helicene

The systematic naming for this class of compounds is based on the number of rings: [n]helicene is the structure consisting of n rings. According to IUPAC, only structures where n is at least 5 are considered helicenes. Some specific compounds also have alternate or trivial names. As the number of rings increases, starting at four, the structure becomes non-planar, but instead the planes of consecutive rings tilt to prevent steric collisions. For helicenes with six benzene units, a 360° turn is completed. In the helicene series the dihedral angles between the extremities increases going from [4]helicene (26°) to [6]helicene (58°) and then decreases again for example in [7]helicene (30°).

Helicenes are notable for having chirality despite lacking both asymmetric carbons and chiral centers. Instead, there is axial chirality, which results from the handedness of the helicity itself. The clockwise and counterclockwise helices are non-superposable. By convention a left-handed helix is minus and labeled (M), a right-handed helix is plus and labeled (P). Evidence from CD spectroscopy suggests left-handed helices are levorotatory and right-handed helices are dextrorotatory.

The stability of the two complementary helical enantiomers with respect to interconversion and the mechanism by which they interconvert depend on n.{{cite journal |title= X-ray and Optical Circular Dichroism as Local and Global Ultrafast Chiral Probes of [12]Helicene Racemization |first1= Victor M. |last1= Freixas |first2= Jérémy R. |last2= Rouxel |first3= Yeonsig |last3= Nam |first4= Sergei |last4= Tretiak |first5= Niranjan |last5= Govind |first6= Shaul |last6= Mukamel |journal = J. Am. Chem. Soc. |year= 2023 |volume= 145 |issue= 38 |pages= 21012–21019 |doi= 10.1021/jacs.3c07032}}

The first helicene structure was reported by Jakob Meisenheimer in 1903 as the reduction product of 2-nitronaphthalene. [5]helicene was synthesized in 1918 by Weitzenböck & Klingler. 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. Since then, several methods for synthesizing helicenes with different lengths and substituents are used. The oxidative photocyclization of a stilbene-type precursor is used most often as the key step. The longest helicene prepared by this method is [16]helicene in 2015.{{cite journal |title= One-Step Synthesis of [16]Helicene |first1= Kazuyuki |last1= Mori |first2= Takashi |last2= Murase |first3= Makoto |last3= Fujita |journal= Angew. Chem. Int. Ed. |volume= 54 |issue= 23 |year= 2015 |pages= 6847-6851 |doi= 10.1002/anie.201502436 }}

In one study, [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:

Image:Helicene olefin Metathesis.png

Other approach is also non-photochemical and is based on assembly of biphenylyl-naphthalenes and their platinum-catalyzed double cycloisomerization leading to various [6]helicenes:

:File:Helicene Cycloisomerization.png

File:Tetrahelicene.jpg|[4]Helicene

File:Pentahelicene.jpg|[5]Helicene

File:Hexahelicene.jpg|[6]Helicene

File:Hexahelicene2.jpg|[6]Helicene, other chirality

File:Heptahelicene.jpg|[7]Helicene

File:Heptahelicene2.jpg|[7]Helicene, other chirality

File:Octahelicene.jpg|[8]Helicene

File:Nonahelicene.jpg|[9]Helicene

File:Decahelicene.jpg|[10]Helicene

File:Undecahelicene.jpg|[11]Helicene

File:Dodecahelicene.jpg|[12]Helicene

File:Tridecahelicene.jpg|[13]Helicene

File:Tetradecahelicene.jpg|[14]Helicene

File:Pentadecahelicene.jpg|[15]Helicene

File:Hexadecahelicene.jpg|[16]Helicene

File:Octadecahelicene.jpg|[18]Helicene

Helicenes have been studied with respect to nonlinear optics, CPL, organocatalysis, conformational analysis, chirality sensing, chemical sensors and hetero-atom substitution.

• Other configurations of consecutively-fused benzene rings:
• Acenes, linear
• Phenacenes, zig-zag
• Circulenes, closed ring

{{reflist|colwidth=30em

|refs=

Helicenes: Synthesis and Applications Yun Shen and Chuan-Feng Chen Chemical Reviews Article ASAP {{doi|10.1021/cr200087r}}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}}

Meisenheimer, J. and Witte, K. (1903), Reduction von 2-Nitronaphtalin. Berichte der deutschen chemischen Gesellschaft, 36: 4153–4164. {{doi|10.1002/cber.19030360481}}

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}}

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}}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}}

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}}

Synthesis of Hexahelicene and 1-Methoxyhexahelicene via Cycloisomerization of Biphenylyl-Naphthalene Derivatives. Storch J., Sýkora J., Čermák J., Karban J., Císařová I., Růžička A. J. Org. Chem. 2009, 74, 3090. {{doi|10.1021/jo900077j}}

One hundred years of helicene chemistry. Part 1: non-stereoselective syntheses of carbohelicenes Marc Gingras Chem. Soc. Rev., 2013,42, 968-1006 {{doi|10.1039/C2CS35154D}}

One hundred years of helicene chemistry. Part 2: stereoselective syntheses and chiral separations of carbohelicenes Marc Gingras, Guy Félix and Romain Peresutti Chem. Soc. Rev., 2013,42, 1007-1050 {{doi|10.1039/C2CS35111K}}

One hundred years of helicene chemistry. Part 3: applications and properties of carbohelicenes Marc Gingras Chem. Soc. Rev., 2013,42, 1051-1095 {{doi|10.1039/C2CS35134J}}

Martin, R. H. (1974), The Helicenes. Angew. Chem. Int. Ed. Engl., 13: 649–660. {{doi|10.1002/anie.197406491}}

Recent Development of Helicene Synthesis Ken Kamikawa Journal of Synthetic Organic Chemistry, Japan Vol. 72 (2014) No. 1 p. 58-67 {{doi|10.5059/yukigoseikyokaishi.72.58}}

Sultam-Based Hetero[5]helicene: Synthesis, Structure, and Crystallization-Induced Emission Enhancement. Tarunpreet S. Virk, Niranjan V. Ilawe, Guoxian Zhang, Craig P. Yu, Bryan M. Wong, Julian M. W. Chan. ACS Omega 2016; 1(6), 1336–1342 {{doi|10.1021/acsomega.6b00335}}

Helquat Dyes: Helicene-like Push–Pull Systems with Large Second-Order Nonlinear Optical Responses

Benjamin J. Coe, Daniela Rusanova, Vishwas D. Joshi, Sergio Sánchez, Jan Vávra, Dushant Khobragade, Lukáš Severa, Ivana Císařová, David Šaman, Radek Pohl, Koen Clays, Griet Depotter, Bruce S. Brunschwig, and Filip Teplý The Journal of Organic Chemistry 2016 81 (5), 1912-1920 {{doi|10.1021/acs.joc.5b02692}}

Synthetic Control of the Excited-State Dynamics and Circularly Polarized Luminescence of Fluorescent “Push–Pull” Tetrathia[9]helicenes Y. Yamamoto, H. Sakai, J. Yuasa, Y. Araki, T. Wada, T. Sakanoue, T. Takenobu, T. Kawai, T. Hasobe, Chem. Eur. J. 2016, 22, 4263. {{doi|10.1002/chem.201504048}}

Tetrathia[7]helicene Phosphorus Derivatives: Experimental and Theoretical Investigations of Electronic Properties, and Preliminary Applications as Organocatalysts D. Dova, L. Viglianti, P. R. Mussini, S. Prager, A. Dreuw, A. Voituriez, E. Licandro, S. Cauteruccio, Asian J. Org. Chem. 2016, 5, 537. {{doi|10.1002/ajoc.201600025}}

Synthesis and Structural Features of Quadruple Helicenes: Highly Distorted π Systems Enabled by Accumulation of Helical Repulsions Takao Fujikawa, Yasutomo Segawa, and Kenichiro Itami

Journal of the American Chemical Society 2016 138 (10), 3587-3595 {{doi|10.1021/jacs.6b01303}}

Controlled Excited-State Dynamics and Enhanced Fluorescence Property of Tetrasulfone[9]helicene by a Simple Synthetic Process Yuki Yamamoto, Hayato Sakai, Junpei Yuasa, Yasuyuki Araki, Takehiko Wada, Tomo Sakanoue, Taishi Takenobu, Tsuyoshi Kawai, and Taku Hasobe The Journal of Physical Chemistry C 2016 120 (13), 7421-7427 {{doi|10.1021/acs.jpcc.6b01123}}

Inherently Chiral Azonia[6]helicene-Modified β-Cyclodextrin: Synthesis, Characterization, and Chirality Sensing of Underivatized Amino Acids in Water Qinfei Huang, Liangwei Jiang, Wenting Liang, Jianchang Gui, Dingguo Xu, Wanhua Wu, Yoshito Nakai, Masaki Nishijima, Gaku Fukuhara, Tadashi Mori, Yoshihisa Inoue, and Cheng Yang The Journal of Organic Chemistry 2016 81 (8), 3430-3434 {{doi|10.1021/acs.joc.6b00130}}

Radical Cation and Neutral Radical of Aza-thia[7]helicene with SOMO–HOMO Energy Level Inversion Ying Wang, Hui Zhang, Maren Pink, Arnon Olankitwanit, Suchada Rajca, and Andrzej Rajca Journal of the American Chemical Society 2016 138 (23), 7298-7304 {{doi|10.1021/jacs.6b01498}}

Electrochemical Capacitive K⁺ EMIS Chemical Sensor Based on the Dibromoaza[7]helicene as an Ionophore for Potassium Ions Detection M. Tounsi, M. BenBraiek, A. Baraket, M. Lee, N. Zine, M. Zabala, J. Bausells, F. Aloui, B. BenHassine, A. Maaref, A. Errachid, Electroanalysis 2016, 28, 2892. {{doi|10.1002/elan.201600104}}

Synthesis and study of the structural properties of oxa[5]helicene derivatives M. Shyam Sundara, Sibaprasad Sahoob, Ashutosh V. Bedekara, Tetrahedron: Asymmetry Volume 27, Issue 16, 1 September 2016, Pages 777–781 {{doi|10.1016/j.tetasy.2016.06.020}}

Synthesis and Photophysical Properties of Aza[n]helicenes Gourav M. Upadhyay, Harish R. Talele, and Ashutosh V. Bedekar The Journal of Organic Chemistry 2016 81 (17), 7751-7759 {{doi| 10.1021/acs.joc.6b01395}}

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; General references

• {{cite book |title= Helicene Chemistry: From Synthesis to Applications |author1= Chuan-Feng |author2= Yun Shen |isbn= 978-3-662-53168-6 |doi= 10.1007/978-3-662-53168-6 |publisher= Springer |year= 2017 |s2cid= 199492403 }}

{{Commons category|Helicenes}}

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Category:Polycyclic aromatic hydrocarbons

Category:Geodesic polyarenes