Review Lecture - The annulenes

Macrocyclic hydrocarbons formally made up of alternating single and double bonds (annulenes) are of interest, since theory predicts such compounds to be aromatic, provided their carbon skeleton can exist in a configuration which is reasonably planar and they contain a closed shell of (4n + 2) π-electrons. Annulenes in which one or more of the double bonds have been replaced by acetylenic linkages are named dehydro-annulenes, and similar criteria for aromaticity apply to such compounds. These speculations have been tested experimentally. A series of annulenes and dehydro-annulenes containing 12, 14, 16, 18, 20, 24 and 30 carbon atoms in the ring have been prepared. The method of synthesis involved the oxidative coupling of certain terminal diacetylenes, whereby macrocyclic 1, 3-diynes were obtained. Prototropic rearrangement of the latter led to dehydro-annulenes, which on catalytic hydrogenation yielded the corresponding annulenes. An annulene or dehydro-annulene, if it is aromatic, should sustain a diamagnetic ring current of π-electrons in an applied magnetic field, a property measurable by the nuclear magnetic resonance (n. m. r.) spectrum. In agreement with theory, those substances expected to be aromatic showed unusual n. m. r. spectra, in which the outer protons appeared at low field and the inner protons at very high field. On the other hand, the spectra of those substances expected to be non-aromatic did not show this behaviour. The theoretical speculations regarding aromaticity have therefore been confirmed, despite the fact that some of the aromatic annulenes and dehydro-annulenes did not exhibit benzene-like chemical behaviour or stability.