Synthesis, oxidation and protonation of octamethyl-1,1′-bipyrrole

Abstract

2,2′,3,3′,4,4′,5,5′-Octamethyl-1,1′-bipyrrole 6 is obtained from 3,4-dimethylhexane-2,5-dione 4 and hydrazine. Treatment of 6 with [NO]BF₄ gives the radical cation 11, while protonation at C_α occurs with HBF₄ to yield the bipyrrolium salt 13. Oxidation with I₂ results in a bipyrrolium iodide [6H⁺] I⁻₃·½I₂ (14). The X-ray structures of 6 and 14 are reported.

Electrochemical oxidation of 6 proceeds in two one-electron steps, but is complicated by coupled slow chemical reactions. The oxidized species are stable on time scales shorter than several seconds. The primarily formed radical cation 11, however, slowly abstracts a hydrogen atom, possibly from the solvent or the supporting electrolyte, to yield protonated bipyrrole 13. The complex reaction mechanism is supported by electrolysis experiments and simulations of cyclic voltammograms.