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The use of visible light to enable small molecule synthesis has grown substantially over the last 15 years. While much of the focus has been on the development of new methods, mechanistic and kinetic studies can provide valuable information about reaction steps and highlight directions for optimization and new methods. This review focuses on reports of visible light, homogenous photoredox reactions that emphasize direct observation of reaction intermediates and/or contain a significant focus on mechanistic and kinetic studies. How these types of studies can improve reaction yields and rates are highlighted. Finally, reaction quantum yields for over 200 photoredox reactions are summarized for the first time. This often-neglected reaction parameter provides valuable insights into the efficiency of photoredox reactions as well as the clues to the underlying mechanism. 

The use of photoredox catalysis for the synthesis of small organic molecules relies on harnessing and converting the energy in visible light to drive reactions. Specifically, photon energy is used to generate radical ion species that can be harnessed through subsequent reaction steps to form a desired product. Cyanoarenes are widely used as arylating agents in photoredox catalysis because of their stability as persistent radical anions. However, there are marked, unexplained variations in product yields when using different cyanoarenes. In this study, the quantum yield and product yield of an α-aminoarylation photoredox reaction between five cyanoarene coupling partners and N-phenylpyrrolidine were characterized. Significant discrepancies in cyanoarene consumption and product yield suggested a chemically irreversible, unproductive pathway in the reaction. Analysis of the side products in the reaction demonstrated the formation of species consistent with radical anion fragmentation. Electrochemical and computational methods were used to study the fragmentation of the different cyanoarenes and revealed a correlation between product yield and cyanoarene radical anion stability. Kinetic modeling of the reaction demonstrates that cross-coupling selectivity between N-phenylpyrrolidine and the cyanoarene is controlled by the same phenomenon present in the persistent radical effect. 

An electron donor – acceptor (EDA) complex forms between 1,4-dicyanobenzene and N -phenylpyrrolidine, which are coupling partners for the α-aminoarylation photoredox reaction. Calculations and experiments demonstrate the EDA complex is a better base than N -phenylpyrroline. A re-analysis of the α-aminoarylation reaction suggests that the EDA complex is a proton acceptor in the reaction.