Diisobutylaluminium hydride


Diisobutylaluminium hydride is a reducing agent with the formula 2, where i-Bu represents isobutyl. This organoaluminium compound was investigated originally as a co-catalyst for the polymerization of alkenes.

Properties

Like most organoaluminum compounds, the compound’s structure is most probably more than that suggested by its empirical formula. A variety of techniques, not including X-ray crystallography, suggest that the compound exists as a dimer and a trimer, consisting 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.
DIBAL can be prepared by heating triisobutylaluminium to induce beta-hydride elimination:
Although DIBAL can be purchased commercially as a colorless liquid, it is more commonly purchased and dispensed as a solution in an organic solvent such as toluene or hexane.

Use in organic synthesis

DIBAL is useful in organic synthesis for a variety of reductions, including converting carboxylic acids, their derivatives, and nitriles to aldehydes. DIBAL efficiently reduces α-β unsaturated esters to the corresponding allylic alcohol. By contrast, LiAlH4 reduces esters and acyl chlorides to primary alcohols, and nitriles to primary amines . DIBAL 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.
Although DIBAL reliably reduces nitriles to aldehydes, the reduction of esters to the same functional group is an infamously finicky reaction which looks useful on paper but often leads to mixtures of alcohol and aldehyde in practice. This problem has been addressed by careful control of the reaction conditions using continuous flow chemistry.

Safety

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