Formal Pericyclic-Coupled Electron Transfer: I. Stepwise Formal Diels-Alder Cycloaddition Enabled by Addition-Coupled Electron Transfer

The formal Diels-Alder (D-A) reaction of alkenes with 1,2-diimine and related substrates is designed and studied by density functional theory (DFT), multi-reference and quasi-classical trajectory calculations in the presence of a benzoquinone-based organic oxidant. The reaction is found to occur through an Addition-Coupled Electron Transfer (ACET) with one electron transferred to the oxidant in one elementary step, affording a carbon-radical, followed by an intramolecular radical addition. The overall barrier is significantly reduced as compared to the traditional concerted D-A reaction. The reaction with two new bond formation events in one elementary step coupled with electron transfer (concerted Pericyclic-Coupled Electron Transfer, cPeriCET) was not found. This work reveals that coupling to electron transfer might be a tool to achieve cycloaddition reactions with unactivated substrates.