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Cubane
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==Reactions and derivatives== Cubane is highly strained, but cannot decompose because [[cubene]] molecules are [[pyramidal alkene]]s, too high-energy for most [[elimination reaction|elimination pathways]]. Certain metals [[metal-ion-catalyzed Ο-bond rearrangement|catalyze Ο-bond rearrangement]] to [[cuneane]]:<ref name=March /><ref name=kindler /> :[[File:Cuban_zu_Cunean.svg|176x176px|class=skin-invert]] With a [[rhodium]] catalyst, cubane first forms ''syn''-tricyclooctadiene, which can thermally decompose to [[cyclooctatetraene]] at 50β60 Β°C.<ref>{{Cite journal |last1=Cassar |first1=Luigi |last2=Eaton |first2=Philip E. |last3=Halpern |first3=Jack |date=1970 |title=Catalysis of symmetry-restricted reactions by transition metal compounds. Valence isomerization of cubane |url=https://pubs.acs.org/doi/abs/10.1021/ja00714a075 |journal=Journal of the American Chemical Society |language=en |volume=92 |issue=11 |pages=3515β3518 |doi=10.1021/ja00714a075 |bibcode=1970JAChS..92.3515C |issn=0002-7863|url-access=subscription }}</ref> :[[File:Cubane_to_cyclooctatetraene.svg|400x400px|class=skin-invert]] The main cubane functionalization challenge is [[C-H bond activation]]. Cubenes still inhibit decomposition during [[radical substitution]], but the reaction offers little control against oversubstitution. In polar reactions, cubane reacts somewhat similarly to [[arene]]s or [[PSEPT|other cluster compounds]]: it [[metalation|metallates]] easily.<ref name=ReactivitySurvey>{{cite website|website=Cubane|first=B.|last=Muir|publisher=[[Imperial College London]]|title=Reactivity|url=https://www.ch.ic.ac.uk/local/projects/b_muir/Cubane/Cubanepro/Reactivity.html|access-date=22 May 2025|url-status=live|archive-url=https://web.archive.org/web/20240119125713/https://www.ch.ic.ac.uk/local/projects/b_muir/Cubane/Cubanepro/Reactivity.html|archive-date=19 Jan 2024}}</ref> Cubane is slightly [[carbon acid|acidic]], deprotonating about 63000 times faster than [[cyclohexane]].<ref>{{cite website|website=Cubane|first=B.|last=Muir|publisher=[[Imperial College London]]|title=Properties|at=The nature of the C–H bond|url=https://www.ch.ic.ac.uk/local/projects/b_muir/Cubane/Cubanepro/Properties.html|access-date=22 May 2025|url-status=live|archive-url=https://web.archive.org/web/20240119125716/https://www.ch.ic.ac.uk/local/projects/b_muir/Cubane/Cubanepro/Properties.html|archive-date=19 Jan 2024}}</ref> Cubane substituents display normal reactivity. For example a [[Curtius rearrangement]] followed by [[organic oxidation]] converts {{chem name|cubane tetra(carbonylchloride)}} to [[tetranitrocubane]].<ref name=ReactivitySurvey/> However, [[electron-rich]] substituents such as [[alcohol (chemistry)|alcohols]] can enable decomposition; they stabilize the cubene intermediate as a [[keto-enol tautomerism|ketone (or equivalent) tautomer]].<ref name=MMisc/> [[Hypercubane]] was predicted to exist in a 2014 publication.<ref>{{cite journal|last=Pichierri|first=F.|journal=Chem. Phys. Lett.|date=2014|volume=612|pages=198β202|doi=10.1016/j.cplett.2014.08.032|title= Hypercubane: DFT-based prediction of an ''O<sub>h</sub>''-symmetric double-shell hydrocarbon|bibcode=2014CPL...612..198P}}</ref><ref>{{Cite web | url=http://www.compchemhighlights.org/2014/12/hypercubane-dft-based-prediction-of-oh.html |title = Hypercubane: DFT-based prediction of an Oh-symmetric double-shell hydrocarbon}}</ref> ===Persubstituted derivatives=== Octaphenylcubane pre-dates the parent compound. Freedman synthesized it from [[tetraphenylcyclobutadiene nickel bromide]] in 1962. It is a sparingly soluble colourless compound that melts at 425β427 Β°C.<ref name= Biegasiewicz /><ref name=freedman1961 /><ref name=freedman1962 /><ref name=freedman1965 /> [[Octanitrocubane]] is a [[green explosive]]. Both [[heptafluorocubane]] and [[octafluorocubane]] were synthesized in 2022 to study octafluorocubane's unusual [[electronic structure]].<ref>{{cite journal |vauthors=Sugiyama M, Akiyama M, Yonezawa Y, Komaguchi K, Higashi M, Nozaki K, Okazoe T |date=August 2022 |title=Electron in a cube: Synthesis and characterization of perfluorocubane as an electron acceptor |journal=Science |volume=377 |issue=6607 |pages=756β759 |doi=10.1126/science.abq0516 |pmid=35951682 |bibcode=2022Sci...377..756S |s2cid=251515925}}</ref> Single-electron reduction to the [[radical anion]] {{chem|C|8|F|8|-}} traps<ref>Pichierri, F. Substituent effects in cubane and hypercubane: a DFT and QTAIM study. ''Theor Chem Acc'' 2017; 136: 114. {{doi|10.1007/s00214-017-2144-5}}</ref> an otherwise-free electron inside the cube, making it the world's smallest box.<ref>{{cite journal |vauthors=Krafft MP, Riess JG |date=August 2022 |title=Perfluorocubane-a tiny electron guzzler |journal=Science |volume=377 |issue=6607 |pages=709 |doi=10.1126/science.adc9195 |pmid=35951708 |bibcode=2022Sci...377..709K |s2cid=251517529|url=https://hal.science/hal-03873082 }}</ref> ===Cubenes and ''poly''-cubylcubane=== Despite their orbital strain, two cubenes have been synthesized, and a third analyzed [[computational chemistry|computationally]]. ''ortho''-{{chem name|Cubene}}, produced via [[lithium-halogen exchange]] followed by elimination,<ref name=MMisc>{{cite website|website=Cubane|first=B.|last=Muir|publisher=[[Imperial College London]]|title=Further topics|url=https://www.ch.ic.ac.uk/local/projects/b_muir/Cubane/Cubanepro/FurtherTopics.html|access-date=22 May 2025|url-status=live|archive-url=http://web.archive.org/web/20240119125713/https://www.ch.ic.ac.uk/local/projects/b_muir/Cubane/Cubanepro/FurtherTopics.html|archive-date=19 Jan 2024}}</ref> was the most pyramidalized alkene ever made at the time of its synthesis;<ref>{{cite journal |title= Cubene (1,2-dehydrocubane) |first1= Philip E. |last1= Eaton |first2= Michele |last2= Maggini |journal= J. Am. Chem. Soc. |year= 1988 |volume= 110 |issue= 21 |pages= 7230β7232 |doi= 10.1021/ja00229a057 |bibcode= 1988JAChS.110.7230E }}</ref> ''meta''-{{chem name|cubene}} is even less stable, and ''para''-{{chem name|cubene}} probably only exists as a [[diradical]] rather than an actual diagonal bond.<ref>{{cite book |title= Strained Hydrocarbons |url= https://archive.org/details/strainedhydrocar00hypo_746 |url-access= limited |editor-first= Helena |editor-last= Dodziuk |chapter= 2.3 A Theoretical Approach to the Study and Design of Prismane Systems |first1= Ruslan M. |last1= Minyaev |first2= Vladimir I. |last2= Minkin |first3= Tatyana N. |last3= Gribanova |publisher= Wiley |year= 2009 |isbn= 9783527627141 |page=55}}</ref> They rapidly undergo [[nucleophilic addition]].<ref name=Cubenes/> Decomposition of cubenes has enabled chemists to synthesize cubylcubane, as well as higher oligomers.<ref name=Cubenes>{{cite journal |last1=Eaton |first1=Philip E. |title=Cubanes: Starting Materials for the Chemistry of the 1990s and the New Century |journal=Angewandte Chemie International Edition in English |date=1992 |volume=31 |issue=11 |pages=1421β1436 |doi=10.1002/anie.199214211 |language=en |issn=1521-3773}}</ref> Per [[X-ray diffraction]], the central cubane-cubane bond is exceedingly short (1.458 Γ ), much shorter than the typical C-C single bond (1.578 Γ ). This is attributed to the fact that the exocyclic orbitals of cubane are [[s orbital|''s''-rich]] and close to the nucleus.<ref>{{cite journal |last1=Gilardi |first1=Richard. |last2=Maggini |first2=Michele. |last3=Eaton |first3=Philip E. |title=X-ray structures of cubylcubane and 2-tert-butylcubylcubane: short cage-cage bonds |journal=Journal of the American Chemical Society |date=1 October 1988 |volume=110 |issue=21 |pages=7232β7234 |doi=10.1021/ja00229a058 |bibcode=1988JAChS.110.7232G |issn=0002-7863}}</ref> The ''oligo''-cubylcubanes are rigid molecular rods considered for [[liquid crystal]] design, but scarcely accessible through conventional [[organic synthesis]]. Absent solubizing groups on the cubane [[monomer]], oligomers with at least 4 units are essentially insoluble.<ref>{{cite website|website=Cubane|first=B.|last=Muir|publisher=[[Imperial College London]]|title=Applications|at=Polymers|url=https://www.ch.ic.ac.uk/local/projects/b_muir/Cubane/Cubanepro/Applications.html|access-date=22 May 2025|url-status=live|archive-url=https://web.archive.org/web/20240508043615/https://www.ch.ic.ac.uk/local/projects/b_muir/Cubane/Cubanepro/Applications.html|archive-date=8 May 2024}}</ref> Poly-cubylcubane is, however, synthesizable via high pressure, solid-state polymerization. It exhibits exceptionally high [[refractive index]].<ref>{{cite journal |last1=Huang |first1=Haw-Tyng |last2=Zhu |first2=Li |last3=Ward |first3=Matthew D. |last4=Wang |first4=Tao |last5=Chen |first5=Bo |last6=Chaloux |first6=Brian L. |last7=Wang |first7=Qianqian |last8=Biswas |first8=Arani |last9=Gray |first9=Jennifer L. |last10=Kuei |first10=Brooke |last11=Cody |first11=George D. |last12=Epshteyn |first12=Albert |last13=Crespi |first13=Vincent H. |last14=Badding |first14=John V. |last15=Strobel |first15=Timothy A. |title=Nanoarchitecture through Strained Molecules: Cubane-Derived Scaffolds and the Smallest Carbon Nanothreads |journal=Journal of the American Chemical Society |date=21 January 2020 |volume=142 |issue=42 |pages=17944β17955 |doi=10.1021/jacs.9b12352 |pmid=31961671 |bibcode=2020JAChS.14217944H |s2cid=210870993 |issn=0002-7863|url=https://par.nsf.gov/servlets/purl/10210835}}</ref>
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