A theoretical study of the reaction between cyclopentadiene and protonatedimine derivatives: A shift from a concerted to a stepwise molecular mechanism

The reaction between cyclopentadiene and protonated pyridine-2-carboxaldehy de imine derivatives has been studied by using Hartree-Fock (HF) and B3LYP methods together with the 6-31G* basis set. The molecular mechanism is step wise along an inverted energy profile. This results from the protonation on both nitrogen atoms of the imine group and the pyridine framework. The fir st step corresponds to the nucleophilic attack of cyclopentadiene on the el ectron-poor carbon atom of the iminium cation group to give an acyclic cati on intermediate, and the second step is associated with the ring closure of this intermediate via the formation of a C-N single bond yielding the fina l cycloadduct. Two reactive channels have been characterized corresponding to the endo and exo approach modes of the cyclopentadiene to the iminium ca tion. The role of the pyridium cation substituent and the nitrogen position (ortho, meta, and para) along the reaction pathway has been also considere d. Solvent effects (dichloromethane) by means of a continuum model have bee n taken into account to model the experimental environment.