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Topic: LiAlH4 problem and question( reduction ester)  (Read 12802 times)

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Offline flaviodan

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LiAlH4 problem and question( reduction ester)
« on: October 06, 2010, 05:52:56 PM »
hi every one,
i'am an italian student of university of chemistry.
During this period i try to make a reduction of a particular aromatic ester using LiAlH4....
so i had a lot of trouble with this reaction due maybe to the bad reactivity of my molecule or due to the old type of LiALH4
But i have a few question about this reactant:

1)first of all i want to know why in many publication of organic synthesis i found alway the esters like ethanol and never like methanol...is ethil ester more reactive in reduction?

2) second i want to now if the reduction can take place at 0 C° or also at rt is great, becouse in a lot of publication i see that they take the reduction in ice.

Thanks all for the answare and sorry for my bad english

Offline Doc Oc

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Re: LiAlH4 problem and question( reduction ester)
« Reply #1 on: October 06, 2010, 06:58:25 PM »
LiAlH4 reductions are tricky, and the problem is almost always in the work-up I've discovered.  To answer your questions:

1) Both ethyl and methyl esters work in the reduction.
2) The cooling of the reaction is to control the exothermic nature of the LiAlH4 reduction, it can get VERY hot.

How are you working up your reaction?  The aluminum salts can make the extraction of your product alcohol very difficult if not impossible.  Typically Rochelle salt is used to chelate them, but I've seen some literature using NaOH (I didn't have good luck with this method).  Long stirring times (12-15 hours) are not uncommon when quenching/chelating the aluminum.

Offline discodermolide

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Re: LiAlH4 problem and question( reduction ester)
« Reply #2 on: October 07, 2010, 02:16:12 AM »
LiAlH4 reductions are tricky, and the problem is almost always in the work-up I've discovered.  To answer your questions:

1) Both ethyl and methyl esters work in the reduction.
2) The cooling of the reaction is to control the exothermic nature of the LiAlH4 reduction, it can get VERY hot.

How are you working up your reaction?  The aluminum salts can make the extraction of your product alcohol very difficult if not impossible.  Typically Rochelle salt is used to chelate them, but I've seen some literature using NaOH (I didn't have good luck with this method).  Long stirring times (12-15 hours) are not uncommon when quenching/chelating the aluminum.

I have found that it is better to employ Lithium borohydride in THF, work up is easy, a quench with acetic acid (care here) extract, neutralise excess acid and evaporate solvent. No problems with aluminum salts.
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Offline orgopete

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Re: LiAlH4 problem and question( reduction ester)
« Reply #3 on: October 07, 2010, 06:55:51 AM »
I've had good success with the procedure in Fieser and Fieser (is it still used?). It is a NaOH addition based upon the wt of LAH. It is simple, easy, and I did not have any trouble isolating my product.
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Offline Doc Oc

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Re: LiAlH4 problem and question( reduction ester)
« Reply #4 on: October 07, 2010, 08:42:20 AM »
Yes orgopete, that's the one.  3x the weight of the LAH as a 15% NaOH solution or something like that.  Like I said, I didn't have good luck with this procedure but I know others (like yourself) who swear by it.

For my problem reaction I quenched with a minimum amount of saturated Rochelle salt solution (only enough to quench the LAH, you know it's done when it stops evolving gas).  Then I diluted the reaction in both ether and 10% Rochelle salt solution and stirred very vigorously overnight.  The layers separated nicely and I got something like 85% yield after flash column.

Offline movies

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Re: LiAlH4 problem and question( reduction ester)
« Reply #5 on: October 07, 2010, 10:06:19 AM »
I have personally always had good luck with the F&F method.  I have used the Rochelle salt procedure too, and it worked.  I find it a little less convenient though.

Offline discodermolide

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Re: LiAlH4 problem and question( reduction ester)
« Reply #6 on: October 07, 2010, 11:59:05 AM »
I have personally always had good luck with the F&F method.  I have used the Rochelle salt procedure too, and it worked.  I find it a little less convenient though.

Believe me LiBH4 is by far better than LiAlH4 and I've used it on ton scale.
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Offline flaviodan

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Re: LiAlH4 problem and question( reduction ester)
« Reply #7 on: October 20, 2010, 05:16:56 PM »
hi all,
sorry for the long absence but in this period i have to work a lot in the lab.
So at the and i found the my LiAlH4 was ,in some way, bad....maybe becouse it wasen't conserved very well so it don't react very well.
I found this becouse we prove another reduction that in the past function, with the same LiAlH4 and the reaction don't start.
So this is the problem...thank to all for the answare

Offline syndakitz

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Re: LiAlH4 problem and question( reduction ester)
« Reply #8 on: October 22, 2010, 09:19:18 AM »
Quote
Believe me LiBH4 is by far better than LiAlH4 and I've used it on ton scale.

Sodium borohydride doesn't reduce esters

Offline Doc Oc

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Re: LiAlH4 problem and question( reduction ester)
« Reply #9 on: October 22, 2010, 09:33:54 AM »
Quote
Believe me LiBH4 is by far better than LiAlH4 and I've used it on ton scale.

Sodium borohydride doesn't reduce esters

No it doesn't, but LITHIUM borohydride does.

Offline discodermolide

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Re: LiAlH4 problem and question( reduction ester)
« Reply #10 on: October 22, 2010, 09:46:38 AM »
Quote
Believe me LiBH4 is by far better than LiAlH4 and I've used it on ton scale.

Sodium borohydride doesn't reduce esters

No it doesn't, but LITHIUM borohydride does.

Sodium borohydride DOES reduce esters. A benzoic acid methyl ester is reducible with NaBH4/THF/MeOH at 55°C
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