Reductions by active metals

Solvent plays a key role in the course of reductions by active metals Changing the solvent of the reaction of hexafluoroacetone with magnesium from tetrahydro turan to dimethylformamide induces bimolecular reduction to the pinacol [5] (equation 2)... 

Reduction by active metals. As previously indicated, the active metal aluminum was first produced by the reduction of anhydrous aluminum chloride by the more active metal potassium ... 

A very detailed description of the chemistry associated with titanium oxygen systems is available [4]. The material is very thermally stable, and extremely resistant toward chemical degradation. It can be partially reduced when heated in the presence of hydrogen or carbon monoxide, the products being either lower oxides or mixtures of titanium carbide and lower oxides. Reduction by active metals (Na, K, Ca, or Mg) can... 

Qualitative Analysis. Nitric acid may be detected by the classical brown-ring test, the copper-turnings test, the reduction of nitrate to ammonia by active metal or alloy, or the nitrogen precipitation test. Nitrous acid or nitrites interfere with most of these tests, but such interference may be eliminated by acidifying with sulfuric acid, adding ammonium sulfate crystals, and evaporating to alow volume. 

Reductive cleavages of carbon-chlorine bonds by active metals and with photochemical activation figure in recent studies aimed at HFCs and HCFCs Sodium amalgam [3J] (equation 25), zinc powder [34] (equation 26), and alumi-mun/tin chloride [35] (equation 26) are all used in conjunction with protic solvents in reactions giving high yields and conversions... 

In addition to Ta205, another oxide is known, Ta02, which may be formed by active-metal reduction las is the tetrachloride), except that the pentoxide is heated with magnesium rather than aluminum. It forms with alkali metals the metatantalates. MTa03, the orthotantalates. MjTaO and pyrotantalates, M4T 207, as well as such polytantalates as MgTaeOju. the latter requiring fusion with the alkali hydroxides,... 

Reactive electrodes refer mostly to metals from the alkaline (e.g., lithium, sodium) and the alkaline earth (e.g., calcium, magnesium) groups. These metals may react spontaneously with most of the nonaqueous polar solvents, salt anions containing elements in a high oxidation state (e.g., C104 , AsF6 , PF6 , SO CF ) and atmospheric components (02, C02, H20, N2). Note that ah the polar solvents have groups that may contain C—O, C—S, C—N, C—Cl, C—F, S—O, S—Cl, etc. These bonds can be attacked by active metals to form ionic species, and thus the electrode-solution reactions may produce reduction products that are more stable thermodynamically than the mother solution components. Consequently, active metals in nonaqueous systems are always covered by surface films [46], When introduced to the solutions, active metals are usually already covered by native films (formed by reactions with atmospheric species), and then these initial layers are substituted by surface species formed by the reduction of solution components [47], In most of these cases, the open circuit potentials of these metals reflect the potential of the M/MX/MZ+ half-cell, where MX refers to the metal salts/oxide/hydroxide/carbonates which comprise the surface films. The potential of this half-cell may be close to that of the M/Mz+ couple [48],... 

The rednctive conpling (see Reductive Coupling) of acenaphthylene by activated metallic ytterbium and samarium yields unra-lanthanidocenes [( ) -Ci2H8)2Ln(THF)2] (Ln = Yb, Sm) (equation 25). 

Although reductions of ketones by active metals in alcohols have been largely supplanted by other procedures in modem synthetic chemistry, these methods still find occasional use. A modification employing K in r-butyl or r-pentyl alcohol has been used for the stereoselective reduction of 7-keto steroids in high yield,and the Li-ethanol and Na-ethanol reductions of 16-keto steroids have been investigated. Both traditional Na-ethanol °" reductions and a variation using Na-propan-2-ol in toluene - have also been used recently in selected systems. 

The position of the heteroatom may also strongly affect the global reactivity of the molecule. Thus, ethers, esters, thioethers, and even amines, that are normally weakly reactive toward the electron, can afford cleavage at the condition to be in an activated position (benzylic or allylic) to the X group (a fast cleavage of the radical anion results in a large shift to less cathodic potentials when the global bielectronic step is considered). In a different way, the reduction of ethers and amines could be achieved only under the conditions of the Birch reaction (i.e. reduction by Li metal in amines or by a solvated electron). These conditions can be electrochemically... 

Reductive Desulfonylations by Active Metals and Salts. Relatively large quantities of reducing agent are required because of the stoichiometric nature of the reactions. Further, the separation of products from large amounts of aqueous solutions of metal salts may be laborious and inefficient. 

Reduction of saturated cyclohexanones by active metals in liquid ammonia frequently affords the most stable isomer (Table l)6,7. 

Most of the metals and metalloids have been isolated from their halides by reaction with sodium. In fact, sodium reduction of metal halides can be considered almost a universal method for preparing metals or metalloids in an elemental state. In many cases, imfavorable economics prevents commercialization of processes involving reduction of a metal halide with an active metal, when compared with processes that involve electrolysis, carbon reduction, or hydrogen reduction, and active metal reduction must be relegated to the role of a laboratory procedure. 

Also, as was the case for alkenes, the carbon-carbon triple bond can be reduced by active metals (e.g., sodium, Na, lithium, and Li) in the presence of a proton donor, such as ethanol (ethyl alcohol, CH3CH2OH). Interestingly, the reduction appears to occur in a series of one-electron transfer steps (first to a radical or radical-Uke species and then to an anion) from the active metal to the alkyne in liquid ammonia (NH3(i)) solvent followed by the transfer of the proton. The alkene produced arises from net antarafacial [or tram-, anti-, or ( )-] hydrogen addition ( nation 6.66). 

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