Electroreduction of 1-methyl 5-nitroindole, 5-nitrobenzofurane, and 5-nitrobenzothiophene in acidic and basic hydroorganic media: Generation and trapping of iminoquinone-type intermediates and electrosynthesis of ring-substituted amino derivatives

Abstract

Preparative electrolysis of 1-methyl-5-nitroindole (1b, X = NCH3), 5-nitrobenzofurane (1c, X = O), and 5-nitrobenzothiophene (1d, X = S) at Hg, in acidic hydromethanolic media, leads to the formation of the corresponding 4-substituted amino derivatives 5, which result from the 100% regioselective addition to iminoquinone-type intermediate 4 of methanol or of any other good nucleophile present in the electrolytic solution. In acidic medium, the iminoquinonium intermediates 4b and 4c were trapped in a cycloaddition reaction with cyclopentadiene added to the electrolysis medium. The regiochemistry of the nucleophilic addition is discussed in light of AM1 calculations. Key words: 1-methyl-5-nitroindole, 5-nitrobenzofurane, 5-nitrobenzothiophene, iminoquinone, electrosynthesis.