Ytterbium dissolving metal reductions

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. ... 

Various processes separate rare earths from other metal salts. These processes also separate rare earths into specific subgroups. The methods are based on fractional precipitation, selective extraction by nonaqueous solvents, or selective ion exchange. Separation of individual rare earths is the most important step in recovery. Separation may be achieved by ion exchange and solvent extraction techniques. Also, ytterbium may be separated from a mixture of heavy rare earths by reduction with sodium amalgam. In this method, a buffered acidic solution of trivalent heavy rare earths is treated with molten sodium mercury alloy. Ybs+ is reduced and dissolved in the molten alloy. The alloy is treated with hydrochloric acid, after which ytterbium is extracted into the solution. The metal is precipitated as oxalate from solution. 

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. 

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