Solvent effects on the proton‐transfer reactions of 7‐hydroxycoumarin and its 4‐methyl derivative with tertiary amines in the ground and excited singlet states [1

Abstract

AbstractUV absorption, fluorescence, and 13C NMR analyses of the proton‐transfer reactions between the title compounds (HC) and tertiary amines showed that the hydrogen‐bonded hydroxycoumarin tautomer anion III (which is stable in the ground state at room temperature) is exclusively produced in the polar aprotic solvent, acetonitrile, containing 1,8‐diazabicyclo[5.4.0]undec‐7‐ene. In addition, HC in the ground state was found to give the hydrogen‐bonded complex I and HC anion II in the presence of the less basic amines, pyridine and triethylamine, respectively, but not to produce the HC tautomer anion III. Both the less polar aprotic solvents (1,4‐dioxane and benzene) and a methyl substituent (that is introduced into the coumarin ring) had a tendency to decrease the thermodynamic stability of a given proton‐transferred ion pair. On the other hand, it was confirmed that the protic polar solvent, methanol (which is able to form a hydrogen‐bonded complex with HC) greatly accelerates the production of the ground‐state tautomer ion pair through the strong hydrogen‐bonding solvation of this ion pair. Close examination of the fluorescence and its lifetime of the HC anion and the HC tautomer anion (the formation of which is markedly enhanced in the excited singlet state) strongly suggested that the former anion serves as a precursor of the latter.