Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: Peterany on December 12, 2012, 12:05:32 PM
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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.
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Are you using the BINAP ligands as described by Feringa in Tetrahedron Asymmetry, 2001, 12, 2497?
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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?
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I second the diethylzinc comment. Very easy to go bad.
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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. ???
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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.
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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.