Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: stevejee on November 06, 2017, 08:53:24 AM
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I've been working on 1,4-addition of Cyanocuprate recently, but having a hard time with reproductibility issue.
Brief procedure is the following :
1. Lithium wire(Flame-dried under Vaccum pump) + Alkyl bromide(-OTBS is there, but still a simple Bromide)
2. CuCN(1equiv, Flame-dried under Vaccum pump) + Solution from 1(2equiv)
3. Add Starting material material in solvent(Ether or THF)
Sometimes, It works fine. Nice Deep-yellow colored Cuprate solution is formed, then reaction works fine.
Otherwise, It turns into Turquoise to blue-colored solution and Starting material remains.
Originally, I thought that water was the problem and flame-dried both Lithium and Copper Cyanide.
Copper cyanide which is used in our lab is not that green-colored, so I think it's safe to think that Impurities in commericial reagent is negligible.
I still couldn't figure out what exactly the problem is.
Is there any factor that I've missed out? Or It's just that my experiment skill is not good enough?
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What did you use: lithium wire + Alkyl bromide or lithium chips prepared from lithium wire, in situ + Alkyl bromide?
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I used Lithium wire, with >98% purity, high sodium.
Wire was washed with Hexane, cut into several pieces before introducing into Rbf with Magnetic Stirrer, sealed with septum, then flame-dried under vaccum pump.
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Oxygen exclusion? I'm not sure I see a way in which oxygen would affect this reaction though.
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Well done but it is critical to thoroughly filter the prepared organolithium solution because the side product LiBr will deactivate the CuCN by forming a CuCN.2LiBr complex instead of the desired RCu(CN)Li.
Besides, the remaining traces of metallic lithium may deactivate the 1,4-addition substrate.
On the other hand, excessive filtering (e.g. through a thick glass wool filter) might lead to significant loss of the prepared organolithium reagent.
The Preparation of n-Butyllithium, J. Am. Chem. Soc., (1949), 71(4), 1499–1500
http://pubs.acs.org/doi/abs/10.1021/ja01172a510
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Oxygen must be excluded for safety reasons because finely divided lithium is highly flammable, as well as to avoid any oxidation of the unsaturated substrate.
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finely divided lithium is highly flammable
It is quite flammable even in large chunks :)
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Well done but it is critical to thoroughly filter the prepared organolithium solution because the side product LiBr will deactivate the CuCN by forming a CuCN.2LiBr complex instead of the desired RCu(CN)Li.
Besides, the remaining traces of metallic lithium may deactivate the 1,4-addition substrate.
On the other hand, excessive filtering (e.g. through a thick glass wool filter) might lead to significant loss of the prepared organolithium reagent.
The Preparation of n-Butyllithium, J. Am. Chem. Soc., (1949), 71(4), 1499–1500
http://pubs.acs.org/doi/abs/10.1021/ja01172a510
Oh...I didn't knew that LiBr deactivates CuCN. I guess that's why it was not working sometimes.
Is there any published paper mentioning that LiBr inhibits 1,4-addition of cuprate? If there is, I would like to know.
Thank you for kind replying.
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I do not know this but what I know is that cyanate anions easily form complexes with metal salts, alkali halides included and that CuCN·2LiCl is a commercial product. As a consequence, any formation of the similar CuCN·2LiBr that is insoluble in organic solvents, will be competitive to the formation of the desired RCu(CN)Li.