Article

Abstract

The [18]paracyclo[14]annulene, 6, a trans-10b,10c-dimethyldihydropyrene bridged by an 18 atom chain at the 2,7-positions, was prepared in 33% yield from 1,14-tetradecanedioyl dichloride and 2,7-bis(hydroxymethyl)-10b,10c- dimethyldihydropyrene (7). Molecular mechanics calculations were carried out to correlate the lowest energy conformers of 6 with the experimental X-ray structure. High field NMR spectra of 6 were fully assigned and correlated with the lower energy conformers, which in solution equilibrate such that the annulene ring can freely spin. The molecule, which is necklace shaped, has the protons H(9/6), and not those at the centre of the chain of the necklace [H(8/7)], closest to the centre of the annulene ring, and most shielded by the ring current. Shielding of various protons of the chain approximately correlates with the distances of their appropriate carbons, averaged over the 10 lowest energy conformers, from the centre of the ring current.Key words: paracyclophane, bridged annulene, molecular mechanics, ring current.