Annulene model, aromaticity

Soncini A, Fowler PW, Jenneskens LW (2005) Angular Momentum and Spectral Decomposition of Ring Currents Aromaticity and the Annulene Model 115 57-79... 

Soncini A, Fowler PW, Jenneskens LW (2005) Angular Momentum and Spectral Decomposition of Ring Currents Aromaticity and the Annulene Model 115 57-79 Stalke D, see Mahalakshmi L (2002) 103 85-116 Stasicka Z, see Jaworska M (2003) 106 153-172 Steed JW, see Turner DR (2004) 108 97-168... 

Angular Momentum and Spectral Decomposition of Ring Currents Aromaticity and the Annulene Model... 

In the fused compounds (241) and (242) the furan ring fails to react as a diene and Diels-Alder reaction with dienophiles occurs on the terminal carbocyclic rings. However, (243) and (244) afford monoadducts with dimethyl fumarate by addition to the furan rings (70JA972). The rates of reaction (Table 2) of a number of dehydroannuleno[c]furans with maleic anhydride, which yield fully conjugated dehydroannulenes of the exo type (247), have been correlated with the aromaticity or antiaromaticity of the products (76JA6052). It was assumed that the transition state for the reactions resembled products to some extent, and the relative rates therefore are a measure of the resonance energy of the products. The reaction of the open-chain compound (250), which yields the adduct (251), was taken as a model. Hence the dehydro[4 + 2]annulenes from (246) and (249) are stabilized compared to (251), and the dehydro[4 ]annulenes from (245) and (248) are destabilized (Scheme 84). 

The conjugated polyene, [18]annulene, IV, is a potential 4m+ 2 aromatic system. Because of steric repulsion between hydrogen atoms inside the ring, the molecule is distorted away from planarity. Nevertheless, its NMR shielding effects indicate an induced ring current in the mean molecular plane, once more in line with the decisive role of o-a-m. All evidence derived from the exclusion principle, conformational rigidity, aromaticity, electronic spectra and NMR shielding is therefore consistent with the alternative picture, and at variance with the conventional model. 

Biphenylene and trisdehydro[12]annulene are representatives of conjugated hydrocarbons with a 4 -membered ring. The pattern of their absorption spectra is completely different from that of benzenoid aromatics. Their HOMO and LUMO are derived from the two NBMOs of an ideal perimeter and both the lowest excited singlet and triplet state can be described by the configuration A ho lu- The transition is symmetry forbidden in molecules of D2h symmetry or higher. Nonradiative decay usually dominates their photophysical properties. Quantum yields of fluorescence and intersystem crossing are low.307 The LCAO version of Platt s perimeter model has been extended to treat conjugated systems with AN jt-electrons derived from [ ]annulenes.308,309... 

Hexadehydro-[ 18]annulene 80>, 15, (see Table 4) possesses no inner protons and the six equivalent outer protons resonate at t 2.98. This represents a downfield shift of over 1 ppm from a model compound, cis-hex-3-ene-l,5-diyne, 9, (t 4.11). This model compound also served to clarify the interpretation of the singlet at t 5.55 in the case of l,5,9-tridehydro-[12]annulene, 1, a molecule for which aromaticity is not predicted. The downfield shift of 15 and the upfield shift of 1 from the acyclic model can then be attributed to a diamagnetic ring current in 15 and a paramagnetic ring current in 1. 

Various substituted l,6-methano-[10]annulenes have been prepared and characterized, and in particular the n.m.r. of such compounds reflects their delocalized nature 17,120,124-127) (a 71-complex formed from reaction of l,6-methano-[10]annulene with hexacarbonylchromium has also been characterized 128>). The X-ray crystallographic analysis 129> of the 2-carboxylic acid derivative 24 shows the bridge angle to be 99.6 °, and thus that the Cio perimeter is even flatter than expected from molecular models. Furthermore, the ten perimeter bonds have lengths which vary over the narrow range 1.38 to 1.42 A, the values being typical of benzenoid aromatic bonds. This lack of appreciable... 

In the series [18]annulene trisulphide, oxide-disulphide, dioxide-sulphide and trioxide, the relationship between planarity and delocalization has again been demonstrated. The trisulphide and oxide-disulphide are predicted from Catalin models to be non-planar and their n.m.r. does reflect the absence of a ring current over the total 18 -re-electron system. Models suggest that the dioxide-sulphide is near-planar and the trioxide planar, and indeed these compounds are aromatic 183,185)... 

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