Wittig Rxn

[MedPR]

I thought the wittig reaction was dependent on a phosphonium ylide with a partial negative carbon attached to it. How does this ylide work for wittig?

How does the carbonyl carbon end up with a partial negative if it has a partial positive due to the oxygens?

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[chiddler]

I think you need to add a strong base to that molecule first to convert the carbon to an anion. Then wittig can occur.

Else i'm not understanding. I don't think i've ever seen a carbon in any sort of double/partial double bond with oxygen holding a negative charge; how do you know it's there?

 

[MedPR]

I think you need to add a strong base to that molecule first to convert the carbon to an anion. Then wittig can occur.

Else i'm not understanding. I don't think i've ever seen a carbon in any sort of double/partial double bond with oxygen holding a negative charge; how do you know it's there?

It's described in a passage as being part of a wittig reaction. It doesn't say anything about a base or what kind of solvent there is. It just says "compound then reacts with (image), which is called a wittig reaction"

 

[chiddler]

It's described in a passage as being part of a wittig reaction. It doesn't say anything about a base or what kind of solvent there is. It just says "compound then reacts with (image), which is called a wittig reaction"

so you're not completely sure if the caranion is with the ester.

it must be added to a base! how else would a carbanion form! but then it may convert the ester into a carboxylic acid which may neutralize the base...

bah. i don't know...nevermind.

 

[MedPR]

so you're not completely sure if the caranion is with the ester.

it must be added to a base! how else would a carbanion form! but then it may convert the ester into a carboxylic acid which may neutralize the base...

bah. i don't know...nevermind.

Well it's strange because I've always imagined wittig reactions to exchange the alkyl on the wittig with an OH group on the substrate. This particular reaction adds only the CO2Et from the wittig as opposed to adding the CHCO2Et.

 

[indianjatt]

Doesn't the Carbon double bonded to the phosphorous have the negative charge? If you draw an equivalence resonance form for the double bond, you can move the two electrons onto the carbon next to the Phosphorous. This can then attack the carbonyl on whatever molecule you have.

 

[MedPR]

Doesn't the Carbon double bonded to the phosphorous have the negative charge? If you draw an equivalence resonance form for the double bond, you can move the two electrons onto the carbon next to the Phosphorous. This can then attack the carbonyl on whatever molecule you have.

Yea that's what you would expect, but the mechanism shows the carbonyl carbon acting as the nucleophile. That's why I was confused by it.

 

[indianjatt]

Where? The one on wikipedia and in my book show the Carbon next to the phosphorous attacking. http://en.wikipedia.org/wiki/Wittig_reaction

Are you talking about the four membered intermediate where the O- attacks the phosphorous and the bond between the C and O forms a double bond with the Carbon and the alkyl chain?

 

[MedPR]

Where? The one on wikipedia and in my book show the Carbon next to the phosphorous attacking. http://en.wikipedia.org/wiki/Wittig_reaction

Are you talking about the four membered intermediate where the O- attacks the phosphorous and the bond between the C and O forms a double bond with the Carbon and the alkyl chain?

 

[indianjatt]

Interesting. The only wittig reaction we learned in my o chem class was one with PPh3 bonded to an alkyl group. This then replaced the carbonyl with an alkene.

 

[MedPR]

Interesting. The only wittig reaction we learned in my o chem class was one with PPh3 bonded to an alkyl group. This then replaced the carbonyl with an alkene.

Yea, that's the only one I've ever seen too. This one is from an passage on a FL I did recently. It just shows you a 7 or 8 step mechanism and asks some questions about it. It even describes this step as a wittig reaction, so it's not like it is some novel reaction we aren't supposed to know. I just thought it was odd and wanted to see if anyone knew anything about it.

 

[sshah92]

Has anyone figured this one out?! I'm thoroughly confused as to why the product includes CO2Et and not CEtOCH. The driving force behind the Wittig is the favorable O-P bond formation... can someone draw a mechanism out for this one?

 

[monkeyvokes]

Has anyone figured this one out?! I'm thoroughly confused as to why the product includes CO2Et and not CEtOCH. The driving force behind the Wittig is the favorable O-P bond formation... can someone draw a mechanism out for this one?

I had an idea. The image above says "step 4," so I'm thinking that steps 1-3 involved turning that terminal alkene into an aldehyde. Then, the wittig reaction would work as it typically does and produce the product depicted.

Do we know if OP's image depicts the starting reagent for step 1 or for step 4?
Because if his pic shows the starting reagent for step 1 and the product for step 4 then I think my idea might work. What does everyone else think?

Any idea where this came from? I'm really curious to see the pull passage..

edit: I don't know if you'd need 3 steps to make a terminal alkene into an aldehyde, though. I think you could just hydrate anti markovnikov with oxymercuration reduction and then oxidize with an oxidizing agent that doesn't fully oxidize to carboxylic acids (the opposite of sodium borohdyrite?)
PCC?