Aromatic Helices Helicenes

Helicenes are benzologues of phenanthrene in which a regular cylindrical helix is formed through an all-ortho annellation of the aromatic rings. The helical structure is a consequence of the repulsive steric overlap of the terminal aromatic nuclei. 

In 1956, M. S. Newman et al. successfully synthesized the first helicene, hexahelicene (91), in a direct way. In the same pioneering paper, they also first described the synthesis of a-2,4,5,7-tetranitro-9-fluorenylidenaminooxy-propionic acid (TAPA, Newman s reagent, 772), which proved to be a Ji-acceptor suitable for enantiomer separation of ji-donors like hexahelicene. 

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The main aspects of the chemical reactivity of helicenes (e.g. electrophilic substitution) equally not deviate from those of planar aromatic compounds, and remarkable reactions of helicenes, which are incidentally found (e.g. the transannular bond formation between a C(l)-substituent and a part of the inner helix) can ultimately be reduced to known principles of aromatic reactivity. 

In l,16-indeno[5,4-c 4, 5 -g]phenanthrene 101) the terminal aromatic rings are spread away from each other, with an interplanar angle of 69.1°. The torsion angle between C(12)-C(ll)-C(ll )-C(12 ) (40.8°) is wide compared to [7]helicene (92) (25°). The strain, in contrast to tribenzo[7]helicene (95) and l,16-dimethyl[6]helicene 100), seems mainly to be localized in the inner part of the helix. 

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