[napoleon79]

I want to make practice of wittig reaction from 5-hydroxyltriphenylphosphonium bromide, KHDMS in toluene, THF, unstaturated conjuntion aldehyde (2-nonenal), -78 oC - 0 oC. But I don't make it. I don't known that what happen is hydroxy group (-OH) for this reaction ?.

please give me some advice ? If you is me, how will you make it ?

thanks 

Now, I understood that Practice in  the lab is more difficult than mechanism in the books.

Hic hic

[opsomath]

KHMDS will immediately react with a hydroxy group to give an alkoxide. Could you correct your chemical name so we know what you're actually doing? Is it 5-hydroxypentyl triphenylphosphonium bromide? You are missing a group.

Anyway, you might have to go with a protecting group or use a less basic reaction. I have used refluxing potassium carbonate in 50% dioxane/water for a one-pot Wittig before, with triphenylmethylphosphonium iodide and p-nitrobenzaldehyde to make p-nitrostyrene. That should be compatible with your alcohol, although it might tear up your unsaturated aldehyde.

[napoleon79]

if I protects -OH group by THP for 5-bromopentanol, it isn't form to THPO-CH2-CH2-CH2-CH2-PPh3 Br. I read some magrazines, some authors talk that (you look under picture).





But my lab don't has PPTS. I have been tried t-BuOK and KHDMS but the product isn't form.
The product want to (5Z,7E)-tetradeca-5,7-dien-1-ol, but this wittig isn't.

thanks

[opsomath]

If you don't have PPTS, what are you using as a catalyst to put the THP group on your alcohol? Have you confirmed that you are making the THP protected product, with spectroscopy or anything? Better check that.

[OC pro]

You have to use 2-fold excess of base!! First equivalent will deprotonate the alcohol, second one will make the phosphonium ylide.

You can use also use a toluene/aq. NaOH solution with phase transfer catalyst. Works always well with my Wittig´s.

[napoleon79]

You have to use 2-fold excess of base!! First equivalent will deprotonate the alcohol, second one will make the phosphonium ylide.

You can use also use a toluene/aq. NaOH solution with phase transfer catalyst. Works always well with my Wittig´s.

Thank you for me some advices.

But I don't try with phase tranfer catalyst, I don't have any document for phase transfer catalyst in wittig reaction.

I need product Z-alkene. If you do by phase tranfer catalyst, How is this alkene ? (Z : E)

If you make wittig reaction with KHDMS/THF, some NaBr ? what happen is this reaction ?

What advantages is phase tranfer catalyst in this wittig reaction of conjugate aldehyde reagent ?

what difference is aldehyde in to bottom then salt and catalyst and on the contrary ? I read some discuss in some forums. I don't understand different them.

[napoleon79]

I am having big problem for wittig. It is wittig reaction : 5-hydroxytrphenylphosphonium and 2-trans-nonenal. I did some wittig reaction that is not bad. But This is no reaction, I don't understand what happen ?
I think that strong base enviroment is R-OH group to R-O- (alkoxide).
I also don't know different between LUMO of aldehyde conjunction and aldehyde unconjunction when we do wittig reaction with HOMO of 5-hydroxytrphenylphosphonium bromide. I don't have avisor sofware for quantum calculation HOMO and LUMO energy of them. If you have any sofware for quantum calculation, please you calculate them for help me.
My professor don't want me protection -OH group.
I am worry my future if i do good this reaction, i will pass this exam of university.

Please help me some advice.

thanks for anyone saw my topic so they want to help me.

thank you   

[napoleon79]

At the moment I have a big problem with wittig reaction. This is wittig reaction between wittig reaction : 5-hydroxytrphenylphosphonium and 2-trans-nonenal. I was success with some other wittig reaction, but in this particular it was not happen. I don’t understand why ?
I think with a strong “base” habitat R-OH group will become R-O (alkoxide). And it will affect to this reaction.
However, I don’t understand why it influences to this reaction and make the reaction failure.
I also don't know different between LUMO of aldehyde conjunction and aldehyde unconjunction when we do wittig reaction with HOMO of 5-hydroxytrphenylphosphonium bromide. I don't have avisor sofware for quantum calculation HOMO and LUMO energy of them. If you have any sofware for quantum calculation, please you calculate them for help me.
My professor don't want me protection -OH group.
I am worry my future if i do good this reaction, i will pass this exam of university.

Please help me some advice.

thanks for anyone saw my topic so they want to help me.

thank you   

[james_a]

Wittig reactions can be difficult when you're dealing with alpha-beta unsaturated systems due to competitive 1,4 addition. The aldehyde *should* win out, you might also be getting addition to the beta carbon.

[napoleon79]

You have to use 2-fold excess of base!! First equivalent will deprotonate the alcohol, second one will make the phosphonium ylide.

You can use also use a toluene/aq. NaOH solution with phase transfer catalyst. Works always well with my Wittig´s.

I don't have experience about toluene/aq. NaOH solution with phase transfer catalyst. If we use this catalyst, it is effect to 5-hydroxytriphenyl phosphonium bromide. Do you have any document about toluene/aq. NaOH solution with phase transfer catalyst, please give me reference. 

thank you for me good advice.