Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: owk9688 on April 10, 2018, 11:20:27 AM
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Hello,
I've never done an oxidation of an alkene before and have also never used mCPBA. The reaction I'm attempting now is to take ortho-allyl Phloroglucinol and oxidize the terminal alkene into an epoxide (so that eventually one of the phenols can open it and I can form a bicyclic structure. It seems straighforward and theres precedent with doing the reaction with ortho-allyl phenol so I feel like it should work and I must be doing something wrong.
Im adding the mCPBA all at once to a stirred solution of my allyl phloroglucinol in Room temperature DCE. The reaction mixture turns deep brown within about 20 seconds after I've added the mCPBA. :-\ My TLC shows complete dissapearance of my product at only 1 hour but the mixture is still this awful looking dark brown. In the crude HNMR I cant see anything that resembles my product. Is there something I'm doing wrong? Should I not be adding the mCPBA all at once?
Any help or suggestions would be appreciated. Thank you!
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Did you also run TLC of the mCPBA and benzoic acid for reference? How old is your mCPBA and how has it been stored?
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I did not run the spots of the mCPBA sample on the TLC as well. I'll definietly do that when I try this again. The mCPBA was about a month old and had been stored in the refrigerator but we had a malfunction that caused it to not be cold for a full week. Thats the only thing I can think of thats been out of the ordinary for our storage of it.
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Could you clarify the starting material a little more? Its a benzene ring with a phenol at 1 and 3, and an allyloxy at 5?
Perhaps you should do it at 0°C and add the MCPBA slowly. Formation of brown garbage smells of polymerization. Also, doing this under air and water free conditions if it isn't already is the first thing I do when a reaction is misbehaving.
My TLC shows complete disappearance of my product at only 1 hour but the mixture is still this awful looking dark brown.
Do you mean starting material disappears after 1 hr? if the product is showing up and then disappearing then I suspect polymerization even more. Poly-phenols like this have funny properties.
My hypothesis is that the phenols are attacking the epoxide intermolecularly via acid-catalyzed epoxide ring opening. The acid catalyst is coming from the benzoic acid byproduct.
One trick I do know though, is for workup, you should first do an aqueous sodium bisulfite wash to reduce the leftover MCPBA to its benzoic acid. Basic workups like aq bicarb don't do a good job of removing MCPBA, but do remove the benzoic acid derivative nicely. I fought this problem for a damn week before an older student dropped a knowledge bomb on me about it.
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Sorry. I’m not sure how to post images here. My substrate is a benzene with phenols at the 1,3, and 5 positions, and an allyl at the 2 position. I suppose side reactions are possible considering aromatic system has 3 strongly activating substituents and one weakly activating one. I will try running it at 0 Celsius and adding the mCPBA portionwise and see if that helps. If not, I may just have to try hydrogen peroxide maybe.
Thank you so much