Polyyne

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Ichthyothereol is a polyyne that occurs in plants in the genus Ichthyothere and is highly toxic to fish Ichthyothereol skeletal.png
Ichthyothereol is a polyyne that occurs in plants in the genus Ichthyothere and is highly toxic to fish

A polyyne is any organic compound with alternating single and triple bonds; that is, a series of consecutive alkynes, (−C≡C−)n with n greater than 1. These compounds are also called polyacetylenes, especially in the natural products and chemical ecology literature, [1] even though this nomenclature more properly refers to acetylene polymers composed of alternating single and double bonds (−CR=CR′−)n with n greater than 1. They are also sometimes referred to as oligoynes, [2] [ needs IPA ] or carbinoids after "carbyne" (−C≡C−)∞, the hypothetical allotrope of carbon that would be the ultimate member of the series. [3] [4] The synthesis of this substance has been claimed several times since the 1960s, but those reports have been disputed. [5] Indeed, the substances identified as short chains of "carbyne" in many early organic synthesis attempts [6] would be called polyynes today.

Contents

The simplest polyyne is diacetylene or butadiyne, H−C≡C−C≡C−H. Along with cumulenes, polyynes are distinguished from other organic chains by their rigidity and high conductivity, [7] both of which make them promising as wires in molecular nanotechnology. Polyynes have been detected in interstellar molecular clouds where hydrogen is scarce.[ citation needed ]

Synthesis

The first reported synthesis of a polyyne was performed in 1869 by Carl Andreas Glaser  [ de ], who observed that copper phenylacetylide (CuC≡C−C6H5) undergoes oxidative dimerization in the presence of air to produce diphenylbutadiyne (C6H5−C≡C−C≡C−C6H5). [4]

Interest in these compounds has stimulated research into their preparation by organic synthesis by several general routes. As a main synthetic tool usually acetylene homocoupling reactions like the Glaser coupling or its associated Elinton and Hay protocols are used. [8] [4] Moreover, many of such procedures involve a Cadiot–Chodkiewicz coupling or similar reactions to unite two separate alkyne building-blocks or by alkylation of a pre-formed polyyne unit. [9] In addition to that, Fritsch–Buttenberg–Wiechell rearrangement was used as crucial step during the synthesis of the longest known polyyne (C44). [10] An elimination of chlorovinylsilanes was used as a final step in the synthesis of the longest known phenyl end-capped polyynes. [11]

Organic and organosilicon polyynes

Using various techniques, polyynes H(−C≡C−)nH with n up to 4 or 5 were synthesized during the 1950s. [12] Around 1971, T. R. Johnson and D. R. M. Walton developed the use of end-caps of the form –SiR3, where R was usually an ethyl group, to protect the polyyne chain during the chain-doubling reaction using Hay's catalyst (a copper(I)–TMEDA complex). [12] [13] With that technique they were able to obtain polyynes like (CH3CH2)3Si(−C≡C−)nSi(CH2CH3)3 with n up to 8 in pure state, and with n up to 16 in solution. Later Tykwinski and co-workers were able to obtain ((CH3)2CH)3Si(−C≡C−)nSi(CH(CH3)2)3 polyynes with chain length up to C20. [14]

A polyyne compound with 10 acetylenic units (20 atoms), with the ends capped by Fréchet-type aromatic polyether dendrimers, was isolated and characterized in 2002. [2] Moreover, the synthesis of dicyanopolyynes with up to 8 acetylenic units was reported. [15] The longest phenyl end-capped polyynes were reported by Cox and co-workers in 2007. [11] As of 2010, the polyyne with the longest chain yet isolated had 22 acetylenic units (44 carbon atoms), end-capped with tris(3,5-di-t-butylphenyl)methyl groups. [10]

Alkynes with the formula H(−C≡C−)nH and n from 2 to 6 can be detected in the decomposition products of partially oxidized copper(I) acetylide ((Cu+)2(−C≡C−) (an acetylene derivative known since 1856 or earlier) by hydrochloric acid. A "carbonaceous" residue left by the decomposition also has the spectral signature of (−C≡C−)n chains. [16]

Organic and organosilicon polyynes Organic polyynes.jpg
Organic and organosilicon polyynes

Organometallics

Organometallic polyynes capped with metal complexes are well characterized. As of the mid-2010s, the most intense research has concerned rhenium (Re(−C≡C−)nRe, n = 3–10), [17] ruthenium (RuRu(−C≡C−)nRuRu, n = 4–10), [18] iron (Fe(−C≡C−)6Fe), [19] platinum (Pt(−C≡C−)nPt, n = 8–14), [20] palladium (Ar(−C≡C−)nPd, n = 3–5, Ar = aryl), [21] and cobalt (Co3C(−C≡C−)nCCo3, n = 7–13) [22] complexes.

Examples of known organometallic polyynes. Organometallic polyynes.tif
Examples of known organometallic polyynes.

Stability

Long polyyne chains are said to be inherently unstable in bulk because they can cross-link with each other exothermically. [5] Explosions are a real hazard in this area of research. [23] They can be fairly stable, even against moisture and oxygen, if the end hydrogen atoms are replaced with a suitably inert end-group, such as tert-butyl or trifluoromethyl. [24] Bulky end-groups, that can keep the chains apart, work especially well at stabilizing polyynes. [2] In 1995 the preparation of carbyne chains with over 300 carbon atoms was reported using this technique. [24] However the report has been contested by a claim that the detected molecules were fullerene-like structures rather than long polyynes. [5]

Polyyne chains have also been stabilised to heating by co-deposition with silver nanoparticles, [25] and by complexation with a mercury-containing tridentate Lewis acid to form layered adducts. [26] Long polyyne chains encapsulated in double-walled carbon nanotubes or in the form of rotaxanes [27] have also been shown to be stable. [28] Despite rather low stability of longer polyynes there are some examples of their use as synthetic precursors in organic and organometallic synthesis. [29]

Structure

Synthetic polyynes of the form R(−C≡C−)nR, with n about 8 or more, often have a smoothly curved or helical backbone in the crystalline solid state, presumably due to crystal packing effects. [30] For example, when the cap R is triisopropylsilyl and n is 8, X-ray crystallography of the substance (a crystalline orange/yellow solid) shows the backbone bent by about 25–30 degrees in a broad arch, so that each C−C≡C angle deviates by 3.1 degrees from a straight line. This geometry affords a denser packing, with the bulky cap of an adjacent molecule nested into the concave side of the backbone. As a result, the distance between backbones of neighboring molecules is reduced to about 0.35 to 0.5 nm, near the range at which one expects spontaneous cross-linking. The compound is stable indefinitely at low temperature, but decomposes before melting. In contrast, the homologous molecules with n = 4 or n = 5 have nearly straight backbones that stay at least 0.5 to 0.7 nm apart, and melt without decomposing. [14]

Natural occurrence

Biological origins

A wide range of organisms synthesize polyynes. [1] [31] These chemicals have various biological activities, including as flavorings and pigments, chemical repellents and toxins, and potential application to biomedical research and pharmaceuticals. In plants, polyynes are found mainly in Asterids clade, especially in the sunflower, carrot, ginseng and bellflower families. However, they can also be found in some members of the tomato, olax, and sandalwood families. [32] The earliest polyyne to be isolated was dehydromatricaria ester (DME) in 1826; however, it was not fully characterized until later. [1] [33]

8,10-Octadecadiynoic acid 8,10-octadecadiynoic acid.png
8,10-Octadecadiynoic acid

The simple fatty acid 8,10-octadecadiynoic acid is isolated from the root bark of the legume Paramacrolobium coeruleum of the family Caesalpiniaceae and has been investigated as a photopolymerizable unit in synthetic phospholipids. [9]

Thiarubrine B Thiarubrine B.svg
Thiarubrine B

Thiarubrine B is the most prevalent among several related light-sensitive pigments that have been isolated from the Giant Ragweed (Ambrosia trifida), a plant used in herbal medicine. The thiarubrines have antibiotic, antiviral, and nematocidal activity, and activity against HIV-1 that is mediated by exposure to light. [34]

Falcarindiol Falcarindiol.svg
Falcarindiol
Oenanthotoxin Oenanthotoxin-structure.png
Oenanthotoxin
Cicutoxin Cicutoxin.svg
Cicutoxin

Polyynes such as falcarindiol can be found in Apiaceae vegetables like carrot, celery, fennel, parsley and parsnip where they show cytotoxic activities. [35] Aliphatic C17-polyynes of the falcarinol type were described to act as metabolic modulators [36] [37] and are studied as potential health-promoting nutraceuticals. [38] Falcarindiol is the main compound responsible for bitterness in carrots, and is the most active among several polyynes with potential anticancer activity found in Devil's club (Oplopanax horridus). Other polyynes from plants include oenanthotoxin and cicutoxin, which are poisons found in water dropwort (Oenanthe spp.) and water hemlock (Cicuta spp.).

Ichthyothereol Ichthyothereol skeletal.png
Ichthyothereol

Ichthyothere is a genus of plants whose active constituent is a polyyne called ichthyothereol. This compound is highly toxic to fish and mammals. [39] Ichthyothere terminalis leaves have traditionally been used to make poisoned bait by indigenous peoples of the lower Amazon basin. [39]

Z-Dihydromatricaria acid Z-dihydromatricaria acid.png
Z-Dihydromatricaria acid

Dihydromatricaria acid is a polyyne produced and secreted by soldier beetles as a chemical defense. [40]

In space

The octatetraynyl radicals and hexatriynyl radicals together with their ions are detected in space where hydrogen is rare. [41] Moreover, there have been claims [42] that polyynes have been found in astronomical impact sites on Earth as part of the mineral chaoite, but this interpretation has been contested. [43] See Astrochemistry.

See also

Related Research Articles

<span class="mw-page-title-main">Alkyne</span> Hydrocarbon compound containing one or more C≡C bonds

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 no other functional groups form a homologous series with the general chemical formula CnH2n−2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature. Like other hydrocarbons, alkynes are generally hydrophobic.

<span class="mw-page-title-main">Polyacetylene</span> Organic polymer made of the repeating unit [C2H2]

Polyacetylene usually refers to an organic polymer with the repeating unit [C2H2]n. The name refers to its conceptual construction from polymerization of acetylene to give a chain with repeating olefin groups. This compound is conceptually important, as the discovery of polyacetylene and its high conductivity upon doping helped to launch the field of organic conductive polymers. The high electrical conductivity discovered by Hideki Shirakawa, Alan Heeger, and Alan MacDiarmid for this polymer led to intense interest in the use of organic compounds in microelectronics. This discovery was recognized by the Nobel Prize in Chemistry in 2000. Early work in the field of polyacetylene research was aimed at using doped polymers as easily processable and lightweight "plastic metals". Despite the promise of this polymer in the field of conductive polymers, many of its properties such as instability to air and difficulty with processing have led to avoidance in commercial applications.

<span class="mw-page-title-main">Carborane</span> Class of chemical compounds

Carboranes are electron-delocalized clusters composed of boron, carbon and hydrogen atoms. Like many of the related boron hydrides, these clusters are polyhedra or fragments of polyhedra. Carboranes are one class of heteroboranes.

In organometallic chemistry, acetylide refers to chemical compounds with the chemical formulas MC≡CH and MC≡CM, where M is a metal. The term is used loosely and can refer to substituted acetylides having the general structure RC≡CM. Acetylides are reagents in organic synthesis. The calcium acetylide commonly called calcium carbide is a major compound of commerce.

An alkyne trimerisation is a [2+2+2] cycloaddition reaction in which three alkyne units react to form a benzene ring. The reaction requires a metal catalyst. The process is of historic interest as well as being applicable to organic synthesis. Being a cycloaddition reaction, it has high atom economy. Many variations have been developed, including cyclisation of mixtures of alkynes and alkenes as well as alkynes and nitriles.

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 synthetic routes such as nucleophilic addition and alpha-hydrogen abstraction. The term carbene ligand is a formalism since many are not directly derived from carbenes and most are much less reactive than lone carbenes. Described often as =CR2, carbene ligands are intermediate between alkyls (−CR3) and carbynes (≡CR). Many different carbene-based reagents such as Tebbe's reagent are used in synthesis. They also feature in catalytic reactions, especially alkene metathesis, and are of value in both industrial heterogeneous and in homogeneous catalysis for laboratory- and industrial-scale preparation of fine chemicals.

<span class="mw-page-title-main">Alkyne metathesis</span>

Alkyne metathesis is an organic reaction that entails the redistribution of alkyne chemical bonds. The reaction requires metal catalysts. Mechanistic studies show that the conversion proceeds via the intermediacy of metal alkylidyne complexes. The reaction is related to olefin metathesis.

<span class="mw-page-title-main">Pauson–Khand reaction</span> Chemical reaction

The Pauson–Khand (PK) reaction is a chemical reaction, described as a [2+2+1] cycloaddition. In it, an alkyne, an alkene and carbon monoxide combine into a α,β-cyclopentenone in the presence of a metal-carbonyl catalyst.

<span class="mw-page-title-main">Schwartz's reagent</span> Chemical compound

Schwartz's reagent is the common name for the organozirconium compound with the formula (C5H5)2ZrHCl, sometimes called zirconocene hydrochloride or zirconocene chloride hydride, and is named after Jeffrey Schwartz, a chemistry professor at Princeton University. This metallocene is used in organic synthesis for various transformations of alkenes and alkynes.

Organogermanium chemistry is the science of chemical species containing one or more C–Ge bonds. Germanium shares group 14 in the periodic table with carbon, silicon, tin and lead. Historically, organogermanes are considered as nucleophiles and the reactivity of them is between that of organosilicon and organotin compounds. Some organogermanes have enhanced reactivity compared with their organosilicon and organoboron analogues in some cross-coupling reactions.

The Wulff–Dötz reaction (also known as the Dötz reaction or the benzannulation reaction of the Fischer carbene complexes) is the chemical reaction of an aromatic or vinylic alkoxy pentacarbonyl chromium carbene complex with an alkyne and carbon monoxide to give a Cr(CO)3-coordinated substituted phenol. Several reviews have been published. It is named after the German chemist Karl Heinz Dötz (b. 1943) and the American chemist William D. Wulff (b. 1949) at Michigan State University. The reaction was first discovered by Karl Dötz and was extensively developed by his group and W. Wulff's group. They subsequently share the name of the reaction.

Copper(I) acetylide, Kupfercarbid or cuprous acetylide, is a chemical compound with the formula Cu2C2. Although never characterized by X-ray crystallography, the material has been claimed at least since 1856. One form is claimed to be a monohydrate with formula Cu
2C
2.H
2O is a reddish-brown explosive powder.

<span class="mw-page-title-main">Linear acetylenic carbon</span> Polymer made of repeating −C≡C− units

Linear acetylenic carbon (LAC), also known as carbyne or Linear Carbon Chain (LCC), is an allotrope of carbon that has the chemical structure (−C≡C−)n as a repeat unit, with alternating single and triple bonds. It would thus be the ultimate member of the polyyne family.

Organogold chemistry is the study of compounds containing gold–carbon bonds. They are studied in academic research, but have not received widespread use otherwise. The dominant oxidation states for organogold compounds are I with coordination number 2 and a linear molecular geometry and III with CN = 4 and a square planar molecular geometry.

<span class="mw-page-title-main">Organomolybdenum chemistry</span> Chemistry of compounds with Mo-C bonds

Organomolybdenum chemistry is the chemistry of chemical compounds with Mo-C bonds. The heavier group 6 elements molybdenum and tungsten form organometallic compounds similar to those in organochromium chemistry but higher oxidation states tend to be more common.

Transition metal carbyne complexes are organometallic compounds with a triple bond between carbon and the transition metal. This triple bond consists of a σ-bond and two π-bonds. The HOMO of the carbyne ligand interacts with the LUMO of the metal to create the σ-bond. The two π-bonds are formed when the two HOMO orbitals of the metal back-donate to the LUMO of the carbyne. They are also called metal alkylidynes—the carbon is a carbyne ligand. Such compounds are useful in organic synthesis of alkynes and nitriles. They have been the focus on much fundamental research.

A metal carbido complex is a coordination complex that contains a carbon atom as a ligand. They are analogous to metal nitrido complexes. Carbido complexes are a molecular subclass of carbides, which are prevalent in organometallic and inorganic chemistry. Carbido complexes represent models for intermediates in Fischer–Tropsch synthesis, olefin metathesis, and related catalytic industrial processes. Ruthenium-based carbido complexes are by far the most synthesized and characterized to date. Although, complexes containing chromium, gold, iron, nickel, molybdenum, osmium, rhenium, and tungsten cores are also known. Mixed-metal carbides are also known.

In organometallic chemistry, a transition metal alkyne complex is a coordination compound containing one or more alkyne ligands. Such compounds are intermediates in many catalytic reactions that convert alkynes to other organic products, e.g. hydrogenation and trimerization.

<span class="mw-page-title-main">Zirconocene</span> Chemical compound

Zirconocene is a hypothetical compound with 14 valence electrons, which has not been observed or isolated. It is an organometallic compound consisting of two cyclopentadienyl rings bound on a central zirconium atom. A crucial question in research is what kind of ligands can be used to stabilize the Cp2ZrII metallocene fragment to make it available for further reactions in organic synthesis.

Titanocene bis(trimethylsilyl)acetylene is a formally titanium(II) organometallic compound with the formula Ti(C5H5)2C2(Si(CH3)3)2. This complex and its zirconium analogue are often referred to as Rosenthal's reagent, after the first chemist to synthesize it, Uwe Rosenthal. This article will discuss its history, synthesis, structure, reactivity, and applications.

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