[junjiefu]

RS-SR + NBS  RSOSR

Disulfide is oxidized to thiosulfinic ester using NBS.
What's the mechanism of this reaction?

[Alwin Kristen]

Radical?

[critzz]

What is the oxygen donor in that reaction?

[junjiefu]

What is the oxygen donor in that reaction?

That's the question that puzzles me the most.
It's an aerobic reaction, so I think the only O donor is the oxygen in the air.

[Dan]

It's an aerobic reaction, so I think the only O donor is the oxygen in the air.

I think it's probably water.

[junjiefu]

It's an aerobic reaction, so I think the only O donor is the oxygen in the air.

I think it's probably water.

Thanks a lot for your reply!

The whole reaction is (from an old paper)

R-S-S-R + MeOH + NBS  [R-S-SO-R]  R-SOOMe

The final product is methyl sulfinate, and RS-SOR is supposed to be the intermediate.

It seems no water is employed, and the only solvent is MeOH.

[Dan]

MeOH could still be the O source.

Start at the beginning - forget about RSS(O)R for now, what do you think happens first when RSSR and NBS react?

[junjiefu]

MeOH could still be the O source.

Start at the beginning - forget about RSS(O)R for now, what do you think happens first when RSSR and NBS react?

Does it form RS⁺Br^-SR? Well, I am not sure. 

[Dan]

Does it form RS⁺Br^-SR? Well, I am not sure. 

Why is the Br charged? Can you draw a curly arrow mechanism?

[junjiefu]

Should it be this?
You are right, the Br is not charged.
What's the following mechanism?

[Dan]

What might be attracted to S⁺?

[junjiefu]

Honestly, I don't know. 

How to analyze which path is possible? Many thanks!

[Dan]

Both possible and reversible.

For your product of methanol addition, what do you think the pKa the OH is? Any bases around?

[orgopete]

I cannot vouch for a complete absence of water, but no matter. One can get there either way. I'd be more inclined to have methanol attack the unoxidized sulfur to give two new sulfides. RSOH is noted to react with itself to form RS(=O)SR + water. So sulfur is still pretty nucleophilic. If no water, then form CH3Br. Give that a go.

[junjiefu]

Both possible and reversible.

For your product of methanol addition, what do you think the pKa the OH is? Any bases around?

Sorry, I don't quite understand what you mean. Can you draw the whole reaction mechanism? 
I really appreciate your kind help. Maybe my organic chemistry is too bad