Alkali metals as reducing agent

We have concentrated our efforts on reducing metal salts in ethereal or hydrocarbon solvents using alkali metals as reducing agents(l 6-54,77). 

Solutions of alkali metals in liquid ammonia are used in organic chemistry as reducing agents. The deep blue solutions effectively contain solvated electrons (p. 126), for example... 

The alkali metal halides, particularly NaCl and KCl, find extensive application in industry (pp. 71 and 73). The hydrides are frequently used as reducing agents, the product being a hydride or complex metal hydride depending on the conditions used, or the free element if the hydride is unstable. Illustrative examples using NaH are ... 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

Special success has followed the use of alkali metal amalgams in tetra-hydrofuran or other ethers (VII, 11, 44) as reducing agents in the syntheses of mono- and polynuclear metal carbonyls, e.g.,... 

Switching from alkali metal to LiAlH as reducing agent for Btp—E—X the first alkyne analogue could be finally isolated and structurally characterized, not for E = Ge and Sn, however, but for E = Pb. While in the Sn case the novel Sn(II) hydride BtpSnH (58) was isolated instead (see Section II), the putative Pb(II) hydride is not stable and dehydrogenates, finally yielding BtpPb=PbBtp (99)95. 

Dialkyl and diaryl ditellurium compounds are easily reduced to tellurols and tellurolates. Alkali metals in liquid ammonia or in an inert organic solvent, sodium borohydride in methanol, ethanol, alcohol/benzene, THF, DMF, or in a basic aqueous medium, lithium aluminum hydride in dioxane or THF/hexamethylphosphoric triamide, and thiourea dioxide in THF/50% aqueous sodium hydroxide have been used as reducing agents (p. 164). The tellurolates are easily oxidized in air. For this reason they are almost always used in situ. 

Since the alkali metals as a class are the strongest chemical reducing agents known, they cannot be prepared by chemical reduction without trie enforcement of a heavy shift in equilibrium conditions as an example, small quantities of sodium metal may be prepared by reducing sodium chloride with calcium metal, distilling away the sodium metal as it is formed. 

The tetrahydroborate salts of alkali metals, M[BH4] (M = Li, Na, K),1 are important because they serve as starting materials for the preparation of other boron hydrides2,3 and because they are used frequently as reducing agents.4 The lithium and sodium salts are prepared on a technical scale.5 9 The tetrahydroborate salts of the alkaline earth metals, M[BH4]2 (M = Mg, Ca, Sr, Ba), have not as yet been used extensively however, calcium bis[tetrahydroborate(l-)], Ca(BH4)2,10 is very soluble in tetrahydrofuran (THF) and it therefore has considerable potential application as a substitute for the lithium and sodium salts. 

It should be mentioned incidental that alkali metal sulfides can also be used as reducing agents for this purpose if they happen to be available cheap. ... 

The equilibrium constant of reaction (1), K = [Cu ][Cu ]/[Cu ], is of the order of 10 thus, only vanishingly small concentrations of aquo-copper(I) species can exist at equilibrium. However, in the absence of catalysts for the disproportionation—such as glass surfaces, mercury, red copper(I) oxide (7), or alkali (311)—equilibrium is only slowly attained. Metastable solutions of aquocopper(I) complexes may be generated by reducing copper(II) salts with europium(II) (113), chromium(II), vanadium(II) (113, 274), or tin(II) chloride in acid solution (264). The employment of chromium(II) as reducing agent is best (113), since in most other cases further reduction to copper metal is competitive with the initial reduction (274). 

The second way to deliver H2 in a reduction is to add two protons and two electrons to a substrate—that is, H2 = 2H + 2e . Reducing agents of this sort use alkali metals as a source of electrons and liquid ammonia (NH3) as a source of protons. Reductions with Na in NH3 are called dissolving metal reductions. 

For reductions carried out in the absence of an added proton donor the usual solvents are liquid NH3, usually with an ethereal cosolvent, or less commonly an ether (THF, DME or diethyl ether). TTie reducing agent is usually one of the common alkali metals (Li, Na, K), although Rb, Cs and alkaline earth metals have also been used. " At least in the case of camphor both types of solvent systems give similar ratios of epimeric alcohols however, product ratios may vaiy as a function of the metal used as reducing agent. In reductions carried out in ethereal solvents the use of ultrasound increases the rate of the reaction, but does not affect the product distribution. ... 

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