Ytterbium ammonia

Ylidenebutenolides, 431 Ytterbium, 517 Ytterbium-Ammonia, 517 Ytterbium chloride, 211... 

Ytterbium-ammonia reductions. Ytterbium possesses reducing properties similar to those of lithium and sodium. It dissolves in liquid ammonia to form a blue solution that is srable for several hours at —33°. This solution reduces arenes to 1,4-dihydroarenes, enones to ketones, and alkynes to rra 5-alkenes. 

Reduction of a variety of organic functional groups has long been carried out using ammonia solutions of alkali metals (Birch and SubbaRao, 1972). Given the strongly electropositive character of lanthanides such as ytterbium (which features a 4f 6s electron configuration), it follows that ytterbium/ammonia solutions should convert a,i8-unsatiu ated ketones to saturated ketones, alkynes to trans-alkenes and aromatics to 1,4-dihydroaromatics (White and Larson, 1978). 

Advantages of the ytterbium/ammonia system include the inertness of ytterbium to water and air relative to alkali metal reagents, and the fact that strongly basic hydroxides can be avoided. 

As was mentioned in section 2.3.3, ytterbium metal dissolves in liquid ammonia to yield ammoniated electrons and Yb " ions. This solution was used by White et al. (1978) to perform reductions of various aromatic systems, similar to the Birch reactions which use lithium or sodium as the metal. The addition of benzoic acid or anisole dissolved in a 10 1 mixture of THF-tert-butyl alcohol, to an ytterbium-ammonia solution gives 1,4-dihydrobenzoic acid (56% yield). Triple bonds are cleanly reduced to trans olefins (i.e. PhC CPh traK5-PhCH=CHPh 75%). The C=C double bonds of conjugated ketones are also reduced by this system. Since the reaction medium initially contains both solvated electrons and Yb + ions it is likely that the above reactions are not directly connected with the presence of divalent ytterbium species. 

Europium and ytterbium are very readily oxidizable and react with 02, and especially with moist air. They rapidly dissolve in dilute mineral acids. Eu and Yb and the alkaline earth metals form, like the alkali metals, deep blue strongly reducing solutions in liquid ammonia. 

Ytterbium melts at 824°C vaporizes at 1,194°C electrical resistivity 25.0 microhm-cm Vickers hardness 21 kg/mm Young s modulus 0.182x10 kg/cm2 shear modulus 0.071x10 kg/cm Poisson s ratio 0.284 magnetic susceptibility 71x10 emu/mol thermal neutron absorption cross section 37 barns reacts slowly with water soluble in dilute acids and ammonia. 

The metal dissolves in dilute and concentrated mineral acids. Evaporation crystallizes salts. At ordinary temperatures, ytterbium, similar to other rare earth metals, is corroded slowly by caustic alkalies, ammonium hydroxide, and sodium nitrate solutions. The metal dissolves in liquid ammonia forming a deep blue solution. 

Solutions of alkali metals in ammonia have been the best studied, but other metals and other solvents give similar results. The alkaline earth metals except- beryllium form similar solutions readily, but upon evaporation a solid ammoniste. M(NHJ)jr, is formed. Lanthanide elements with stable +2 oxidation states (europium, ytterbium) also form solutions. Cathodic reduction of solutions of aluminum iodide, beryllium chloride, and teUraalkybmmonium halides yields blue solutions, presumably containing AP+, 3e Be2, 2e and R4N, e respectively. Other solvents such as various amines, ethers, and hexameihytphosphoramide have been investigated and show some propensity to form this type of solution. Although none does so as readily as ammonia, stabilization of the cation by complexation results in typical blue solutions... 

Metals. Lanthanide metals are also considered as valuable precursors. For example, alkoxides derived from cheap and low-boiling-point alcohols have been alternatively synthesized from metals in the presence of HgQ2 as catalyst [133]. Representative and specific methods of preparation include transmetalla-tion reactions (Eq. 7-9) [134], using ammonia solutions of ytterbium and europium as synthetic reagents (Eq. 10) [135] and the generation of thiolate complexes from disulfides (Eq. 11) [136],... 

A common precursor to LnN are the simple inorganic amides Ln(NH2)x (x = 2, 3) which can be placed between the nitrides and the alkyl substituted amides. Their main use lies in the synthesis of other solid materials like lanthanide hydroxides [21,33], carbides [34] or above-mentioned nitrides. Very recently solutions of europium and ytterbium in liquid ammonia have been rediscovered as synthetic tools (Sect. 7.1). 

In 1956 it was found that europium and ytterbium dissolve in liquid ammonia with the characteristic deep blue color known for the alkali and alkaline earth metals [36-40]. This behavior arises from the low density and high volatility of those metals compared to the other lanthanide elements [41]. Samarium, which normally also occurs in the divalent oxidation state, does not dissolve under... 

Recently solutions of europium and ytterbium in liquid ammonia have been rediscovered as a precursor system to highly pure exchanged complexes [270], The active species in these reactions are the hexaammoniates and the only byproducts are hydrogen and ammonia (Eq. 23a-c). 

Solutions of ytterbium in liquid ammonia are capable of reducing aromatic systems to 1,4-dihydroaromatics or alkynes to trans alkenes [293]. The a,fi-unsaturated cholest-4-en-3-one was reduced to eholestanone in 80% yield by threefold excess of ytterbium in ammonia followed by oxidation with Jones reagent (Scheme 16). In the absence of a proton source, (HOEt) and THF as co-solvent, the pinacol dimer was obtained as the major product. 

Scheme 16. Solutions of ytterbium in ammonia as synthetic agents...

Reaction of cyclooctatetraene with solutions of ytterbium and 2 moles of potassium in liquid ammonia gives K2 [M(C8H8)2] the solvate [K(dme)]2 [Yb(C8Hs)2] (DME = 1,2-dimethoxyethane) has a sandwich uranocene-type structure for the anion. 

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. ... 

The reduction of disubstituted acetylenes with ytterbium in liquid ammonia also produces trans-a -kenes in good yields. This reducing system does reduce some double bonds, such as the strained double bond in norbomadiene however, in general, carbon-carbon double bonds do not undergo reduction with this reducing system. The expense of powdered metallic ytterbium does not make this a very practical reducing agent for synthetic purposes. 

Ytterbium tends to be more reactive than other lanthanoid elements. It is usually stored in sealed containers to keep it from reacting with oxygen in the air. It also reacts slowly with water and more rapidly with acids and liquid ammonia. 

Europium and ytterbium (R/zeolite) introduced into K+-exchanged Y-zeolite by impregnation from liquid ammonia solutions similarly show various catalytic reactivities with changes in evacuation temperatures (Baba et al. 1992). The changes in the chemical state of rare earths with the evacuation temperature are studied by IR and X-ray absorption... 

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