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. With only a small group of reactions that display chain characteristics, we are still unsure of how prevalent photochemical chain reactions are or how to tune a reaction to proceed through this specific mechanism. Spectroscopic and photochemical analytical techniques are increasingly integral to enhancing our understanding of modern and classical organic transformations. These and other studies have revealed information imperative for extrapolating the kinetics of these systems. Increasing our understanding of these chain reactions in photochemistry provides a promising avenue to increasing efficiency for these reactions, especially when scaling up reactions that have been exclusively performed in the academic research labs. Our current investigation aims to further elucidate the photochemical chain mechanism, specifically the factors influencing its success in the Ru-catalyzed Diels-Alder cycloaddition. Our focus centers on Ru(II) catalyzed [4+2] cycloaddition reaction. Following our kinetic characterization for the cycloaddition between anethole and isoprene, this study explores the substrate scope for sucessful chain propagation and the imapct of electron upconversion, previously described by Alabugin and coworkers. Mechanistic studies were performed utilizing transient absorption spectroscopy (TAS) in combination with quantum yield measurements and computational analysis to measure the chain lifetime and recognize the connection of this [4+2] cycloaddition to electron upconversion.