Helicene

Helicenes in organic chemistry are ortho-condensed polycyclic aromatic compounds in which benzene rings or other aromatic compounds are angularly annulated so as to give helically shaped molecules. The chemistry of helicenes has attracted continuing attention because of their unique structural, spectral, and optical features.

Helicenes are notable for having chirality while lacking both asymmetric carbons and chiral centers. Helicenes' chirality results from the fact that clockwise and counterclockwise helices are non-superimposable.

Background
The first [6]helicene (also called hexahelicene) was synthesized by means of a synthetic scheme that closed the two central rings by Friedel-Crafts cyclization of carboxylic acid compounds by M. S. Newman and D. Lednicer in 1956. Since then, several methods for synthesizing helicene have been reported. Today, the synthesis of helicenes with different lengths and substituents is possible. The oxidative photocyclization of a stilbenes-type precursor is used most often as the key step. The longest helicene, [14]helicene, was prepared in 1975 by utilizing this method.

In one study [5]helicene was synthesized in an olefin metathesis reaction of a divinyl compound (prepared from 1,1'-bi-2-naphthol in several steps), with Grubb's second generation catalyst:



Polyacenes are the linear 1,3-fused or meta-analogues of helicenes.

Circulenes
Closed rings consisting of benzenes are called circulenes. [5]circulene or corannulene, [6]circulene or coronene and [7]circulene have been synthesized in the laboratory. These compounds belong to are larger class of geodesic polyarenes. Whereas the 5 and 6 homologues are bowl-shaped molecules, [7]circulene has a unique saddle-shaped structure (compare to cone and partial cones in calixarenes).

Sulflower
A stable octacirculene based on thiophene not containing any hydrogen has been reported with the name sulflower (sunflower plus sulfur). With molecular formula (C2S)8 the compound is considered an analogue of carbon sulfide :



Its synthesis (a variation of the Ferrario reaction reaction) is based on deprotonation of a tetrathiophene with lithium diisopropylamide followed by reaction with elemental sulfur to a sulfur substituted intermediate followed by vacuum pyrolysis. The molecule is flat and together with the 9-membered homologue at a local strain energy minimum.