DFT studies on tautomeric preferences: Proton transfer in 1,5-bis(pyridin-2-yl)- and 1,5-bis(quinolin-2-yl)pentane-2,4-diones

Multiple basic and acidic centers in the molecules of the title compounds make them susceptible to the proton transfers of many different types. DFT calculations show that 1,5-bis(pyridin-2-yl)pentane-2,4-dione tautomeric forms are very unstable. On the other hand, symmetric dienol (among the pyridin-2-yl derivatives) and dienaminone (among the quinolin-2-yl derivatives) tautomers were found to be the most stable. Although stability of the tautomer depends on aromaticity of its molecule and on strength and number of the intramolecular hydrogen bonds, relation between these variables seems to be complex.