Pyrazoles, N-oxidation

Section 2.1 is devoted to the aromatic pyrazole 1-oxides. Further structures derived from pyrazole and embedding an N-O bond are the nonaromatic 1,2-dioxides also called diazapentalene 4,4 -dioxides 75-78, the pyrazoline 1-oxides also called diazapentalene 4-oxides 79-84, and the 2-substituted pyrazoline 1-oxides represented in Scheme 22 by the parent structure 85. Structures 77, 78, 79, 81, and 82 are known and are discussed in Section 2.2 but since the present review deals with aromatic /V-oxides the nonaromatic N-oxides will only be discussed when their chemistry relates them to the aromatic pyrazole N-oxides. 

Some examples of the nitration of pyrazole N-oxides are known [75, 76]. The result of nitration is determined by the ratio of the components in the nitrating mixture (Scheme 10). 

The N- hydroxypyrazoles (523 R = H) and the pyrazole N- oxides (268 Section 4.04.2.1.3 (xiii)) have been reduced to pyrazoles by means of zinc in acetic acid and catalytic hydrogenation, respectively (75MI40402). 

C7H11N02 cis-2-amino-4-cyclohexene-1-carboxylic acid 54162-90-2 497.65 43.658 2 11391 C7H12N202 1-methoxy-3,4,5-trimethyl pyrazole-N-oxide 39753-42-9 459.15 39.973 2... 

Achtung Pyrazol-N-oxide konnen sich explosionsartig zersetzen. 

Achtung Pyrazol-N-oxide sind starke Oxidationsmittel und konnen sich explosionsartig zerset-zen1366. 

Pyrazole N-oxides (429) are formed by the action of nitrile oxides RCNO (R = Ph, Ac, or C02Et) on the imidoyl-substituted oxosulphonium ylide... 

Oxidation of N -substituted pyrazoles to 2-substituted pyrazole-l-oxides using various peracids (30) facilitates the introduction of halogen at C, followed by selective nitration at C. The halogen atom at or is easily removed by sodium sulfite and acts as a protecting group. Formaldehyde was... 

In 2000, it was proposed that the regioselectivity of the [3 + 2] cycloaddition of fullerenes could be modified under microwave irradiation. Under conventional heating, N-methylazomethine yhde and fullerene-(C7o) gave three different isomeric cycloadducts because of the low symmetry of C70 vs. Ceo. Using microwave irradiation and o-dichlorobenzene as a solvent, only two isomers were obtained, the major cycloadduct 114 being kinetically favored (Scheme 39) [75]. The same authors had previously reported the 1,3-dipolar cyclo addition of pyrazole nitrile oxides, generated in situ, to Geo under either conventional heating or microwave irradiation. The electrochemical characteristics of the cycloadduct obtained with this method made this product a candidate for photophysical apphcations [76]. 

Electroactive 3-(N-phenylpyrazolyl)fullereno[l,2-r/]isoxazolines have been synthesized by using 1,3-dipolar cycloaddition of pyrazole nitrile oxides, generated in situ, to Cgo at elevated temperature or microwave irradiation. The cyclic voltammetry measurements show a strong donor pyrazole ring, and a better acceptor ability of the fullerene moiety than the parent C60 (538). Treating fullerene Cgo with mesitonitrile oxide in toluene gives fullerene-nitrile oxide adduct, which is supposed to be useful for electrical and optical components (539). 

Other heterocyclic N-oxides have been found to display biological activities that may depend on their proven or hypothesised ability to release NO these include 4/ /-pyrazol-4-oTie 1,2-dioxides 71, 2H-1,2,3-triazole 1-oxides 72, benzotetrazine 1,3-dioxides 73 and 1,2,3-benzotriazine 3-oxides 74. These systems have been less extensively studied than the furoxan and 1,2-diazetine dioxide systems discussed above. 

Dioximes are known to generate isoxazoles and related compounds when oxidized with IBTA. However, these reactions are of limited use because of formation of side products. For example, oxidation of dioximes of /B-diketones 159 gives rise to a mixture of 3,5-disubstituted oxazoles 160 and pyrazole-di-N-oxides 161 (82MI1). In another case, oxidation of dioximes 162 affords a mixture of the isomeric dihydroisoxazolo-isoxazoles 163 and pyridazine dioxides 164 (76S837 79JOC3524 82MI1). 

Af-Hydroxypyrazoles 38 and their N -oxides 39 are converted by f-BuOCI to the 4-chloro-47/-pyrazoles 40 and 41, respectively25,26,56 (Scheme 14). Unlike Scheme 6, the reaction proceeds even when R1 and R are alkyl. When... 

Cyanations of benzimidazolium or benzimidazole N-oxides have been reported (93CHE127). Cyanopyrazoles are formed by irradiation of pyrazoles in the presence of cyanide ions in photosubstitution reactions. 

The basicity of the aromatic amine iV-oxide can be substantially varied by the introduction of substituents on the aromatic ring. IR spectra of free N-oxides display a prominent band between 1200 and 1300 cm-1, attributable to the nitrogen-oxygen stretching frequency v(NO). The more activating the substituent, the lower the energy of the absorption. Upon coordination, v(NO) is decreased by up to 60 cm-1. IR data for pyrazole and pyridine derivatives are given in Table 42.281-286... 

This review constitutes the first comprehensive description of the preparation, properties, and reactions of 5-membered heteroaromatic N-oxides derived from pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, and tetrazole. 

N-Oxide Azaheterocycle Azole Pyrazole Imidazole Triazole Tetrazole N-Oxidation Cyclization Functionalization Activation Regioselectivity One pot... 

Substituted pyrazole 1-oxides 74 can be prepared by N-oxidation of 1-substituted pyrazoles 89, by A/-alkylation of 1 -hydroxypyrazoles 90, or by cyclization of 1,3-oximimines, conjugated oximenamines, or conjugated 1,3-nitrosoimines. 

N-Alkylation of 1-hydroxypyrazoles 90 produces 2-substituted pyrazole 1-oxides 91 (Scheme 26). Competing O-alkylation of the... 

---