Synthesis and Isolation of Antiaromatic Expanded Azacoronene via Intramolecular Vilsmeier-Type Reaction

Abstract

An antiaromatic cation of the expanded hexapyrrolohexaazacoronene (HPHAC) <b>1</b>^<b>+</b> was synthesized by a Vilsmeier-type reaction of the partially unfused HPHAC <b>2</b>. X-ray diffraction analysis revealed the formation of a seven-membered ring with a methyne linkage between the pyrrole moieties. Although <b>1</b>^<b>+</b> is a monocation, upfield shifts of the peripheral ethyl protons were clearly observed in the ¹H NMR spectra, indicating 24π antiaromaticity. Global antiaromaticity was also supported by nucleus-independent chemical shift and anisotropy of the induced current density calculations. Cation <b>1</b>^<b>+</b> displayed two reversible oxidations and one irreversible reduction in the cyclic voltammetry measurements. Treatment of <b>1</b>^<b>+</b> with NOSbF₆ gave aromatic trication <b>1</b>^<b>3+</b> with 22π-electron conjugation.