Transition-metal-based homogeneous catalysis

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). 

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)... 

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. 

Comparing the different subareas of catalysis, all have common characteristics, but also significant differences are visible. Our area of main interest is homogeneous catalysis based on transition metal catalysts. In general, these homogeneous catalysts (most often the precatalyst) are molecularly defined. 

The manifold intermediates in homogeneous transition-metal catalysis are certainly metal complexes and therefore show a behaviour like ordinary coordination compounds associations of phosphorus donors open up multifarious additional controls. Both, substrates and P ligands are Lewis bases that we have to consider and that compete at the coordination centers of the metal, leading to competitive, non-competitive or uncompetitive activation or inhibition processes in analogy to the terminology of enzyme chemistry... 

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