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

Abstract

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 TiO 2 powders (Degussa P-25, Ishihara ST-01, and HyCOM TiO 2) loaded with platinum (Pt), rhodium (Rh), or palladium (Pd). A common feature of the results of experiments using a wide variety of metal-loaded TiO 2 photocatalysts is that the rate of PCA formation ( r PCA) was greatly reduced when higher optical purity of PCA (OP PCA), i.e., enantio excess of the l-isomer of PCA, was obtained; higher r PCA was achieved by the use of Pt-loaded TiO 2 powders, while these powders gave relatively low OP PCA. Selectivity of PCA yield ( S PCA), i.e., amount of PCA production based on l-Lys consumption, also tended to increase with decrease in OP PCA, giving a master curve in the plots of OP PCA versus S PCA. Among the TiO 2 powders used in this study, HyCOM TiO 2 showed relatively high OP PCA and S PCA but not optimum S PCA and OP PCA 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 (NH 3), a by-product. It was observed for several photocatalysts that the 15N proportion ( P 15) in PCA was almost equal to OP PCA, 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 P 15 in NH 3 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 OP PCA and S PCA obtained for various TiO 2 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 S PCA was relatively low, photocatalysts, favoring the reduction of α-CSB rather than ε-CSB, gave higher OP PCA but lower S PCA, since some ε-CSB remained unreduced to give racemic PCA. In contrast, at higher S PCA, both CSBs were reduced nonselectively and OP PCA was found to be determined mainly by the selectivity in the oxidation stage. The relatively low yield of molecular hydrogen (H 2) when higher S PCA was achieved is consistent with the mechanism in which H 2 liberation occurs instead of the reduction of CSBs by e −. Thus, the general tendency of plots between OP PCA and S PCA could be explained by the above-described redox-combined mechanism of photocatalysis.