[15576-93-9]  · AlBr2H  · Dibromoalane  · (MW 187.80)

(reagent for reduction of acetals;1,2 selective carbonyl reductions3,4)

Physical Data: mp -15 °C; bp 95 °C under high vacuum.5

Solubility: sol benzene, chloroform, ether, THF.

Form Supplied in: not commercially available.

Analysis of Reagent Purity: normally prepared in situ as a solution in ether and used without purification or analysis; however, the bis-THF adduct has been characterized by NMR, IR, and elemental analysis.6

Preparative Methods: generally prepared in situ from Lithium Aluminum Hydride (1 equiv) and Aluminum Bromide (3 equiv) in ether at 0 °C;2,3 alternatively, the reagent can be prepared in pure form by reaction of Aluminum Hydride (1 equiv) with AlBr3 (2 equiv) in ether followed by vacuum distillation.5 The bis-THF adduct [Br2AlH.2(THF)] is a stable crystalline compound (mp 64-67 °C) which can be prepared by either of the methods described above (substituting THF for ether as the solvent).6

Purification: by distillation; see above.

Handling, Storage, and Precautions: generally prepared in situ and used immediately; handle under dry nitrogen in a fume hood.

Reduction of Acetals.

Dibromoalane (1) is a useful reagent for the reductive cleavage of acetals to the corresponding hydroxy ethers. For example, reduction of a bicyclic acetal with (1) in ether afforded the cis and trans hydroxy ethers (cis:trans = 97:3) in 72% yield (eq 1).1 Note that substitution of Dichloroalane or Diisobutylaluminum Hydride (DIBAL) for (1) in this reaction led to a decrease in the cis:trans product ratio (possibly due to the higher reaction temperatures required for these less reactive reagents).

Chiral acetals can be reduced with dibromoalane to afford the corresponding ether with varying degrees of stereoselectivity. For example, reduction of the chiral acetal derived from cyclohexyl methyl ketone and (-)-(2R,4R)-2,4-pentanediol afforded the hydroxy ether as a 23:1 mixture of diastereomers in 99% yield (eq 2).2 Dibromoalane generally gives better results than dichloroalane or DIBAL but stereoselectivity is somewhat lower with less hindered substrates. Note that the ethers generated by this reduction can subsequently be converted to the corresponding alcohol via an oxidation-elimination sequence (eq 3).2 The net result of the overall sequence is thus a stereoselective reduction of the original ketone.

Selective Carbonyl Reduction.

Dibromoalane (1), in combination with a hindered alkoxyaluminum compound, is an excellent reagent for the selective reduction of a hindered ketone in the presence of a less-hindered ketone (eq 4).3 The less hindered of the two ketones forms a complex with the alkoxyaluminum additive (e.g. Methylaluminum Bis(2,6-di-t-butyl-4-methylphenoxide)), leaving the more hindered ketone accessible for reduction by the hydride reagent. A variety of hydride reagents may be used but (1) usually gives the best selectivity.3

In combination with Copper(I) Iodide, (1) can also effect 1,4-reduction of a,b-unsaturated ketones. However, lithium aluminum hydride/copper iodide is generally superior for this purpose.4

Related Reagents.

Dichloroalane; Diisobutylaluminum Hydride; Lithium Aluminum Hydride-Copper(I) Iodide.

1. Ishihara, K.; Mori, A.; Yamamoto, H. T 1990, 46, 4595.
2. (a) Mori, A.; Fujiwara, J.; Maruoka, K.; Yamamoto, H. TL 1983, 24, 4581. (b) Mori, A.; Fujiwara, J.; Maruoka, K.; Yamamoto, H. JOM 1985, 285, 83.
3. Maruoka, K.; Araki, Y.; Yamamoto, H. JACS 1988, 110, 2650.
4. Ashby, E. C.; Lin, J. J.; Kovar, R. JOC 1976, 41, 1939.
5. Wiberg, E.; Schmidt, M. ZN(B) 1951, 6, 459.
6. Schmidt, D. L.; Flagg, E. E. IC 1967, 6, 1262.

Timothy A. Blizzard

Merck Research Laboratories, Rahway, NJ, USA

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