Parallel Charge-Transfer Mechanisms Allow for Diversifying the Choices for Photoredox Catalysts

Photochemistry provides access to reactive intermediates that are often inaccessible by any other means. Most organic molecules, however, are colorless ultraviolet absorbers. Therefore, photocatalysts absorbing in the visible spectral region are essential for transferring the required energy and charges to make challenging chemical transformations possible. A selection of a photocatalyst for driving oxidative or reductive reactions is crucial and is commonly based on their electrochemical potentials as well as the potentials of the starting materials. This selection, however, sometimes proves limiting and misleading, especially when the thermodynamic driving forces of the charge-transfer steps are relatively small. Here, we show that porphyrinoids with differences in their electrochemical potentials exceeding 0.5 V can photocatalyze the same model reaction of N-alkyl-2,4,6-triphenylpyridinium salt with alkynyl p-tolylsulfone to form the same alkylated alkynyl product in similar yields. Our studies reveal that switching between parallel reaction pathways makes the attainment of these conversion efficiencies possible. Electron-rich catalysts drive the formation of alkyl radicals principally via a photoinduced electron transfer to the pyridinium ion and a sequential hole transfer recovers their ground states, i.e., PET-HT mechanism. Conversely, a photoinduced hole transfer dominates the initial formation of the reduced forms of electron-deficient porphyrins that then transfer electrons to the pyridinium salt to release the same alkyl radicals, i.e., PHT-ET mechanism. This discovery demonstrates a paradigm where reaction mechanism adjust to the electronic properties of catalysts and opens doors for transformative diversification and broadening of the applicability of photochemical transformations.

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