Reducing Metals

1 Thermal Reactions The use of reducing metals (M ) allows the radical formation 

SCHEME 9.10 Synthesis of 79 and 80 derivatives by intramolecular H-atom migration. 

The reduction of ROOR via ET from M produces the cleavage of the o-bond (RO-OR), thereby liberating the corresponding alkoxide anion RO , the radical RO, and A novel strategy for 

A related approach used catalysis by FeCl, t-BuOOH (TBHP) as oxidant, and 1,8-diazabicycloundec-7-ene (DBU) as ligand. Several dialkyl ethers, sulfides, and amines have been used to generate alkyl radicals, which add to A-substituted A-phenylacrylamide [56]. This initiation methodology (M +/ROOR) was also used in the synthesis of 6-arylated phenanthridines [57, 58], biaryls [59], fluorenones and xanthones [60], etc. 

Using a similar approach, a mild and efficient visible Ught-mediated diarylation of V-substituted V-phenylacrylamides 93 with ArN +X 94 was developed to afford 3,3-disubstituted oxindoles 95 by constructing two C—C bonds in one step via an into-intramolecular sequence (Eq. 9.17) [66]. 

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Ammonia will reduce metallic oxides which are reduced by hydrogen (for example copper(II) oxide, CuO, lead(II) oxide, PbO), being itself oxidised to nitrogen ... 

Common reducing agents are hydrogen in the presence of metallic or complex catalysts (e.g. Ni, Pd, Pt, Ru, Rh), hydrides (e.g. alanes, boranes, LIAIH, NaBHJ, reducing metals (e.g. Li, Na, Mg, Ca, Zn), and low-valent compounds of nitrogen (e.g. NjHj, NjHJ, phosphorus (e.g. triethyl phosphite, triphenyiphosphine), and sulfur (e.g. HO-CHj-SOjNa = SFS, sodium dithionite = Na S O. ... 

Catalytic hydrogenation is mostly used to convert C—C triple bonds into C C double bonds and alkenes into alkanes or to replace allylic or benzylic hetero atoms by hydrogen (H. Kropf, 1980). Simple theory postulates cis- or syn-addition of hydrogen to the C—C triple or double bond with heterogeneous (R. L. Augustine, 1965, 1968, 1976 P. N. Rylander, 1979) and homogeneous (A. J. Birch, 1976) catalysts. Sulfur functions can be removed with reducing metals, e. g. with Raney nickel (G. R. Pettit, 1962 A). Heteroaromatic systems may be reduced with the aid of ruthenium on carbon. 

The use of reducing metals nowadays is mainly restricted to acyloin and pinacol coupling reactions (see p. 53f.) and Birch reductions of arenes (A.A. Akhrcm, 1972 see p. 103f.) and activated C—C multiple bonds (see p. 103f.). 

A traditional method for such reductions involves the use of a reducing metal such as zinc or tin in acidic solution. Examples are the procedures for preparing l,2,3,4-tetrahydrocarbazole[l] or ethyl 2,3-dihydroindole-2-carbox-ylate[2] (Entry 3, Table 15.1), Reduction can also be carried out with acid-stable hydride donors such as acetoxyborane[4] or NaBHjCN in TFA[5] or HOAc[6]. Borane is an effective reductant of the indole ring when it can complex with a dialkylamino substituent in such a way that it can be delivered intramolecularly[7]. Both NaBH -HOAc and NaBHjCN-HOAc can lead to N-ethylation as well as reduction[8]. This reaction can be prevented by the use of NaBHjCN with temperature control. At 20"C only reduction occurs, but if the temperature is raised to 50°C N-ethylation occurs[9]. Silanes cun also be used as hydride donors under acidic conditions[10]. Even indoles with EW substituents, such as ethyl indole-2-carboxylate, can be reduced[ll,l2]. 

The process can be operated in two modes co-fed and redox. The co-fed mode employs addition of O2 to the methane/natural gas feed and subsequent conversion over a metal oxide catalyst. The redox mode requires the oxidant to be from the lattice oxygen of a reducible metal oxide in the reactor bed. After methane oxidation has consumed nearly all the lattice oxygen, the reduced metal oxide is reoxidized using an air stream. Both methods have processing advantages and disadvantages. In all cases, however, the process is mn to maximize production of the more desired ethylene product. 

Alkaline permanganate pretreatment of steel for the removal of heat scale and smut prior to acid pickling results in faster descaling and reduced metal attack (see Metal surface treatments Metal treatments). Stainless steel alloys can also be cleaned by alkaline permanganate followed by pickling in nonoxidi2ing acids (260). 

The essential operations of an extractive metallurgy flow sheet are the decomposition of a metallic compound to yield the metal followed by the physical separation of the reduced metal from the residue. This is usually achieved by a simple reduction or by controlled oxidation of the nonmetal and simultaneous reduction of the metal. This may be accompHshed by the matte smelting and converting processes. 

Reduction processes are characterized either by the reducing agent selected or by the physical state of the metallic product. The separation of reaction products determines the choice and design of the furnace. Reduction processes are classified according to the physical state of the reduced metal. 

Acid Coolers. Cast Hon trombone coolers, once the industry standard (101), are considered obsolete. In 1970, anodically passivated stainless steel sheU and tube acid coolers became commercially available. Because these proved to have significant maintenance savings and other advantages, this type of cooler became widely used. Anodic passivation uses an impressed voltage from an external electrical power source to reduce metal corrosion. 

Includes only hydrogen-reduced metal powder and chemicals. 

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

A Japanese patent has claimed improvements in the direct condensation of menadione with phytyl chloride in the presence of a reducing metal such as 2inc or iron powder (30). Tin chloride has been reported to be a useful catalyst for this condensation (31,32). 

Condensate systems can be chemically treated to reduce metal corrosion. Treatment chemicals include neutralising amines, filming amines, and oxygen scavenger-metal passivators. 

Semiconductor and Solar Cells. High purity (up to 99.9%) antimony has a limited but important appHcation in the manufacture of semiconductor devices (see Semiconductors). It may be obtained by reduction of a chemically purified antimony compound with a high purity gaseous or soHd reductant, or by thermal decomposition of stibine. The reduced metal may be further purified by pyrometaHurgical and zone melting techniques. 

Direct Reduction with Metals. PoUucite can be directly reduced by heating the ore in the presence of calcium to 950°C in a vacuum (20), or in the presence of either sodium or potassium to 750°C in an inert atmosphere (21). Extraction is not complete. Excessive amounts of the reducing metal is required and the resultant cesium metal is impure except when extensive distiUation purification is carried out. Engineering difficulties in this process are significant, hence, this method is not commerciaUy used. 

Platinum and Platinum Alloys. Platinum has excellent resistance to strong acids and, at elevated temperatures, to oxidation. Under reducing conditions at high temperatures it must be protected from low-fusing elements or their oxides. Easily reduced metals at high temperatures may form low-fusing alloys with platinum. 

Deposition reactions for some reducing agents are given in Table 1 hydrogen is a principal by-product of each reduction. Elemental phosphoms or boron is codeposited with the reduced metal from hypophosphite, borohydride, or organoborane baths (15). Other minor reactions can also occur (18). All of these reductions can be viewed as dehydrogenation reactions (16,19). 

Whenever insoluble anodes are used, the pH of the plating solution decreases along with the metal ion concentration. In some plating baths, a portion of the anodes is replaced with insoluble anodes in order to prevent metal ion buildup or to reduce metal ion concentration. Lead anodes have been used in acid copper sulfate baths, and steel anodes have been used in alkaline plating baths. 

Precautions must be taken to ensure good electrical contact with the specimen, to avoid contamination of the solution with easily reducible metal ions, and to ensure that inhibitor decomposition has not occurred. Instead of using 2 mL of any proprietaiy inhibitor, 0.5 g/L of inhibitors such as diorthotolyl thiourea or qmuonne ethiodide can be used. 

To reduce metal attack and fumes, use an acid inhibitor during acid pickling, A 0.01 % concentration is recommended,... 

Second, in addition to the base material changes, many of today s combustors also have Thermal Barrier Coatings (TBCs), which have an insulation layer of the total thickness used is 0.015-0.025 inch (0.4-0.6 mm) and are based on Zr02-Y203 and can reduce metal temperatures by 90-270 °F (50-150 °C). 

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