RE: ORGLIST: Platinum

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From: Wim Verschueren (Wim.Verschueren$##$tibotec.be)
Date: Fri Jan 26 2001 - 10:57:13 EST


Hello,

Phenyl (and other aromatic systems) rings are mostly not reduced by
platinum. For this rhodium is the catalyst of choice.
Rhodium, platinum, ruthenium and palladium can easily be weighed in open air
but must be mixed with the solvent under nitrogen. This is best accomplished
by flushing the flask with nitrogen adding the catalyst and then the solvent
in this order.The highest risk of combustion occurs after the hydrogenation
when the catalyst is filtered off.The catalyst is then saturated with
hydrogen and wet from the solvent.I have seen spontaneous combustion several
times so the risk is real.
Raney nickel is the worst catalyst in this manner.This will always ignite in
contact with air and is therefore kept under water. When filtering keep in
mind that also the water is removed...
Filtration off all this catalysts must always be performed under nitrogen
and flush also the flask with nitrogen while filtering.

With platinum generally more selectivity is possible. (for instance to keep
aromatic chlorines or bromine)In most cases imines and hydrazones will be
reduced but aldehydes and cbz often not when a catalyst poison like
thiophene is added.

Wim Verschueren

===========================================
Wim Verschueren
Tibotec
Medicinal Chemistry Department
Gen. De Wittelaan L11B21
2800 Mechelen Belgium
e-mail:Wim.Verschueren$##$Tibotec.be
Tel: (32)15/293152
Fax: (32)15/293192
============================================



-----Original Message-----
From: owner-orglist$##$dq.fct.unl.pt [mailto:owner-orglist$##$dq.fct.unl.pt]On
Behalf Of Jonas Nilsson
Sent: Friday, January 26, 2001 4:29 PM
To: Multiple recipients of list orglist
Subject: Re: ORGLIST: Platinum


Is it true that phenyl rings might be readily reduced at atmospheric
pressure by H2/Pt?
/jN
 _____________________ _____________________
| Jonas Nilsson | | |
|Linkoping University | | Telephone |
| IFM | | --------- |
| Dept. of Chemistry | | work: +46-13-285690 |
| 581 83 Linkoping | | fax: +46-13-281399 |
| Sweden | | home: +46-13-130294 |
|_____________________| |_____________________|
----- Original Message -----
From: "Chris Borella" <cborella$##$ic.sunysb.edu>
To: "Jonas Nilsson" <jonni$##$ifm.liu.se>
Sent: Friday, January 26, 2001 4:07 PM
Subject: Re: ORGLIST: Platinum


> Hi,
> I have used Pt on carbon a couple of times and never had any problem with
it
> combusting. As far as the selectivity I am not sure but it will
hydrogenate
> most phenyl rings to cyclohexanes readily.

> From: Jonas Nilsson <jonni$##$ifm.liu.se>
> To: Multiple recipients of list orglist <orglist$##$dq.fct.unl.pt>
> Sent: Friday, January 26, 2001 3:19 AM
> Subject: ORGLIST: Platinum
>
> > Hi
> > When handling Palladium (5% on activated carbon) in small amounts
(<10mg),
> > my experience tells me that there is seldom any problem with spontanious
> > combustion, and the need for caution is quite limited.
> >
> > Now, I need to use Platinum (5% on activated carbon) and there is a
large
> > warninglabel stating spontanious combustion on the package. Is the need
> for
> > caution more pronounced for Platinum than for Palladium?
> >
> > And another question:
> > Is there a general trend of activity or selectivity that you know of
using
> > Palladium or Platinum for catalytic hydrogenation. The functionalities
> which
> > are the concern are Cbz protectiongroups, aldehydes, imines and
> hydrazones.
> > Any difference in reactivity using Pd or Pt?
> >
> > /jN


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