LiAlH*

The results of the research on LiAlH have been reviewed in a number of review articles (for reviews see [1-7]). This is the easily commercially available alanate with the highest theoretical storage capacity which equals 10.6 wt%H2 (Table 1.4 in Sect. 1.1). Such a high capacity makes it a potentially very attractive hydride for solid state hydrogen storage. 

It is well established in the literature [67, 70, 85-89] that hydrogen desorbs from a pure, uncatalyzed LiAlH hydride in a three-step decomposition very similar to that of NaAlH, the first of which goes through the melting of LiAlH  

On the other hand, from the Kissinger analysis shown in Fig. 1.28 in Sect. 1.4.1, we calculated that the activation energy of decomposition for the reaction (Rib) from the liquid equals -122 kJ/mol. This is slightly higher than the value obtained by Andreasen. 

Another factor which suppresses melting of LiAlH is catalysts. Andreasen [67] used 1 min ball milling to disperse 2 mol%TiCl3 l/3AlCl3 in LiAlH. DSC experiments with the heating rates 3-5°C showed only two endothermic reactions, the 

Andreasen et al. [86] also found that ball milling increased the rate constant, k, in the JMAK equation (Sect. 1.4.1), of reaction (Rib) in solid state but virtually had no effect on the rate constant of reaction (R2). They also showed that the reaction constant, k, of reaction (Rib) in solid state increases with decreasing grain size of ball-milled LiAlH within the range 150-50 mn. Andreasen et al. concluded that the reaction (Rib) in solid state is limited by a mass transfer process, e.g., long range atomic diffusion of Al while the reaction (R2) is limited by the intrinsic kinetics (too low a temperature of decomposition). In conclusion, one must say that ball milling alone is not sufficient to improve the kinetics of reaction (R2). A solution to improvement of the kinetics of reaction (R2) could be a suitable catalytic additive. 

LiAlD4 enhanced by VC13 occurred during the first week after milling at temperature in the range of 20-50°C. 

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Lithium aluminium hydride, LiAlH, is a very active reducing agent, and is used particularly for the ready reduction of carboxylic acids (or their esters) to primary alcohols R-COOH -> R CH,OH. 

Lithium aluminium hydride LiAlH is a useful and conveuient reagent for the selective reduction of the carbonyl group and of various other polar functional groups. It is obtained by treatment of finely powdered lithium hydride with an ethereal solution of anhydrous aluminium chloride ... 

The conversion of carboxylic acid derivatives (halides, esters and lactones, tertiary amides and lactams, nitriles) into aldehydes can be achieved with bulky aluminum hydrides (e.g. DIBAL = diisobutylaluminum hydride, lithium trialkoxyalanates). Simple addition of three equivalents of an alcohol to LiAlH, in THF solution produces those deactivated and selective reagents, e.g. lithium triisopropoxyalanate, LiAlH(OPr )j (J. Malek, 1972). 

If it is necessary to reduce one group in a given molecule without affecting any other unprotected reducible group, the following reactivity orders for ease of reduction toward catalytic hydrogenation, LiAlH, and diborane may serve as a guideline. 

Another possibility for asymmetric reduction is the use of chiral complex hydrides derived from LiAlH. and chiral alcohols, e.g. N-methylephedrine (I. Jacquet, 1974), or 1,4-bis(dimethylamino)butanediol (D. Seebach, 1974). But stereoselectivities are mostly below 50%. At the present time attempts to form chiral alcohols from ketones are less successful than the asymmetric reduction of C = C double bonds via hydroboration or hydrogenation with Wilkinson type catalysts (G. Zweifel, 1963 H.B. Kagan, 1978 see p. 102f.). 

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Primary and secondary amines also react with epoxides (or in situ produced episulfides )r aziridines)to /J-hydroxyamines (or /J-mercaptoamines or 1,2-diamines). The Michael type iddition of amines to activated C—C double bonds is also a useful synthetic reaction. Rnally unines react readily with. carbonyl compounds to form imines and enamines and with carbo-tylic acid chlorides or esters to give amides which can be reduced to amines with LiAlH (p. Ilf.). All these reactions are often applied in synthesis to produce polycyclic alkaloids with itrogen bridgeheads (J.W. Huffman, 1967) G. Stork, 1963 S.S. Klioze, 1975). 

The previous product was added to LiAlH (6 eq.) in THF. The solution was heated at reflux for 1 h. The excess hydride was destroyed by dropwise addition of water and the resulting mixture filtered through Celite. The filtrate was diluted with EtOAc, washed with brine and dried (Na2S04). The product was an oil (3.4 g, 98%). 

Acetamidothiazoles are reduced with LiAlH to 2-alkylamino-thiazoles in good yields (81, 130, 477, 478), (see also Section I.l.E). 

Acetyl chlotide is reduced by vatious organometaUic compounds, eg, LiAlH (18). / fZ-Butyl alcohol lessens the activity of LiAlH to form lithium tti-/-butoxyalumium hydtide [17476-04-9] C22H2gA102Li, which can convert acetyl chlotide to acetaldehyde [75-07-0] (19). Triphenyl tin hydtide also reduces acetyl chlotide (20). Acetyl chlotide in the presence of Pt(II) or Rh(I) complexes, can cleave tetrahydrofuran [109-99-9] C HgO, to form chlorobutyl acetate [13398-04-4] in about 72% yield (21). Although catalytic hydrogenation of acetyl chlotide in the Rosenmund reaction is not very satisfactory, it is catalyticaHy possible to reduce acetic anhydride to ethylidene diacetate [542-10-9] in the presence of acetyl chlotide over palladium complexes (22). Rhodium trichloride, methyl iodide, and ttiphenylphosphine combine into a complex that is active in reducing acetyl chlotide (23). 

Although a few simple hydrides were known before the twentieth century, the field of hydride chemistry did not become active until around the time of World War II. Commerce in hydrides began in 1937 when Metal Hydrides Inc. used calcium hydride [7789-78-8J, CaH2, to produce transition-metal powders. After World War II, lithium aluminum hydride [16853-85-3] LiAlH, and sodium borohydride [16940-66-2] NaBH, gained rapid acceptance in organic synthesis. Commercial appHcations of hydrides have continued to grow, such that hydrides have become important industrial chemicals manufactured and used on a large scale. 

However, when it is obtained by pyrolysis of diethylmagnesium or by reaction of diethylmagnesium and LiAlH (11), it is very reactive with both air and water. This difference in reactivity mainly results from the much finer particle size of the product obtained by the pyrolysis route. 

LiAlH is soluble in ethers, 35-40 g/100 g diethyl ether at 25°C. Solubihty in THF, the other common solvent for LiAlH, is 13 g/100 g at 25°C. Polyethylene glycol diaLkyl ethers are also good solvents. 

Preparation. Commercial manufacture of LiAlH uses the original synthetic method (44), ie, addition of a diethyl ether solution of aluminum chloride to a slurry of lithium hydride (Fig. 2). 

The stoichiometry (4 mol lithium hydride to 1 mol LiAlH ) makes this an inherently expensive process, even though high yields of pure product are obtained. For large-scale production, metathesis from NaAlH is economically preferred. 

The reactions of NaAlH are the same as those of LiAlH. However, it is much less soluble THF is the only good solvent. Heterogeneous reductions in good yield with NaAlH in hydrocarbon media have been reported (49). 

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