Cubane is a synthetic

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...

compound with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

. It consists of eight

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

atoms arranged at the corners of a

cube A cube or regular hexahedron is a three-dimensional space, three-dimensional solid object in geometry, which is bounded by six congruent square (geometry), square faces, a type of polyhedron. It has twelve congruent edges and eight vertices. It i ...

, with one

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

atom attached to each carbon atom. A solid

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

line 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". 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 In geometry, a tetrahedron (: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (geometry), vertices. The tet ...

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 A regular octahedron has 24 rotational (or orientation-preserving) symmetries, and 48 symmetries altogether. These include transformations that combine a reflection and a rotation. A cube has the same set of symmetries, since it is the polyhedr ...

. 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 Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

for hydrocarbon molecules. The resulting high

energy density In physics, energy density is the quotient between the amount of energy stored in a given system or contained in a given region of space and the volume of the system or region considered. Often only the ''useful'' or extractable energy is measure ...

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.

Synthesis

The classic 1964 synthesis starts with the conversion of 2-cyclopentenone to 2-bromo cyclopentadienone: Cyclopentenone to 2-bromocyclopentadienone

Allylic In organic chemistry, an allyl group is a substituent with the structural formula . It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, . In 1844, Theodor Wertheim isolat ...

bromination In chemistry, halogenation is a chemical reaction which introduces one or more halogens into a chemical compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers, drugs ...

with ''N''-bromosuccinimide in

carbon tetrachloride Carbon tetrachloride, also known by many other names (such as carbon tet for short and tetrachloromethane, also IUPAC nomenclature of inorganic chemistry, recognised by the IUPAC), is a chemical compound with the chemical formula CCl4. It is a n ...

followed by addition of molecular bromine to the

alkene In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...

gives a 2,3,4-tribromocyclopentanone. Treating this compound with

diethylamine Diethylamine is an organic compound with the formula . It is classified as a secondary amine. It is a flammable, volatile weakly alkaline liquid that is miscible with most solvents. It is a colorless liquid, but commercial samples often appear br ...

diethyl ether Diethyl ether, or simply ether, is an organic compound with the chemical formula , sometimes abbreviated as . It is a colourless, highly Volatility (chemistry), volatile, sweet-smelling ("ethereal odour"), extremely flammable liquid. It belongs ...

causes elimination of two equivalents of

hydrogen bromide Hydrogen bromide is the inorganic compound with the formula . It is a hydrogen halide consisting of hydrogen and bromine. A colorless gas, it dissolves in water, forming hydrobromic acid, which is saturated at 68.85% HBr by weight at room temper ...

to give the diene product. CubaneSynthesis

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 Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...

to 3. In the next step, the ''endo'' isomer 3 (with both

groups in close proximity) forms the cage-like isomer 4 in a photochemical +2

cycloaddition In organic chemistry, a cycloaddition is a chemical reaction in which "two or more Unsaturated hydrocarbon, unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of th ...

. The bromoketone group is converted to ring-contracted

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...

5 in a

Favorskii rearrangement The Favorskii rearrangement is principally a rearrangement of cyclopropanones and α-halo ketones that leads to carboxylic acid derivatives. In the case of cyclic α-halo ketones, the Favorskii rearrangement constitutes a ring contraction. This ...

with

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which utili ...

. Next, the thermal

decarboxylation Decarboxylation is a chemical reaction that removes a carboxyl group and releases carbon dioxide (CO2). Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain. The reverse process, which is ...

takes place through the

acid chloride In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group . Their formula is usually written , where R is a side chain. They are reactive derivatives of carboxylic acids (). A specific example o ...

(with

thionyl chloride Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately Volatility (chemistry), volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a Halogenation, chlorinating reagen ...

) and the ''tert''-butyl perester 6 (with ''tert''-butyl hydroperoxide and

pyridine Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom . It is a highly flammable, weak ...

) 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. Cuban4

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).

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: : Cuban_zu_Cunean

With a

rhodium Rhodium is a chemical element; it has symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion-resistant transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring isot ...

catalyst, cubane first forms ''syn''-tricyclooctadiene, which can thermally decompose to

cyclooctatetraene 1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as nnulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because of ...

at 50–60 °C. : Cubane_to_cyclooctatetraene

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 Aromatic compounds or arenes are organic compounds "with a chemistry typified by benzene" and "cyclically conjugated." The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were ...

s or other cluster compounds: it metallates easily. Cubane is slightly

acidic An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen cation, H+), known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid. The first category of acids are the ...

, deprotonating about 63000 times faster than

cyclohexane Cyclohexane is a cycloalkane with the molecular formula . Cyclohexane is non-polar. Cyclohexane is a colourless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cyclohexan ...

. Cubane substituents display normal reactivity. For example a Curtius rearrangement followed by

organic oxidation Organic reductions or organic oxidations or organic redox reactions are redox reactions that take place with organic compounds. In organic chemistry oxidations and reductions are different from ordinary redox reactions, because many reactions carr ...

converts to tetranitrocubane. However,

electron-rich Electron-rich is jargon that is used in multiple related meanings with either or both kinetic and thermodynamic implications: * with regards to electron-transfer, electron-rich species have low ionization energy and/or are reducing agents. Tetr ...

substituents such as

alcohols In chemistry, an alcohol (), is a type of organic compound that carries at least one hydroxyl () functional group bound to a Saturated and unsaturated compounds, saturated carbon atom. Alcohols range from the simple, like methanol and ethanol ...

can enable decomposition; they stabilize the cubene intermediate as a ketone (or equivalent) tautomer. Hypercubane was predicted to exist in a 2014 publication.

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 Quantum chemistry, also called molecular quantum mechanics, is a branch of physical chemistry focused on the application of quantum mechanics to chemical systems, particularly towards the quantum-mechanical calculation of electronic contributions ...

. Single-electron reduction to the radical anion traps an otherwise-free electron inside the cube, making it the world's smallest box.

Cubenes and ''poly''-cubylcubane

Despite their orbital strain, two cubenes have been synthesized, and a third analyzed computationally. ''ortho''-, produced via lithium-halogen exchange followed by elimination, was the most pyramidalized alkene ever made at the time of its synthesis; ''meta''- is even less stable, and ''para''- probably only exists as a

diradical In chemistry, a diradical is a chemical species, molecular species with two electrons occupying molecular orbitals (MOs) which are degenerate energy level, degenerate. The term "diradical" is mainly used to describe organic compounds, where most ...

rather than an actual diagonal bond. They rapidly undergo

nucleophilic addition In organic chemistry, a nucleophilic addition (AN) reaction is an addition reaction where a chemical compound with an electrophilic double or triple bond reacts with a nucleophile, such that the double or triple bond is broken. Nucleophilic addit ...

. Decomposition of cubenes has enabled chemists to synthesize cubylcubane, as well as higher oligomers. Per

X-ray diffraction X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...

, 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. The ''oligo''-cubylcubanes are rigid molecular rods considered for

liquid crystal Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal can flow like a liquid, but its molecules may be oriented in a common direction as i ...

design, but scarcely accessible through conventional

organic synthesis Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...

. Absent solubizing groups on the cubane

monomer A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization. Classification Chemis ...

, oligomers with at least 4 units are essentially insoluble. Poly-cubylcubane is, however, synthesizable via high pressure, solid-state polymerization. It exhibits exceptionally high

refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

References

{{reflist, colwidth=30em , refs= {{cite journal, last1 = Biegasiewicz , first1 = Kyle , last2 = Griffiths , first2 = Justin , last3 = Savage , first3 = G. Paul , last4 = Tsanakstidis , first4 = John , last5 = Priefer , first5 = Ronny , year = 2015 , title = Cubane: 50 years later , journal = Chemical Reviews, volume = 115 , issue = 14 , pages = 6719–6745 , doi=10.1021/cr500523x , pmid=26102302 {{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 book, first1=Michael B., last1=Smith, first2=Jerry, last2=March, title=March's Advanced Organic Chemistry, url=https://archive.org/details/organicchemistry00mich_115, url-access=limited, edition=5th, publisher=John Wiley & Sons, date=2001, pag
1459
isbn=0-471-58589-0 {{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