Lithium tetrahydridoaluminate

LiAlH4, lithium tetrahydridoaluminate ("lithium aluminium hydride . so-called) is an excellent reducing agent in ether solution for both organic and inorganic compounds it may be used to prepare covalent hydrides SiH ether, for example... 

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.)... 

Lithium tetrahydroaluminate Lithium tetrahydridoaluminate Sodium tetrahydroborate Sodium tetrahydridoborate Lithium trialkoxyhydridoaluminate Sodium bis(2-methoxyethoxy)dihydro-aluminate Diisobutylalane Tributylstannane... 

After the amalgam is cooled, 400 mL of tetrahydrofuran (THF) distilled from lithium tetrahydridoaluminate is added to the flask, followed by 50.0 g (142 mmole) of W(CO)6. "Caution. The THF should be checked for peroxide and high water content before LiAlH4 is added. The distillation procedure is given in Reference 6. 

Grignard reagents and lithium tetrahydridoaluminate achieved SN2 displacement of fluoride anion (Eq. 100). Displacement of a second fluoride ion occurred with excess reducing agent, and upon the action of butyllithium. These reactions have not found extensive use in target synthesis. 

Reductive defluorination reactions have also been described in ether, difluoroallylic alkoxides undergo stereoselective reduction (Eq. 138) to the E-mono-fluoro derivatives upon treatment with lithium tetrahydridoaluminate [354]. Sodium borohydride [355] and Red-Al [346] have also been used to achieve this transformation. 

Primary and secondary alkyl bromides, iodides, and sulfonates can be reduced to the corresponding alkanes with LiBHEt3 (superhydride) or with lithium aluminum hydride (LiAlH4, other names lithium tetrahydridoaluminate or lithium alanate). If such a reaction occurs at a stereocenter, the reaction proceeds with substantial or often even complete stereoselectivity via backside attack by the hydride transfer reagent. The reduction of alkyl chlorides to alkanes is much easier with superhydride than with LiAlH4. The same is true for sterically hindered halides and sulfonates ... 

The tetrahydrofuran used in the synthesis is dried immediately prior to use and is purified by atmospheric-pressure distillation over sodium tetrahydridoaluminate(l — ) under a nitrogen atmosphere. The use of lithium tetrahydridoaluminate(l —) is to be avoided because of reports that it explodes on distillation. The following precautions should be observed when distilling tetrahydrofuran over sodium tetrahydridoaluminate(l —). 

Sodium tetrahydridoaluminate(l —) has been found to be superior to lithium tetrahydridoaluminate(l-) as a drying agent over which tetrahydrofuran is distilled. Tetrahydrofuran distills at 65°, a temperature at which lithium tetrahydridoaluminate( 1 -) is already decomposing slowly. As tetrahydrofuran is distilled from a flask containing lithium tetrahydrido-aluminate(l —) the temperature of the pot increases and can reach a temperature of 90°, at which lithium tetrahydridoaluminate(l-) decomposes rapidly and has been known to detonate. On the other hand, sodium tetra-hydridoaluminate(l-) decomposes at 185°, a temperature far above temperatures expected in the distillation flask during distillation. Under no condition should the distillation of tetrahydrofuran over sodium tetra-hydridoaluminate(l —) be allowed to proceed to dryness. Preferably the distillation should be stopped just as the solvent layer reaches the top of the heating mantle. 

Diethyl ether (Fisher Certified) was distilled over lithium tetrahydrido-aluminate(l - ) immediately prior to use. It is perfectly safe to distill diethyl ether over lithium tetrahydridoaluminate(l -) since diethyl ether boils (34°) sufficiently below the decomposition point of lithium tetrahydrido-aluminate(l —) provided normal precautions are taken. See Synthesis 3. 

The checkers noted a delay in the formation of solid magnesium dihydride, which did not begin to precipitate until nearly all the lithium tetrahydridoaluminate(l —) had been added, t The diethyl ether was determined by difference in the gravimetric analysis. 

The solution of aluminum trihydride used in this synthesis is prepared by the reaction of 100% sulfuric acid with lithium tetrahydridoaluminate(l —) in dry tetrahydrofuran.3 Under nitrogen flow, a stoichiometric amount of the sulfuric acid is added dropwise by syringe at 0° to a solution of lithium tetrahydridoaluminate(l —) in dry tetrahydrofuran. The apparatus used is just like that shown in Fig. 2, except that an ice bath is used to cool the reaction flask and thus prevent ether cleavage. As the sulfuric acid is added to the lithium tetrahydridoaluminate(l —), a precipitate of lithium sulfate forms and hydrogen is evolved. For this reason, the reaction must be carried out in a hood. After all the sulfuric acid has been added, the resulting slurry is stirred for 2 hr, then filtered in a glove box.4,5 The filtrate, a clear solution of aluminum trihydride in tetrahydrofuran, is stored in the refrigerator at -20° until it is needed. 

The reaction of methyllithium with dimethylzinc in 1 1 and 2 1 molar ratios in diethyl ether solution produces lithium tri- and tetramethyl-zincates.1 Two equivalents of lithium trimethylzincate( 1 -) react with three equivalents of lithium tetrahydridoaluminate(l -), and one equivalent of lithium tetramethylzincate(2 —) reacts with two equivalents of lithium tetra-hydridoaluminate(l —) to give lithium trihydridozincate( 1 -) and lithium tetrahydridozincate(2 —), respectively, in quantitative yields. Both reactions proceed in diethyl ether at room temperature and are complete within 5 min. The compounds are air-sensitive and therefore must be prepared in an airfree atmosphere (nitrogen or argon). They may be used as reducing agents in organic syntheses. 

The diethyl ether used herein is purified and dried prior to use by atmospheric-pressure distillation over lithium tetrahydridoaluminate( 1 — ) under a nitrogen atmosphere. The lithium tetrahydridoaluminate(l —) should be added to the diethyl ether in a nitrogen-filled dry box, and the distillation should not be carried out past the point where the distillation flask is one-quarter full. 

The crude Na[Zn2(CH3)2H3] solution in THF may be used as a reducing agent.4 Like lithium tetrahydridoaluminate(l —), the zinc reagent reduces aliphatic ketones and esters to the corresponding alcohols and aliphatic nitriles to amines. However, of the two, the zinc compound is a somewhat milder reducing agent as shown by the reduction of benzonitrile to the corresponding imine under conditions in which lithium tetrahydrido-aluminate(l —) yields the amine. 

While lithium tetrahydridoaluminate(l -) is used extensively as a reducing agent,1 relatively little is known about the utility of amine and/or amino complexes (compounds of the general formulas H3A1(NR3) and H3 A1-... 

Lithium tetrahydridoaluminate(l —) may be used as obtained commercially. Trimethylammonium chloride is dried under vacuum at 100° for 1 hr. The benzene solvent should be dried by heating at the reflux temperature for a period in excess of 1 hr over lithium tetrahydrido-aluminate(l —) followed by distillation under a nitrogen atmosphere. In a nitrogen-filled dry box, 47.8 g (0.50 mol) of trimethylammonium chloride is transferred to a Schlenk tube. To a 1000-mL three-necked flask containing a large, egg-shaped stirring bar, 22.8 g (0.60 mol) of lithium tetrahydrido-aluminate(l —) is added. A 20% excess of lithium tetrahydrido-aluminate(l —) is used in order to eliminate the possibility of obtaining... 

Lithium tetrahydridogallate(l —) is a moisture-sentitive solid similar in many respects to lithium tetrahydridoaluminate(l —). It reacts with protic solvents, but is soluble without reaction in a variety of ethers. The solid decomposes slowly at room temperature, but may be stored with decomposition at — 78° under an inert atmosphere or in vacuum. Ether solutions are stable for at least several days. A variety of gallium hydride derivatives may be prepared from Li[GaH4], including (CH3)3N GaH3.1,2,6 Infrared and Raman data have been reported for [GaH4]-.7... 

The solvents should be free of air and water. Pyridine and 1,5-cyclooctadiene were distilled under nitrogen. Tetrahydrofuran was distilled from lithium tetrahydridoaluminate(l —) and stored over sodium in a nitrogen atmosphere. Pentane was dried over molecular sieve and sparged with nitrogen. 

Caution. Care should be taken in drying the diethyl ether with lithium tetrahydridoaluminate —) because of the danger of fire if the ether is very wet. See also Synthesis 1 for additional information. 

The tetrahydrofuran is dried and purified by shaking with sodium hydroxide pellets, refluxing 24 hr over sodium metal, and distilling in an inert atmosphere from lithium tetrahydridoaluminate or sodium diphenylketyl immediately before use. Other solvents are dried and purified in the usual way. 2 Alkyl or aryl halides are also freshly distilled. 

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