Lithium aluminium hydride in purification

Lithium aluminium hydride, in purification Lithium hydride... 

Lead dioxide as oxidizing agent for conversion of 2-hydroxy-3-methyl-benzoic add to 2-hydroxyisophthalic add, 40, 48 Lithium aluminium hydride, in purification of 1,2-dimethoxyethane, 41, 97... 

Basic solid liquid two-phase conditions with f-butyl peroxide and N-benzylquininium chloride convert cyclohex-2-enone preferentially into the 2(S),3(S)-oxirane (20% ee) which, upon purification and treatment with hydrazine, yields (S)-cyclohex-2-enol [7]. This reaction contrasts with the direct reduction of cyclohex-2-enone to the /J-isomer by lithium aluminium hydride in the presence of quinine [20]. 

This group of reagents is commercially available in large quantities some of its members - notably lithium aluminium hydride (LiAlH4), calcium hydride (CaH2), sodium borohydride (NaBH4) and potassium boro-hydride (KBH4) - have found widespread use in the purification of chemicals. 

The ethylmagnesium bromide is prepared in dry tetrahydrofuran and stored no longer than 1 week in a 250-ml. tube fitted with a 3-way vacuum stopcock and a dropping buret. The solution is decanted into the buret, and the correct volume is transferred to the reaction flask with positive nitrogen pressure. The tetrahydrofuran is purified by distillation from lithium aluminium hydride. See Org. Syn., 46, 105 (1966), for warning regarding the purification of tetrahydrofuran. 

Reduction (lithium aluminium hydride/tetrahydrofuran) of the tetraester 34 to the tetraol 35, followed by chlorination (thionyl chloride), afforded 36 in good yield. This tetrachloride was then subjected to base-promoted P-elimination (potassium rerf-butoxide) giving the desired bisdiene 37 in quantitative yield without purification. The sensitivity of 37 toward both thermal and photochemical degradation and its propensity to polymerize necessitated its immediate use following its preparation. 

The reaction between 3-bromo-l-propanol and the aqueous CN" solution was completed in 7 minutes at 90 °C. The hydrolysis was completed in only 9 minutes at 150 °C giving an 80% yield of the lactone. After a 2 minute reduction at room temperature with lithium aluminium hydride, the product was obtained in 10 minutes by treatment with methanesulphonyl chloride at 110 °C. The overall reaction and purification to pharmaceutical standards was complete in about 70 minutes and the decay-corrected isolated yield was approximately 5%. This was adequate for tracer studies in both monkeys and human subjects. ... 

The purification of diethyl ether (see Chapter 4) is typieal of liquid ethers. The most conunon contaminants are the alcohols or hydroxy compounds from which the ethers are prepared, their oxidation products (e.g. aldehydes), peroxides and water. Dialkyl ethers form peroxides much more readily than other ethers, e.g. ethyl phenyl ethers, on standing in air. Peroxides, aldehydes and alcohols can be removed by shaking with alkaline potassium permanganate solution for several hours, followed by washing with water, concentrated sulfurie acid [CARE], then water. After drying with calcium chloride, the ether is distilled. It is then dried with sodium or with lithium aluminium hydride, redistilled and given a final fractional distillation. The drying process is repeated if necessary. 

In addition to water, this solvent is almost invariably contaminated with peroxide and acetaldehyde. For removal of the peroxide impurity, dioxan is treated with tin(II) chloride and then distilled. Subsequently, the above process is repeated with anhydrous potassium hydroxide for removal of the acetaldehyde. Finally, a molecular sieve with a pore size of 0.4 nm may be used to bind traces of water. This procedure is followed again by distillation. In this way the purification of dioxane may be carried out in three steps, with triple distillation. For simplification, refluxing with lithium aluminium hydride may be attempted, followed by distillation which frees the dioxane from all three impurities in one step. 

Apart from water, the most frequent impurity in this solvent is the peroxide, but it may also contain other organic impurities. Refluxing with lithium aluminium hydride and subsequent distillation may be employed for simultaneous dehydration and purification in one step. 

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