Diisobutylaluminium hydride

Diisobutylaluminium hydride, DIBAL, DIBAL-H or DIBAH, is a reducing agent with the formula (i-Bu₂AlH)₂, where i-Bu represents isobutyl (-CH₂CH(CH₃)₂). This organoaluminium compound was investigated originally as a co-catalyst for the polymerization of alkenes.^[1]

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Properties

Like most organoaluminum compounds, the compound’s structure is more than that suggested by its empirical formula. It is dimeric, consisting of a pair of tetrahedral aluminium centers sharing bridging hydride ligands. Hydrides are small and, for aluminium derivatives, are highly basic, thus they bridge in preference to the alkyl groups.

DIBAH can be prepared by heating triisobutylaluminium (itself a dimer) to induce beta-hydride elimination:^[2]

(i-Bu₃Al)₂ → (i-Bu₂AlH)₂ + 2 (CH₃)₂C=CH₂

Although DIBAH can be purchased commercially as a colorless liquid, it is more commonly purchased and dispensed as a solution in organic solvents such as toluene.

Use in organic synthesis

DIBAH is useful in organic synthesis for a variety of reductions, including converting esters and nitriles to aldehydes.^[3] By contrast, LiAlH₄ reduces esters and acyl chlorides to primary alcohols, and NaBH₄ fails to reduce most esters. DIBAH reacts slowly with electron poor compounds, and more quickly with electron rich compounds. Thus, it is an electrophilic reducing agent whereas LiAlH₄ can be thought of as a nucleophilic reducing agent.

Safety

DIBAH, like most alkylaluminium compounds, reacts violently with air and water, potentially leading to fires.

References

1. ^ K. Ziegler, H. Martin and F. Krupp (1960). "Metallorganische Verbindungen, XXVII Aluminiumtrialkyle und Dialkyl-Aluminiumhydride Aus Aluminiumisobutyl-Verbindungen". Justus Liebigs Annalen der Chemie 629 (1): 14-19. doi:10.1002/jlac.19606290103.
2. ^ Eisch, J. J. Organometallic Syntheses Volume 2, Academic Press: New York, 1981. ISBN 0-12-234950-4.
3. ^ Galatsis, P. “Diisobutylaluminum Hydride” in Encyclopedia of Reagents for Organic Synthesis John Wiley & Sons: New York, 2001. doi:10.1002/047084289X.rd245