Butalene and related compounds: aromatic or antiaromatic?

Area of Science:

• Computational chemistry
• Organic chemistry
• Quantum chemistry

Background:

• Condensed cyclobutadienoid systems are of interest due to their unique electronic properties.
• Understanding aromaticity and antiaromaticity in fused ring systems is crucial for predicting chemical behavior.

Purpose of the Study:

• To investigate the electronic and structural properties of butalene, bicyclobutadienylene, and dicyclobutenobutalene using computational methods.
• To assess the aromaticity/antiaromaticity of these compounds and compare them to known systems.
• To study the ring-opening mechanisms and associated energy barriers.

Main Methods:

• Density functional theory (DFT) calculations were employed.
• Energetic and magnetic criteria were used to evaluate aromaticity.
• Transition states for ring-opening reactions were located and characterized.

Main Results:

• Butalene (3) was found to be planar and aromatic.
• Bicyclobutadienylene (12) is nonplanar and not aromatic, exhibiting less antiaromaticity than cyclobutadiene.
• Dicyclobutenobutalene (20) shows increased antiaromaticity, with properties resembling a fusion of aromatic and antiaromatic components.
• Ring-opening transition states for butalene and bicyclobutadienylene are conrotatorily twisted.
• The ring-opening barrier for bicyclobutadienylene is significantly higher than for butalene.
• Ring inversion is the primary barrier for the ring-opening of dicyclobutenobutalene.

Conclusions:

• The study provides a detailed computational analysis of three condensed cyclobutadienoid systems.
• Aromaticity and antiaromaticity trends were elucidated, offering insights into structure-property relationships.
• Distinct ring-opening pathways and energy barriers were identified, contributing to the understanding of their reactivity.