Energetic, geometric and magnetic analysis of all-syn,trans cyclic cyclopropanes

Structural, energetic and magnetic properties of a series of cyclic cyclopropanes have been investigated on the DFT (B3LYP/6-31G(d)) level. The cyclization of these oligomeric all-syn,trans cyclopropanes, designated [N]rondelanes, should be possible on energetic grounds, since no substantial destabilization energies were calculated for the larger homologues. However, the high strain for the smaller rondelanes might prevent their successful synthesis. Thermochemical reactions, structural data, and NICS values were investigated in order to test for homoaromatic stabilization. The careful choice of a homoisodesmic reaction revealed weak aromatic stabilization energies for [3] and [7]rondelane, 2.3 kcal/mol and 2.2 kcal/mol, respectively. However, we could not observe a pattern according to the Hückel rule and the stabilization energies are small compared to the stabilization of cyclopropane due to σ-aromaticity. Calculated structural data indicate homoconjugation for the smaller homologues. [3]Rondelane showed a NICS value of −2.54 indicating a weak neutral homoaromaticity.