Catalysis by transition metals

Transition metal ions inhibit alcohol oxidation by reacting with hydroxyperoxy radicals (see Sect. 2.6), viz. 

As well as terminating chains, these ions may initiate chains by reactions with H202 and RH, viz. 

Acceleration of oxidation will be observed only when initiation is predominant over termination, i.e. when 

The first example of a heterogeneously catalyzed hydroamination of an alkene appeared in a 1929 patent in which it is claimed that NH, reacts with ethylene (450°C, 20 bar) over a reduced ammonium molybdate to give EtNH2 [24]. An intriguing reaction was also reported by Bersworth, who reacted oleic acid with NH3 in the presence of catalysts like palladium or platinum black or copper chromite to give the hydroamination product in quantitative yields [25]. However, this result could not be reproduced [26]. 

Teter et al. filed a series of patents aimed at the production of organic compounds containing nitrogeif or the production of nitriles and amines from ammonia and olefins by passing mixtures of olefin and NH3 over transition metals, mainly cobalt deposited on various supports at 250-370°C and 100-200 bar [27-43]. With cobalt on asbestos, a mixture of amine, nitrile, olefin hydrogenation product, polymers, and cracking products is obtained (Eq. 4.1) [31]. 

At higher temperatures, propene and NHj react over basic catalysts to afford a mixture of nitriles (Eq. 4.2) [42]. 

It was thought that propionitrile came from dehydrogenation of the anti-Markovnikov hydroamination product, w-PrNH2. Propionitrile can break down to ethylene and HCN, the former reacting with NH3 to generate acetonitrile via ethyl-amine, the latter adding to propene to form the butyronitriles [26, 37]. 

PhNH2 reacts with ethylene in the presence of alkali metals, e.g., sodium deposited on alumina, to afford the hydroamination product in good yield but with a low turnover frequency (TOE = mol of product synthesized per mol of catalyst in 1 h) (Eq. 4.3) [44]. 

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The development of methods for aromatic substitution based on catalysis by transition metals, especially palladium, has led to several new methods for indole synthesis. One is based on an intramolecular Heck reaction in which an... 

A discussion of the different types of solute-solute and solute-solvent interactions acting in homogeneous catalysis by transition metal complexes. E. Cesarotti, R. Ugo and L. Kapan, Coord. Chem. Rev., 1982,43, 275-298 (47). 

Homogeneous catalysis by transition metal clusters has been reviewed from the perspective of the specific transformations.Examples of very mixed-metal clusters catalyzing processes homogeneously are collected in Table IX. As is generally the case with homogeneous catalysis, the catalytic precursor is well defined, but the nature of the active catalyst is unclear. 

Mo03 (bipy)] [Mo03 (H20) ] Inorganic Chemistry, 49, 6865—6873. Moreno-Manas, M. and Pleixats, R. (2003) Formation of carbon-carbon bonds under catalysis by transition-metal nanopartides. Accounts of Chemical Research, 36, 638—643. 

This chapter focuses exclusively on microwave heterogeneous catalysis. Microwave homogeneous catalysis by transition metal complexes is treated in Chapt. 11, phase transfer catalysis in Chapt. 5, catalytic reactions on graphite in Chapt. 7, photocataly-tic reactions in Chapt. 14, and catalytic synthesis oflabeled compounds in Chapt. 13. 

Catalysis by Transition Metal Ions and Complexes in Hydrocarbon Oxidation by Dioxygen... 

CATALYSIS BY TRANSITION METAL IONS AND COMPLEXES IN HYDROCARBON OXIDATION BY DIOXYGEN... 

Many publications report on the effect that acids and bases can have on catalysis by transition metal complexes, and in most cases positive effects are reported. 

Catalysis by transition metals in liquid-phase oxidation has been thor- oughly investigated. The roles of other ions have not been sufficiently studied. This paper is concerned with catalysis by hydrogen ions and some anions, in the chain oxidation of secondary alcohols such as cyclohexanol and 2-propanol. Secondary alcohols, because of their polarity, are convenient for studying ionic homolytic reactions and their role in chain oxidation. 

Catalytic Decomposition of Hydrogen Peroxide by Ferrous Ions Catalysis by Transition Metal Ions and Complexes in Liquid-Phase Oxidation of Hydrocarbons and Aldehydes by Dioxygen... 

Homogeneous catalysis by transition metal complexes almost always involves processes in which product-catalyst separation and catalyst recycling are important issues. For years, researchers have worked to find effective ways to isolate metal-complex catalysts in phases separate from those containing the catalyst, usually by anchoring the metal complex to a solid surface. As summarized by Driessen-Holscher, it is now evident that the method that has met with most practical success in this direction involves the use of multiple liquid phases. For example, rhodium complexes with water-soluble sulfonated ligands are used to catalyze alkene hydroformyla-tion, and the aqueous-phase catalyst and the organic products are easily separated as insoluble liquid phases. 

Several other low temperature investigations, two in air (41, 55) and two in steam (37,44) > are interesting from the standpoint of showing the complications that arise when diffusion, catalysis, and product inhibition of the surface become important. Long and Sykes (46, 47) have studied catalytic effects for C -f H20 and C -f C02 reactions and propose slightly different mechanisms for catalysis by transition metals and by alkali metals. 

The mechanism of the asymmetric induction in asymmetric catalysis by transition metal complexes is only speculation. In principle, the asymmetric induction can result from direct interaction between ligand and substrate as proposed, for instance, by McQuillin et al. (32) for the asymmetric hydrogenation. But it might take place simply by interaction between substrate and chiral transition metal atoms without... 

After the first successful asymmetric hydroformylation, although in low optical yields, the reaction was further investigated by different groups all over the world. The results have been rather disappointing from a synthetic point of view, as in a few cases only, optical yields as high as 30 to 50% have been achieved. However, some interesting information has been obtained, both on the mechanism of hydroformylation and on the basic aspects of homogeneous asymmetric catalysis by transition metal complexes. 

Coordination can also lead to activation of the substrate molecule via cleavage (e.g., H2, 02) or via redistribution of electron density (e.g., CO, olefin, RCN). Catalysis by transition metal complexes can be further divided into two... 

Because of their importance in the mechanism of oxidation reactions, much attention has been paid to the kinds of oxygen species present on the surface of these catalysts. A general review concerning oxygen in catalysis by transition metal oxides has recently been presented by Bielanski and Haber.5 In this section the most important results of the main methods of investigation are reviewed. 

The move toward catalytic reactions is reflected in the increase in the number of chapters in this book on the topic compared to the first edition. The trend has been observed by noted chemists in the previous decade. Professor Seebach, for example, in 1990 stated the primary center of attention for all synthetic methods will continue to shift toward catalytic and enantioselective variants indeed, it will not be long before such modifications will be available for every standard reaction. 6 Professor Trost in 1995 was a little more specific with catalysis by transition metal complexes has a major role to play in addressing the issue of atom economy—both from the point of view of improving existing processes, and, most importantly, from discovering new ones. 7 However, the concept can be extended to biological and organic catalysts and to those based on transition metals. 

Hennig, H. (1999) Homogeneous photo catalysis by transition metal complexes. Coordination Chemistry Reviews, 182, 101-123. 

B. Mechanisms of Redox Catalysis by Transition Metal Complexes-... 

Liquid phase oxidation of hydrocarbons by molecular oxygen forms the basis for a wide variety of petrochemical processes,3 "16 including the manufacture of phenol and acetone from cumene, adipic acid from cyclohexane, terephthalic acid from p-xylene, acetaldehyde and vinyl acetate from ethylene, propylene oxide from propylene, and many others. The majority of these processes employ catalysis by transition metal complexes to attain maximum selectivity and efficiency. 

Hill CL, Prosser-McCartha CM. Homogeneous catalysis by transition metal oxygen anion clusters. Coord Chem Rev 1995 143 407-55. 

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