Photocatalytic Reductive C–C Coupling Driven by Light-Induced Electron Transfer/Phase Migration of a Redox Mediator in a Biphasic Solution

Natural photosynthesis establishes highly efficient molecular-conversion reactions driven by solar light energy, based on the combination of ingenious parts, such as light capturing, charge separation and migration, and catalysis in the reaction center. In addition to extensive studies on artificial photosynthetic systems mimicking the natural system, photoredox catalysis has recently had great impact on the field of photochemical organic synthesis. In any case, photoinduced electron transfer from/to electron donor(s)/acceptor(s) is a key initial step. One of the most important issues in the construction of highly efficient artificial photocatalytic systems is to suppress undesired backward electron transfer that decreases total efficiency. Herein, we developed a new photocatalytic system using biphasic solution media to mitigate such backward charge recombination. Two-immiscible solution composed of water and 1,2-dichloroethane (DCE) was employed with ferrocenium/ferrocene (Fc⁺/Fc) electron mediator. The change in dissolubility of Fc⁺/Fc (hydrophilic/hydrophobic) by photocatalytic electron transfer is expected to play an important role for the spatial charge separation to suppress the backward electron transfer. We demonstrated photocatalytic reductive coupling of benzyl bromide by utilizing the photoinduced inter-liquid phase migration of Fc⁺/Fc; suppressing the undesired backward charge recombination significantly.<br/>Visible-light irradiation to a biphasic solution that composed of water and DCE solution containing an Ir(III)-photosensitizer, Fc, and benzyl bromide (Bn-Br) facilitated the reductive coupling of Bn-Br to dibenzyl (Bn₂). Given the energy diagrams of the photosensitizer and Fc, the Fc should act as an electron donor for the photoexcited Ir(III)-photosensitizer. The reduced Ir(III) complex has an enough potential for one-electron reduction of benzyl bromide forming Bn₂. Importantly, Fc⁺, generated by photooxidation, migrates to the aqueous phase due to the drastic change in its partition coefficient compared to that of Fc. On the other hand, visible-light irradiation to the same solution without water phase did not give any product. This finding indicates that the phase migration of Fc⁺ across a biphasic solution suppresses the unfavorable backward charge recombination, which enables the photoreduction of Bn-Br. The results on the coupled phase migration/photoinduced electron transfer prompted us to further investigate design principles in order to improve the efficiency of this photocatalytic reaction. The co-existence of anions can further modify the driving force of phase migration of Fc⁺ depending on their hydrophilicity; the best photocatalytic activity was obtained with 99% yield after continuous photocatalysis for 90 min in the presence of NBu₄⁺Br^– compared to 25% one without additive. Thus, the liquid-liquid phase migration of the mediator results in effective charge separation, which leads to facilitate the reduction of Bn-Br in the DCE phase.