The 2,4,6,8-Tetramethylhomotropyliumdication.

Area of Science:

• Organic Chemistry
• Physical Chemistry
• Computational Chemistry

Background:

• The synthesis of the Hückel-aromatic 1,3,5,7-tetramethylcyclooctatetraene dication (3) was reported in 1976.
• NMR spectroscopy revealed an unexpected rearrangement of dication 3 to dication 5 at -20°C.
• This rearrangement has remained unaddressed in scientific literature for fifty years.

Purpose of the Study:

• To propose a mechanistic pathway for the observed rearrangement of 1,3,5,7-tetramethylcyclooctatetraene dication (3) to 1,3,5,7-tetramethylbicyclo[3.3.0]-dication (5).
• To investigate the relative stability between the rearranged dication 5 and its unknown isomer, 2,4,6,8-tetramethylbicyclo[3.3.0]dication (6).

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy was used to observe the rearrangement.
• Computational chemistry methods were employed to predict energy differences between dication isomers.
• Mechanistic analysis was performed to elucidate the rearrangement pathway.

Main Results:

• Dication 3 was observed to rearrange to dication 5 at -20°C.
• Dication 6 is predicted to be significantly more stable than dication 5 (34.4 kcal/mol lower).
• Dication 6 is also predicted to be more stable than the starting dication 3 (40.9 kcal/mol lower).

Conclusions:

• The unexpected rearrangement of a Hückel-aromatic dication to a bicyclic structure highlights novel reactivity.
• The proposed mechanism provides insight into the transformation of cyclooctatetraene derivatives under stable ion conditions.
• Computational predictions suggest the existence of a more stable isomeric bicyclic dication, meriting further investigation.