Tautomeric equilibrium of cyclic β-ketoester enamines

Abstract

This paper is dedicated to Professor Camille Sandorfy on the occasion of his 65th birthdayMARC LABELLE and DENIS GRAVEL. Can. J . Chem. 63, 1884Chem. 63, (1985.The conjugated-unconjugated tautomerism of P-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.MARC LABELLE et DENIS GRAVEL. Can. J. Chem. 63, 1884Chem. 63, (1985.L'equilibre tautomkrique des formes conjuguCes et dCconjuguees des Cnamines de p-cCtoesters a Ct C Ctudite. Plusieurs Cnamines de diverses grandeurs de cycle a I'amine et au cttoester et divers groupes alkyles I'ester ont Ct C prCparCes. Les proportions de formes conjuguCes et dCconjuguCes a l'equilibre sont rapportCes. Les rksultats sont expliquCs 21 partir de trois effets: (I) llinstabilitC relative des formes conjuguCes est due a une importante interaction stkrique; (2) les proportions des deux formes sont surtout controlCes par la grandeur des cycles impliquCs, en raison de differences d'Cnergie entre les structures hybrides de rksonance; (3) la nature du groupe alkyle de I'ester influence le rapport des deux formes a cause d'une diffCrence de IibertC de rotation dans la forme conjuguCe. Le degre d'instabilitt des doubles liens exocycliques a des cycles a 5, 6 et 7 chainons est discutC.In the course of our study on the scope and limitations of P-ketoester enamines in organic synthesis ( I ) , we became interested in investigating the factors which control the equilibrium between the conjugated and unconjugated forms in P-ketoester enamines (Scheme 1). An examination of the literature revealed that the presence, at equilibrium, of significant amounts of the unconjugated form is not a phenomenon limited to P-ketoester enamines (2-4) but has also been observed with P-ketoamide enamines (5-7), P-diketone enamines (4, 8, 9), P-ketosulfone enamines (lo), and P-ketothioamide enamines (7). The presence of this equilibrium has also been frequently noted in the results of enamine acylations (3, 5, 8, 10, 1 1) which often give some 1,3-diacylated product, resulting from acylation of the unconjugated form of the monoacylated product.The only systematic study dealing with the equilibrium between the conjugated and unconjugated forms in P-dicarbonyl enamines has been published by Colonna et al. (3). In this paper, the authors report the observed equilibrium proportions of conjugated and unconjugated forms ...

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