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 ^[1].

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.

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Background

In one study ^[2] [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

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 ^[4] :

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.

References