Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: darkdevil on June 17, 2017, 09:53:39 PM
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Hi there,
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I have a bottle of "a little bit day-olded" isopropylmagnesium chloride. I tried to do a homocoupling reaction of 3-bromothiophene to give 3,3'-bithiophene with Ni(dppp)Cl2 as catalyst , but the yield is lower than expected.
Here is how I did the reaction:
~~ All flasks are protected under Argon~~
1) I first treated 3-bromothiophene with isopropylmagnesium chloride at low temperature using THF as solvent
2) After the metallation is completed, I transferred the Grignard reagent to another flask with 3-bromothiophen, Ni(dppp)Cl2 and anhydrous THF. The addition was dropwise and I observed heat release instantly
3) I then let the reaction to carry out for 6 hours at room temperature (while the literature suggested refluxing for 20 hours)
4) I then added water to quench the reaction and purify my products
5) I obtained correct NMR for 3,3'-bithiophene but the yield is much lower than literature which is over 80% while I got below 10%
Since the first step is to generate the 3-thiophenyl magnesium chloride by metallation of my 3-bromothiophene, I am not sure if it is related to the day old bottle of my isopropylmagnesium chloride or not. I can see salt inside the sealed bottle. Can I add this reagent in a little bit of excess?
Or do I really need to reflux the reaction mixture? Has anyone have experience working with isopropylmagnesium chloride?
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If you dont have a way to monitor the last step I suggest you follow the recipe and reflux the solution.
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Why would you not:
A. reflux it?
B. let it go for longer?
Theres usually a reason people do things in the literature.
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because I observed that there is no more heat release. I am not sure if the reaction is still undergoing
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If you are concerned about complete conversion to the desired grignard, you can run a test conversion with a D2O quench and analyze by NMR to check for % conversion.
Monitoring the Ni step is a little tricky, but you could probably do it by taking out aliquots and checking by NMR.
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Why not just follow the published procedure??
If it is not complete after 20 hours reflux keep going and check by TLC for anymore conversion. If none then work up.
If there are any problems, I would suspect the catalyst.
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GC would be good to monitor the reaction.
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Please forgive my ignorance, but what is the advantage of Grignard reactants over a Wurtz coupling here?
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The use of a Ni catalyst suggests that this is a Kumada type coupling.
See http://www.organic-chemistry.org/namedreactions/kumada-coupling.shtm (http://www.organic-chemistry.org/namedreactions/kumada-coupling.shtm)
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I have experience of this kind of reaction using Isopropyl magnesium bromide (also called turbo grignard Reagent).
Make sure your apparatus are dry and also used anhydrous solvent and finally try to use fresh Isopropyl magnesium bromide bottle. Presence of little moisture in the reaction mixture definitely would destroy grignard reagent and in my opinion this may be the one reason for lower yield.
Try to conduct this reaction under inert atmosphere and if you have access to Schlenk line that would be best.
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A turbo Grignard reagent is one co-mixed with lithium chloride. A term coined by its discoverer, Paul Knochel, from München.