A computational investigation on the sequential rearrangement mechanism of 2-allyl-2,4,5-hexatrienaldehyde involving [1,5]-hydrogen migration and 8π-electrocyclization

The sequential rearrangement reaction mechanism of the 2-allyl-2,4,5-hexatrienaldehyde has been studied at the unrestricted Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional (UB3LYP)/6-31G**, Complete Active Space Self-Consistent Field (CAS (8,8))/6-31G**, Configuration Interaction with Single and Double Excitations (CISD)//UB3LYP/6-31G** and the second-order perturbation theory based on the CASSCF reference wave function (CASPT2)//CAS(8,8)/6-31G** levels. Two pathways have been found to be involved for this reaction. The first pathway includes four processes of the rotation of the C3-C4 single bond, the stepwise [2 + 2] cycloaddition, the [1,5]-hydrogen migration, and the ring opening isomerization, while the second pathway undergoes only two processes of the [1,5]-hydrogen migration and the 8 pi-electrocyclization. The calculation results indicate that the second pathway is favorable, in good agreement with the recent experimental observation. The whole reaction is stepwise and strong exothermic. (c) 2007 Wiley Periodicals, Inc.