Crystal structure of a cyclotetramer from a strained cyclic allene

Area of Science:

• Organic Chemistry
• Organometallic Chemistry
• Crystal Engineering

Background:

• Cyclopropa[l]phenanthrene derivatives are complex polycyclic aromatic hydrocarbons.
• Cycloheptatetraenes are uncommon cyclic allenes with strained structures.
• Cyclotetramerization reactions are rare, especially for highly strained systems.

Purpose of the Study:

• To synthesize and characterize a novel dibenzoannellated cycloheptatetraene.
• To investigate the cyclotetramerization behavior of this strained cyclic allene.
• To elucidate the structural and electronic properties of the cyclotetramer product.

Main Methods:

• Low-temperature treatment of 1,1-dibromo-1a,9b-cyclopropa[l]phenanthrene with butyllithium and copper(II) chloride in tetrahydrofuran (THF).
• Single-crystal X-ray diffraction analysis to determine the structure of the cyclotetramer.
• Density Functional Theory (DFT) calculations (B3LYP/6-31+G**) to study electronic structure and reaction pathways.

Main Results:

• Successful synthesis of a dibenzoannellated 1,2,4,6-cycloheptatetraene.
• Observation of a rare cyclotetramerization reaction forming a "formal" 2 + 2 + 2 + 2 cyclotetramer.
• Crystal structure revealed a central, zigzag-folded eight-membered ring with axial hydrogen atoms and exocyclic double bonds.
• Computational analysis indicated a significantly distorted, strained cyclic allene potentially formed via ring expansion of a cyclopropylidene intermediate.

Conclusions:

• The study presents a novel synthetic route to a complex dibenzoannellated cycloheptatetraene.
• The observed cyclotetramerization highlights unusual reactivity for strained cyclic allenes.
• Structural and computational data provide insights into the formation and properties of highly strained polycyclic systems.