Mechanism bimetallic

Natta and his associates have postulated a mechanism involving chain propagation from an active center formed by the chemisorption of an electropositive metal alkyl of small ionic radius on the cocatalyst surface. This yields an electron-deficient bridge complex such as I, and chain growth then emanates from the C-Al bond. 

It is suggested that the nucleophilic olefin forms a jt-complex with the ion of the transition metal, and following a partial ionization of the alkyl bridge, the monomer is included in a six-membered ring transition state. The monomer is then 

While a limited amount of experimental evidence does lend support to the bimetalUc concept, majority opinion favors the second and simpler alternative, the monometalhe meehanism. 

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The bimetallic mechanism is illustrated in Fig. 7.13b the bimetallic active center is the distinguishing feature of this mechanism. The precise distribution of halides and alkyls is not spelled out because of the exchanges described by reaction (7.Q). An alkyl bridge is assumed based on observations of other organometallic compounds. The pi coordination of the olefin with the titanium is followed by insertion of the monomer into the bridge to propagate the reaction. 

Jacobsen developed a method employing (pybox)YbCl3 for TMSCN addition to meso-epoxides (Scheme 7.22) [46] with enantioselectivities as high as 92%. Unfortunately, the practical utility of this method is limited because low temperatures must be maintained for very long reaction times (up to seven days). This reaction displayed a second-order dependence on catalyst concentration and a positive nonlinear effect, suggesting a cooperative bimetallic mechanism analogous to that proposed for (salen)Cr-catalyzed ARO reactions (Scheme 7.5). 

In contrast, Cozzi and Umani-Ronchi found the (salen)Cr-Cl complex 2 to be very effective for the desymmetrization of meso-slilbene oxide with use of substituted indoles as nucleophiles (Scheme 7.25) [49]. The reaction is high-yielding, highly enantioselective, and takes place exclusively at sp2-hybridized C3, independently of the indole substitution pattern at positions 1 and 2. The successful use of N-alkyl substrates (Scheme 7.25, entries 2 and 4) suggests that nucleophile activation does not occur in this reaction, in stark contrast with the highly enantioselective cooperative bimetallic mechanism of the (salen)Cr-Cl-catalyzed asymmetric azidolysis reaction (Scheme 7.5). However, no kinetic studies on this reaction were reported. 

The actual mechanism by which the propagation occurs and the factors governing the formation of stereoregular polymers is state debatable. Among the several mechanisms proposed, the bimetallic mechanism of Natta and the monometallic mechanism of Cossee have received much attention. Cossee s... 

Natta s bimetallic mechanism stipulates that when the catalyst and cocatalyst components are mixed, the chemisorption of the aluminium alkyl (electropositive in nature) occurs on the titanium chloride solid surface which results in the formation of an electron-deficient bridge complex of the structure shown... 

Propagation proceeds through an enolate species in the bimetallic mechanism described by Eq. 8-73 in which two metallocene species are involved—one is a neutral enolate, and the other is the corresponding metallocenium cation [Collins et al., 1994 Li et al., 1997]. In LV, the propagating chain is coordinated at the neutral transition metal center (Zr3) and monomer... 

In this mechanism, the catalytically active rhodium(I) derivative is a bimetallic compound thereby providing active sites on two adjacent rhodium atoms. The methods of enzyme kinetics (12) yield the following rate equation for this bimetallic mechanism ... 

Coates and coworkers have carried out kinetic studies of the alternating copolymerization of CHO and C02 catalyzed by several of the P-diiminate zinc derivatives [29]. These authors have proposed a bimetallic mechanism to be operative, which is consistent with their experimental observations, including the large differences in activity noted for a series of structurally closely related catalysts. It was proposed that one zinc center would coordinate and activate the epoxide substrate, while the second zinc center would provide the propagating carbonate species to ring-open the epoxide. This proposal is represented by the transition state depicted in Figure 8.3a. 

Variations in monometallic and bimetallic mechanisms have been proposed. The monometallic mechanisms seem to be inherently simpler than bimetallic mechanisms except for requiring a migration step. On the other hand, bimetallic mechanisms that consider the presence of the activator in the polymerisation system can be more convincing in some instances. 

Most reaction models which describe the mechanism of diene polymerization by Nd catalysts have been adopted from models developed for the polymerization of ethylene and propylene by the use of Ti- and Ni-based catalysts systems. A monometallic insertion mechanism which accounts for many features of the polymerization of a-olefins has been put forward by Cossee and Arlman in 1964 [624-626]. Respective bimetallic mechanisms date back to Patat, Sinn, Natta and Mazzanti [627,628]. The most important and generally accepted mechanisms for the polymerization of dienes by Nd-based catalysts are discussed in the following. 

With increasing zirconium concentration the molecular weight decreases nearly linearly. This leads to the conclusion that chain transfer occurs via a bimetallic mechanism. 

From an applied perspective, metals are probably the most important surfaces in SOMC. Whereas in SOMC on oxides we introduce the active site, in SOMC on metals the metals themselves are the active catalysts. Organometallic chemistry provides the means to modify the often unselective metal surface by introduction of organometallic complexes (most often chemically inert) to the surface and eventually by transforming the initially obtained species by thermal or chemical means. For example, the hydrogenolysis of cyclic hydrocarbons, in particular cyclohexane, on the surface of unmodified Ir particles supported on a silica carrier has been studied and clearly indicated to bimetallic mechanism of C-C bond cleavage (Scheme 4) [24, 25]. 

The mechanism of the Jacobsen HKR and ARO are analogous. There is a second order dependence on the catalyst and a cooperative bimetallic mechanism is most likely. Both epoxide enantiomers bind to the catalyst equally well so the enantioselectivity depends on the selective reaction of one of the epoxide complexes. The active species is the Co(lll)salen-OH complex, which is generated from a complex where L OH. The enantioselectivity is counterion dependent when L is only weakly nucleophilic, the resolution proceeds with very high levels of enantioselectivity. 

While the exact nature of the active material present is still poorly understood, it is thought that the catalytic material produced from aluminum triethyl and titanium trichloride has a bridged structure as shown (first structure. Fig. 22.5). A bimetallic mechanism, originally proposed by Natta, illustrates a probable mode of progression of a substituted vinylic monomer to stereoregular polymer. 

FIGURE 22.5 A bimetallic mechanism for coordination (Ziegler-Natta) polymerization. 

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