Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: stevet on April 25, 2010, 01:22:47 PM
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Hi All.
Has anyone ever experienced problems when doing an mCPBA epoxidation of a (terminal) alkene, namely that no epoxide is formed while all of the starting alkene has reacted (spot disappeared from TLC). There were three products, one most likely the benzoic acid (white crystalline solid),m while the other two, i have as of yet been unable to identify (got proton NMR, no carbon, 2D yet), though they are definitely not the desired epoxide.
Just wondering if anyone has ever experienced this problem before. The procedure I followed involved addition of mCPBA to the alkene in solution, leaving for 18 hours, then workup with sulfite and then bicarbonate.
Thanks!
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Could you draw the structure of starting material? Maybe it does react but sort of differently.
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Here it is. One of the possible products i thought may have formed could have been the benzyl ester of the benzoic acid. I guess it could also be possible that the benzoic acid attacked the epoxide, opening it up...but i don't really think thats too likely...
Cant seem to upload an image, but here it is in line struture form...
CH2=CH-CH2-CH(CH3)-CH2-O-CH2-Ph
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From where did u got the benzoic acid as by product?
Is it benzoic acid or m-chlorobenzoic acid?
Since MCPBA pure form cannot be used,always u will have m-chlorobenzoic acid around 10% in MCPBA.
Did u checked the reaction after short period?
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Sorry, just abbreviating, i do mean the m-chlorobenzoic acid. The mCPBA that I used had no indication of the content of m-chlorobenzoic acid (its from Aldrich), and I assumed it was pure. While doing research beofre I did the reaction, I did see that it is most preparations do contain the normal acid, though on the container, it claims to be 99% pure (from what i remember).
I did check the reaction periodically, and funny enough, a spot appeared where i expected the epoxide to be on a tlc plate (had prepared some in very low yeilds using a different method, so knew where to look), though from NMR, this that product (after purification) was definitely not the epoxide.
So...I'm still confused :P
When you say that the mCPBA cannot be used alone, what did you mean? Is the m-chlorobenzoic acid's presence necessary for the reaction?
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My mistake, the mCPBA is only about 77%, just checked. I still used quite an excess (about 2.3) equivalents compared to the alkene.
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Do you add any base to it? I have only done one alkene epoxidation with mCPBA in my life, but I added excess KHCO3 to it and ran the rxn in dichloromethane.
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Journal of Organic Chemistry, 1996 , vol. 61, # 21 p. 7402 - 7407
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"When you say that the mCPBA cannot be used alone, what did you mean? Is the m-chlorobenzoic acid's presence necessary for the reaction? "
No!
Pure mCPBA is not available, as it can be detonated by shock or by sparks always it is sold(even mCPBA from ALDRICH) with m-chlorobenzoic acid(around10-15%)!!!!
And i am sorry,as i couldn't give any suggestion on your reaction.
Propably with in couple of days i hope i can....
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I had difficulty using mCPBA for preparing methylglycidate actually. The reaction was just fairly lousy, despite my best efforts and repeating numerous literature preps. I ultimately went with a bleach based oxidation instead. Hope this helps....
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Thanks for the help everyone!
Journal of Organic Chemistry, 1996 , vol. 61, # 21 p. 7402 - 7407
Will have a look at that journal article.
Do you add any base to it? I have only done one alkene epoxidation with mCPBA in my life, but I added excess KHCO3 to it and ran the rxn in dichloromethane.
The methods that I have looked at have so far added the excess sodium bicarbonate after the reaction is completed, and this procedure has been done in our lab before with good results. I have however seen some references where the bicarbonate as added to the initial reaction mixture, so I may give that another go and see what happens.
I had difficulty using mCPBA for preparing methylglycidate actually. The reaction was just fairly lousy, despite my best efforts and repeating numerous literature preps. I ultimately went with a bleach based oxidation instead. Hope this helps....
yeah, at this rate, this is what i am considering... ;D
Again, thanks to everyone for the *delete me*
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Two things, one titrate the MCPBA. I don't recall the reagents, but someone probably can, bisulfite, thiosulfate or something like that and titrate to starch/iodine endpoint.
Second, if you just want the product, form the bromohydrin with NBS/H2O and eliminate with NaOH to make the epoxide.
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Hi!
I know it's been some years since you posted this, but I seem to be having the same problem you had... I am trying to do the epoxidation on a terminal double bond, using only mCPBA, but I get many products instead of one majoritory product. I haven't characterized them yet, but I don't believe it's the epoxide...
I was wondering which solution worked for you back then, would it be better to add excess base such as NaHCO3 to the reaction, or trying the Jacobsen epoxidation with bleach?
Thanks!!
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Peracids tend to be more reactive towards electron rich alkenes so terminal C=C bonds are not favoured.
I believe better results are achieved by using hydrogen peroxide as your oxidant but I have no protocol for that.
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As Orgopete said, that is what I do for epoxides.
If you would like to try it, I have a procedure I use that you can follow.
Let me know. It is high yielding.
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As Orgopete said, that is what I do for epoxides.
If you would like to try it, I have a procedure I use that you can follow.
Let me know. It is high yielding.
I'm having the same problem epoxides, when I reaction , got several products , one majority . Then I purified and got 4 compounds , none corresponded to epoxidized compound I wanted , were derived from MCPBA .
Any recommendation?
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Can you show the reaction?
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Then I purified and got 4 compounds , none corresponded to epoxidized compound I wanted , were derived from MCPBA .
Any recommendation?
If you can identify the products you have, you can figure out what the problem is and think about how it can be solved.
Common problems are unwanted oxidation of other functional groups or epoxide cleavage (by water, chlorobenzoic acid or neighbouring groups).
Acid-catalysed epoxide cleavage can often be suppressed by adding a weak insoluble base (e.g. bicarb) to neutralise the equivalent of chlorobenzoic acid produced in the reaction. Problems with neighbouring groups may require protection strategies. It's difficult to offer specific advice until there is enough data to diagnose the problem.