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In Clayden's Organic Chemistry (Second Edition) it is written that borane is useful in chemoselectively reducing the most nucleophilic carbonyl to its corresponding alcohol. It is later said that DIBAL reacts similarly and is useful in reducing esters to aldehydes, but that they will not proceed to the alcohol.

How are these differences in reactivity rationalised? I tried to draw analogies from the comparison of $\ce{LiAlH4}$ and $\ce{NaBH4}$ but since $\ce{LiAlH4}$ is the stronger reductant, I would have expected DIBAL to be the more reactive species!

Mechanisms

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    $\begingroup$ DIBAL will reduce esters to alcohols. In fact with certain substrates it can be sufficiently challenging to stop at the aldehyde that the easiest option is to DIBAL all the way to the alcohol and then oxidise back up to the desired aldehyde (Swern, DMP, TPAP etc) $\endgroup$
    – NotEvans.
    Commented Feb 6, 2018 at 23:10
  • $\begingroup$ Then why does it seem to so often be hailed as the answer to this exact problem. From MasterOrganicChemistry.com, for example, ''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". $\endgroup$
    – Jacob
    Commented Feb 6, 2018 at 23:19
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    $\begingroup$ Because undergrad organic chemistry is full of "lies". Sad but true... DIBAL+Swern is a really common sequence and you'll see it in many total syntheses. Lindlar is pretty good but the fully hydrogenated alkane is often a byproduct and if you leave it stirring too long, you will only get the alkane. Also, borane - a Lewis acid - should react with the most nucleophilic carbonyl, not least (typo, I guess?) $\endgroup$
    – orthocresol
    Commented Feb 6, 2018 at 23:21
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    $\begingroup$ The challenge, of course, is that aldehydes are more electrophilic than esters. Your hope here is that you can trap something at the same oxidation state as the aldehyde to prevent further reaction. I guess, with DIBAL, in some cases, the aluminum oxygen adduct may be stable enough to prevent further reaction. Alternatively, you can go with the Weinreb amide strategy, which captures the aldehyde equivalent as an imine. $\endgroup$
    – Zhe
    Commented Feb 7, 2018 at 0:07
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    $\begingroup$ You are right. My suspicion is that it depends on the conditions (temperature, solvent) and the stability of the tetrahedral intermediate (your penultimate structure in both schemes). I think there is some possibility of aggregation with the aluminium adduct so it may not necessarily be sufficient to look at a monomeric species. Anything beyond this would be pure speculation from me. $\endgroup$
    – orthocresol
    Commented Feb 7, 2018 at 0:08

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