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Topic: borane-THF as reducing agent  (Read 22160 times)

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Offline Hand15

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borane-THF as reducing agent
« on: July 30, 2008, 01:51:54 PM »
Well... in organic chemistry I've learned that BH3-THF could be used for hydroboration.
But somehow in last week experiment, the borane-THF is used as reducing agent for carbonyl compound and I have no idea about it as reducing agent. Could someone explain to me what is the limitation of BH3-THF compared to lithium aluminium hydride and Sodium Borohydride?

Btw.. those BH3-THF is generated by the reaction of BF3 etherate with NaBH4 and the reacted molecules only contain carboxylic acid and ester functional group (so it is impossible to be reduced by sodium borohydride).

The molecules to be reduced is:

Offline azmanam

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Re: borane-THF as reducing agent
« Reply #1 on: July 30, 2008, 02:53:36 PM »
See attached for why NaBH4 is better for esters, and why borane (BH3-THF or CB-H, which is catechol borane, also a boron-based reducing agent, http://en.wikipedia.org/wiki/Catecholborane) is better for acids.  The borane helps make the acid more susceptible to nucleophilic attack by the H- from another molecule of borane.
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Offline azmanam

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Re: borane-THF as reducing agent
« Reply #2 on: July 30, 2008, 02:54:35 PM »
basically, both LAH, NaBH4, and BH3 all have nucleophilic hydride (H-).  It's just a matter of tuning the reactivity to reduce an ester or an acid.
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Offline Hand15

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Re: borane-THF as reducing agent
« Reply #3 on: July 30, 2008, 06:28:53 PM »
See attached for why NaBH4 is better for esters, and why borane (BH3-THF or CB-H, which is catechol borane, also a boron-based reducing agent, http://en.wikipedia.org/wiki/Catecholborane) is better for acids.  The borane helps make the acid more susceptible to nucleophilic attack by the H- from another molecule of borane.

Strange... I thought NaBH4 could only be used for reduction of ketone and aldehyde. Are you sure it could reduce the ester?

Btw... base on your reasoning, BF3-THF would reduce the acid and not the ester, is it right?
Thanks.

Offline azmanam

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Re: borane-THF as reducing agent
« Reply #4 on: July 31, 2008, 07:42:54 AM »
Yes, NaBH4 will work on esters.  See chart here, and write up here.  LAH may be faster, but is more flammable.  Just depends on what specific reaction you're running.

Yes, BH3-THF will reduce an acid selectively in the presence of an ester.
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Offline azmanam

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Re: borane-THF as reducing agent
« Reply #5 on: July 31, 2008, 10:24:04 AM »
Perhaps more importantly, LAH will reduce BOTH acids and esters.  NaBH4 will not.
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Offline Hand15

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Re: borane-THF as reducing agent
« Reply #6 on: August 11, 2008, 10:17:43 PM »
Thanks ;D

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