It pretty much depends on what you are hydrogenating. For example, if you are hydrogenating an isolated olefinic double bonds which reduces already at
1 atm, increasing the pressure is not going to help much. Nitroarenes also don't need increased pressure and 1 atm does just fine. But if you are
reducing groups difficult to reduce using Pd-C (e.g., nitriles, oximes, debenzylations, some aryl alkyl ketones, dechlorinations), then it is well
worth speeding up the reaction by increasing the pressure. But in essence, whatever works in a Parr shaker apparatus (designed for 60 PSI = 4.14 bar)
will work also using a balloon as long as you load it up with some more catalyst (and/or use the 10% Pd-C) to compensate for the slowing down.
On the other hand, hydrogenations in autoclaves at >100 bar with heating up are something quite different. Usually things that refuse to get
hydrogenated in a good ol'shaker apparatus, will eventually give up under such forcing conditions. But these are usually employed when using catalysts
like copper chromite, or for the reduction of benzene rings with Ni or Pt based catalysts.
For the more common transformations, you can always choose a more appropriate catalyst instead of forcing conditions. For example, if neutral or basic
conditions are tolerated, you can often choose Raney Nickel over Pd-C. As an example, nitriles reduce sluggishly with Pd-C in the presence of HCl, but
the same transformation can be performed better with Raney Ni in the presence of ammonia at the same pressure. Furthermore, since the Raney Ni is
cheap, you can use lots of it, thus making the reaction proceed faster at a much lower pressure.
In the case of beta-nitrostyrenes, I would expect a slight complication when the reaction is too slow. Lowering the pressure without increasing the
catalyst load, will slow down the hydrogenation, but it will not slow down any side reactions that do not involve hydrogen (like the hydrolysis of
some intermediate, for example). I believe this might be the reason why the Japonese authors perform the reaction at 0 °C. I also believe this is
why, in the above mentioned patent, increased temperatures can be used to force the reaction on a nitropropene - no acid is being used. This probably
reduces the selectivity for the primary amine, but at least it allows for harsher conditions. The claim that the reduction goes better with a mixture
of Ni and Pd based catalysts might have a rationale as well. The Pd catalyst might reduce to a certain stubborn intermediate which is on turn further
reduced by the Ni catalyst (the oxime?). |