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Topic: mCPBA oxidation of terminal alkene  (Read 4174 times)

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Offline owk9688

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mCPBA oxidation of terminal alkene
« on: April 10, 2018, 11:20:27 AM »
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!

Offline Doc Oc

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Re: mCPBA oxidation of terminal alkene
« Reply #1 on: April 10, 2018, 01:25:43 PM »
Did you also run TLC of the mCPBA and benzoic acid for reference? How old is your mCPBA and how has it been stored?

Offline owk9688

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Re: mCPBA oxidation of terminal alkene
« Reply #2 on: April 10, 2018, 02:21:06 PM »
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.

Offline wildfyr

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Re: mCPBA oxidation of terminal alkene
« Reply #3 on: April 10, 2018, 02:48:45 PM »
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.


Quote
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.

Offline owk9688

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Re: mCPBA oxidation of terminal alkene
« Reply #4 on: April 10, 2018, 08:39:12 PM »
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

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