Electron has long been recognized for its potential to catalyze chemical reactions, which is referred to as redox catalysis. While acid/base catalysis involves addition/removal of a positively charged atomic particle, proton, an electronically opposite mode of activation is enabled by redox catalysis. Although acid/base catalysis can only trigger polarity-driven reactions, both polarity-driven and radical-driven reactions are made possible by redox catalysis, since single electron transfer (SET) should be involved. Radical-driven reactions were somewhat less familiar in the field of synthetic organic chemistry, however, recent developments in photocatalysis and electrocatalysis have brought them to the forefront. Redox-catalyzed reactions can be categorized into two groups, one is the reactions triggered by oxidative SET and the other is those triggered by reductive SET, while both processes are net redox neutral. In this article, we focus on oxidative SET-triggered cycloadditions, including [2+2] and Diels-Alder.

Oxidative SET from bench stable neutral molecules produces radical cation species, which can be used as unique reactive intermediates. Electron-rich arenes and alkenes are common precursors for the production of radical cations, generating aryl radical cations and alkenyl radical cations by oxidative SET. We have developed oxidative SET-triggered [2+2] and Diels-Alder cycloadditions, where enol ether radical cations and styrene radical cations are used as reactive intermediates. Our reactions are designed based on redox tag processes, where the importance of an intramolecular SET is highlighted. Both [2+2] and Diels-Alder cycloadditions are triggered by oxidative SET from electron-rich alkenes, producing enol ether radical cations and styrene radical cations, respectively. After the intermolecular trapping, they form aryl radical cations through intramolecular SET, which are finally reduced by starting alkenes to complete net redox neutral processes and run catalytic cycles. Intramolecular SET in redox tag processes is further studied by density functional theory (DFT) calculations.