Catalyst transition metal derivatives

Ziegler catalysts Complex catalysts prepared by interaction between an organometallic derivative and a transition metal derivative. A typical catalyst is the product of the interaction of TiCU and AIBU3. These catalysts polymerize olefins, particularly ethylene, to polyolefins, the polymerization generally being in a siereoregular manner. 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

Because of the importance of olefin metathesis in the industrial production of olefins and polymers, many different catalysts have been developed. Almost all of these are transition metal-derived, some rare exceptions being EtAlCl2 [758], Me4Sn/Al203 [759], and irradiated silica [760]. The majority of catalytic systems are based on tungsten, molybdenum, and rhenium, but titanium-, tantalum-, ruthenium-, osmium-, and iridium-based catalysts have also proven useful for many applications. 

Group 6 transition metal derivatives composed of Mo and W have been widely used as catalysts for the polymerization of... 

Both heterogeneous and homogeneous disproportionation catalysts are known. All contain a transition metal component with derivatives of Mo, W, and Re being the most important. Heterogeneous catalysts are generally metal oxides deposited on a support such as silica or alumina (1, 4). Homogeneous catalysts in general require a non-transition metal derivative as cocatalyst (2, 3). 

Although there is dispute about the exact oxidation state of titanium in the active species [Ti(III) or Ti(IV)], it was suggested, from the results of ESR measurements, that Ti(III) species form highly active sites for producing syndiotactic polystyrene in styrene polymerisation systems with the TiBz4—[Al(Me)0]x catalyst [50]. The moderately low catalyst activity is attributable to the stability of the benzyl transition metal derivatives towards reduction. 

No reactions of the type shown in Eq. (18) have been reported to be catalyzed by transition metal derivatives. Sufficiently vigorous conditions (or the proper catalyst) have not been investigated or the ring strain inher-... 

Although the thermal decomposition of diazo compounds has received an extraordinary amount of attention, nevertheless, most of the studies were run in the presence of a catalyst (generally, copper metal or a copper salt), and therefore the catalyzed decomposition of diazo compounds by transition metal derivatives constitutes one of the most intensely investigated fields of organic chemistry. The use of such catalysts has resulted in a considerable improvement of the efficiency and selectivity of many carbene reactions (see Section 9). 

The synthesis of the polyacetylene powder has been known since the late 1950s, when Natta used transition metal derivatives that have since become known as Ziegler-Natta catalysts. The characterization of this powder was difficult until Shirakawa and coworkers [18] succeeded in synthesizing lustrous, silvery, polycrystalline films of polyacetylene (which has become known as Shirakawa polyacetylene) and in developing techniques for controlling the content of cis and trans isomers ... 

The NR MS method has made significant contributions to organometallic and coordination chemistry.The main purpose of the majority of NR MS experiments in this field is to test whether or not a particular neutral metal derivative is stable in the gas phase. Such observation is a good indication for the possible involvement of these species in the reactions taking place in the gas phase (such as interstellar processes) and their likely formation and stability in the condensed phase. The importance of the latter is indisputable because metal complexes are involved as reactants or catalysts in numerous chemical processes. These studies have been performed on a variety of metal complexes, such as very simple two-atomic species, metal solvates, and polyatomic transition metal derivatives. 

Representative examples of the recent generation of metathesis catalysts are shown below (Fig. 2.29). Notice that these transition-metal derivatives contain an M=C bond. Such compounds are known as metal carbenes [18-20]. 

The application of polymer-supported catalysts has now been extended to the synthesis of complexes between transition metal derivatives and structurally ordered macromolecular ligands to give catalytic systems exhibiting high activity and stereoselectivity. Polystyrene and polymethacrylate resin and polystyrene-divinylbenzene-polystyrene-polybutadiene block copolymers, as well as vinyl-functionalized polysiloxanes grafted onto silica, are very suitable polymers for heterogenization of mostly Pt and Rh complexes. Moreover, polyamides exhibit much higher thermal stability than conventional polystyrene supports (114). 

M catalyst transition metal alkyl derivatives of main group metal... 

Homogeneity of the reaction medium. The reaction medium is often heterogeneous, at least in the first step of the reaction. This may result in an ill-defined distribution of the end-groups and the location of the catalyst is often based on pure assumptions. The case of transition metal derivatives is particularly critical because their unoccupied orbitals accept the doublets of donating atoms, such as oxygen atoms in aliphatic polyethers. 

The coordinated diphosphines are quite robust and can be stored for a long period of time in air, and they provide easy access to the free chiral phosphine ligands via a simple liberation procedure. This also allows the synthesis of various transition metal derivatives for purposes such as catalyst screening. 

Section 14 15 Coordination polymerization of ethylene and propene has the biggest eco nomic impact of any organic chemical process Ziegler-Natta polymer ization IS carried out using catalysts derived from transition metals such as titanium and zirconium tt Bonded and ct bonded organometallic com pounds are intermediates m coordination polymerization... 

Cuprous salts catalyze the oligomerization of acetylene to vinylacetylene and divinylacetylene (38). The former compound is the raw material for the production of chloroprene monomer and polymers derived from it. Nickel catalysts with the appropriate ligands smoothly convert acetylene to benzene (39) or 1,3,5,7-cyclooctatetraene (40—42). Polymer formation accompanies these transition-metal catalyzed syntheses. 

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