Aluminum Hydride1

AlH3

[7784-21-6]  · AlH3  · Aluminum Hydride  · (MW 29.99)

(reducing agent for many functional groups; used in hydroalumination of alkynes; allylic rearrangements)

Alternate Name: alane.

Physical Data: colorless, nonvolatile solid in a highly polymerized state; mp 110 °C (dec). X-ray data have also been obtained.2

Solubility: sol THF and ether; precipitates from ether after standing for approximately 30 min depending upon method of preparation.

Analysis of Reagent Purity: hydride concentration can be determined by hydrolyzing aliquots and measuring the hydrogen evolved.3

Preparative Methods: can be prepared by treating an ether solution of Lithium Aluminum Hydride with Aluminum Chloride (eq 1).4 This affords an ether solution of AlH3 after precipitation of LiCl. Solutions have to be used immediately otherwise AlH3 precipitates as a white solid which consists of a polymeric material with ether. The solvent can be removed and the solid redissolved in THF.5 Alternatively, THF solutions can be prepared directly according to one of the reactions shown in eqs 2-4.5,6

Handling, Storage, and Precautions: solutions of AlH3 are not spontaneously inflammable.3 However, since AlH3 has reactivity comparable to LiAlH4, one should follow similar handling and precautions as those exercised for LiAlH4. Solutions of AlH3 are prepared in situ but are known to degrade after 3 days, and long term storage of solutions is not possible. Use in a fume hood.

Functional Group Reductions.

Reductions by alane take place primarily by a two-electron mechanism.7,8 However, evidence for a SET pathway exists.9 AlH3 will reduce a wide variety of functional groups.4 These include aldehydes, ketones, quinones, carboxylic acids, anhydrides, acid chlorides, esters, and lactones, from which the corresponding alcohol is the isolated product. Amides, nitriles, oximes, and isocyanates are reduced to amines. Nitro compounds are inert to AlH3. Sulfides and sulfones are unreactive, but disulfides and sulfoxides can be reduced. Tosylates are also not reduced by AlH3.

The reduction of ketones with AlH3 has selectivity different from other hydride reagents (eqs 5 and 6).10 The hydroxymethylation of ketones via a two-step procedure has also been accomplished (eq 7).11 The conversion of a,b-unsaturated ketones to allylic alcohols can be carried out with very good selectivity using AlH3 (eq 8);12 however, DIBAL is the reagent of choice for this transformation (see Diisobutylaluminum Hydride).12c

Carboxylic acids and esters are reduced more rapidly by AlH3 than by LiAlH4, whereas the converse is true for alkyl halides. As a result, acids and esters can be reduced in the presence of halides (eq 9). In addition, esters can be reduced in the presence of nitro groups (eq 10). This stands in contrast to LiAlH4 in which nitro groups are reduced. Acetals can also be reduced to the half protected diol as illustrated in eq 11.13

In reductions of amides to amines there is a competition between C-O and C-N bond cleavage which depends upon the reaction conditions. This complication does not occur with AlH3. A quantitative yield of amine is obtained with short reaction times. Conjugated amides can be cleanly reduced to the allylic amine (eq 12).6

The less basic AlH3 appears to be better than LiAlH4 for reducing nitriles with relatively acidic a-hydrogens to amines, and enolizable keto esters to diols (eq 13).6

The reduction of b-lactams to azetidines can be accomplished with AlH3 (eq 14),14 while ring opening was observed with LiAlH4. AlH3 can also convert enamines to the corresponding alkenes (eq 15).15

While alkyl halides are usually inert to AlH3, the reduction of cyclopropyl halides to cyclopropanes (eq 16)16 and glycosyl fluorides to tetrahydropyrans (eq 17)17 are known.

Desulfurization of sultones is rapid and proceeds in good yield with AlH3 while LiAlH4 affords poor yields with long reaction times (eq 18).18

Epoxide Ring Opening.

With most epoxides, hydride attack occurs at the least sterically hindered site to give the corresponding alcohol (eq 19).19 However, due to the electrophilic nature of AlH3 compared to LiAlH4, it is possible for ring opening to occur at the more hindered site. With phenyl substituted epoxides, mechanistic studies have shown that attack at a benzylic carbenium ion or a 1,2-hydride shift followed by hydride attack gives products with the same regiochemistry but with different stereochemistry (eq 20).6,20 The stereoselectivity of AlH3 mediated epoxide openings has been studied in depth.21

Hydroalumination.

The addition of AlH3 across a triple bond has been shown to occur in propargylic systems.22 When the reactions are quenched with Iodine, AlH3 gives the 2-iodo-(E)-alkene while LiAlH4 gives the 3-iodo-(E)-alkene (eq 21). AlH3 can also be used in conjunction with Titanium(IV) Chloride to carry out a reaction similar to a hydroboration.23 Thus 1-hexene is converted to hexane upon aqueous work-up or to the corresponding alcohol upon exposure of the reaction intermediate to oxygen (eq 22). Similar results were obtained with nonconjugated dienes.

Allylic Rearrangements (SN2).

The SN2 displacement of a good leaving group to give the rearranged allylic system can be carried out with AlH3.24 This reaction appears not to be sterically demanding as a variety of displacements are possible (eq 23).

Preparation of allenes from propargylic systems can also be accomplished.24 Most systems show a preference for syn elimination; however, mesylates prefer an anti mode of elimination (eq 24). This same procedure has been used to prepare fluoroallenes (eq 25).25

Dialkylaluminum hydrides also behave as hydroalumination reagents (see Diisobutylaluminum Hydride) and are more commonly used than AlH3.


1. (a) Gaylord, N. G. Reduction With Complex Metal Hydrides; Interscience: New York, 1956. (b) Semenenko, K. N.; Bulychev, B. M.; Shevlyagina, E. A, RCR 1966, 35, 649. (c) Rerick, M. N. Reduction Techniques and Applications in Organic Synthesis, Augustine, R. L., Ed.; Dekker: New York, 1968. (d) Cucinella, S.; Mazzei, A.; Marconi, W. ICA Rev. 1970, 51. (e) Walker, E. R. H. CSR 1976, 5, 23. (f) Brown, H. C.; Krishnamurthy, S. T 1979, 35, 567. (g) Hajos, A. Complex Hydrides and Related Reducing Agents in Organic Synthesis; Elsevier: Amsterdam, 1979. (h) Seyden-Penne, J. Reduction by the Alumino- and Borohydrides in Organic Synthesis; VCH: New York, 1991.
2. Turley, J. W.; Rinn, H. W. IC 1969, 8, 18.
3. Yoon, N. M.; Brown, H. C. JACS 1968, 90, 2927.
4. Finholt, A. E.; Bond, A. C.; Schlesinger, H. I. JACS 1947, 69, 1199.
5. (a) Brown, H. C.; Yoon, N. M. JACS 1966, 88, 1464. (b) Browner, F. M.; Matzek, N. E.; Reigler, P. F.; Rinn, H. W.; Roberts, C. B.; Schmidt, D. L.; Snover, J. A.; Terada, K. JACS 1976, 98, 2450.
6. Ashby, E. C.; Sanders, J. R.; Claudy, P,; Schwarts, P. JACS 1973, 95, 6485.
7. Laszlo, P.; Teston, M. JACS 1990, 112, 8751.
8. (a) Park, S.-U.; Chung, S.-K.; Newcomb, M. JOC 1987, 52, 3275. (b) Yamataka, H.; Hanafusa, T. JOC 1988, 53, 773.
9. (a) Ashby, E. C.; Goel, A. B. TL 1981, 22, 4783. (b) Ashby, E. C.; DePriest, R. N.; Pham, T. N. TL 1983, 24, 2825. (c) Ashby, E. C.; DePriest, R. N.; Goel, A. B.; Wenderoth, B.; Pham, T. N. JOC 1984, 49, 3545. (d) Ashby, E. C.; Pham, T. N. JOC 1986, 51, 3598. (e) Ashby, E. C.; Pham, T. N. TL 1987, 28, 3197.
10. (a) Ayres, D. C.; Sawdaye, R. JCS(P2) 1967, 581. (b) Ayres, D. C.; Kirk, D. N.; Sawdaye, R. JCS(P2) 1970, 505. (c) Guyon, R.; Villa, P. BSF 1977, 145. (d) Guyon, R.; Villa, P. BSF 1977, 152. (e) Martinez, E.; Muchowski, J. M.; Velarde, E. JOC 1977, 42, 1087.
11. Corey, E. J.; Cane, D. JOC 1971, 36, 3070.
12. (a) Jorgenson, M. J. TL 1962, 559. (b) Brown, H. C.; Hess, H. M. JOC 1969, 34, 2206. (c) Wilson, K. E.; Seidner, R. T.; Masamune, S. CC 1970, 213. (d) Dilling, W. L.; Plepys, R. A. JOC 1970, 35, 2971. (e) Ashby, E. C.; Lin, J. J. TL 1976, 3865.
13. (a) Danishefsky, S.; Regan, J. TL 1981, 22, 3919. (b) Takano, S.; Akiyama, M.; Sato, S.; Ogasawara, K. CL 1983, 1593. (c) Richter, W. J. JOC 1981, 46, 5119.
14. Jackson, M. B.; Mander, L. N.; Spotswood, T. M. AJC 1983, 36, 779.
15. Coulter, J. M.; Lewis, J. W.; Lynch, P. P. T 1968, 24, 4489.
16. Muller, P. HCA 1974, 57, 704.
17. Nicolaou, K. C.; Dolle, R. E.; Chucholowski, A.; Randal, J. L. CC 1984, 1153.
18. (a) Wolinsky, J.; Marhenke, R. L.; Eustace, E. J. JOC 1973, 38, 1428. (b) Smith, M. B.; Wolinsky, J. JOC 1981, 46, 101.
19. Maruoka, K.; Saito, S.; Ooi, T.; Yamamoto, H. SL 1991, 255.
20. Lansbury, P. T.; Scharf, D. J.; Pattison, V. A. JOC 1967, 32, 1748.
21. Elsenbaumer, R. L.; Mosher, H. S.; Morrison, J. D.; Tomaszewski, J. E. JOC 1981, 46, 4034.
22. Corey, E. J.; Katzenellenbogen, J. A.; Posner, G. H. JACS 1967, 89, 4245.
23. Sato, F.; Sato, S.; Kodama, H.; Sato, M. JOM 1977, 142, 71.
24. Claesson, A.; Olsson, L.-I. JACS 1979, 101, 7302.
25. Castelhano, A.; Krantz, A. JACS 1987, 109, 3491.

Paul Galatsis

University of Guelph, Ontario, Canada



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