Indazolium salts

The pyrazolinones (11) (pyrazolinethiones and iminopyrazolines) correspond to the replacement of R or R in (22) by 0 , and are normally represented by their neutral structure, but it is important to bear in mind their dipolar formula (26) when discussing their reactivity. Similarly, the indazolinones (12) (27) result from the indazolium salts (23). 

The Af-methyl chemical shifts of quaternary pyrazolium and indazolium salts have been discussed in connection with the resonance effect of the heterocycle (74JHC1011). 

Simple HMO calculations (68JCS(B)725) satisfactorily account for the UV spectra of a great number of pyrazoles substituted by methyl and phenyl groups. The spectra of pyrazolium and indazolium salts (free bases in IN HCl) have been compared with calculated transitions (Pariser-Parr-Pople method) (74MI40403). 

Figure 22 represents the main consequences on pyrazole reactivity when a positive charge is present at the 2-position of the nucleus. A similar situation occurs in the indazolium salts, which thermally decompose into an alkylindazole and an alkyl halide, a reaction sequence described by von Auwers. 

The acidic character of the hydrogen atoms of C-methyl groups linked to the pyrazolium ring (Figure 22 Section 4.04.2.1.1(11)) facilitates a number of reactions difficult to carry out with neutral pyrazoles. Since efficient methods of dealkylation have been described (Section 4.04.2.3.lO(ii)), the synthesis via the pyrazolium salt is a useful alternative. The same behaviour is observed for indazolium salts, for example, nucleophilic addition to aromatic aldehydes (78JOC1233). 

N. F. Haley, 3-Methyl-2,l-benzisoxazolium, benzisothiazolium, and indazolium salts as new active-methyl compounds, J. Org. Chem., 43, 1233-1237 (1978). 

The nucleus of indazole, like that of pyrazole, is stable to reduction, so that treatment of the amido derivative (23) with LAH leads only to the amine (24).Indazolium salts (25) are more reactive and undergo reduction with potassium borohydride to give indazolines (26). ... 

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