Abstract
The title compounds are available by a twofold cyclizing Wittig reaction of (all-E)-3,3'-(hexa-1,3,5-triene-1,6-diyldifuran-5,2-diyl)bis[prop-2-enal] (4) with (all-E)-(hexa-1,3,5-triene-1,6-diyl)bis(furan-5,2-diylmethylene)-bis[triphenylphosphonium] dibromide (7). Two conformational isomers 2a/2a' of (Z,E,E,E,E,Z,E,E,E,E)-tetraepoxy[36]annulene (6.4.6.4) are obtained. The oxidation of 2a/2a' ...
Abstract
The title compounds are available by a twofold cyclizing Wittig reaction of (all-E)-3,3'-(hexa-1,3,5-triene-1,6-diyldifuran-5,2-diyl)bis[prop-2-enal] (4) with (all-E)-(hexa-1,3,5-triene-1,6-diyl)bis(furan-5,2-diylmethylene)-bis[triphenylphosphonium] dibromide (7). Two conformational isomers 2a/2a' of (Z,E,E,E,E,Z,E,E,E,E)-tetraepoxy[36]annulene (6.4.6.4) are obtained. The oxidation of 2a/2a' yields two (E,E,Z, E,E,E,E,Z,E,E)-tetraoxa[34]porphyrin(6.4.6.4) dications 3a/3a', which are conformers, too. The oxidation of 2a/2a' is accompanied by the isomerization of four ethen-1,2-diyl bridges. The reduction of the dications 3a/3a' leads to the new (E,E,Z,E,E,E,E,Z,E,E)-tetraepoxy[36]ann (2b) and (E,E,E,Z,E,E,E,E,Z,E)-tetraepoxy[36]annulene(6.4.6.4) (2c). In 2b as well as in 2c, both 1,3-butadiene-1,4-diyl bridges are rotating until -90 degrees. The Delta delta values, ie., the maximum delta difference of the 'inner' and 'outer' perimeter protons of 3a/3a' (26.62 and 25.32 ppm) are of the same size as the Delta delta value of the tetramethyl[34]porphyrin(5.5.5.5) dication (1; Delta delta = 25.3 ppm); therefore, they might be called 'superheteroaromatic' too. The Delta delta values of the tetraepoxy[36]annulenes(6.4.6.4) (2a-c; Delta delta = 2.3-3.3 ppm) establish that they are still paratropic; they represent the most expanded antiaromatic systems yet known.
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