Sigmatropic hydrogen shifts in aryltetraphenylcyclopentadienes

Resumo

Dynamic NMR has revealed intramolecular migrations of hydrogen atom over the periphery of the five-membered ring in 5-(p-tolyl)-1,2,3,4-tetraphenylcyclopentadiene in a deuteronitrobenzene solution with energy barrier ΔG₁₈₀^≠ = 24.8 kcal/mol. Quantum-chemical DFT calculations B3LYP/6-311++G** have shown that such migrations in 1,2,3,4,5-pentaphenylcyclopentadiene in the gas phase occur in a chiral conformation of propeller type by the mechanism of 1,5-sigmatropic hydrogen shifts with retention of configuration through asymmetric transition state with energy barrier ΔE_ZPE^≠ = 25.9 kcal/mol. Enantiomers P and M can readily interconvert into each other (ΔE_ZPE^≠ = 3.9 kcal/mol) owing to synchronous flip rotations of the phenyl groups.