Metathesis catalysts, acetylene olefin

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. 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

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. 

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... 

Ruthenium catalysts, coordinated with an N-heterocyclic carbene allowed for the ROMP of low-strain cyclopentene and substituted cyclopentenes (10,23). Suitable ruthenium and osmium carbene compounds may be synthesized using diazo compounds, by neutral electron donor ligand exchange, by cross metathesis, using acetylene, cumulated olefins, and in an one-pot method using diazo compounds and neutral electron donors (24). The route via diazo compounds is shown in Figure 1.7. 

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] ... 

Apart from the present polymerization of substituted acetylenes, olefin metathesis is known to be catalyzed specifically by W and Mo catalysts 67 69). In olefin meta-... 

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). 

Katz, T. J. Reactions of acetylenes and alkenes induced by catalysts of olefin metathesis. NATO ASI Ser., Ser. C1989, 269, 293-304. 

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. 

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... 

The pentacarbonylrhenium halides, first prepared by Hieber, are starting materials for the syntheses of many novel rhenium carbonyl compounds. Photochemical, vibrational, and kinetic " properties of these molecules have been studied. A rhenium carbonyl halide-alkyl aluminum halide system polymerizes acetylene and is a useful olefin-metathesis catalyst. " ... 

Well-defined Ru carbene catalysts, which are well known as very active catalysts for olefin metathesis reactions, have been elucidated to polymerize un- and substituted acetylenes such as a,co-diynes, propiolic acid esters, and DPAs. Grubbs 1st, 2nd-, and 3rd- (23-25, respectively. Figure 8) generation catalysts... 

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). 

The variety of olefins (and acetylenes) which undergo metathesis and the variety of active heterogeneous and homogeneous catalysts which have been used make it difficult to propose one general mechanism for this reaction. There are currently several possibilities which are being considered. These are shown in Fig. 11 (54, 55). 

C-E bond formation via hydroalumination, 10, 859 C-E bond formation via hydroboration, 10, 842 olefin cross-metathesis, 11, 195 terminal acetylene silylformylation, 11, 478 Chemspeed automated synthesizer, for high-throughput catalyst preparation, 1, 356 Chini complexes, characteristics, 8, 410 Chiral bisphosphanes, in hydrogenations on DIOP modification, 10, 7... 

---