{{Short description|Class of chemical compounds}} right|400px|the first four radialenes'''{{chem name|[n]Radialenes}}''' are alicyclic organic compounds containing n cross-conjugated exocyclic double bonds.<ref name=patai/><ref name=IyodaReview /><ref name=gholami /><ref name=maas /> The double bonds are commonly alkene groups but those with a carbonyl (C=O) group are also called radialenes.<ref name=hyoda /> For some members the unsubstituted parent radialenes are elusive but many substituted derivatives are known.

Radialenes are related to open-chain dendralenes and also to compounds like butadiene and benzene which also consist of a ring of sp<sup>2</sup> hybridized carbon atoms.

Radialenes are investigated in organic chemistry for their unusual properties and reactivity but have not ventured outside the laboratory. Reported uses are as experimental building blocks for novel organic conductors and ferromagnets.<ref name=Patra /> The first radialene called ''hexaethylidencyclohexane'' was synthesised in 1961.<ref name=Hoppf />

==Conformation== [3] and [4]radialenes are expected to have a planar molecular geometry with all carbon atoms in the same plane. This is verified experimentally in ''hexamethyl[3]radialene'' and ''{{chem name|cyclobutanetetraonetetrakis(hydrazone)}}''. ''{{chem name|Decamethyl[5]radialene}}'' has a twist envelope geometry with C<sub>2</sub> symmetry while a chair conformation is calculated for [6]radialene and found experimentally for ''hexa-(ethylidene)cyclohexane''

Due to their specific pi-electron distributions, hydrocarbons such as perylene and triphenylene are not considered radialenes. One study <ref name=Patra /> describes a [6]radialene composed of thiophene units:<ref name=comment />

:400px|Planar radialene Patra 2007|class=skin-invert-image

This compound is reported as planar with D<sub>3h</sub> symmetry (X-ray diffraction) but not aromatic: the carbon-carbon bond lengths are unusually long (145 pm vs. 140 pm for benzene) and the calculated NICS value is close to zero.

An analogous [8]radialene which adopts a saddle conformation with D<sub>2d</sub> symmetry<ref>Zhao, C., Xu, L., Wang, Y., Li, C., & Wang, H. (2014). Recent Progress in the Synthesis and Application of Saddle-shaped Cyclooctatetrathiophenes and Their Derivatives. Chinese Journal of Chemistry, 33(1), 71&ndash;78. https://doi.org/10.1002/cjoc.201400524</ref> is an intermediate in the synthesis of sulflower (C<sub>8</sub>S<sub>8</sub>):

:400px|Sulflower synthesis|class=skin-invert-image

==Synthesis and properties== The parent [3], [4], [5] and [6]radialenes polymerize when in contact with oxygen.

===[3]Radialenes=== [3]Radialene or '''trimethylenecyclopropane''' was synthesised in 1965.<ref name=dorko /><ref name=hencher /><ref name=bally /> Reported derivatives are triquinocyclopropanes,<ref name=west /><ref name=zecher /><ref name=benham /> salts of trimethylenecyclopropane dianions,<ref name=fukunaga /> {{chem name|tris(thioxanthen-9-ylidene)cyclopropane}},<ref name=sugimoto /> {{chem name|tris(fluoren-9-ylidene)cyclopropane}}<ref name=iyoda /> and {{chem name|hexakis(trimethylsilylethynyl)[3]radialene}}.<ref name=lange /> Phosphorus derivatives (based on {{chem name|4,5,6-triphospha[3]radialene}}) have also been reported.<ref name=Miyake /><ref name=miyake2 /><ref name=miyahara /><ref name=sasamori2015 /> Phospharadialenes have been investigated as quantum efficiency improvers in solar cells <ref name=hopf2012 /> {{chem name|Hexakis[4-(diarylamino)phenyl][3]radialene}} derivatives have been investigated for their low oxidation potentials.<ref name=matsumoto />

===[4]Radialenes=== The unsubstituted [4]radialene has been prepared in an elimination reaction of ''{{chem name|cis,trans,cis-tetra(bromomethyl)cyclobutane}}'' with sodium methoxide in ethanol.<ref name=griffin />

[[Image:-4-radialeneSynthesis.png|400px|center|[4]radialene synthesis]]

Hydrogenation with platinum on carbon gives ''cis,cis,cis-tetramethylcyclobutane'' in accordance with the proposed structure.

===[5]Radialenes=== Successful low-temperature synthesis of the parent compound [5]radialene was reported in 2015.<ref name=mackay />

===[6]Radialenes=== The parent [6]radialene is unstable and polymerises immediately on formation. It has been synthesised from 1,5,9-cyclododecatriyne, 1,3,6-tri(chloromethyl)mesitylene and tricyclobutabenzene.<ref name=barkovich /><ref name=harruff /><ref name=barkovich2 /><ref name=schiess />

Only substituted [6]radialenes exist as stable compounds. Stable derivatives are the hexamethyl substituted,<ref name=Hoppf/><ref name=hoppf2 /> dodecamethyl substituted <ref name=iyoda2 /> and hexabromo substituted <ref name=stanger /> radialene.

A trisalkoxy-substituted radialene has also been reported,<ref name=shinozaki /> the central ring adopting a non-planar twist-boat conformation:

:Hexaradialene derivative Shinozaki 2010

==Uses== Radialenes have been researched as a potential way to access complex synthetic molecules <ref name=richthofen/><ref name=Januszewski/> and in polymer synthesis.<ref name=hollis /><ref name=hollis2 />

==References== {{reflist|colwidth=30em |refs= <ref name=patai>''The Chemistry of Dienes and Polyenes, Volume 1 The Chemistry of Dienes and Polyenes Volume 121'' Patai's Chemistry of Functional Groups Zvi Rappoport Ed. Wiley, 1997</ref> <ref name=hyoda>''Effect of Overcrowding in [n]Radialenes on the Synthesis of Bis[4]radialenes''Menahem Kaftory, Mark Botoshansky, Shunji Hyoda, Toshihiro Watanabe, and Fumio Toda J. Org. Chem.; '''1999'''; 64(7) pp 2287 - 2292; (Article) {{doi|10.1021/jo9818677}}</ref> <ref name=Patra>''Planar [6]Radialenes: Structure, Synthesis, and Aromaticity of Benzotriselenophene and Benzotrithiophene'' Asit Patra, Yair H. Wijsboom, Linda J. W. Shimon, and Michael Bendikov Angew. Chem. Int. Ed. '''2007''', 46, 8814 –8818 {{doi|10.1002/anie.200703123}}</ref> <ref name=Hoppf>{{Cite journal| doi = 10.1002/hlca.19610440206| title = Zur Kenntnis der Hexaalkylbenzole. 3. Mitteilung. Über einen neuen Kohlenwasserstoff C<sub>18</sub>H<sub>24</sub> (Hexaäthylidencyclohexan)| year = 1961| last1 = Hopff | first1 = H.| last2 = Wick | first2 = A. K.| journal = Helvetica Chimica Acta| volume = 44| issue = 2| pages = 380 }}</ref> <ref name=dorko>''The Preparation and Properties of Trimethylenecyclopropane'' Ernest A. Dorko Journal of the American Chemical Society 1965 87 (23), 5518-5520 {{doi|10.1021/ja00951a067}}</ref> <ref name=hencher>''The molecular structure of trimethylenecyclopropane'' E.A. Dorko, J.L. Hencher, S.H. Bauer Tetrahedron Volume 24, Issue 6, 1968, Pages 2425–2434 {{doi|10.1016/S0040-4020(01)82515-1}}</ref> <ref name=bally>Bally, T. and Haselbach, E. (1978), ''Tris (methylidene)-cyclopropane (“[3]Radialene”). Part 2. Electronic states of the molecular cation and revised UV.-absorption spectrum of the parent neutral. HCA'', 61: 754–761. {{doi|10.1002/hlca.19780610223}}</ref> <ref name=west>''Synthesis of a Triquinocyclopropane'' Robert. West and David C. Zecher Journal of the American Chemical Society 1967 89 (1), 152-153 {{doi|10.1021/ja00977a033}}</ref> <ref name=zecher>''Diarylquinocyclopropenes and triquinocyclopropanes'' Robert West and David C. Zecher Journal of the American Chemical Society 1970 92 (1), 155-161 {{doi|10.1021/ja00704a025}}</ref> <ref name=benham>''Polyanthraquinocyclopropanes, dianthraquinocyclopropanone, and dianthraquinoethylene. Synthesis and properties'' Judith L. Benham, Robert West, and John A. T. Norman Journal of the American Chemical Society 1980 102 (15), 5047-5053 {{doi|10.1021/ja00535a037}}</ref> <ref name=fukunaga>''Negatively substituted trimethylenecyclopropane dianions'' Tadamichi Fukunaga Journal of the American Chemical Society 1976 98 (2), 610-611 {{doi|10.1021/ja00418a050}}</ref> <ref name=iyoda>Iyoda, M., Otani, H. and Oda, M. (1988), ''Tris(fluoren-9-ylidene)cyclopropane, a Novel [3]Radialene''. Angew. Chem. Int. Ed. Engl., 27: 1080–1081. {{doi|10.1002/anie.198810801}}</ref> <ref name=mackay>''[5]Radialene'' Emily G. Mackay, Christopher G. Newton, Henry Toombs-Ruane, Erik Jan Lindeboom, Thomas Fallon, Anthony C. Willis, Michael N. Paddon-Row, and Michael S. Sherburn Journal of the American Chemical Society Article ASAP {{doi|10.1021/jacs.5b07445}}</ref> <ref name=sugimoto>Sugimoto, T., Misaki, Y., Kajita, T., Nagatomi, T., Yoshida, Z.-i. and Yamauchi, J. (1988), Tris(thioxanthen-9-ylidene)cyclopropane, and Its Radical Cation and Dication. Angew. Chem. Int. Ed. Engl., 27: 1078–1080. {{doi| 10.1002/anie.198810781}}</ref> <ref name=lange>Lange, T., Gramlich, V., Amrein, W., Diederich, F., Gross, M., Boudon, C. and Gisselbrecht, J.-P. (1995), ''Hexakis(trimethylsilylethynyl)[3]radialene: A Carbon-Rich Chromophore with Unusual Electronic Properties''. Angew. Chem. Int. Ed. Engl., 34: 805–809. {{doi|10.1002/anie.199508051}}</ref> <ref name=griffin>''The Chemistry of Photodimers of Maleic and Fumaric Acid Derivatives. IV.1 Tetramethylenecyclobutane'' Gary W. Griffin and Laurence I. Peterson J. Am. Chem. Soc.; '''1962'''; 84(17) pp 3398 - 3400; {{doi|10.1021/ja00876a033}}</ref> <ref name=barkovich>{{Cite journal| doi = 10.1021/ja00425a046| title = 1,5,9-Cyclododecatriyne. Synthesis and conversion to intermediate 1,2:3,4:5,6-tricyclobutabenzene| year = 1976| last1 = Barkovich | first1 = A. J.| last2 = Vollhardt | first2 = K. P. C.| journal = Journal of the American Chemical Society| volume = 98| issue = 9| pages = 2667 }}</ref> <ref name=harruff>{{Cite journal| doi = 10.1021/ja00477a055| title = Hexaradialene: precursors and structure| year = 1978| last1 = Harruff | first1 = L. G.| last2 = Brown | first2 = M.| last3 = Boekelheide | first3 = V.| journal = Journal of the American Chemical Society| volume = 100| issue = 9| pages = 2893 }}</ref> <ref name=barkovich2>{{Cite journal| doi = 10.1021/ja00467a036| title = Hexaradialene| year = 1977| last1 = Barkovich | first1 = A. J.| last2 = Strauss | first2 = E. S.| last3 = Vollhardt | first3 = K. P. C.| journal = Journal of the American Chemical Society| volume = 99| issue = 25| pages = 8321 }}</ref> <ref name=schiess>{{Cite journal| doi = 10.1002/hlca.19780610232| title = Hexakis (methylidene)-cyclohexane (?[6]Radialene?). Chemical and spectral properties| year = 1978| last1 = Schiess | first1 = P.| last2 = Heitzmann | first2 = M.| journal = Helvetica Chimica Acta| volume = 61| issue = 2| pages = 844 }}</ref> <ref name=hoppf2>{{Cite journal| doi = 10.1002/hlca.19610440104| title = Zur Kenntnis der Hexaalkylbenzole. 2. Mitteilung. Seitenkettenhalogenierungen von Hexaäthylbenzol| year = 1961| last1 = Hopff | first1 = H.| last2 = Wick | first2 = A. K.| journal = Helvetica Chimica Acta| volume = 44| issue = 1| pages = 19 }}</ref> <ref name=iyoda2>{{Cite journal| doi = 10.1021/ja00233a028| title = A new approach to the construction of radialenes by the nickel-catalyzed cyclooligomerization of [3]cumulenes (butatrienes)| year = 1988| last1 = Iyoda | first1 = M.| last2 = Tanaka | first2 = S.| last3 = Otani | first3 = H.| last4 = Nose | first4 = M.| last5 = Oda | first5 = M.| journal = Journal of the American Chemical Society| volume = 110| issue = 25| pages = 8494 }}</ref> <ref name=stanger>{{Cite journal| doi = 10.1002/chem.19970030207| title = Hexabromotricyclobutabenzene and Hexabromohexaradialene: Their Nickel-Mediated One-Pot Syntheses and Crystal Structure| year = 1997| last1 = Stanger | first1 = A.| last2 = Ashkenazi | first2 = N.| last3 = Boese | first3 = R.| last4 = Bläser | first4 = D.| last5 = Stellberg | first5 = P.| journal = Chemistry: A European Journal| volume = 3| issue = 2| pages = 208–211 | pmid = 24022949}}</ref> <ref name=shinozaki>{{Cite journal| doi = 10.1002/anie.200907305| pmid = 20333636| year = 2010| last1 = Shinozaki | first1 = S.| last2 = Hamura | first2 = T.| last3 = Ibusuki | first3 = Y.| last4 = Fujii | first4 = K.| last5 = Uekusa | first5 = H.| last6 = Suzuki | first6 = K.| title = Hexaradialenes by Successive Ring Openings of Tris(alkoxy-tricyclobutabenzenes): Synthesis and Characterization| journal = Angewandte Chemie International Edition in English| volume = 49| issue = 17| pages = 3026–3029 }}</ref> <ref name=comment>Coupling reaction reagent bis(1,5-cyclooctadiene)nickel(0). Equal amount of tetramer formed</ref> <ref name=richthofen>''Synthesis and Diels–Alder Reactions of a Benzo[5]radialene Derivative'' Andreas A. von Richthofen, Liliana Marzorati, Lucas C. Ducati, and Claudio Di Vitta Organic Letters 2014 16 (15), 4020-4023 {{doi|10.1021/ol5018432}}</ref> <ref name=Januszewski>Januszewski, J. A., Hampel, F., Neiss, C., Görling, A. and Tykwinski, R. R. (2014), ''Unexpected Formation of a [4]Radialene and Dendralenes by Addition of Tetracyanoethylene to a Tetraaryl[5]cumulene''. Angew. Chem. Int. Ed., 53: 3743–3747. {{doi|10.1002/anie.201309355}}</ref> <ref name=Miyake>Miyake, H., Sasamori, T. and Tokitoh, N. (2012), ''Synthesis and Properties of 4,5,6-Triphospha[3]radialene''. Angew. Chem. Int. Ed., 51: 3458–3461. {{doi|10.1002/anie.201200374}}</ref> <ref name=miyake2>''The 4,5,6-triphospha[3]radialene dianion: a phosphorus analogue of the deltate dianion. A NICS(0)πzz examination of their aromaticity'' Hideaki Miyake, Takahiro Sasamori, Judy I-Chia Wu, Paul v. R. Schleyer and Norihiro Tokitoh Chem. Commun., 2012,48, 11440-11442 {{doi|10.1039/C2CC35978B}}</ref> <ref name=miyahara>''The first x-ray analysis of a phospha[3]radialene'', Polyhedron Volume 11, Issue 3, 1992, Pages 385-387 Ikuko Miyahara, Atsuhiro Hayashi, Ken HirotsuMasaaki Yoshifuji, Hideki Yoshimura, Kozo Toyota {{doi|10.1016/S0277-5387(00)83187-3}}</ref> <ref name=hollis>''2-D Coordination Polymers of Hexa(4-cyanophenyl)[3]-radialene and Silver(I): Anion···π-Interactions and Radialene C−H···Anion Hydrogen Bonds in the Solid-State Interactions of Hexaaryl[3]-radialenes with Anions'' Courtney A. Hollis, Lyall R. Hanton, Jonathan C. Morris, and Christopher J. Sumby Crystal Growth & Design 2009 9 (6), 2911-2916 {{doi|10.1021/cg9002302}}</ref> <ref name=hollis2>''Two-Dimensional and Three-Dimensional Coordination Polymers of Hexakis(4-cyanophenyl)[3]radialene: The Role of Stoichiometry and Kinetics'' Courtney A. Hollis, Stuart R. Batten, and Christopher J. Sumby Crystal Growth & Design 2013 13 (6), 2350-2361 {{doi|10.1021/cg400036x}}</ref> <ref name=IyodaReview>Iyoda, M., Yamakawa, J. and Rahman, M. J. (2011), ''Conjugated Macrocycles: Concepts and Applications''. Angew. Chem. Int. Ed., 50: 10522–10553. {{doi|10.1002/anie.201006198}}</ref> <ref name=matsumoto>''Triarylamines on [3]Radialene Scaffold: Novel [3]Radialene-based, Multistep, Wide-range Redox Systems with Remarkably Low Oxidation Potentials'' Kouzou Matsumoto, Nao Yamada, Tetsuya Enomoto, Hiroyuki Kurata, Takeshi Kawase, Masaji Oda Chemistry Letters Vol. 40 (2011) No. 9 P 1033-1035 {{doi|10.1246/cl.2011.1033}}</ref> <ref name=hopf2012>Hopf, H. (2012), ''Phospharadialenes—A New Kid in Town''. Angew. Chem. Int. Ed., 51: 11945–11947. {{doi|10.1002/anie.201206101}}</ref> <ref name=gholami>''Oligomeric and Polymeric Systems with a Cross-conjugated π-Framework'' Mojtaba Gholami and Rik R. Tykwinski Chemical Reviews 2006 106 (12), 4997-5027 {{doi|10.1021/cr0505573}}</ref> <ref name=maas>Hopf, H. and Maas, G. (1992), ''Preparation and Properties, Reactions, and Applications of Radialenes''. Angew. Chem. Int. Ed. Engl., 31: 931–954. {{doi|10.1002/anie.199209313}}</ref> <ref name=sasamori2015>''Photochemical (E)–(Z) Isomerization of the P=C Double Bond in Triphospha[3]radialene–[M(CO)5] (M = W, Cr) Complexes'' Takahiro Sasamori, Koki Hirano, Hideaki Miyake, Norihiro Tokitoh Chemistry Letters Vol. 44 (2015) No. 9 P 1240-1242 {{doi|10.1246/cl.150422}}</ref> }}

Category:Vinylidene compounds Category:Hydrocarbons Category:Cyclic compounds