Paracyclophane
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In
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
, a cyclophane is a
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 ...
consisting of an
aromatic In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...
unit (typically a
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 ...
ring) and a
chain A chain is a serial assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a rope in that it is flexible and curved in compression but linear, rigid, and load-bearing in tension. A ...
that forms a
bridge A bridge is a structure built to Span (engineering), span a physical obstacle (such as a body of water, valley, road, or railway) without blocking the path underneath. It is constructed for the purpose of providing passage over the obstacle, whi ...
between two non-adjacent positions of the aromatic ring. More complex derivatives with multiple aromatic units and bridges forming cagelike structures are also known. Cyclophanes are well-studied examples of strained organic compounds.


Cyclophanes


Structures

Paracyclophanes adopt the
boat conformation Cyclohexane conformations are any of several three-dimensional shapes adopted by cyclohexane. Because many chemical compound, compounds feature structurally similar six-membered Ring (chemistry), rings, the structure and dynamics of cyclohexane a ...
normally observed in
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 ...
s. Smaller value of n lead to greater distortions.
X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
on ' aracyclophane' shows that the aromatic bridgehead carbon atom makes an angle of 20.5° with the plane. The
benzyl In organic chemistry, benzyl is the substituent or molecular fragment possessing the structure . Benzyl features a benzene ring () attached to a methylene group (). Nomenclature In IUPAC nomenclature, the prefix benzyl refers to a substituent ...
carbons deviate by another 20.2°. The carbon-to-carbon bond length alternation has increased from 0 for
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 ...
to 39 pm. Despite their distorted structures, cyclophanes retain their
aromaticity In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugation alone. The e ...
, as determined by UV-vis spectroscopy.


Reactivity

With regards to their reactivity, cyclophanes often exhibit diene-like behavior, despite evidence for aromaticity in even the most distorted cyclophane. This highly distorted cyclophane photochemically converts to the Dewar benzene derivative. Heat reverses the reaction. With
dimethyl acetylenedicarboxylate Ethane ( , ) is a naturally occurring organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural ...
, etacyclophane rapidly undergoes the Diels-Alder reaction. A non-bonding nitrogen to arene distance of 244 pm is recorded for a pyridinophane and in the unusual
superphane Superphane is a 6-fold bridged cyclophane with all arene positions in the benzene dimer taken up by ethylene spacers. The compound has been of some scientific interest as a model for testing aromaticity and was first synthesised by Virgil Boekelhei ...
the two benzene rings are separated by a mere 262 pm. Other representative of this group are in-methylcyclophanes, in-ketocyclophanes and ''in'',''in''-Bis(hydrosilane).


NMR properties

The
proton NMR Proton nuclear magnetic resonance (proton NMR, hydrogen-1 NMR, or 1H NMR) is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the stru ...
spectra of cyclophanes have been intensively examined to gain insights into the aromaticity of the benzene ring. Also of great interest is the shielding effects of the aromatic ring on the hydrocarbon strap. Generally the aromatic protons appear near their usual positions around 7.2 ppm, indicating that even with severe distortions, the ring retains aromaticity. The central methylene protons in the aliphatic bridge are shielded to a position of around - 0.5 ppm.


Synthesis

aracyclophane can be synthesized beginning with the Bamford–Stevens reaction to form the spiro
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 ( ...
1 in ''scheme 3'', rearranging in a
pyrolysis Pyrolysis is a process involving the Bond cleavage, separation of covalent bonds in organic matter by thermal decomposition within an Chemically inert, inert environment without oxygen. Etymology The word ''pyrolysis'' is coined from the Gree ...
reaction through the
carbene In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a Valence (chemistry), valence of two and two unshared valence electrons. The general formula is or where the R represents substituents or hydrogen atoms. Th ...
intermediate 4. A separate route to the Dewar form involves a Ag+-induced
rearrangement reaction In organic chemistry, a rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another at ...
of the bicyclopropenyl compound 7. Metacyclophanes are generally less strained and thus more easily prepared than paracyclophanes. Shown below is the route to a 414]metaparacyclophane in ''scheme 4'' featuring a in-situ Ramberg-Bäcklund Reaction converting the
sulfone In organic chemistry, a sulfone is a organosulfur compound containing a sulfonyl () functional group attached to two carbon atoms. The central hexavalent sulfur atom is double-bonded to each of two oxygen atoms and has a single bond to each of ...
3 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 ...
4.


Naturally occurring cyclophanes

A few cyclophanes exist in nature. One example of a metacyclophane is
cavicularin Cavicularin is a natural phenolic secondary metabolite isolated from the liverwort '' Cavicularia densa''. This macrocycle is unusual because it was the first compound isolated from nature displaying optical activity solely due to the presence of ...
. Haouamine A is a paracyclophane found in a certain species of
tunicate Tunicates are marine invertebrates belonging to the subphylum Tunicata ( ). This grouping is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ...
. Because of its potential application as an anticancer
drug A drug is any chemical substance other than a nutrient or an essential dietary ingredient, which, when administered to a living organism, produces a biological effect. Consumption of drugs can be via insufflation (medicine), inhalation, drug i ...
it is also available from
total synthesis Total synthesis, a specialized area within organic chemistry, focuses on constructing complex organic compounds, especially those found in nature, using laboratory methods. It often involves synthesizing natural products from basic, commercially ...
via an
alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...
-
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 ...
Diels-Alder reaction in the crucial step with expulsion of carbon dioxide (''scheme 5''). In this compound the deviation from planarity is 13° for the benzene ring and 17° for the bridgehead carbons. An alternative cyclophane formation strategy in ''scheme 6'' was developed based on
aromatization Aromatization is a chemical reaction in which an aromaticity, aromatic system is formed from a single nonaromatic precursor. Typically aromatization is achieved by dehydrogenation of existing cyclic compounds, illustrated by the conversion of cycl ...
of the ring well after the formation of the bridge. Two additional types of cyclophanes were discovered in nature when they were isolated from two species of
cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
from the family ''Nostocacae''. These two classes of cyclophanes are both ,7paracyclophanes and were named after the species from which they were extracted: cylindrocyclophanes from ''Cylindrospermum lichenforme'' and nostocyclophanes from ''Nostoc linckia''.


.naracyclophanes

200px, Superphane. A well studied member of the .naracyclophane family is .2aracyclophane. One method for its preparation is by the 1,6-Hofmann elimination of 4-methylbenzyltrimethylammonium hydroxide: The .2aracyclophane-1,9-diene has been applied in ROMP to a
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. ...
with alternating cis-alkene and trans-alkene bonds using Grubbs' second generation catalyst: The driving force for ring-opening and polymerization is strain relief. The reaction is believed to be a
living polymerization In polymer chemistry, living polymerization is a form of chain growth polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transf ...
due to the lack of competing reactions. Because the two benzene rings are in close proximity this cyclophane type also serves as guinea pig for
photochemical Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400  nm), visible (400–750&nb ...
dimerization In chemistry, dimerization is the process of joining two identical or similar molecular entities by bonds. The resulting bonds can be either strong or weak. Many symmetrical chemical species are described as dimers, even when the monomer is u ...
reactions as illustrated by this example: The product formed has an octahedrane skeleton. When the
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 ...
group is replaced by a
methylene group A methylene group is any part of a molecule that consists of two hydrogen atoms bound to a carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—m ...
no reaction takes place: the dimerization requires through-bond overlap between the aromatic
pi electron In chemistry, pi bonds (π bonds) are covalent chemical bonds, in each of which two lobes of an orbital on one atom overlap with two lobes of an orbital on another atom, and in which this overlap occurs laterally. Each of these atomic orbitals ...
s and the sigma electrons in the C-N bond in the reactants
LUMO In chemistry, HOMO and LUMO are types of molecular orbitals. The acronyms stand for ''highest occupied molecular orbital'' and ''lowest unoccupied molecular orbital'', respectively. HOMO and LUMO are sometimes collectively called the ''frontie ...
.


Janusene

The symmetrical molecule .3rthocyclophane, also known as janusene, is a cyclophane that contains 4 benzene rings in a cleft-shaped arrangement. First synthesized in 1967 by Stanley J. Cristol through the
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 ...
of
anthracene Anthracene is a solid polycyclic aromatic hydrocarbon (PAH) of formula C14H10, consisting of three fused benzene rings. It is a component of coal tar. Anthracene is used in the production of the red dye alizarin and other dyes, as a scintil ...
and dibenzobarrelene, the molecule has been used to study stacking and interactions between cations and pi orbitals, particularly with silver ions. Derivatives and complexes of janusene have been created to study cation-pi interactions, transannular interactions in similar rigid
aromatic In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...
molecules, and systems that depend on carbon-carbon distances. Various synthetic methods for producing janusene have been developed since the original cycloaddition reaction was discovered, including microwave assisted reactions 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 ...
transfer from 5,6,7,8-tetrafluorobenzobarrelene.


Phanes

Generalization of cyclophanes led to the concept of
phanes In Orphic cosmogony Phanes (, genitive ) or Protogonos () is a primeval deity who was born from the cosmic egg at the beginning of creation. He is referred by various names, including Erikepaios "Power" () and Metis "Thought". Mythology ...
in the IUPAC nomenclature. Some example systematic phane names are: * 4etacyclophane is 1(1,3)-benzenacyclopentadecaphane * .2'aracyclophane (or .2aracyclophane) is 1,4(1,4)-dibenzenacyclohexaphane In "1(1,3)-benzenacyclopentadecaphane", the "1" refers to the first position of the ring as a "superatom", the "(1,3)" describes the "meta" location, "benzena" refers to the ring, and the "pentadeca" (15) describes the chain length counting the ring as one atom.


See also

* Cycloparaphenylene, cyclic all-para-linked phenyl groups. *
Calixarenes A calixarene is a macrocycle or cyclic oligomer based on a methylene-linked phenols. With hydrophobic cavities that can hold smaller molecules or ions, calixarenes belong to the class of cavitands known in host–guest chemistry. Nomenclature ...


General sources

* B. H. Smith, Bridged Aromatic Compounds, Academic Press, New York, 1964. * P. M. Keehn, S. M. Rosenfeld (eds.), Cyclophanes, Vols. 1 and 2, Academic Press,New York, 1983. * F. Vögtle, F., G. Hohner, Top. Curr. Chem. 1978, 74, 1. * F. Vögtle, P. Neumann, Top. Curr. Chem. 1983, 113, 1; 1985, 115, 1.


References

{{Authority control Cyclophanes