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Topic: Why this reaction doesn't go far??  (Read 3345 times)

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Offline Peterany

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Why this reaction doesn't go far??
« on: December 12, 2012, 12:05:32 PM »
I'm trying conjugate addition with 2-cyclohexanone and diethylzinc.
I'm using Cu(OTf)2 and phosphoamidite ligand, as catalyst.
It's very well-known reaction, high yield and ee% (I remember max. 99%) .
and procedure also is easy to get.
Following the procedure, however, starting enone remains still..  :-[ :'(
I suspected the starting didn't fully converted because of complexation of Cu and ligands.
But, it's only a suggestion. Anyone can find what's the problem here??
Any comments would be greatly appreciated. Thanks.


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A solution of Cu(OTf), (0.020 mmol) and ligand (0.040 mmol) in toluene (5 mL) was stirred for 1 h. The colorless solution was cooled (- 30 °C) and enone (1.0 mmol) and
1.5 equivalent of diethyllzinc solution were added. After 3 h at - 30 °C the reaction mixture was poured into 25 mL of 1 N HCI and extracted with diethyl ether (2 x 25 mL). The combined organic layers were washed with brine (25 mL), dried (MgSO,), filtered, and evaporated to give the crude 1,4-products. After purification by chromatography (SiO,. hexane:diethyl ether 5: 1) the ee valueswere determined.

Offline discodermolide

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Re: Why this reaction doesn't go far??
« Reply #1 on: December 12, 2012, 12:10:41 PM »
Are you using the BINAP ligands as described by Feringa in Tetrahedron Asymmetry, 2001, 12, 2497?
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Offline OC pro

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Re: Why this reaction doesn't go far??
« Reply #2 on: December 12, 2012, 03:40:18 PM »
Problem could be with diethyl zinc. This stuff is difficult to handle and can be spoiled easily making the reaction a complete fail. How fresh was your Zn(Et)2?

Offline stewie griffin

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Re: Why this reaction doesn't go far??
« Reply #3 on: December 12, 2012, 10:01:52 PM »
I second the diethylzinc comment. Very easy to go bad.

Offline Peterany

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Re: Why this reaction doesn't go far??
« Reply #4 on: December 13, 2012, 05:35:51 AM »
Problem could be with diethyl zinc. This stuff is difficult to handle and can be spoiled easily making the reaction a complete fail. How fresh was your Zn(Et)2?


I'm using commercial Et2Zn, 1.0 M in hexane (Aldrich)

I tried with reagents opened almost 1 year ago at first,
and tried with new Et2Zn. But, It didn't show big difference.

So, I'm finding a clue why this reaction doesn't work.  ???

Offline Peterany

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Re: Why this reaction doesn't go far??
« Reply #5 on: December 13, 2012, 05:45:13 AM »
Are you using the BINAP ligands as described by Feringa in Tetrahedron Asymmetry, 2001, 12, 2497?

Actually, I'm using this one.
This ligand also was reported by Feringa.

Offline OC pro

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Re: Why this reaction doesn't go far??
« Reply #6 on: December 13, 2012, 08:15:04 AM »
Solvents and whole set-up is dry? Do you use nitrogen or argon? Do you add the Zn(Et)2 dropwise and very slowly (temperature control)?
It could also be a difference in which solution your diethylzinc has to be handled. To which solution does the paper refer when diethylzinc solution is mentioned. I mean the reactivity and solubility comparing hexane and diethyl ether can be very different.

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