Apolyyne is anyorganic compound with alternatingsingle andtriple bonds; that is, a series of consecutivealkynes,(−C≡C−)_n withn greater than 1. These compounds are also calledpolyacetylenes, especially in the natural products and chemical ecology literature,^[1] even though this nomenclature more properly refers toacetylene polymers composed of alternating single and double bonds(−CR=CR′−)_n withn greater than 1. They are also sometimes referred to asoligoynes,^{[2][needs IPA]} orcarbinoids 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.

The simplest polyyne isdiacetylene or butadiyne,H−C≡C−C≡C−H. Along withcumulenes, polyynes are distinguished from other organic chains by their rigidity and high conductivity,^[7] both of which make them promising as wires inmolecular nanotechnology. Polyynes have been detected in interstellarmolecular clouds wherehydrogen is scarce.^{[citation needed]}

The first reported synthesis of a polyyne was performed in 1869 byCarl Andreas Glaser [de], who observed that copper phenylacetylide (CuC≡C−C₆H₅) undergoesoxidativedimerization 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 byorganic synthesis by several general routes. As a main synthetic tool usually acetylene homocoupling reactions like theGlaser coupling or its associated Elinton and Hay protocols are used.^[8][4] Moreover, many of such procedures involve aCadiot–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]

Using various techniques, polyynesH(−C≡C−)_nH withn up to 4 or 5 were synthesized during the 1950s.^[12] Around 1971,T. R. Johnson andD. R. M. Walton developed the use of end-caps of the form –SiR₃, where R was usually anethyl group, to protect the polyyne chain during the chain-doubling reaction usingHay's catalyst (acopper(I)–TMEDAcomplex).^[12][13] With that technique they were able to obtain polyynes like(CH₃CH₂)₃Si(−C≡C−)_nSi(CH₂CH₃)₃ withn up to 8 in pure state, and withn 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-typearomaticpolyetherdendrimers, was isolated and characterized in 2002.^[2] Moreover, the synthesis of dicyanopolyynes with up to 8 acetylenic units was reported.^[15] The longestphenyl 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 withtris(3,5-di-t-butylphenyl)methyl groups.^[10]

Alkynes with the formulaH(−C≡C−)_nH andn from 2 to 6 can be detected in the decomposition products of partially oxidizedcopper(I) acetylide ((Cu⁺)₂(⁻C≡C⁻) (an acetylene derivative known since 1856 or earlier) byhydrochloric acid. A "carbonaceous" residue left by the decomposition also has the spectral signature of(−C≡C−)_n chains.^[16]

Organometallic polyynes capped with metal complexes are well characterized. As of the mid-2010s, the most intense research has concernedrhenium (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] andcobalt (Co₃C(−C≡C−)_nCCo₃,n = 7–13)^[22] complexes.

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 andoxygen, if the end hydrogen atoms are replaced with a suitably inertend-group, such astert-butyl ortrifluoromethyl.^[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 werefullerene-like structures rather than long polyynes.^[5]

Polyyne chains have also been stabilised to heating by co-deposition withsilver nanoparticles,^[25] and by complexation with amercury-containingtridentateLewis acid to form layeredadducts.^[26] Long polyyne chains encapsulated in double-walledcarbon nanotubes or in the form ofrotaxanes^[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]

Synthetic polyynes of the formR(−C≡C−)_nR, withn 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 istriisopropylsilyl andn 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 withn = 4 orn = 5 have nearly straight backbones that stay at least 0.5 to 0.7 nm apart, and melt without decomposing.^[14]

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 inAsterids clade, especially in thesunflower,carrot,ginseng andbellflower families. However, they can also be found in some members of thetomato,olax, andsandalwood families.^[32] The earliest polyyne to be isolated was dehydromatricaria ester (DME) in 1826; however, it was not fully characterized until later.^[1][33]

The simple fatty acid 8,10-octadecadiynoic acid is isolated from the root bark of the legumeParamacrolobium coeruleum of the familyCaesalpiniaceae and has been investigated as aphotopolymerizable unit in syntheticphospholipids.^[9]

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

Polyynes such asfalcarindiol can be found inApiaceae vegetables likecarrot,celery,fennel,parsley andparsnip where they show cytotoxic activities.^[35] AliphaticC₁₇-polyynes of the falcarinol type were described to act as metabolic modulators^[36][37] and are studied as potential health-promotingnutraceuticals.^[38] Falcarindiol is the main compound responsible for bitterness incarrots, and is the most active among several polyynes with potential anticancer activity found inDevil's club (Oplopanax horridus). Other polyynes fromplants includeoenanthotoxin andcicutoxin, which are poisons found inwater dropwort (Oenanthe spp.) andwater hemlock (Cicuta spp.).

Ichthyothere is a genus of plants whose active constituent is a polyyne calledichthyothereol. This compound is highlytoxic tofish andmammals.^[39]Ichthyothere terminalisleaves have traditionally been used to make poisoned bait by indigenous peoples of the lowerAmazon basin.^[39]

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

Theoctatetraynyl radicals andhexatriynyl 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 mineralchaoite, but this interpretation has been contested.^[43] SeeAstrochemistry.

• Cyanopolyyne
• Enediyne
• Enyne

• Polyynes
• Conjugated hydrocarbons
• Astrochemistry

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