[nielsgeode]

Hi,

Let me first introduce myself because I'm new here
My name is Niels and I'm a 24 year old chemistry student doing my master research project on organometallic chemistry (group 5 metals Nb and Ta specific) in the Netherlands.

My problem is this: I'm currently synthesizing a substituted phenyllithium salt. I do the synthesis as follows:

Ar-Br + Me-Li ==> Ar-Li + Me-Br
I'm afraid the following will happen:
Ar-Li + Me-Br ==> Ar-Me + Li-Br

Does anybody has experience with these kinds of reaction. How well does the formed methylbromide reacts with the product? Is it likely to happen? I'm asking this because I've already done the reaction twice and I cannot explain my NMR spectra and my yield is way too high (~150%). I'm 100% sure I did not make any calculation errors so I think there is (some) LiBr in my product. If I add one eq of LiBr my yield becomes about 95% which is more logical.

The other thing is:
The substituded phenyl is a fulvene and I add 2 eq of MeLi (one to make the Ar-Li and one to add over the double bond of the fulvene). Would it help if I first make the Ar-Li by adding 2 eq of Li(s), isolating that one and then adding 1 eq of MeLi to add over the fulvene C=C bond?

I've searched in the literature for a couple of hours but still I haven't found anything really useful for this...

thanx, 
Niels

[g-bones]

I personally have not had a problem with metal halogen exchange products reacting with each other.  Whenever I create a lithium salt I am usually trapping it with an electrophile after the exchange is complete and dont have much crossover (although always look out for direct attack of your Me-Li to the electrophile, try using exactly one eq. or even .9 eq and take the hit on the yield).  in your case I would say you can have a concern because MeBr is a very reactive compound.  to avoid this, you should maybe try nBuli, sec-BuLi or maybe even t-BuLi as your lithium source (i personally use nBuLi and havent had a problem). 
Isolation of organolithiums is probably pretty difficult, I would say if you are using it for some sort of nucleophilic attack you should probably make it one pot.  Just perform the metal-halogen exchange for a certain amount of time (necessary to completely exchange) and add your electrophile directly.  hope this helps 

[nielsgeode]

Thanx for you reply 

1) I do have to isolate the lithium salt it because I let it react with NbCl3Me2 which needs to be made made in-situ:

NbCl5 + ZnMe2 ==> NbCl3Me2

I have to use MeLi because I add MeLi over the double bond (I want a methyl, not a butyl)

I was thinking of something else: the yield of about 150% is evidence that there is still Br- present.

Would it be possible to quench about 20mg of my product in water and then do a test which is specific for the presence of Br- ions in aqueous solution?

[nielsgeode]

I found it 
just acidify the solution and add silver nitrate. Now hoping that this also works for very small concentrations....

[g-bones]

sounds like a good idea... new idea came to me as well

If you wanted to make both in situ you could set up two flasks side by side form the aryl-lithium salt, and canula it over into the flask with your metal complex that is being made insitu as well.  this way both can be made in situ .  I am assuming you are using schlenk techniques so when you are ready to canula the aryl lithium salt into your metal solution, place the canula into the aryl-lithium solution with your argon or nitrogen input into that flask with only an outlet in the other, this way the pressure will push the solution into the metal solution.  If you need to add it dropwise, thats a little bit more complicated and a little reading in an air-sensitive techniques resource... have fun   

[nj_bartel]

Couldn't you avoid some difficulties by just working in a dry box?

[nielsgeode]

Hi all,

first of al: of course I use schlenk techniques, I use them on a daily basis so that is no problem to me 

second: I found out what happens:


Instead of B I get C

This is why I came up with a different route which I tried today:


According to literature (J Organomet Chem 193 (1980) 283 for those interested) the ortholithiobromobenzene is stable at about -110. At -90 degrees celcius 90% decomposes in ~30 min. I done reactions of Ar-Li compounds with fulvenes previously, mostly at -60 degrees celcius. I kept this one between -110 and -100 for about 1,5h taking a sample evere 20 min. I checked wih GC/MS and I have absolutely no product. ZERO. 

Another options is 1,2-dilithiobenzene but I do not want to make that because it goes via a organomercury compound 

I just came up with two other options (just on paper, no idea if they work):



and



Anyone any ideas for these or other suggestions?