1-Methylpyrazole 2-oxide, nitration

Pyrazoles can undergo nitration at several positions 4-bromo-l-methylpyrazole yields the 3,5-dinitro product. 1-Methylpyrazole 2-oxide yields the 5-nitro derivative. 

We have already noted (Section 4.04.2.1.4(xi)) that alkyl groups on pyrazoles are oxidized with permanganate to carboxylic acids. Silver nitrate and ammonium persulfate transform 4-ethyl-1-methylpyrazole (436) into the ketone (437) (72JHC1373). The best yield was obtained starting with the alcohol (438) and using an acid dichromate solution as oxidizing agent. 

Several pieces of evidence can be used to determine whether the nitration of a given substrate occurs through the conjugate acid (SH+) or through the small amount of neutral substrate in equilibrium with it these approaches include the determination of the rate profile and comparisons with the reactivities of model compounds (Ridd, 1971b). From such evidence, it appears that a number of nitration reactions involve the small amount of neutral substrate in equilibrium with the conjugate acid the substrates include 2,6-dichloropyridine, pyridine-1-oxide, l-methylpyrazole-2-oxide, 3-methyl-2-pyridone, acetophenone and p-nitroaniline. These reactions have been reviewed recently (Hoggett et al., 1971 see especially Chap. 8). However, for the majority of these substrates, the... 

The nitration of l,4,5-trimethylimidazole-3-oxide and l-methylpyrazole-2-oxide may also occur at about the encounter rate (Ferguson et al., 1977). 

In contrast to the results for pyrazole, I-methylpyrazole jV-oxide (7.49) nitrates at the 5-position (72TL2771), which may be attributed to conjuga-tive ( + M) electron release from oxygen to the 5-position. Some 3-nitration could therefore also be expected, but none was reported. 

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