Solvent effects dissolving metal reduction

In the key step, the glycosyl bromide (10) was treated with Zn in acetic acid in conditions typical of those of dissolving metal reduction, and this has led to the speculation that the elimination occurs by addition of two electrons from the metal to the C-1 carbonium ion formed by ionization of the halide, followed by elimination of the adjacent acetoxy group. Alternatively, organozinc intermediates could be involved. Protic solvents are necessary for the reaction and acetic acid can be replaced by ethanol or 1-propanol but if still milder conditions are required, the elimination can be effected with trimethyl phos-phite.20... 

Lithium hydride is perhaps the most usehil of the other metal hydrides. The principal limitation is poor solubiUty, which essentially limits reaction media to such solvents as dioxane and dibutyl ether. Sodium hydride, which is too insoluble to function efficiently in solvents, is an effective reducing agent for the production of silane when dissolved in a LiCl—KCl eutectic at 348°C (63—65). Magnesium hydride has also been shown to be effective in the reduction of chloro- and fluorosilanes in solvent systems (66) and eutectic melts (67). 

Reduction of groups attached to amines (aromatic rings and carbon-carbon double and triple bonds) can be effected by methods previously discussed. These include (a) catalytic hydrogenation with hydrogen gas (H2) over a metal catalyst such as platinum (Pt), nickel (Ni), or palladium (Pd) (Equation 10.37) and (b) dissolving metals such as sodium (Na) in an alcohol solvent/reactant (Equation 10.38). Some nitrogenous materials have been known to poison some catalysts and thus experimentation is often necessary. 

Many investigators have actively studied the electrochemical reduction of C02 using various metal electrodes in organic solvents because these solvents dissolve much more C02 than water. With the exception of methanol, however, no hydrocarbons were obtained. The solubility of C02 in methanol is approximately 5 times that in water at ambient temperature, and 8-15 times that in water at temperatures below 0°C. Thus, studies of electrochemical reduction of C02 in methanol at —30°C have been conducted.148-150 In methanol-based electrolytes using Cs+ salts the main products were methane, ethane, ethylene, formic acid, and CO.151 This system is effective for the formation of C2 compounds, mainly ethylene. In the LiOH-methanol system, the efficiency of hydrogen formation, a competing reaction of C02 reduction, was depressed to below 2% at relatively negative potentials.152 The maximum current efficiency for hydrocarbon (methane and ethylene) formation was of 78%. 

Useful solvents must themselves resist oxidation or reduction, should dissolve suitable ionic solutes and nonelectrolytes, and in addition should be inexpensive and obtainable in high purity. Kratochvil indicated that the most potentially useful solvents are those that have a dielectric constant greater than about 25 and have Lewis-base properties. Some solvents meeting these criteria are acetonitrile, dimethyl-sulfoxide, dimethylformamide, dimethylacetamide, propylene carbonate, ethylene carbonate, formamide, sulfolane, and y-butyrolactone. Solvents of the Lewis-base type show specific solvation effects with many metal cations (Lewis acids). Thus acetonitrile functions as a Lewis base toward the silver ion. At the same time it reacts but little with the hydrogen ion. 

The Clemmensen reduction of aldehydes and ketones to methyl or methylene groups takes place by heating with zinc and hydrochloric acid. A non-miscible solvent can be used and serves to keep the concentration in the aqueous phase low, and thus prevent bimolecular condensations at the metal surface. The choice of acid is confined to the hydrogen halides, which appear to be the only strong acids whose anions are not reduced with zinc amalgam. The Clemmensen reduction employs rather vigorous conditions and is not suitable for the reduction of polyfunctional molecules, such as 1,3- or 1,4-diketones, or of sensitive compounds. However, it is effective for simple compounds that are stable to acid (7.38). A modification under milder conditions uses zinc dust and HCl dissolved in diethyl ether (ethereal HCl). Other methods for converting C=0 to CH2 are described in Schemes 7.87 and 7.105. 

ABSTRACT. Toluene radical anion, generated by dissolving potasssium metal in toluene by the assistance of dicyclohexano-18-crown-6, has been proved to be especially effective for reductive removal of fluorine atom from unactivated alkyl fluorides that resist common reduction conditions. Stereochemical and mechanistic aspects of the present method is discussed. In connection with the preparation of substrates the effect of dipolar aprotic solvents on the nucleophilic fluorination with potassium fluoride/dicyclohexano-18-crown-6 system was also examined, and sulfolane or N,N-dimethylformamide was shown to be a solvent of choice. 

RuCls 3H2O is the most common ruthenium compound containing chlorine with stable properties. It easily dissolves in water and is cheaper than other ruthenium compounds. In the past, the ruthenium catalysts were prepared by impregnation with RuCIs as the precursor and water as solvent. However, the chlorine of remnant after reduction can poison the ruthenium catalysts when a metal oxide is adopted as a support. The poison effect of chlorine is not so obvious for ruthenimn catalysts with activated carbon as support. ... 

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