Do Alkoxycarbonyl Substituents Stabilize Small Cycloalkane Rings?

Abstract

The standard enthalpies of formation ΔHf°(g) of mono‐ and gem‐di(alkoxycarbonyl)‐substituted cyclopropanes 1, cyclobutanes 2 and cyclopentanes 3 have been calculated from the standard enthalpies of combustion ΔHc°, which were measured calorimetrically, in combination with the standard enthalpies of vaporization ΔHvap°. The latter were obtained for 1a−c, 2b−c and 3b−c from the temperature dependence of the vapor pressures, which were measured in a flow system. Contrary to suggestions in the literature, only weak stabilization (≤ 8 kJ · mol−1) of small rings by gem‐alkoxycarbonyl substituents was observed. In this context, we give a revised value for the enthalpy of combustion of methyl cyclobutanecarboxylate. It is concluded that the known high rates of ring closure to gem‐dialkoxycarbonyl cyclopropanes are not attributable to a ‘stabilizing effect’ resulting from conjugation between the alkoxycarbonyl substituents and the cyclopropane ring, as has been suggested in the literature. The operation of a Thorpe‐Ingold‐ or gem‐dimethyl‐type effect would seem to offer a more satisfactory interpretation.

Cubane, Cuneane, and Their Carboxylates: A Calorimetric, Crystallographic, Calculational, and Conceptual Coinvestigation

“…Accepting the enthalpies of formation of the parent cycloalkanes from ref and that of the methyl carboxylate esters from ref , we find a reaction exothermicity of −7.7 ± 1.9 kJ·mol -1 . The strain energy difference between dimethyl 1,4-cubanedicarboxylate and dimethyl 2,6-cuneanedicarboxylate is expected to be 15.4 kJ·mol -1 higher than that of the difference between cubane and cuneane.…”

“…However, what is more important in the current context is the assumption that these species have the same strain energy. f Value taken from ref . g Value taken from ref .…”

gem-Disubstituent Effect: Theoretical Basis and Synthetic Applications

“…Interestingly, in several cases the increase of the rate of cyclization has not been explained by the Thorpe−Ingold theory (angle compression) . For example, Rüchardt et al first attributed the high rate of ring closure of gem - dialkoxycarbonyl cyclopropanes to the ring stabilization, which arises from the conjugative interaction between the electron-withdrawing substituent and the ring (Scheme ) . However, they noted that the stabilization energy of these small rings upon gem -dialkoxycarbonyl substitution was <8 kJ mol -1 .…”

“…2 For example, Ru ¨chardt et al first attributed the high rate of ring closure of gem- dialkoxycarbonyl cyclopropanes to the ring stabilization, which arises from the conjugative interaction between the electron-withdrawing substituent and the ring (Scheme 7). 37 However, they noted that the stabilization energy of these small rings upon gemdialkoxycarbonyl substitution was <8 kJ mol -1 . Thus, the authors reasoned that the observed rate enhancement must come from the decrease of the nonbonding interactions and rotational entropy, as proposed by Allinger and Zalkow for the case of gem-dimethyl substitution on medium rings.…”

Interplay of thermochemistry and Structural Chemistry, the journal (volume 18, 2007) and the discipline

“…However, few examples exist of compounds with gem-electron withdrawing substituents. A recent study is that of the biscarbomethoxy derivative within the context of other cycloalkane carboxylates and dicarboxylates [263].…”