I've been occupied all morning with voodoo. Well, the technical name for it is catalytic hydrogenation, but let's call it for what it is: witchcraft. It's a widely used reaction in organic chemistry, and you can use it to reduce all kinds of different functional groups on your molecules. But once you get off the well-traveled roads, it's all jungle drums at midnight.
One reason we chemists like this reaction so much is that it's simple. You add some dark insoluble powder to your compound - which is some metal like palladium, platinum, nickel or the like, adsorbed onto carbon black or another solid. Then you add solvent and put the whole thing under an atmosphere of hydrogen gas. That soaks into the metal particles, your compound sits on them and gets magically reduced, and after a while you filter everything off and there's your clean, transformed product.
Most of the time. You'll note that I've skipped over a lot of variables there. For one thing, there's the choice of metal catalysts. Pt and Pd get the most use, but they come on a variety of solid supports. Carbon, alumina, barium sulfate, calcium carbonate. . .they all act differently. And don't stop with those guys: nickel's not to be ignored, then rhodium's available, and even ruthenium if you want to crank up the pressure. The pressure of all that hydrogen, there's another variable. Just a balloon on top, atmospheric pressure? Or put in a thick glass bottle on a shaker and turn it up to 50 pounds per square inch? Higher, in a metal apparatus? And what temperature did you have in mind? Ambient, or would you like to heat things up? Remember, as the pressure goes up, so does the temperature you can run the solvents up to.
Ah yes, the solvents. A lot of the time you see this work done in methanol or ethanol, but the reactions will often go quite differently in ethyl acetate or even something less polar. I've even seen some done in dichloromethane, although that somehow just seems wrong. Acids often have a profound effect on things, particularly if there's a basic amine in your compound.
And I haven't mentioned poisoned catalysts yet, have I? A bit of lead, or the addition of (non-protonated) amines or sulfur-containing compounds can dial down the reactivity of a lot of these metals - often down to zero, but sometimes to a useful level that you can't reach any other way. And then there's transfer hydrogenation, where you don't use the gas itself, but let some other compound give up hydrogen inside the reaction and transfer it over to your substrate. Paraformaldehyde, formic acid, phosphites, cyclohexene - all of those will work, and they can all work differently.
So. . .how many variations are we up to? Do you want to use 5% palladium on carbon in methanol, room temperature at 50 psi? Or platinum oxide in acetic acid at 50 degrees? Rhodium on alumina, ethanol, 100 psi at 100 C? Or wet 10% platinum catalyst with formic acid? That should get you started on this simple, well-known reaction. I've run 22 of them in the last two days, with the assistance of the H-Cube reactor, and I have to say: I'm about hydrogenated out.