Experimental Wittig reaction question

[faumember89]

Hey everybody,

 

I'm an undergraduate organic chemistry student at FAU, and part of my research is new experimental methods. All that's fine, but my problem is much more basic at the moment. I'm doing a basic Wittig reaction to produce stilbenes, and the reaction is essentially thus: Dissolve a small mass of benzyltriphenylphosphonium chloride in methylene chloride, add in a catalytic amount of solid sodium hydroxide (there is a color change to bright yellow/orange), stir, and then over time add a stoichiometric amount of benzaldehyde. The reaction takes 15-30 minutes as the color fades away, and the products are cis- and trans- stilbenes.

 

My problem is that I keep getting unreacted benzaldehyde in the product, and I don't know how to purify it out. I've tried doing the reaction with a slight excess of benzyltriphenylphosphonium chloride, I've tried letting the reaction run overnight, and no matter what it seems that I keep getting benzaldehyde in the product. Is there something I can do to ensure that all the benzaldehyde reacts? If there's nothing I can do procedurally, is there any technique I can use to purify out the benzaldehyde afterwards?

 

Thanks in advance.

[Tartaglia]

Use a stronger base or a solvent in which NaOH is more soluble - thf for instance.

Afterwards how about adding a mild oxidant and washing the carboxylic acid out with base

[louis wu]

You need a solvent extraction. Dry your product thoroughly; then from memory something like propan-2-ol or t-butanol might work. Aldehydes do not like polar solvents.

Recrystallise the products from the alcohol.

[UC]

Catalytic NaOH? But you need to deprotonate the benzyl carbon stoichiometrically. You end up with NaCl and H2O from the reaction as the base is consumed. Consider moving to a stronger base such as KOt-Bu or KH, which may necessitate a change in solvent.

 

As for purification, there's always flash chromatography should all else fail. It may also be possible to gently oxidize the benzaldehyde and then shake the organic layer with 1M bicarbonate. Unless you have lots of polar sidechains on the stilbenes, they should greatly favor the organic layer, while the now benzoic acid will be readily removed as the benzoate

[faumember89]

Thanks for the replies, everyone.

 

Do we know for certain that benzyltriphenylphosphonium chloride is soluble in THF? If so, that could help a lot, because it is definitely the case that the NaOH pellets never fully dissolve in the DCM.

 

Oxidation sounds like a definite possibilty towards eliminating the excess benzaldehyde, so I'll look into that as well if I continue to have extra.

[Tartaglia]

Thf is quite a good solvent for these sort of things. You only have to get a little bit in solution anyway.

I think the important point here is the need for a stoichiometric amount of base which UC pointed out and I (ashamedly) missed.

[faumember89]

Yeah, I was following an experiment my prof gave me and assumed it was catalytic because I didn't really think about the mechanism of reaction, but you guys are definitely right. Gonna try it with THF sometime this week and I'll post some exuberant thanks if all goes well!

 

My next step is to make allenes and cumulated systems via the same procedure, the reaction with benzaldehyde is just a mock-up to get the procedure down and my experimental methods polished.

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I'd like to report that the benzyltriphenylphosphonium chloride is poorly soluble in distilled THF as compared to a similar volume of methylene chloride, and I'm not particularly sure why seeing as I thought most reactions that used DCM as a solvent work even better in THF... I can't even get 0.5g of the ylide salt to dissolve in about 15mL of the THF.

[Tartaglia]

It is really the solubility of the base (NaOH, but it would be better to use a stronger one) which thf will increase. Thf is a coordinating solvent and will bind to Na+/K+.It is also not an electrophile and so will not undergo substitution reactions which CH2Cl2 can.

 

I think bearing in mind you must use a stoichiometric amount of base, I would simply use a standard procedure, which presumably you can look up.

 

A long shot here - you aren't doing this for Mike Turner at Manchester are you?

[faumember89]

I understand and believe you that THF will be a better solvent for NaOH but I need a solvent in which benzyltriphenylphosphonium chloride and NaOH will be soluble, although there may not be one. I've been looking online and most standard procedures use aqueous 10M NaOH and do the reaction in methylene chloride, but they have some delivery method to get the hydroxide ions into the methylene chloride, something called "aquilat 336" which we don't have in my lab.

 

No, I don't work in Manchester, I'm an undergraduate student at Florida Atlantic University.

[akcapr]

what is the color of your final solution? use only a solution of hydroxide and add that to your phospine chloride, this will create the wittig reagent. then, add your methylene chloride and your aldehyde. this should work properly. and then extraction can be used to isolate the product