Photocatalytic Synthesis of Pipecolic Acid from Lysine on TiO₂: Effects of the Structure of Catalysts and Adsorbed Species on Chiral Selectivity

Abstract

A one-step photocatalytic synthesis of l-pipecolic acid (l-PCA) from l-lysine (l-Lys) has been investigated by in situ infrared (IR), circular dichroism, and 1H NMR spectroscopy. Chiral selectivity toward l-Lys was found to be governed by the structure of adsorbed l-Lys and the structure of TiO2. Both anatase and rutile were not able to catalyze the reaction in a solvent-free environment. In situ IR studies revealed that l-Lys adsorbed on these TiO2 in a dicationic form of which the positive −NH3 + functional groups are not able to interact with photogenerated holes. Interestingly, P-25 TiO2, which contains 80 wt % anatase and 20 wt % rutile, adsorbs l-Lys in a zwitterionic form, allowing its hydrogen in the α-amino group to be abstracted by the photogenerated hole, producing racemic PCA. The addition of H2O as a solvent allowed l-Lys to be adsorbed on P-25 in an anionic form, producing l-PCA as a major product. This study demonstrates that the reaction environment has a direct impact on the structure of adsorbed species that controls the selectivity of a chiral synthesis.

Cell-free biocatalytic syntheses of l-pipecolic acid: a dual strategy approach and process intensification in flow

“…16 Alternatively, photocatalysis using a TiO 2 catalyst allowed the production of racemic and L-PA with moderate enantiomeric excess but still 40% final yield. 17 As an alternative, new approaches mimicking the natural pathways have gained momentum in the last years. PA pathways have been studied for years now in plants and animals, due to their relationship with lysine metabolism defects.…”