Polyyne

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

In chemistry, a polyyne is any organic compounds with alternating single and triple bonds; that is, (−C≡C−)
n with n greater than 1. The simplest example is diacetylene or buta-1,3-diyne, H−C≡C−C≡C−H.

These compounds have also been called oligoynes,^[1] or carbinoids after "carbyne" (−C≡C−)
∞, the hypothetical allotrope of carbon that would be the ultimate member of the series.^[2][3] The synthesis of this substance has been claimed several times since the 1960s, but those reports have been disputed.^[4] Indeed the substances identified as short chains of "carbyne" in many early organic synthesis attempts^[5] would be called polyynes today.

Polyynes should not be confused with polyacetylenes, polymers formally obtained by polymerization of acetylene and its derivatives, whose backbones have alternating single and double bonds n.

Along with cumulenes, polyynes are distinguished from other organic chains by their rigidity, which makes them promising for molecular nanotechnology. Polyynes have been detected in interstellar molecular clouds where hydrogen is scarce.

History

The first reported synthesis of polyynes was performed in 1869 by Carl Glaser, who observed that copper(I) phenylacetylide undergoes oxidative dimerization in the presence of air to produce diphenylbutadiyne.^[3]

Using various techniques, polyynes H(−C≡C−)
nH with n up to 4 or 5 were synthesized during the 1950s.^[6] Around 1971, T. R. Johnson and D. R. M. Walton developed the use of end-caps of the form −SiR
3, 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).^[6][7] With that technique they were able to obtain polyynes like Et
3Si–(C≡C)
m–SiEt
3 with m up to 8 in pure state, and with m up to 16 in solution.

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+
)
2C2−
2 (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.^[8]

Stability

Long polyyne chains are said to be inherently unstable in bulk because they can cross-link with each other in an exothermal (indeed explosive) reaction.^[4] Explosions are a real hazard in this area of research.^[9] They can be fairly stable, even against moisture and oxygen, if they are end-capped with suitable groups inert groups (such as tert-butyl or trifluoromethyl) rather than hydrogen atoms,^[10] especially bulky ones that can keep the chains apart.^[1] In 1995 the preparation of carbyne chains with over 300 carbons was reported using this technique.^[10] However the report has been contested by a claim that the detected molecules were fullerene-like structures rather than long polyynes.^[4]

A polyyne compound with 10 acetylenic units (20 atoms), with the ends capped by Fréchet-type aromatic polyether dendrimers, was isolated and characterized in 2004.^[1] As of 2010, the polyyne with the longest chain yet isolated had 22 acetylenic units (44 atoms), end-capped with tris(3,5-di-t-butylphenyl)methyl groups.^[11]

Structure

Synthetic polyynes of the form R−(−C≡C−)
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.^[12]

Natural occurrence

Biological origins

A wide range of organisms synthesize polyacetylenes.^[13][14] and many are associated with medicinal properties. The acetylenic fatty acid 1 is isolated from the root bark of the legume Paramacrolobium caeruleum of the Showy Mistletoe (Loranthaceae) family. The stems and leaves of members of this family have been used for the treatment of cancer in Indonesia.

The naturally occurring pigment thiarubrine B (2) has been isolated from the Giant Ragweed (Ambrosia trifida). Plants containing this type of compound have been used to treat skin infections and intestinal parasites by native people in Africa and Canada.

The inner bark and roots of Devil's club (Oplopanax horridus) is used by native Americans to treat a variety of ailments. One of the polyynes isolated is oplopandiolacetate (3). Dihydromatricaria acid (4) is a polyyne obtained from the soldier beetle. Other polyynes from plants include oenanthotoxin, cicutoxin, and falcarinol.

Some fungal melanins are pure polyyne.^{[citation needed]}

Polyynes can be found in Apiaceae vegetables like carrot, celery, fennel, parsley and parsnip where they show cytotoxic activities.^[15]

Ichthyothere is a genus of plants whose active constituent is a polyyne called ichthyothereol. This compound is so toxic to fish that they will jump out of the water if Ichthyothere terminalis leaves are used as bait.

Because of the potential medicinal properties of these polyynes, their synthetic pathways are being studied with the hope that these can be replicated industrially by organic synthesis. Many such procedures involve a Cadiot–Chodkiewicz coupling.

In space

The octatetraynyl radicals and hexatriynyl radicals together with their ions are detected in space where Hydrogen is rare. See Astrochemistry.

See also

• Cyanopolyyne

References