Spin-Coupled Study of the Electronic Mechanism of the Hetero-Diels−Alder Reaction of Acrolein and Ethene

Abstract

Modern valence bond theory, in its spin coupled form, is used to elucidate the electronic rearrangements that take place during the course of the gas-phase hetero-Diels-Alder cycloaddition reaction of s-cis-acrolein (cis-1-oxabutadiene or cis-propeneal) and ethene. It is found that the most dramatic changes to the electronic structure occur in a relatively narrow interval of the reaction pathway soon after the transition state and that the system passes through a geometry at which it can be considered to be significantly aromatic. Although concerted, the reaction is markedly asynchronous, with the breaking of the carbon-oxygen pi bond, and the formation of the new carbon-oxygen sigma bond, "lagging behind" somewhat the other bond-making and bond-breaking processes.