[bharathi]

hi all,
I tried to reduce a ketone to hydrocarbon using sodiumborohydride & acetic acid and in the course of experimentation observed the following. The substrate is charged in the solvent and sodiumborohydride is added. To this acetic acid is added during which temperature rose from 10°C to 28°C and froathing is observed. Froathing could be due to H2 gas liberation . What is the complex that forms due to interaction between NaBH4&aceticacid? Is the exotherm due to this interaction or due to the reaction of the soformed complex with the amide?Can some one let me know if this complex is stable? I am not clear of the mechanism that is driving this reaction.
Plz help

Regards,
Bharathi

[GSR]

hi,

the reaction is  mediated by Sodium triacetoxy borohydride (STAB),NaBH(OAc)3 is formed from the reaction of sodium borohydride with three moles of acetic acid in an inert solvent such as toluene.   If a carboxylic acid is used as the solvent for a NaBH4 reaction, then the reaction will be  effective. its generally used for reductive amination reactions. Sodium triacetoxy borohydrides (STAB) is more selective a reducing agent than NaBH4 : at pH = 5-6, it reduces imines and aldehydes whereas it does not reduce ketones. STAB is ideal for reductive amination reaction using aliphatic and aromatic aldehydes as well as aliphatic, cyclic and bicyclic ketones. One of the true strengths of STAB is its ability to accomplish reductive aminations with weakly basic amines, for example, aromatic amines containing withdrawing groups such as -NO2 and -CN.
apart from acetic acid, Other acids such as ethyl and propionic acids, may also be used to obtain comparable reducing agents.

hope this is helpful.

GSR

[GSR]

in addition to the above reply,
 A  review of the NaBH(OAc)3 method was published in
J. Org. Chem. 1996, 61, 3849-3862

GSR

[movies]

Very nice explanation!

My old advisor did a lot of the early work with reductive amination using STAB.  STAB can also often be used to selectively reduce aldehydes in the presence of ketones.  Here is another review:

Chem. Soc. Rev. 1998, 395.