Annulenes ring currents

Something interesting happens when we go beyond benzene to apply the aromatic ring current test to annulenes... 

FIGURE 13 10 More shielded (red) and less shielded (blue) protons in (a) [18]annulene and (b) [16]annulene The induced magnetic field associated with the aromatic ring current in [18]annulene shields the inside protons and deshields the out side protons The opposite occurs in [16]annulene which is antiaromatic... 

The 7/NMR spectrum displays signals of shielded protons = - 2.35, integral level 1) and of deshielded ones = 10.45 and 9.39, integral levels 1 1). This reflects a ring current due to aromaticity as described for annulenes and porphyrins in section 2.5.2. To conclude, the reaction involves an oxidative cyclisatlon of 2,5-bis(2-pyrrolylmethyl)-17/-pyrrole 2 with 47/-trlazole-3,5-dlaldehyde 3 to the corresponding 2,3-diazaporphyrin 4, following the 3-t-l pathway of porphyrin synthesis. Two non-equivalent tautomers may exist these are the diaza[ 18]annulene 4a and the tetraaza[18]annulene 4b. 

Annulene is a very imstable compound that undergoes cyclization to bicyclic isomers and can be kept only at very low temperature. The NMR spectrum has been studied at low temperature." Besides indicating the double-bond geometry shown in the stmcture below, the spectrum reveals a paramagnetic ring current, the opposite to what is observed for aromatic systems. This feature is quite characteristic of the [4 ]annulenes and has been useful in characterizing the aromaticity or lack of it in annulenes." ... 

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. ... 

As we have seen (p. 59), these systems are expected to be not only nonaromatic but actually antiaromatic. The chief criterion for antiaromaticity in annulenes is the presence of a paramagnetic ring current, which causes protons on the outside of the ring to be shifted upfield while any inner protons are shifted downfield, in sharp... 

The fact that many 4 systems are paratropic even though they may be nonplanar and have unequal bond distances indicates that if planarity were enforced, the ring currents might be even greater. That this is true is dramatically illustrated by the NMR spectrum of the dianion of 83 (and its diethyl and dipropyl homologs). We may recall that in 83, the outer protons were found at 8.14-8.67 8 with the methyl protons at —4.25 8. For the dianion, however, which is forced to have approximately the same planar geometry but now has 16 electrons, the outer protons are shifted to about -3 8 while the methyl protons are found at 21 8, a shift of 258 We have already seen where the converse shift was made, when [16]annulenes that were antiaromatic were converted to 18-electron dianions that were aromatic. In these cases, the changes in NMR chemical shifts were almost as dramatic. Heat of combustion measurements also show that [16]annulene is much less stable than its dianion. 

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... 

NMR data for aza[18]annulene 234 show a wide separation between the centers of the inner and outer proton lH NMR multiplets (Ad =11 ppm), indicative of a strong diamagnetic ring current.270 271 The bis-dehydro system 235 with 22 jr-electrons in the aromatic ring is diatropic.272... 

A bispyridine derivative of dibenzo-hexaaza[18]-annulene 236 was prepared by Bell and Guzzo.275 Fur an building blocks were used in the syntheses of the diatropic compounds 237 and 238.276-278 The chemical shifts of the indicated hydrogens show that the diamagnetic ring current decreases in the order 237 > 238 (Scheme 83). 

The diatropism of diazapyraceheptylenes (106), which follows from a comparison of the PMR spectra with that of 105a (Table VI), may be due to the effect of the cyano groups on the electronic configuration of the 7r-system. The chemical shift difference between the inside and outside protons clearly reveals the expected diamagnetic ring current in the annulene derivatives (110). The coupling constants of the seven-membered rings of 110 show that this part of the molecule is planar and delocalized. 

For example, the question of a paramagnetic ring current in pyracylene 1C. A. Coulson and R. B. Mallion, J. Am. Chem. Soc. 98, 592 (1976)1, which is closely related to hydrazino-bridged [ 12]annulenes, treated in Section VIII,A. 

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