Transition metal-based homogeneous catalysts

To date, many soluble transition metal-based homogeneous catalysts including POMs have been developed for H202- and 02-based green oxidations. They are usually dissolved in reaction solutions, making all catalytic sites accessible to... 

The asymmetric (bio)catalytic reduction of C=C-bonds goes in hand with the creation of (up to) two chiral centers and is thus one of the most widely employed strategies for the production of chiral compotmds. Whereas cis-hydrogenati(Mi using transition-metal based homogeneous catalysts has been developed to an impressive standard,stereocomplementary asymmetric trans-hydrogenation is still at the stage of development [967]. 

Two important points to note are that, unlike transition metal-based homogeneous catalysts where the metal ions can have a wide range of oxidation states, the lanthanides are almost always in the 3+ oxidation state. Complexes 2.64 and 2.65 are no exceptions. Second, valence electron count for the metal has no significance for complexes of the/elements. 

As the first transition metal-based homogeneous catalysis of hydroamination, in the early 1970s Coulson from the Du Pont laboratories had described the addition of secondary aliphatic amines to ethylene in the presence of various rhodium compounds [15, 16]. Definite results were reported with RhCl3 3 H2O as pre-catalyst in tetrahydrofuran as solvent under starting ethylene pressures of 5-14 MPa at 180-200 °C for different secondary amines (Table 3). 

However, compared to the early transition metal-based olefin catalysts, the research on late transition metal catalysts has just begun. Moreover, catalysts based on transition metals are rarely used for polymerizations in SCCO2 and certainly not for olefin polymerization. In this study, we have used a homogeneous diimine palladium complex, also known as the Brookhart system (see Fig. 8.4) [14, 15]. The catalyst has been synthesized according to literature procedures [14, 15] for more details see De Vries [9]. 

It is difficult to summarize all the progress of the homogeneous catalytic aldol reaction due to the space limitations. With representative or selected examples, this article focuses on those efforts in homogeneous catalysis and will be discussed with selected examples according to the different types of catalysts il) transition metals based Lewis acids, such as palladium, copper, iron, etc (2)... 

Transition metal catalyzed aldol reactions are attractive methods due to their high catalytic activity, privileged chelation effects of controlling stereoselectivity, and mild or neutral reaction conditions. Except group 5-7, most of transition metals have been shown as efficient catalyst in homogeneous aldol reactions with variants of substrates. Thus and correspondingly, the catalytic aldol reactions will be emphasized herein with representative transition metal based complexes in group 4 and 8-11. 

Odenkirk, W., Whelan, J., and Bosnich, B., Homogeneous catalysis. A transition metal based catalyst for the Mukaiyama crossed-aldol reaction and catalyst deactivation by electron transfer, Tetrahedron Lett., 33, 5729, 1992. 

Polymerization of olefins such as styrene is promoted by acid or base or sodium catalysts, and polyethylene is made with homogeneous peroxides. Condensation polymerization is catalyzed by acid-type catalysts such as metal oxides and sulfonic acids. Addition polymerization is used mainly for olefins, diolefins, and some carbonyl compounds. For these processes, initiators are coordination compounds such as Ziegler-type catalysts, of which halides of transition metals Ti, V, Mo, and W are important examples. 

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