A polyyne is any organic compound with alternating Single bond and ; that is, a series of consecutive , with n greater than 1. These compounds are also called , especially in the natural products and chemical ecology literature,[
]
or carbinoids after "carbyne" , the hypothetical allotrope of carbon that would be the ultimate member of the series.[
][ In Avancés récentes en chimie des acétylènes – Recent advances in acetylene chemistry
] The synthesis of this substance has been claimed several times since the 1960s, but those reports have been disputed.[ Indeed, the substances identified as short chains of "carbyne" in many early organic synthesis attempts]
The simplest polyyne is diacetylene or butadiyne, . Along with , polyynes are distinguished from other organic chains by their rigidity and high conductivity,[
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Synthesis
The first reported synthesis of a polyyne was performed in 1869 by , who observed that copper phenylacetylide () undergoes oxidation dimerization in the presence of air to produce diphenylbutadiyne ().[
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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.[ 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.][ In addition to that, Fritsch–Buttenberg–Wiechell rearrangement was used as crucial step during the synthesis of the longest known polyyne ().][
] 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 with n up to 4 or 5 were synthesized during the 1950s.[ Around 1971, T. R. Johnson and D. R. M. Walton developed the use of end-caps of the form –, where R was usually an ethyl group, to protect the polyyne chain during the chain-doubling reaction using Hay's catalyst (a copper–TMEDA complex).]
A polyyne compound with 10 acetylenic units (20 atoms), with the ends capped by Fréchet-type aromatic polyether , was isolated and characterized in 2002.[ Moreover, the synthesis of dicyanopolyynes with up to 8 acetylenic units was reported.]
with the formula and n from 2 to 6 can be detected in the decomposition products of partially oxidized copper(I) acetylide ( (an acetylene derivative known since 1856 or earlier) by hydrochloric acid. A "carbonaceous" residue left by the decomposition also has the spectral signature of chains.[
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Organometallics
Organometallic polyynes capped with metal complexes are well characterized. As of the mid-2010s, the most intense research has concerned rhenium (, n = 3–10),
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.[ Bulky end-groups, that can keep the chains apart, work especially well at stabilizing polyynes.][ In 1995 the preparation of carbyne chains with over 300 carbon atoms was reported using this technique.][
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Polyyne chains have also been stabilised to heating by co-deposition with silver nanoparticles,
Structure
Synthetic polyynes of the form , 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.[
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Natural occurrence
Biological origins
A wide range of organisms synthesize polyynes.[Stavholt, K., and N. A. Sorensen. 1950. Studies relating to naturally-occurring Acetylene Compounds: V. Dehydro Matricaria Ester (Methyl n-decene-triynoate) from the Essential Oil of Artemisia vulgaris L. Acta Chemica Scandinavia 4.]
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 .
Thiarubrine B is the most prevalent among several related light-sensitive 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.
Ichthyothere is a genus of plants whose active constituent is a polyyne called ichthyothereol. This compound is highly toxic to fish and mammals.
Dihydromatricaria acid is a polyyne produced and secreted by as a chemical defense.
In space
The octatetraynyl radicals and hexatriynyl radicals together with their ions are detected in space where hydrogen is rare.
See also
