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. |