Calcium dissolving metal reductions

One of die most popular reactions in organic chemistry is dissolving metal reductions [1-3], Two systems are frequently used - sodium dissolved in ammonia with alcohol and lithium dissolved in alkylamines [4]. Although calcium is seldom used, it has been successfully applied to the reduction of a variety of compounds and functional groups [5], including aromatic hydrocarbons, carbon-carbon double and triple bonds, benzyl ethers, allyl ethers, epoxides, esters, aliphatic nitriles, dithianes, als well as thiophenyl and sulfonyl groups. 

Addition of 1 mol of hydrogen to the carbon-carbon triple bond can be accomplished stereospecifically. Catalytic reduction leads to the cis isomer. This is most often carried out using Lindlar catalyst, a lead-poisoned palladium-on-calcium carbonate preparation. Palladium on BaS04 is an alternative. Some examples are recorded in Scheme 3.10. Numerous other catalyst systems have been employed to effect the same reduction. Many specific cases are cited in reviews of catalytic hydrogenations. If the trans alkene is desired, the usual method is a dissolving-metal reduction in ammonia. This reaction is believed to involve two successive series of reduction by sodium and protonation ... 

A variety of ketones or aldehydes may be reduced to an alcohol using dissolving metal reduction. Alcohols other than ethanol are used as solvents, and low boiling amines such as methylamine or dimethylamine can be used in place of ammonia. Other alkali metals such as lithium or calcium also work. Lithium in a mixture of methylamine and ethanol, and calcium metal in methylamine may also be used in these reduction reactions, primarily on large-scale reactions such as those found in industrial laboratories or factories. 

An alternative method of reducing the aromatic ring relies on metals (sodium, lithium calcium, etc.), in a so-called dissolving metal reduction. An example is the... 

The reduction of a carbon-carbon multiple bond by the use of a dissolving metal was first accomplished by Campbell and Eby in 1941. The reduction of disubstituted alkynes to c/ s-alkenes by catalytic hydrogenation, for example by the use of Raney nickel, provided an excellent method for the preparation of isomerically pure c -alkenes. At the time, however, there were no practical synthetic methods for the preparation of pure trani-alkenes. All of the previously existing procedures for the formation of an alkene resulted in the formation of mixtures of the cis- and trans-alkenes, which were extremely difficult to separate with the techniques existing at that time (basically fractional distillation) into the pure components. Campbell and Eby discovered that dialkylacetylenes could be reduced to pure frani-alkenes with sodium in liquid ammonia in good yields and in remarkable states of isomeric purity. Since that time several metal/solvent systems have been found useful for the reduction of C=C and C C bonds in alkenes and alkynes, including lithium/alkylamine, ° calcium/alkylamine, so-dium/HMPA in the absence or presence of a proton donor,activated zinc in the presence of a proton donor (an alcohol), and ytterbium in liquid ammonia. Although most of these reductions involve the reduction of an alkyne to an alkene, several very synthetically useful reactions involve the reduction of a,3-unsaturated ketones to saturated ketones. ... 

In the synthesis of the antidepressant (-)-Rolipram, Meyers et al. [26] tried to convert bicyclic lactam 39 to hydroxylactam 40 by use of dissolved metals. The increased yield of 40 on going from Li -> K -> Na parallels fhe reduction potential of the metals Li (3.0), K (2.9), and Na (2.7). The reduction potential of calcium is known to be even lower. When 39 is treated with calcium metal (10 equiv.) in liquid ammonia, fhe desired 40 is produced in 84% yield (Scheme 4.11). The same type of reduction is applicable to the conversion of 41 to 42 [27]. Furthermore, treatment of tricyclic lactam 43 wifh EtsSiH and I ifTi gives poor isolated yield (22%) of the polar diamino alcohol 44. In contrast, calcium-ammonia reduction of 43 produces N-unsubstituted hydroxylactam 45 in an excellent yield. 

Carbon-carbon double bonds are often stable to dissolved metals. Considerable amounts of norbornane (54) are obtained when either norbomadiene (53) or nor-bornene (56) is reduced by calcium in methylamine-ethylenediamine (Scheme 4.15) [35]. The C-C double bonds in both substrates are highly strained, which enables the reduction to proceed smoothly. In the reduction of diene 53, a tricyclic compound 55 is produced as a by-product in 18% yield. 

In addition to being more selective, dissolved calcium metal functions in a similar way to lithium and sodium metals towards organic functional groups [45]. Tab. 4.2 lists reductions giving the same products by the three dissolved metals. Among these, calcium affords the highest yields for some substrates (entries 1-3). The compounds in Tab. 4.2 include an aldehyde, indole [46], aryl ketone, enone, naphthalene [47], pyridine N-oxide [48], benzyl alcohol, styrene, and buckminster-fullerene. 

Hydrides. Zirconium hydride [7704-99-6] in powder form was produced by the reduction of zirconium oxide with calcium hydride in a bomb reactor. However, the workup was hazardous and many fires and explosions occurred when the calcium oxide was dissolved with hydrochloric acid to recover the hydride powder. With the ready availabiHty of zirconium metal via the KroU process, zirconium hydride can be obtained by exothermic absorption of hydrogen by pure zirconium, usually highly porous sponge. The heat of formation is 167.4 J / mol (40 kcal/mol) hydrogen absorbed. 

Although the exact chemical mechanism for the direct oxide reduction reaction has not yet been fully characterized, it has been well established that the reaction goes to completion when excess calcium is present, sufficient CaCl2 is available to dissolve the CaO produced, and adequate stirring is used. As calcium metal is soluble to about 1 wt% in CaC12 at 835°C, excess Ca insures that the reaction is driven to completion by mass-action effects. 

Direct Oxide Reduction. In DOR, plutonia is reduced with calcium metal to form plutonium metal and calcium oxide.2 3 The reaction takes place in a CaCl2 solvent which dissolves the calcium oxide and allows the plutonium metal to coalesce in the bottom of the crucible. 

Actinide metals with lower vapor pressures (Th, Pa, and U) cannot be obtained by this method since no reductant metal exists which has a sufficiently low vapor pressure and a sufficiently negative free energy of formation of its oxide. For the large-scale production of U, Np, and Pu metals, the calciothermic reduction of the actinide oxide (Section II,A) followed by electrorefining of the metal product is preferred (24). In this process the oxide powder and solid calcium metal are vigorously stirred in a CaCl2 flux which dissolves the by-product CaO. Stirring is necessary to keep the reactants in intimate contact. 

Stable hydrosols may be obtained similarly by reduction of arsenious oxide, dissolved in aqueous sodium hydroxide containing some other protective colloid such as gelatin or egg-albumin, by means of alkaline pyrogallol.6 Salts of metallic acids, such as sodium antimonate or calcium plumbate,-with or without the addition of protalbic acid, may also be employed as protective colloids.1... 

In the modern Hunt-Douglas process the ore is leached with dilute sulphuric acid, and the copper converted into cupric chloride by addition of ferrous chloride or calcium chloride. The use of the calcium salt entails removal of the calcium sulphate by filtration. The cupric salt is precipitated as cuprous chloride by reduction with sulphur dioxide, and the precipitate is converted into metallic copper by treatment with iron, or into cuprous oxide by the action of milk of lime. In this process the amount of iron needed is proportionately small, ferric hydroxide is not precipitated, and silver is not dissolved. 

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