ORGLIST: Hydrogenation of benzylcaronyloxy(CBZ) protection group

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From: Jonas Nilsson (jonni$##$ifm.liu.se)
Date: Thu Aug 30 2001 - 09:59:13 EDT


Hi.
I'm having some problems with catalytic hydrogenation of Z-protected
amidine.
In a series of compounds I'm currently working on, my final step is
deprotection of the amidine.

The procedure involves treating my compounds with hydrogen gas in the
presence of Palladium 10% on activated carbon. I use ethanol with water
(5-10%) as solvent. Most of the compounds in the series goes without prob=
lem
giving the desired amidine in good yield. However the last one seems almo=
st
impossible to deprotect in good yield.

This is to me quite confusing since there is very little difference in th=
e
structures, and no difference at all in the CBZ-part.

The general reaction look like this:

XN-NRCO-CH2-C(OR)-R-Ar-C(=NZ)-NH2 =>
                        R-X-NH-NRCO-CH2-C(OR)-R-Ar-C(=NH)-NH2

For the X part I've used:
alkyl-
dialkyl-
aryl-
dialyl-
acyl-
R-NH-CO-

They all worked fine. My latest addition was X=Ph-CH2-SO2- and this has
given me some problems.
From a 1/2-1 hour reduction I was now forced to reduce for 2 days with 3
additions of Pd/C to achieve approx. 50% of product among with 20%
startingmaterial and 10% of product missing the X part completly.

So, we all know that sulfur poisons palladium, but I've never heard of an
oxidized sulfur working as a poison.

I now turn to you and hope that you may shed some light on this problem

Best regards
Jonas Nilsson
__________________
Jonas Nilsson
IFM Kemi
Linköpings Universitet
013-285690

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