Polyyne

The first reported synthesis of a polyyne was performed in 1869 by , who observed that copper phenylacetylide () undergoes oxidative dimerization in the presence of air to produce diphenylbutadiyne (). An elimination of chlorovinylsilanes was used as a final step in the synthesis of the longest known phenyl end-capped polyynes. Organic and organosilicon polyynes Using various techniques, polyynes {{chem2|H(\sC\tC\s)_{n}H}} with n up to 4 or 5 were synthesized during the 1950s. With that technique they were able to obtain polyynes like {{chem2|(CH3CH2)3Si(\sC\tC\s)_{n}Si(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 {{chem2|((CH3)2CH)3Si(\sC\tC\s)_{n}Si(CH(CH3)2)3}} polyynes with chain length up to C20. The longest phenyl end-capped polyynes were reported by Cox and co-workers in 2007. polyynes Organometallics Organometallic polyynes capped with metal complexes are well characterized. As of the mid-2010s, the most intense research has concerned rhenium ({{chem2|Re(\sC\tC\s)_{n}Re}}, n = 3–10), ruthenium ({{chem2|RuRu(\sC\tC\s)_{n}RuRu}}, n = 4–10), iron (), platinum ({{chem2|Pt(\sC\tC\s)_{n}Pt}}, n = 8–14), palladium ({{chem2|Ar(\sC\tC\s)_{n}Pd}}, n = 3–5, Ar = aryl), and cobalt ({{chem2|Co3C(\sC\tC\s)_{n}CCo3}}, n = 7–13) complexes. polyynes. == Stability ==

Long polyyne chains are said to be inherently unstable in bulk because they can cross-link with each other exothermically. Explosions are a real hazard in this area of research. 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. However the report has been contested by a claim that the detected molecules were fullerene-like structures rather than long polyynes. and by complexation with a mercury-containing tridentate Lewis acid to form layered adducts. Long polyyne chains encapsulated in double-walled carbon nanotubes or in the form of rotaxanes have also been shown to be stable. Despite rather low stability of longer polyynes there are some examples of their use as synthetic precursors in organic and organometallic synthesis. ==Structure==

Synthetic polyynes of the form {{chem2|R(\sC\tC\s)_{n}R}}, 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. 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. ==Occurrence==

Biological origins A wide range of organisms synthesize polyynes. 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 the euasterids, 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. The earliest polyyne to be isolated was dehydromatricaria ester (DME) in 1826; however, it was not fully characterized until later. 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. 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. Polyynes such as falcarindiol can be found in Apiaceae vegetables like carrot, celery, fennel, parsley and parsnip where they show cytotoxic activities. Aliphatic -polyynes of the falcarinol type were described to act as metabolic modulators and are studied as potential health-promoting nutraceuticals. 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.). Ichthyothere is a genus of plants whose active constituent is a polyyne called ichthyothereol. This compound is highly toxic to fish and mammals. Ichthyothere terminalis leaves have traditionally been used to make poisoned bait by indigenous peoples of the lower Amazon basin. In space The octatetraynyl radicals and hexatriynyl radicals together with their ions are detected in space where hydrogen is rare. Moreover, there have been claims that polyynes have been found in astronomical impact sites on Earth as part of the mineral chaoite, but this interpretation has been contested. See Astrochemistry. ==See also==