Studies on Basicities of Aminotropone and Aminotropolone Derivatives

Abstract

It has been clarified that tropone⁴⁾ (Ia) is stabilized by the resonance effect of the structure Ib, from some experimental data, for example, dipole moment, ⁵⁾ electron diffraction, ⁶⁾ and IR spectrum.⁷⁾ In aminotropone and aminotropolone derivatives, contribution of the structure Ib should result in the decrease of basicity of the amino group. The pK_a values of aminotropones and aminotropolones^11-18) (III to XIII) were measured by the spectroscopic method⁸⁾ in order to investigate the contribution of the structure of Ib type for stabilization of the compounds III to XIII. It was found that the basicity of aminotropones and aminotropolones (Table II) tends to rise as the distance between the amino group and the carbonyl group increases, and is comparable to those of cyanoaniline, ¹⁹⁾ chloroaniline, ²⁰⁾ and bromoaniline.²¹⁾ Tautomerism of asymmetric aminotropolones was also discussed. By comparison of the UV spectra of aminotropolones with those of aminotropones, it was concluded that 3-aminotropolone (XI) and 4-aminotropolone (XII) exist predominantly as the tautomeric structure of 2-aminotropone type (XIa) and 3-aminotropone type (XIIa), respectively, in water.