DMF as a wittig solvent

[big314mp]

We just did a Wittig reaction in Ochem lab, and the solvent we used was DMF.

 

Looking at the structure of DMF shows it has a carbonyl, similar to the carbonyl of the substrate the Wittig reagent was attacking (cinnamaldehyde).

 

My question is, why doesn't the DMF react with the wittig reagent and ruin the reaction?

[hermanntrude]

wittig reagents generally react faster with aldehydes than with ketones IIRC

 

edit: it might even be that wittigs don't react with ketones at all... i'd have to look it up... i think they do, but they'd react preferentially with an aldehyde

[Tartaglia]

Wittig's definitely react with ketones.

 

The reason that DMF probably didn't react is there is a contributing resonance form of the type Me2N+=C(O-)H, which renders it less susceptible to nucleophilic attack

[big314mp]

Thanks!

 

So its just that the nitrogen slows down the attack on DMF, so the other aldehyde is preferentially reacted.

[Tartaglia]

Probably, though I'm far from certain.

 

Amides can react with Wittig reagents in the usual fashion. There is an example on page 853 of "Advanced Organic Chemistry" (3rd edition) by Jerry March, but I suspect the rate is significantly slower