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Topic: Epoxidation issue.  (Read 2769 times)

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Offline Zipzap

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Epoxidation issue.
« on: September 24, 2011, 01:27:37 AM »
So here's my situation:

I have to give the product of this compound (http://imgur.com/4VgLG) when it is subjected to epoxidation with MCPBA, and justify my answer.

This is what I thought was the product: (http://imgur.com/2TWpl)

However, assuming what I did was right, I'm stuck on justifying why it ends up this way. Does the fact that the double bond I chose had a more substituted carbon have anything to do with it? Could it also have something to due with greater electron density? I really need *delete me*  :D  :o

Offline Dan

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Re: Epoxidation issue.
« Reply #1 on: September 24, 2011, 08:52:46 AM »
If you can't justify it, why did you suggest it? Did you randomly select a product and then try to justify it?

You should really think about the justification before suggesting the answer.

Think about nucleophiles and electrophiles. Is mCPBA a nucleophilic or electrophilic oxidising agent?
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Offline Zipzap

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Re: Epoxidation issue.
« Reply #2 on: September 24, 2011, 11:09:03 AM »
I'd say it's electrophilic in nature, since the compound is attacking it to get that oxygen atom.

Now that I think about it, this should be my answer: http://imgur.com/DPAGa

That double bond that I chose is the most electron rich out of the double bonds there. Thus, the more nucleophilic electron rich double bonds are epoxidized more rapidly. It is the least substituted out of the alkyl substituents, making it the one that is more electron donating.

Is this right?

Offline HonestDoctor

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Re: Epoxidation issue.
« Reply #3 on: September 24, 2011, 03:10:36 PM »
Wouldn't your first one be right since you would have a tertiary carbocation in your epoxidation which is much more stable?

Offline Dan

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Re: Epoxidation issue.
« Reply #4 on: September 25, 2011, 07:04:16 AM »
I'd say it's electrophilic in nature, since the compound is attacking it to get that oxygen atom.

the more nucleophilic electron rich double bonds are epoxidized more rapidly.

You're right, the most electron rich alkene will be oxidised, but the regioselectivity you first suggested was correct. The other two alkenes are relatively electron poor - can you see why?

You will also have to justify diastereoselectivity...
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Offline Zipzap

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Re: Epoxidation issue.
« Reply #5 on: September 25, 2011, 10:02:42 AM »
Wait, now that I think about electron rich bonds, shouldn't I have chosen that very top double bond? According to my notes, that's the one that's going to react the fastest.

As for diastereoselectivity, I seem to be at an absolute loss. Can you perhaps give me a useful hint?

Offline HonestDoctor

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Re: Epoxidation issue.
« Reply #6 on: September 25, 2011, 10:57:37 AM »
I'd say it's electrophilic in nature, since the compound is attacking it to get that oxygen atom.

the more nucleophilic electron rich double bonds are epoxidized more rapidly.

You're right, the most electron rich alkene will be oxidised, but the regioselectivity you first suggested was correct. The other two alkenes are relatively electron poor - can you see why?

You will also have to justify diastereoselectivity...


By other two, do you mean the 2 double bonds on the left? Could the double bond with the oxygen on the bottom left affect the double bond one carbon over in terms of slightly pulling the electrons?

Offline Dan

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Re: Epoxidation issue.
« Reply #7 on: September 25, 2011, 12:14:12 PM »
Sorry, I should've been clearer. The alkene on the far right is the most electron rich. The other two are conjugated with the ketone and therefore electron poor - consider the reactivity of a,b-unsaturated carbonyls - they are electrophiles.

For diastereoselectivity I would make a model.
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