Cesium reduction

Cesium, first discovered by Bunsen and Kirchoff ia 1860 while examining spring water, was the first element discovered spectroscopically (1). The name, comes from the Latin caesius, sky blue, and refers to the characteristic blue spectral lines of the element. Cesium salts were not successfully reduced to metal until 1881. Electrolysis of the molten chloride did not yield cesium metal under the same conditions that led to the reduction of the other alkaU metal chlorides. 

Cesium was first produced ia the metallic state by electrolysis of a molten mixture of cesium and barium cyanides (2). Subsequentiy the more common thermochemical—reduction techniques were developed (3,4). There were essentially no iadustrial uses for cesium until 1926, when it was used for a few years as a getter and as an effective agent ia reduciag the electron work function on coated tungsten filaments ia radio tubes. Development of photoelectric cells a few years later resulted ia a small but steady consumption of cesium and other appHcations for cesium ia photosensing elements followed. 

There are three basic methods of converting poUucite to cesium metal or compounds direct reduction with metals decomposition with bases and acid digestion. In each case grinding of the ore to 75 p.m precedes conversion. 

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. 

With the nitro group successfully introduced, the aromatic fluoride substituent in 11 was ready to undergo the nucleophilic aromatic substitution with the hydrox-ypyridine 9. The reaction proceeded smoothly in DMF at 55 °C using an equimolar amount of cesium carbonate as the base and provided a 90% isolated yield of 23 after crystallization. With compound 23 in hand, only the reduction of the nitro... 

Fluoride ion is effective in promoting the reduction of aldehydes by organosil-icon hydrides (Eq. 161). The source of fluoride ion is important to the efficiency of reduction. Triethylsilane reduces benzaldehyde to triethylbenzyloxysilane in 36% yield within 10-12 hours in anhydrous acetonitrile solvent at room temperature when tetraethylammonium fluoride (TEAF) is used as the fluoride ion source and in 96% yield when cesium fluoride is used.83 The carbonyl functions of both p-anisaldehyde and cinnamaldehyde are reduced under similar conditions. Potassium bromide or chloride, or tetramethylammonium bromide or chloride are not effective at promoting similar behavior under these reaction conditions.83 Moderate yields of alcohols are obtained by the KF-catalyzed PMHS, (EtO SiH, or Me(EtO)2SiH reduction of aldehydes.80,83,79... 

Diphenylsilane catalyzed by various salts promotes the 1,2-reduction of cinnamaldehyde.318 Cesium fluoride catalysis is particularly effective.320 It is possible to stop these reactions at the silyl ether stage.73,320 The 1,2-reduction of citral is accomplished in high yield with diphenylsilane and Wilkinson s catalyst (Eq. 262) 435 Interestingly, the trialkylsilanes, ethyldimethylsilane and triethylsilane, give high yields of the 1,4-reduction product whereas diisopropylsilane gives a 1 1 mixture of 1,2- and 1,4-reduction (Eq. 263)435... 

The 1,4-reduction of a,/3-unsaturated aldehydes is best carried out with diphenylsilane in the presence of zinc chloride and tetrakis(triphenylphosphine) palladium436 or a combination of triethylsilane and tris(triphenylphosphine) chlororhodium 437 Other practical approaches use phenylsilane with nickel (0) and triphenylphosphine438 and diphenylsilane with cesium fluoride.83 It is possible to isolate the initial silyl enol ether intermediate from the 1,4-hydrosilylation of o, /3-unsaturated aldehydes (Eq. 264).73,411 The silyl enol ethers are produced as a mixture of E and Z isomers. 

A platinum (0.05 wt%)-impregnated ETS-lO(Cs) sample showed spectra similar to that of ETS- 10(Cs) (g... < gxx. gyy). except that the Ti3+ signal intensity increased by a factor of about 2.4 compared with that of the ETS-lO(Cs) sample. Although the reduction in Ti3+ intensity by Cs is attributed to greater stabilization of Ti4+ ions by the more basic and larger Cs atoms, the increase in the intensity induced by platinum is attributed to better activation of the reductant molecules (H2) by platinum and the consequently greater reduction of Ti4+ to Ti3+. In other words, both cesium and platinum influence the reducibility... 

Azomethine ylides. Reduction of oxazolium salts (1) with phenylsilane and cesium fluoride provides an unstable 4-oxazoline (2), which can react as an azomethine ylide with a dipolarophile such as DM AD to give a dihydropyrrole (3). ... 

Metals react with nonmetals. These reactions are oxidation-reduction reactions. (See Chapters 4 and 18). Oxidation of the metal occurs in conjunction with reduction of the nonmetal. In most cases, only simple compounds will form. For example, oxygen, 02, reacts with nearly all metals to form oxides (compounds containing O2-). Exceptions are the reaction with sodium where sodium peroxide, Na202, forms and the reaction with potassium, rubidium, and cesium where the superoxides, K02, Rb02, and Cs02 form. 

Stereospecific ketone reduction was also observed (Giordano et al. 1985) with potassium, rubidium, and cesium (but not with sodium) in tertiary alcohols (but not in secondary or primary alcohols). The undesirable dimerization probably proceeds more readily in the case of sodium. Tertiary alcohols are simply more acidic than primary or secondary alcohols. It is reasonable to point out that the ketone-to-alcohol reduction of 3a-hydroxy-7-oxo-5p-cholic acid by alkali metals is a key step in the industrial synthesis of 3a,7p-dihydroxy-5p-cholic acid. 

Rubidium does not exist in its elemental metallic form in nature. However, in compound forms it is the 22nd most abundant element on Earth and, widespread over most land areas in mineral forms, is found in 310 ppm. Seawater contains only about 0.2 ppm of rubidium, which is a similar concentration to lithium. Rubidium is found in complex minerals and until recently was thought to be a rare metal. Rubidium is usually found combined with other Earth metals in several ores. The lepidolite (an ore of potassium-lithium-aluminum, with traces of rubidium) is treated with hydrochloric acid (HCl) at a high temperature, resulting in lithium chloride that is removed, leaving a residue containing about 25% rubidium. Another process uses thermochemical reductions of lithium and cesium ores that contain small amounts of rubidium chloride and then separate the metals by fractional distillation. 

One problem in refining cesium is that it is usually found along with rubidium therefore, the two elements must be separated after they are extracted from their sources. The main process to produce cesium is to finely grind its ores and then heat the mix to about 600°C along with liquid sodium, which produces an alloy of Na, Cs, and Ru, which are separated by fractional distillation. Cesium can also be produced by the thermochemical reduction of a mixture of cesium chloride (CsCl) and calcium (Ca). 

Cesium is obtained from its ore pollucite. The element in pure form may be prepared by several methods (i) electrolysis of fused cesium cyanide, (ii) thermal reduction of cesium chloride with calcium at elevated temperatures, and (iii) thermal decomposition of cesium azide. It is stored under mineral od. The element must be handled under argon atmosphere. 

Cesium is highly reactive. It is the most electropositive metal-more electropositive and reactive than other alkali metals of lower atomic numbers. The standard redox potential E° for the reduction Cs+ -i- e — Cs is -3.026 V. It reacts explosively with water, forming cesium hydroxide, CsOH and hydrogen ... 

Fischwick has detailed a rapid synthetic approach to the imidazole based y-lactam alkaloid, (+/—) cynometiine (67), isolated from the stem bark of Cynometra hankei, and which has been shown to be a potential analgesic (16). Cesium fluoride induced formation of the ylide, from precursor 68, followed by cycloaddition to alkene 69 furnished the required adduct 70 in 71% yield as a 4 1 diastereomeric mixture in favor of the desired isomer. Deprotection of the thioketal followed by NaBH4 reduction delivered the desired racemic product (Scheme 3.17). 

The enantiomeric synthesis of rranj-3,4-disubstituted tetrahydrothiophenes using a sulfur ylide cycloaddition has been reported <990L1667>. The sulfur ylide derived from the action of cesium fluoride on sulfide 111 underwent an asymmetric cycloaddition with chiral a,p-unsaturated camphorsultam amide 112 giving tetrahydrothiophene 113 (80% de). The configuration was confirmed by cleavage of the chiral auxiliary followed by reductive desulfurization with Raney-Ni which gave known carboxylic acid 114. 

To purify the cesium (rubidium) from the admixture of the zirconium used for reduction, repeat the distillation from one ampoule into another one. For this purpose, carefully heat the alkali metal in the ampoule with a gas burner until boiling begins and perform distillation slowly (during 0.5 hour). After this, seal the ampoule with the metal. Write the equation of the reaction. 

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