Development of Chiral N-Heterocyclic Photoredox Catalysts for Asymmetric Photo-mediated Synthesis

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In traditional catalysis, the catalyst supports each step of the catalytic cycle and serves as a reaction companion. In contrast, photoredox catalysts in many cases initiate the formation of a reactive radical species that reacts off without catalyst association. However, some of the mechanisms of chiral induction known from transition metal and organocatalysis are already applied for activating substrates with light and controlling stereoselective product formation. Unlike conventional photoredox catalysts that often require the use of a co-catalyst for catalyst recovery, organic from pyrimidopteridine tetraones act as dual photoredox catalysts and hydrogen atom transfer catalysts and can be regenerated in the absence of co-catalysts or sacrificial reagents. Pyrimidopteridine tetraones have a flavin-like structure but exhibit significantly higher excited-state redox potentials and high stability. In particular, the excited-state reduction potential of greater than +2.0 V vs SCE in acetonitriles enables the one-electron oxidation of a wide range of organic substrates. Previous studies showed that neither the absorption nor the ground-state or excited-state electrochemical properties are drastically affected by changes in the N substituents. The solid evidence that pyrimidopteridine tetraones play a dual role as a strong excited-state oxidant and as a HAT catalyst suggests that the stereoselectivity of both steps can be controlled by a single chiral catalyst. Therefore, the implementation of chiral side chains based on short peptides composed of artificial and natural amino acids should lead to potent chiral organic photoredox catalysts that allow both stereoselective radical reaction and stereospecific hydrogen atom transfer through steric and non-covalent interactions. The triad of modulation via quantum chemical calculations, peptide and heterocycle synthesis, and application in the synthesis of organic molecular building blocks is complemented by extensive spectroscopic studies.