Metal catalysis concepts

Extending the same concept of a planar chiral nucleophilic or basic heterocyclic Fe-sandwich complex, aza-ferrocenes 65 were prepared. The latter have also been successfully applied as bidentate ligands in transition metal catalysis [85]. 

In the sixties of past century, a few patents issued to Bergbau Chemie [5,48,49] and to Mobil Oil [50-52], respectively described the use of CFPs as supports for catalytically active metal nanoclusters and as carriers for heterogenized metal complexes of catalytic relevance. For the latter catalysts the term hybrid phase catalysts later came into use [53,54], At that time coordination chemistry and organo-transition metal chemistry were in full development. Homogeneous transition metal catalysis was expected to grow in industrial relevance [54], but catalyst separation was generally a major problem for continuous processing. That is why the concept of hybrid catalysis became very popular in a short time [55]. 

The term chemistry in interphases was first introduced in the field of reverse-phase chromatography [41], In 1995 Lindner et al. transferred the concept to the area of transition metal catalysis [42] and in a recent review the concept is explained in detail [43], The interphase is defined as a region within a system in which the stationary and a mobile component penetrate on a molecular level without the formation of a homogeneous mixture. In these regions the reactive centre on the stationary phase... 

Since 2000, the field of asymmetric catalysis has bloomed extensively (and perhaps unexpectedly) with the introduction of a variety of metal-free catalysis concepts that have collectively become known as organocatalysis. Perhaps more impressive... 

Concepts for Transition Metal Catalysis Using Ionic Liquids 111... 

In the following sub-chapters two selected examples will be presented to illustrate general concepts for transition metal catalysis in ionic liquids. In both examples the role of the ionic liquid is different being alternatively used mainly in its function as ligand precursor or selective extraction solvent respectively. 

Other Transition Metals. The rather general validity of the chlorosilane-based catalysis concept is further substantiated by some recent examples of pinacol coupling processes catalyzed either by low-valent vanadium ([CpV(CO)4] cat., Zn, chlorosilane) [16j or low-valent samarium (Sml2 cat.. Mg, chlorosilane) [17]. Likewise,... 

Bifiinctional conjunctive reagents (BCRs), such as a 1,3-substituted 2-methylenepropane (96), can serve as effective TMM synthons via transition metal catalysis. In this case, the transition metal complex can promote the elimination of the elements X—Y from (96) to form a coordinated TMM species. Cycli-zation of this intermediate with an alkene thus produces the methylenecyclopropane product and also regenerates the catalyst (Scheme 5). In the last decade a number of BCRs utilizing this concept, or some modifications thereof, has been developed for various TMM synthons. ... 

Critical emphasis is placed on the industrial importance of homogeneous catalysis as defined above and to the discussion of possibilities and limitations of the manufacturing processes. Because of rapid developments and a vast amount of literature, an unbiased assessment is difficult and so misinterpretations may arise. We hope that this does not happen, and we have endeavoured to offer the reader a useful compilation of the manifold concepts and applications of organo-metallic catalysis. 

In addition to the synthesis of specific ligands and the resulting catalysts new ideas and concepts begin to combine with transition metal chemistry to open up new areas in homogeneous catalysis, like catalysis under supercritical conditions, colloidal catalysis , organometallic electrocatalysis and multi-metallic catalysis " ( cooperative catalysis ). 

Triazoles are generally prepared by the cycloaddition of an alkyne with an azide, which can be promoted thermally or by metal catalysis. This cycloaddition reaction of an azide and an alkyne is a typical Click reaction, that is, a highly efficient and reliable general reaction that can be used for linking moieties for various purposes, such as in combinatorial chemistry. Opinion is divided on whether Click is a useful or necessary concept but it seems to have become associated, almost synonymously, but incorrectly, solely with this triazole synthesis. 

The notion that chemical reactions in the aqueous phase may be important to atmospheric chemistry dates back at least 30 yr, when Junge and Ryan (1958) called attention to the great potential of cloud water for the oxidation of dissolved S02 by heavy-metal catalysis. At that time the process appeared to be the only viable oxidation mechanism for atmospheric S02. Later, when the concept of OH radical reactions gained ground, the gas-phase oxidation of S02 by OH was recognized to be equally important. The recent revival of interest in aqueous phase reactions is connected with efforts to achieve a better understanding of the origins of rainwater acidity. An oxidation of N02 to nitric acid also takes place in cloud water. Contrary to previous ideas, however, this process was recently shown to have little influence on atmospheric reactions of N02. 

Some Frequently Used Concepts of Metal Catalysis... 

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