[SinkingTako]

So I found this reaction somewhere.


And this is my proposed mechanism, which is similar to Favorskii. But there is definitely something wrong, as somewhere there is either a formation of an alkoxide or a carboanion, which isn't favourable. What's wrong? And how does this reaction take place? Thanks!

Apologies for the image being the wrong way... Can't edit it at the moment...

[discodermolide]

Bases are known to abstract the beta proton of epoxides to give the allylic alcohol. In your case with the Grignard I think you could expect simple epoxide ring opening to give the corresponding cyclohexanol.

[SinkingTako]

Bases are known to abstract the beta proton of epoxides to give the allylic alcohol.

why is the beta position of the epoxide prone to proton extraction?

Yes,  I think the formation of cyclohexanol is the main reaction. Though this reaction also takes place. This is the part I'm confused about.

It's from
http://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/orgmetal.htm
Just above the section on metal exchange reactions.

[discodermolide]

Well you have a strained ring system here, intact a god leaving group in breaking one of the epoxide oxygen-carbon bonds.
Imagine a metal coordinated to the oxygen atom, this induces a partial positive charge on the O atom, making the protons alpha and beta a bit more acidic. Abstraction of the alpha proton is probably sterically hindered, but removal of the beta proton does not suffer from this problem. So the base removes the trans, anti-periplanar H atom to the C-O bond to give the allylic alcohol.

[SinkingTako]

Thank you! Okay i get the epoxide part now. Though I still don't get how to do the rearrangement part. Any tip? Thanks!

[discodermolide]

I don't see the rearrangement happening, the intermediate would be too strained to form. All that will happen here is either the allylic alcohol will form or the epoxide opens to give the cyclohexanol.

[orgopete]

That is not how the reaction takes place. Proton abstraction would consume Grignard and no further reaction will take place. This is a Lewis acid catalyzed reaction. If MgBr2 were added to the reaction, the fraction of rearrangement will increase. The product forms by addition to the intermediate aldehyde. See http://www2.ups.edu/faculty/hanson/c251lab.03/ibuprofen/ibuprofen.htm

[clarkstill]

Yeah, this reaction goes via a Lewis acid mediated Pinacol-type rearrangement, followed by addition to the aldehyde...

[SinkingTako]

Okay,  thank you! Is is something like this?

[Benzene]

I looked at the link you gave, I have no idea how this CH₃MgBr reaction proceeds to the 1-hydroxy-ethylcyclopentane.
I would say that the mechanism you have drawn cannot be correct, because the rearrangement you have drawn proceeds through a cation in a solution containing methyl magnesium bromide.
I like your first mechanism better.

[orgopete]

I would say that the mechanism you have drawn cannot be correct, because the rearrangement you have drawn proceeds through a cation in a solution containing methyl magnesium bromide.
I like your first mechanism better.

I cannot say categorically the mechanism is correct, but it is consistent with an increase in MgBr2 increases rearrangement.

Although it is unusual to see a carbocation in Grignard mechanism, let me remind you that formal charges are simply bookkeeping devices as no change in charge ever occur, protons are positive and electrons negative. The charge being written is more commonly an indicator of the importance or motivation of one chemical species over another. A Markovnikov addition could be written as concerted (as you might find with hydroboration), or bromide addition to generate the more stable carbanion, or by protonation. While Markovnikov himself simply noted the halogen was added to the more substituted carbon, we now believe this occurs because the key step is the addition of a proton to an alkene. Initial protonation certainly it is more plausible as bromide is a much weaker base than a carbanion and its formation from a bromide would require climbing a large energy hill. Therefore writing a carbocation is telling you the magnesium is pulling the electrons away from the carbon. While that is unusual, given the greater strain of an epoxide, greater electrophilicity of magnesium, seems plausible.

Disco referred to the allyl alcohol formation from an epoxide. If the mechanism were to proceed by this mechanism, this mechanism consumes Grignard. Although a carbanion can be recovered at a later stage, doing so invites formation of by-products to compete with the methyl anion. Base opening of an epoxide is a known reaction catalyzed by diethylamide. It does not give an aldehyde. The rearrangement of an epoxide can occur with a Lewis acid, including Grignard reagents, but not alkyl lithium reagents. One might think if the rearrangement were base catalyzed, this rearrangement could be found with other bases.

[Benzene]

I would say that the mechanism you have drawn cannot be correct, because the rearrangement you have drawn proceeds through a cation in a solution containing methyl magnesium bromide.
I like your first mechanism better.

I cannot say categorically the mechanism is correct, but it is consistent with an increase in MgBr2 increases rearrangement.

Although it is unusual to see a carbocation in Grignard mechanism, let me remind you that formal charges are simply bookkeeping devices as no change in charge ever occur, protons are positive and electrons negative. The charge being written is more commonly an indicator of the importance or motivation of one chemical species over another. A Markovnikov addition could be written as concerted (as you might find with hydroboration), or bromide addition to generate the more stable carbanion, or by protonation. While Markovnikov himself simply noted the halogen was added to the more substituted carbon, we now believe this occurs because the key step is the addition of a proton to an alkene. Initial protonation certainly it is more plausible as bromide is a much weaker base than a carbanion and its formation from a bromide would require climbing a large energy hill. Therefore writing a carbocation is telling you the magnesium is pulling the electrons away from the carbon. While that is unusual, given the greater strain of an epoxide, greater electrophilicity of magnesium, seems plausible.

Disco referred to the allyl alcohol formation from an epoxide. If the mechanism were to proceed by this mechanism, this mechanism consumes Grignard. Although a carbanion can be recovered at a later stage, doing so invites formation of by-products to compete with the methyl anion. Base opening of an epoxide is a known reaction catalyzed by diethylamide. It does not give an aldehyde. The rearrangement of an epoxide can occur with a Lewis acid, including Grignard reagents, but not alkyl lithium reagents. One might think if the rearrangement were base catalyzed, this rearrangement could be found with other bases.

can you get a reference for this rearrangement? It makes sense to me that as the Lewis acid conc. increases you will see this rearrangement product, but I would like to see someones work on it.

[orgopete]

I don't know if the reference will come through. I searched for MgBr2 rearrangement of cyclohexene oxide. If the reference page from Google books fails, you can repeat the search. Reference Comprehensive...