Chemistry Forums for Students > Organic Chemistry Forum for Graduate Students and Professionals

Stubborn Cbz??

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aranel:
i have this compound with a cbz which refuses to be cleaved off. tried 33% HBr in AcOH. even left overnight. got my still protected compound back, and a tiny amount of deprotected compound. tried hydrogenation with Pd/C. no change. added some more Pd, thinking the catalyst might have been poisoned or so. nothing.

how could this happen? any ideas?? or what else i could try you think?  ???

Doc Oc:
How old is your Pd/C?  That stuff does go bad so if you have an old bottle it might be considerably degraded.  Either add a lot more (which should be no problem because you just filter the Pd off at the end anyways) or you can try a different Pd source, I've actually done this deprotection with Pd(OH)2 and a hydrogen balloon.

aranel:
it is less than 2 years old. is that enough time for the Pd to go bad?

i thought that for the HBr in AcOH i used. so ordered a new bottle. but made hardly any improvement.

so you think it must be something with the reagents right? i cant see any reason for the compound to be stable to those conditions :-\

Doc Oc:
Yes, I would say that's enough time for the Pd to go bad, but it depends on how it's been stored.  If it's just sitting on a shelf, then it's probably bad.  If you're careful with it, parafilm it, and throw it in a dessicator, then maybe the substrate is stubborn.  Either *Ignore me, I am impatient* the amount or try a different Pd source.

Haven't done HBr deprotection so don't know about the ins & outs of that.

This definitely does sound reagent based and the Pd/C immediately sent up a red flag for me because I know from experience that stuff doesn't keep.  However, if multiple deprotection methods aren't working, it could be substrate problems.  If you're allowed, maybe posting the rest of the structure would help (although I don't think it should matter that much).

aranel:
not in a dessicator, but parafilmed.

okay then i would go for a different Pd source. if still not working, then think of any other problems releated to substrate.

thank you!

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