1-Phenyl-1,2-cyclohexadiene: generation, interception by activated olefins, dimerisation and trimerisation

Area of Science:

• Organic Chemistry
• Reaction Mechanisms
• Synthetic Chemistry

Background:

• 1-phenyl-1,2-cyclohexadiene is a reactive intermediate.
• Understanding its reactivity is crucial for synthetic applications.
• Previous studies on similar dienes provide context.

Purpose of the Study:

• To investigate the synthesis of 1-phenyl-1,2-cyclohexadiene from various precursors.
• To explore the cycloaddition reactivity of 1-phenyl-1,2-cyclohexadiene with activated olefins.
• To elucidate the structures and formation mechanisms of its dimer and trimer.

Main Methods:

• Synthesis of four distinct precursors: 6,6-dibromo-1-phenylbicyclo[3.1.0]hexane, (1alpha,5alpha,6alpha)-6-bromo-6-fluoro-1-phenylbicyclo[3.1.0]hexane, 1-bromo-2-phenylcyclohexene, and 1-bromo-6-phenylcyclohexene.
• Interception of the generated 1-phenyl-1,2-cyclohexadiene with activated olefins (styrene, 1,1-diphenylethene, indene, furan, 2,5-dimethylfuran).
• Characterization of cycloadducts, dimer, and trimer, including X-ray crystal diffraction for the trimer structure.

Main Results:

• All four precursors successfully yielded 1-phenyl-1,2-cyclohexadiene.
• Cycloaddition reactions with activated olefins resulted in [2+2] and [4+2] cycloadducts, with regioselectivity dependent on the diene's phenyl group.
• Dimerization and trimerization occurred in the absence of trapping reagents, forming triphenylene derivatives with unusual structures, suggesting phenyl group participation.

Conclusions:

• The study demonstrates efficient generation of 1-phenyl-1,2-cyclohexadiene from multiple precursors.
• The reactivity patterns in cycloadditions and dimerization/trimerization provide insights into the behavior of this strained diene.
• Diradical intermediates are proposed for both cycloaddition and dimerization/trimerization pathways, with X-ray data confirming a unique trimer structure.