Trisdehydro annulenes

Treatment of 1,5,9-decatriyne (162) with copper(ii) acetate in pyridine yielded the 20-membered cyclic dimer 163 and the 30-membered cyclic trimer 166 . The bisdehydro[20]annulene 164 and the trisdehydro[30]annulene 167 were obtained by the prototropic rearrangement from 163 and 166, respectively. Half-reduction of the dehydroannulenes (164 and 167) yielded [20]- (165) and [30]- (160) annulenes, respectively . 

It has been found that 155 can be converted into 3-nitro-l,3,7-trisdehydro-[I8]annulene (170) on brief treatment with copper(ii) nitrate in acetic anhydride at room temperature. The monoacetyl derivative of 155 was also prepared. ... 

Hexakisdehydro[18]annuIene (178), a [4/i + 2]dehydroannulene having a similar geometry to l,5,9-trisdehydro[12]annulene (172), has been synthesized according to the following sequence of reactions ... 

The bisdehydro[12]annuIene 171 and the trisdehydro[12]annulene 172 gave the corresponding diatropic dianions (249 and 250) on treatment with potassium . Although, as already mentioned, 171 shows high conformational mobility, the... 

It has been predicted theoretically that planar [4/i-t-2]annulcnes will be aromatic up to [22]annulene, but that [26]annulene will no longer be aromatic . The findings that monodehydro[22]annulene , [22]annulene and tetra-r-butylbisdehydro-[22]annulene (289) are diatropic are consistent with the theoretical prediction. Trisdehydro[26]annulene (292) and monodehydro[26]annulene (293) prepared... 

Biphenylene and trisdehydro[12]annulene are representatives of conjugated hydrocarbons with a 4 -membered ring. The pattern of their absorption spectra is completely different from that of benzenoid aromatics. Their HOMO and LUMO are derived from the two NBMOs of an ideal perimeter and both the lowest excited singlet and triplet state can be described by the configuration A ho lu- The transition is symmetry forbidden in molecules of D2h symmetry or higher. Nonradiative decay usually dominates their photophysical properties. Quantum yields of fluorescence and intersystem crossing are low.307 The LCAO version of Platt s perimeter model has been extended to treat conjugated systems with AN jt-electrons derived from [ ]annulenes.308,309... 

Reduction of 1,5-bisdehydro- and 1,5,9-trisdehydro[12]annulenes (80] and of octadehydro[24]annulene [81] with alkali metal.s provides radical anions and dianions. These dianions are diatropic, having gained two electrons compared to the paratropic parent dehydroannulenes, and, as with the corresponding annulenes, there is a dramatic change in the chemical shifts. For example, in the case of 1,5-hisdehydro[12]annulene the signal for the inner hydrogen shifts from 5 17.6 to 5 -6.88 on reduction. 

Compounds (X) (prepared as an intermediate in the preparation of [18]annulene see above) and (XIV) are known, respectively, as 1,7,13-trisdehydro[18]annulene and 1,5-bisdehydro[12]annulene. 

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