Variations on the Bergman Cyclization Theme: Electrocyclizations of Ionic Penta-, Hepta-, and Octadiynes

Area of Science:

• Computational Chemistry
• Organic Chemistry
• Theoretical Chemistry

Background:

• The Bergman cyclization of hexa-3-ene-1,5-diyne to p-benzyne is a known reaction.
• This reaction has potential applications as an antitumor agent due to low cyclization barrier and diradical stability.
• Ionic extensions of this cyclization have not been thoroughly investigated.

Purpose of the Study:

• To theoretically investigate ionic extensions of the Bergman cyclization.
• To explore the electrocyclizations of penta-1,4-diyne anion, hepta-1,6-diyne cation, and octa-1,7-diyne dication.
• To analyze the cyclization barriers and product stability of these ionic species.

Main Methods:

• Employed the spin-flip formulation of equation-of-motion coupled cluster theory with single and double substitutions (EOM-SF-CCSD).
• Calculated cyclization barriers for anionic, cationic, and dicationic diyne systems.
• Analyzed the electronic structure and aromaticity of the resulting diradical products.

Main Results:

• The penta-1,4-diyne anion exhibits a high cyclization barrier (+66 kcal mol⁻¹).
• The hepta-1,6-diyne cation and octa-1,7-diyne dication show low energy cyclization barriers via a previously unreported triplet pathway.
• Significant aromaticity was observed in the triplet diradical products of the cationic cyclizations.

Conclusions:

• Ionic extensions of the Bergman cyclization, particularly cationic species, offer energetically favorable reaction pathways.
• The triplet pathway for hepta-1,6-diyne cation and octa-1,7-diyne dication cyclization is a key finding.
• The resulting aromatic triplet diradicals warrant further investigation for potential applications.