Quantum chemical study of the stereochemistry of double bond migration in 2-vinylnorbornane on palladium surface

Quantum-chemical modeling of the mechanism of isomerization of endo/exo-isomers of 2-vinylnorbornane (VNB) into Z/E-isomers of 2-ethylidene-norbornane (ENB) has been carried out using the DFT-PBE method. For each isomerization reaction, two 4-step routes including VNB adsorption, ENB desorption, and H atom detachment and attachment steps, the sequence of which depends on the type of route, have been considered. In the “allylic” route, the H atom is first cleaved off, leading to the formation of the allylic intermediate (C₇H₁₀)·CHCH₂. In the “ethylidene” route, the H atom is initially attached to the terminal C atom, forming the ethylidene intermediate C₇H₁₁C·HCH₃. According to calculations, an allyl intermediate is formed from adsorbed VNB with a small activation barrier, which binds strongly to the surface. The observed activation energy of almost all routes considered is determined by the energy of this intermediate. In the absence of hydrogen, the allylic intermediates will deactivate the active centers of the catalyst. The experimentally observed stereoselectivity is determined by a thermodynamic factor, namely the relative energy difference between the adsorbed endo/exo-isomers of VNB* and the desorbed Z/E-isomers of ENB. The formed E-ENB from endo-VNB and Z-ENB from exo-VNB have moderate adsorption energies and their desorption is found to be thermodynamically favorable.