Barrelene is a

bicyclic A bicyclic molecule () is a molecule that features two joined rings. Bicyclic structures occur widely, for example in many biologically important molecules like α-thujene and camphor. A bicyclic compound can be carbocyclic (all of the ring ...

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

with

chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...

C₈H₈ and

systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature. A semisystematic name or semitrivi ...

bicyclo .2.2cta-2,5,7-triene. First synthesized and described by

Howard Zimmerman Howard E. Zimmerman (July 5, 1926 – February 12, 2012) was a professor of chemistry at the University of Wisconsin–Madison. He was elected to the National Academy of Sciences in 1980 and the recipient of the 1986 American Institute of Chemis ...

in 1960, the name derives from the resemblance to a

barrel A barrel or cask is a hollow cylindrical container with a bulging center, longer than it is wide. They are traditionally made of wooden stave (wood), staves and bound by wooden or metal hoops. The word vat is often used for large containers ...

, with the staves being three

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon–carbon bond, carbon–carbon doub ...

units attached to two

methine In organic chemistry, a methine group or methine bridge is a trivalent functional group , derived formally from methane. It consists of a carbon atom bound by two single bonds and one double bond, where one of the single bonds is to a hydrogen. ...

groups. It is the formal Diels–Alder adduct of

benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...

and

acetylene Acetylene (Chemical nomenclature, systematic name: ethyne) is a chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is u ...

. Due to its unusual

molecular geometry Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that det ...

, the compound is of considerable interest to theoretical chemists.

Iptycene An iptycene is an aromatic compound composed of varying number of arene subunits bound to a bridged bicyclo-octatriene core structure. They are formally derivatives of barrelene. The first and simplest iptycene molecule is triptycene. The first ...

s, with the alkene groups part of an

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, are related compounds. It is also a starting material for many other organic compounds, such as

semibullvalene Bullvalene is a hydrocarbon with the chemical formula . The molecule has a cage-like structure formed by the fusion of one cyclopropane and three cyclohepta-1,4-diene rings. Bullvalene is unusual as an organic molecule due to the and bonds for ...

Synthesis

The original Zimmerman synthesis modified in 1969 starts from

coumalic acid Coumalic acid is an organic compound with the molecular formula C6H4O4. Its melting point is around 210 °C. In laboratory coumalic acid may be obtained by self-condensation of malic acid Malic acid is an organic compound with the molecul ...

:Reaction scheme:

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 ...

(1) takes place at 650 °C with

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

to give α-

pyrone Pyrones or pyranones are a class of heterocyclic chemical compounds. They contain an unsaturated six-membered ring, which has one oxygen atom and a carbonyl functional group. There are two isomers, denoted as 2-pyrone and 4-pyrone. The 2-pyrone ...

(2). The reaction with

methyl vinyl ketone Methyl vinyl ketone (MVK, IUPAC name: butenone) is the organic compound with the formula CH3C(O)CH=CH2. It is a reactive compound classified as an enone, in fact the simplest example thereof. It is a colorless, flammable, highly toxic liquid wi ...

(3) is a tandem Diels–Alder/retro-Diels–Alder/Diels–Alder sequence, which yields di-

ketone In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...

5 as a mixture of two

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...

s. It is possible to convert the endo isomer 5b to the exo isomer 5a by an

epimerization In stereochemistry, an epimer is one of a pair of diastereomers. The two epimers have opposite configuration at only one stereogenic center out of at least two. All other stereogenic centers in the molecules are the same in each. Epimerization is t ...

process through the

enol In organic chemistry, enols are a type of functional group or intermediate in organic chemistry containing a group with the formula (R = many substituents). The term ''enol'' is an abbreviation of ''alkenol'', a portmanteau deriving from "-ene ...

. The ketone groups are converted to

oxime In organic chemistry, an oxime is an organic compound belonging to the imines, with the general Chemical formula, formula , where R is an organic Side chain, side-chain and R' may be hydrogen, forming an aldoxime, or another organic functional g ...

groups in 6 by reaction with

hydroxylamine Hydroxylamine (also known as hydroxyammonia) is an inorganic compound with the chemical formula . The compound exists as hygroscopic colorless crystals.Greenwood and Earnshaw. ''Chemistry of the Elements.'' 2nd Edition. Reed Educational and Prof ...

and then to the

tosylate In organic chemistry, a toluenesulfonyl group (tosyl group, abbreviated Ts or TosIn this article, "Ts", unless otherwise stated, means tosyl, not tennessine.) is a univalent functional group with the chemical formula . It consists of a tolyl ...

groups in 7 by reaction with

tosyl chloride 4-Toluenesulfonyl chloride (''p''-toluenesulfonyl chloride, toluene-''p''-sulfonyl chloride) is an organic compound with the formula CH3C6H4SO2Cl. This white, malodorous solid is a reagent widely used in organic synthesis. Abbreviated TsCl or To ...

. A basic

Beckmann rearrangement The Beckmann rearrangement, named after the German chemist Ernst Otto Beckmann (1853–1923), is a rearrangement reaction, rearrangement of an oxime functional group to substituted amides. The rearrangement has also been successfully performed on ...

takes the scheme to give

amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula , where R, R', and R″ represent any group, typically organyl functional group, groups or hydrogen at ...

8 and its

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

to the di-

amine In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...

9 takes place with

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula . It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly corrosive base (chemistry), ...

. Finally, a

Hofmann elimination Hofmann elimination is an elimination reaction of an amine to form alkenes. The least stable alkene (the one with the fewest substituents on the carbons of the double bond), called the Hofmann product, is formed. This tendency, known as the Hofm ...

through

ammonium salt Ammonium is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged (cationic) molecular ion with the chemical formula or . It is formed by the addition of a proton (a hydrogen nucleus) to ammonia (). Ammonium i ...

10 gives the barrelene 11. : Synthesis of barrelene

Many alternative routes have been devised since then, one of them starting from

benzene oxide Oxepine is an oxygen-containing heterocycle consisting of a seven-membered ring with three double bonds. The parent C6H6O exists as an equilibrium mixture with benzene oxide. The oxepin–benzene oxide equilibrium is affected by the ring substi ...

: :

An alternate route that allows synthesis of the parent barrelene system and a variety of substituted barrelenes has also been reported.

Barrelene reactions

Barrelene is

hydrogenated Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organi ...

with hydrogen gas and

Adams' catalyst Adams Pearmain, also called Adam's Parmane, is a cultivar of apple. It was introduced to the Royal Horticultural Society, Horticultural Society of London in 1826 by Robert Adams, under the name Norfolk Pippin. The fruit is large, varying from t ...

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

to the fully saturated bicyclo .2.2octane.

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

bromine Bromine is a chemical element; it has chemical symbol, symbol Br and atomic number 35. It is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between th ...

tetrachloromethane 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 ...

gives a di-bromo adduct because a

coupling reaction In organic chemistry, a coupling reaction is a type of reaction in which two reactant molecules are bonded together. Such reactions often require the aid of a metal catalyst. In one important reaction type, a main group organometallic compound o ...

intervenes: :

Epoxidation In organic chemistry, an epoxide is a cyclic ether, where the ether forms a three-atom ring: two atoms of carbon and one atom of oxygen. This triangular structure has substantial ring strain, making epoxides highly reactive, more so than other ...

of barrelene with

oxone Potassium peroxymonosulfate is widely used as an oxidizing agent, for example, in pools and spas (usually referred to as monopersulfate or "MPS"). It is the potassium salt of peroxymonosulfuric acid. Potassium peroxymonosulfate per se is rarely e ...

gives the trioxatrishomobarrelene which on rearrangement with

boron trifluoride Boron trifluoride is the inorganic compound with the formula . This pungent, colourless, and toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds. Structure and bonding The g ...

(driving force:relief of

strain energy In physics, the elastic potential energy gained by a wire during elongation with a tensile (stretching) or compressive (contractile) force is called strain energy. For linearly elastic materials, strain energy is: : U = \frac 1 2 V \sigma \v ...

) converts into the trioxatrishomocubane: : This compound can be envisioned as a

cubane Cubane is a synthetic hydrocarbon compound with the Chemical formula, formula . It consists of eight carbon atoms arranged at the corners of a Cube (geometry), cube, with one hydrogen atom attached to each carbon atom. A solid crystalline substanc ...

with three oxygen atoms inserted into three opposite edges or as 9-crown-3 capped by two

units. The molecule is

chiral Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek language, Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is dist ...

and the separate

enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...

s have been isolated. Certain barrelenes have been used as a

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 ...

in a ring opening metathesis polymerization: : The catalyst is a

Schrock carbene A transition metal carbene complex is an organometallic compound featuring a divalent carbon ligand, itself also called a carbene. Carbene complexes have been synthesized from most transition metals and f-block metals, using many different synthe ...

(a ''molybdenum bis-(hexafluoro-tert-butoxy) carbene catalyst'') and the long alkyl chain attached to the monomer is required for solubility. Oxidation of the polymer with

DDQ 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (or DDQ) is the chemical reagent with formula C6Cl2(CN)2O2. This oxidant is useful for the dehydrogenation of alcohols, phenols, and steroid ketones. DDQ decomposes in water, but is stable in aqueous mine ...

affords the naphthalene pendant of

poly(p-phenylene vinylene) Poly(''p''-phenylene vinylene) (PPV, or polyphenylene vinylene) is a Conducting polymers, conducting polymer of the rigid-rod polymer family. PPV is the only polymer of this type that can be processed into a highly ordered crystalline thin film. ...

Isopentane Isopentane, also called methylbutane or 2-methylbutane, is a branched-chain saturated hydrocarbon (an alkane) with five carbon atoms, with formula or . Isopentane is a volatile and flammable liquid. It is one of three structural isomers with t ...

solutions of barrelene undergo photolytic

isomerisation In chemistry, isomerization or isomerisation is the process in which a molecule, polyatomic ion or molecular fragment is transformed into an isomer with a different chemical structure. Enolization is an example of isomerization, as is tautomeri ...

when

acetone Acetone (2-propanone or dimethyl ketone) is an organic compound with the chemical formula, formula . It is the simplest and smallest ketone (). It is a colorless, highly Volatile organic compound, volatile, and flammable liquid with a charact ...

is added as a

photosensitizer Photosensitizers are light absorbers that alter the course of a photochemical reaction. They usually are catalysts. They can function by many mechanisms; sometimes they abstract an electron from the substrate, and sometimes they abstract a hydro ...

to produce

. Prolonged irradiation results in further isomerisation to form

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 ...

. :

Notes

{{reflist, group=note

References

Cycloalkenes Bicyclic compounds