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1911
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[*] posted on 22-7-2011 at 07:03
Transfer hydrogenation


I have done some investigation about catalytic hydrogenation. And now it seems as I don't have access to the palladium metal (or rather to the economic resources to obtain it:)) that I have to go with Raney nickel. However the reactions applying Raney nickel as a catalyst need to be carried out at much higher temperatures and pressures of H2 which aren't practical for a normal amateur chemist so I'm wondering if transfer hydrogenation of simple carboxylic acids with either hydrazine or isopropanol with Raney nickel would proceed at ambient pressure and sensible temperatures.
I was unable to find any relevant information on google or previous threads about temperature and pressure needed for CTH so any help would be appreciated.
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[*] posted on 22-7-2011 at 08:49


It would make it easier to answer your question if you told us what type of functionality you want to reduce. ;)

Here is an example of a nitrile to aldehyde reduction with Ni-Al alloy at normal pressure using formic acid as hydrogen donor. If you read the whole article you will see that Raney nickel can work too. I would attach the articles referred there but I don't have J. Chem. Soc available online.

http://orgsyn.org/orgsyn/prep.asp?prep=cv6p0631
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[*] posted on 22-7-2011 at 10:52


You may be interested in reading up on Urushibara nickel. It's easier to prepare than Raney nickel, and has a lot of the same properties. Of course, people here tend to not have such good luck with it, BUT if nothing else, it's an easy way to make nickel powder. When making Raney nickel, it's kind of important to start with nickel powder so you get a homogenous alloy, otherwise most of your nickel will be in chunks.

Once you've made the nickel-aluminum alloy, you can then use that directly as your catalyst in the reaction with formic acid in a sealed vessel. The aluminum will react with the acid to generate hydrogen, which will raise the pressure in the vessel and provide an additional source of hydrogen for the reaction.

Reducing carboxylic acids isn't easy though. There's a reason LAH is used so much for that purpose, because few other things work.

You might also want to look into Kolbe electrolysis. This can allow you to substitute a carboxyl group for some other group, by selecting some other acid. ie, you can do the electrolysis in GAA to replace the carboxyl group with a methyl group.
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[*] posted on 22-7-2011 at 11:27


Quote: Originally posted by 1911  
hydrogenation of simple carboxylic acids

Rhenium oxides are the best catalysts for the reduction of acids to alcohols. Water aids the reduction. Ruthenium catalysts and mixed ruthenium-rhenium catalysts can be used; but under more vigorous conditions.
For simple carboxylic acids, rhenium catalysts are the best. But for small-scale preparations, LiAlH4 of borohydrides are probably more-suitable. I attach a paper on Rhenium oxides anyway. There are a number of other papers in this series which are very good also.
Nickel won't work under sensible conditions; nor will palladium to my knowledge.



Attachment: Rhenium Oxide.pdf (138kB)
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[Edited on 22-7-2011 by tmb]
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[*] posted on 22-7-2011 at 12:32


Thank you for your answers.
Kolbe electrolysis isn't actually a reduction but decarboxylation. With two substrates it produces a mix of three different products so the yield would be really low.
Lithium aluminium hydride was on my mind too but I find that way too expensive for any scaled up production.
Also as I find palladium too expensive I wouldn't even look towards rhenium. I don't even know where to get such metal.
Now it looks that I have two options: either CTH with unknown catalyst and hydrogen donator or reacting the acid with thionyl chloride followed by reduction with sodium borohydride (which could prove to be quite expensive too for I would have to make that hydride myself:().

So I would still give the transfer hydrogenation a thought.
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[*] posted on 22-7-2011 at 13:02


Catalytic reduction is the most cost-effective means of reducing large quantities; despite the expense of the metal. Ruthenium trichloride is not very expensive, and ruthenium catalysts will work under easily-achievable conditions on the free-acid. If you could convert the acid to an ester, this would of course widen your options - both for catalytic and borohydride reductions. Ruthenium has a tendency to destroy phenyl groups, however - either by reduction to the cyclohexane, or to the carboxylic acid - so depending upon your unnamed substrate, this may not be suitable. Catalysts may also be regenerated - whereas hydrides cannot.
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[*] posted on 22-7-2011 at 13:08


True, kolbe electrolysis isn't a reduction per se, but depending on what you need, it can give you the same products as a reduction. Typically, you use one of the acids in excess, which can be practical if the acid is really cheap like GAA. Since the acetic-acetic reaction product is ethane, that just evaporates, and because there is way more acetic acid than your other acid, there won't be too much of its dimer.

Rhenium is definitely hard to find in small quantities. I wonder if it would be practical to buy like a kilogram and sell it in small portions on ebay?
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