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Topic: Favorskii rearrangement  (Read 5161 times)

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Offline Hunt

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Favorskii rearrangement
« on: August 24, 2008, 07:06:55 AM »
I'm supposed to find the product of rearrangement the chemical compound shown below undergoes when exposed to a nucleophile OEt-. I thought that favorskii rearrangment could occur since there is an alpha hydrogen. If this is correct, then is the mechanism below right ? What makes no sense to me is the intermediate formed in the 3rd step. The 2nd image shows a different intermediate formed, which should be more stable. The carboanion formed is tablized by Br an electron withdrawing group. In the 1st case, the carboanion is less stable since it is attached to alkyl electron donating groups. The soln manual shows , however, that the product formed is the 1st one. Why is this so ? Perhaps the whole mechanism is incorrect ?

P.S. I skipped the details when writing the mechanism to save time but I think there should be no confusion.

1st mechanism :



2nd mechanism :



Offline aldoxime_amine

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Re: Favorskii rearrangement
« Reply #1 on: August 29, 2008, 11:58:43 AM »
I think first 2 steps should be accoding to mechanism 1 due to less steric hindrance of that alpha H. Net 2 steps should be by mechanims 2 with the same reason you gave.

Offline Hunt

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Re: Favorskii rearrangement
« Reply #2 on: August 29, 2008, 01:38:17 PM »
Thanks for confirming aldoxime_amine . Is it possible that the the size of Br- must be taken into account ? In the sense that a Br- anion is large ( like bulky groups ... ) and may de-stabalize the carboanion and so product 1 is obtained instead of product 2?

Offline chittimallasanthoshkumar

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Re: Favorskii rearrangement
« Reply #3 on: September 02, 2008, 10:38:03 AM »
Hi

Please see the attachment before going through the explanation. Probable reason for obtaining the product as shown in your scheme 1 might be as follows; There are 2 possible structures for the starting material as shown in the attached doc file. I presume that compound 1 should be more stable due to the SP2 hybridization of carbonyl compound which has least 1,3 diaxial interactions compared to compound 3. At this point let us forget about compound 3 and 4 of the doc file. Now after the attack of the nucleophile, to get relieved from the 1,3-diaxial interactions the bond should have broken and furnished the observed product as shown in scheme 1.

Offline Hunt

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Re: Favorskii rearrangement
« Reply #4 on: September 03, 2008, 06:36:54 PM »
Hello chittimallasanthoshkumar ,

I cant say that I understood your explanation but thanks anyway. Perhaps I'll give it some thought later.

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