Aluminium metal hydride

Metal alkyl and aryl oxyhydrides are similar to alkoxides in that they involve a metal hydride attached to the oxygen from an alkyl or aryl alcohol. Examples include the aluminium metal hydride sodium bis(2-methoxyethoxy) aluminohydride and lithium aluminium-tri-tert-butoxyhydride. They are dangerous fire risks in contact with water and can decompose to evolve hydrogen gas. 

Boron forms a whole series of hydrides. The simplest of these is diborane, BjH. It may be prepared by the reduction of boron trichloride in ether by lithium aluminium hydride. This is a general method for the preparation of non-metallic hydrides. 

Industrially, chlorine is obtained as a by-product in the electrolytic conversion of salt to sodium hydroxide. Hazardous reactions have occuned between chlorine and a variety of chemicals including acetylene, alcohols, aluminium, ammonia, benzene, carbon disulphide, diethyl ether, diethyl zinc, fluorine, hydrocarbons, hydrogen, ferric chloride, metal hydrides, non-metals such as boron and phosphorus, rubber, and steel. 

Hydrogen can be prepared by the reaction of water or dilute acids on electropositive metals such as the alkali metals, alkaline earth metals, the metals of Groups 3, 4 and the lanthanoids. The reaction can be explosively violent. Convenient laboratory methods employ sodium amalgam or calcium with water, or zinc with hydrochloric acid. The reaction of aluminium or ferrosilicon with aqueous sodium hydroxide has also been used. For small-scale preparations the hydrolysis of metal hydrides is convenient, and this generates twice the amount of hydrogen as contained in the hydride, e.g. ... 

N. G. Gaylord, Reduction with Complex Metal Hydrides, Interscience, New York, 1956, KM6 pp. J. S. Pizey, Lithium Aluminium Hydride, Ellis Horwood, Ltd., Chichester, 1977,... 

Eliel, E. L., and M. N. Rerick Reduction with Metal Hydrides, VIII. Reduction of Ketones and Epimerization of Alcohols with Lithium-Aluminium Hydride — Aluminium Chloride. J. Amer. chem. Soc. 82, 1367 (1960). 

A number of complex metal hydrides such as lithium aluminium hydride (LiAlH4, abbreviated to LAH) and sodium borohydride (NaBHj) are able to deliver hydride in such a manner that it appears to act as a nucleophile. We shall look at the nature of these reagents later under the reactions of carbonyl compounds (see Section 7.5), where we shall see that the complex metal hydride never actually produces hydride as a nucleophile, but the aluminium hydride anion has the ability to effect transfer of hydride. Hydride itself, e.g. from sodium hydride, never acts as a nucleophile owing to its small size and high charge density it always acts as a base. Nevertheless, for the purposes of understanding the transformations. 

Whilst the complex metal hydride is conveniently regarded as a source of hydride, it never actually produces hydride as a nucleophile, and it is the aluminium hydride anion that is responsible for... 

It is quite difficult to reduce benzene or pyridine, because these are aromatic stmctures. However, partial reduction of the pyridine ring is possible by using complex metal hydrides on pyridinium salts. Hydride transfer from lithium aluminium hydride gives the 1,2-dihydro derivative, as predictable from the above comments. Sodium borohydride under aqueous conditions achieves a double reduction, giving the 1,2,5,6-tetrahydro derivative, because protonation through the unsaturated system is possible. The final reduction step requires catalytic hydrogenation (see Section 9.4.3). The reduction of pyridinium salts is of considerable biological importance (see Box 11.2). 

The most useful reagents for reducing aldehydes and ketones are the metal hydride reagents. Complex hydrides are the source of hydride ions, and the two most commonly used reagents are NaBlTj and LiAlH4. Lithium aluminium hydride is extremely reactive with water and must be used in an anhydrous solvent, e.g. dry ether. 

The simple, salt elimination reaction of Equation (8.1) has been employed for amides of all the group 13 metals. In addition, it is currently the only well-established route to M(I) metal amides where M = Ga or Tl. The alkane elimination route of eqn. (8.2) is generally employed only for M = Al or Ga. This synthetic approach is also used for the metal imides (RMNR )n where a primary amine H2NR is the reactant. The use of metal hydrides, of which Equation (8.3) is but one example, is limited mainly to aluminium and, to a lesser extent, gallium because of the decreased stability of the heavier metal hydrides. 

Catalytic reduction of steroid epoxides received considerable attention before the development of complex metal hydride reducing agents. Hydrogenation of 3 ,4a-epoxy steroids over platinum in acetic acid (Eq. 360), for example, gives rise to a mixture of 3 -hydroxy and 3 -acetoxy steroids.Reductive cleavage thus occur in the same direction as with lithium aluminium hydride in this particular instance —t.r. it gives an axis alcohol. 

Metal hydrides. Lithium hydride, sodium hydride, potassium hydride and lithium aluminium hydride all react violently with water liberating hydrogen the heat of reaction may cause explosive ignition. Excess metal hydride from a reaction must be destroyed by the careful addition of ethyl acetate or acetone. 

Following the procedure for the transformation of acetate (12) to (16), the acetate (13) was converted to trans-acetate (18). Reduction of (18) with metal hydride followed by oxidation of the resulting alcohol with pyridinum chlorochromate produced ketone (11), which afforded alcohol (17) by reduction with lithium aluminium hydride. 

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