Nature of the ring-closure process along the rearrangement of octa-1,3,5,7-tetraene to cycloocta-1,3,5-triene from the perspective of the electron localization function and catastrophe theory.

We analyze the behavior of the energy profile of the ring-closure process for the transformation of (3Z,5Z)-octa-1,3,5,7-tetraene 5 to (1Z,3Z,5Z)-cycloocta-1,3,5-triene 6 through a combination of electron localization function (ELF) and catastrophe theory (CT). From this analysis, concepts such as bond breaking/forming processes, formation/annihilation of lone pairs, and other electron pair rearrangements arise naturally through the reaction progress simply in terms of the different ways of pairing up the electrons. A relationship between the topology and the nature of the bond breaking/forming processes along this rearrangement is reported. The different domains of structural stability of the ELF occurring along the intrinsic reaction path have been identified. The reaction mechanism consists of six steps separated by fold and cusp catastrophes. The transition structure is observed in the third step, d(C1-C8) = 2.342 Å, where all bonds have topological signature of single bonds (C-C). The "new" C1-C8 single bond is not formed in transition state and respective catastrophe of the ELF field (cusp) is localized in the last step, d(C1-C8) ≈ 1.97 Å, where the two monosynaptic nonbonding basins V(C1) and V(C8) are joined into single disynaptic bonding basin V(C1,C8). The V(C1,C8) basin corresponds to classical picture of the C1-C8 bond in the Lewis formula. In cycloocta-1,3,5-triene 6 the single C1-C8 bond is characterized by relatively small basin population 1.72e, which is much smaller than other single bonds with 2.03 and 2.26e.

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

The reaction between cyclopentadiene and protonated pyridine-2-carboxaldehyde imine derivatives has been studied by using Hartree-Fock (HF) and B3LYP methods together with the 6-31G basis set. The molecular mechanism is stepwise along an inverted energy profile. This results from the protonation on both nitrogen atoms of the imine group and the pyridine framework. The first step corresponds to the nucleophilic attack of cyclopentadiene on the electron-poor carbon atom of the iminium cation group to give an acyclic cation 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 final cycloadduct. Two reactive channels have been characterized corresponding to the endo and exo approach modes of the cyclopentadiene to the iminium cation. The role of the pyridium cation substituent and the nitrogen position (ortho, meta, and para) along the reaction pathway has been also considered. Solvent effects (dichloromethane) by means of a continuum model have been taken into account to model the experimental environment.