Comments on: Sodium Borohydride (NaBH4) Reduction of Aldehydes and Ketones

By: James Ashenhurst

James Ashenhurst — Wed, 09 Oct 2024 09:51:12 +0000

Hi – there’s no clear reason why the meso or dl- will be major or minor. You might find that it depends on solvent, temperature, different reagent, and so on. The ratio will likely be close to but not exactly 50/50. My short answer is that there’s no way to predict in advance which product will be major in a situation like this.

By: amel_baros

amel_baros — Mon, 30 Sep 2024 21:30:27 +0000

For an experiment my class is doing, we’re making (1R,2S / meso) and (1R,2R) hydrobenzoin from Benzil (a ketone) and NaBH4 catalyst.
My question is: How do you predict which products will “major” or “minor” (plentiful, rarer), from mechanism alone? All your examples had the 2x- bonded Oxygen stay in the same place. Is that for convenience, or do these oxygens really tend to face the same direction (relative to the two ~R groups) while the new H tends to attach opposite the new OH group?

By: James Ashenhurst

James Ashenhurst — Wed, 19 Jun 2024 02:16:26 +0000

In reply to Namy. Yes, that's an acid base reaction between basic hydride (from BH4) and the moderately acidic glycerol. For this reason we often cool the reaction mixture in an ice bath (or dry ice bath) and use a slight excess of borohydride.

By: Namy

Namy — Tue, 18 Jun 2024 01:15:16 +0000

Hi, I’m doing an etherification reaction for polyglycerol and notice large amount of bubble (and fizzy!) as soon as NaBH4 comes in contact with glycerol. Is there any possible reaction occurs? (glycerol that we used is 99.7% so, technically, there should be very few amount of water presence)

Thank you!

By: James Ashenhurst

James Ashenhurst — Tue, 10 Oct 2023 01:49:38 +0000

In reply to Kostas. Reaction should be very fast, in particular in basic conditions since base will catalyze conversion of the anomeric center to the ring-opened aldehyde form (which is reduced). I'm not sure what procedure you're referencing, but it's generally very quick.

By: Kostas

Kostas — Mon, 09 Oct 2023 18:32:43 +0000

Can the reduction reaction take place in basic conditions? For example for the reduction of sugars to their respective polyols, sodium borohydride is used with ammonium hydroxide or ammonium acetate but it is suggested to wait for at least 12 hours. Is it because the reaction is slow under basic conditions without the presence of H+?

By: James Ashenhurst

James Ashenhurst — Wed, 27 Sep 2023 18:52:16 +0000

In reply to Kyuchul Lee. There are many, many preparations out there for reductive amination with NaBH4. Byproducts are generally not a problem. Is there something specific you're seeing?

By: James Ashenhurst

James Ashenhurst — Wed, 27 Sep 2023 18:51:02 +0000

In reply to muon davan. Well, probably has something to do with higher electronegativity of boron relative to aluminum plus poorer Al-H orbital overlap versus B-H overlap

By: James Ashenhurst

James Ashenhurst — Wed, 27 Sep 2023 18:49:55 +0000

In reply to Nithil.

Are you sure it’s NaBH4 and not something like lithium triethylborohydride (Super-Hydride) which is much more reactive? I’ve seen examples of it reducing secondary alkyl halides, but not tertiary.
e.g. https://www.sigmaaldrich.com/deepweb/assets/sigmaaldrich/marketing/global/documents/157/575/acta-vol12.pdf
see page 7 on that pdf

By: James Ashenhurst

James Ashenhurst — Wed, 27 Sep 2023 18:46:36 +0000

In reply to Deepanshu Goyal. BH3 would be the first byproduct. I didn't go into more detail than that. In air, BH3 will rapidly oxidize to B(OH)3