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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: kriggy on January 06, 2015, 09:38:05 AM

Title: wittig reaction?
Post by: kriggy on January 06, 2015, 09:38:05 AM

I found this reaction and I just want to make sure, is this just a wittig? I didnt think that PPh₃ is strong enough base to form the ylide needed for this reaction - to remove the Cl^- Or is it just easier since it is double alkylation of PPh₃ thus not needing a base?
Or could it be done with bromoform in a similar manner?
I suppose that dichlormethane doesnt react because bromine is better leaving group-and increasing the temperature/time could make it react too?
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FNTs9oJM.png&hash=2440af28d4de74c9ce47d7305d129c3a133ef335)


And one more: is there a way to convert ketone/aldehyde into halogenderivate (C=O  :rarrow: C-X) in one step? Or do I need reduction+substitution?

Title: Re: wittig reaction?
Post by: Rutherford on January 06, 2015, 10:12:27 AM

You don't need a base. The triphenyl phosphine is in excess, one molecule makes S_N2 on the carbon, the other on the bromine atom of the intermediate.

Title: Re: wittig reaction?
Post by: critzz on January 06, 2015, 03:44:41 PM

It's a Corey-Fuchs reaction, which is indeed a similar to a wittig reaction. http://www.organic-chemistry.org/namedreactions/corey-fuchs-reaction.shtm

Quote from: kriggy on January 06, 2015, 09:38:05 AM

And one more: is there a way to convert ketone/aldehyde into halogenderivate (C=O  :rarrow: C-X) in one step?

I'm actually not sure, probably reduction/substitution.

Title: Re: wittig reaction?
Post by: AlphaScent on January 07, 2015, 09:30:16 AM

critzz:

In the link you posted it states that the ylid adds to an aldehyde.  Does it always have to be an aldehyde? 

In the given scheme it only says R1.  Can we assume it is a hydrogen or does the reaction proceed with alkyl substituents?

Title: Re: wittig reaction?
Post by: discodermolide on January 07, 2015, 11:19:59 AM

Effectively it has to be an aldehyde. You need the beta H for elimination to the bromoalkyne which then undergoes a halogen-metal exchange to give the acetylide anion. This can be quenched with electrophiles.
I suppose you could call the first step a Wittig variation.

Title: Re: wittig reaction?
Post by: AlphaScent on January 07, 2015, 11:33:06 AM

Thanks Disco!

I realized that once I read a bit further and looked at the mechanism.  My fault.

Title: Re: wittig reaction?
Post by: kriggy on January 07, 2015, 12:54:42 PM

Thanks guys :)

Quote from: discodermolide on January 07, 2015, 11:19:59 AM

Effectively it has to be an aldehyde. You need the beta H for elimination to the bromoalkyne which then undergoes a halogen-metal exchange to give the acetylide anion. This can be quenched with electrophiles.
I suppose you could call the first step a Wittig variation.

But I could do it with ketone If I dont want to make the alkyne (for example for some substitution etc..)?

Title: Re: wittig reaction?
Post by: discodermolide on January 07, 2015, 02:24:51 PM

Sure it should work with a ketone. Study the mechanism in the link given above by critzz.

Title: Re: wittig reaction?
Post by: Plumbum on January 08, 2015, 08:40:58 AM

If this was an corey fuchs, which i suppose because i looked it up ;) why isnt this product weve got here reacting to an alkyne? I thought thats the whole point of  corey fuchs.

Title: Re: wittig reaction?
Post by: discodermolide on January 08, 2015, 08:54:38 AM

Quote from: Plumbum on January 08, 2015, 08:40:58 AM

If this was an corey fuchs, which i suppose because i looked it up ;) why isnt this product weve got here reacting to an alkyne? I thought thats the whole point of  corey fuchs.

To do that you need a strong base and a proton on the carbon beta to the bromine. This, as posted, is just a "bromination".