Metathesis catalysts, acetylene

CH3OC//2CH2OCH3), S 1.28 (s, 9H, CC(CH3)3). The complex Cl3(dme)W= CC(CH3)3 is an important intermediate for preparing olefin and acetylene metathesis catalysts.3,4 It is also a useful starting material for preparing certain tungstenacyclobutadiene, /3-cyclopropenyl, and 5-cyclopentadienyl... 

On the basis of the fact that tungsten(VI) alkylidene complexes will metathesize olefins one might predict that acetylenes should be metathesized by tungsten(VI) alkylidyne complexes (29). Acetylene metathesis is not unknown, but the catalysts are inefficient and poorly understood (30, 31). 

Initially alkynes were polymerised by trial and error with the use of Ziegler type recipes and the mechanism for these reactions may well be an insertion type mechanism. Undefined metathesis catalysts of ETM complexes were known to give poly-acetylene in their reaction with alkynes (acetylene) [45] and metallacycles were proposed as intermediates. Since the introduction of well-defined catalysts far better results have been obtained. The mechanism for this reaction is shown in Figure 16.24 [46], The conductive polymers obtained are soluble materials that can be treated and deposited as solutions on a surface. 

Poly (acetylenes) [16], There are several catalysts available for polymerization of substituted acetylenes. Whereas Ziegler-Natta catalysts are quite effective for polymerization of acetylene itself and simple alkylacetylenes, they are not active towards other substituted acetylenes, e.g. phenylacetylenes. Olefin-metathesis catalysts (Masuda, 1985 Masuda and Higashimura, 1984, 1986) and Rh(i) catalysts (Furlani et al., 1986 Tabata, 1987) are often employed. In our experience, however, many persistent radicals and typical nitrogen-containing functional groups serve as good poisons for these catalysts. Therefore, radical centres have to be introduced after construction of the polymer skeletons. Fortunately, the polymers obtained with these catalysts are often soluble in one or other organic solvent. For example, methyl p-ethynylbenzoate can be polymerized to a brick-coloured amorph- See the Appendix on p. 245 of suffixes to structural formula numbers. 

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. 

X. POLYMERIZATION OF ACETYLENES BY OLEFIN METATHESIS CATALYSTS A. Proof of Mechanism... 

Acetylenes copolymerize with each other and with cycloalkenes under the influence of olefin metathesis catalysts. With non-living systems it is possible to make statistical... 

Acetylenic monomers also appeared to undergo polymerisation with conventional olefin metathesis catalysts. This relates to monosubstituted highly branched alkylacetylenes and arylacetylenes as well as disubstituted acetylenes (internal alkynes) [16-18], It has been demonstrated that acetylene itself may also be polymerised using olefin metathesis catalysts [19,20]. The polymerisation of alkynes [scheme (2)] involves a metathesis reaction [scheme (5) of Chapter 2] analogously to that of cycloolefins [21] ... 

The mechanism of polymerisation of alkynes with metathesis catalysts requires that the original triple bond of the acetylenic monomer becomes a single bond in the polymer [scheme (5) of Chapter 2], in contrast to the insertion mechanism of acetylene polymerisation with Ziegler-Natta catalysts, where the triple bond becomes a double bond [scheme (1)]. Ideas about the mechanism of metathesis polymerisation of cycloolefins suggested that isolable metal carbenes might promote the polymerisation of cycloolefins suggested that isolable metal carbenes might promote the polymerisation of alkynes, as indeed turned out to be true, as several metal carbenes were found [22-24] to cause alkyne polymerisation. 

As in the case of the ring-opening metathesis polymerisation of cycloolefins, an important matter is the control of polymerisation to prepare acetylenic polymers having precise structures. A living polymerisation is of practical importance in the synthesis of monodisperse polymers, such as terminally functionalised polymers and block copolymers. The metathesis catalysts that promote the living polymerisation of acetylene [42] and acetylenic monomers include M0OCI4 SnBu EtOFkNbCls and Ta, Mo and W alkylidenes [84, 133, 152, 153]. 

The reaction of the rhenium alkylidyne complex 277 with diisopropyl-acetylene and with diethylacetylene [Eq. (196)] demonstrates the sensitivity of metathesis reactions toward steric factors (57). With diisopropylace-tylene an alkylidyne complex is obtained whereas the reaction with diethylacetylene gives a metallacyclobutadiene. In the metathesis reactions the alkyne with the bulkiest groups cleaves most easily from intermediate metallacyclobutadiene complexes. The rhenacyclobutadienes with the smallest substituents thus become sinks and slow down the effective rate of metathesis. The alkylidyne alkylidene rhenium complex 278 is an active olefin metathesis catalyst (52). Reaction with hexene transforms the neo-pentylidene group into a propylidene group as shown in Eq. (197). 

Diynes, such as dipropargyl derivatives, are amenable to cyclopolymerization giving high-molecular-weight polymers (eq. (11), where X = O, S, R2Si, C(C02Et)2, etc). In the presence of an olefin metathesis catalyst, acetylenes copolymerize with each other and with cyclic alkenes. 

Figure 10-10 Polymerization of 1,6-diynes using a molybdenum alkylidene catalyst [Rp is (CFjljCHjC] [67]. The 1,6-diyne monomer is drawn in two different exaggerated conformations to illustrate that head-tail polymerization leads to six-membered rings, and tail-tail polymerization leads to five-membered rings. See Fig. 10-8 for a more mechanistic diagram of acetylene metathesis.

Note that TiCl4/AIEt3 is acting here as a classical metathesis catalyst, in contrast to its behavior as a Ziegler-Natta catalyst with acetylene. 

Two mechanisms have been proposed for acetylene and substituted acetylene polymerization by transition metal catalysts one is the metal-alkyl mechanism and the other is the metal-carbene mechanism. In general, it has been proposed that the polymerization of acetylenes by Ziegler-Natta catalysts proceeds by the metal-alkyl mechanism, while the metal-carbene mechanism has been accepted for the polymerization of substituted acetylenes by metathesis catalysts whose main components are halides or complexes of group 5 and 6 transition metals. The latter will be discussed in Section III. 

It was first observed by Woon (1974) and Farona (1974) that acetylenes could be polymerized by catalysts of the type Mo(CO)3(toluene). This was followed by the discovery that conventional metathesis catalysts such as M0CI5 (Masuda 1974) and WCls (Navarro 1976 Masuda 1976), with or without a cocatalyst, could also bring about polymerization of acetylenes. At first there was some doubt as to whether these polymerizations were being propagated by the metathesis mechanism (Scheme 10.2) or whether a Ziegler-Natta mechanism was operating. However, the observation that metal carbene complexes could react with acetylenic molecules to form simple adducts as in reaction (20) (Fischer, H. 1980), and the fact that such complexes could initiate the polymerization of acetylenes, albeit somewhat slowly, but cleanly and in fair yield, soon allayed these doubts. 

In Table 10.1 are simimarized the alkylacetylenes that have been polymerized by metathesis catalysts. The halides NbCls, TaCls, M0CI5, and WCl can be used without a cocatalyst because acetylenes themselves react readily with the halide to generate an initiating metal carbene complex, perhaps through a sequence such as eqns. (23a) or (23b) (Weiss 1986b). 

Acetylenes may be regarded as the first members of the series their polymerization using olefin metathesis catalysts is described in Ch. 10. There is no recorded attempt to polymerize cyclopropene with metathesis catalysts the product would probably be cyclohexa-1,4-diene rather than polymer. 

Recently. Shirakawa et al. also synthesized mono-substituted acetylenes with liquid crystallinemoieties by Ziegler—Natta and metathesis catalysts (see Chart... 

Compounds of type 7 proved to be remarkably active catalysts for the metathesis of internal olefins. [44,68,69] The activity of such species for the metathesis of ordinary internal olefins (e.g., c 5 -2-pentene) appeared to maximize for the OCMe(CF3)2 species. New alkylidene complexes such as W(NAr)(CHPh)[OCMe(CF3)2]2 could be isolated, and in some cases trigonal bipyramidal (TBP) tungstacyclobutane intermediates were stable enough to be observed and isolated. On the basis of this work it was proposed that the rate of reaction of alkylidene complexes with olefins correlated directly with the electron-withdrawing ability of the alkoxide, as found in acetylene metathesis systems described earlier. In many circumstances trigonal bipyramidal or square pyramidal tungstacyclobutane intermediates could be observed. [44] In any system in which ethylene could be formed, unsubstituted metallacycles could... 

Oh, S.Y., R. Ezaki, K. Akagi, and H. Shirakawa. 1993. Polymerization of monosuhstituted acetylenes with a liquid-crystalline moiety by Ziegler-Natta and metathesis catalysts. J Polym Sci Polym Chem 31 2977. 

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