Substituted alkynes, polymerization

Polymerization of alkynes by Ni" complexes produces a variety of products which depend on conditions and especially on the particular nickel complex used. If, for instance, O-donor ligands such as acetylacetone or salicaldehyde are employed in a solvent such as tetrahydrofuran or dioxan, 4 coordination sites are available and cyclotetramerization occurs to give mainly cyclo-octatetraene (cot). If a less-labile ligand such as PPhj is incorporated, the coordination sites required for tetramerization are not available and cyclic trimerization to benzene predominates (Fig. A). These syntheses are amenable to extensive variation and adaptation. Substituted ring systems can be obtained from the appropriately substituted alkynes while linear polymers can also be produced. 

On the other hand polysilylalkynes with phenyl or allyl substituents are converted with triflic acid into polymeric alkynylsilyltriflates. These polymers react with many acidic element hydrogen compounds or lithium element compounds with formation of silicon element bonds. Thus we found an easy approach to numerous new functional substituted alkynes [12], Eq.(9) shows selected examples of this reaction type. 

In 1975, it was discovered that WCk, which is a typical metathesis catalyst, is capable to catalyze the polymerization of phenylacetyl-ene. Subsequently, various substituted acetylenes have been polymerized by this type of catalyst. In 1983, poly(l-(trimethylsilyl)-l-propyne)) was synthesized in the presence of Tads and NbCls (35). The alkyne polymerization has many similarities with ROMP. 

The polymerization of substituted alkynes is postulated to proceed either by the metathesis mechanism or by an insertion mechanism (18). Numerous alkyne derivates have been shown to polymerize in the presence of group V, VI, and VIII transition metal catalysts. 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

The isolation of these closely related thiolate complexes hints at an important role for 172-vinyl ligands in reactions which lead to net ligand substitution at metal. The SR bridge between Cp and W may resemble a snapshot along a reaction path for alkyne insertion into a M—L bond or for transfer of L from an T 2-vinyl to metal (97). A mechanism for alkyne polymerization based on rj2-vinyl intermediates has also been constructed (186). 

Metals of Groups 5 and 6 (Nb, Ta, Mo and W) are known to form carbene complexes and are widely used in olefin metathesis [99, 100, 111]. Therefore, the polymerization of substituted alkynes with catalysts based on these metals is assumed... 

Another example of self-assembly of porphyrin-containing polymer was illustrated by Li et al.73 Polyacetylene functionalized with fullerene and zinc porphyrin pendant groups were synthesized by polymerizing the corresponding fullerene/porphyrin substituted alkyne monomers with rhodium(I) norbomadiene catalyst (Scheme 5.5).74 Polymers with different ratio of C60 and porphyrin were synthesized. The polymers showed photocurrent response when the thin films were irradiated with white light, which was due to the electron transfer from the photo-excited porphyrin to the C60 units. In addition, the copolymers aggregated into ellipse-shaped nanorod structures with a diameter of approximately 100 nm and a length of... 

Finally, one may note the curious behaviour of alkynes. If Mo(CO)6/non-4-yne is irradiated at room temperature and an excess of 3-chlorophenol then added, there is rapid metathesis to give oct-4-yne and dec-5-yne with nearly 100% selectivity (Mortreux 1977). In contrast, W(CO)6/CCl4//iv (350 ran) induces polymerization of hept-2-yne while causing metathesis of pent-2-ene present in the same reaction mixture (Stockel 1978). The difference in behaviour of the two systems presumably lies in the ability of Mo(CO)6/non-4-yne// v to generate a metal carbyne on addition of the phenol, whereas W(CO)6/CCl4/Av gives only a metal carbene. Photoassisted polymerization of terminal alkynes takes place with WCV/iv in hydrocarbon solutions (Landon 1985) photocatalyzed polymerization of substituted alkynes is induced by W(CO)6/SnCl4// v (Tamura 1994). For related systems, see Ch. 10. 

Figure 11. Metallocenyl-substituted 1-alkynes polymerized via metathesis polymerization.

A variety of heteroatom substituted 1-alkynes were polymerized with the three carbyne W(VI) complexes. These catalysts tolerate in alkyne polymerization reactions more heteroatom substituents than in olefin metathesis [4, 5, 6, 7], Table 3. 

TABLE 3. Polymerization of heteroatom substituted alkynes with Cl3(dme)W=C Bu (Cl), Np3WsC Bu (Np) and ( BuO)3WsC Bu ( BuO) solvent CH2CI2 temperature 25°C molar ratio W/alkyne = 1/100 yield and molecular weight of the polyalkynes after 24 h n.s. = not soluble in THF, CH2CI2. 

Scheme XX. Possible products from a polymerization of substituted alkynes.

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