[Baerchen]

Hi

Has anyone prepared Rieke Mg before? I wish to use for a Grignard reaction where I hope to form the Grignard at a low temp. It's my last resort as so far I either cannot initiate Grignard formation, or I get the Wurtz-coupling product.

Looking at the Rieke Mg lit, I think I'll try to form Mg* from MgCl2, Li metal and naphthalene.

Any tips from someone who has actually done this?
Thanks!

[OC pro]

Sigma-Aldrich offers a 2.5M solution of Rieke-Magnesium in THF.
As alternative you can find a very nice procedure at www.orgsyn.org. It works fine. I have done it several years back.
But, if you can spend the money I would first try the solution from Aldrich.

[stewie griffin]

Just a quick check but I assume you've tried to activate the Mg with either iodine or 1,2-dibromoethane?
I haven't made Rieke Mg, though I have made Rieke Zn from Li, naphthalene (catalyitc) and ZnCl₂. It's always nice to buy it if you can, but it's expensive. The total prep time for me my second time around was less than 4 hours start to finish.
If the Mg prep is anything like the Zn prep you'll probably want a sonicator (I have to constantly sonicate the lithium and naphthalene together throughout the prep to get it to stay green which indicates the formation of the lithium naphthalide) and a syringe pump (the ZnCl₂ was added to the lithium naphthalide over 1.5 hrs). I also fused the ZnCl₂ three times to make sure it was anhydrous.

[Baerchen]

Thank you both for your reply. My supervisor wasn't keen on purchasing Rieke Mg but I'm somewhat confident I can make it with the procedure which sounds very similar to what Stewie said with ZnCl2. Sonication is a good tip too, I'll keep it in mind.

Yes, I have tried I2 and dibromoethane to initiate Mg turnings, as well as dry stirring the Mg. They work just fine as long as I heat the flask as well. Then the Grignard forms and couples to itself within minutes. I am hoping Rieke Mg will work at a low temp so I can avoid forming the coupled product.

Thanks again, I'll report how it goes if I think of anything useful to others!

[OC pro]

@ Baerchen: There is another option to avoid Wurtz coupling:

Switch to diethyl ether. Use a 5-fold excess of Magnesium in combination with a high degree of dilution (~0.05M). Very slow addition of the alkyl halide at room temperature. That will suppress the Wurtz coupling.
 

[Nebelhom]

It's recently been shown that you can use anhydrous LiCl to get a Grignard to form relatively easily. I've never tried it myself, but I know someone who swears by it.

http://onlinelibrary.wiley.com/doi/10.1002/anie.200454084/abstract

You just have to make sure that LiCl is bone-dry and under inert atmosphere as it is quite hygroscopic. Use Schlenk tubes if you have any.

[gerrytheone]

Hi

Has anyone prepared Rieke Mg before? I wish to use for a Grignard reaction where I hope to form the Grignard at a low temp. It's my last resort as so far I either cannot initiate Grignard formation, or I get the Wurtz-coupling product.

Looking at the Rieke Mg lit, I think I'll try to form Mg* from MgCl2, Li metal and naphthalene.

Any tips from someone who has actually done this?
Thanks!

[dunno260]

I had a bear of a time trying to make a prenyl grignard.  The best result I had was using diethyl ether, a 5 fold excess of magnesium, slow addition at 0 degrees celcius, concentration at 0.1N, and using the chloride as the halogen and not the bromide (never succeeded with the bromide).  Ended up abandoning it to some different methods though for a couple of additional reasons besides the challenges.