Cyclophane ring currents

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 reduction of a series of cyclophanes (21-29), which are dosely related but structurally different, helped understanding of the effects of the ring size, the nature of the aromatic unit, planarity and heteroatoms on the ring current [100]. A prerequisite in all these model cydophanes was the existence of [4n] re-electrons in the perimeter 24 re-electrons in 21, 24, 25 and 26, 28 re-electrons in 22 and 27, 32 re-electrons in 23 36 re-electrons in 28 and 20 re-electrons in 29. All of the neutral cyclophanes 21-29 show chemical shifts that are characteristic of olefinic and aromatic protons and only weak indications of anisotropy effects can be observed. 

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