The 2,4,6,8-Tetramethylhomotropyliumdication

In 1976 George A. Olah et al. synthesized the Hückel-aromatic 1,3,5,7-tetramethylcyclo-octatetraene dication 3 under stable ion conditions. As observed by NMR spectroscopy, 3 rearranges at -20°C to the 1,3,5,7-tetramethylbicyclo[3.3.0]-dication 5. This is an unexpected result that has not been commented upon in the original papers or considered in the literature for half a century. We propose a mechanism for the rearrangement of 3 to 5 and discuss the relative instability of the latter with respect to the isomeric 2,4,6,8-tetramethylbicyclo[3.3.0]dication 6. The unknown isomeric dication 6 is predicted to be 34.4 kcal mol^-1 lower in energy than 5 and 40.9 kcal mol^-1 lower in energy than 3.