In a blatant plug for the Reagent Guide, each Friday I profile a different reagent that is commonly encountered in Org 1/ Org 2. Version 1.2 just got released, with a host of corrections and a new page index.
Also known as: DIBAL-H, DIBAH
What it’s used for: DIBAL is a strong, bulky reducing agent. It’s most useful for the reduction of esters to aldehydes. Unlike lithium aluminum hydride, it will not reduce the aldehyde further if only one equivalent is added. It will also reduce other carbonyl compounds such as amides, aldehydes, ketones, and nitriles.
Similar to: LiAlH4, LiAl(Ot-Bu)3H
Like Lindlar’s catalyst, DIBAL is most notable for what it does not do. It reduces esters, but not to alcohols – it stops at the aldehyde stage. Let’s have a look.
Keeping the temperature low (–70°C) tends to keep a lid on the reactivity here. So long as the temperature is kept here for the duration of the experiment and only one equivalent of DIBAL is added, the aldehyde is obtained.
In addition, DIBAL can do all the reductions that NaBH4 does, so ketones and aldehydes are reduced to secondary and primary alcohols, respectively.
Finally, DIBAL will also do partial reductions of nitriles to imines. The imines are then hydrolyzed to aldehydes upon addition of water. In this respect DIBAL again differs from LiAlH4, which will reduce nitriles all the way to amines.
The mechanism for reduction by DIBAL is a little bit unusual compared to NaBH4. Whereas NaBH4 is considered a “nucleophilic” reductant – that is, it delivers hydride (H-) directly to a carbonyl carbon, DIBAL is an “electrophilic” reductant. That is, the first step in the reaction is coordination of a lone pair from the carbonyl oxygen (a nucleophile) to the aluminum (electrophile). It is only after coordinating to its carbonyl host that DIBAL delivers its hydride to the carbonyl carbon, resulting in formation of a neutral hemiacetal intermediate that is stable at low temperatures. Quenching of the reaction then breaks down the hemiacetal, resulting in isolation of the aldehyde. (“real life” disclaimer: this reaction looks great on paper but can sometimes be difficult to achieve in practice).
The same mechanism is in effect in the reduction of nitriles to imines (and then on to aldehydes). Coordination of the nitrile nitrogen to aluminum is followed by delivery of hydride, and from there, addition of water leads to hydrolysis of the imine and subsequent isolation of the aldehyde.
P.S. You can read about the chemistry of DIBAL and more than 80 other reagents in undergraduate organic chemistry in the “Organic Chemistry Reagent Guide”, available here as a downloadable PDF.