[napoleon79]

K2CO3 as acid scavenger. I don't know mechanism of acid scavenger in 5-bromopentanol. When I do 5-hydroxypentyltriphenylphosphonium bromide, I try to add K2CO3. The product isn't bistriphenylphosphonium bromide that it is form 5-hydroxypentyltriphenylphosphonium bromide.

Thank you for me some advices

Please you give me mechanism of acid scavenger.

 

[napoleon79]

The mixture is reflux in CH3CN solution. The mixture is PPh3 and BrCH2CH2CH2CH2CH2OH & K2CO3 (Scavenger acid). I don't understand mechanism it form 5-hydroxypentyltriphenylphosphonium bromide but it is form bis-triphenylphosphonium bromide.

thanks

[discodermolide]

The mixture is reflux in CH3CN solution. The mixture is PPh3 and BrCH2CH2CH2CH2CH2OH & K2CO3 (Scavenger acid). I don't understand mechanism it form 5-hydroxypentyltriphenylphosphonium bromide but it is form bis-triphenylphosphonium bromide.

thanks

Is it to scavenge HBr produced by the cyclisation of your hydroxy-pentylbromide? (which would give the pyran?)

[napoleon79]

Ok. I agree with you. If I give K2CO3 in the mixture then cyclisation will not be form tetrahydropyran. So the product is not form bis-triphenylphosphonium bromide.
But I don't know mechanism of it. what happen ?

What pH is K2CO3 in CH3CN solution ?

If K2CO3 reacts with HBr of cyclisation 5-bromopentanol, this reaction will be form tetrahydropyran. So the journal is wrong. The product is bis-triphenylphosphonium bromide.

My practice in my lab. I give K2CO3 into the mixture of PPh3, CH3CN, 5-bromopentanol. The product is 5-hydroxypentylphosphonium bromide. But I don't know explain this mechanism.

Thanks 

[orgopete]

I'm not sure what is being attempted here. I also haven't synthesized a Wittig reagent before, so I am only guessing. I don't see why K2CO3 would have been added. This is what I would expect. If you were to react triphenylphosphine with the bromoalcohol, it should form the phosphonium salt. I am guessing this may be insoluble in the acetonitrile, but I don't know that. If it were insoluble, I don't know why potassium carbonate would have been added as it too would be insoluble. At this point, it would be a mixture that I would not want.

Re: mechanism, this should be a simple SN2 substitution reaction.

Perhaps someone with experience in preparing phosphonium salts for Wittig reactions could provide some help here.

[discodermolide]

Normally you just have to take the halide and Ph3P in a solvent and heat if required. The phosphonium salt usually precipitates out.
In this case the bromide would be prone to cyclise to the pyran.

I would personally protect the OH group as a THP ether or a TBDMS ether (TBDMSCl/Imidazole/DMF, RT), then make the salt, do the Wittig and remove the OH protection.