Probing the Fluxional Bonding Nature of Rapid Cope rearrangements in Bullvalene C10H10 and Its Analogs C8H8, C9H10, and C8BH9

Abstract

Bullvalene C10H10 and its analogs semibullvalene C8H8, barbaralane C9H10, and 9-Borabarbaralane C8BH9 are prototypical fluxional molecules with rapid Cope rearrangements at finite temperatures. Detailed bonding analyses performed in this work reveal the existence of two fluxional π-bonds (2 2c-2e π → 2 3c-2e π → 2 2c-2e π) and one fluxional σ-bond (1 2c-2e σ → 1 4c-2e σ → 1 2c-2e σ) in their ground states and transition states, unveiling the universal π + σ double fluxional bonding nature of these fluctuating cage-like species. The highest occupied natural bond orbitals (HONBOs) turn out to be typical fluxional bonds dominating the dynamics of the systems. The 13C-NMR and 1H-NMR shielding tensors and chemical shifts of the model compound C8BH9 are computationally predicted to facilitate future experiments.

Control of Dynamic sp3-C Stereochemistry

“…As a result, the Cope rearrangement inverts simultaneously some, or all, of the four or five stereogenic centres present in the structure. Given that the rearrangement of BB is known to proceed with a remarkably low Gibbs free energy of activation, ΔG ‡ , of 32.3 kJ•mol −1 (Table S3), [31][32][33][34][35] chiral 9-BB, 3-BB, or 2,4-BB derivatives should undergo rapid enantiomerization.…”

“…Using these parameters, the automerization of BB is predicted to proceed with an activation energy, ΔE ‡ , of 41.8 kJ•mol −1 , which is ~10 kJ•mol −1 higher than the experimentally measured 32 ΔG ‡ of 32.3 kJ•mol −1 , in keeping with previous DFT investigations. 33,34 DFT methods systematically overestimate the energy barrier to Cope rearrangement of barbaralanes, but nevertheless allow useful comparisons of trends in activation energies and are known to predict accurately the relative ground-state energies of isomers. 33,34 The computationally predicted ΔE ‡ values for 1 (41.8 kJ•mol −1 ) and 2 (42.9 kJ•mol −1 ) are very similar to BB, indicating that the hydroxyl or ester group substitutions at position 9 do not significantly change the rapid kinetics.…”

“…33,34 DFT methods systematically overestimate the energy barrier to Cope rearrangement of barbaralanes, but nevertheless allow useful comparisons of trends in activation energies and are known to predict accurately the relative ground-state energies of isomers. 33,34 The computationally predicted ΔE ‡ values for 1 (41.8 kJ•mol −1 ) and 2 (42.9 kJ•mol −1 ) are very similar to BB, indicating that the hydroxyl or ester group substitutions at position 9 do not significantly change the rapid kinetics. The absence of the mirror plane in 1 is evident (Fig.…”

External Electric Fields Interrupt the Concerted Cope Rearrangement of Semibullvalene

“…We took interest in the degenerate Cope rearrangement (DCR) of SBV, − hereafter referred to as the DCR of SBV, because of its small activation barrier and an experimental Δ G ⧧ = 5.5 kcal mol –1 at low temperature (130 K) . This unique property makes SBV a prime example of a fluxional molecule that showcases “shapeshifting” properties and other interesting phenomena like heavy-atom tunneling. − Additionally, its highly strained ring system and aromatic TSS makes SBV a theoretical/experimental curiosity and an ideal case study to explore the effects of EEFs on its reactivity. For further in-depth discussion on the nature of the DCR of SBV, see the Supporting Information.…”

Control of dynamic sp3-C stereochemistry

“…

remarkably low free energy of activation, ΔG ‡ , of 32.3 kJ mol −1 (Supplementary Table 7) [34][35][36][37][38] , chiral 9-BB, 3-BB or 2,4-BB derivatives should undergo rapid enantiomerization.

…”