Tests for aromaticity applied to the pentalenoquinones — A computational study

Abstract

Criteria for aromaticity and antiaromaticity were applied to the four pentalenoquinones, 1,2-, 1,5-, 1,4-, and 1,6-pentalenoquinone, i.e., bicyclo[3.3.0]octa-4,6,8-triene-2,3-dione (7a), bicyclo[3.3.0]octa-3,5,8-triene-2,7-dione (7b), bicyclo[3.3.0]octa-1(5),3,7-triene-2,6-dione (7c), and bicyclo[3.3.0]octa-1(5),3,6-triene-2,8-dione (7d). Geometry optimizations and frequency calculations were done with the pBP/DN* DFT method as implemented in Spartan, and single-point HF/3-21G calculations to obtain Löwdin bond orders (Spartan), as well as HF/6-31G* NICS calculations (Gaussian 98) were also carried out. Geometries and bond orders, chemical hardness, and NICS values gave no definite indication of aromatic or antiaromatic character. However, homodesmotic ring-opening reactions to give acyclic analogues indicated that 7a and 7b are nonaromatic (resonance energies -11 and 5 kJ mol -1 ) while 7c and 7d are antiaromatic (resonance energies -83 and -54 kJ mol -1 ). The resonance energies were obtained with the aid of an estimate of the strain energy of the molecules 7 (86 kJ mol -1 ) by a novel extrapolation procedure on hydropentalenes. Calculated pBP/DN* activation energies for Diels-Alder reactions with ethyne and ethene placed 7a and 7b in an "unreactive" class similar to 1,3-butadiene and fulvene, and 7c and 7d in a "reactive" class, similar to cyclopentadienone.Résumé : On a appliqué les critères d'aromaticité et d'antiaromaticité aux quatre 1,2-, 1,5-, 1,4-et 1,6pentalénoquinones, soit les bicyclo[3.. On a effectué les optimisations de la géométrie et les calculs de fréquences à l'aide de la méthode « pBP/DN* DFT » telle qu'implémentée dans Spartan; on a aussi effectué des calculs ponctuels HF/3-21G pour obtenir les ordres de liaison de Löwdin (Spartan) ainsi que des calculs « NICS » au niveau HF/6-31G* (Gaussian 98). Les géométries et les ordres de liaison, la dureté chimique et les valeurs « NICS » ne donnent aucune indication concernant le caractère aromatique ou antiaromatique des composés. Toutefois, des réactions d'ouverture de cycle homodesmotiques conduisant à la formation d'analogues acycliques indiquent que les composés 7a et 7b ne sont pas aromatiques (énergies de résonance de -11 et 5 kJ mol -1 ) alors que les composés 7c et 7d sont antiaromatiques (énergies de résonance de -83 et -54 kJ mol -1 ). Les énergies de résonance ont été obtenues à l'aide d'une évaluation de l'énergie de tension des molécules 7 (86 kJ mol -1 ) par le biais d'une nouvelle méthode extrapolation appliquée aux hydropentalènes. Les énergies d'activation calculées par la méthode pBP/DN* pour les réactions de Diels-Alder avec l'éthyne et l'éthène suggèrent que les composés 7a et 7b font partie de la classe de composés non réactifs, comme le buta-1,3-diène et le fulvène alors que les composés 7c et 7d font partie de la classe des composés réactifs, semblables à la cyclopentadiénone. Scheme 3. Selected C-C bond lengths (Å) and (parentheses)Löwdin bond orders of transition states and products from Diels-Alder reactions of 7a-7b, with ...

Energetic Aspects of Cyclic Pi-Electron Delocalization: Evaluation of the Methods of Estimating Aromatic Stabilization Energies

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