Aluminum Hydride Alane

The trimethylamine adduct of aluminum hydride (alane) has been of recent interest as a precursor for the chemical vapor deposition (CVD) of aluminum metal1 and aluminum gallium arsenide thin films.2 Because of the absence of aluminum-carbon covalent bonds in the precursor, carbon incorporation in the resulting films can be suppressed significantly. In addition, the deposition temperature can be lowered. 

Al—Ti Catalyst for cis-l,4-PoIyisoprene. Of the many catalysts that polymerize isoprene, four have attained commercial importance. One is a coordination catalyst based on an aluminum alkyl and a vanadium salt which produces /n j -l,4-polyisoprene. A second is a lithium alkyl which produces 90% i7j -l,4-polyisoprene. Very high (99%) i7j -l,4-polyisoprene is produced with coordination catalysts consisting of a combination of titanium tetrachloride, TiCl, plus a trialkyl aluminum, R Al, or a combination of TiCl with an alane (aluminum hydride derivative) (86—88). 

Closely related to, but distinct from, the anionic boron and aluminum hydrides are the neutral boron (borane, BH3) and aluminum (alane, A1H3) hydrides. These molecules also contain hydrogen that can be transferred as hydride. Borane and alane differ from the anionic hydrides in being electrophilic species by virtue of the vacant p orbital and are Lewis acids. Reduction by these molecules occurs by an intramolecular hydride transfer in a Lewis acid-base complex of the reactant and reductant. 

Lithium aluminum hydride and alanes are frequently used for the preparation of hydrides of other metals. Diethylmagnesium is converted to magnesium hydride [777], trialkylchlorosilanes are transformed to trialkylsilanes... 

The pyridine ring is easily reduced in the form of its quaternary salts to give hexahydro derivatives by catalytic hydrogenation [446], and to tetrahydro and hexahydro derivatives by reduction with alane aluminum hydride) [447], sodium aluminum hydride [448], sodium bis 2-methoxyethoxy)aluminum hydride [448], sodium borohydride [447], potassium borohydride [449], sodium in ethanol [444, 450], and formic acid [318]. Reductions with hydrides give predominantly 1,2,5,6-tetrahydro derivatives while electroreduction and reduction with formic acid give more hexahydro derivatives [451,452]. 

Tetrahydrofuran itself is not entirely inert to some hydrides although it is a favorite solvent for reductions with these reagents. A mixture of lithium aluminum hydride and aluminum chloride produced butyl alcohol on prolonged refluxing in yields corresponding to the amount of alane generated [633]. 

Reagents of choice for reduction of epoxides to alcohols are hydrides and complex hydrides. A general rule of regioselectivity is that the nucleophilic complex hydrides such as lithium aluminum hydride approach the oxide from the less hindered side [511, 653], thus giving more substituted alcohols. In contrast, hydrides of electrophilic nature such as alanes (prepared in situ from lithium aluminum hydride and aluminum halides) [653, 654, 655] or boranes, especially in the presence of boron trifluoride, open the ring in the opposite direction and give predominantly less substituted alcohols [656, 657,658]. As far as stereoselectivity is concerned, lithium aluminum hydride yields trans products [511] whereas electrophilic hydrides predominantly cis products... 

Chemical reduction of aromatic aldehydes to alcohols was accomplished with lithium aluminum hydride [5i], alane [770], lithium borohydride [750], sodium borohydride [757], sodium trimethoxyborohydride [99], tetrabutylam-monium borohydride [777], tetrabutylammonium cyanoborohydride [757], B-3-pinanyl-9-borabicyclo[3.3.1]nonane [709], tributylstannane [756], diphenylstan-nane [114], sodium dithionite [262], isopropyl alcohol [755], formaldehyde (crossed Cannizzaro reaction) [i7i] and others. 

Many more examples exist for reduction of the carhonyl only. Over an osmium catalyst [763] or platinum catalyst activated by zinc acetate and ferrous chloride [782] cinnamaldehyde was hydrogenated to cinnamyl alcohol. The same product was obtained by gentle reduction with lithium aluminum hydride at —10° using the inverse technique [609], by reduction with alane (prepared in situ from lithium aluminum hydride and aluminum chloride)... 

Acetals of aldehydes are usually stable to lithium aluminum hydride but are reduced to ethers with alane prepared in situ from lithium aluminum hydride and aluminum chloride in ether. Butyraldehyde diethyl acetal gave 47% yield of butyl ethyl ether, and benzaldehyde dimethyl acetal and diethyl acetal afforded benzyl methyl ether and benzyl ethyl ether in 88% and 73% yields, respectively [792]. 

Lithium aluminum hydride reduces preferentially the carbonyl function (p. 98) but alanes prepared by reactions of aluminum hydride with two equivalents of isopropyl or /er/-butyl alcohol or of diisopropylamine reduce the conjugated double bonds with high regioselectivity in quantitative yields [871] (p. 121). 

Reduction of unsaturated ketones to unsaturated alcohols is best carried out Nit v complex hydrides. a,/3-Unsaturated ketones may suifer reduction even at the conjugated double bond [764, 879]. Usually only the carbonyl group is reduced, especially if the inverse technique is applied. Such reductions are accomplished in high yields with lithium aluminum hydride [879, 880, 881, 882], with lithium trimethoxyaluminum hydride [764], with alane [879], with diisobutylalane [883], with lithium butylborohydride [884], with sodium boro-hydride [75/], with sodium cyanoborohydride [780, 885] with 9-borabicyclo [3.3.1]nonane (9-BBN) [764] and with isopropyl alcohol and aluminum isopro-... 

The reagent of choice for the reduction of ketals to ethers is alone prepared in situ from lithium aluminum hydride and aluminum chloride in ether. At room temperature ethers are obtained in 61-92% yields [792, 934]. Cyclic ketals prepared from ketones and 1,2- or 1,3-diols afford on hydrogenolysis by alanes alkyl P- or y-hydroxyalkyl ethers in 83-92% yields [792]. 

The reason why the carbonyl group in -santonin remained intact may be that, after the reduction of the less hindered double bond, the ketone was enolized by lithium amide and was thus protected from further reduction. Indeed, treatment of ethyl l-methyl-2-cyclopentanone-l-carboxylate with lithium diisopropylamide in tetrahydrofuran at — 78° enolized the ketone and prevented its reduction with lithium aluminum hydride and with diisobutyl-alane (DIBAL ). Reduction by these two reagents in tetrahydrofuran at — 78° to —40° or —78° to —20°, respectively, afforded keto alcohols from several keto esters in 46-95% yields. Ketones whose enols are unstable failed to give keto alcohols [1092]. 

High yields of amines have also been obtained by reduction of amides with an excess of magnesium aluminum hydride (yield 100%) [577], with lithium trimethoxyaluminohydride at 25° (yield 83%) [94] with sodium bis(2-methoxy-ethoxy)aluminum hydride at 80° (yield 84.5%) [544], with alane in tetra-hydrofuran at 0-25° (isolated yields 46-93%) [994, 1117], with sodium boro-hydride and triethoxyoxonium fluoroborates at room temperature (yields 81-94%) [1121], with sodium borohydride in the presence of acetic or trifluoroacetic acid on refluxing (yields 20-92.5%) [1118], with borane in tetrahydrofuran on refluxing (isolated yields 79-84%) [1119], with borane-dimethyl sulflde complex (5 mol) in tetrahydrofuran on refluxing (isolated yields 37-89%) [1064], and by electrolysis in dilute sulfuric acid at 5° using a lead cathode (yields 63-76%) [1120]. 

Reduction of o /i-unsatin-ated lactams, S,6-dihydro-2-pyridones, with lithium aluminum hydride, lithium alkoxyaluminum hydrides and alane gave the corresponding piperidines. 5-Methyl-5,6-dihydro-2-pyridone (with no substituent on nitrogen) gave on reduction with lithium aluminum hydride in tetrahydrofuran only 9% yield of 2-methylpiperidine, but l,6-dimethyl-5,6-dihydro-2-pyridone and 6-methyl-l-phenyl-5,6-dihydro-2-pyridone afforded 1,2-dimethylpiperidine and 2-methyl-1-phenylpiperidine in respective yields of 47% and 65% with an excess of lithium aluminum hydride, and 91% and 92% with alane generated from lithium aluminum hydride and aluminum chloride in ether. Lithium mono-, di- and triethoxyaluminum hydrides also gave satisfactory yields (45-84%) [7752]. 

Reductions of alkyl pyridones with lithium aluminum hydride or alane are very complex and their results depend on the position of the substituents and on the reducing reagent. Since the pyridones can be viewed as doubly unsaturated lactams with a,/J- and )i, -conjugated double bonds, the products result from all possible additions of hydride ion 1,2,1,4 or 1,6. Consequently the products of reduction are alkylpiperidines and alkylpiperideines with double bonds in 3,4 or 4,5 positions [449, 7755]. 

Amides containing nitro groups are reduced to diamino compounds with alane. A, A -Dimethyl-p-nitrobenzamide, on reduction with lithium aluminum hydride in the presence of sulfuric acid in tetrahydrofuran, gave 98% yield of dimethyl-p-aminobenzylamine [1117]. 

Even better yields are obtained with alane produced in situ from lithium aluminum hydride and 0.5mol of 100% sulfuric acid in tetrahydrofuran [994], or 1 mol of aluminum chloride in ether [787] Procedure 17, p. 208). One or 1.3 mol of the alane is used per mole of the nitrile and the reduction is carried out at room temperature. Comparative experiments showed somewhat higher yields of amines than those obtained by lithium aluminum hydride alone [787]. 

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