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.Key words: aromaticity, pentalenoquinones, DFT, hardness, NICS, homodesmotic, resonance energy, bicyclo[3.3.0]octatrienediones.