Aromaticity in bridged heteroannulenes

Abstract

The aromatic stabilities of bridged hetero[n]annulenes (n = even, odd) and the parent structures of hetero‐[n]annulenes and hetero[n]polycycles have been predicted by the topological resonance energy, TRE, approach. The TRE criterion places bridged structures in all cases studied between the hetero[n]annulenes and hetero[n]polycyclic systems. The extra aromatic stability of the bridged heteroannulenes compared to heteroannulenes is attributed to homoaromatic interaction across the ring at the bridge sites. Available experimental evidence supports the theoretical results.

Synthesis and Diatropicity of trans-2‘,5‘,10b,10c-Tetramethylfurano[3,4-e]-10b,10c-dihydropyrene. A Valence Isomerization To Form a Novel Isoannulenofuran at the Expense of Two Benzene and One Furan Rings

“…The fusion of these heterocycles at the 3,4-positions to unsaturated carbocycles, i.e., isoannelation, has also been examined. Results from the calculations of the topological resonance energy per π-electron for the series of isoannelated compounds 4 − 6 again indicate that 5 is the least aromatic. The participation of oxygen in the aromaticity of the resulting isoannelated macroring thus seems to be the least effective of the selected heteroatoms.…”

Isoindole and isomeric heterocyclic donating substituents in ruthenium(II)nitrosyl complexes with large first hyperpolarizabilities and potential two-photon absorption capabilities: a computational approach

“…This should likely depend on the availability of the heterocyclic amine system (R) and the possibility of its modification for further coupling with the terpyridine moiety. To reach these target species, several strategies are currently being the subject of intense investigation, notably indenopyridines (17)(18)(19)(20)(21)(22) [35], isoindoles (1) and pyridoisoindoles (13-14) [36]. A literature survey reveals that the cyclopentaquinolines species precursors of molecules 6-8 were never approached, either theoretically or experimentally.…”

“…It is noteworthy that expressions (3) and (4) It was previously shown by theoretical calculations using several approaches and then proved with X-ray diffraction analysis and mass spectrometry that the nitrogen atom of the isoindole is strongly involved in the π-delocalized structure [21,22]. Owing to an electron-rich o-quinonic structure, isoindole can act as an efficient electron-donating moiety.…”

Synthesis and Diatropicity of trans-N-Cyclohexyl-2‘,5‘,10b,10c-tetramethyl- pyrrolo[3,4-e]-10b,10c-dihydropyrene. The First Example of an Iso[17]annulenopyrrole

“…None of the members of the 18π 3 has been isolated, and thus, the above order of aromaticity could not be verified experimentally. The syntheses of iso[17]annulenofuran 4 and iso[17]annulenothiophene 11 5 have, however, been reported. The difference in chemical shifts between the internal and external protons of 4 and 5 are <0.35 ppm, indicating that there is hardly any detectable ring current (aromatic character) in these systems.…”

“…The iso[17]annulenofuran 8 exhibits only <7% of the ring current observed in the parent [14]annulene 12 . Before this work, there was no known example of an iso[17]annulenopyrrole . It would thus be interesting to determine experimentally whether a significant increase in diatropicity (aromaticity) is observed going from 8 to 10 (or a derivative) similar to that predicted theoretically going from 3a to 3c .…”