Induced ring current

Annulene and dehydro[22]annulene are also diatropic. A dehydroben-zo[22]annulene has been prepared that has eight C=C units, is planar and possesses a weak induced ring current. In the latter compound there are 13 outer protons at 6.25-8.45 8 and 7 inner protons at 0.70-3.45 5. Some aromatic bridged [22]annu-lenes are also known. The [26]annulene has not yet been prepared, but several dehydro[26]annulenes are aromatic.Furthermore, the dianion of 1,3,7,9,13,15, 19,21-octadehydro[24]annulene is another 26-electron system that is aromatic. Ojima and co-workers prepared bridged dehydro derivatives of [26], [30], and [34]annulenes. All of these are diatropic. The same workers prepared a bridged tetradehydro[38]annulene, which showed no ring current. On the other hand, the dianion of the cyclophane 89 also has 38 perimeter electrons, and this species is diatropic. ... 

The anisotropy of the magnetic susceptibility of a cyclic conjugated system, attributable to induced ring currents in its rc-electron network, is one of the important quantities indicative of 7t-electron delocalization. The method used for the calculation of the magnetic susceptibilities of nonalternant hydrocarbons is the London-Hoarau method taken together with the Wheland-Mann SCF technique . The resonance integral is assumed again to be of exponential form but... 

Using similar methodology, macrocycle 126 was prepared, as well as the unusual monoene 127 [76]. Considerable debate in the literature over the last thirty years has focused on whether dehydrobenzoannulenes are able to sustain induced ring currents [5al. Although fusion of arenes to the annulenic core provides rigidity and stability, this also weakens the diatropicity/paratropicity of the macrocycle significantly. Until quite recently, the number of planar systems available for study was limited however, with the the addition of 123 and 126, the series of alkyne-linked, tribenzo-fused dehydroannulenes is complete from... 

Aromaticity has been long recognized as one of the most useful theoretical concepts in organic chemistry. It is essential in understanding the reactivity, structure and many physico-chemical characteristics of heterocyclic compounds. Aromaticity can be defined as a measure of the basic state of cyclic conjugated TT-electron systems, which is manifested in increased thermodynamic stability, planar geometry with non-localized cyclic bonds, and the ability to sustain an induced ring current. In contrast to aromatic compounds there exist nonaromatic and antiaromatic systems. Thus, pyrazine (69)... 

The key underlying assumption of all the NMR studies of the homotropenylium systems is that there is an induced ring current when the ions are in a magnetic field and that this effects the two C(8) protons in a different manner. Dauben, Wilson and Laity measured the diamagnetic susceptibility of 11 and showed that it has a susceptibility exaltation which is similar in magnitude to that of the tropylium ion88. 

Up to this time there has been no report of the experimental determination of the structure of the parent homotropenylium ion. The three simplest systems that have been studied are 18, 19 and the iron complex 20. Cations 18 and 19 each have an oxygen-containing electron-donor substituent and, as such, appear to have smaller induced ring currents than the parent ion. In fact 18 and 19 have almost identical chemical shift differences (A<5 = 3.10 ppm) between the two C(8) protons. In the case of 20, A<5 is very small and it was considered to be a non-cyclically delocalized model for the bicyclo[5.l.OJheptadienyl cation69. 

There is limited experimental magnetic information on norcaradienes and most of the currently available information comes from theory. Pikulik and Childs examined the H NMR spectra of the type 110 and pointed out that the chemical shifts of the C(7) and C(l)/C(6) proton resonances were anomalous as compared to complexes of related cyclopropyl compounds such as 111 and 112246. They argued that the chemical shifts of 110 could be accounted for on the basis of an induced ring current. 

Aromaticity is a manifestation of electron delocalization in closed circuits, either in two or in three dimensions. This results in energy lowering, often quite substantial, and a variety of unusual chemical and physical properties. These include a tendency toward bond length equalization, unusual reactivity, and characteristic spectroscopic features. Since aromaticity is related to induced ring currents, magnetic properties are particularly important for its detection and evaluation. 

Applying the criterion of the induced ring current, 15 Elvidge16 obtained the following comparative percentages of aromaticity benzene, 100 thiophene, 75 pyrrole, 59 and furan, 46. However, other authors,17-19 using the same method, arrived at somewhat different results. 

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. 

It has been suggested that this attenuation of A<5 with substitution is attributable to a reduction in cyclic electron delocalization in the homoaromatic ring and a consequent attenuation in the induced ring current. However, as will be shown later, substitution also results in some fairly major changes in the structure of the homotropenylium ion and the impact of these structural changes on the relative position of the C(8) protons and/or ring current have not been disentangled. 

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