Phenylenes, circular

An oligoalkyne polycyclization route to any circular phenylene is conceptually different from those developed for the other topologies, as it requires the elaboration of a suitably functionalized dehydrobenzannulene, a significant synthetic 

To summarize this section, to date 19 phenylenes have yielded to synthesis. They can be divided broadly into 5 families ( of examples) linear (3), angular/ helical (7), zigzag (2), bent (3) and branched (4). Their topologies have been accessed through 26 different routes, 15 of which involved in the crucial step an all-intramolecular cobalt-catalyzed cyclization, 9 used intermolecular variants and 2 a combination of the two strategies. 

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Circular phenylenes have the distinguishing characteristic of a resonance picture that includes forms that encompass both the irmer and outer peripheral loops, a phenomenon described as superdelocalization [81]. This class of circular phenylenes remains elusive [67, 82]. The simplest member of this series that does not suffer from additional circular strain is [6]phenylene 77d (Scheme 4.18), also christened antikekulene [57] to highlight its relationship to kekulene, its aU-benzenoid relative with an equal number of rings [83]. In antikekulene, avoidance of (benzo)cyclobutadienoid local circuits is expected to enhance the contribution of the potentially superdelocalized resonance form depicted for the structure in Scheme 4.18, albeit with the added and destabilizing feature that both inside and outside peripheries contain a (4n) electron count. 

A Marietta, D Gongalvcz, ON Oliveira Jr., RM Faria, and FEG Guimaraes, Circular dichroism and circularly polarized luminescence of highly oriented Langmuir-Blodgett films of poly(/j-phenylene vinylene), Synth. Met., 119 207-208, 2001. 

Figure 4.2. Simple phenylene topologies (a) linear [4]-, (b) angular [4]-, (c) zigzag [4]-, (d) branched [4]-, (e) (mixed) bent [4]-, and (f) circular [6 phenylene.

Figure 4.3. Phenylenes and topologically related PAHs (a) linear [/VJphenylenes and polyacenes (b) angular/zigzag [/VJphenylenes and polyphenanthrenes/helicenes (c) circular [6]phenylene sheet and graphite (d) archimedene (C120) and fullerene C o).

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