Racemization of Secondary-Amine-Containing Natural Products Using Heterogeneous Metal Catalysts

Heterogeneously catalyzed racemization reactions of the secondary amines (S)-1-methyl-6,7-dimethoxy-1,2,3,4-tetrahydroi-soquinoline (salsolidine) and (S)-1-methyl-1,2,3,4-tetrahydro-beta-carboline were investigated using Pd, Pt, and Ir on carbon or Al2O3 supports. A comparison of kinetics and deactivation on selected platinum and iridium catalysts was performed. Furthermore, the relative stabilities of (S)-salsolidine and the corresponding imine on Pt(111) and Ir(111) surfaces were analyzed by density functional theory calculations. The racemization was faster on platinum and took place without detectable byproduct formation. Iridium, however, proved reusable and, in contrast to the platinum catalyst, deactivation at low catalyst concentration was not observed. Also, the physisorption of (S)-salsolidine and the imine was stronger on platinum than on iridium.