Pyrazoles, nitro derivatives

In neutral media the band of pyrazole nitro derivatives is in the 250-290 nm region in acid media it is shifted by 30 nm to the short-wave region and in alkaline media a 40 nm shift to the long-wave region occurs [246,1115,1116,1119-1126], These shifts are caused by protonation and deprotonation of nitropyrazoles that is widely used in studying the acid-base properties of nitroazoles since the existence of these shifts allows spectrophotometrical determination of the pand pA s + values of these compounds. 

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. 

Acylindazoles are good acylating agents, particularly the 2-acetyl-7-nitro derivative (131). The acetylation of pyrazole by this compound proceeds quickly in chloroform and more slowly (about 15 min) in DMSO (66BSF3041). 

The creation of the N—N bond as the last step of the ring synthesis is common in indazoles and very rare in pyrazoles. In indazoles this method is well known (type B synthesis (67HC(22)l), for example, the dehydration of oximes (570) with acetic anhydride yields 1-acetylindazoles (571), and in basic medium the indazole 1-oxides (573) are formed from the nitro derivatives (572). 

The syntheses shown in Scheme 26 (71CB1573) are based on dibenzo-/3-tropolone (108) and its enamines. Condensation of 108 and benzoin in the presence of ammonia affords pyrrole 109. The morpholino enamine gives pyrazole 110 via benzoylation, hydrolytic elimination of morpholine, and cyclization, whereas diazo-group transfer onto the anilino enamine leads to triazole 111. Dione 108 and p-nitrophenylazide in one step give predominantly the p-nitro derivative of 111 (92G249). 

Casoni (59G1539) has reported that 3-methyl-1-phenylpyrazole yields mainly the 4-nitro derivative with some l-(4-nitrophenyl) product, using nitric acid in acetic anhydride, whereas nitration in sulfuric acid gives 4-nitro-l-(4-nitrophenyl)pyrazole. 

Nitration of l-methyl-4-(2,4-dinitrophenyl)pyrazole (7.47) occurs 25% at the 3-position and 15% at the 5-position of the pyrazole ring, whereas the 2,4,6-trinitro homologue (7.48) nitrates only in the 3-position (70JHC707 71JHC293), probably because of the increased electron withdrawal in 7.48 across the high-order 4,5-bond (cf. the explanation given in the paper that the 5-nitro derivative was formed, but that its rate of decomposition was greater than its rate of formation )... 

Derivatives of pyrazole containing a nitro group at position 3 or 5 were obtained in this way [393 100], Nevertheless, the diazotiation of 3,5-dimethyl-4-aminopyra-zole does not lead to the formation of nitro derivatives [398],... 

The construction of a heterocyclic ring from two reagents, one of which contains a nitro group, is widely used in the synthesis of the nitro derivatives of pyrazole, isoxazole, and 1,2,3-triazole. Thus, for example, the reaction of sodionitromalonal-dehyde with substituted hydrazines leads to the corresponding derivatives of 4-nitropyrazole [33, 61, 471 173] (Scheme 63). 

Varvounis and co-workers (01JHC1065) (Scheme 60) used 50% aqueous nitric acid for the nitration of pyrazol-3-ones 181a-c. The 4-nitro derivatives 182a-c were obtained in 80, 78 and 70% yield, respectively. 

The VNS is the reaction of choice for incorporation of a-sulfonylalkyl substituents into nitroarenes and their heteroanalogues. Particularly accessible and useful are nitroarylmethyl phenyl sulfones and their heteroanalogues that are efficiently produced in the VNS reactions of carbanions of chloromethyl aryl sulfones with a great variety of nitroarenes and nitroheteroarenes. Nitro derivatives of heterocycles, such as pyrrole [54,55], furan [54], thiophene [54], imidazole [106, 112, 113], pyrazole [114], pyridine [57], indole [115], indazole [116, 117], benzimidazole [118], benzotriazole [119], benzofuroxan [120], quinoline [121], and porphyrins [122, 123], have been shown to enter this reaction. 

Nitro groups are easily reduced, catalytically or chemically, to give amino compounds, e.g. 4-nitroisothiazoles give the corresponding 4-amino derivatives (72AHC(i4)i). In the pyrazole series, intermediate nitroso compounds can be isolated. Nitrosoimidazoles are also relatively stable. 

A similar treatment of the Schiff s base derived from 3-nitro-2-thienylcarbaldehyde (564) with triethyl phosphite gave the thieno[3,2-h]pyrazole (565) (78CC453). ... 

From UV studies of 4-phenyl-, 4-nitro- and 4-nitroso-pyrazoles, Habraken et al. (67RTC1249,72JHC939) conclude that the 4-pyrazolyl group acts as an electron-donating group. UV spectra of pairs of 1-aryl- and 2-aryl-indazoles and their utility in the determination of isomeric structures are discussed in (67BSF2619) many other UV data on indazole derivatives can be found in (71PMH(3)67). 

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