Issue in Honor of Prof. Rosa Ma Claramunt ARKIVOC 2014 (ii) 57-70
On the solvatochromism, dimerization and tautomerism of indazole
Javier Catalán
Departamento de Química Física Aplicada, Universidad Autónoma de Madrid, Cantoblanco, E-28024 Madrid, Spain E-mail:
[email protected]
Dedicated to Prof. Rosa Maria Claramunt on the occasion of her 65th anniversary
Abstract
A careful study of the solvatochromism of indazole was for the first time conducted in this work. The study revealed that indazole solvatochromism is governed mainly by the polarizability of the medium and, to a lesser extent, by its acidity and basicity. Based on the results of the solvatochromic analysis and their processing with Abe’s model, this polar compound undergoes no significant change in dipole moment from its ground electronic state to its first excited state, which contradicts the predictions of other authors. Indazole dimerizes in both 2-methylbutane and 1-chlorobutane. However, neither its monomer nor its dimer undergoes tautomerization to 2H-indazole in the ground state in response to an increase in dipolarity of the medium. Also, based on experimental evidence, no tautomerization occurs in the first excited singlet or triplet of the compound.
Keywords: Indazole, solvatochromism, dimerization, tautomerization.
Introduction
Indazole is a polar asymmetric bicyclic diazole heterocycle with a dipole moment in the gas phase of 1.76 D.1 It contains a basic pyridine nitrogen with an intrinsic basicity in the gas phase of 214.2 kcal/mol and an acidic pyrrole nitrogen with an intrinsic basicity in protonated form of
348.4 kcal/mol.2 The fact that these two nitrogen sites are located in adjacent positions in the molecular structure (see Scheme 1) allows indazole at low concentrations in mildly acid or basic media to form 1H and 2H tautomers via acid–base interactions (see Scheme 1),3-10 as well as symmetrical dimers by hydrogen bonding.11,12 Page 57 ©ARKAT-USA, Inc.
