Nitration by Other Nitrating Agents

In addition to the sulfuric-nitric acid mixture, acetylnitrate (or a mixture of acetic anhydride with nitric acid) is used quite widely for the introduction of a nitro group into the azole ring. Azoles not containing a pyrrole nitrogen atom are nitrated by acetylnitrate in the same way as by the sulfuric-nitric acid mixture. Data have been published on the nitration of isoxazoles [176,181-183,185,292,293] and thiazoles [203, 206, 215] by acetylnitrate. 1-Substituted pyrazoles [48, 61, 294-301], imidazoles [ 116,133, 302-305], and isoxazoles [306] are nitrated by the nitric acid-acetic anhydride system in the same way as by the sulfuric-nitric acid mixture. 

The A-nitro derivatives of imidazole can only be isolated in the case where the imidazole ring contains strong electron-withdrawing substituents (e.g., N02) [79, 322-329], In other cases the C-nitro derivatives are formed directly [101,133, 330-335], It was suggested that the A-nitro derivative is formed during the nitration of the intermediate 1,2,3-triazole [336], However, no evidence was presented for this mechanism. 

The kinetics of the nitration of azoles by acetylnitrate has not been studied systematically. Mention has only been made of the complex nature of the reaction kinetics and the low yield of the nitration products, which hinders correct interpretation of the results [56], 

4-Nitro-5-nitrimino-1H-1,2,4-tria/olc (46%) forms on the nitration of 5-nitrimino-1,47/-1,2,4-triazole by HN03/Ac20 only at -8 to -10°C [340], The following nitration of the product gives l,l -dinitro-3,3 -azo-l,2,4-triazole. 

However, it was not possible to obtain l-trityl-2-nitroimidazole by this method as, for example, also during the nitration of the lithium derivative of 1-tritylimida-zole by nitronium tetrafluoroborate [356], This is probably due to the elimination of the triphenylmethyl group during nitration. However, it was possible to obtain l-trityl-2-nitroimidazole by using propyl nitrate as nitrating agent [356], The product 

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