Electrochemically-Induced Radical Cation Diels-Alder Reactions Assisted By "Redox Tag Strategy"

Abstract

Diels-Alder (DA) reaction is among the most important reaction for constructing carbon-carbon bond in organic chemistry. The mechanisms into it are well understood; diene and dienophile should have complementary electronic characters for smooth reaction. In the situation where both substrates are electron-rich, significantly higher temperature or longer time are required. One of the most effective solution for such electronically-mismatched DA is umpolung process, which can give highly electrophilic radical cation from otherwise nucleophilic electron-rich starting substrates by one electron oxidation. Namely, the radical cations generated from electron-rich olefins can readily afford the DA adduct with nucleophilic electron rich dienes.1 Since electrochemical method has proven to be the simplest approach for the umpolung reaction, anodic method has been developed for the radical cation-mediated DA (rDA).2 Our group have been focused on the anodically-induced C-C bond forming reactions using lithium perchlorate nitromethane electrolyte solution system.3 In particular, [2 + 2] cycloaddition reaction4 between enol ether and unactivated terminal olefin is initiated by one electron oxidation of enol ether to give corresponding radical cation. The reaction allow for the mechanistic study of radical cation species, providing information, for example, intramolecular electron transfer properties5 and functionality of aromatic radical cation intermediates.6 In the talk to be presented, anodically-induced rDA (Scheme 1) will be reported especially to electrochemical condition, scope and limitations, and new design of the rDA based on the previous study of radical cation properties.