The effect of oxygen and the reduction temperature of the Pt/Al2O3 catalyst in enantioselective hydrogenation of 1-phenyl-1,2-propanedione

The effect of oxygen and catalyst reduction temperature in enantioselective hydrogenation of 1-phenyl-1,2-propanedione over commercial Pt/Al2O3 cataly st was investigated. Dichloromethane was used as solvent. The catalyst was modified in situ with (-)-cinchonidine. Relatively high enantiomeric excess es (65%) of (R)-1-hydroxy-1-phenyl-2-propanone were obtained with the solve nt used as received, i.e. containing traces of dissolved oxygen and other i mpurities. Dichloromethane dissociated partially on the Pt/Al2O3 surface ca using desorption of methane, ethene and HCl from the catalyst during TPD ac cording to mass spectrometric analysis. Under anaerobic conditions the reac tion rate was low giving only about 40% enantiomeric excesses of (R)-1-hydr oxy-1-phenyl-2-propanone. When injecting 5 mm(3) of oxygen into the reactor a beneficial effect was observed (i.e. higher reaction rate and enantiomer ic excess) in comparison with anaerobic conditions. Poisoning effect of oxy gen was observed when injecting 500 mm(3) of oxygen into the reactor. Effec t of catalyst reduction temperature was studied at three different temperat ures (170, 400 and 455 degrees C). Highest reaction rates and enantiomeric excesses were obtained with the catalyst reduced at 400 degrees C. Methane was desorbed from the catalyst at temperatures between 263 and 383 degrees C which could be the explanation for the lower activity of the catalyst red uced at 170 degrees C. It was demonstrated that small amounts of oxygen can have a beneficial effect in enantioselective hydrogenation of 1-phenyl-1,2 -propanedione and also that catalyst reduction temperature plays an importa nt role in obtaining high enantiomeric excesses. (C) 2000 Elsevier Science B.V. All rights reserved.