Aromatic Rings and Aromatic Rods: Nonplanar Character of an Indeno-dehydro[14]annulene

Abstract

Since the concept of aromaticity has been proposed to be generalizable to acetylenic rods (‘linear ring’ of [2]annulene), p-diisopropyl-tetraphenyl-carbo-benzene (C₄₈H₃₄) and an indenone-fused isopropyl-triphenyloctadehydro[14]annulene (C₄₂H₂₆O) can be regarded as based on heptacyclic aromatic cores. The formation and X-ray crystal structures of both products are described. The latter has been obtained as a reductive rearrangement product of a transient isopropyl-pentaoxy[5]pericyclyne devised as a putative precursor of a carbo-fulvene target. A mechanism accounting for this peculiar transformation is proposed. Deviation from global planarity is measured by a 6° angle between the mean plane of the indenone bicycle and that of 13 atoms of the [14]annulenic macrocycle, forming dihedral angles with the local plane of the isopropyl-substituted sp² vertex of 16° and 15°, respectively. The magnetic aromaticity of the carbo-benzene and indeno-octadehydro[14]annulene products is evaluated by NICS calculations.

Key words

alkyne - [14]annulene - aromaticity - carbo-benzene - indenone

• References and Notes

• 1 The larger the number of atoms prone to undergo slight strains vs. the ideal VSEPR configuration (Gillespies’ valence shell electron pair repulsion model), the larger the allowed global out-of-plane deformation of the ring, the angular strain being spread out over all the endocyclic atoms.

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