To what extent can nine-membered monocycles be aromatic?

Paul Von Ragué Schleyer, L. Nyulászi, T. Kárpáti

Research output: Contribution to journalArticle

3 Citations (Scopus)

Abstract

Ab initio and density functional computations have been used to characterize the heteronins. New targets for synthesis are suggested on the basis of the results. The aromaticity of the heteronins was evaluated via numerous criteria such as magnetic properties (NICS and 8H), geometric indices (Bird, BDSHRT and HOMA), aromatic stabilization energies (ASE), and the barriers to planarity. Along with the experimentally characterized cyclononatetraenyl (1CH-) and azonide (1N-) anions, the phosphonide ion (1P-, which has not been reported before), favors planar (C2v) symmetry. All the other heteronins are nonplanar in order to alleviate ring strain. Comparisons of the neutral planar heteronins (constrained to C2v symmetry) show the extent to which heteroatoms like P and S, as well as N and O (in oxonin, 10), can participate in the cyclic electron delocalization. Due to hyperconjugation, the C2v-symmetric 9,9-distannylcyclononatetraene 1C(SnH3)2 is moderately aromatic, in contrast to 1CH2, which is a nonaromatic, conjugated system. The planar C2v-symmetric 1BH and 1AlH and 1CF2 are antiaromatic, like the planar form of 1CH+. Annelation with three-membered rings resulted in planar heteronins. The sole exception was phosphonin (8PH), but the barrier to planarity decreased to 1.8 kcal/mol in 8PR, due to the bulky R: 2,6-ditert-butylphenyl substituent at the phosphorus atom. Among the 8π heteronins, only the previously reported 1CH+ is Möbius-aromatic; 1BH, 1AlH and 1CF2 also have C2 structures but are nonaromatic due to large torsional angles which preclude cyclic delocalization. (

Original languageEnglish
Pages (from-to)1923-1930
Number of pages8
JournalEuropean Journal of Organic Chemistry
Issue number10
Publication statusPublished - May 2003

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Birds
Phosphorus
Anions
Magnetic properties
Stabilization
Electrons
Ions
Atoms
oxonin

Keywords

  • Aromaticity
  • Heterocycles
  • Nine-membered rings

ASJC Scopus subject areas

  • Organic Chemistry

Cite this

To what extent can nine-membered monocycles be aromatic? / Von Ragué Schleyer, Paul; Nyulászi, L.; Kárpáti, T.

In: European Journal of Organic Chemistry, No. 10, 05.2003, p. 1923-1930.

Research output: Contribution to journalArticle

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AB - Ab initio and density functional computations have been used to characterize the heteronins. New targets for synthesis are suggested on the basis of the results. The aromaticity of the heteronins was evaluated via numerous criteria such as magnetic properties (NICS and 8H), geometric indices (Bird, BDSHRT and HOMA), aromatic stabilization energies (ASE), and the barriers to planarity. Along with the experimentally characterized cyclononatetraenyl (1CH-) and azonide (1N-) anions, the phosphonide ion (1P-, which has not been reported before), favors planar (C2v) symmetry. All the other heteronins are nonplanar in order to alleviate ring strain. Comparisons of the neutral planar heteronins (constrained to C2v symmetry) show the extent to which heteroatoms like P and S, as well as N and O (in oxonin, 10), can participate in the cyclic electron delocalization. Due to hyperconjugation, the C2v-symmetric 9,9-distannylcyclononatetraene 1C(SnH3)2 is moderately aromatic, in contrast to 1CH2, which is a nonaromatic, conjugated system. The planar C2v-symmetric 1BH and 1AlH and 1CF2 are antiaromatic, like the planar form of 1CH+. Annelation with three-membered rings resulted in planar heteronins. The sole exception was phosphonin (8PH), but the barrier to planarity decreased to 1.8 kcal/mol in 8PR, due to the bulky R: 2,6-ditert-butylphenyl substituent at the phosphorus atom. Among the 8π heteronins, only the previously reported 1CH+ is Möbius-aromatic; 1BH, 1AlH and 1CF2 also have C2 structures but are nonaromatic due to large torsional angles which preclude cyclic delocalization. (

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