Other Metals as Catalysts

The iridium-catalyzed azide-alkyne cycloaddition, [ Ir(cod)Cl 2], reaction of electron-rich internal thioalkynes, in various polar and nonpolar solvents, including water, under mild reaction conditions (2mol% of catalyst and room temperature), gives the corresponding 1,4-disubstituted 1,2,3-triazole system in 22-98% yields [61]. 

Au/Ti02 can serve as an active catalyst in the Huisgen [3-h2] cycloaddition in aqueous medium, yielding 1,4-disubstituted 1,2,3-triazoles regioselectively as the sole product, in short reaction times (45 min) with good to excellent yields. The catalyst may be recycled up to five times [62]. 

Ag(I) acetylide has been thought to catalyze the Huisgen cycloaddition however, the addition of copper(I) salts is required to accomplish the cycloaddition and to obtain 1,4-disubstituted 1,2,3-triazoles [63]. Despite this, homogeneous complexes of type [Ag (L2)(X )]catalyze the acetylene-azide cycloaddition. When Af,Af-diisopropyl(2-diisopropylphosphanyl)benzamide was used as ligand, the reaction conditions based on 2-2.5 mol% of catalyst, for 24 h, at 90 C in toluene lead to conversions higher than 99% [64]. 

Charcoal impregnated with zinc is able to catalyze, in a 10 mol% loading, the cycloaddition of organic azides and alkynes in DMF at 50 C, to provide the corresponding 1,4-disubstituted 1,2,3-triazoles and 1,4,5-trisubstituted 1,2,3-triazoles in good to excellent yields (64-95%). The catalyst can be recovered by filtration and reused at least, for three more cycles [65]. 

Bimetallic catalysts in which one metal can fine-tune or modify the catalytic properties of the other metal offer a considerable promise. In a Cu-Mn bimetallic catalyst, the reaction between CuO and Mn20g through electron transfer from Mn to the formation of a Mn ion and a stabilized Cu ion may take place [66]. 

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Research on the production of formaldehyde from methane, propane, and the hydrocarbon mixtures encountered in natural gas have been described by Bibb. Studies involving ethane, propane, and higher paraffins are also reported by Wiezevich and Frolich, who used iron, nickel, aluminum, and other metals as catalysts and employed pressures up to 135 atmospheres. 

Of little use commercially except as a route to anthraquinone. For this purpose it is oxidized with acid potassium dichromate solution, or better, by a catalytic air oxidation at 180-280 C, using vanadates or other metal oxide catalysts. 

Use of alcohol as a solvent for carbonylation with reduced Pd catalysts gives vinyl esters. A variety of acrylamides can be made through oxidative addition of carbon monoxide [630-08-0] CO, and various amines to vinyl chloride in the presence of phosphine complexes of Pd or other precious metals as catalyst (14). 

Although the aminolysis of esters to amides is auseful synthetic operation, usually it presents some disadvantages in terms of drastic reaction conditions, long reaction times or strong alkali metal as catalyst, which are usually not compatible with other functional groups in the molecule [6]. For this reason, enzymatic aminolysis of carboxylic acid derivatives offers a clean and ecological way for the preparation of different kind of amines and amides in a regio-, chemo-, and enantioselective manner. 

Other metal oxide catalysts studied for the SCR-NH3 reaction include iron, copper, chromium and manganese oxides supported on various oxides, introduced into zeolite cavities or added to pillared-type clays. Copper catalysts and copper-nickel catalysts, in particular, show some advantages when NO—N02 mixtures are present in the feed and S02 is absent [31b], such as in the case of nitric acid plant tail emissions. The mechanism of NO reduction over copper- and manganese-based catalysts is different from that over vanadia—titania based catalysts. Scheme 1.1 reports the proposed mechanism of SCR-NH3 over Cu-alumina catalysts [31b],... 

The first reported work on the kinetics of hydrogenolysis reactions of simple hydrocarbons appears to be that of Taylor and associates at Princeton (2-4, 14, 15), primarily on the hydrogenolysis of ethane to methane. The studies were conducted on nickel, cobalt, and iron catalysts. More recently, extensive studies on ethane hydrogenolysis kinetics have been conducted on all the group VIII metals and on certain other metals as well (16,28-83). 

Cyclopropanation reactions can be promoted using copper or rhodium catalysts or indeed systems based on other metals. As early as 1965 Nozaki showed that chiral copper complexes could promote asymmetric addition of a carbenoid species (derived from a diazoester) to an alkene. This pioneering study was embroidered by Aratani and co-workers who showed a highly enantioselective process could be obtained by modifying the chiral copper... 

A highly interesting class of catalysts is represented by bimetallic systems, which in many important catalytic processes show improved activity or selectivity compared with catalysts involving only one metal. Understanding their better performance is still a challenge. One metal can tune and/or modify the catalytic properties of the other metal as the result of both electronic or/and structural effects. Several mechanisms for synergism can be proposed, but it is difficult to assess their relative importance. It is clear that each metal can play a very important role in proper circumstances [41]. 

The gas-phase oxidation of ethylene to ethylene oxide over a supported silver catalyst was discovered in 1933 and is a commercially important industrial process. Using either air or oxygen, the ethylene oxide is produced with 75% selectivity at elevated temperatures (ca. 250 °C). Low yields of epoxides are obtained with propylene and higher alkenes so that other metal-based catalysts are used. A silver-dioxygen complex of ethylene has been implicated as the active reagent.222... 

Transition metals are active as catalysts because of their capacity to chemisorb atoms, given that the main role of transition metals as catalysts is to atomize gaseous molecules, such as H2, 02, N2, and CO, thereby providing atoms to other reactants and reaction intermediates [27],... 

Other metal-centered catalysts that have been studied include (te)strapped chiral porphyrins derived from L-proline, which can induce modest (< 30%) enantioselectivity <02EJIOC1666>. Supported amidate-bridged platinum blue complexes, which have not yet been applied to chiral epoxidation, but which show promise of utilizing molecular oxygen as the terminal oxidant, have... 

Moore compared the technological branch of solar energy conversion, essentially photovoltaics, with the biological branch. He explained how a standard fuel cell that operates on oxygen and hydrogen produces water and electromotive force. A typical human-engineered fuel cell operates at 50-60 percent power conversion efficiency and uses platinum or other noble metals as catalysts. 

A number of other mechanistic sequences are thought to occur in alkyne trimerizations mediated by other metal systems. Catalysts derived from Co2(CO)s follow a characteristic sequence of steps that involves exclusively dinuclear complexes (Scheme 26). The nature of the final (alkyne)3C02(CO)4 intermediate, a so-called flyover complex, is supported by both X-ray crystal structure data as well as its involvement in forming other products besides benzenes. These dinuclear cobalt systems are note-... 

Considerable interest in industry has been paid to the oxidative carbonylation of styrene as a promising method for producing cinnamic acid derivatives (eq. (7)). As catalyst systems, ordinary Pd complexes in the presence of Cu salts as well as other metals as co-catalysts have been used [37]. 

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