M. Sigalov, A. Afonin, I. Sterkhova
Jul 5, 2019
Citations
0
Influential Citations
8
Citations
Journal
The Journal of organic chemistry
Abstract
2-[(2H-Indazol-3-yl)methylene]-1H-indene-1,3(2H)-dione 6 and (E)-2-[(2H-indazol-3-yl)methylene]-2,3-dihydro-1H-inden-1-one 7 have been synthesized. In the crystal, the NH hydrogen atom of 6 is disordered between the N(1) and N(2) atoms with the population ratio of 0.69:0.31. Molecule 7 crystallizes in two tautomeric polymorphs: 7-1H tautomer (yellow) and 7-2H tautomer (red). Both 6 and 7 form centrosymmetric dimers in the crystal with the monomeric units linked by С=О···H···N bifurcated hydrogen bonds in 6 and N-H···N hydrogen bonds in 7. According to 1H and 13C NMR data, in DMSO-d6 solution the 6-1H tautomer predominates, whereas in less polar CDCl3 or CD2Cl2 the 6-2H tautomer is stabilized by strong N-H···O=C intramolecular hydrogen bond. Compound 7 in DMSO or ethanol solutions exists in the form of 7-1H and 7-2H tautomers. On the example of 7-2H tautomer it was shown for the first time that the 2H tautomers of 3-substituted indazoles can be stabilized by an intermolecular hydrogen bond and may remain in aprotic solvents almost indefinitely. However, in the open air or in water, fast 2H→1H tautomerization occurs. As follows from DFT calculations, the high stability of the 2H form in solution is due to the formation of centrosymmetric dimers, which are more stable than the corresponding dimers of the 1H tautomer.