[AlphaScent]

First time doing this reduction.  Any hints to a successful reaction? My main thought is what is the best way to filter out the Pd/C after the reaction is complete?


 

[Doc Oc]

Filter through Celite and wash the Celite a couple times.  You should be able to evaporate the solvent and get good clean product.

[AlphaScent]

Sweet, thank you!  Seems simple enough.  I do P-2 nickel all the time, but that is on non-conjugated, internal alkynes.  I will post back with my results.  By the way doc, do you know anything about rate?  Im not worried about over hydrogenating, obviously, but was wondering in your experience how long this reduction took? (alkene to alkane)

[orgopete]

Don't let your filter cake become dry. It could ignite any residual solvent.

[fledarmus]

There is a safe way of removing active catalysts, and many unsafe ways. The unsafe ways usually cause fires.

First, be sure that you do a thorough job of purging the hydrogen gas.

Second, If you have a convenient way to filter through celite in an inert gas atmosphere, do so.

Third, as Orgopete says, don't let your catalyst dry out. Have plenty of solvent on hand so that you can add more to wash the filter cake before it starts to dry. Also have water on hand - as soon as you have finished washing your filter cake (leaving some solvent so it doesn't dry out), turn off the vacuum, move your funnel to another container, and add water immediately. Transfer the catalyst and filter cake in water to your heavy metal waste bottle, and be sure there is enough water in the bottle to cover the solid contents. Don't let the waste dry out either.

I've seen more than one hood destroyed working in pharmaceutical research companies because people were careless with hydrogenation catalysts. Pay attention to how you handle them. They are safe and convenient to work with, as long as you treat them with respect.

[Dan]

A nice alternative to Celite is glass microfibre pads (e.g. Whatman GF/B). Less hassle.

I can only echo the importance of purging the hydrogen thoroughly and having water on hand.

[OC pro]

Have done a lot of reductions on molar scale in 6l flasks (100l+ of H2 had to be added, was not funny).
Stirring is also a most important point. As fast as possible and with larger scale of course a mechanic stirrer is favoured.
The best way to make it is like this:
Evacuate a double-necked flask (properly stoppered) 3 times and backfill with argon or nitrogen. Then add catalyst. Then slowly rinse solvent (methanol and ethanol tend to ignite very sponateously) at the glasswall. Once the Pd is covered, you can add more solvent. You can also use DCM/MeOH mixtures (when your starting material does not dissolve well in Methanol). Then its even easier since DCM will not burn.
Then after Pd has been slurried, add your starting material (in solution or neat) and evacuate 3 times and always backfill with hydrogen (stirring can be applied to provide better mixing).
Reduction of alkenes and alkynes are usually very fast within minutes but the prob is always that you won´t see it in TLC. Therefore, I would suggest stirring overnight as long as there are no functional groups which can be harmed.
After the synthesis is finished, filter over Celite and wash with solvent. In most cases no chromatography is needed.

 

[408]

Having only ever worked with Pd/C (20%), which are the catalysts which can catch fire?  I have never seen this happen with this system.

[discodermolide]

They all catch fire when they dry out. Raney nickel is especially bad for this. After a hydrogenation and filtering off the catalyst it should not be sucked to dryness otherwise it will ignite. Keep water handy to cover it up after the filtration and washing is complete.

[Dan]

I've had a small fire with Pd black (due improper precautions/naivety early in my career), but never had any any problems with Pd on C.

[orgopete]

I'm with disco on this one. What I think makes a fire so insidious is one can become lazy because they don't have a fire. I've had Pd/C sparking. I quickly quenched before a fire.

[Dan]

What I think makes a fire so insidious is one can become lazy because they don't have a fire.

This is absolutely true, and this kind of negligence is the reason I had a fire as a student (nothing more than a candle sized flame, it was small scale). Needless to say, some of my attitudes changed for the better on that day.

[curiouscat]

What I think makes a fire so insidious is one can become lazy because they don't have a fire.

This is absolutely true, and this kind of negligence is the reason I had a fire as a student (nothing more than a candle sized flame, it was small scale). Needless to say, some of my attitudes changed for the better on that day.

I think most finely divided metals are in general a fire hazard. Activation only makes things more exciting.

[OC pro]

When adding a second portion of Pd/C into the flask I had a decent flash coming out. No fun in a 6l flask! Stoppered with nitrogen and then it went all fine.
When filtering I always had a watchglass nearby to cover the funnel. The fire stopped very soon.

[Dan]

When adding a second portion of Pd/C into the flask I had a decent flash coming out.

Yes, this is one of the most risky stages. I know a lot of people who've had a pop adding an extra portion of the catalyst without purging with an inert gas first (this was how I had my small accident as well). I also recommend adding additional portions of catalyst as a slurry in the reaction solvent.