Tetradehydro annulene

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. ... 

Tridehydro[18]annulene here and in the sequel refers to the symmetrical isomer shown in the formula, admixed with smaller quantities of an unsymmetrical isomer and tetradehydro[18]annulene.6 These can be separated by chromatography on alumina coated with 20% silver nitrate, but this is unnecessary for the synthesis of [18]annulene sinceallthree substances give this annulene on catalytic hydrogenation.8... 

For the full electronic spectra of tridehydro[18]annulene and tetradehydro[24]annulene in isooctane, see Reference 3. 

Myers et al. were able to trap 1,5-naphthyne (53), which is formed in solution from l,2,6,7-tetradehydro[10]annulene (66) even at -40 °C. The intermediate... 

Wolovsky describes the oxidative coupling of 1,5-hexadiyne, followed by prototropic rearrangement by heating with potassium /-butoxide in f-butanol-benzene at 100°. The resulting mixture of unsaturated hydrocarbons was easily separated by elution chromatography on a column of alumina impregnated with 20% silver nitrate into the tetradehydro[18]annulene (111) and the two isomeric tridehydro[18]annulenes (1) and (11). 

The outer protons of 1,3,7,13-tetradehydro-[18]annulene 107>, 16, (see Table 4) resonate in a range encompassing the chemical shift of the hexadehydro-[18]annulene, and, as expected, the two inner protons of this compound are shielded, and absorb as two quartets at t 7.66 and t7.80. The structure given in Table 4 is the most likely 10 ) of the four structures possible ) for a tetradehydro-[18]annulene containing three cis and two trans double bonds. 

The possibility of varying the sequence of a given number of trans and cis double bonds around a ring (within limits set by the positions of the acetylene bonds) has been referred to for 1,3,7,13-tetradehydro-[18]annulene, 16. In the case of l,7,13-tridehydro-[18]annulene 110> this phenomenon has lead to the isolation of two distinct isomers 107> of this compound, isomer I, 18, and isomer II, 19 (see Table 4). 

Figure 13-2 X-ray crystal structures of perethynylated hexadehydro[18]annuIene 48b at 100 K (A) and tetradehydro[12]annulene 47 a at room temperature (B).

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