Synthesis and solvatochromic and acid-base reactions of a betaine and salts of 4-N-pyridiniumcatechol

Abstract

Oxidation of catechol by phenyliodosyldiacetate in the presence of pyridine gives 4-N-pyridiniumcatechol salts whose structures are confirmed by an independent synthesis from 2,2-dimethyl-5-aminobenzodioxole. The spectroscopically determined ionization constants for the 4-N-pyridiniumcatechol cation depend on the nature of the buffer cation solution (sodium, ammonium, tetraethylammonium). The large solvatochromic shift in the long wavelength absorption band of the betaine of 4-N-pyridinium-catechol followed the empirical scale of solvent polarity E_T ^N. Introduction of the N-pyridinium group in position 4 increases the acidity of the catechol by 2.7pK units, which is an almost identical effect to the introduction of a 4-nitro group. However, the solvatochromism for the anion of 4-nitrocatechol is insignificant. The compounds were characterized by their ¹H NMR and IR spectra.