Understanding the electronic reorganization in the thermal isomerization reaction of trans-3,4-dimethylcyclobutene. Origins of outward Pseudodiradical {2n+2} torquoselectivity

The thermal isomerization reaction of trans-3,4-dimethylcyclobutene (1,2,3,4-DMC) to produce the isomer (2E, 4E)-hexadiene have been studied using density functional theory at the B3LYP/6-31+G level. For this reaction, two different channels of the conrotatory torquoselectivity allowing the formation of the two isomeric (E,E) and (Z,Z) have been characterized. The isomer (Z,Z) occurs through the inward conrotatory mechanism, whereas the isomer (E,E) occurs through the outward conrotatory mechanism. The outward conrotatory is favored by 11.3 kcal/mol with respect to inward conrotatory one. This behavior is consistent with the thermodynamic properties: enthalpy, free energy, and entropy calculated in both reaction pathways. The topological analysis of the electron localization function at the outward conrotatory transition state explicated the electronic reorganization through a pseudodiradical {2n + 2} process and not a pericyclic reorganization. (c) 2012 Wiley Periodicals, Inc.