Indazole, acidity tautomerism

Aromatic pyrazoles and indazoles, in the broad sense defined in Sections 4.04.1.1.1 and 4.04.1.1.2, will be discussed here. Tautomerism has already been discussed (Section 4.04.1.5) and acid-base equilibria will be considered in Section 4.04.2.1.3. These two topics are closely related (Scheme 10) as a common anion (156a) or a common cation (156b) is generally involved in the mechanism of proton transfer (e.g. 78T2259). For aromatic pyrazoles with exocyclic conjugation there is also a common anion (157) for the three tautomeric forms... 

Photophysical experiments and theoretical calculations have been used to determine the tautomeric equilibrium between IH- and 2/f-indazoles (15a) and (15b) both in the ground (So) and in the excited state (S,) <94JPC10606>. The measurement of sublimation and solution enthalpies in water at 25°C, basicities in the excited state of 1-methylindazole (pAa = 1.61) and 2-methylindazole (pA a = 3.00), and MP2//6-31G //6-31G calculations of the difference in energy between their conjugated acids in the ground state allows the complete thermodynamic cycle to be established. Indazole in the excited state is 3.0 pA a units more acid (pA a = 1108) and 1.8 pK units more basic... 

Indazole, = 1-25, is less basic than pyrazole but a stronger N-H acid, pKa = 13.86 [121]. The tautomerism of indazole is a special case inasmuch as the 2//-indazole possesses an o-... 

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