[zq]

I am trying to selectively protect the benzimidazole NH in the presence of a phenolic OH grup in the same molecule. I tried BOC anhydride without a base but it did not work. Can you please help me in this regard?
I need a protecting group for the benzimidazole NH which is stable to a base (K2CO3 or Cs2CO3) as I have to do O-alkylation of the free phenolic OH group in the next step.

[agrobert]

The reason you BOC reaction did not work (in general) is because you need a base (NaHCO₃) typically when your amine is hindered or not primary.  The N-H is not very nucleophilic and NaHCO₃ will not deprotonate your phenolic moeity.  You need to understand functionality before running a reaction.  Generally the proton that is abstracted from benzimidazole is in the 2 posiiton.

That being said your nitrogen is not nucleophilic.  I don't think you need to protect it, but you need to make sure whatever reaction you run for the phenolic group you do not deprotonate the 2 position which is a very stable anion.

Also think about this, if you were to make the BOC protected phenol how stable do you really think it would be, not very.

Lots of chemistry and problems going on with this simple system.  Have further questions?  Think about it.

Andrew

[zq]

Thank you very much.

[zq]

Now I could successfully selectively protect the benzimidazole NH in the presence of phenolic OH with BOC anhydride (1 eq) using triethylamine (1 eq) and DMAP (1 eq) in DMF (rt, 1 h, excellent yield). Thanks again Andrew.

[agrobert]

Now I could successfully selectively protect the benzimidazole NH in the presence of phenolic OH with BOC anhydride (1 eq) using triethylamine (1 eq) and DMAP (1 eq) in DMF (rt, 1 h, excellent yield). Thanks again Andrew.

Good Job.  In the future a system with TEA and DMAP requires only a catalytic amount of DMAP, generally 0.2 equiv or less.  Of course the reaction will proceed with TEA alone but it will take longer.  Just to let you know because DMAP is somewhat expensive and 1 equiv is not really necessary.