4- -nitro-1 //-pyrazole

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. 

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). 

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 basicities of indazole (1.31), 1-methyl (0.42) and 2-methyl (2.02) derivatives and of eight other substituted indazoles have been measured (67BSF261 The effect of substituents in the 3-position is similar in pyrazoles and indazoles with Api values as follows Me, 0.80 and 0.86 Cl, -3.01 and -2.98 Br, -2.85 and -2.82, respectively. A nitro group in the homocycle has an expected base-weakening effect of -2 pK units, whether it is at the 5- or the 6-position. 

A Hammett relationship of the form ApK = 5.8am has been proposed for 4-substituted pyrazoles (74TL1609) in order to explain the effect of 4-nitro ApK = 4.5, am = 0.71) and 4-diazo groups (Apiifa = 10.0, am = 1.76). The acidity constants of a series of pyrazolidine-3,5-diones have been determined (75AJC1583) and the 4- -butyl-1,2-diphenyl derivative phenylbutazone has a pK of 4.33. 

Nitration of 4-(2-thienyl)- (301) and 4-(3-thienyl)-pyrazoles (302) mainly occurs on the thiophene ring, but when acetyl nitrate is used as the nitration agent small quantities of products nitrated on the pyrazole ring are isolated (position of the nitro group uncertain) (80CS( 15)102). Pyrazol-l -ylpyridines (303) undergo electrophilic reactions (bromination, chlorination and nitration) preferentially in the pyrazole ring. Thus, the nitration of (303 R = R = = H) either with a mixture of nitric acid and sulfuric acid at 10-15 °C or with... 

In pyrazoles the two most important effects are produced by nitro groups and by quaternization (Section 4.02.3.1.7). Both enhance considerably the reactivity of a second substituent, for example, that of a methyl group towards aldehydes or of a chloro substituent towards amines. 

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). 

Pyrazole, 3-nitro- H NMR, 5, 184 (79MI40407) pACa, 5, 225 (B-76MI40402, 73JHC1055) Pyrazole, 3(5)-nitro-... 

Pyrazole, 1 -(2-methylaminophenyl)-synthesis, 5, 283 Pyrazole, l-methyl-5-nitro-2-oxide... 

However, in the presence of the electron-withdrawing nitro group, hydration occurs already under the conditions of its reduction. In this case, the direction of addition of the water molecule is preserved. The yield of l-methyl-4-phenylacetyl-pyrazole was 90% [99JCS(P1)3713] (Scheme 87). 

It is known that the ability of nitrotolane to cyclize depends on electronic factors (69MI2) hence l,3-dimethyl-4-nitro-5-phenylethynylpyrazole, whose acetylene group is in the most electron-accepting position of the pyrazole ring, i.e., favorable for nucleophilic addition, was introduced into the reaction of cyclization. Thus,... 

In Nitro-pyrazol kann die Nitro-Gruppe durch Zinn(II)-chlorid in verdunnter Kalilauge zur Nitroso-Gruppe ([Nitroso-pyrazol) reduziert werden (s. Bd. X/l, S. 1064). 

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