pH-Controlled selective synthesis of lactate from pyruvate with a photoredox system of water-soluble zinc porphyrin, an electron mediator and platinum nanoparticles dispersed by polyvinylpyrrolidone

Abstract

Photoredox catalytic systems are attracting widespread attention not only in the fields of water splitting into hydrogen and oxygen and CO₂ reduction, but also in the field of organic synthesis for sustainable chemicals such as biodegradable polymers and their raw materials. As an example of the development of a photoredox catalyst system for organic synthesis, visible-light driven selective pyruvate reduction to lactate (a raw material for biodegradable polymers) with a system of triethanolamine (TEOA), zinc tetraphenylporphyrin tetrasulfonate (ZnTPPS), methylviologen (MV²⁺) and colloidal platinum nanoparticles dispersed by polyvinylpyrrolidone (Pt-PVP) has been reported previously. In this work, the improvement of visible-light driven selective lactate synthesis with this system was attempted by tuning an electron mediator or controlling the pH. The radical cation species is expected to be more effective in reducing pyruvate to lactate than hydride species in this system. From the result of the pH controlling experiment, it was found that the protonated TEOA (TEOAH⁺) is strongly involved in the pyruvate reduction to lactate with this system under steady-state irradiation. Moreover, racemic lactate was synthesized and no enantioselectivity was observed in this system.