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Author: Subject: Catalytic Hydrogenation comments......
zed
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[*] posted on 18-9-2008 at 16:07


I've had access to Parr type hydrogenators. Always preferred Brown's method. Quiet, unobtrusive, very effective. At least, very effective for what I wanted to do.

http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1962/84/i14/...

A very clean, low-tech way to produce a lot of product with minimal equipment. Uses a fair amount of Sodium Borohydride, if you are using NaBH4 as your only hydrogen source. But, you can just use Borohydride to generate your catalyst, then use H2 from a different source to run your hydrogenation.

[Edited on 18-9-2008 by zed]
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[*] posted on 18-9-2008 at 16:44


Quote:
Originally posted by zed
I've had access to Parr type hydrogenators. Always preferred Brown's method. Quiet, unobtrusive, very effective. At least, very effective for what I wanted to do.


..........may I ask what's Brown's method?.......solo




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zed
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[*] posted on 18-9-2008 at 16:49


I just added a link to the original post.

Another good point about Brown, you can calculate the amount of hydrogen used, by observing the amount of Borohydride solution that has been consumed.

To produce a really large quantity of product, because the vessel is stationary and accessible.....If you construct it with a few extra inlet and outlet tubes, you can siphon-filter-out reduced product, and inject more material for reduction, while retaining your hydrogen atmosphere and catalyst. So, multiple runs can be made....Until, of course, your catalyst becomes poisoned.







[Edited on 18-9-2008 by zed]
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[*] posted on 19-9-2008 at 11:54


Quote:
Originally posted by gsd
For reduction of C = O to C- H.

2) U-Ni-A : Was nearly as good, could be recycled 10 times but T/P conditons were higher as compared to Pd/C

3) Raney Ni: Could complete the reduction at all. Inspite of using drastic T/P conditions , the reduction proceeded only upto C - OH


What temperatures, pressures and mixing conditions were you using? How much catalyst was used? How was the U-Ni-A activated? Acetic acid or hydrochloric acid and for how long time? Was there any other functional groups pressent in the substrate? Details please!
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[*] posted on 20-9-2008 at 00:08


Back to Solo's dilemma; I've also been wondering how to calculate how much H2 is used in the Parr. The gauges are fine for observing when no more gas is taken up but getting from there to a precise calculation of moles used didn't seem practical. However I don't know how I missed the standardization procedures. I have Parr manuals too thanks to Sauron. Maybe the one posted by Wizard is more up to date. Anway the procedure claims a 0.1 mole sensitivity at least and I'm not after more than that as precision. Solo's article was interesting too in that it's a lot like the original Ogata method but sort of a one pot approach in the hydrogenator if I got it right. In Resolution of Ephedrine, Ogata reacted p2p with Meam over Hg amalgam and then put the product in the hydrogenator. Brown's method is very interesting but would it work on benzillic hydroxyls which can be reduced under medium pressure with Pd/C?

[Edited on 20-9-2008 by chemrox]




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[*] posted on 21-9-2008 at 19:55


Depends which Parr you are using. If you are using the High Pressure Reactor, you might just be able to calculate the unfilled volume of the reactor...Fill it with enough high pressure H2 to accomplish your reduction, then control the rate of reduction by controlling the solutions temperature and speed of agitation.

As for Brown's method. It supposedly produces an unusually active catalyst; the more active your catalyst, the less pressure your hydrogenation will require. I don't know what pressures you are working at, but If you have to use significant pressure for your reduction, then the glass apparatus that Brown uses, isn't the way to go. Still, there is no reason that Brown's method, or a modification of it, can't be used in a metal vessel.

Another interesting hydrogenation catalyst, that might work under moderate conditions, depending on your substrate, is Tungsten Carbide. It likes acidic conditions, and it may require less heat and pressure than Raney Nickel. It likes to work at 5 to 10 Atmospheres, and the more pressure you give it, the less heat it requires. Quite active, once you get it going. Reported to resist poisoning.

I would imagine that a Parr shaker with a metal bottle (rather than glass), should be able to easily utilize Tungsten Carbide as a hydrogenation Catalyst.

Now that prices for Platinum and even Palladium, have skyrocketed, maybe Carbide deserves a look. Not much literature on it, or at least I haven't seen much.

I don't know what reactivity it has with Benzylic Alcohols, might be useless. But, it isn't impossible that it would be effective at reducing Nitrostyrenes to amines. Keep the pressure up, the acidity high, and temperature down....and it has a good chance of working.


US Patent. 5,646,085 Catalyst for Preparation of aromatic Nitro compounds.
The key passage being on page 5, paragraph 7.

"When work is performed in a liquid medium that is acidic and reactive (said medium being homogeneous or heterogeneous and having an acid titre equalling at least 0.1N and preferably1N), IT IS NOT NECESSARY TO INCREASE THE PRESSURE MUCH ABOVE 2 ATMOSPHERES IN ORDER TO REACH THE (ACTIVATION) THRESHOLD, AND THIS CAN BE DONE AT RELATIVELY LOW TEMPERATURE (ABOUT 50 DEGREES C.).

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sec...

[Edited on 21-9-2008 by zed]
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[*] posted on 21-9-2008 at 22:28


Hum, aromatic nitro compounds are much more easily reduced than aliphatic, or nitrostyrenes under cat hydrogeantion conditions... I don't think that statement can be applied to nitrostyrenes, which require very high catalyst loadings. There have been a few sparse claims about atm hydrogeantion to the amine, but it wasn't backed up with any hard info or details... Aliphatic nitro compouds could be more inclined to be reduced under tese condiitons, considering that they behave pretty well under cTH conditions, the CTH donors could be swapped for atmospheric H2.

Two other interesting catalyst would be Cu/SiO2 and Ni/C IMHO.

Cu/SiO2 is very easily made (I have made a few grams a few months ago), and is apparently very good at reducing the double bond in a,b-unsaturated ketones. Maybe by analogy itcould be used to reduced nitrostyrenes to aliphatic nitroalkanes? It can also be used to deoxygenate aromatic keto,nes to alkanes, as a Clemmensen or Wolf-kisher but in much smoother condiitons (atm H2, toluene, 90°C 1/1-5/1 w/w sub/cat)... A very promising path,there is a series of papers avaible the ref forum on this catalyst.

Ni/C is said to be also pretty easy to prepare, and surprisingly doesn't require excessive pressures and temps to get working (although the authors sued to it for various coupling, and dehalogeantion of aryl chlorides). The cheapness of the catalyst could make it a viable alternative to the common palaldium and Pt.




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[*] posted on 22-9-2008 at 01:03


Quote:
Originally posted by Barium
If you can make a really active Raney nickel you should be able to use it at such low pressures for the reduction of a imine.


Do you have the details for making a really active Raney nickel?

And does this differ a lot from the average fresh commercial Raney nickel?




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[*] posted on 22-9-2008 at 03:22


Preparation of the very active W-6 Raney Nickel from OrgSyn:

Raney Nickel, W-6

I think this the most active Raney nickel commonly used. Maybe more active catalyst can be prepared with additives or special conditions..




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[*] posted on 22-9-2008 at 23:37


I think for nitrostyrenes you're going to use LAH. I don't see a way to use the medium pressure Parr. Maybe high pressure (?) I don't know .. can't go there with my gear.



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[*] posted on 23-9-2008 at 00:33


Chemrox, what are you talking about? Where did high pressure come from?
beta-Nitrostyrenes can be hydrogenated at 1 atm or more to either alpha-phenylacetaldoximes or beta-phenylethylamines depending on conditions and catalyst loading/type. No high pressure is needed and even Pd-C works even though it sucks in regard of oxime reductions (which means only beta-unsubstituted bet-nitrostyrenes can be reduced all the way to the amine since ketoximes are generally unreducible with Pd-C).




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[*] posted on 23-9-2008 at 22:40


I didn't bring it up ..I don't know anything about using high pressure H2.. someone mentioned it ala Parr. No idea why.



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