Silver byproduct

Another common method of introducing NHG ligands has been to isolate silver carbene complexes first, and then react them with a platinum salt. Normally, silver oxide is reacted with an imidazolium salt, before the introduction of the platinum source at ambient or low temperature insoluble silver byproducts are conveniently filtered off. A conceptually similar approach is the prior formation of a tungsten or molybdenum pentacarbonyl carbene complex, and reaction of this with a platinum precursor. The room-temperature reaction with, for example, PtCl2(NGPh)2 results in the release of carbon monoxide and the precipitation of an undefined tungsten or molybdenum compound, which may easily be removed by filtration. 

Silver is widely distributed in sulfide ores of which silver glance (argentite), Ag2S, is the most important. Native silver is sometimes associated with these ores as a result of their chemical reduction, while the action of salt water is probably responsible for their conversion into hom silver , AgCl, which is found in Chile and New South Wales. The Spanish Americas provided most of the world s silver for the three centuries after about 1520, to be succeeded in the nineteenth century by Russia. Appreciable quantities are now obtained as a byproduct in the production of other metals such as copper. 

Most silver is nowadays produced as a byproduct in the manufacture of non-ferrous metals such as copper, lead and zinc, when... 

Ethylene oxide is an important intermediate for ethylene glycol (antifreeze) and for plastics, plasticizers, and many other products [R.A. van Santen and H.P.C.E. Kui-pers, Adv. Catal. 35 (1987) 265]. In Chapter 1 we explained that the replacement of the traditional manufacturing process - which generated 1.5 mole of byproducts per 1 mole of epoxide - by a catalytic route based on silver catalysts is a major success story with respect to clean chemistry (Fig. 9.16). 

In step 1, only minor changes in time and temperature were made. In step 2, it was found that the addition of diethylamine led to a decrease in dimeric byproducts. An improved ratio of the intermediate (with the iodide moiety para rather than ortho to the methoxy group) was attained with a reagent far less expensive than the silver acetate used in the preliminary synthetic route. 

Insoluble impurities fall to the floor of the cell as anode slime. Despite the derogatory name, this material contains precious metals such as gold, silver, and platinum. Anode slime from the electrorefining of nickel11 at Sudbury, Ontario, is a significant source of platinum and palladium as byproducts ( 0.34 g Pt and 0.36 g Pd per metric ton of ore), whereas deposits in the Bushveld complex (Transvaal, South Africa) are so rich in platinum-group metals (Ru, Os, Rh, Ir, Pd, Pt) that the associated Co, Ni, and Cu recovered are considered to be by-products of the lucrative platinum production (4.78 g Pt and 2.03 g Pd per metric ton of ore).7... 

Vinyltrimethylstannanes react with xenon difluoride in dichloromethane at room temperature in the presence of equimolar (or a 50 Vo excess) of silver trifluoromethanesulfonate and a catalytical (0.1 equiv) quantity of 2,6-di-/e/7-butyl-4-methylpyridine to form the corresponding vinyl fluorides in high to moderate yields.54 57 The substitution reaction is tolerant to various functional groups, such as ketones, esters, carbamates, ketals, ethers, phenol rings and tertiary alcohols. As byproducts corresponding alkenes have been detected due to pro-tiodestannylation. 

Researchers at Solvay reported in a patent that aluminum trichloride can be activated with metal halides [e. g., silver chloride or iron(III) chloride] and acid salts NaF nllF.33 Thus, 1,2-dichloro-l.l,2-trifluoroethane (19) has been converted into 2,2-dichloro-l.l.Ttrifluoroethane (20), and l,1.2-trichloro-l,2.2-trifluoroethane (1) into l.1.l-trichloro-2.2.2-trifluoroethane (2) with reduced byproduct formation. 

Silver is widely distributed throughout the world. It rarely occurs in native form, but is found in ore bodies as silver chloride, or more frequently, as simple and complex sulfides. In former years, simple silver and gold-silver ores were processed by amalgamation or cyanidation processes, The availability of ores amenable to treatment by these means has declined. Most silver is now obtained as a byproduct or coproduct from base metal ores, particularly those of copper, lead, and zinc. Although these ores are different in mineral complexity and grade, processing is similar. 

All the ores are concentrated in complex mills by selective froth flotation lu produce individual copper, zinc, lead, and, infrequently, silver concentrates. The copper and lead concentrates are smelted to produce lead and copper bullions from which silver is recovered by electrolytic or fine refining. The silver bearing zinc concentrates are commonly processed by leaching and electrolytic methods. Silver is ultimately recovered as a byproduct from zinc plant residues. Canada is a leading silver mining country. Other important sources of silver are Mexico, die United States, Peru, the former U.S.S.R., and Australia. See also Mineralogy. 

The roaster product is lcachcd with spent electrolyte 1 sulfuric acid) under near-neutral conditions to dissolve most of the zinc, copper, and cadmium, but little of the iron. The leach residue solids are releached in hot, strong add to dissolve more zinc, since it attacks the otherwise insoluble zinc femtes. The iron which is also dissolved in this second leach is then precipitated as jarosite, goethite, or hematite. The development of these iron predpitation techniques permitted the use of the hot. strong acid leach and an increase 111 zinc extraction from about 87% to greater Ilian 95%. Simultaneously, the hot acid leach frequently generates a leach residue rich enough in lead and silver to provide significant byproduct value, as well as increased recovery of cadmium and copper. 

Primary alkyl halides can also be alkynylated by silver acetylides. Isabelle and coworkers reported the reaction of methyl iodide, ethyl iodide and <7rmethyl iodide with several silver acetylides to give disubstituted alkynes.116 The authors preferred a non-radical-mediated mechanism for this reaction, as neither methane nor ethane, expected byproducts of a radical reaction, were observed. 

Toward this end, Woerpel and Nevarez examined the possibility of di-tert-butylsilylene transfer from cyclohexene silacyclopropane 58 to imine 169a (Scheme 7.48).123 Thermolysis produced a mixture of silaaziridine 170a and an imine-dimer byproduct (171). The results by Brook and coworkers suggested that if the temperature of silylene transfer were lowered, isolation of 170a without formation of byproduct 171 would be possible. As anticipated, exposure of cyclohexene silacyclopropane 58 to imine 169a in the presence of substoichiometric amounts of silver triflate produced only 170a. This silaazridine could be purified by bulb-to-bulb distillation to afford the product in 80% yield. Copper salts required... 

Although the main routes to propylene oxide formation are not based on direct catalytic oxidation of propylene, the direct epox-idation of propylene on silver would be financially preferable if high yield and selectivity to propylene oxide could be achieved. Similarly to ethylene oxidation on silver part of the undesirable byproduct CO2 comes from the secondary oxidation of propylene oxide (2,3). The kinetics of the secondary silver catalyzed oxidation of propylene oxide to CO2 and H2O have been studied by very few investigators (2). 

The nature of the working silver catalyst was different during methanol oxidation and ethene oxidation reactions as a result of variations in the reaction conditions (Wachs, 2002). In methanol oxidation (at 600 °C), H 2 was a major by-product, and Raman spectroscopy showed that the silver catalyst was essentially reduced and contained only trace amounts of atomic oxygen in the subsurface. During ethene oxidation (at ca. 230 °C), H2 was not formed as a byproduct. The absence of H2 and the lower reaction temperature during ethene oxidation result in a silver surface with atomic oxygen species. 

Tsuruya and co-workers (83,84) recently reported that addition of alkaline earth metals (e.g., Ca, Sr, and Ba) to an Ag/SiOi catalyst by a coimpregnation method enhanced the catalytic activity of the partial oxidation of benzyl alcohols into benzaldehydes, with production of only small amounts of byproducts (carbon dioxide, toluene, and benzene). The formation of carbonaceous material was thought to be inhibited by the alkaline earth metals, which also helps to disperse the metallic silver and facilitate oxygen adsorption. This effect causes the formation of an oxygenated silver surface that is generally believed to be responsible for the partial oxidation of benzyl alcohol. 

A 2 1 mixture of silver carboxy late and iodine is well known to be a good oxidant in the transformation of enol silyl ethers into the corresponding a-acyloxy carbonyl compounds (139). Five- and six-membered ring enol silyl ethers serve as the best substrates, with larger ring systems forming a-iodo carbonyl compounds as byproducts. 

Ring expansion reactions of the type shown in equations (17) and (18), in which a more equitable distribution of carbon atoms between two rings of a polycyclic hydrocarbon is produced, provide the link between the decyclization reactions above and the annulation reactions considered later. It would seem that the greater the strain in the reactant, the larger the number of potential catalysts. The complexes of seven different metals catalyze the reaction shown in equation (17). Of the possible catalysts, only the silver (I) salts bring about any byproduct formation. The isomerization of [l.l.ljpropellane (equation 18) is catalyzed by [RhCl(PPh3)3]. ... 

In the same manner, dimethylsulfide substitution proved to be a practical procedure for the preparation of NHC-gold complexes. Good yields were obtained for the synthesis of monocarbene Au(I) complexes (104) from [(Me2S)AuCl] and one equivalent of free NHC (Scheme 17) iHios jjjg steric hindrance of NHCs was found to be of critical importance, since NHCs with smaller nitrogen substituents such as Mes, wo-propyl or n-butyl led to a lower yield and formation of a byproduct identified as the biscarbene complex [(NHC)2Au]+ Cl. Fortunately, the silver-mediated transmetallation was an efficient alternative route for these NHCs (see Section 2.6). When the same reaction was performed with two equivalents of in situ generated NHCs, the cationic biscarbene complex (105) [(NHC)2Au]X (X = Cl, Br, orPFe) was obtained as the only product. ... 

N-Iodosuccinimide has been prepared only by the action of iodine on N-silver succinimide. The present procedure is essentially that of Djerassi and Lenk, with the modification that dioxane is the reaction medium instead of acetone dioxane gives a better yield without formation of a lachrymatory byproduct. 

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