Concerted Bond Formation in Diels-Alder Reactions: A Configuration Analysis

The concerted bond formation in the cycloaddition reaction between butadiene and ethylene, which is regarded as a prototypical model of the Diels-Alder reaction, was examined through an electron configuration analysis. We found that the weight of a crossed two-electron transfer configuration, in which one electron is transferred from the HOMO of butadiene to the LUMO of ethylene and another electron is transferred from the HOMO of ethylene to the LUMO of butadiene, increases gradually along the reaction coordinate and becomes one of the major configurations in the product region. This configuration involves mutual electron transfer between the two subsystems and gives rise to an electronic configuration that is formally equivalent to that produced by polarization within each isolated subsystem, although such polarization is symmetry-forbidden along the reaction pathway. Thus, the crossed two-electron transfer configuration provides a symmetry-allowed route to realize a polarization-like electronic configuration within each subsystem. These results demonstrate how localized bonding representations used in conventional organic electron theory emerge from orbital interactions and establish a direct connection between frontier-orbital descriptions and resonance-like electron redistribution during the concerted bond formation.