Crucial Roles of Leaving Group and Open-Shell Cation in Photoreaction of (Coumarin-4-yl)methyl Derivatives.

Photoreactions of (coumarin-4-yl)methyl derivatives have been extensively studied in many fields of chemistry, including organic synthesis and photoinduced drug delivery systems. The identification of the reaction intermediates involved in the photoreactions is crucial not only for elucidating the reaction mechanism but also for the application of the photoreactions. In this study, the photoreactions of 7-diethylamino(coumarin-4-yl)methyl thioester [-SC(O)CH], thionoester [-OC(S)CH], and ester [-OC(O)CH] were investigated to clarify the intermediary species and their chemical behavior. While a radical pair [i.e., 7-diethylamino(coumarin-4-yl)methyl radical and CHC(O)S] plays an important role in the photoreactions of and , an ion pair [i.e., 7-diethylamino(coumarin-4-yl)methyl cation, and CHCO] was the key in the photoreaction of . O-isotope-labeling of revealed a negligible recombination process within the ion pair. The unprecedented observation was rationalized by the open-shell character of the 7-diethylamino(coumarin-4-yl)methyl cation, whose formation was confirmed through product analysis and transient absorption spectroscopy.