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[[File:Hexahelicene-from-xtal-3D-vdW.png|right|150px|Hexahelicene]]
[[File:Hexahelicene-from-xtal-3D-vdW.png|right|150px|Hexahelicene]]
'''Helicenes''' in [[organic chemistry]] are [[aromatic ortho substituent|ortho-condensed]] [[Polycyclic compound|polycyclic]] [[Aromaticity|aromatic compounds]] in which [[Benzene|benzene rings]] or other aromatics are angularly [[annulation|annulated]] to give [[helix|helically]]-shaped [[molecules]]. The chemistry of helicenes has attracted continuing attention because of their unique structural, [[Spectroscopy|spectral]], and [[Optics|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''' in [[organic chemistry]] are [[aromatic ortho substituent|ortho-condensed]] [[Polycyclic compound|polycyclic]] [[Aromaticity|aromatic compounds]] in which [[Benzene|benzene rings]] or other aromatics are angularly [[annulation|annulated]] to give [[helix|helically]]-shaped [[molecules]]. The chemistry of helicenes has attracted continuing attention because of their unique structural, [[Spectroscopy|spectral]], and [[Optics|optical]] features.<ref>''Helicenes: Synthesis and Applications'' Yun Shen and Chuan-Feng Chen Chemical Reviews Article ASAP {{DOI|10.1021/cr200087r}}</ref> <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 {{doi|10.1021/jo048707h}}</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 helices are non-[[superimposition|superimposable]] – this is an example of [[axial chirality]].
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 helices are non-[[superimposition|superimposable]] – this is an example of [[axial chirality]].


==Background==
==Background==
[[Image:-6-helicene.svg|right|100px|[6]helicene]]
⚫
[[Image:-6-helicene.svg|right|100px|[6]helicene]] The first [6]helicene (also called ''hexahelicene'') was [[Chemical synthesis|synthesized]] by [[Melvin Spencer Newman|M. S. Newman]] and D. Lednicer in 1956 via a scheme that closed the two central rings by [[Friedel-Crafts reaction|Friedel-Crafts cyclization]] of [[carboxylic acid]] compounds.<ref>Newman, M. S.; Lednicer, D. ''J. Am. Chem. Soc.'', '''1956''', ''78'', 4765-4770</ref> Since then, several methods for synthesizing helicenes have been reported. Today, the synthesis of helicenes with different lengths and [[substituent]]s is possible. The oxidative [[Electrocyclic reaction|photocyclization]] of a [[stilbene]]-type [[Precursor (chemistry)|precursor]] is used most often as the key step. The longest helicene, [14]helicene, was prepared in 1975 by this method.
The first helicene structure was reported by [[Jakob Meisenheimer]] in 1903 as the reduction product of [[2-nitronaphtalene]] <ref>Meisenheimer, J. and Witte, K. (1903), ''Reduction von 2-Nitronaphtalin''. Berichte der deutschen chemischen Gesellschaft, 36: 4153–4164. {{doi|10.1002/cber.19030360481}}</ref>. [5]helicene was synthesised in 1918 by Weitzenböck & Klingler. <ref>''Synthese der isomeren Kohlenwasserstoffe 1, 2–5, 6-Dibenzanthracen und 3, 4–5, 6-Dibenzphenanthren'' Richard Weitzenböck and Albert Klingler Monatshefte für Chemie / Chemical Monthly Volume 39, Number 5, 315-323, {{DOI|10.1007/BF01524529}} </ref> The first [6]helicene (also called ''hexahelicene'') was [[Chemical synthesis|synthesized]] by [[Melvin Spencer Newman|M. S. Newman]] and D. Lednicer in 1955 via a scheme that closed the two central rings by [[Friedel-Crafts reaction|Friedel-Crafts cyclization]] of [[carboxylic acid]] compounds. <ref> A NEW REAGENT FOR RESOLUTION BY COMPLEX FORMATION; THE RESOLUTION OF PHENANTHRO-[3,4-c]PHENANTHRENE Melvin S. Newman, Wilson B. Lutz, and Daniel Lednicer Journal of the American Chemical Society 1955 77 (12), 3420-3421 {{DOI|10.1021/ja01617a097}}</ref> <ref> ''The Synthesis and Resolution of Hexahelicene'' Melvin S. Newman and Daniel Lednicer Journal of the American Chemical Society 1956 78 (18), 4765-4770
⚫
{{DOI|10.1021/ja01599a060}}</ref> Since then, several methods for synthesizing helicenes have been reported. Today, the synthesis of helicenes with different lengths and [[substituent]]s is possible. The oxidative [[Electrocyclic reaction|photocyclization]] of a [[stilbene]]-type [[Precursor (chemistry)|precursor]] is used most often as the key step. The longest helicene, [14]helicene, was prepared in 1975 by 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]] (BINOL) in several steps), with [[Grubbs' catalyst|Grubbs' second generation catalyst]]:
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''' {{doi|10.1002/anie.200504150}}</ref> [5]helicene was synthesized in an [[olefin metathesis]] reaction of a divinyl compound (prepared from [[1,1'-bi-2-naphthol]] (BINOL) in several steps), with [[Grubbs' catalyst|Grubbs' second generation catalyst]]:


[[Image:Helicene olefin Metathesis.png|center|500px|Helicene synthesis by olefin metathesis]]
[[Image:Helicene olefin Metathesis.png|center|500px|Helicene synthesis by olefin metathesis]]

Revision as of 18:36, 29 December 2011

Hexahelicene
Hexahelicene

Helicenes in organic chemistry are ortho-condensed polycyclic aromatic compounds in which benzene rings or other aromatics are angularly annulated to give helically-shaped molecules. The chemistry of helicenes has attracted continuing attention because of their unique structural, spectral, and optical features.[1] [2]

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 – this is an example of axial chirality.

Background

[6]helicene
[6]helicene

The first helicene structure was reported by Jakob Meisenheimer in 1903 as the reduction product of 2-nitronaphtalene [3]. [5]helicene was synthesised in 1918 by Weitzenböck & Klingler. [4] The first [6]helicene (also called hexahelicene) was synthesized by M. S. Newman and D. Lednicer in 1955 via a scheme that closed the two central rings by Friedel-Crafts cyclization of carboxylic acid compounds. [5] [6] Since then, several methods for synthesizing helicenes have been reported. Today, the synthesis of helicenes with different lengths and substituents is possible. The oxidative photocyclization of a stilbene-type precursor is used most often as the key step. The longest helicene, [14]helicene, was prepared in 1975 by this method.

In one study,[7] [5]helicene was synthesized in an olefin metathesis reaction of a divinyl compound (prepared from 1,1'-bi-2-naphthol (BINOL) in several steps), with Grubbs' second generation catalyst:

Helicene synthesis by olefin metathesis
Helicene synthesis by olefin metathesis

Polyacenes are the linear 1,3-fused or meta-analogues of helicenes. Conceptually related compounds are the circulenes.

External links

Wikimedia Commons has media related to Helicenes.

References

  1. ^ Helicenes: Synthesis and Applications Yun Shen and Chuan-Feng Chen Chemical Reviews Article ASAP doi:10.1021/cr200087r
  2. ^ 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 doi:10.1021/jo048707h
  3. ^ Meisenheimer, J. and Witte, K. (1903), Reduction von 2-Nitronaphtalin. Berichte der deutschen chemischen Gesellschaft, 36: 4153–4164. doi:10.1002/cber.19030360481
  4. ^ Synthese der isomeren Kohlenwasserstoffe 1, 2–5, 6-Dibenzanthracen und 3, 4–5, 6-Dibenzphenanthren Richard Weitzenböck and Albert Klingler Monatshefte für Chemie / Chemical Monthly Volume 39, Number 5, 315-323, doi:10.1007/BF01524529
  5. ^ A NEW REAGENT FOR RESOLUTION BY COMPLEX FORMATION; THE RESOLUTION OF PHENANTHRO-[3,4-c]PHENANTHRENE Melvin S. Newman, Wilson B. Lutz, and Daniel Lednicer Journal of the American Chemical Society 1955 77 (12), 3420-3421 doi:10.1021/ja01617a097
  6. ^ The Synthesis and Resolution of Hexahelicene Melvin S. Newman and Daniel Lednicer Journal of the American Chemical Society 1956 78 (18), 4765-4770 doi:10.1021/ja01599a060
  7. ^ 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 doi:10.1002/anie.200504150
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