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Author Topic: Di-grignard of p-bromobenzene  (Read 3177 times)

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mattfred

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Di-grignard of p-bromobenzene
« on: April 13, 2009, 04:57:35 PM »

I believe that I have made a di-grignard by refluxing Mg and p-bromobenzene in anhydrous THF. The solution appears cloudy orange as if some precipitate has formed from the initially clear orange/brown solution. I originally intended to cannulate the product into the next step of the reaction, but I am concerned after some research about the mono-grinard and starting materials, as well as possible side products.

Does anyone have any experience/advice for the workup of this compound? Am I better off just waiting until the finished product to purify? Does the appearance of my compound seem appropriate?
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azmanam

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Re: Di-grignard of p-bromobenzene
« Reply #1 on: April 13, 2009, 05:30:43 PM »

Aryl grignard do change colors.  mesityl grignard is somewhat greenish.  I wouldn't be too put off by the color.

by digrignard do you mean a dialkylmagnesium compound, R2Mg?  That is probably not likely to happen.  Magnesium undergoes oxidative addition into the R-X bond, which forms the RMgX compound and oxidizes Mg from 0 to +2.  For a second alkyl group to add, the RMgX would have to either undergo another oxidative add'n (which would push Mg to +4... unlikely).  alternatively, two grignard reagents could transmetallate: RMgX + RMgX -->  R2Mg + MgX2.  I don't know how likely that would be, but I doubt it would occur to a large extent... maybe with enough heat, but I still don't see it happening...




wait.  upon closer reading, you say p-bromobenzene.  what is the bromine para to?  do you mean p-DIbromobenzene?  That changes the question completely.  I don't have any experience with grignard SMs with 2 reactive sites.  I'll defer if that's what you're actually talking about.
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mattfred

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Re: Di-grignard of p-bromobenzene
« Reply #2 on: April 13, 2009, 05:43:11 PM »

Yeah...sorry... don't know how I missed that. It's definitely p-dibromobenzene. I meant di-grignard as in both bromine positions react. I was expecting a color change, which I saw quite rapidly to an orangish/brown, but I am concerned about how cloudy the solution is. I stopped the stirring and it does appear to be a precipitate, which makes cannulation more difficult (maybe impossible? I don't know much about it).

Thanks for the rapid response and pointing out my mistake.
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Squirmy

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Re: Di-grignard of p-bromobenzene
« Reply #3 on: April 14, 2009, 01:14:17 AM »

Maybe you're making the di-Grignard and it's not very soluble. How are you trying to use it?  Can you get away with adding your other reactant to the Grignard mixture?

Quote
alternatively, two grignard reagents could transmetallate: RMgX + RMgX -->  R2Mg + MgX2.  I don't know how likely that would be, but I doubt it would occur to a large extent.

2 RMgX ::equil::  MgR2  +  MgX2

This definitely does occur and is known as the Schlenk equilibrium. You can drive the equilibrium to the right if you use the right solvent (usually 1,2-dimethoxyethane or dioxane), which precipitates out the MgX2 salt via a chelate. You can even buy MgBu2
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azmanam

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Re: Di-grignard of p-bromobenzene
« Reply #4 on: April 14, 2009, 01:35:54 AM »

Quote
This definitely does occur and is known as the Schlenk equilibrium. You can drive the equilibrium to the right if you use the right solvent (usually 1,2-dimethoxyethane or dioxane), which precipitates out the MgX2 salt via a chelate. You can even buy MgBu2

yeah, that makes sense.  I was misunderstanding the question at first.  I usually hear of dialkylzinc reagents more than dialkylmagnesium reagents.  Sorry for muddying the waters.

More generally, I've made vinyl Grignard a number of times, and I rarely get a homogenous solution.  I typically see unreacted magensium metal still swimming around (even when vinyl bromide is in excess...) and the Grignard typically takes on sort of a gelatinous form after a while.  Reagent is still good, and I typically make it in excess and syringe out what I need.

Ps, I often titrate it with salicylaldehyde phenylhydrazone before I use it.  That way I know what the titer is and I can make sure I'm adding the correct stoichiometry.

http://dx.doi.org/10.1021/jo982433e
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mattfred

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Re: Di-grignard of p-bromobenzene
« Reply #5 on: April 14, 2009, 06:22:26 AM »

Maybe you're making the di-Grignard and it's not very soluble. How are you trying to use it?  Can you get away with adding your other reactant to the Grignard mixture?

That's what I was wondering. The procedure I'm taking this from cannulates the THF solution into the next reagent. It is carbon disulfide. I just assumed that they did this because CS2 or the product reacts with magnesium in some way. The product is a dithioacid which is (supposedly) very strongly chelating. I might be able to remove it with a strong acid, but I'm not really sure.

I could try using 2:1 equivalents, but I want to try and get as much of the di-grignard as possible. I used 10:1 in this reaction.
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Squirmy

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Re: Di-grignard of p-bromobenzene
« Reply #6 on: April 14, 2009, 08:19:07 AM »

Honestly, I'm not certain whether it would/should be a problem. Adding CO2 to a Grignard usually isn't a problem, so I say go for it. Also, your starting materials are relatively cheap, so it's not that big a loss even if it doesn't work.
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mattfred

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Re: Di-grignard of p-bromobenzene
« Reply #7 on: April 14, 2009, 12:32:01 PM »

Haha good point. Thanks for the advice.
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