Quantum Chemical Reaction Path and Transition State for a Model Cope (and Reverse Cope) Elimination

Abstract

Ab initio and density functional calculations have been performed on a simple model of the Cope and reverse Cope elimination reactions. The correlational energies have been taken into account up to the MP4(SDQ)/6-311G** and CCSD(T)/6-311++G** levels for RHF methods or using different (nonlocal) exchange and correlation functionals for the density functional theory. The calculated activation energies and free energies for both hydrogenated and deuterated reagents were found to be in good agreement with the available experimental data as well as primary kinetic isotope effects. The bond order analysis predicts an almost completely synchronous reaction path for correlated methods and a little advanced H transfer for the HF method. The intrinsic reaction coordinate path following method shows that although the reaction is concerted, the H transfer slightly precedes the C−N bond breaking. Modeling the solvent effect explained the solvent dependence of the Cope (reverse Cope) products equilibrium. Any very significant hydrogen tunneling could be excluded from the shape of the Born−Oppenheimer potential energy surface and from the good agreement between the calculated and measured primary kinetic isotope effects. From these results and the computed minimal energy path, a refined picture of both the Cope and reverse Cope eliminations implying a one-step slightly dissymmetric reaction mechanism could be proposed.