[Nescafe]

Hi,

Looking at the swern oxidation one of the key steps on the mechanism is deprptonation of the methyl proton in the sulfur ylide, taking into account that the pka of a methyl proton is 50, that of DMSO methyl is 35, how is it that triethyl amine (pka 10) is able to abstract that proton? Do I have the pka of the sulfur ylide wrong? I know it's around 35 for DMSO but perhaps is more acidic in the ylide?

Nescafé

[orgopete]

I don't know the pKa, but it is more acidic.

[discodermolide]

I do not think it is the methyl group that is deprotonated.
The Swern oxidation uses DMSO/Oxalyl chloride.
Hint: Take a look at the mechanism.

[Nescafe]

I do not think it is the methyl group that is deprotonated.
The Swern oxidation uses DMSO/Oxalyl chloride.
Hint: Take a look at the mechanism.

I did, prior to posting the question.

http://www.organic-chemistry.org/namedreactions/swern-oxidation.shtm

Nescafé.

[discodermolide]

I do not think the sulphur ylide is deprotonated. The triethylamine removes the S⁺-O-C(R)₂H proton, forming the carbonyl and eliminating dimethyl sulphide.
As you say triethylamine is not strong enough to remove one of the methyl protons of the S-Me group.

[orgopete]

I do not think the sulphur ylide is deprotonated. The triethylamine removes the S⁺-O-C(R)₂H proton, forming the carbonyl and eliminating dimethyl sulphide.
As you say triethylamine is not strong enough to remove one of the methyl protons of the S-Me group.

It appears you are suggesting that of R₂CHOS⁺(CH₃)₂, the R₂CH- is the most acidic and deprotonation occurs by triethylamine over the sulfonium methyl groups. I think that is incorrect.

I was taught the ylide mechanism in grad school, but I don't have the references to prove it. However, if you have a reference to direct deprotonation, please cite it.

[discodermolide]

I too was taught the ylide mechanism, but it doesn't mean I think it is correct. You have said this yourself many times about other aspects of organic chemistry.
I do not have access to the literature. However, do you think that the pKa of the ylide methyl hydrogen moves all the way down to that of triethylamine? Is there any literature you have that can demonstrate this, I don't have any, and with no access I can't remedy this. This elimination via the ylide requires a "5-membered ring intermediate" how likely is that in this case?
Do you even require to deprotonate this methyl group to eliminate DMS?

[Dan]

There has long been good evidence for the ylide mechanism. Deuterium labels α to the OH are incorporated in the eliminated DMS - which supports D abstraction by an ylide, and not direct deprotonation.

See for example Torssell, K. Tet. Lett. 1966, 7, 4445-4451

To my knowledge, there are no reports of evidence for a direct deprotonation as you suggest. I am not suggesting this means there aren't any though...

[discodermolide]

Thank you Dan.