Polyacetylenes cycloaddition

Reactions of Cjq with metal carbene complexes also yield the [6,6] methano-fullerenes [392]. These adducts are probably not formed via a carbene addition, but via a formal [2-1-2] cycloaddition under formation of a metalla cyclobutane intermediate. The Fischer carbene complex [mefhyl(methoxymethylene)]pentacarbonyl chromium can be utilized to prepare l,2-mefhyl(methoxymethano)-fullerene in 20% yield [392]. A tungsten carbene complex was primarily used to initiate the formation of a polyacetylene polymer, but it was discovered that addition of to the complex-polymer-mixture improves the polymerization and dramatically increases the catalytic activity of the carbene complex [393]. can be integrated into the polymer via carbene addition. 

The low reactivity of 2 towards C-C double bonds is also seen in the reaction with a diacetylene linked by a C-C double bond. Again it can be assumed that the reaction sequence is initiated by a four-fold cycloaddition of four molecules of 2 followed by a rearrangement to furnish the two disilabicyclohexadiene units linked by a C-C double bond 10 (Eq. 4). In both molecules the C-C double bonds adopt an all-frans arrangement [6]. These compounds behave like polyacetylenes, where the dlX-trans form is more stable then the all-cis configuration [7]. 

Synthesis of polyacetylene is not a trivial process and this is due to the reactions of acetylene. Polymerization of acetylene can be accompanied by a variety of side reactions, thus dimerization and formation of benzene and cyclooctatetraene, cycloadditions and isomeriza-tions, as well as further reactions of the initial products are anticipated. In attempts to remove catalyst residues and impurities produced by such side reactions different methods have been adopted using a variety of purification procedures. Furthermore, polyacetylene is insoluble, infusible and susceptible to fairly rapid atmospheric degradation. Its insolubility and infusibility not only make purification difficult, but also precludes most conventional fabrication methods and procedures for modifying the polymer s bulk morphology. Hence it is important to produce high-purity material by methods which will allow convenient fabrication and control of the bulk morphology. In... 

The Diels-Alder reaction (Scheme 3.1) of several dienophiles with polyacetylene was investigated. The success of such reactions of an insoluble polymer will, of course, be limited by the diflusion of the reagent into the porous structure, and high conversion yields would only be expected for highly reactive dienophiles. The conversion may also be subject to the influence of statistical effects, since this cycloaddition reaction requires the participation of four adjacent carbon centres along the polyacetylene backbone. The maximum random occupation of the carbon centres for an irreversible reaction of this type has been calculated to be 80%. 

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