Helicene 2192951 220259042 2008-06-19T00:27:40Z SmackBot 433328 Date the maintenance tags or general fixes [[Image:Hexahelicene-3D-vdW.png|right|150px|Hexahelicene]] '''Helicenes''' in [[organic chemistry]] are [[aromatic ortho substituent|ortho-condensed]] [[polycyclic]] [[Aromaticity|aromatic compounds]] in which [[Benzene|benzene rings]] or other aromatic compounds are angularly [[annulation|annulated]] so as to give [[helix|helically]] shaped [[molecules]]. The chemistry of helicenes has attracted continuing attention because of their unique structural, [[spectral]], and [[optical]] features <ref>''Diels-Alder Additions of Benzynes within Helicene Skeletons'' David Zhigang Wang, Thomas J. Katz, James Golen, and Arnold L. Rheingold J. Org. Chem.; '''2004'''; 69(22) pp 7769 - 7771 [http://pubs3.acs.org/acs/journals/doilookup?in_doi=10.1021/jo048707h DOI Graphical Abstract]</ref>. Helicenes are notable for having [[Chirality (chemistry)|chirality]] while lacking both [[tetrahedral molecular geometry|asymmetric carbons]] and [[chiral center]]s. Helicenes' chirality results from the fact that clockwise and counterclockwise [[helix|helices]] are non-[[superimposition|superimposable]]. ==Background== [[Image:-6-helicene.svg|right|100px|[6]helicene]] The first [6]helicene (also called ''hexahelicene'') was [[synthesized]] by means of a synthetic scheme that closed the two central rings by [[Friedel-Crafts reaction|Friedel-Crafts cyclization]] of [[carboxylic acid]] compounds by M. S. Newman and D. Lednicer in 1956.{{Fact|date=June 2008}} Since then, several methods for synthesizing helicene have been reported. Today, the synthesis of helicenes with different lengths and [[substituent]]s is possible. The oxidative [[Electrocyclic reaction|photocyclization]] of a [[stilbenes]]-type [[wiktionary:Precursor|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 <ref>''Preparation of Helicenes through Olefin Metathesis '' Shawn K. Collins, Alain Grandbois, Martin P. Vachon, Julie Côté [[Angewandte Chemie International Edition]] Volume 45, Issue 18 , Pages 2923 - 2926 '''2006''' [http://dx.doi.org/10.1002/anie.200504150 Abstract]</ref> [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]]: [[Image:Helicene olefin Metathesis.png|center|500px|Helicene synthesis by olefin metathesis]] [[Polyacene]]s are the linear 1,3-fused or meta-[[analog (chemistry)|analogue]]s of helicenes. ==Circulenes== [[Image:-7-circulene.svg|right|100px|[7]circulene]]Closed rings consisting of [[benzene]]s are called '''circulenes'''. [5]circulene or [[corannulene]], [6]circulene or [[coronene]] and [7]circulene <ref>''Extended systems of closed helicene. Synthesis and characterization of [7] and [7.7]-circulene'' Koji Yamamoto Pure & Appl. Chern., Vol. 65, No. 1, pp. 157-163, '''1993'''. [http://www.iupac.org/publications/pac/1993/pdf/6501x0157.pdf Online article]</ref> have been synthesized in the laboratory. These compounds belong to are larger class of [[geodesic polyarene]]s. Whereas the 5 and 6 [[Homologous series|homologue]]s are bowl-shaped molecules, [7]circulene has a unique [[saddle]]-shaped structure (compare to cone and partial cones in [[calixarene]]s). ===Sulflower=== A stable octacirculene based on [[thiophene]] not containing any hydrogen has been reported with the name '''sulflower''' ([[sunflower]] plus [[sulfur]]). With [[molecular formula]] (C<sub>2</sub>S)<sub>8</sub> the compound is considered an analogue of [[carbon sulfide]] <ref>''Communication Sulflower: A New Form of Carbon Sulfide'' Konstantin Yu. Chernichenko, Viktor V. Sumerin, Roman V. Shpanchenko, Elizabeth S. Balenkova, Valentine G. Nenajdenko [[Angewandte Chemie International Edition]] Volume 45, Issue 44 , Pages 7367 - 7370 '''2006''' {{DOI|10.1002/anie.200602190}}</ref> : [[Image:SulflowerSynthesis.png|center|500px|Sulflower synthesis]] Its synthesis (a variation of the [[Ferrario 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. ==External links== * [http://www.npi.gov.au/database/substance-info/profiles/74.html National Pollutant Inventory - Polycyclic Aromatic Hydrocarbon Fact Sheet] == References == {{Reflist}} [[Category:Polycyclic aromatic hydrocarbons]] [[Category:geodesic polyarenes]]