Tropolones, aromatic character

X-ray analysis of 2-methoxy-4-hydroxy-5//-l-benzazepin-5-one (a benzazatropolone), prepared by methylation of the corresponding 4-hydroxy-l-benzazepin-2,5-dione with Meerwein s reagent, demonstrates the presence of a planar seven-membered ring but, in contrast to tropolone, little 71-electron delocalization.17 Likewise, ll//-dibenz[f>,e]azepin-ll-ones display no significant aromatic character.18 In contrast, 7-chloro-8//-thieno[3,2-c]azepin-8-one (12) has azepine ring hydrogen resonances at 8.7 and 9.02 ppm that indicate a substantial contribution from the polar zwitterionic mesomer 13.19... 

Another seven-membered ring that shows some aromatic character is tropone (44). This molecule would have an aromatic sextet if the two C=0 electrons stayed away from the ring and resided near the electronegative oxygen atom. In fact, tropones are stable compounds, and tropolones (45) are found in nature. However, analyses of dipole moments, NMR spectra, and X-ray diffraction measurements show that tropones and tropolones display appreciable bond alternations. ... 

These molecules must be regarded as essentially nonaromatic, although with some aromatic character. Tropolones readily undergo aromatic substitution, emphasizing that the old and the new definitions of aromaticity are not always parallel. In sharp contrast to 44, cyclopentadienone (46) has been isolated only in an argon matrix below 38 Above this temperature, it dimerizes. Many earlier attempts to prepare it were unsuccessful. As in 44, the electronegative oxygen atom draws electron to itself, but in this case it leaves only four electrons and the molecule is unstable. Some derivatives of 46 have been prepared. ... 

Although the chemical properties of tropolones were at one stage associated with aromatic character in the seven-membered ring this interpretation is unnecessary and indeed unreasonable since electrophilic attack on a tropylium-style ring is not to be expected. 

Some compounds have partial aromatic or antiaromatic character due to the presence of a minor aromatic or antiaromatic resonance structure. Tropolone (cy-cloheptadienone) is much more stable than one would expect from a highly unsaturated ketone because its C-O resonance structure is aromatic. On the other hand, cyclopentadienone is extremely unstable because its C-O resonance structure is antiaromatic. 

The simplicity of the tropolone molecule belies its fascinating character in both solution and the solid state. The aromatic properties of these compounds make varied interactions observable in the solid state including hydrogen tunnelling, tt-tt stacking and bromo... bromo interactions. The addition of a metal centre does not suppress this character, but can instead add further dimensions to the complexes formed. Palladium(II) complexes with tropolone can result in a myriad products, in which tropolone can act as a source of bromo moieties to assist in the reaction path. Although only a small fraction of our research team s work has been presented, the... 

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