[12]Annulynes.

Area of Science:

• Organic Chemistry
• Aromaticity Studies
• Annulene Synthesis

Background:

• Benzene's dehydro analogue, o-benzyne ([6]annulyne), exists as a single isomer.
• The synthesis and properties of larger dehydroannulenes remain less explored.
• Understanding annulene isomers is crucial for probing aromaticity and electronic structure.

Purpose of the Study:

• To synthesize and characterize various isomers of [12]annulyne.
• To investigate the electronic properties and stability of these [12]annulyne isomers.
• To explore the behavior of [12]annulynes and their reduced forms.

Main Methods:

• Self-condensation of 1,5-hexadiyne using potassium tert-butoxide.
• Complete dehydrohalogenation of hexabromocyclododecene.
• 1H NMR spectroscopy to analyze ring current effects and proton resonances.
• Electrochemical reduction to form dianions.

Main Results:

• Two isomers of [12]annulyne (3,11-di-trans and 5,9-di-trans) were synthesized via self-condensation, exhibiting weak paratropic ring currents and oxygen sensitivity.
• A third, less stable isomer (3,9-di-trans-[12]annulyne) was obtained through dehydrohalogenation.
• Both synthesized isomers could be reduced to their respective, stable, diatropic dianions.
• A tight association between potassium cations and alkyne p(y)-orbitals caused unusually low-field proton NMR resonances.

Conclusions:

• Multiple isomers of [12]annulyne can be synthesized, expanding the scope of dehydroannulene chemistry.
• The electronic properties, including aromaticity (manifested as diatropicity in dianions), are isomer-dependent.
• The interaction of counterions with annulene systems significantly influences their spectroscopic properties.