jwarr
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quenching with ethyl formate?
Hi all,
I'm looking at the following procedure and I'm a bit confused. It's a procedure for the reduction of an aromatic nitrile to the aldehyde using
DIBAL-H.
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Reduction of Nitrile 15b. A solution of 2.32 g of nitrile 15b in 120 ml of hexane was cooled to -70" and 24.6 ml of 1 M DIBAL-H in hexane was added.
The mixture was stirred at -70" for 30 min and at ambient temperature for 5 hr, where upon 2 ml of ethyl formate was added and stirring was continued
for 1 hr. The mixture was poured into saturated ammonium chloride solution and, after
20 min, aqueous sulfuric acid was added and the product was isolated with ether,17b affording 2.31 g (96%) of aldehyde 16b.
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Can someone explain this workup to me? Every DIBAL reduction I've ever run is quenched first with MeOH in an aprotic solvent, then H2O in MeOH, then
the resulting aluminum gel is filtered over celite and extracted with ether.
I don't understand why they are adding ethyl formate.
[Edited on 28-2-2013 by jwarr]
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Dr.Bob
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I know that in many LAH reactions, ethyl acetate is added, which gets reduced, providing ethanol as the main byproduct. The main intent is to destroy
any remaining LAH in a slow, controlled manner, so that you are quenching a lithium salt of an alcohol, not LAH, and no hydrogen is evolved in that
step. If you add an aqueous quench directly, then the LAH or DiBAL can react to form hydrogen and heat at the same time, which can be quite exciting
if done too fast (seen it happen, don't need to see it again).
Using methanol would work fine, but it has a lower flash point and higher reactivity that ethyl acetate. I would guess that ethyl formate is being
used for the same reason, but creates methanol and ethanol as products. You could likely use methanol instead on that scale, but at larger scales,
most people use ethyl acetate as it is less energetic a reaction, and that procedure is well known to most large scale process chemists.
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jwarr
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Ah, thank you. Wouldn't this create solubility issues in the subsequent extraction? That is, the product may be soluble in the aqueous layer due to
the ethanol?
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Dr.Bob
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You said that before "Every DIBAL reduction I've ever run is quenched first with MeOH in an aprotic solvent." So if methanol worked before, why
would ethanol be worse? You are only talking about 2ml ethyl formate in ~140 ml hexane, not going to be a huge issue. And not all of the ethyl
formate will react. I like ethyl acetate as it is more common, forms traces of ethanol (which is less polar than methanol, and much tastier) and the
un-reacted EtOAc won't bother the extraction at all.
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