A case study of antiaromaticity: carbomethoxy cyclopropenyl anion

Abstract

The simplest ideas of antiaromaticity refer to regular monocyclic systems and the eigenfunctions of the HückelHamiltonian for 4n π electrons in such systems. The antiaromaticity is expressed in the energy penalty for such idealizedsystems relative to the Hückel energy for 2n noninteracting π pairs. Observed systems seldom achieve the regular planargeometry assumed in this picture, owing to their ability to ease the antiaromaticity penalty by departures from theregular geometry and also by export of the 4n π electrons’ charge to substituents. In this report we estimate numericalvalues for the stabilization derived from such departures from the structure and the charge distribution of the idealizedantiaromatic cyclopropenyl anion for a specific case, 3-dehydro-3-methyl carboxylate cyclopropenyl anion 1(–) usingthe thermochemical scheme CBSQB3 supplemented by CCSD(T) calculations. According to the isodesmic reaction, theanion 1(–) is destabilized by about 10–15 kcal/mol relative to the saturated 3-dehydro-3-methylcarboxylate cyclopropylanion 2(–). We propose that the anion relieves a portion of the antiaromatic destabilization by (a) pyramidalizationof one carbon of the ring, and (b) export of negative charge into the ester substituent. Both of these responses areexpressed in the equilibrium structure of the anion. In the course of the study we estimate the acidity of several relatedanions and the enthalpy of formation of their neutral conjugate acids, and describe the interconversion of 1 to thedehydrotriafulvalene anion 3(–) by reaction with CO2 .