Dibal Mechanism

[Gaetan Boren]

Themost notable reaction of DIBAL is the reduction of esters to aldehydes. Unlike lithium aluminum hydride (LiAlH4), which reduces esters to primary alcohols, reductions with DIBAL can stop at the aldehyde stage if the temperature is kept very low.

The advantage of DIBAL here is that it is more efficient. If we use LiAlH4, we obtain a primary alcohol, which we would then have to oxidize up to the aldehyde using a reagent such as PCC, Dess-Martin periodinane, or the Swern oxidation. (See post: Alcohol Oxidation)

The mechanism for reduction of esters to aldehydes with DIBAL is roughly similar to the familiar addition-elimination mechanism of nucleophilic acyl substitution, with a slightly modified first step. (See post: Nucleophilic Acyl Substitution)

The carbonyl oxygen of esters is a Lewis base. So the first step is coordination of the Lewis basic carbonyl oxygen to the Lewis acidic aluminum, to give a species with a negative formal charge on aluminum.

Reduction of nitriles follows a similar mechanism to that for the reduction of esters. Coordination of the Lewis-basic nitrile nitrogen to aluminum is followed by delivery of hydride to the nitrile carbon (form C-H, break C-N (pi) ). This is another example of an addition mechanism.

DIBAL is a useful reagent for the partial reduction of carboxylic acid derivatives. A successful DIBAL reduction of an ester to an aldehyde will save an extra step relative to LiAlH4 followed by oxidation.

How can Dibal H reduce an aldehyde and a ketone to their corresponding alcohols at once which are present in a single compound but not ester to alcohol provided 1 equivalent? (Since, DIBAL H has only 1 hydride)

DIBAL can in fact reduce esters to alcohols even with only one equivalent; what happens first is the conventional hydride reduction. The second reduction is a type of Meerwein-Ponndorf-Verley reduction where there is hydride transfer from the isobutyl group.

You can break the aluminum complex by refluxing in methanol. DIBAL 2 eq. at zero deg reduces ester to alcohol. 1 Eq DIBAL at -78 reduces esters to aldehyde. those reactions are clean. yields are more than 90 %. after refluxing with methanol one can do celite filtration and concentrate the methanol extract..no need to even do a column. Of course there should be no other competing groups.

DIBAL reduction produces O(-) which itself is a strong base. The formation of the C-O double bond along with expulsion of CH3O(-) is therefore not accompanied by a huge energy barrier. CH3O(-) is not a great leaving group, but still a much better one than H(-) or most carbon-based leaving groups.

If you want the aldehyde, you MUST keep it at -78. A non polar solvent is ideal. Avoid ethereal solvents (they tend to coordinate to the aluminum and this can affect reactivity), DCM should be fine. Toluene can present solubility problems, but might be OK for you at -78. If you are aiming for the alcohol, then warming is fine.

No, DIBAL-H does not react with carboxylic acids. You have two options: perform an esterification of your -COOH to -COOMe for example and then use DIBAL-H, or you can use the -COOH directly with another reagent like LiAlH4 or NaBH4.

i used 6 year old DIBAL for opening up my isopropyldene ring itwas working fantastic but when i used new Dibal it producing unusual reports does anyone feel any prb like this

what can be degraded product of DIBAL-H ?

It can be referred to as DIBAH, DIBAL, or DIBAL-H. Usually when discussing it amongst themselves, organic chemists call it DIBAL because it rolls more easily off the tongue than either of the other two.

Somebody recommended to me that the Fieser work up followed by a filtration over Celite should remove the Al salts emulsion just as well. I am probably going to use this method as a first choice from now on, because trying to extract my product from Rochelle salts solution caused me a lot of grief!

Also, having re-read this site properly, I will now always use H2O to quench my DIBAL reductions of nitriles. I mistakenly used methanol earlier on and looking at the mechanism of imine hydrolysis above, that might explain why I got a nasty mixture of crap when I quenched the imine with Methanol! Guess I can just pour the reaction on to water at 0 Celsius, rather than quenching direct at -70.

$\textDiBAl-H$ reduces by electrophilic attack and provides hydrogen by $\textH^-$ transfer. Unlike $\textLiAlH_4$ it does not directly provide $\textH^-$ 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 $\textDiBAl-H$, can someone propose a mechanism for it?

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:

One of the more useful reaction involving nitriles is their hydrolysis to form carboxylic acids. This reaction occurs in either acidc or basic aqueous solutions with slight differences in each mechanism. In the case of acid catalysis, the nitrile becomes protonated. Protonation increases the electrophilicity of the nitrile so that it will accept water, a poor nucleophile. With base catalyzed hydrolysis, the strongly nucleophilic hydroxide anion is capable of directl addition to the carbon-nitrogen triple bond. During both mechanisms an amide intermediate is formed which usually is not isolated.

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The catalytic hydrogenation of nitriles is often the most economical route available for the production of primary amines.[3] Catalysts for the reaction often include group 10 metals such as Raney nickel,[4][5][6] palladium black, or platinum dioxide.[1] However, other catalysts, such as cobalt boride, also can be regioselective for primary amine production:

Such reactions proceed via enamine intermediates.[8] The most important reaction condition for selective primary amine production is catalyst choice.[1] Other important factors include solvent choice, solution pH, steric effects, temperature, and the pressure of hydrogen.

Nitriles can also be converted to aldehydes by reduction and hydrolysis. The Stephen aldehyde synthesis uses Tin(II) chloride and hydrochloric acid to yield an aldehyde via the hydrolysis of a resulting iminium salt. Aldehydes can also form using a hydrogen donor followed by in-situ hydrolysis of an imine. Useful reagents for this reaction include formic acid with a hydrogenation catalysis[12] or metal hydrides, which are used to add one mol of hydrogen to the nitrile. For example, sodium borohydride reduces nitriles in alcoholic solvents with a CoCl2 catalyst or Raney nickel.[13]

The hydride reagent Diisobutylaluminium hydride, or DIBAL-H, is commonly used to convert nitriles to the aldehyde.[14] Regarding the proposed mechanism, DIBAL forms a Lewis acid-base adduct with the nitrile by formation of an N-Al bond. The hydride is then transferred to the carbon of the nitrile. Aqueous workup produce the desired aldehyde and ammonia.[15]

Compared to other direct reductions of carboxylic acids or carboxylic acid derivates such as using DIBAL-H or Rosenmund conditions, the Fukuyama Reduction is a mild alternative, offering outstanding functional group tolerance (see recent literature).

An initial oxidative addition of Pd(0) to the C(sp2)-S bond is followed by transmetallation of the resultant acylpalladium species with Et3SiH. Reductive elimination from the acylpalladium hydride leads to the desired aldehyde.

On the basis of this mechanism, it was surmised that substitution of Et3SiH by an appropriate organometallic reagent would provide access to ketones. Extensive screening of various transition metal catalysts and organometallic reagents have revealed suitable conditions, which are currently used in theFukuyama-Coupling.

If the desired product is an aldehyde, a milder reducing agent is needed which can stop the reduction at the aldehyde oxidation stage. For this purpose, bis(2-methylpropyl)aluminum hydride or diisobutylaluminum hydride abbreviated DiBAL or DiBAl-H can be used as a reducing agent.

The reaction is performed in hexane at low temperature (-78C) to prevent further reduction of the aldehyde. Because at the higher temperature DiBAl can reduce aldehydes and ketones to alcohols.

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