A Synthetic, X-ray, NMR Spectroscopy and DFT Study of β-Naphthil Dihydrazone, Di(β-naphthyl)acetylene, Tetra(β-naphthyl)cyclopentadienone, and Hexa(β-naphthyl)-benzene: C6 (C10 H7 )6 Is a Disordered Molecular Propeller.

Treatment of β-naphthil dihydrazone, 1, with silver oxide yields di(β-naphthyl)acetylene, 2, which undergoes Diels-Alder cycloaddition with tetra(β-naphthyl)cyclopentadienone, 4, to give hexa(β-naphthyl)benzene, 5, upon decarbonylation. Molecules 1, 2 and 4 have been characterised by X-ray crystallography, but hexa(β-naphthyl)benzene exhibits rotational disorder of the peripheral substituents. Nevertheless, calculations at the density functional level reveal the favoured structure of 5 to be a molecular propeller, in which the eight possible rotamers are essentially iso-energetic. Variable-temperature NMR spectroscopy studies yield a naphthyl rotational barrier of approximately 17 kcal mol-1 , similar to that previously found for meta-substituted phenyl groups. Enantiomerisation of hexa(β-naphthyl)benzene engendered by rotation of a single naphthyl ring has been studied by DFT calculations, and the process has been represented pictorially.