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Nicodem
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In that US3187047 there is no literal mention of water as such being added intentionally and separately. It only describes what materials the
autoclave "was charged with". Those skilled in the art will recognize that the amount of water is roughly that which inevitably comes with the Raney
nickel suspension (300 mL water per 500 g Raney Ni). I take it that you never worked with Raney nickel? Well, this catalyst is prepared or acquired in
the form of an aqueous slurry which must never be allowed to dry or else it immediately catches fire.
The little amount of water introduced this way will undoubtedly have influence on the ketone/ketimine equilibrium, especially because the equilibrium
for ammonia derived ketimines is already in favor of the ketone, but this must not be much of a problem since ketones reduce more slowly than
ketimines/ketiminiums over Raney Ni. The fast equilibration is assured by the use of the buffer system. So, as it is usually the case with reductive
aminations and many other reactions, the unfavored equilibrium of one reaction is compensated by the favored equilibrium of the other reaction, so
that the selectivity is as it is.
In any case, the effect of water content on the reaction selectivity can in no way be compared between:
a) ammonia and methylamine (or different amines in general), because the ketone/ketimine equilibrium is totally different;
b) ketones and aldehydes (or even among different types in each class) for reasons similar as above;
c) different types of reductive aminations, because the substrate selectivity for the ketone/ketimine/ketiminium is widely different among the
reducing reagents (such as for example, between borohydride and hydrogen over Raney Ni; or think of extremes such as NaCNBH3 vs. NaBH4 if you prefer).
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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chemrox
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Thanks for the explanation. Why are you calling the last two examples "extremes?" Did you mean in terms of selectivity or had you something else in
mind?
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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chemrox
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I used a Zn dust procedure for a dehydrochlorination and the workup was ghastly!
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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chemrox
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Here's a review of STAB applications
Attachment: STAB review.pdf (1.9MB)
This file has been downloaded 8815 times
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Palladium
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Quote: Originally posted by Nicodem  |
In that US3187047 there is no literal mention of water as such being added intentionally and separately. It only describes what materials the
autoclave "was charged with". Those skilled in the art will recognize that the amount of water is roughly that which inevitably comes with the Raney
nickel suspension (300 mL water per 500 g Raney Ni). I take it that you never worked with Raney nickel? Well, this catalyst is prepared or acquired in
the form of an aqueous slurry which must never be allowed to dry or else it immediately catches fire.
The little amount of water introduced this way will undoubtedly have influence on the ketone/ketimine equilibrium, especially because the equilibrium
for ammonia derived ketimines is already in favor of the ketone, but this must not be much of a problem since ketones reduce more slowly than
ketimines/ketiminiums over Raney Ni. The fast equilibration is assured by the use of the buffer system. So, as it is usually the case with reductive
aminations and many other reactions, the unfavored equilibrium of one reaction is compensated by the favored equilibrium of the other reaction, so
that the selectivity is as it is.
In any case, the effect of water content on the reaction selectivity can in no way be compared between:
a) ammonia and methylamine (or different amines in general), because the ketone/ketimine equilibrium is totally different;
b) ketones and aldehydes (or even among different types in each class) for reasons similar as above;
c) different types of reductive aminations, because the substrate selectivity for the ketone/ketimine/ketiminium is widely different among the
reducing reagents (such as for example, between borohydride and hydrogen over Raney Ni; or think of extremes such as NaCNBH3 vs. NaBH4 if you prefer).
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If you say that this water is nessecary to prevent the RaNi from selfignition, I believe so. I have never used RaNi. Still, I'm a bit confused; I have
actaully seen RaNi reductions in US Patents before, where it was pointed especially that no water except that of being produced by the reaction itself
was allowed.
I also know that amination with ammonia or methylamine can't be compared, but my point was that I have seen water in both - I can't just find this US
Patent I have read with 40% methylamine water solution. Anyway, the secondary amine seems to have easier being formed in many aspects - maybe I'm
wrong.
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Nicodem
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| Quote: Originally posted by Palladium | | Still, I'm a bit confused; I have actaully seen RaNi reductions in US Patents before, where it was pointed especially that no water except that of
being produced by the reaction itself was allowed. |
Would you kindly stop it with this annoying habit and start using references? I can't do guesswork all the time. This last guesswork of mine is in
that perhaps you saw a Raney Ni catalysed reductive amination with alcoholic ammonia in the absence of any buffer system. In such a case, the ammonia
would better be as anhydrous as possible and in a very large excess or else a poor primary vs. secondary amine selectivity is likely to occur.
| Quote: | | I also know that amination with ammonia or methylamine can't be compared, but my point was that I have seen water in both - I can't just find this US
Patent I have read with 40% methylamine water solution. Anyway, the secondary amine seems to have easier being formed in many aspects - maybe I'm
wrong. |
I already addressed this exact thing in my previous post. Water is of little issue with the more nucleophilic amines that relatively readily form
imines. This should be kind of obvious. That's why I said that it makes no sense to compare reductive aminations with amines with those that use
ammonia.
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Palladium
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I'm sorry for the unpleasant behaviour I've punished everybody with here. I think your humour is the highest priority for this change I immediately
will do.
http://www.google.com/patents/US4766245?printsec=abstract#v=...
Here we got a US Patent where they using RaNi anhydrous. I'm very clear over that this is performed with ammonia, not its salt. The point I would like
to reach here, is that those skilled in the art succeeds in avoiding water even though it's RaNi. They are drying the RaNi here, and this was my
thought about the former US Patent - because of this I believe that the amount of water is negligible in US Patent 3,187,047. Probably it's
intentionally.
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Nicodem
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You are now comparing two very different reactions. The US3187047 descrbes a reductive amination of ketones with ammonia (in the form of the acetate)
using hydrogenation over Raney Ni, but the US4766245 describes the classical alkylation of ammonia with alcohols catalysed with some "almost" Raney Ni
type of catalyst. This last is a common industrial reaction for preparing simple amines from alcohols, though in this specific case it is done on an
oligomeric alcohol. This is not a reductive amination of ketones even though part of the reaction mechanism is very similar. The principle on which
this type of reactions are based is called borrowing hydrogen.
Raney Ni can be made almost anhydrous relatively simply. Everyone can have his own technique to do this, but what I do is to hold a strong magnet on
the outer side of the autoclave in order to thoroughly wash the commercial catalyst slurry by decantation once or twice with the solvent I use for the
hydrogenation. This way you don't need to wash it on a filter under inert atmosphere, yet still don't risk to dry it out and start a fire. The magnet
is for forcing the sedimentation of the smaller particles which otherwise easily go into suspension and could be lost while decanting. Otherwise it is
not really necessary.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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