[Hand15]

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:

[azmanam]

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.

[azmanam]

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.

[Hand15]

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.

[azmanam]

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.

[azmanam]

Perhaps more importantly, LAH will reduce BOTH acids and esters.  NaBH4 will not.

[Hand15]

Thanks