Helicene

{{Short description|Class of chemical compounds}} right|thumb|150px|Hexahelicene [[File:Helicenegeneral.svg|right|thumb|150px|General structure formula of [''n'']helicene]] In organic chemistry, '''helicenes''' are ortho-condensed polycyclic aromatic compounds in which benzene rings or other aromatics are angularly annulated to give helically-shaped chiral molecules.<ref name=GoldBook>{{GoldBookRef|title=helicenes|file=H02762}}</ref> The chemistry of helicenes has attracted continuing attention because of their unique structural, spectral, and optical features.<ref name=martin /><ref name=shen /><ref name=zhigang /><ref name=gingras /><ref name=gingras2 /><ref name=gingras3 /><ref name=kamikawa />

==Structure and properties== 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.<ref name=GoldBook/> 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''.<ref>{{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 |pmid= 37704187 |bibcode= 2023JAChS.14521012F |osti= 2305719 }}</ref>

==Synthesis== The first helicene structure was reported by Jakob Meisenheimer in 1903 as the reduction product of 2-nitronaphthalene.<ref name=meisenheimer /> [5]helicene was synthesized in 1918 by Weitzenböck & Klingler.<ref name=weitzenbock /> 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.<ref name=lutz /><ref name=newman /> 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.<ref>{{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 |pmid= 25907312 |bibcode= 2015ACIE...54.6847M }}</ref>

In one study,<ref name=collins /> [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:

center|500px|Helicene synthesis by olefin metathesis

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:<ref name=storch />

:center|500px

== Applications == Helicenes have been studied with respect to nonlinear optics,<ref name=coe /> CPL,<ref name=yamamoto/><ref name=sakai /> organocatalysis,<ref name=dova/> conformational analysis,<ref name=fujikawa /> chirality sensing,<ref name=Huang /> chemical sensors<ref name=Tounsi/> and hetero-atom substitution.<ref name=Wang2016 /><ref name=sundara /><ref name=Upadhyay/><ref name=virk />

Cobalt-[9]helicene is one of a rare class of multidecker metallocene polymers in which the metal atoms are laterally displaced.<ref>{{Cite journal|journal=Chemical Reviews|year=1999|volume=99|title=Organometallic polymers with transition metals in the main chain|first=Paul|last=Nguyen|first2=Paloma|last2=Gómez-Elipe|first3=Ian|last3=Manners|orig-date=7 Dec 1998|doi=10.1021/cr960113u|publisher=American Chemical Society|pp=1526–1527}}</ref>

== See also == * Other configurations of consecutively-fused benzene rings: ** Acenes, linear ** Circulenes, closed ring ** Phenacenes, zig-zag

== References == {{reflist|colwidth=30em |refs= <ref name=shen>{{cite journal | last1=Shen | first1=Yun | last2=Chen | first2=Chuan-Feng | title=Helicenes: Synthesis and Applications | journal=Chemical Reviews | date=2012 | volume=112 | issue=3 | pages=1463–1535 | doi=10.1021/cr200087r | pmid=22017405 | bibcode=2012ChRv..112.1463S }}</ref><ref name=zhigang>{{cite journal | last1=Wang | first1=David Zhigang | last2=Katz | first2=Thomas J. | last3=Golen | first3=James | last4=Rheingold | first4=Arnold L. | title=Diels−Alder Additions of Benzynes within Helicene Skeletons | journal=The Journal of Organic Chemistry | date=2004 | volume=69 | issue=22 | pages=7769–7771 | doi=10.1021/jo048707h | pmid=15498014 }}</ref> <ref name=meisenheimer>{{cite journal | last1=Meisenheimer | first1=Jakob | last2=Witte | first2=Klaus | title=Reduction von 2-Nitronaphtalin | journal=Berichte der Deutschen Chemischen Gesellschaft | date=1903 | volume=36 | issue=4 | pages=4153–4164 | doi=10.1002/cber.19030360481 }}</ref> <ref name=weitzenbock>{{cite journal | last1=Weitzenböck | first1=Richard | last2=Klingler | first2=Albert | title=Synthese der isomeren Kohlenwasserstoffe 1, 2?5, 6-Dibenzanthracen und 3, 4?5, 6-Dibenzphenanthren | journal=Monatshefte für Chemie | date=1918 | volume=39 | issue=5 | pages=315–323 | doi=10.1007/BF01524529 }}</ref> <ref name=lutz>{{cite journal | last1=Newman | first1=Melvin S. | last2=Lutz | first2=Wilson B. | last3=Lednicer | first3=Daniel | title=A New Reagent for Resolution by Complex Formation: The Resolution of Phenanthro-[3,4-''c'']phenanthrene | journal=Journal of the American Chemical Society | date=1955 | volume=77 | issue=12 | pages=3420–3421 | doi=10.1021/ja01617a097 }}</ref><ref name=newman>{{cite journal | last1=Newman | first1=Melvin S. | last2=Lednicer | first2=Daniel | title=The Synthesis and Resolution of Hexahelicene | journal=Journal of the American Chemical Society | date=1956 | volume=78 | issue=18 | pages=4765–4770 | doi=10.1021/ja01599a060 | bibcode=1956JAChS..78.4765N }}</ref> <ref name=collins>{{cite journal | last1=Collins | first1=Shawn K. | last2=Grandbois | first2=Alain | last3=Vachon | first3=Martin P. | last4=Côté | first4=Julie | title=Preparation of Helicenes through Olefin Metathesis | journal=Angewandte Chemie International Edition | date=2006 | volume=45 | issue=18 | pages=2923–2926 | doi=10.1002/anie.200504150 | pmid=16568482 | bibcode=2006ACIE...45.2923C }}</ref> <ref name=storch>{{cite journal | last1=Storch | first1=Jan | last2=Sýkora | first2=Jan | last3=Čermák | first3=Jan | last4=Karban | first4=Jindřich | last5=Císařová | first5=Ivana | last6=Růžička | first6=Aleš | title=Synthesis of Hexahelicene and 1-Methoxyhexahelicene via Cycloisomerization of Biphenylyl-Naphthalene Derivatives | journal=The Journal of Organic Chemistry | date=2009 | volume=74 | issue=8 | pages=3090–3093 | doi=10.1021/jo900077j | pmid=19284794 }}</ref> <ref name=gingras>{{cite journal | last1=Gingras | first1=Marc | title=One hundred years of helicene chemistry. Part 1: Non-stereoselective syntheses of carbohelicenes | journal=Chem. Soc. Rev. | date=2013 | volume=42 | issue=3 | pages=968–1006 | doi=10.1039/C2CS35154D | pmid=23151799 | bibcode=2013CSRev..42..968G }}</ref> <ref name=gingras2>{{cite journal | last1=Gingras | first1=Marc | last2=Félix | first2=Guy | last3=Peresutti | first3=Romain | title=One hundred years of helicene chemistry. Part 2: Stereoselective syntheses and chiral separations of carbohelicenes | journal=Chem. Soc. Rev. | date=2013 | volume=42 | issue=3 | pages=1007–1050 | doi=10.1039/C2CS35111K | pmid=23151610 | bibcode=2013CSRev..42.1007G }}</ref> <ref name=gingras3>{{cite journal | last1=Gingras | first1=Marc | title=One hundred years of helicene chemistry. Part 3: Applications and properties of carbohelicenes | journal=Chem. Soc. Rev. | date=2013 | volume=42 | issue=3 | pages=1051–1095 | doi=10.1039/C2CS35134J | pmid=23151680 | bibcode=2013CSRev..42.1051G }}</ref> <ref name=martin>{{cite journal | last1=Martin | first1=Richard Henri | title=The Helicenes | journal=Angewandte Chemie International Edition in English | date=1974 | volume=13 | issue=10 | pages=649–660 | doi=10.1002/anie.197406491 }}</ref> <ref name=kamikawa>{{cite journal | last1=Kamikawa | first1=Ken | title=Recent Development of Helicene Synthesis | journal=Journal of Synthetic Organic Chemistry, Japan | date=2014 | volume=72 | pages=58–67 | doi=10.5059/yukigoseikyokaishi.72.58 }}</ref> <ref name=virk>{{cite journal | last1=Virk | first1=Tarunpreet Singh | last2=Ilawe | first2=Niranjan V. | last3=Zhang | first3=Guoxian | last4=Yu | first4=Craig P. | last5=Wong | first5=Bryan M. | last6=Chan | first6=Julian M. W. | title=Sultam-Based Hetero[5]helicene: Synthesis, Structure, and Crystallization-Induced Emission Enhancement | journal=ACS Omega | date=2016 | volume=1 | issue=6 | pages=1336–1342 | doi=10.1021/acsomega.6b00335 | doi-access=free | pmid=31457199 | pmc=6640820 }}</ref> <ref name=coe>{{cite journal | last1=Coe | first1=Benjamin J. | last2=Rusanova | first2=Daniela | last3=Joshi | first3=Vishwas D. | last4=Sánchez | first4=Sergio | last5=Vávra | first5=Jan | last6=Khobragade | first6=Dushant | last7=Severa | first7=Lukáš | last8=Císařová | first8=Ivana | last9=Šaman | first9=David | last10=Pohl | first10=Radek | last11=Clays | first11=Koen | last12=Depotter | first12=Griet | last13=Brunschwig | first13=Bruce S. | last14=Teplý | first14=Filip | title=Helquat Dyes: Helicene-like Push–Pull Systems with Large Second-Order Nonlinear Optical Responses | journal=The Journal of Organic Chemistry | date=2016 | volume=81 | issue=5 | pages=1912–1920 | doi=10.1021/acs.joc.5b02692 | pmid=26844587 }}</ref> <ref name=yamamoto>{{cite journal | last1=Yamamoto | first1=Yuki | last2=Sakai | first2=Hayato | last3=Yuasa | first3=Junpei | last4=Araki | first4=Yasuyuki | last5=Wada | first5=Takehiko | last6=Sakanoue | first6=Tomo | last7=Takenobu | first7=Taishi | last8=Kawai | first8=Tsuyoshi | last9=Hasobe | first9=Taku | title=Synthetic Control of the Excited-State Dynamics and Circularly Polarized Luminescence of Fluorescent "Push–Pull" Tetrathia[9]helicenes | journal=Chemistry – A European Journal | date=2016 | volume=22 | issue=12 | pages=4263–4273 | doi=10.1002/chem.201504048 | pmid=26863928 | bibcode=2016ChEuJ..22.4263Y }}</ref> <ref name=dova>{{cite journal | last1=Dova | first1=Davide | last2=Viglianti | first2=Lucia | last3=Mussini | first3=Patrizia R. | last4=Prager | first4=Stefan | last5=Dreuw | first5=Andreas | last6=Voituriez | first6=Arnaud | last7=Licandro | first7=Emanuela | last8=Cauteruccio | first8=Silvia | title=Tetrathia[7]helicene Phosphorus Derivatives: Experimental and Theoretical Investigations of Electronic Properties, and Preliminary Applications as Organocatalysts | journal=Asian Journal of Organic Chemistry | date=2016 | volume=5 | issue=4 | pages=537–549 | doi=10.1002/ajoc.201600025 }}</ref> <ref name=fujikawa>{{cite journal | last1=Fujikawa | first1=Takao | last2=Segawa | first2=Yasutomo | last3=Itami | first3=Kenichiro | title=Synthesis and Structural Features of Quadruple Helicenes: Highly Distorted π Systems Enabled by Accumulation of Helical Repulsions | journal=Journal of the American Chemical Society | date=2016 | volume=138 | issue=10 | pages=3587–3595 | doi=10.1021/jacs.6b01303 | pmid=26918641 | bibcode=2016JAChS.138.3587F }}</ref> <ref name=sakai>{{cite journal | last1=Yamamoto | first1=Yuki | last2=Sakai | first2=Hayato | last3=Yuasa | first3=Junpei | last4=Araki | first4=Yasuyuki | last5=Wada | first5=Takehiko | last6=Sakanoue | first6=Tomo | last7=Takenobu | first7=Taishi | last8=Kawai | first8=Tsuyoshi | last9=Hasobe | first9=Taku | title=Controlled Excited-State Dynamics and Enhanced Fluorescence Property of Tetrasulfone[9]helicene by a Simple Synthetic Process | journal=The Journal of Physical Chemistry C | date=2016 | volume=120 | issue=13 | pages=7421–7427 | doi=10.1021/acs.jpcc.6b01123 }}</ref> <ref name=Huang>{{cite journal | last1=Huang | first1=Qinfei | last2=Jiang | first2=Liangwei | last3=Liang | first3=Wenting | last4=Gui | first4=Jianchang | last5=Xu | first5=Dingguo | last6=Wu | first6=Wanhua | last7=Nakai | first7=Yoshito | last8=Nishijima | first8=Masaki | last9=Fukuhara | first9=Gaku | last10=Mori | first10=Tadashi | last11=Inoue | first11=Yoshihisa | last12=Yang | first12=Cheng | title=Inherently Chiral Azonia[6]helicene-Modified β-Cyclodextrin: Synthesis, Characterization, and Chirality Sensing of Underivatized Amino Acids in Water | journal=The Journal of Organic Chemistry | date=2016 | volume=81 | issue=8 | pages=3430–3434 | doi=10.1021/acs.joc.6b00130 | pmid=27002931 }}</ref> <ref name=Wang2016>{{cite journal | last1=Wang | first1=Ying | last2=Zhang | first2=Hui | last3=Pink | first3=Maren | last4=Olankitwanit | first4=Arnon | last5=Rajca | first5=Suchada | last6=Rajca | first6=Andrzej | title=Radical Cation and Neutral Radical of Aza-thia[7]helicene with SOMO–HOMO Energy Level Inversion | journal=Journal of the American Chemical Society | date=2016 | volume=138 | issue=23 | pages=7298–7304 | doi=10.1021/jacs.6b01498 | pmid=27219299 | bibcode=2016JAChS.138.7298W }}</ref> <ref name=Tounsi>{{cite journal | last1=Tounsi | first1=Moncef | last2=Ben Braiek | first2=Mourad | last3=Baraket | first3=Abdoullatif | last4=Lee | first4=Michael | last5=Zine | first5=Nadia | last6=Zabala | first6=Miguel | last7=Bausells | first7=Joan | last8=Aloui | first8=Faouzi | last9=Ben Hassine | first9=Béchir | last10=Maaref | first10=Abderrazak | last11=Errachid | first11=Abdelhamid | title=Electrochemical Capacitive K<sup>+</sup> EMIS Chemical Sensor Based on the Dibromoaza[7]helicene as an Ionophore for Potassium Ions Detection | journal=Electroanalysis | date=2016 | volume=28 | issue=12 | pages=2892–2899 | doi=10.1002/elan.201600104 | hdl=10261/168001 | hdl-access=free }}</ref> <ref name=sundara>{{cite journal | last1=Shyam Sundar | first1=M. | last2=Sahoo | first2=Sibaprasad | last3=Bedekar | first3=Ashutosh V. | title=Synthesis and study of the structural properties of oxa[5]helicene derivatives | journal=Tetrahedron: Asymmetry | date=2016 | volume=27 | issue=16 | pages=777–781 | doi=10.1016/j.tetasy.2016.06.020 }}</ref> <ref name=Upadhyay>''Synthesis and Photophysical Properties of Aza[n]helicenes'' Gourav M. 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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 }}

Category:Polycyclic aromatic hydrocarbons Category:Geodesic polyarenes