Circumambulatory Rearrangements of 5-Halo-1,2,3,4,5-pentaphenylcyclopentadienes

Abstract

DFT quantum chemical calculations at the CAM-B3LYP/Def2TZVP level of theory showed that intramolecular migrations of halogens in 5-halo-1,2,3,4,5-pentaphenylcyclopentadienes (C5Ph5Hlg, Hlg = F, Cl, Br, I) involve chiral conformation of their molecules with a propeller arrangement of phenyl groups via 1,5-sigmatropic shifts around the five-membered ring through asymmetric transition states with energy barriers ΔE≠ZPE of 42.5 (F), 26.2 (Cl), 20.2 (Br), and 15.2 kcal/mol (I). The results were consistent with the data of dynamic NMR spectroscopy. The P and M enantiomers are readily interconvertible (ΔE≠ZPE = 1.7–3.7 kcal/mol) by way of synchronous flips of the phenyl groups. The calculated barriers to alternative 1,3-halogen shifts in C5Ph5Hlg are considerably higher than those for 1,5-shifts: ΔE≠ZPE = 60.7 (F), 38.6 (Cl), 32.0 (Br), and 27.9 kcal/mol (I).