Retro [2 + 2 + 2] ring opening in tricyclobutabenzene derivatives. Thermochemistry and reaction barriers. A theoretical hybrid density functional study.
The starting materials, transition states, and products of the concerted retro [2 + 2 + 2] ring-opening reactions of several tricyclobutabenzene derivatives substituted by double-bonded substituents were calculated at the B3LYP/6-31G theoretical level. It was found that the geometries of the products (cyclododecatriyne derivatives) are governed by the electronegativity of the substituents and hyperconjugation effects. The transition-state geometries of all the derivatives are similar. It was found that the retro [2 + 2 + 2] ring-opening reactions are endothermic (ca. 30-85 kcal mol(-1), depending on the substituents) with high barriers, ranging between ca. 95 and 115 kcal mol(-1). It is predicted that the tricyclobutabenzene derivatives under study will be stable when prepared and that they can be made by [2 + 2 + 2] intramolecular ring closure of the respective cyclododecatriyne derivative, especially with the use of transition metals as mediators or catalysts.
Stanger
Amnon
A
Department of Chemistry, The Institute of Catalysis Science and Technology, and The Lise-Meitner-Minerva Center for Computational Quantum Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel. stanger@tx.technion.ac.il