Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: subro on July 03, 2015, 02:42:43 PM
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I have to propose a proper mechanism for this reaction
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This is my attempt. I'm not sure if it's right, so i prefer to ask for your wisdom. What do you think?
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Hint: MgBr2 is a Lewis acid and the intermediate can be written as a carbocation.
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Hint: MgBr2 is a Lewis acid and the intermediate can be written as a carbocation.
Aah
And now?
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If the bromide reacted, the product would be the bromo-alcohol. It isn't.
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Aah, ok. And now?
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Either something like that or a hydride migration (pinacol-like). The oxygen of ether is probably more basic than bromide. Even though bromide has a net negative charge, it is a very weak base. Bromide cannot deprotonate hydronium ions.
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Thanks to Orgopete's suggestions, the last mechanism is correct. Except that the carbocation is a stronger elctrophil than proton cation and therefore, deprotonation can occur without bromide attack.
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Thanks to Orgopete's suggestions, the last mechanism is correct. Except that the carbocation is a stronger elctrophil than proton cation and therefore, deprotonation can occur without bromide attack.
So, what is the proper mechanism? I don't get how can the H be eliminated without using the bromide ion. Maybe the ether's oxygen abstracts them? Or does the carbocation migrate to the carbonyl carbon?
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H-C-C(+)-O-Mg(-)Br2 → H(+) + C=C-O-Mg(-)Br2 → C=C-OH + MgBr2 → H-C-C=O + MgBr2
Is it more clear, now?
What was missing before, is that oxygen is a Bronsted (Lewis) base and MgBr2 is a Bronsted (Lewis) acid and thus, a negative charge must be drawn on the Mg atom of the -O-MgBr2 adduct. There is no leaving of a bromide from MgBr2.
-O- + MgBr2 → =O(+)-Mg(-)Br2
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Thanks to Orgopete's suggestions, the last mechanism is correct. Except that the carbocation is a stronger elctrophil than proton cation and therefore, deprotonation can occur without bromide attack.
So, what is the proper mechanism? I don't get how can the H be eliminated without using the bromide ion. Maybe the ether's oxygen abstracts them? Or does the carbocation migrate to the carbonyl carbon?
In order to complete the mechanism, I wanted a hydride migration, similar to a pinacol rearrangement. No elimination.
If we want to be technical about this problem, look at the following:
http://pubs.acs.org/doi/abs/10.1021/ja01580a046
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Not a migration but a hydrogen cation elimination, followed by double bond formation and enol –ketone isomerization.
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Thanks to Orgopete's suggestions, the last mechanism is correct. Except that the carbocation is a stronger elctrophil than proton cation and therefore, deprotonation can occur without bromide attack.
So, what is the proper mechanism? I don't get how can the H be eliminated without using the bromide ion. Maybe the ether's oxygen abstracts them? Or does the carbocation migrate to the carbonyl carbon?
In order to complete the mechanism, I wanted a hydride migration, similar to a pinacol rearrangement. No elimination.
If we want to be technical about this problem, look at the following:
http://pubs.acs.org/doi/abs/10.1021/ja01580a046
I get it now. Many thanks to both of you ^^