Homogeneous transition metal catalyst

Homogeneous transition-metal catalysts are usually chosen because of their high selectivity. A distinction is made between chemoselectivity, regioselectivity, and stereoselectivity, of which a special form is enantioselectivity. The latter is e.specially important in the production of pharmaceuticals. 

V,iV, A "-trialkylated 1,3,5-triazacyclohexanes (R3TAC) have been used extensively as ligands for homogeneous transition metal catalysts. The protonolysis of diethylzinc by the addition of protonated A,/V7v/V"-alkylated... 

The screening of the catalytic activity in these reactions is made possible by the readily available library of various heterogeneous and homogeneous transition metal catalysts. The use of microwaves ensures that two reactions, each requiring different times to reach equilibrium under thermal conditions, can now be completed within a very short interval. 

In this chapter and in Chapters 10-12, we will review and validate some methods for asymmetric (transfer) hydrogenation of carbon-oxygen and carbon-carbon double bonds catalysed by non-metallic systems, homogeneous transition metal catalysts and biocatalysts. Reduction of carbon-nitrogen double bond systems will be reported in another volume of this series. 

In the search to develop new materials for immobilization of homogeneous transition metal catalyst to facilitate catalyst-product separation and catalyst recychng, the study of dendrimers and hyperbranched polymers for application in catalysis has become a subject of intense research in the last five years [68], because they have excellent solubility and a high number of easily accessible active sites. Moreover, the pseudo-spherical structure with nanometer dimensions opens the possibility of separation and recycling by nanofiltration methods. Although dendrimers allow for controlled incorporation of transition metal catalysts in the core [69] as well as at the surface [70], a serious drawback of this approach is the tedious preparation of functionalized dendrimers by multi-step synthesis. 

Characteristic features attributed to homogeneous transition metal catalysts are their superior selectivity and activity compared with their heterogeneous counterparts. However, a serious shortcoming of homogeneous catalysis is the cumbersome separation of the catalyst from reaction products and the quantitative recovery of the catalyst in an active form. 

C. Masters, Homogeneous Transition Metal Catalysts, Chapman and Hall, London, 1981. 

PIONEERING INDUSTRIAL PROCESSES USING HOMOGENEOUS TRANSITION METAL CATALYSTS... 

Cokoja M, Bruckmeier C, Rieger B et al (2011) Transformation of carbon dioxide with homogeneous transition-metal catalysts a molecular solution to a global challenge Angew Chem Int Ed 50(37) 8510-8537... 

Alkane Functionalization with Homogenous Transition Metal Catalysts I 37... 

The activation and transformation of C-H bonds in alkanes by homogeneous transition metal catalysts should to be a topic of further research. No processes are close to commercialization on a technical scale. So far the organometallic approach by Shilov is one of the most promising ways of producing a practical system [55]. 

Homogeneous transition metal catalysts are used in such industrial processes as ... 

The BASF cobalt/iodide catalyzed process for methanol carbonylation was quite quickly superseded by a rhodium/iodide catalyzed process discovered at Monsanto and first commercialized in 1970 at a plant in Texas City. The Monsanto process was a significant advance and became one of the few large tonnage processes to use a homogeneous transition metal catalyst. It was later... 

The same CALB preparation was appUed in many dynamic kinetic resolutions combining two types of catalysts with each other. In the presence of homogeneous transition metal catalysts that catalyze the racemization and heterogeneous acids or bases or immobilized transition metals Novozym 435 was not deactivated [1, 26-28]. This is all the more remarkable since the reactions catalyzed by these catalysts include redox reactions at elevated temperatures (>60°C). When Novozym 435 was applied for the enantioselective synthesis of cyanohydrin acetates (10) from aliphatic aldehydes (7), good results were achieved (Scheme 2.2) for this dynamic kinetic resolution (DKR) [29]. Here NaCN is used as the base for the dynamic racemic formation and degradation of the cyanohydrins (6 and 8). 

Homogeneous transition metal catalysts, in a broad sense, are offsprings of the field of organometallic complex chemistry. Here complexes and reactions with CO and H are among the most studied areas. A wealth of information is available. 

In actual homogeneous catalyzed processes, a homogeneous transition-metal catalyst is in equilibrium with its free transition-metal center and ligands. An excess of ligands is normally added [1] to decrease die amount of fiee transition-metal which negatively influences the selectivity of the reaction. Therefore, to apply Reverse Flow Adsorption, a combination of two ad rbents has to be used to reversibly adsorb the transition-metal center and the excess of ligands. 

To apply Reverse Flow Adsorption, a combination of two adsorbents has to be used for the reversible adsorption of a homogeneous transition-metal catalyst. The transition-metal center can be adsorbed by a suitable ligand immobilized onto a solid carrier, while the ligand is adsorbed by an immobilized transition-metal. Two groups of adsorbents have been studied, based on the HSAB predictions ( i the interactions with the Co(ll) transition-metal center or PPhj ligands ... 

The hydrogenation of isolated aromatic rings is not an easy task with homogeneous transition metal catalysts. Several complexes, however, have been successfully used for this purpose. 

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