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Diisobutylaluminium hydride(DIBAL-H) reduces only carboxylic acid, ester or both?

The answer given was that it only reduces ester.

Is it correct that under any condition carboxylic acids are not reduced by DIBAL-H?

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    $\begingroup$ Bulky groups are there for selectivity, so while it might do that, it's not supposed to. $\endgroup$ – Mithoron Dec 26 '17 at 15:35
  • $\begingroup$ Yes I think it does. Like BH3, DIBAL-H is an electrophilic reducing agent. In other words, it is best able to reduce electron-rich compounds like amides, acids and esters. I have seen various sources saying that too. However, it is best known for its reduction of the ester to the aldehyde and not the alcohol. $\endgroup$ – Tan Yong Boon Nov 6 '18 at 6:16
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    $\begingroup$ Conditions matter a lot in this question. Are you using a full equivalent or two equivalents? Your answer might be very different with 10 equivalents... $\endgroup$ – Zhe Jan 5 at 2:49
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Does diisobutylaluminium hydride (DIBAL-H) reduce only carboxylic acids? or only carboxylic acid esters? or reduce both both types of compounds?

The answer is yes, it does reduce both. Not only them, it also reduces nitriles to aldehydes, and is a more selective reagent than lithium aluminum hydride (LAH) in the reduction of nitriles (Ref.1). About reduction of carbozylic acid, Miller et al. (Ref.1) found that DIBAL-H can be used to reduce benzoic acid to give 72% yields of benzyl alcohol in 1959. In same publication, they have also shown that esters can be reduced to corresponding alcohols as well.

In reduction of nitriles to aldehydes, following results were achieved and compared with the result obtained by Amundsen and Nelson (Ref.2) using $\ce{LiAlH4}$:

Benzonitrile Reduction

The summery of reduction of carboxylic acids and their esters are illustrated in following scheme, where two different groups (Ref.1 & Ref.3) achieved two different results for ester reduction under different conditions:

Acid&Ester Reduction


References:

  1. A. E. G. Miller, J. W. Biss, and L. H. Schwartzman, “Reductions with Dialkylaluminum Hydrides,” J. Org. Chem. 1959, 24(5), 627–630 (DOI: 10.1021/jo01087a013).
  2. L. H. Amundsen, L. S. Nelson, “Reduction of Nitriles to Primary Amines with Lithium Aluminum Hydride,” J. Am. Chem. Soc. 1951, 73(1), 242–244 (DOI: 10.1021/ja01145a082).
  3. L. I. Zakharkin, I. M. Khorlina, “Reduction of esters of carboxylic acids directly into aldehydes with diisobutylaluminum hydride,” Tetrahedron Letters 1962, (14), 619-620 (https://doi.org/10.1016/S0040-4039(00)70918-X).
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My book (Cengage for JEE Adv) doesn't mention anything about carb acids, but I was able to find this website that says DiBAL-H reduces carb acids to aldehydes and alcohols, but requires 2-3 equivalents, at a high temperature.

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Why DIBAL is weaker than LiAlH4 . as per mechanism mentioned in the site https://www.entrancei.com/questions-lialh4-reaction-and-mechanism LiAlH4 is H- Donor , the RDS is H- release if we take DIBAL it consist of two electron donating group makes it e rich so i think it can donate H- faster than LiAlH4 . secondly i have seen in many book it can reduce all functional group which are reduce by LAH .

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