σ-Bishomoconjugation (σ-Bishomoaromaticity) in 4C/3(2)e Cations-Scope and Limitations

Area of Science:

• Organic Chemistry
• Computational Chemistry
• Physical Chemistry

Background:

• Seco-dodecahedradiene 1 exhibits varying distances between double bond carbons (2.90–3.20 Å) and pyramidalization angles (14.9–35.5°).
• Understanding electron delocalization in polycyclic systems is crucial for predicting chemical reactivity and stability.

Purpose of the Study:

• To define the geometrical prerequisites for in-plane (sigma)-homoconjugative (sigma-bishomoaromatic) electron delocalization.
• To investigate the stability and electronic properties of 4-carbon/3-electron (4C/3e) radical cations and 4-carbon/2-electron (4C/2e) dications derived from seco-dodecahedradiene.

Main Methods:

• Computational modeling to analyze geometric parameters and electron delocalization.
• Matrix isolation techniques to study radical cation stability.
• Superacid media experiments to probe dication behavior.

Main Results:

• The study clarifies the geometric conditions necessary for sigma-bishomoaromaticity in seco-dodecahedradiene cations.
• The 4C/3e radical cation of seco-dodecahedradiene was found to be persistent in a matrix.
• The 4C/2e dication is an unstable intermediate, readily converting to a stable bisallylic dication.

Conclusions:

• The geometrical factors influencing sigma-homoconjugation in seco-dodecahedradiene cations are now better understood.
• The distinct stability of the 4C/3e radical cation versus the 4C/2e dication highlights the impact of electron count on aromaticity and reactivity.