Theoretical study on the intramolecular [2+2+2] reaction mechanism

Abstract

The intramolecular [2+2+2] reaction is one of the famous pericyclic reactions. Although reaction mechanism is important and has attracted a lot of attention from both experimental and theoretical researcher, the reaction mechanism is still not clear. In this study, we have studied the ring-opening reaction mechanism of tris-cyclopropacyclohexane (TPH) and tris-cyclobutacyclohexane (TBH) to interpret the intramolecular [2+2+2] reaction mechanism. On each system, we calculated the ring-opening reaction pathways from each structural isomer because there are many structural isomers for reactant. All structures were determined with analytically calculated energy gradients at the CASSCF method with the 6-31G* basis set, the energy was estimated by a MRMP2 method with the 6-311+G** basis set. The reaction from TBH has lager activation energy barriers than the reaction from TPH. The C-C Bond of TPH is easier to cleave than that of TBH because the ring distortion of a three-membered ring is larger than that of a four-membered ring. In addition, it was found that the conformation of the central six-membered ring affects the activation energy and the reaction mechanism.