Enantioselective Hydrogenation on Palladium: Limitations of Continuous Fixed-Bed Reactor Operation

The palladium-catalyzed continuous enantioselective hydrogenation of 4-methoxy-6-methyl-2-pyrone 1 to the corresponding 5,6-dihydropyrone 2 was studied in a fixed-bed reactor. Steady-state enantiodifferentiation was achieved by feeding the chiral modifier (cinchonine). The influence of reaction conditions on the rate and enantiomeric excess (ee) was investigated under differential and integral reactor conditions. Optimization of the enantioselectivity afforded 84.5% ee to (R)-2 at 1% conversion and 100% chemoselectivity. The attempt to produce high yield of (R)-2 failed since increasing conversion resulted in further hydrogenation of 2 to the tetrahydropyrone derivative 3, and a rapid loss of ee due to hydrogenation of cinchonidine. The poor chemo- and enantioselectivities at higher conversion are attributed to the high Pd/1 and Pd/cinchonine ratios in the fixed-bed reactor. It seems to be a general feature of Pt- and Pd-catalyzed enantioselective hydrogenations that the necessary cinchona alkaloid/reactant ratio is remarkably higher in a continuous reactor than in a slurry reactor.