Catalytic applications, transition metal copper

Many transition-metal complexes have been widely studied in their application as catalysts in alkene epoxidation. Nickel is unique in the respect that its simple soluble salts such as Ni(N03)2 6H20 are completely ineffective in the catalytic epoxidation of alkenes, whereas soluble manganese, iron, cobalt, or copper salts in acetonitrile catalyze the epoxidation of stilbene or substituted alkenes with iodosylbenzene as oxidant. However, the Ni(II) complexes of tetraaza macrocycles as well as other chelating ligands dramatically enhance the reactivity of epoxidation of olefins (90, 91). 

Titanium is one of the most important transition metals used in catalytic enantioselective reactions. Whereas rhodium, palladium, copper and ruthenium are rather rare in Nature, and the depletion of natural resources is evoked for these, titanium does not suffer from lack of availability. In fact, it is the 9th most abundant element on Earth and one of the cheapest transition metals. The products resulting from the hydrolysis of titanium complexes are nontoxic and do not cause any environmental problems. This low toxicity has allowed titanium to be used for multiple applications, including medical uses (prostheses, sun screens, etc.). 

Various methodologies for catalytic direct arylations via C—H bond activation employing transition metals other than palladium have been developed in recent years. In particular, rhodium- and ruthenium-based complexes have enabled the development of promising protocols for catalytic direct arylations. Whilst rhodium catalysts were found broadly applicable to the direct aryiation of both arenes, as well as heteroarenes, ruthenium-catalyzed chelation-assisted C—H bond function-ahzations could be used for the conversion of a variety of attractive organic electrophiles. In addition, inexpensive copper and iron salts have recently been shown as economically attractive alternatives to previously developed more expensive catalysts. Given the economically and environmentally benign features of selective C—H bond functionalizations, the development of further valuable protocols is expected in this rapidly evolving research area. 

Supramolecular chemistry has been a very popular research topic for three decades now. Most applications are foreseen in sensors and opto-electronical devices. Supramolecular catalysis often refers to the combination of a catalyst with a synthetic receptor molecule that preorganizes the substrate-catalyst complex and has also been proposed as an important possible application. The concept, which has proven to be powerful in enzymes, has mainly been demonstrated by chemists that investigated hydrolysis reactions. Zinc and copper in combination with cyclodextrins as the receptor dramatically enhance the rate ofhydrolysis. So far, the ample research devoted to transition metal catalysis has not been extended to supramolecular transition metal catalysis. A rare example of such a supramolecular transition metal catalyst was the results of the joined efforts of the groups of Nolte and Van Leeuwen [SO], They reported a basket-shaped molecule functionalized with a catalytically active rhodium complex that catalyzed hydrogenation reactions according to the principles of enzymes. The system showed substrate selectivity, Michaelis Menten kinetics and rate enhancement by cooperative binding of substrate molecules. The hydroformylation of allyl catachol substrates resulted in a complex mixture of products. 

On the basis of the wide catalytic applications of NHC transition metal complexes [25], Nolan and coworkers have thoroughly studied the catalytic activity in CuAAC reactions of well-defined copper(I) complexes with general formula [CuX(NHC)]. Organic solvents, mixtures of EtOH/water, and pure water have been used as reaction media. In particular, it has been reported that complexes [CuBr(SIMes)] (1 in Fig. 15.1, SIMes=iVAf-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene)) and [CuI(IAd)] (2 in Fig. 15.1, IAd=iVAf-adamantyl imidazol-2-ylidene) show a remarkable activity for the synthesis of a... 

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