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 ...

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The isomerization of oxirene (oxacyclopropene) (1) to ketene, dimethyloxirene (7) to dimethylketene via the oxo carbene ("ketocarbene"), and the retro-Diels–Alder extrusion of oxirene and dimethyloxirene from their formal adducts (9 and 24, respectively) with benzene were studied computationally. All species were optimized at the MP2(fc)/6–31G(df,p) level; the species involving 1 were also subjected to MP2(fc)/6–31G(df,p) frequency and single-point CCSD(T)/6–31G(df,p) calculations. At the CCSD(T)/6–31G(df,p)//MP2(fc)/6–31G(df,p) level 1 isomerized to ketene in one step with a barrier of 2.8 kJ mol–1 and a reaction energy of –320.6 kJ mol–1. The extrusion of 1 from 9 had a late transition state and activation and reaction energies of 264.2 and 214.2 kJ mol–1, respectively, cf. cyclopropene extrusion from its adduct (192.3 and 95.9 kJ mol–1), indicating an antiaromatic destabilization energy of 214.2 – 95.9 = 118 kJ mol–1 for 1. The carbene 8 from ring-opening of 7 lay 10.9 kJ mol–1 above 7 (CCSD(T)/6–31G(df,p)//MP2(fc)/6–31G(df,p)), but the transition state could not be found; 8 isomerized to dimethylketene (252.7 kJ mol–1 below 7) with a barrier of 16.4 kJ mol–1, and to s-(Z)- and s-(E)-butenone with barriers of 28.5 and 35.4 kJ mol–1, respectively. The UV (TDDFT, B3P86/6–311++G**//MP2(fc)/6–31G(df,p)) spectra of 1 and 7 were calculated. Discrepancies were seen between the calculated IR spectra of 7 (bis(trifluoromethyl)oxirene) and perfluoro ethyl methyloxirene, and those attributed to these species in earlier matrix-isolation work. Key words: oxirene, dimethyloxirene, ab initio, retro-Diels–Alder, Diels–Alder.

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