Home > Community > What is the mechanism for the hydrolysis of the boron-alkoxide complex in NaBH4 reduction?
Upvote

27

Downvote
+ Alcohols
+ Chemistry
+ Redox
Posted by
Mike Bauer

What is the mechanism for the hydrolysis of the boron-alkoxide complex in NaBH4 reduction?

Cara Holms  Follow

For an acidic work up:

  1. Protonate an oxygen.
  2. Fragment the tetraalkyl borate into the trialkyl borate and your product alcohol.
  3. Add water to the trialkyl borate and remove the proton.
  4. Repeat.

More

Upvote

VOTE

Downvote
Chas Alston  Follow

I don’t know whether the mechanism of hydrolysis of the boronic acid esters has been studied. However, the dissociative mechanism immediately suggests itself if only by comparison to similar structures.

Hydrolysis mechanism

Since the workup is typically written $\ce{H+/H2O}$, you may consider some buffered source of external protons. These can protonate any of the alcoholates which can then dissociate. (The $\ce{B=O}$ double bond I drew in should be considered no more than additional stabilisation from oxygen lone pairs; boron is electron-deficient by definition.) Another water molecule can associate: the first bond is cleaved. Repeat three times and end up with four molecules of alcohol and tetrahydroxidoborate $\ce{[B(OH)4]-}$.

More

Upvote

VOTE

Downvote
Fred Jenkins  Follow
3rd species, boron should be negatively charged. Are those arrows dative bonds?More
Upvote

VOTE

Downvote
Jaap Folmer  Follow
@orthocresol The wrong charge also explains why the chemical warning wouldn’t go away ;)More
Upvote

VOTE

Downvote
Erik Bartsch  Follow
You could just drop the double bond in the second intermediate...More
Upvote

VOTE

Downvote
Jonathon Howard  Follow
Yes and yes. Which I chose mainly to avoid adding even more charges …More
Upvote

VOTE

Downvote