(perfluoroalkyl)-l,3,4-oxadiazoles have a high thermal stability and are suitable for use as heating-bath liquids and solvents for fluorinated polymers. 

The 2,5-perfluoroalkyl-l,3,4-oxadiazoles show an extraordinarily high thermal stability. Among other possible applications, they can be used as bath liquids and solvents for highly fluorinated polymers.38 In the last few years numerous 1,3,4-oxadiazole derivatives... 

Perfluoroalkyl-1,2,4-oxadiazoles 240 underwent an uncommon reaction with hydrazine to afford the corresponding triazoles 241 <03JOC605>. 

N-Amino-i-triazoles also can be obtained by a ring transformation of some heterocycles. For instance, hydrazine converts perfluoroalkyl 1,3,4-oxadiazoles, under mild conditions via the acyclic intermediate 168, into... 

While 2,5-bis(perfluoroalkyl)-l,3,4-oxadiazoles 3 react with hydrazine to give the N2-(x-hy-drazonoperfluoroalkyl)perfluoroalkylhydrazides 4 (see Section 5.1.1.2.1.), 2,5-bis(trifluoro-methyl)-l,3,4-oxadiazole (3, RF = CF3) reacts with hydrazine to afford the dihydro-1,2,4,5-tetrazine 5 which is oxidized by iron(lll) chloride to give 3,6-bis(trifluoromethyl)-1,2,4,5-tetrazine (6, RF = CF3).164... 

An example of a photochemical conversion of one fluorinated heterocyclic system (fluoropyrazole) to another (fluoroimidazole) was described above. In an another example of photochemical isomerization to access fluorinated heterocyclic systems, irradiation of perfluoroalkylated 3-A-methylamino-oxadiazoles 143 in the presence of an excess of methylamine produces perfluoroalkylated 1,2,4-triazoles 144 along with the isomeric 1,3,4-oxadiazole 145. The formation of an intermediate exo-cyclic diazerene 146 or the open-chain carbodiimide 147 was proposed (Fig. 3.87). If the irradiation is carried out in the presence of methanol and triethylamine. 

Buscemi, S. Pace, A. Pibiri, I. Vivona, N. Spinelli, D. Fluorinated heterocyclic compounds. An expedient route to 4-perfluoroalkyl-l,2,4-triazoles via an unusual hydrazinolysis of 5-perfluoroalkyl-l,2,4-oxadiazoles first examples of an ANRORC-like reaction in 1,2,4-oxadiazole derivatives. J. Org. Chem. 2003, 68, 605-608. 

The natural development of the study of this class of compounds has been the synthesis of protic ILs. In this context, a series of 18 samples were prepared by appropriately mixing acid and basic building blocks (Figure 17). By using di- or monocarboxyhc perfluoroalkylic acids, in combination with 1,2,4-oxadiazoles functionalized with either two pyridines or one pyridine and a fluorinated chain, it is possible to obtain symmetric and nonsymmetric fluorous architectures, respectively (2012PCP14306). Many... 

---