1.2.4- Oxadiazoles, photoinduced

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>. 

A series of 3-amino (or 3-N-substituted amino)-l,2,4-oxadiazoles 108 can be obtained in moderate to high yields by the photoinduced rearrangement of 3-acylamino-l,2,5-oxadiazoles (furazans) 106 irradiated in the presence of an excess of ammonia or aliphatic amines [64—66]. The reaction follows a fragmentation-cycloaddition route, with the initial formation of a nitrile and a nitrile-oxide the latter is attacked by a nitrogen nucleophilic reagent (ZH in Scheme 12.29), and the open-chain interme-... 

Buscemi, S., Vivona, N. and Caronna, T. (1996) Photoinduced molecular rearrangements. The photochemistry of some 1,2,4-oxadiazoles in the presence of nitrogen nucleophiles. Formation of 1,2,4-triazoles, indazoles, and benzimidazoles. Journal of Organic Chemistry, 61 (24), 8397-8401. 

Dopants also influence the emission processes from PPVs. Improved red dopants have been based on pyran dyes" while Qo doping appears to be variable" "". Doping with electron transport materials such as oxadiazoles give polymers with balanced properties for hole transport". The avoidance of low molecular weight material in the synthesis of cyano based PPVs is important" as are head to head and tail to tail chain sequences in thiophene based polymers. Head to tail tetramer sequences were the most fluorescent. Metal ion doped PPV s are claimed to be good chemosensors and broad emission is observed from titania doped PPV . Electron rich dopants enhance the emission in the red region while electro and photoinduced infrared bands from PPV are similar . ... 

Amino-5-phenyl-l,2,4-oxadiazole and 1 gave enamino-ketone 188 that on irradiation with a low-pressure mercury lamp gave benzimidazole 189 as a result of a photoinduced rearrangement of the 1,2,4-oxadiazole ring (88JHC1551) (Scheme 32). 

Other imidazole syntheses include the photoinduced rearrangement of 1,2,4-oxadiazole-substituted enaminones (88JHC1551) and the condensation of 1,2-phenylenediamine-derived enaminones with triethyl orthoformate (89H281). 

Most modem syntheses of 1,2,4-oxadiazoles are classifiable as belonging to one of the two classical strategies described above (Schemes 1 and 2). Protected precursors are sometimes used, particularly when the lateral moiety in the final oxadiazole ring is sensitive to the reaction conditions (2012JOC4832). In some cases, alternative methodologies have been developed to generate particularly reactive precursors in situ for example, a photoinduced retro-cycloaddition reaction of acylamino furazan 8... 

For a review, see Vivona, N. and Buscemi, S., Photoinduced molecular rearrangements of O-N bond containing five-membered heterocycles. An assay for 1,2,4- and 1,2,5-oxadiazoles, Heterocycles, 41, 2095, 1995. 

Buscemi, S. and Vivona, N., Heterocyclic photorearrangements. Photoinduced rearrangements of 1,2,4-oxadiazoles substituted by an XYZ side chain sequence,/. Heterocyclic Chem., 25,1551,1988. 

Buscemi, S., Macaluso, G., and Vivona, N., Heterocyclic photorearrangements — photochemical behavior of some 3-acetylamino-5-aryl-l,2,4-oxadiazoles. A photoinduced iso-heterocydic rearrangement, Heterocycles, 29,1301, 1989. 

Buscemi, S., Pace, A., Vivona, N., Caronna, T., and GaHa, A., Photoinduced single electron transfer on 5-aryl-1,2,4-oxadiazoles some mechanistic investigations in the synthesis of quanazolin-4-ones, /. Org. Chem., 64, 7028, 1999. 

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