(Invited) Identification of a Self-Photosensitizing Hydrogen Atom Transfer Organocatalyst System

Abstract

The carbon-hydrogen (C-H) bond is a fundamental chemical bond that constitutes organic molecules, and its direct conversion leads to highly efficient molecular synthesis. In recent years, the hydrogen atom transfer (HAT) catalysis using the energy of visible light, has been attracting attention. However, existing methods require the use of photoredox catalysts such as organic molecules with complex structures and expensive metal complexes. In this study, we developed an organocatalytic system that mimics the electron transfer process of enzymes in vivo and promotes functionalization of stable C-H bond without photoredox catalyst. We designed a thiophosphoric acid catalyst that contains both a redox-active binaphthyl moiety and a sulfur atom. We found that this catalyst forms charge-transfer complexes with electron-deficient heteroaromatics and catalytically produces thyil radical via multi-step electron transfer under visible light irradiation.Based on the above design, we decided to investigate the formation of active HAT catalytic species in the absence of photoredox catalysis by the Minisci-type reaction using aldehydes and N-heteroaromatic rings. The results of the study confirmed that 29% of the product was obtained when using binaphtyl thiophosphoric acid (TPA) catalyst, as expected. Yield improved to 80% when an electron-donating group was introduced to the binaphthyl skeleton to enhance the donor-acceptor interaction. Changing the binaphthyl skeleton and thiophosphoric acid moieties markedly reduced reactivity, indicating that the presence of a sulfur atom in addition to a rigid binaphthyl skeleton is essential for the efficient reaction progress. With this optimized condition, alkylation of N-heteroaromatic rings by C-H bond activation of alcohols was found to proceed. Dehydrogenation of alcohols and benzylation of imines were also successfully achieved by using N-heteroaromatic catalyst. The mechanism of the electron transfer process has been confirmed by spectroscopic and computational methods, and will be presented in detail in the lecture. Figure 1