Abstract
The simplest ideas of antiaromaticity refer to regular monocyclic systems and the eigenfunctions of the H\{u}ckel Hamiltonian for 4n $\pi $ electrons in such systems. The antiaromaticity is expressed in the energy penalty for such idealized systems relative to the H\{u}ckel energy for 2n noninteracting $\pi $ pairs. Observed systems seldom achieve the regular planar geometry assumed in this picture, owing to their ability to ease the antiaromaticity penalty by departures from the regular geometry and also by export of the 4n $\pi $ electrons' charge to substituents. In this report we estimate numerical values for the stabilization derived from such departures from the structure and the charge distribution of the idealized antiaromatic cyclopropenyl anion for a specific case, 3-dehydro-3-methyl carboxylate cyclopropenyl anion 1(-) using the thermochemical scheme CBSQB3 supplemented by CCSD(T) calculations. According to the isodesmic reaction, the anion 1(-) is destabilized by about 10--15 kcal/mol relative to the saturated 3-dehydro-3-methylcarboxylate cyclopropyl anion 2(-). We propose that the anion relieves a portion of the antiaromatic destabilization by (a) pyramidalization of one carbon of the ring, and (b) export of negative charge into the ester substituent. Both of these responses are expressed in the equilibrium structure of the anion. In the course of the study we estimate the acidity of several related anions and the enthalpy of formation of their neutral conjugate acids, and describe the interconversion of 1 to the dehydrotriafulvalene anion 3(-) by reaction with CO$_{2}$.
Highlights
Aromaticity is an organizing principle in chemical theory that is associated with a number of interrelated properties of molecules. 1,2 These properties generally depart from what might be expected from compounds composed of localized pair bonds, that is, distinct bond lengths characteristic of single, double, and triple connections; simple additive representations of molecular properties including enthalpy and magnetic susceptibility; and reactivity and spectra attributable to the behavior of local features
This is familiar from molecular mechanics, which takes a strain-free system with electrostatic interactions as the zero-energy reference point; group additivity methods, which take noninteracting fragments as the reference point; the Hückel model, which begins with identical interactions between adjacent π centers; and the hierarchy of isodesmic reactions, which incorporate a zeroth order assumption that similar fragments have like energy in various molecules, a “transferability” assumption
The sequence of basis sets used in the extrapolation, which include diffuse members, allows a reasonable treatment of anions, as attested by the accuracy of electron affinity values observed for this method. 22−24
Summary
Aromaticity is an organizing principle in chemical theory that is associated with a number of interrelated properties of molecules. 1,2 These properties generally depart from what might be expected from compounds composed of localized pair bonds, that is, distinct bond lengths characteristic of single, double, and triple connections; simple additive representations of molecular properties including enthalpy and magnetic susceptibility; and reactivity and spectra attributable to the behavior of local features (functional groups and chromophores). To characterize antiaromaticity we will rely on the time-honored device of referring to idealized systems This is familiar from molecular mechanics, which takes a strain-free system with electrostatic interactions as the zero-energy reference point; group additivity methods, which take noninteracting fragments as the reference point; the Hückel model, which begins with identical interactions between adjacent π centers; and the hierarchy of isodesmic reactions, which incorporate a zeroth order assumption that similar fragments have like energy in various molecules, a “transferability” assumption. We will evaluate the relative enthalpies of the alternative products of protonation shown in Reaction 3 With these results and comparative studies on cyanosubstituted derivatives we can describe the twofold response (by geometric distortion and charge export) of the anion 3-dehydro-3-cyclopropenyl methyl carboxylate 1(–) to the stress derived from the presence of four π electrons in its three-membered ring. We will sketch the reaction profile for Reaction 4, following a mechanistic suggestion by Sachs and Kass. 19
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