Aromaticity and Curvature in Heteroacepentalenes

Abstract

Heteroatom-substituted acepentalene derivatives which are isoelectronic with the known acepentalenediide dianion are nonplanar, fused aromatic tricycles which are hemifullerenes of the corresponding C20 heteroanalogue. Depending on the number and position of heteroatoms, they may be anionic, neutral, or cationic. A nucleus-independent chemical shift study indicates that substitution of the central carbon of the acepentalenediide system with N or O results in a substantial increase in aromaticity. Peripheral aza substitution on the other hand tends to increase curvature and decrease aromaticity. Alkylation or protonation at the central position of asymmetrically substituted heteroacepentalenides leads to chiral, bowl-shaped, 10 pi aromatic species.

A Zwitterionic, 10 π Aromatic Hemisphere

“…Theoretical modeling of 10 confirms the structural assignment based on spectroscopic data and gives useful insights into its electronic properties. Compound 10 had been part of a previous study on the aromaticity and curvature in heteroacepentalenes . There, a DFT model (B3LYP/6‐311++G(d,p)) determined that 10 would be aromatic by virtue of a high degree of C−C bond equalization (Δ r =0.0237 Å) and the low torsion angle between p‐orbital axis vectors ( τ POAV2 =14.3°), despite its nonplanar character.…”

Nonplanar Butterfly‐Shaped π‐Expanded Pyrrolopyrroles

“…Curved polyarenes offer au nique arena to study unprecedented structure-function relationships, such as properties that emerge from the effects of curvature on aromaticity. [43][44] Indeed, theseP AHs exhibit unique reactivity,s uch as supramolecular aggregation in the anionic state and complexation with transition metals. [45] However,m osto ft hem are pure hydrocarbons and only af ew contain any aromatic heterocyclic moieties, [46][47][48] other than porphyrinoids.…”

A Zwitterionic, 10 π Aromatic Hemisphere

“…Theoretical modeling of 10 confirms the structural assignment based on spectroscopic data and gives useful insights into its electronic properties.Compound 10 had been part of ap revious study on the aromaticity and curvature in heteroacepentalenes. [17] There,aDFT model (B3LYP/6-311+ ++ +G-(d,p)) determined that 10 would be aromatic by virtue of ahigh degree of CÀCbond equalization (Dr = 0.0237 )and the low torsion angle between p-orbital axis vectors (t POAV 2 = 14.38 8), despite its nonplanar character. While NMR modeling by the gauge-independent atomic orbital (GIAO) method gives data in good agreement with the experimental 1 Ha nd 13 Cshifts for 10 and 12 (see the Supporting Information), the correspondence to the respective 14 Ns ignals at 144.6 and 103.5 ppm, vs.1 47.3 and 101.3 (calculated) was exceptional.…”