mnick12
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home brew silica supported hydrogenation catalysts
Hello everyone heres what I have been working on lately, its a continuing project and I am open to suggestions.
Attachment: for sci mad.pdf (280kB)
This file has been downloaded 1951 times
I really have to give thanks to Klute (where has he been!?) for the inspiration. If you haven't already go look at his thread about Cu on SiO2.
comments and suggestions are welcome.
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DJF90
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Very nice mnick, its just a shame it didnt work. If you still have some of the pre-reduced catalyst on hand would it be possible for you to attempt
the reduction again, if only to observe your expected colour change from dark brown to black? You might try using a foil tee-pee as advocated by Peach
here, it supposedly works very well.
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smuv
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You stirred it right?
Can you describe your hydrogenation apparatus a little better, I am guessing you used a balloon on top of that upper fitting?
The only thing I can add, is pd/c would make short work of this at atmospheric pressure, it might not do it with 100% selectivity, but I imagine the
selectivity would still be quite high if done under mild conditions. Pd/c is very bad at reducing aliphatic ketones. Running the reduction in a
non-polar solvent will further reduce ketone reduction, a little base such as triethylamine will also improve selectivity but might poison the
catalyst too much.
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Magpie
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A noble effort. Do you have some specific ideas on why the catalyst was not reduced?
The single most important condition for a successful synthesis is good mixing - Nicodem
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mnick12
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DJF90- I dont have any catalyst left from the first batch but I do still have all the precursors necessary for its preparation. Also just for the heck
of things I prepared a similar catalyst who's metal composition was 100% copper, I have not yet properly tested it but I intend to in the near future.
But you are right re-trying this experiment would be a good idea. Also the foil teepee sounds like a good idea.
smuv- You are correct, the apparatus consisted of a 250ml rbf (probably a little to big) that T-bore vacuum adapter and a balloon full of H2. The
flask was evacuated a few times then filled with H2, it was then heated under a over pressure of H2. After a while the heat was lowered and a slight
vacuum was re-applied next the substrate was added in solution using a syringe. Next an H2 balloon was placed on and the whole thing was heated for a
certain time. Oh and yes I did stir but I noticed stirring tended to cake up the catalyst on the sides of the flask. I suppose I will finally have to
give in and buy some Pd/C.
Magpie- Good question, this has been something I have been thinking about and I do a have a few ideas. First of all my H2 source was Zn w/ H2SO4 and
as you know H2S is one of the by products of this reaction, and sulfur compounds especially sulfides tend to be very effective catalyst poisons.
Luckily I remembered this during my initial experimentation and switched to NaOH w/ Al as my H2 source which when properly dried is a pretty clean
source of H2, still no reduction occurred. My second thought and perhaps more likely cause was my use of coballt as the primary metal in my catalyst,
after reading some patents and that neet book about catalysts in the scimad library, I discovered cobalt oxides do not reduce to the active metal at
an appreciable rate at temperatures under 400c. So for all my work it could be that I just didnt heat it hot enough. One thing worth mentioning is all
of those metals are variable meaning effective catalysts should be able to be prepared from compositions that are primarily nickel of copper, which is
good since both those metals reduce at a much lower temperature.
thanks again
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Random
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Maybe Nickel Oxide could be reduced with charcoal to make nickel on charcoal catalyst or could you try urashibara nickel catalyst, there is procedure
on rhodium (you just need nickel salt, hydroxide and zinc dust - maybe magneisum dust could also work).
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mnick12
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I've tried urushibara nickel and have had mixed results.
On the subject of nickel oxide I am certain reducing nickel oxide with charcoal would not give a very active catalyst. In order to be effective,
catalysts must meet certain criteria, this includes things like particle size, shape, porosity, and its overall ability to absorb and react with
hydrogen to form the hydride complex necessary for reductions. That is why the author(s) of the patents I based my experiment on co-precipitate the
metal salts onto a porous carrier such as silica gel, diatamaceous earth, alumina hydrate etc. Doing so allows greater control over the average size
of the catalyst and overall metal %. Simply reducing nickel oxide with charcoal may indeed produce nickel, but not in the state necessary for allowing
hydrogenations to occur.
thanks for the idea though
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DJF90
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The following paper may be of interest. Although applied for the conjugate reduction of cinnamate esters, I suspect it may work nicely for the
substrates you're interested in. The solvent they use is dioxane, and whilst you may be able to make some from ethylene glycol, you may find another
solvent will work nicely (I would suggest THF as a starting point).
http://ifile.it/k2dcnte
[Edited on 24-3-2011 by DJF90]
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smuv
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The thing is, it is much easier to get selective reduction of olefin vs carbonyl for cases of carboxylates/esters as opposed to aldehydes/ketones. It
could work, but...there is a good chance it won't provide good selectivity.
Honestly, my recommendation is what lately I have told a lot of people about catalytic hydrogenations. Go to a library, flip through one of the many
good reviews on catalytic hydrogenations. No doubt, many reviews have actual sections just relating to reduction of α,β-unsaturated
carbonyls. I guarantee, no weird catalysts/systems are needed. The great thing is, that heterogeneous catalytic hydrogenation has not changed much
in 50 years...the fundamentals have simply been forgotten.
I also highly recommend, trying an easy hydrogenation, such as styrene to start out. Setup is important, poor setups (among many things) cause
hydrogen leaks and makes it hard to track the reaction. Just the fact that, no mention of vacuum grease was made in the writeup, tells me this was
doomed to failure before it even began.
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DJF90
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Of course, but even in papers dedicated specifically to the reduction of such substrates (i.e. those mentioned by the thread author) overreduction is
still a problem. The attached paper lends itself nicely to the reduction of both compounds, although experimental details are a little vague. The
paper it references on the topic is of little help either. However, they claim a 99% yield of rheosmin from the respective enone using Pd/C in
methanol with 0.1eq AcOH and a hydrogen balloon for 24hrs. Zingerone seems to always be accompanied by its alcohol using this methodology and others
(10mol% raney nickel in methanol or ethanol as example, see US2381210 (attached)).
[Edited on 24-3-2011 by DJF90]
Attachment: Zingerone-Rheosmin paper.pdf (93kB)
This file has been downloaded 1580 times
Attachment: US2381210A.pdf (346kB)
This file has been downloaded 646 times
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smuv
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Good find with that first article. But honestly, if the OP runs this reduction in toluene or toluene with a minimum amount of ether to confer
solubility with say 0.05g 5% wet Pd/C per gram substrate and he doesn't get 100% selectivity for the a,b-saturated ketone, I will buy this whole forum
a beer.
The authors might be good synthetic chemists, but they don't know a lot about palladium reductions. Polar solvents (methanol), Acids (AcOH) and
protic solvents (methanol) are all good promoters of carbonyl reduction. I think the only reason they got good selectivity for rheosmin with such
unselective conditions is that it must be inactive towards reduction (which is more the rule than anything for non aromatic ketones, ex. you can't
reduce acetone or acetaldehyde with Pd/c, but benzaldehyde is a sinch with some over-reduction to toluene if you don't pick your conditions
correctly).
Also, this is stepwise, if a burrette is used to track H2 uptake, their will be no a,b-saturated alcohol.
[Edited on 3-25-2011 by smuv]
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DJF90
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Again smuv, I agree. I recall seeing that aromatic ketones quite readily undergo deoxygenation using Pd/C, AcOH, H2. Klute's work should be followed
if Pd/C is used as the catalyst, at least as a guide. However I beleive the OP's intention was to circumvent the necessity for such a catalyst, and
use something much more available. In that case, assuming Raney Nickel is no problem, I suggest that the patent might be the best guideline.
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