Hexadehydro annulenes

Diercks, R. VoUhardt, K. P. C. Tris(benzocyclobutadieno)benzene, the triangular [4]phenylene with a completely bond-fixed cyclohexatriene ring cobalt-catalyzed synthesis from hexaethynylbenzene and thermal ring opening to l,2 5,6 9,10-tribenzo-3,4,7,8,ll,12-hexadehydro[12]annulene, 7. Am. Chem. Soc. 1986,108, 3150-3152. 

Hydrogenation of cyclicpolyacetylene, 14 [1,3,7,9,13,15-hexadehydro (18) annu-lene], over Pd-on-BaS04 in benzene inhibited by a drop of quinoline provides an improved synthesis of 18-annulene ... 

Hexadehydro-[ 18]annulene 80>, 15, (see Table 4) possesses no inner protons and the six equivalent outer protons resonate at t 2.98. This represents a downfield shift of over 1 ppm from a model compound, cis-hex-3-ene-l,5-diyne, 9, (t 4.11). This model compound also served to clarify the interpretation of the singlet at t 5.55 in the case of l,5,9-tridehydro-[12]annulene, 1, a molecule for which aromaticity is not predicted. The downfield shift of 15 and the upfield shift of 1 from the acyclic model can then be attributed to a diamagnetic ring current in 15 and a paramagnetic ring current in 1. 

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. 

Dehydroannulenes reported by Sondheimer and co-workers include l,3,7,9,13,15-hexadehydro-[18]-annulene (14), in which the protons (all outside ) resonate at low field, suggesting the presence of a diamagnetic aromatic ring current. In comparison, the protons of... 

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).

The synthesis of the hexadehydro[18]annuIene (27), and hence of [18]-annulene, has been improved. Although cycloheptyne has not been... 

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