Catalytic processes alkene polymerization

The 7t ligands play important roles in a large number of homogeneous catalytic processes. Alkene polymerization and a variety of other reactions involve alkene coordination (see Chapters 6 and 7). As the name suggests, CO is the main ligand in carbonylation reactions (see Chapter 4). All four ligands CO, alkene, H , and PR3, play important parts in hydroformylation reactions (see Chapter 5). 

The elimination of a hydrogen atom positioned on a carbon to the central metal constimtes an important reaction in transition metal catalysis. In the classical example, an alkylmetal intermediate is reversibly converted to an alkene and a metaUiydride (scheme 1.12). Despite the fact, that the resulting hydridometal complex can be exploited in various catalytic processes including polymerization reactions, [57] cycloisomerizations, [58] annulations, [59] etc., the ]S-hydride elimination is often considered undesired in transition metal catalyzed cross couplings. Thus, efforts have often been concentrated towards the prohibition of this fundamental reaction [60]. Nevertheless, the ]S-hydride elimination is a vital transformation in a number of catalytic processes including the ene-yne coupling reported by Trost [61] and Skrydstrup, [62] oxidation of alcohols, [63] the Heck reaction etc [64]. 

To date, the most frequently used ligand for combinatorial approaches to catalyst development have been imine-type ligands. From a synthetic point of view this is logical, since imines are readily accessible from the reaction of aldehydes with primary or secondary amines. Since there are large numbers of aldehydes and amines that are commercially available the synthesis of a variety of imine ligands with different electronic and steric properties is easily achieved. Additionally, catalysts based on imine ligands are useful in a number of different catalytic processes. Libraries of imine ligands have been used in catalysts of the Strecker reaction, the aza-Diels-Alder reaction, diethylzinc addition, epoxidation, carbene insertions, and alkene polymerizations. 

Alkenes are known to form d - complexes with low valent transition metal ions (or atoms), thus stabilizing their low valent complexes (152). Complexes of this type are key intermediates in a variety of catalytic processes, e.g., hydrogenations, polymerizations,... 

Despite its inherent difficulties, carbometallation has, in fact, played important roles in catalytic asymmetric carbon-carbonal bond formation. Isotactic and syndiotactic alkene polymerization involving both heterogeneous and homogeneous Ti and Zr catalysts must involve a series of face-selective carbometallation processes, although the main stereochemical concern in poly(alkene) formation is diastereoselectivity rather than enantioselectivity. This fascinating topic, however, is outside the scope of this chapter, and the readers are referred to Chapter 11 and other previous reviews [6]. 

Another important reaction typically proceeding in transition metal complexes is the insertion reaction. Carbon monoxide readily undergoes this process. Therefore, the insertion reaction is extremely important in organoiron chemistry for carbonylation of alkyl groups to aldehydes, ketones (compare Scheme 1.2) or carboxylic acid derivatives. Industrially important catalytic processes based on insertion reactions are hydroformylation and alkene polymerization. 

The examples discussed so far are all transition metal complexes. As we will see later (Chapters 4-9), most homogeneous catalytic processes are indeed based on transition metal compounds. However, catalytic applications of rare earth complexes have also been reported, although so far there has not been any industrial application. Of special importance are the laboratory-scale uses of lanthanide complexes in alkene polymerization and stereospecific C-C bond formation reactions (see Sections 6.4.3 and 9.5.4). 

Alkyl complexes are intermediates in a number of homogeneous catalytic processes, such as carbonylation, alkene polymerization, hydrogenation, etc. Allyl... 

Polymerization of alkene monomers, with or without functional groups, are very important industrial processes. Until recently the use of homogeneous catalysts was restricted to relatively small-volume production of specialty dimers and oligomers. The manufacture of the two largest-tonnage plastics— polyethylene and polypropylene—has so far been based on heterogeneous catalytic processes. 

Organometallic compounds are used widely as homogeneous catalysts in the chemical industry (see Topic J5). For example, if the alkene insertion reaction continues with further alkene inserting into the M-C bond, it can form the basis for catalytic alkene polymerization. Other catalytic cycles may include oxidative addition and reductive elimination steps as described in Topic H9. Figure 2 shows the steps involved in the Monsanto acetic acid process, which performs the conversion... 

The key step in nearly all of the catalytic processes to be discussed is olefin insertion into a metal hydride [Eq. (2)] or organometallic species [Eq. (3)]. These hydrometallation and carbometallation processes also form the basis for the polymerization of alkenes. Olefin insertions generally occur with the same regiose-lectivity as hydroboration reactions [9], with the bulky metal and associated ligands residing at the least hindered site of the two carbon reactive unit. 

The insertion of alkenes into M-H or M-C bonds is one of the common methods used for the generation of metal alkyl complexes and is a crucial step in a variety of catalytic processes, most importantly the Ziegler-Natta polymerization of olefins. In these intramolecular reactions, a coordinated alkene undergoes a concerted 1,2-insertion via a planar... 

This chapter will briefly review the fundamental organometallic reactions that play a key role in almost all metal-catalyzed processes. It will then apply these reaction steps to explain currently accepted mechanisms for some major catalytic cycles hydrogenation, hydroformylation, methanol carbonylation, Pd-catalyzed coupling reactions, and alkene polymerization and metathesis. Each of these catalytic reactions is covered in considerably more detail in later chapters, so the discussion here will be limited to relating and using the various fundamental reactions to build up and describe multistep catalytic cycles. 

Supported metal complex catalysts for alkene polymerization. Supported chromium complexes on silica have been used for many years in the Phillips process for ethylene polymerization, and promoters are not required. Like these supported complexes, the classical TiClj Ziegler polymerization catalysts have also long been viewed as presenting surface catalytic sites that are well described as molecular analogues. 

The role of transition-metal carbonyls and particularly those of the Group 6 metals in homogeneous photocatalytic and catalytic processes is a matter of considerable interest [1]. UV irradiation especially provides a simple and convenient method for generation of thermally active co-ordinately unsaturated catalyst for alkenes or alkynes transformation. By using tungsten and molybdenum carbonyl compounds as catalysts, alkenes and alkynes can be metathesized, isomerised and polymerized. Photocatalytic isomerization of alkenes in the presence of molybdenum hexacarbonyl was observed by Wringhton thirty years ago [2]. Carbonyl complexes of molybdenum catalyze not only... 

Naphthaleneytterbium demonstrates a high activity not only in stoichiometric reactions but also in many catalytic processes. It catalyzes at room temperature the polymerization of styrene, methylmetacrylate, ethylacrylate, isoprene, ethylene oxide, the copolymerization of ethylene oxide with styrene, piperilene and carbon dioxide [65], the hydrogenation of alkenes, alkynes, ketones, aldehydes [78], the formation of alkylenecarbonates from epoxides and CO2 [65]. 

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