Abstract:

The structure of the bisannelated semibullvalene 11, which was originally predicted to be in a symmetrical homoaromatic ground state, was investigated by ultralow-temperature diffraction experiments down to 15 K and by related HF calculations. High-resolution X-ray data taken at 15 K (sin /_max = 1.074 Å^-1) provided a detailed electron density distribution that allowed a full topological analysis. The experimental and HF electron densities for 11 are in excellent accord. The electron density regions between C(2,8) clearly reveal a "normal cyclopropyl -bond" and very sparse density between C(4,6) corresponding with little or no interaction between these latter two centers. These results certainly confirm that in the solid state 11 is not homoaromatic. The experimental data at 15 K provide limiting values of structural parameters for a single tautomer of 11. In particular, the C(2,8) and C(4,6) distances are extremely useful base values for a bisannelated semibullvalene with an exceptionally low activation barrier to the Cope rearrangement.