Tautomeric structures and proton transfer in 2-(2-pyridyl)indan-1,3-dion and -diimine derivatives

Abstract

The structures of products obtained by the condensation reaction of 2-pyridyl-1,3-indandione with anilines were revealed to be imine-enamine tautomeric forms rather than imine-imine forms. The hydrogen of the enamine NH is hydrogen-bonded intramolecularly with the nitrogen in the pyridine ring. Based on variable temperature NMR spectra, the NH proton was found to be in dynamic 1,5-migration, which is synchronized with internal rotation around the pivot-bond connecting the pyridine ring and the 1,3-indan moiety. That is, the hydrogen-bonding site is in rapid exchange accompanied by 1,5-proton transfer and internal rotation. On the other hand, for the derivative bearing no pyridine ring at the 2-position, 1,5-proton transfer is very fast in NMR time-scale. Based on an X-ray analysis, the structure of 2-pyridyl-1,3-indandione was determined to be a betaine form including a pyridinium ion. The structures of the other related compounds are also reported.