[RaInBowDaSh1488]

Reductions are pretty easy in general and a reaction I am quite familiar with. Lately however, I've been trying to carry out a reduction that has been a bit problematic.

I am reducing a rather non-polar acid with LAH. The reducing agent in question is of good quality, I've used in frequently recently and it is effective. This particular acid is displaying some weird behavior.

First off, even when I quench the reaction slowly, it displays a tendency to absorb or evaporate the THF solvent. This is something I have never seen before. I'm keeping the flask cool in ice, but the THF just disappears. I'm using a relatively small amount of LAH, actually the smallest amount I've every used, and a good amount of THF. This behavior is really puzzling.

Additionally, this reaction seems to take forever and never go to completion. Most carboxylic acids I've reduced go to completion in less then 3 hours. Is it abnormal for a carboxylic acid to not be fairly quickly reduced in excess LAH? I am considering refluxing it and allowing it to sit for 12 or more hours.

Also, I'd appreciate any tips on keeping track of the progress of a reduction by TLC. All the insoluble LAH floating around really makes TLC a pain. It blocks up pipettes, messes up the TLC plate and sometimes even reduces the silica. I'd love to try and keep from wasting plates and pipettes and I'm sure there is a more effective way of taking TLC's during a reduction.

Thanks!

[camptzak]

maybe if you tried to run the reaction in DMSO the LAH would solubilize better and the reaction rate would increase.

[Dan]

I'd give borane·THF a shot (other functionality permitting).

[orgopete]

I'd give borane·THF a shot (other functionality permitting).

I second this. I've never reduced an acid or amide with LAH, but looking at experiments, they look difficult. I've noticed additional additions of LAH or extended refluxing. Mechanistically, I always thought it should be difficult to eliminate lithium oxides, a necessary step in the reduction.

[RaInBowDaSh1488]

Thanks for the advice! I will certainly be considering borane·THF for this reaction. I've reduced a number of acids with only LAH fairly easily before, but I guess I have just been lucky in the past. I think borane·THF should work nicely though.