Circumambulatory Rearrangements of 5-Halo-1,2,3,4,5-pentaphenylcyclopentadienes
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umambulatory Rearrangements of 5-Halo-1,2,3,4,5-pentaphenylcyclopentadienes G. A. Dushenkoa,*, I. E. Mikhailova, and V. I. Minkina a
Institute of Physical and Organic Chemistry, Southern Federal University, Rostov-on-Don, 344090 Russia *e-mail: [email protected] Received July 2, 2020; revised July 14, 2020; accepted July 18, 2020
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). Keywords: halopentaphenylcyclopentadienes, circumambulatory rearrangement, molecular propellers, chirality, DFT calculations
DOI: 10.1134/S1070428020100127 Structure determination and study of structural nonrigidity of pentaarylcyclopentadienyl halides are of great importance since these compounds are precursors to a wide range of cyclopentadiene derivatives with practically useful properties, including complexes that are prototypes of molecular motors [1–5]. In addition, numerous metal complexes with pentaarylcyclopentadienyl ligands are widely used to catalyze asymmetric cycloadditions and a number of other important reactions [1, 6] and are highly efficient luminophores and compounds with nonlinear optical properties [7–9]. Pentaphenylcyclopentadiene itself is a fluorescent molecular rotor whose emission is induced by intra- and intermolecular interactions between phenyl rings [10]. By using 1H and 13C NMR techniques we previously revealed and studied intramolecular migrations of halogens in halocyclopentadiene derivatives. It was found that chlorine and bromine atoms migrate around the five-membered ring of tolyltetraphenylcyclopentadiene with energy barriers ΔG≠298 of 25.9 and 18.1 kcal/mol, respectively [11–13]. The migrations of chlorine and bromine around pentamethyl cyclopentadienepentacarboxylate and tetramethyl alkylcyclopentadienetetracarboxylates were characterized by barriers of 25.7–27.3 and 16.2–22.9 kcal/mol, respectively
[14]. Circumambulatory rearrangements of 5-iodocyclopentadiene are much faster, and the corresponding energy barrier is 14 kcal/mol [15]. According to DFT calculations, 5-iodocyclopentadiene in a rotating electric field behaves as a molecular rotor in which the iodine atom rapidly moves in one direction around the five-membered ring (k298 = 630 s–1) [16]. Rearrangements involving halogen migrat
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