Mechanism metathesis

Ring-Opening Polymerization. Ring-opening polymerization of cycloolefins in the presence of tungsten- or molybdenum-based catalysts proceeds by a metathesis mechanism (67,68). 

Alkanes, dipurin-8-yl-synthesis, 5, 574 Alkanes, poly-N-pyrazoIyl-synthesis, 5, 320 Alkanoic acids, tetrazolyl-anti-inflammatory activity, 5, 835 Alkanoic acids, 4-thienyi-cyclization, 4, 905-906 Alkene metathesis mechanism, 1, 668 Alkenes activated... 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

There are of course borderline cases when the reacting hydrocarbon is acidic (as in the case of 1-alkynes) a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also react with dihydrogen according to a o-bond metathesis mechanism. However, for the late elements an alternative exists in the form of an oxidative addition reaction. This alternative does not exist for d° complexes such as Sc(III), Ti(IV), Ta(V), W(VI) etc. and in such cases o-bond metathesis is the most plausible mechanism. 

Lou X, Detrembleur C, Lecomte P, Jerome R (2002) Novel unsaturated E-caprolactone polymerizable by ring-opening metathesis mechanisms. e-Polymers no 034... 

Considering the metathesis mechanism (Scheme 1) in the polymerization of acetylene compounds, an ideal initiator composed of transition metals has a carbene ligand, which promisingly achieves precisely controlled polymerization. 

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. 

Besides the cr-bond metathesis mechanism proposed by Tilley23 for the dehydrogenative coupling of silanes, a Zr(II) pathway25 and a silylene mechanism26 have been proposed based on the nature of the products. The dehydrogenative polymerization of 1,2,3-trimethyltrisilane or of a mixture of diastereomers of 1,2,3,4-tetramethyltetrasilane showed evidence that, besides Tilley s mechanism, a further mechanism is present. The product formation can be explained by a silylene mechanism where the silylenes are formed by a-elimination from the silyl complexes by a new type of /J-elimination which involves Si—Si bond cleavage (/F-bond elimination) as described in Scheme 727. 

As a result, it could be shown that both in the solid state and in solution the [ R2P(CH2)nPR2-K2P XRu=CHR]+cation dimerizes to form dicationic, dinuclear complexes [ R2P(CH2)nPR2-/c2P XRu=CHR 2]2+. In different crossover and trapping experiments, monomeric species could be found in solution. During the next pre-equilibrium step these monomeric complexes should form an olefin jt-complex. The product formation presumably proceeds via a metallacyclobutane according to the Chauvin metathesis mechanism (Figure 3.17) [48],... 

Woo s combinative catalyst system of CpjMCE/Hydride is different from the catalyst systems using Cp2MCl2/2 alkyllithiums of Corey, Tanaka, and Harrod. Real catalytic species in the dehydrocoupling of hydrosilanes could be a metallocene hydride based on a sigma-bond metathesis mechanism.12,1315 Inorganic hydrides effectively produce a metallocene hydride whereas alkyllithium can produce a metallocene hydride via... 

The most widely accepted mechanism for Group 4 metallocene promoted dehydrocoupling reactions is the a-bond metathesis mechanism proposed by Tilley as shown in Fig. 3 3h,3°,30,4°,96 reac on of the metallocene... 

By contrast, the Fischer carbyne tnms-bromotetracarbonyl(phcnylmcthyli-dene)tungsten, Br(CO)4W = CPh [140-142], will polymerise monosubstituted acetylenes by a metathesis mechanism [143] but will not give transalkylidynation products. The Fischer carbyne and related carbynes are effective catalysts for the metathesis polymerisation of not only monosubstituted acetylenes, whose polymerisation can be brought about by a number of catalysts [108, 109, 118, 120-125, 128-134], but also of acetylenes polymerised previously by only a few or no catalysts, i.e. even disubstituted acetylenes [112-115, 126, 127, 144-148] and unsubstituted acetylene [19, 20, 42, 119]. 

The cyclopolymerisation of 1,6-heptadiyne derivatives such as diethyl dipro-pargyl malonate by a Schrock carbene, [Me(CF3)2CO]2Mo(=NAr)=CHMe3, via a metathesis mechanism, proceeds in a living fashion to provide a conjugated polymer having both five-and six-membered rings, which is shown schematically below [154, 155] ... 

Metathetical polycondensation of acyclic dienes has not been successful with conventional catalysts used for the ring-opening metathesis polymerisation of cycloolefins, which is due to the fact that Lewis acids are usually present, and produce deleterious side reactions [13,16,17]. Only Lewis acid-free, well-defined catalysts have been successfully applied for acyclic diene metathesis polycondensation the key success has been to choose catalysts that obviate other pathways not involving the metathesis mechanism [18-20]. It was Wagener et al. [16,21] who first were able to convert an acyclic a, co-diene (1,9-decadiene), by using an acid-free metal alkylidene catalyst, to a high molecular weight... 

The developments over the past few years in the understanding of organometallic mechanisms and structure has led to a new generation of catalysts for the polymerization of cyclic olefins by the ring opening metathesis mechanism. These catalysts provide the means to control molecular weight and molecular weight distribution and to introduce functionality into polyolefins. 

Scheme 2 shows a simplified description of the ring-closing metathesis mechanism [3]. In the first step, one double bond of starting diene 13 forms with the catalyst 20 via the -complex... 

Various investigators have examined the nature of reactive intermediates. As has already been mcmioned. olefins, especially ethylene [SO-54) and alcohols [56, can be incorporated into Fischer-Tropsch products. A metathesis mechanism was proposed to account for the formation of propene and methane from ethylene according to Equations (18) and (19) [55. ... 

These features have signiHcam implications as far as the metathesis mechanism is concerned (Section 13.1). 

Marshall has pointed out the striking similarity between (his mechanism and the uJefin metathesis mechanism. 

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