Polyyne cyclic

D. R. M. Walton. The early polyyne papers were by Jones and by Bohlmann, and to my knowledge, the crystal structures were not published, the latest papers are by Diederich and deal with cyclic carbon oxides. Don t press me for details over bond lengths, however there is a tendency towards bond equalization and the curvature is there. 

The chemistry of acyclic, synthetic polyacetylenes will be considered in this chapter. Cyclic and naturally occuring polyacetylenes are covered in separate chapters. Although most of the discussion is directed toward conjugated polyynes, reactions of non-conjugated derivatives are included in which interactions between triple bonds play an important role. Earlier work in the area has been reviewed thoroughly and consequently in this chapter attention is directed mainly toward some of the more recent advances. 

In 1961, Wotiz and Dale independently reported the new syntheses of non-conjugated cyclic polyynes. When a,a -dibromides are treated with a mixture of sodium acetylide and disodium acetylide in liquid ammonia, cyclic diynes (129) are obtained together with the linear polyynes (130). Sodium acetylide, being a chain terminator, prevents the formation of large amounts of linear polyynes. 1,8-Cyclo-tetradecadiyne (129, n = 5) and the 22-membered dioxadiyne (131) were obtained by this method. 

Coupling by copper(II) acetate in pyridine is also a useful procedure for active alkynes such as phenyl-acetylene and conjugated polyynes. This method is also a useful procedure for making cyclic alkynes. 

Increasing interest has been focused on the acetylene-based polymer chemistry relevant to cyclic polyynes — the topic of this chapter [1]. Cyclic polyynes are unique members in the family of sp-carbon allotropes. Like linear polyynes, they are composed of unsaturated carbon atoms covalently... 

In this chapter, keeping in mind the generation, characterization, and reactions of the cyclic polyynes, the interplay of organic chemistry and carbon cluster science during the last decade is presented. First, following short historical remarks (Section 6.2.1), recent research activity on the production of cyclo[ ]carbons from well-defined organic precursors is surveyed (Section 6.2.2). Second, major structural and electronic properties of mono-cyclic carbon clusters are presented in the context of theoretical considerations (Section 6.2.3), followed by observational results of photoelectron spectroscopy (Section 6.2.4). Third, considerations on the infrared activity of cyclic Cio will be presented (Section 6.2.5). Finally, this chapter ends with experimental as well as theoretical proposals for the structures of multicyclic polyynes (Section 6.3) and their relevance to the formation of fullerenes, in particular from polycyclic polyynes (Section 6.4). 

The stability of Cio suggests a monocyclic form because linear Cio, whether cumulenic or polyynic, must be subjected to the attack by hydrogen atoms according to the discussion stated above. More convincing evidence for the cyclic form is desired, however. 

Figure 3.43 Metalotalyzed growth on the open end of a nanotube according to the scooter mechanism (a) insertion of C2-units into carbon-metal bonds, (b) complexing of the metal atom by a cyclic polyyne.

A second hypothesis on the attachment of metal atoms postulates the existence of a cyclic polyyne on the rim of the growing nanotube where the metal atom moves along (Figure 3.43b). Yet this kind of structure is logical only for armchair nanotubes as they alone can form a respective polyyne without building up too much strain. 

Linear C species may be represented simply as shown in Fig. 1-6. For an even number of carbons, the simplest electronic structure may be either a dicarbene-cumulene structure, or a diradical-polyyne . The corresponding cyclic structures will be nonlinear and strained, but formally possess closed-shell cumulene or polyyne structures. These differ by having all bond lengths equal, or alternating bond lengths, respectively. For it = odd, the linear structures may be of the dicarbene-cumulene or tetraradical-polyyne type. The cyclic isomers may be cumulene or carbene-polyyne . 

Figure 1-6 Electronic structures of odd and even length (n) linear and cyclic polyynes.

Enyne metathesis RCM can be performed in tandem with RCM using appropriately spaced diene-yne substrates. Many examples of this reaction featuring various degrees of complexity have been reported. Representative examples of tandem enyne metathesis-RCM are depicted in Scheme 30 and include (i) formation of the fused bicyclic compound 257 from dienyne 255 for securinine total synthesis/ (ii) synthesis of cyclic ethers (e.g., 261 and 262) from diene-alkynes (e.g, 258) and control of the product distribution through alkene substitution pattern,and (iii) double tandem RCM-enyne metatheses (conversion of 263 into 266) of appropriate polyene-polyyne systems. 

The small clusters of carbon, with a few added elements, can resemble any of these forms. Pure carbon clusters, up to ten atoms or so, are either linear or cyclic, most strongly resembling the graphite or polyyne forms. Larger cluster sizes, or impurity atoms, stabilize the diamond form. The graphite and polyyne forms have in common a conjugated tt electron system, so these small carbon clusters and their relatives may be cultivated into wires for nanometer-scale electronics. 

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