Melgar
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Exclusive Pd/C pressure reactions that are easily tested.
I'm working on a conceptual H2 pressure reactor that uses an air rifle pump to create the necessary pressures, and I'm looking for some easily-tested
reactions. Cleaving a benzyl group from an amine seems pretty easy, and everything I could find said that Pd/C was the way to do it, although
reaction times were long:
http://www.synarchive.com/protecting-group/Amine_Benzylamine
Then this came up, claiming that CTH on Pd/C was much faster and more efficient, which is hard to deny:
https://erowid.org/archive/rhodium/chemistry/cth.n-debenzyla...
It seems that every time I find a reaction that needs H2 and Pd/C, I find another paper that claims that CTH results in better yields in a fraction of
the reaction time. Is there any reaction that this isn't true of? And also, for Pd/C, whether with H2 or NH4COOH, you can use the salts that form
when you alkylate with benzyl chloride, correct? A lot of reactions are done with HCl present, although the papers I've read don't explicitly say
whether they're starting with the HCl salts or not. If the acidity is a problem, that could be altered with KOH or triethylamine or similar, right?
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Dr.Bob
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The problem is that most reactions are highly dependent on substituents, protecting groups, if any, and other factors that make every reaction unique.
Also heterogeneous catalysts are quite variable, one batch of Pd/C might work in 1 hour, another batch might take 10 hours under the same
conditions. Trace impurities of sulphur and other catalyst poisons can also make a huge effect. I have run both gas phase and transfer
hydrogenations, it is hard to predict which will work best, often either can work fine, but in some cases there are good reasons for using one or the
other. The key is that you can use a vast excess of material for a CTH, but only a limited amount of gas pressure in normal equipment. And HCl can
affect compounds wildly, again dependent on the pKa of the compound and other factors. Some pyridine compounds can be hydrogenated better in HCl,
not sure if that is solubility influenced or what.
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clearly_not_atara
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You could try zincocene/thiamin, or more generally zincocene/heterocarbene which only works with hydrogen. This paper provides an example, although
this is not the only geometry:
http://onlinelibrary.wiley.com/doi/10.1002/anie.201303968/fu...
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Melgar
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Quote: Originally posted by Dr.Bob  | | The problem is that most reactions are highly dependent on substituents, protecting groups, if any, and other factors that make every reaction unique.
Also heterogeneous catalysts are quite variable, one batch of Pd/C might work in 1 hour, another batch might take 10 hours under the same
conditions. Trace impurities of sulphur and other catalyst poisons can also make a huge effect. I have run both gas phase and transfer
hydrogenations, it is hard to predict which will work best, often either can work fine, but in some cases there are good reasons for using one or the
other. The key is that you can use a vast excess of material for a CTH, but only a limited amount of gas pressure in normal equipment. And HCl can
affect compounds wildly, again dependent on the pKa of the compound and other factors. Some pyridine compounds can be hydrogenated better in HCl,
not sure if that is solubility influenced or what. |
I was thinking to use benzylamine compounds, because the benzyl group is a common protecting group for amines, and the recommended way to remove it is
Pd/C. It wouldn't be a common protecting group if reaction times could vary anywhere from 1-10 hours, would it? Of they would recommend some other
way of removing it, no?
I was kind of hoping for really basic, easy reactions, where perhaps you'd be able to immediately smell the product if you had it, and if you couldn't
smell the product, then the reaction obviously failed, and you could skip the chromatography. It'd also be nice if I could get prepared Pd/C to use
at first, so I could tell whether or not preparing my own Pd/C was the source of the problem. Fortunately, it's available for about $20/gram at 10%
Pd on eBay, which is acceptable, I guess.
The first step in the process of learning something is admitting that you don't know it already.
I'm givin' the spam shields max power at full warp, but they just dinna have the power! We're gonna have to evacuate to new forum software!
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