Tautomeric equilibrium of cyclic β-ketoester enamines

Abstract

The conjugated–unconjugated tautomerism of β-ketoester enamines has been investigated. Enamines with different amine ring size, ketone ring size, and ester alkyl group have been prepared. Equilibrium ratios of the conjugated and unconjugated forms have been determined. A three-point rationalization is presented: (1) the relative instability of the conjugated form is due to a large steric interaction; (2) the ring size combination largely determines the equilibrium ratio because of energy differences between the resonance hybrid structures of the conjugated forms; (3) the ester alkyl group slightly affects the equilibrium ratio, through a difference in rotational freedom in the conjugated form. The degree of instability of double bonds exocyclic to 5-, and 6-, and 7-membered rings is discussed.