Cubane
{{Short description|Organic compound (C8H8) with a cube carbon structure}}
{{chembox
| Watchedfields = changed
| verifiedrevid = 443545029
| Name = Cubane
| ImageFileL1 = Cuban.svg
| ImageNameL1 = Structural formula of cubane
| ImageClassL1 = skin-invert
| ImageFileR1 = Cubane molecule ball.png
| ImageNameR1 = Ball-and-stick model of cubane
| PIN = Cubane{{cite book | title = Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book) | publisher = The Royal Society of Chemistry | date = 2014 | location = Cambridge | page = 169 | doi = 10.1039/9781849733069-FP001 | isbn = 978-0-85404-182-4 | quote = The retained names adamantane and cubane are used in general nomenclature and as preferred IUPAC names.}}
| SystematicName = Pentacyclo[4.2.0.02,5.03,8.04,7]octane
| OtherNames =
|Section1={{Chembox Identifiers
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 119867
| PubChem = 136090
| InChIKey = TXWRERCHRDBNLG-UHFFFAOYAL
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/C8H8/c1-2-5-3(1)7-4(1)6(2)8(5)7/h1-8H
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = TXWRERCHRDBNLG-UHFFFAOYSA-N
| CASNo_Ref = {{cascite|correct|??}}
| CASNo = 277-10-1
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = Z5HM0Q7DK1
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 33014
| SMILES = C12C3C4C1C5C2C3C45
| InChI = 1/C8H8/c1-2-5-3(1)7-4(1)6(2)8(5)7/h1-8H
}}
|Section2={{Chembox Properties
| Formula = {{chem2|C8H8}}
| MolarMass = 104.15 g/mol
| Density = 1.29 g/cm3
| Appearance = Transparent{{cite web | url=https://www.ch.ic.ac.uk/local/projects/b_muir/Cubane/Cubanepro/Start.html | title=Start }} crystalline solid
| MeltingPtC = 133.5
| BoilingPtC = 161.6
}}
|Section8={{Chembox Related
| OtherFunction = Cuneane
Dodecahedrane
Tetrahedrane
Prismane
Prismane C8
| OtherFunction_label = hydrocarbons
| OtherCompounds = Octafluorocubane
Octanitrocubane
Octaazacubane
Mirex
}}
}}
Cubane is a synthetic hydrocarbon compound with the formula {{chem2|C8H8}}. It consists of eight carbon atoms arranged at the corners of a cube, with one hydrogen atom attached to each carbon atom. A solid crystalline substance, cubane is one of the Platonic hydrocarbons and a member of the prismanes. It was first synthesized in 1964 by Philip Eaton and Thomas Cole. Before this work, Eaton believed that cubane would be impossible to synthesize due to the "required 90 degree bond angles".{{cite book |last1=Teachers |first1=University of New South Wales Summer School for Chemistry |title=Approach to Chemistry: Lectures and Workshop Reports of the ... Summer School for Chemistry Teachers |date=1963 |publisher=The University |page=98 |url=https://books.google.com/books?id=cFA0AQAAIAAJ |language=en}} "This compound was described only a few months ago and, curiously enough, it is quite easy to make, although only a year ago I would have predicted that it would be difficult, or even impossible, to synthesize."{{cite book |last1=Moore |first1=John W. |last2=Stanitski |first2=Conrad L. |last3=Jurs |first3=Peter C. |title=Chemistry: The Molecular Science |date=2002 |publisher=Harcourt College Publishers |isbn=978-0-03-032011-8 |page=372 |url=https://books.google.com/books?id=XjcvAQAAIAAJ |language=en}} "This sharp bond angle creates severe bond strain in cubane, a compound thought previously impossible to synthesize because of the required 90° bond angles." The cubic shape requires the carbon atoms to adopt an unusually sharp 90° bonding angle, which would be highly strained as compared to the 109.45° angle of a tetrahedral carbon. Once formed, cubane is quite kinetically stable, due to a lack of readily available decomposition paths. It is the simplest hydrocarbon with octahedral symmetry.
Having high potential energy and kinetic stability makes cubane and its derivative compounds useful for controlled energy storage. For example, octanitrocubane and heptanitrocubane have been studied as high-performance explosives. These compounds also typically have a very high density for hydrocarbon molecules. The resulting high energy density means a large amount of energy can be stored in a comparably smaller amount of space, an important consideration for applications in fuel storage and energy transport. Furthermore, their geometry and stability make them suitable isosteres for benzene rings.{{Cite journal |last1=Wiesenfeldt |first1=Mario P. |last2=Rossi-Ashton |first2=James A. |last3=Perry |first3=Ian B. |last4=Diesel |first4=Johannes |last5=Garry |first5=Olivia L. |last6=Bartels |first6=Florian |last7=Coote |first7=Susannah C. |last8=Ma |first8=Xiaoshen |last9=Yeung |first9=Charles S. |last10=Bennett |first10=David J. |last11=MacMillan |first11=David W. C. |date=June 2023 |title=General access to cubanes as benzene bioisosteres |journal=Nature |language=en |volume=618 |issue=7965 |pages=513–518 |doi=10.1038/s41586-023-06021-8 |pmid=37015289 |issn=1476-4687|pmc=10680098 |bibcode=2023Natur.618..513W }}
Synthesis
The classic 1964 synthesis starts with the conversion of 2-cyclopentenone to 2-bromocyclopentadienone:
File:Cyclopentenone to 2-bromocyclopentadienone.png
Allylic bromination with N-bromosuccinimide in carbon tetrachloride followed by addition of molecular bromine to the alkene gives a 2,3,4-tribromocyclopentanone. Treating this compound with diethylamine in diethyl ether causes elimination of two equivalents of hydrogen bromide to give the diene product.
File:CubaneSynthesis.png{{clear left}}
The construction of the eight-carbon cubane framework begins when 2-bromocyclopentadienone undergoes a spontaneous Diels-Alder dimerization. One ketal of the endo isomer is subsequently selectively deprotected with aqueous hydrochloric acid to 3.
In the next step, the endo isomer 3 (with both alkene groups in close proximity) forms the cage-like isomer 4 in a photochemical [2+2] cycloaddition. The bromoketone group is converted to ring-contracted carboxylic acid 5 in a Favorskii rearrangement with potassium hydroxide. Next, the thermal decarboxylation takes place through the acid chloride (with thionyl chloride) and the tert-butyl perester 6 (with tert-butyl hydroperoxide and pyridine) to 7; afterward, the acetal is once more removed in 8. A second Favorskii rearrangement gives 9, and finally another decarboxylation gives, via 10, cubane (11).
A more approachable laboratory synthesis of disubstituted cubane involves bromination of the ethylene ketal of cyclopentanone to give a tribromocyclopentanone derivative. Subsequent steps involve dehydrobromination, Diels-Alder dimerization, etc.{{Cite journal|doi=10.1071/C97021|title=Dimethyl Cubane-1,4-dicarboxylate: A Practical Laboratory Scale Synthesis|year=1997|last1=Bliese|first1=Marianne|last2=Tsanaktsidis|first2=John|journal=Australian Journal of Chemistry|volume=50|issue=3|page=189}}{{Cite web|author =Fluorochem, Inc|date=July 1989|title=Cubane Derivatives for Propellant Applications|url=https://apps.dtic.mil/sti/pdfs/ADA210368.pdf|url-status=live|archive-url=https://web.archive.org/web/20210709185435/https://apps.dtic.mil/sti/pdfs/ADA210368.pdf |archive-date=2021-07-09 }}
The resulting cubane-1,4-dicarboxylic acid is used to synthesize other substituted cubanes. Cubane itself can be obtained nearly quantitatively by photochemical decarboxylation of the thiohydroxamate ester (the Barton decarboxylation).{{Cite journal |last=Eaton |first=Philip E. |date=1992 |title=Cubane: Ausgangsverbindungen für die Chemie der neunziger Jahre und des nächsten Jahrhunderts |url=https://onlinelibrary.wiley.com/doi/10.1002/ange.19921041105 |journal=Angewandte Chemie |language=de |volume=104 |issue=11 |pages=1447–1462 |doi=10.1002/ange.19921041105|bibcode=1992AngCh.104.1447E |url-access=subscription }}
Reactions and derivatives
Cubane is highly strained, but cannot decompose because cubene molecules are pyramidal alkenes, too high-energy for most elimination pathways. Certain metals catalyze σ-bond rearrangement to cuneane:
With a rhodium catalyst, cubane first forms syn-tricyclooctadiene, which can thermally decompose to cyclooctatetraene at 50–60 °C.{{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=3515–3518 |doi=10.1021/ja00714a075 |bibcode=1970JAChS..92.3515C |issn=0002-7863|url-access=subscription }}
:File:Cubane_to_cyclooctatetraene.svg
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 arenes or other cluster compounds: it metallates easily.{{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}} Cubane is slightly acidic, deprotonating about 63000 times faster than cyclohexane.{{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}}
Cubane substituents display normal reactivity. For example a Curtius rearrangement followed by organic oxidation converts {{chem name|cubane tetra(carbonylchloride)}} to tetranitrocubane. However, electron-rich substituents such as alcohols can enable decomposition; they stabilize the cubene intermediate as a ketone (or equivalent) tautomer.
Hypercubane was predicted to exist in a 2014 publication.{{cite journal|last=Pichierri|first=F.|journal=Chem. Phys. Lett.|date=2014|volume=612|pages=198–202|doi=10.1016/j.cplett.2014.08.032|title= Hypercubane: DFT-based prediction of an Oh-symmetric double-shell hydrocarbon|bibcode=2014CPL...612..198P}}{{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}}
=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 425–427 °C.
Octanitrocubane is a green explosive.
Both heptafluorocubane and octafluorocubane were synthesized in 2022 to study octafluorocubane's unusual electronic structure.{{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=756–759 |doi=10.1126/science.abq0516 |pmid=35951682 |bibcode=2022Sci...377..756S |s2cid=251515925}} Single-electron reduction to the radical anion {{chem|C|8|F|8|-}} trapsPichierri, 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}} an otherwise-free electron inside the cube, making it the world's smallest box.{{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 }}
=Cubenes and ''poly''-cubylcubane=
Despite their orbital strain, two cubenes have been synthesized, and a third analyzed computationally. ortho-{{chem name|Cubene}}, produced via lithium-halogen exchange followed by elimination,{{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}} was the most pyramidalized alkene ever made at the time of its synthesis;{{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= 7230–7232 |doi= 10.1021/ja00229a057 |bibcode= 1988JAChS.110.7230E }} 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.{{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}} They rapidly undergo nucleophilic addition.
Decomposition of cubenes has enabled chemists to synthesize cubylcubane, as well as higher oligomers.{{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=1421–1436 |doi=10.1002/anie.199214211 |language=en |issn=1521-3773}} Per X-ray diffraction, the central cubane-cubane bond is exceedingly short (1.458 Å), much shorter than the typical C-C single bond (1.578 Å). This is attributed to the fact that the exocyclic orbitals of cubane are s-rich and close to the nucleus.{{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=7232–7234 |doi=10.1021/ja00229a058 |bibcode=1988JAChS.110.7232G |issn=0002-7863}}
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.{{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}} Poly-cubylcubane is, however, synthesizable via high pressure, solid-state polymerization. It exhibits exceptionally high refractive index.{{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=17944–17955 |doi=10.1021/jacs.9b12352 |pmid=31961671 |bibcode=2020JAChS.14217944H |s2cid=210870993 |issn=0002-7863|url=https://par.nsf.gov/servlets/purl/10210835}}
See also
References
{{reflist|colwidth=30em
|refs=
{{cite journal|title=Cubane|first1=Philip E.|last1=Eaton|first2=Thomas W.|last2=Cole|journal=J. Am. Chem. Soc.|date=1964|volume=86|issue=15|pages=3157–3158|doi=10.1021/ja01069a041|bibcode=1964JAChS..86.3157E }}
{{cite journal|title=The Cubane System|first1=Philip E.|last1=Eaton|first2=Thomas W.|last2=Cole|journal=J. Am. Chem. Soc.|date=1964|volume=86|issue=5|pages=962–964|doi=10.1021/ja01059a072|bibcode=1964JAChS..86..962E }}
{{cite journal|title=Studien über den Mechanismus chemischer Reaktionen, XXIII. Hydrierungen von Nitrilen unter Verwendung von Terpenen als Wasserstoffdonatoren|first1=K.|last1=Kindler|first2=K.|last2=Lührs|journal=Chem. Ber.|volume=99|date=1966|pages=227–232|doi=10.1002/cber.19660990135}}
{{cite journal|title=Tetraphenylcyclobutadiene Derivatives. II.1 Chemical Evidence for the Triplet State|first=H. H.|last=Freedman|journal=J. Am. Chem. Soc.|date=1961|volume=83|issue=9|pages=2195–2196|doi=10.1021/ja01470a037|bibcode=1961JAChS..83.2195F }}
{{cite journal|title=Tetraphenylcyclobutadiene Derivatives. IV.1 "Octaphenylcubane"; A Dimer of Tetraphenylcyclobutadiene|first1=H. H.|last1=Freedman|first2=D. R.|last2=Petersen|journal=J. Am. Chem. Soc.|date=1962|volume=84|issue=14|pages=2837–2838|doi=10.1021/ja00873a046|bibcode=1962JAChS..84.2837F }}
{{cite journal|title=Structure of the Dimer of tetraphenylcyclobutadiene|first1=G. S.|last1=Pawley|first2=W. N.|last2=Lipscomb|first3=H. H.|last3=Freedman|journal=J. Am. Chem. Soc.|date=1964|volume=86|issue=21|pages=4725–4726|doi=10.1021/ja01075a042|bibcode=1964JAChS..86.4725P }}
}}
External links
- [http://www.synarchive.com/syn/14 Eaton's cubane synthesis at SynArchive.com]
- [http://www.synarchive.com/syn/189 Tsanaktsidis's cubane synthesis at SynArchive.com]
- [http://www.ch.ic.ac.uk/local/projects/b_muir/Cubane/Cubanepro/Start.html Cubane chemistry at Imperial College London]
Category:Polycyclic nonaromatic hydrocarbons
Category:Theoretical chemistry
Category:Substances discovered in the 1960s