Annulenes large ring

Dianions of several of the large ring annulenes have also been prepared.23 The 4n + 2 system [18]annulene, which has outer protons at 8 = +9.3 ppm and inner protons at S = — 3 ppm, is converted by potassium to the dianion, a 4n system with outer protons at S = — 1 ppm and inner protons at S = +29 ppm, the lowest field resonance known for a proton bound to carbon. (The largest known upfield shift, S = — 9 ppm, occurs for the inner protons of an 18 7r electron (4n + 2) monoanion.24)... 

Large-Ring Annulenes Like cyclooctatetraene, larger annulenes with (4A0 systems do not show antiaromaticity because they have the flexibility to adopt nonplanar conformations. Even though [12]annulene, [16]annulene, and [20]annulene are (4A0 systems (with N = 3,4, and 5, respectively), they all react as partially conjugated polyenes. 

Hiickel s rule also predicted aromatic stability for certain large ring polyenes called annuknes. Fully conjugated [14]-annulene and [18]-annulene do have aromatic properties, especially at lower temperatures (Figure 12), as does [22]-annulene. They contain fourteen, eighteen, and twenty-two tt electrons, respectively, corresponding to values of 4n -I- 2 where n = 3, 4, and 5, and all the molecules are planar. The [12]-, [16]-, [20]-, and [24]-annulenes, on the other hand, do not obey Hiickel s rule, and they are not aromatic. 

Hiickel s rule also predicted aromatic stability for certain large ring polyenes called annuknes. Fully conjugated [14]-annulene and [18]-annulene do have aromatic properties, especially at lower temperatures (Figure 12), as does... 

Before I960 the only annulenes that were available to test Hiickel s predictions were benzene and cyclooctatetraene. During the 1960s, and largely as a result of research by E Sondheimer, a number of large-ring annulenes were synthesized, and the predictions... 

We also learned in Section 14.7C that internal hydrogens of large-ring aromatic compounds such as [18]annulene, because of their position, are highly shielded by this induced field. They therefore absorb at unusually low frequency, often at negative delta values. 

Turning to the ring current definition of aromaticity, the chemical shift calculations for different structures for [18]annulene can now be carried out in a straightforward manner by quantum mechanics. These calculations predict quite a large ring current, and the chemical shifts are substantially larger for the inner protons if there is bond alternation than if the bonds were to be equivalent. (By equivalent here is meant similar to those in benzene and to each other, and the quantum mechanical stationary... 

Annulenes as large as the 30-membered ring have been prepared, and many dehydroannulenes, whicfr contain one or more triple bonds, are also known. 

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