[stevejee]

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?

[pgk]

What did you use: lithium wire + Alkyl bromide or lithium chips prepared from lithium wire, in situ + Alkyl bromide?

[stevejee]

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.

[wildfyr]

Oxygen exclusion? I'm not sure I see a way in which oxygen would affect this reaction though.

[pgk]

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

[pgk]

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.

[Borek]

finely divided lithium is highly flammable

It is quite flammable even in large chunks

[stevejee]

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.

[pgk]

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.