The Fault Line in Prototropic Tautomerism

Azaindolizine proton cranes attached to 7-hydroxyquinoline and 3-hydroxypyridine: a comparative theoretical study

“…7HQ (Scheme 3(A)) is a tautomeric molecule, where the K-tautomer, although stabilized by re-arrangement of double bonds, is substantially less stable compared to the E-form 40,41 and cannot be observed in most organic solvents. Although, depending on the classification, this kind of prototropic tautomerism can be attributed to amide-iminol 42 or N-type, 43 in the discussion below, for the sake of simplicity and continuity with the seminal works in this field; 34,44 enol and keto tautomers are used to denote whether a C-OH or CQO group is present in the corresponding structure. The K-form in 3-hydroxypyridine (3HP), as a result of a simple intramolecular neutralization reaction, possesses a zwitterionic structure (Scheme 3(B)), which means substantially less stability in most organic solvents.…”