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

Abstract

An antiaromatic cation of the expanded hexapyrrolohexaazacoronene (HPHAC) 1 + was synthesized by a Vilsmeier-type reaction of the partially unfused HPHAC 2. X-ray diffraction analysis revealed the formation of a seven-membered ring with a methyne linkage between the pyrrole moieties. Although 1 + is a monocation, upfield shifts of the peripheral ethyl protons were clearly observed in the 1 H NMR spectra, indicating 24π antiaromaticity. Global antiaromaticity was also supported by nucleus-independent chemical shift and anisotropy of the induced current density calculations. Cation 1 + displayed two reversible oxidations and one irreversible reduction in the cyclic voltammetry measurements. Treatment of 1 + with NOSbF 6 gave aromatic trication 1 3+ with 22π-electron conjugation.

Aromatic Nanosandwich Obtained by σ-Dimerization of a Nanographenoid π-Radical

“… 37 These alterations have produced π-conjugated systems exhibiting variable emission behavior, 31 susceptibility to multiple oxidation 33 or reduction, 36 , 37 and tunable aromaticity. 32 , 34 The present design combines radial fusion of electron-deficient subunits with edge expansion, to produce an odd-electron donor–acceptor π system, which undergoes spontaneous σ-dimerization. The resulting dimer is thermodynamically stabilized by balancing dispersion, distortion, and covalent forces, but can be cleaved into the π radicals using photochemical excitation.…”

Recent Advances in Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds

“… Reagents and conditions: (a) 16 NOSbF 6 (2.0 equiv) and DCM, rt, 10 min; (b) 18 3,4-dihexylpyrrole, NaH, DMF, rt; (c) FeCl 3 , CH 3 NO 2 , rt; (d) I 2 (excess), reflux, under N 2 flow; (e) 17 AgNO 2 (9.7 equiv), DCM, rt; (f) 21 DMF or N , N- dimethylbenzamide, POCl 3 , KPF 6 , DCE; (g) NOSbF 6 or BAHA, DCM. …”

“…This approach was followed in the synthesis of an antiaromatic expanded azacoronene cation [ 13.12 ] + reported by the Uno group in 2019. 21 Synthesis of the partially fused 13.11 was carried out under standard conditions while controlling the amount of oxidant employed. The Vilsmeier–Haack reaction of 13.11 using DMF and POCl 3 gave an excellent yield of the intramolecularly cyclized [ 13.12a ] + , rather than α-formyl derivatives of 13.11 .…”

Ring Contraction of Metallacyclobutadiene to Metallacyclopropene Driven by π- and σ-Aromaticity Relay

“…They also have weak and broad absorption band tails at 700-800 nm, which are typical bathochromic shifts of NIR bands attributed to the decrease in optical band gaps, in accordance with the antiaromatic character. 48,49 The NIR absorption of 2a-2c exhibits photothermal properties; for example, the temperature of the solution containing 1.00 mg mL -1 for 2c increased from 25.5 °C to 46.6 °C within 7 min under irradiation at 808 nm by a laser at a laser power density of 1.0 W cm -2 (Figure 4d). Metallacycles 3a-3c gave rise to two obvious absorption peaks at wavelengths of ~340 and ~510 nm.…”