Tropolone definition

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. ... 

The a-substituted tropolonates 2 which are tris chelate complexes with a M06 coordination core have received considerable study by DNMR.27, 4S 46) The complexes are of the M(A-B)3 and M(A-B )3 types and therefore the averaging sets of Eaton efa/.26, 27 in Table 1 are applicable. The metal ions and a-R-substituents used in these studies include M = Al(III), Ga(III), Co(III), V(III), Mn(III), Ru(III), Rh(III) and Ge(IV) R = isopropyl (C3H7) and isopropenyl (C3HS), however, only complexes of Al(III), Ga(III), and Co(III) have yielded definitive mechanistic information.27, 45 46> On the basis of line shape changes of the methyl resonances these complexes can be classed kinetically as follows stereochemically nonrigid complexes which attain the fast-exchange limit of inversion and/or isomerization... 

The proton NMR spectra of the tetrakistropolonates of zirconium and hafnium in dimethyl sulfoxide has also been examined. The spectra are consistent with a complex in which all four ligands are equivalent, but there is no evidence for rapid exchange between the chelates and free ligands (379). Similarity in chemical shifts for the tropolone and chelate resonances, however, makes a definitive statement difficult. Attempts to form a 10-coordinate species with tropolone were unsuccessful (377). 

Tropolone is another famous nonalternant system whose aromaticity has long been disputed. Recent studies of its anion and two conformers of protonated tropolone have given us a good opportunity to analyze how the aromaticity is affected by the protonation. The HOMA for tropolone is equal to 0.642. Its anion is definitely less aromatic, HOMA = 0.176, whereas cis and trans conformers of protonated... 

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