Modeling of the mechanism of endo—exo-isomerization of dicyclopentadiene hydrogenation products in the presence of AlCl3

Abstract

The quantum chemical modeling (PBEO/cc-pVTZ) of the catalytic endo—exo-iso-merization of products of dicyclopentadiene hydrogenation, tetrahydrocyclopentadiene, tetrahydrodicyclopentadiene (THDCPD) and dihydrodicyclopentadiene (DHDCPD), showed that the reactions include three steps: hydride ion transfer from endo-THDCPD (endo-DHDCPD) molecule to AlCl₃, [1,2]-sigmatropic rearrangement of the carbocation, and backward hydride ion transfer from [AlHCl₃]⁻ to the carbocation. The energy profiles and mechanisms of THDCPD and DHDCPD isomerization do not differ substantially. The maximum Gibbs activation energy (ΔG^≠₂₉₈) is achieved in the H⁻ transfer step. The solvation effects (CH₂Cl₂, CCl₄) result in an appreciable decrease in the energy barrier, and the influence of CH₂Cl₂ turns out to be more significant. The introduction of three solvent molecules into the system leads to an additional decrease in ΔG^≠₂₉₈ of the hydride ion transfer step (38.2 kcal mol⁻¹). It has been shown for the first time that the first reaction step can involve the H atom in both exo- and endo-position. The energy barrier of the exo-H⁻ transfer step is lower (by 1.5 kcal mol⁻¹). The results obtained indicate a principal possibility of endo—exo-isomerization in the product of incomplete dicyclopentadiene hydrogenation, which can be of practical significance. However, it should be taken into account that the isomerization of DHDCPD on AlCl₃ can be impeded by the formation of η²-complexes.