Effect of Cr¹R²/P⁺R¹R² Exchange on the Degenerate Cope Rearrangement of Barbaralane

GRAPHICAL ABSTRACT

Abstract

Theoretical calculations of the degenerate Cope rearrangement at the DFT (B3LYP/6-31+G*) level reveal that CR¹R²/P⁺R¹R² exchange in barbaralanes results in raising the activation enthalpies by 2.5–4.8 kcal mol⁻¹. Substitution at the 2- and 6-positions by the -C≡N groups brings down the activation enthalpies by 3.5–4.2 kcal mol⁻¹, whereas substitution at these positions by the –CF₃ groups raises the activation barriers by ca. 1.8 kcal mol⁻¹ _. Methylene chloride as solvent does not affect the activation enthalpies. The NICS values and the ¹H NMR chemical shifts as well as the bond distances in the transition structures of the Cope rearrangement of 9-substituted barbaralanes and their 9-phospha-analogues confirm their aromatic character. The Natural Bond Orbital (NBO) analysis reveals the existence of conjugative and hyperconjugative interactions in these molecules, which become much stronger in the respective transition structures in the –CN substituted derivatives, but are much weaker in the -CF₃ substituted derivatives. Extensive delocalization of 6 electrons resulting from conjugative and hyperconjugative interactions combined with the fast degenerate Cope rearrangement confer bishomoaromatic character on these molecules.

Keywords:

• Bishomoaromaticity
• 9-substututed barbaralanes
• 9-phosphabarbaralanonium ions
• degenerate Cope rearrangement
• DFT calculations
• NBO analysis