Troubles with thiols

[kelloggs93]

Good morning! I'm a  organic chemistry PhD student and I'm new member of this forum

In the last months I had troubles with a reaction: it was a coupling between a primary amine and the Mercaptopropionic Acid. I obtained the producted that I want, but the yield were very bad (5-10%), and I really don't know why. Did you know if the free thiol terminal groups could give me some troubles? Could this be trapped in the MgSO4 that I use to dry the organic phase after the estraction? Because I really don't know where I can lose everything.

I always work with cuopling reaction usind HATU, HOAt and Collidinen, but I never had a free thiol terminal group.

 

Thanks to everyone!

[hypervalent_iodine]

Unless you are using a lot of MgSO₄, I don't see it being the problem. The thiol may be a problem. Have you looked through the other parts of your reaction to see what else is present? 

[kelloggs93]

Yes, tomorrow I'll try not to use MgSO4, but I just want to understand what happened

I looked to the other parts, sometimes we saw the starting materials, sometimes some subproducts of the coupling reagents (and we always see them when we use the copuling reagents) and one time a understandable product (we did mass analisys, we thougth that It could be the thioester or the disulfide bridge, but it wasn't).

I'll hope that without using MgSO4 everything it could be more understandable.

[hypervalent_iodine]

I honestly don’t think that removing MgSO4 is going to change much given how low your yields are. How soluble is everything in your reaction? Have you tried altering the conditions at all? How do you monitor the reaction?

[kelloggs93]

The condition of reaction are:

- 1 equivalent of amine (important, is a TFA salt)

-1.2 equivalent of mercaptopropionic acid

-2 equivalent of Collidinen

-2 equivalent of DIPEA

-1.1 equivalent of HATU

-1.1 equivalent of HOAt

-Solvent: DMFdry/DCMdry 1/3 (in order to have all the coumpond soluble)

I tried different procecure, the last time I did: In a two-necked flask I put the amine, the coupling reagents and the base. I put the solvents, I degased them with three cycle of Vacuum/Argon, and I put the flask in the ice fot 15 minutes. After this, I added the mercaptopropionic acid - I made other three cycle of Vacuum/Argon and I made the reaction go to rt.

Quenching: I removed the DCM under vacuum, then do three extraction with EtoAc. I controlled that all the product went in the organic phase with TLC, and the dried the organic phase with MgSO4.

Purification: reverse phase cromatography, solvent water+0.1%TFA and ACN.

I thought that the MgSO4 could be the problem because on TLC after the extraction I saw the product: so I thougt that maybe the thiol could make a complex with Mg2+, but I'm not sure, so I asked for help

[hypervalent_iodine]

I meant temp and time used. 

What is the mass recovery like from the column?

6 hours ago, kelloggs93 said:

thought that the MgSO4 could be the problem because on TLC after the extraction I saw the product: so I thougt that maybe the thiol could make a complex with Mg2+, but I'm not sure, so I asked for help

 

You mean in the combined organic layers prior to drying? If you get product out after drying, I’m not sure that means anything. Is there anything in your aqueous layer that could be product? You can switch to Na2SO4 if you’re concerned.