Pyrazolines, oxidation reduction

These dyes are used for electrochromic display. Many compounds show color changes by oxidation-reduction cycle. The biologen type (7) yields radical cations by reduction and displays brilliant color. In comparison to tungsten oxide, which shows a blue color by reduction, the response is faster. By changing the substituents, many colors can be obtained. Pyrazoline-type compounds (8), which possess a nitrogen heterocycle, can also display black. 

Hi) Electrochemical reactions and reactions with free electrons Electrochemical oxidation of 3-methyl-l-phenylpyrazole gave the 3-carboxylic acid whereas electrochemical reduction (Section 4.04.2.1.6(i)) of l,5-diphenyl-3-styrylpyrazole produced the A -pyrazoline (B-76MI40402) with concomitant reduction of the exocyclic double bond (343). 

Another 2,3-bis(phenylhydrazone), namely, l,5 4,6-dianhydro-2,3-hexodiulose bis(phenylhydrazone) (62a), was also found to cyclize to a pyrazoline (63a) by the action of copper(II) sulfate or benzalde-hyde, which usually convert osazones into aldosuloses or osotria-zoles.65 Reduction of this bicyclic compound (63a) led to the pyrazo-lidine 64, and periodate oxidation of 63a gave the expected aldehyde 65. 

The reduction of 4-nitroso-3-pyrazolin-5-ones has already been discussed in connection with the synthesis of 4-dialkylamino-3-pyra-zolin-5-ones. This reduction occurs readily to give the corresponding amine when catalytic hydrogenation or various chemical combina-tions461,092,1174 1175 are used. Oxidation of nitroso to nitro occurs in the presence of excess of nitrous acid or by use of nitric acid.984-992... 

Pyrazole resistant to oxidation and reduction reaction due to loss of aromaticity, but may be hydrogenated catalytically, first to pyrazoline, and then to pyrazolidine. Both of these compounds are stronger bases than p3rrazole. 

Results of a theoretical study of the mechanism of Br2 oxidation of IH-pyrazolines to pyrazoles suggest that a bromo substituted 2- or 1-pyrazoline is involved in the reaction. Br2 and SbCls oxidize bis(4-t-butylphenyl)nitroxyl (A) to the corresponding oxoammonium salts. The tribromide salt on heating is converted into a mixture of bromodiphenylamines which is also formed from A as a result of three-electron reductive bromination. ... 

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