Understanding the Propagation Step in a Photoredox Cycloaddition Chain Reaction

Photoredox catalysis has become increasingly significant in academic and industrial processes, replacing harsh reaction conditions and high temperatures with visible light. Photoredox chain reactions that proceed via dark electron or hole "catalysis" represent an intriguing pathway to achieving high-productivity photoredox reactions. Though photoredox chain reactions are known, our understanding of how to design reactions that exhibit chain behavior remains limited. In particular, a key step in designing a chain reaction is a dark propagation step, where an energetically upconverted electron or hole initiates a subsequent cycle of product generation. This study combines quantum yield (QY) measurements, transient absorption spectroscopy (TAS), electrochemical investigations, kinetic modeling, and computational studies of a ruthenium-catalyzed photoredox chain [4 + 2] cyclization between trans-anethole and various dienes to interrogate the kinetics of the propagation step. That data demonstrate that the free energy for the propagation step (ΔG _prop) is the key kinetic descriptor for the reaction. There is a linear relationship between ΔG _prop and the rate constant for propagation, and ΔG _prop is a good predictor for the QY of the reaction. In addition, ΔG _prop correlates with the oxidation potential of the diene, offering a simple molecular predictor of chain behavior.