Ligand modifications

In comparison to heterogeneous catalyzed reactions, homogeneous catalysis offers several important advantages. The catalyst complex is usually well defined and can be rationally optimized by ligand modification. Every metal center can be active in the reaction. The reaction conditions are usually much milder (T usually < 200 °C), and selectivities are often much higher than with heterogeneous catalysts. 

In 1968 Wilkinson discovered that phosphine-modified rhodium complexes display a significantly higher activity and chemoselectivity compared to the first generation cobalt catalyst [29]. Since this time ligand modification of the rhodium catalyst system has been the method of choice in order to influence catalyst activity and selectivity [10]. 

Figure 7. Typical examples of ligand modifications to improve the solubility of complexes in scCC>2...

Other special probe properties can be built in by suitable ligand modification. For example, 4,7-dihydroxy-l,10-phenanthroline complexes of Ru(II)... 

Figure 12.1 General representation of an ECE pincer ligand coordinated to a metal fragment (MX L ), including the different functions that can be regulated by appropriate ligand modifications.

The control of reactivity to achieve specific syntheses is one of the overarching goals of organic chemistry. In the decade since the publication of the third edition, major advances have been made in the development of efficient new methods, particularly catalytic processes, and in means for control of reaction stereochemistry. For example, the scope and efficiency of palladium- catalyzed cross coupling have been greatly improved by optimization of catalysts by ligand modification. Among the developments in stereocontrol are catalysts for enantioselective reduction of ketones, improved methods for control of the... 

With j -branched aluminium alkyls like Al(i-Bu)3, reduction is often the main reaction [52]. Ligand modification can be used to increase the amount of reduction still further, and also to control the diastereoselectivity. In this respect, the phenoxide-modified complex 7 appears to be particularly effective [53]. A recurring problem in diastereoselective reductions is that the product can epimerize through MPV reduction (see next section) of the starting material [54]. The kinetics of this complicated system have been analyzed in terms of the iso-inversion principle [55]. 

When the transfer reaction competes successfully with further insertion, as in the case of nickel, dimerization becomes the dominant transformation. When metal hydride elimination, in turn, is slow relative to insertion, polymeric macromolecules are formed. Ligand modification, the oxidation state of the metal, and reaction conditions affect the probability of the two reactions. Since nickel hydride, like other metal hydrides, catalyzes double-bond migration, isomeric alkenes are usually isolated. 

B. K. Brandley, M. Kiso, S. Abbas, P. Nikrad, O. Srivastava, C. Foxall, Y. Oda, and A. Hasegawa, Structure-function studies of seleciin carbohydrate ligands. Modification to fucose, sialic acid and sulphate as a sialic acid replacement, Glycobiology 3 633 (1993),... 

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