Mechanisms of the hydrogen atom and the phenyl group migration in the molecule of heptaphenylcycloheptatriene

Abstract

In terms of the density functional theory using the B3LYP functional, 1,2,3,4,5,6,7-heptaphenylcycloheptatriene was shown to be the most stable in the boat conformation of the cycloheptatriene ring with the H atom in the equatorial position. 1,5-Sigmatropic shifts of the H atom along the seven-membered ring perimeter take place when it is in the axial position through the asymmetric transition state with the barrier ΔE ≠ ZPE = 28.7 kcal mol−1. The H atom can attain the axial position upon inversion of the seven-membered ring, which is accompanied by the orthogonal turn of the phenyl group at the sp3-hybridized C atom (ΔE ≠ ZPE = 22.6 kcal mol−1). The energy barrier to the circular rearrangement of the H atom (ΔE ≠ ZPE = 32.2 kcal mol−1) explains formation of isomers during the high-temperature synthesis of di(p-tolyl)pentaphenylcycloheptatriene. The barrier to the 1,5-sigmatropic shifts of the phenyl group is 19.7 kcal mol−1 higher than that for the competing shifts of the H atom.