Lithium borohydride solubilities

Unlike many other borohydrides, lithium borohydride is highly soluble ia ethers including aUphatic ethers, THF, an d polyglycol ethers. It is also very soluble ia amines and ammonia. Dissolution ia water and lower aUphatic alcohols leads to extensive decomposition and hydrogen evolution. 

Other reagents used for reduction are boranes and complex borohydrides. Lithium borohydride whose reducing power lies between that of lithium aluminum hydride and that of sodium borohydride reacts with esters sluggishly and requires refluxing for several hours in ether or tetrahydrofuran (in which it is more soluble) [750]. The reduction of esters with lithium borohydride is strongly catalyzed by boranes such as B-methoxy-9-bora-bicyclo[3.3.1]nonane and some other complex lithium borohydrides such as lithium triethylborohydride and lithium 9-borabicyclo[3.3.1]nonane. Addition of 10mol% of such hydrides shortens the time necessary for complete reduction of esters in ether or tetrahydrofuran from 8 hours to 0.5-1 hour [1060],... 

Lithium borohydride, LiBH4. Mol. wt. 21.79, m.p. 284° dec., extremely hygroscopic, soluble in lower primary amines and ethers (diethyl ether, 4 g./lOO g. tetrahydrofurane, 21 g./100 g.) soluble (with reaction) in lower alcohols soluble in water with slow decomposition LiBHi-b 2 H2O— LiB02-F 4 H2 +heat. Supplier Alfa. 

For reduction, lithium borohydride [38] has an advantage over sodium boro-hydride, as it is readily soluble in simple ether solvents. Lithium borohydride reduces selectively the esters in the presence of other reducible groups. The reduction of esters proceeds smoothly but relatively slowly. However, it is found [39] that in the presence of 10 mol% of lithium-9-boratabicydo[3.3.1]nonane, Li 9-BBNH, or lithium triethylborohydride, LiEtjBH strongly catalyzes the reduction of esters by lithium borohydride in diethyl ether. Catalytic effects have also been observed with B-OMe-9-BBN, LiB(OMe)2-9-BBN, and LiEt3BOMe (Table 25.12) [39]. 

The tetrahydridoborate ion, as "sodium borohydride" NaBH is soluble in water and is similarly an excellent reducing agent in this solvent. (Lithium tetrahydridoaluminate cannot be used in water, with which it reacts violently to give hydrogen.)... 

Reductions with hydrides and complex hydrides are usually carried out by mixing solutions. Only sodium borohydride and some others are sometimes added portionwise as solids. Since some of the complex hydrides such as lithium aluminum hydride are not always completely pure and soluble without residues, it is of advantage to place the solutions of the hydrides in the reaction flask and add the reactants or their solutions from separatory funnels or by means of hypodermic syringes. 

Use of more efficient solvents (tetrahydrofuran, isopropyl ether, dimethoxyethane) or more soluble metal hydride reagents (sodium borohydride, lithium tributoxy aluminum hydride, sodium bis(2-methoxyethyl) aluminum hydride) favors the alternative reduction pathway to the hydroquinone. 

Disodium ditelluride can be prepared from tellurium and sodium in liquid ammonia, lithium triethylborohydride in THF, sodium borohydride in ethanol or THF" lithium or sodium in ethylenediamine , sodium hydride in DMF , hydrazine hydrate in aqueous sodium hydroxide , thiourea dioxide in DMF/THF/aqueous sodium hydroxide, and sodium formaldehyde sulfoxylate (Rongalite C) " in aqueous sodium hydroxide. The reducing agents compatible with organic solvents provide reaction media in which long-chain aliphatic halides are soluble. Such halides are insoluble in liquid ammonia, or aqueous media. 

Considering the extreme reactivity of most hydrides (such as sodium hydride and lithium aluminum hydride) toward water, sodium borohydride is somewhat surprisingly sold as a stabilized aqueous solution 14 molar in sodium hydroxide containing 12% sodium borohydride. Unlike lithium aluminum hydride, sodium borohydride is insoluble in ether and soluble in methanol and ethanol. 

Diethylene glycol dimethyl ether ( diglyme, bp 161°) is an excellent water-miscible solvent. It is, for example, one of the few solvents other than water in which sodium borohydride is reasonably soluble. Its high boiling point is often a convenience for example, a low-boiling ketone or aldehyde may be reduced in this solvent by lithium aluminum hydride, a high-boiling alcohol (such as butyl... 

The reactions are commonly initiated with AIBN at ca. 80 °C in a solvent such as benzene or toluene, but alkyl halides have also been reduced at -60 °C with tributyltin hydride under sonication conditions.133-134 Some reductions of halides have been carried out by generating the tin hydride in situ from a molar equivalent of the reducing agent [lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane)] and a catalytic amount of organotin hydride, halide, or oxide. Reduction of halides has also been carried out under aqueous conditions, using 4,4 -azobis(4-cyanovaleric acid) (ACVA) as a water-soluble radical initiator.60... 

This solid, which is stable in dry air up to 300°, is a less powerful reducing agent than lithium aluminium hydride, from which it differs also by being soluble in hydroxylic solvents and to a lesser extent in ether-type solvents. Sodium borohydride forms a dihydrate melting at 36-37°, and its aqueous solutions decompose slowly unless stabilised to above pH 9 by alkali. (For example, a useful sodium borohydride solution is one that is nearly saturated at 30-40° and containing 0.2% sodium hydroxide.) Its solubility in water is 25, 55 and 88g per lOOmL... 

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