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$\text{DiBAl-H}$ reduces by electrophilic attack and provides hydrogen by $\text{H}^-$ transfer. Unlike $\text{LiAlH}_4$ it does not directly provide $\text{H}^-$ ion. So it should not undergo acid base reaction with carboxylic acid. But my friend argued that it undergoes acid base reaction. I can't find any reaction mechanism for reduction of carboxylic acid with $\text{DiBAl-H}$, can someone propose a mechanism for it?

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Reduction of esters of carboxylic acids directly to aldehydes is of great synthetic interest. Zakharkin and Khorlina in 1962 (Ref.1) has shown that, diisobutylaluminum hydride (DIBAL-H) could be very useful in reducing esters to aldehyde in high yields at low temperatures (about $\pu{-70 ^oC}$). They also found that solvents affect reduction significantly. For example, the reduction reaction in toluene or hexane proceeds to give corresponding aldehydes in yields l0-15% more than those in ether as the solvent. Previously, in 1959, Miller et al. (Ref.2) found that DIBAL-H is a more selective reagent than lithium aluminum hydride in the reduction of nitriles to aldehydes, and it can also be used to reduce benzoic acid to give 72% yields of benzyl alcohol (they also showed esters can be reduced to corresponding alcohols). The conditions and yields of these references are summarized in this page. In Ref.2, during acid reduction, Miller et al. have used 3 equivalents of DIBAL-H and observed that a gas (hydrogen) evaluation upon the addition of the hydride to benzoic acid until one mole of the reducing agent had reacted. Thereafter, the addition of more DIBAL-H gave no gas. Thus, the first reaction is, presumably an acid-base reaction. However, DIBAL-H here should be acting as a Lewis-acid instead of a base, as illustrated in scheme 1:

DIBAL as Lewis Acid

The ist mono-adduct, then further reacted with second DIBAL-H to give di-adduct, which upon acid hydrolysis would give expected aldehyde (Scheme 2):

Carb Acid Reduction


References:

  1. 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).
  2. 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).
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  • $\begingroup$ +1 that's exactly what I was looking for, thanks $\endgroup$ – Loop Back Feb 9 at 3:34
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At lower temperatures (around -78oC), carboxylic acids (with 2 equiv. of DIBAL-H) is reduced by DIBAL-H

enter image description here

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  • $\begingroup$ What does aqueous base have to do with the question? Two equivalents of DIBAL-H may be required. One as a base; one as a reductant. The bis-aluminum complex (lower right corner) must be stable prior to work up at low temperature. $\endgroup$ – user55119 Feb 7 at 23:41
  • $\begingroup$ @Sarahjane this does not answer the initial part of my question. Further this mechanism does not seem to be correct. 1 eq. of DiBAl-H can reduce ester, here's the mechanism ( i.stack.imgur.com/1Dwzk.jpg ). So why does carboxylate Ion require 2 eq. of DiBAl-H inspite being more electron rich? $\endgroup$ – Loop Back Feb 8 at 4:26

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