Photocatalytic redox-combined synthesis of l-pipecolinic acid from l-lysine by suspended titania particles: effect of noble metal loading on the selectivity and optical purity of the product

Photocatalytic (>300 nm) conversion of l-(S)-lysine (l-Lys), in its neutralized aqueous solution, into l-pipecolinic acid (l-PCA) under deaerated conditions at 298 K was investigated in detail using suspended TiO2 powders (Degussa P-25, Ishihara ST-01, and HyCOM TiO2) loaded with platinum (Pt), rhodium (Rh), or palladium (Pd). A common feature of the results of experiments using a wide variety of metal-loaded TiO2 photocatalysts is that the rate of PCA formation (rPCA) was greatly reduced when higher optical purity of PCA (OPPCA), i.e., enantio excess of the l-isomer of PCA, was obtained; higher rPCA was achieved by the use of Pt-loaded TiO2 powders, while these powders gave relatively low OPPCA. Selectivity of PCA yield (SPCA), i.e., amount of PCA production based on l-Lys consumption, also tended to increase with decrease in OPPCA, giving a master curve in the plots of OPPCA versus SPCA. Among the TiO2 powders used in this study, HyCOM TiO2 showed relatively high OPPCA and SPCA but not optimum SPCA and OPPCA simultaneously. In order to interpret such relations, the mechanism of stereoselective synthesis of the l-isomer of PCA (l-PCA) was investigated using isotope-labeled α-15N-l-lysine with quantitative analysis of incorporation of 15N in PCA and ammonia (NH3), a by-product. It was observed for several photocatalysts that the 15N proportion (P15) in PCA was almost equal to OPPCA, suggesting that oxidative cleavage by photogenerated positive holes of the ε-amino moiety of l-Lys gave optically pure l-PCA through retention of chirality at the α-carbon in the presumed intermediate, a cyclic Schiff base (α-CSB), which undergoes reduction by photoexcited electrons into PCA. From P15 in NH3 and PCA, the selectivity of oxidation between α and ε-amino groups in l-Lys by photoexcited positive holes (h+) and the efficiency of reduction of α-CSB (produced via ε-amino group oxidation to give optically pure PCA) and ε-CSB (produced via α-amino group oxidation to give racemic PCA) by photoexcited electrons (e−) were calculated. The former was found to be independent of the kind of photocatalyst, especially the loaded metal, while the latter was influenced markedly only by the loaded metal. It was clarified that OPPCA and SPCA obtained for various TiO2 powders used in the present study were strongly governed by the reduction stage, i.e., the efficiency of reduction of two types of CSB. When SPCA was relatively low, photocatalysts, favoring the reduction of α-CSB rather than ε-CSB, gave higher OPPCA but lower SPCA, since some ε-CSB remained unreduced to give racemic PCA. In contrast, at higher SPCA, both CSBs were reduced nonselectively and OPPCA was found to be determined mainly by the selectivity in the oxidation stage. The relatively low yield of molecular hydrogen (H2) when higher SPCA was achieved is consistent with the mechanism in which H2 liberation occurs instead of the reduction of CSBs by e−. Thus, the general tendency of plots between OPPCA and SPCA could be explained by the above-described redox-combined mechanism of photocatalysis.