Chapter 10. Aromatic compounds

Abstract

It has been suggested' that the contribution to the magnetic susceptibility component perpendicular to the molecular plane serves as a more reliable aromaticity index than that based on magnetic anisotropy or n.m.r. data, and that high magnetic anisotropy does not necessarily imply aromaticity. The approach seems similar to that in more recent papers by Flygare and co-workers* than those cited where the importance of relating to a hypothetical localized model is also emphasized. A small advanced text on aromaticity3 is worthy of attention, particularly for its laudable separation of experimental and theoretical aspects. The full details of the preparation of cycloocta[def]biphenylene have a~peared.~" This molecule, which might be expected to have simultaneously olefinic, aromatic, and antiaromatic properties, exhibits a marked paramagnetic ring shift, indicative of the planar 4n7r rings, but is thermally stable and seems to exhibit a high reactivity towards electrophiles. A related compound (1) has three linearly fused 4n7r rings, is thermally stable, and is d i a t r o p i ~. ~~ It is concluded that the application of the simple n.m.r. criterion of aromaticity to polycyclic systems is hazardous. The importance of the local anisotropic contribution to proton shieldings has also been emphasized, this being held responsible for the observed shifts in paratropic [4n]annulenes.'