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Author: Subject: A puzzle from the Mitsunobu reaction
allchemy
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[*] posted on 24-12-2011 at 06:45
A puzzle from the Mitsunobu reaction


My work is to synthesis a ether by Mitsunobu reaction(triphenylphosphine and DIAD mole ratio is about 1:1),in the work up procedure ,triphenylphosphine oxide and diisopropyl hydrazodicarboxylate is precipitated from the solution , triphenylphosphine oxide content is > 70% and diisopropyl hydrazodicarboxylate is 23% by GC analysis from this precipitates, in the filtrate, triphenylphosphine oxide content is between 10%-12% and diisopropyl hydrazodicarboxylate is < 4% and triphenylphosphine is about 5%.The total triphenylphosphine oxide is far outweigh diisopropyl hydrazodicarboxylate ,why is not approach to 1:1 ?

[Edited on 24-12-2011 by allchemy]

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[Edited on 25-12-2011 by allchemy]

[Edited on 25-12-2011 by allchemy]

[Edited on 25-12-2011 by allchemy]
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Dr.Bob
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[*] posted on 24-12-2011 at 08:53


Perhaps your triphenylphosphine is already partly oxidized at the start. We found that older bottles tend to have some oxidation occur just from air. I don't remember the exact details, but that can cause both poor yields, and harder workups.

We also tried the pyridine version of the Ph3P, which is supposed to extract into acid, but we had poor luck getting that to work. What we did have some luck with was using triphenylphosphine bound to resin, but that cost is very high unless you can recycle the resin back and reuse it. And of course the DIAD goes bad fast also if not stored cold enough. They are great reactions for small scale, but very hard to scale up without a large cost and purification battle.
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allchemy
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[*] posted on 24-12-2011 at 18:05
Revise my writing error


Triphenylphosphine(not phenol,I have got it wrong ) and DIAD mole ratio is about 1:1. In the termination,the total triphenylphosphine oxide and diisopropyl hydrazodicarboxylate should also approach 1:1,however, the total triphenylphosphine oxide is far outweigh diisopropyl hydrazodicarboxylate. I'm puzzled.

My product is belong to fine chemicals,easy to purificate.

DIAD goes bad fast if not stored cold enough,is it true?]

[Edited on 25-12-2011 by allchemy]
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allchemy
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[*] posted on 24-12-2011 at 18:40


The manufacturer said that the DIAD purity is >98%
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ThatchemistKid
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[*] posted on 25-12-2011 at 11:33


Are you sure the DIAD is not a solution of 40% concentration in toluene...generally from what I have seen that is how DEAD is sold. DIAD maybe the same. Also DIAD does decompose if not stored in a fridge though not as fast as DEAD.
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[*] posted on 28-12-2011 at 06:38


Quote: Originally posted by allchemy  
The manufacturer said that the DIAD purity is >98%


It was that pure (maybe, they tend to exaggerate purity IMO) when it was manufactured. From that date on, the purity will go down, faster if not stored cold.

The same goes for the triphenylphosphine, it will start oxidizing from the moment it is manufactured unless kept away from oxygen, and most plastic bottles don't keep out oxygen well. Even glass ones will let it in when opened. So its purity will drop over time.

Some chemicals, like benzoic acid, phenol, most salts, and non redux active chemicals will stay pure upon storage. Most amines, phosphines, azo compounds, aldehydes, and other reactives will degrade in storage, some alarmingly fast. I was surprised to learn how fast fluorobenzylamines (I don't remember which isomers we checked) would degrade, even in new, "sealed", bottles, when we tested them by GC-MS over a few weeks. They had air oxidized to the imine, which then reacted with the amine to form new products, which then formed new peaks. Within a month, the material had gone from about 96% pure (label said 99%) to under 70% pure, with a number of secondary amines, carbonates (from absorbing CO2 in air), imines, and complex crap in it.

So don't think the purity on the label is good forever. It might not even be right with a new bottle.
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