Polyyne

In chemistry, a polyyne (/ˈpɒliaɪn/) 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. The simplest example is diacetylene or butadiyne, H−C≡C−C≡C−H. In chemistry, a polyyne (/ˈpɒliaɪn/) 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. The simplest example is diacetylene or butadiyne, H−C≡C−C≡C−H. These compounds have also been called oligoynes, or carbinoids after 'carbyne' (−C≡C−)∞, the hypothetical allotrope of carbon that would be the ultimate member of the series. 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 would be called polyynes today. Polyynes are distinct from polyacetylenes, polymers formally obtained by polymerization of acetylenes. The backbones of polyacetylenes have alternating single and double bonds n. In biochemistry and plant biology, 'polyacetylene' is routinely used to describe naturally occurring polyynes and related species. 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. The first reported synthesis of a polyyne was performed in 1869 by Carl Glaser, who observed that copper phenylacetylide (CuC2C6H5) undergoes oxidative dimerization in the presence of air to produce diphenylbutadiyne (C6H5C4C6H5). 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 Glaser, Elinton or 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 (C44). An elimination of chlorovinylsilanes was used as a final step in the syntesis of the longest known phenyl end-capped polyynes. Using various techniques, polyynes H(−C≡C−)nH 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 −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). With that technique they were able to obtain polyynes like Et3Si−(C≡C)n−SiEt3 with n up to 8 in pure state, and with n up to 16 in solution.Later Tywkinski and co-workers were be able to obtain iPr3Si−(C≡C)n−SiiPr3 polyynes with chain length up to C20. 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. Morevoer, the synthesis of dicyanopolyynes with up to 8 acetylenic units was reported. The longest phenyl end-capped polyynes were reporoted by Cox and co-workers in 2007. 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. 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.