Investigation of induced currents in cyclic forms of ortho-acylphenols and lithium analogues: does the lithium cation contribute to aromatic π-electron delocalisation?

Abstract

Quasi-rings constructed as cyclic beta-ketoenol or lithium beta-ketoenolate moieties fused to aromatic molecules such as benzene, naphthalene and phenanthrene have geometric consequences for the conjoined aromatic similar to those caused by fusion of a benzene ring. This similarity is particularly marked in the case of lithium-bonded derivatives. Energies of bond separation reactions and the HOMA (harmonic-oscillator model of aromaticity) geometry-based index of pi-electron delocalisation show further similarities between quasi-rings and benzene. However, application of the ipsocentric approach to calculation of induced current density, shows that neither hydrogen nor lithium quasi-rings support pi-electron ring currents. It is argued that the geometric effects stem from electrostatic interaction of Li(+), and to a lesser extent H(+), with oxygen neighbours, causing a partial equalisation of CO bond lengths and knock-on effects on CC bonds, in the quasi-ring. The observed geometric effects do not arise from aromatic pi-electron delocalisation in the quasi-ring but are closer to the well-known push-pull effect between the electron-donating -OH and electron-accepting -CO groups, leading to reduction in bond-length alternation.