Cycloalkynes
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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 cycloalkyne is the cyclic
analog Analog or analogue may refer to: Computing and electronics * Analog signal, in which information is encoded in a continuous variable ** Analog device, an apparatus that operates on analog signals *** Analog electronics, circuits which use analog ...
of 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 ...
(). A cycloalkyne consists of a closed
ring (The) Ring(s) may refer to: * Ring (jewellery), a round band, usually made of metal, worn as ornamental jewelry * To make a sound with a bell, and the sound made by a bell Arts, entertainment, and media Film and TV * ''The Ring'' (franchise), a ...
of
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 containing one or more
triple bond A triple bond in chemistry is a chemical bond between two atoms involving six Electron pair bond, bonding electrons instead of the usual two in a covalent bond, covalent single bond. Triple bonds are stronger than the equivalent covalent bond, sin ...
s. Cycloalkynes have a general formula Because of the linear nature of the alkyne unit, cycloalkynes can be highly strained and can only exist when the number of carbon atoms in the ring is great enough to provide the flexibility necessary to accommodate this
geometry Geometry (; ) is a branch of mathematics concerned with properties of space such as the distance, shape, size, and relative position of figures. Geometry is, along with arithmetic, one of the oldest branches of mathematics. A mathematician w ...
. Large alkyne-containing carbocycles may be virtually unstrained, while the smallest constituents of this class of molecules may experience so much strain that they have yet to be observed experimentally. Cyclooctyne () is the smallest cycloalkyne capable of being isolated and stored as a stable compound. Despite this, smaller cycloalkynes can be produced and trapped through reactions with other organic molecules or through
complexation A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or ...
to
transition metals In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...
.


Examples

File:Cyclopropyne.png, Cyclopropyne File:Cyclobutyne.svg,
Cyclobutyne Cyclobutyne (C4H4) is a hydrocarbon molecule containing a triple bond within a four carbon atom ring. This cycloalkyne is very unstable due to its high ring strain and has not been isolated in the pure state. However, osmium coordination complexe ...
File:Cyclopentyne.svg, Cyclopentyne File:Cyclohexyne.svg, Cyclohexyne File:Cycloheptyne.svg, Cycloheptyne File:Cyclooctyne.svg,
Cyclooctyne Cyclooctyne is the cycloalkyne with a formula . Its molecule has a ring of 8 carbon atoms, connected by seven single bonds and one triple bond. Cyclooctyne is the smallest cycloalkyne that is stable enough to be isolated, although the chemical i ...


Background

Due to the significant geometric constraints imposed by the functionality, cycloalkynes smaller than cyclodecyne () result in highly strained structures. While the cyclononyne () and
cyclooctyne Cyclooctyne is the cycloalkyne with a formula . Its molecule has a ring of 8 carbon atoms, connected by seven single bonds and one triple bond. Cyclooctyne is the smallest cycloalkyne that is stable enough to be isolated, although the chemical i ...
() are isolable (though strongly reactive) compounds, cycloheptyne (), cyclohexyne () and cyclopentyne () only exist as transient reaction intermediates or as
ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
s coordinating to a metal center. There is little experimental evidence supporting the existence of
cyclobutyne Cyclobutyne (C4H4) is a hydrocarbon molecule containing a triple bond within a four carbon atom ring. This cycloalkyne is very unstable due to its high ring strain and has not been isolated in the pure state. However, osmium coordination complexe ...
() or cyclopropyne (), aside from studies reporting the isolation of an
osmium Osmium () is a chemical element; it has Symbol (chemistry), symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a Abundance of elements in Earth's crust, trace element in a ...
complex Complex commonly refers to: * Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe ** Complex system, a system composed of many components which may interact with each ...
with cyclobutyne ligands. Initial studies which demonstrated the transient intermediacy of the seven-, six- and five-membered cycloalkynes relied on trapping of the high-energy alkyne with a suitable reaction partner, such as a cyclic
diene In organic chemistry, a diene ( ); also diolefin, ) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alk''ene'' units, with the standard prefix ''di'' of systematic nome ...
s or
diazo In organic chemistry, the diazo group is an organic moiety consisting of two linked nitrogen atoms at the terminal position. Overall charge-neutral organic compounds containing the diazo group bound to a carbon atom are called diazo compounds ...
compounds to generate the Diels–Alder or
diazoalkane 1,3-dipolar cycloaddition The Diazoalkane 1,3-dipolar cycloaddition is a 1,3-dipolar cycloaddition (an organic reaction) between a 1,3-dipole diazo compound (notably diazomethane) and a dipolarophile. When the dipolarphile is an alkene, the reaction product is a pyrazolin ...
products, respectively. Stable small-ring cycloalkynes have subsequently been isolated in complex with various
transition metals In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...
such as
nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...
,
palladium Palladium is a chemical element; it has symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas (formally 2 Pallas), ...
and
platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...
. Despite long being considered to be chemical curiosities with limited synthetic applications, recent work has demonstrated the utility of strained cycloalkynes in both
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 ...
of complex
natural products A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical s ...
and bioorthogonal chemistry.


Angle strain

Angle strain In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal. Strain is most commonly discussed for small rings such as cyclopropanes and cyclobutanes, whose internal angles are su ...
in cycloalkynes arises from the deformation of the bond angle which must occur in order to accommodate the molecular geometry of rings containing less than ten carbons. The strain energies associated with cyclononyne () and cyclooctyne () are approximately 2.9 kcal/mol and 10 kcal/mol, respectively. This upwards trend in energy for the isolable constituents of this class is indicative of a rapid escalation of angle strain with an inverse correlation to ring size. Analysis by
photoelectron spectroscopy Photoemission spectroscopy (PES), also known as photoelectron spectroscopy, refers to energy measurement of electrons emitted from solids, gases or liquids by the photoelectric effect, in order to determine the binding energies of electrons in t ...
has indicated that the alkyne bond in small cyclic systems is composed of two non-degenerate Ï€ bonds â€“ a highly reactive strained bond perpendicular to a lower-energy Ï€ bond. ''Cis''-bending of the bond angle results in the drastic lowering of the energy of the lowest unoccupied molecular orbital, a phenomenon which accounts for the reactivity of strained cycloalkynes from the perspective of
molecular orbital theory In chemistry, molecular orbital theory (MO theory or MOT) is a method for describing the electronic structure of molecules using quantum mechanics. It was proposed early in the 20th century. The MOT explains the paramagnetic nature of O2, whic ...
.


Synthesis

Initial efforts toward the synthesis of strained cycloalkynes showed that cycloalkynes could be generated via the elimination of
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 ...
from 1-chloro-cycloalkene in modest yield. The desired product could be obtained as a mixture with the corresponding
allene In organic chemistry, allenes are organic compounds in which one carbon atom has double bonds with each of its two adjacent carbon atoms (, where R is hydrogen, H or some organyl group). Allenes are classified as diene#Classes, cumulated dienes ...
as the major product. Further work in this area was aimed at developing milder reaction conditions and generating more robust yields. To circumvent the generation of the undesired
allene In organic chemistry, allenes are organic compounds in which one carbon atom has double bonds with each of its two adjacent carbon atoms (, where R is hydrogen, H or some organyl group). Allenes are classified as diene#Classes, cumulated dienes ...
, the Kobayashi method for
aryne In organic chemistry, arynes and benzynes are a class of highly Reactivity (chemistry), reactive chemical Chemical species, species derived from an aromatic ring by removal of two substituents. Arynes are examples of didehydroarenes (1,2-didehydro ...
generation was adapted for the synthesis of cycloalkynes. More recently, a superior method for generating strained cycloalkynes was developed by Fujita. It involves base induced
β-elimination In chemistry, dehydrohalogenation is an elimination reaction which removes a hydrogen halide from a substrate. The reaction is usually associated with the synthesis of alkenes, but it has wider applications. Dehydrohalogenation from alkyl halide ...
of
vinyl Vinyl may refer to: Chemistry * Polyvinyl chloride (PVC), a particular vinyl polymer * Vinyl cation, a type of carbocation * Vinyl group, a broad class of organic molecules in chemistry * Vinyl polymer, a group of polymers derived from vinyl ...
iodonium A halonium ion is any onium ion containing a halogen atom carrying a positive charge. This cation has the general structure where X is any halogen and no restrictions on R, this structure can be cyclic or an open chain molecular structure. Halon ...
salts. The vinyl iodonium proved to be a particularly useful synthetic precursor to strained cycloalkynes due to its high reactivity which arises from the strong electron withdrawing ability of the positively charged iodine species as well as the
leaving group In organic chemistry, a leaving group typically means a Chemical species, molecular fragment that departs with an electron, electron pair during a reaction step with heterolysis (chemistry), heterolytic bond cleavage. In this usage, a ''leaving gr ...
ability of the iodonium. In addition to the elimination-type pathways described, cycloalkynes can also be obtained through the oxidation of cyclic with
mercury oxide Mercury oxide can refer to: * Mercury(I) oxide (mercurous oxide), Hg2O * Mercury(II) oxide (mercuric oxide), HgO See also *Montroydite, the mineral form of mercury(II) oxide {{Short pages monitor