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 .