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

Hai Dang Nguyen; Manabu Abe

doi:10.1021/jacs.4c02880

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

J Am Chem Soc. 2024 Apr 17;146(15):10993-11001. doi: 10.1021/jacs.4c02880. Epub 2024 Apr 5.

Authors

Hai Dang Nguyen¹, Manabu Abe^{1

2}

Affiliations

¹ Department of Chemistry, Graduate School of Advance Science and Engineering, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Hiroshima, Japan.
² Hiroshima Research Center for Photo-Drug-Delivery Systems (Hi-P-DDS), Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Hiroshima, Japan.

PMID: 38579283
DOI: 10.1021/jacs.4c02880

Abstract

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 1a [-SC(O)CH₃], thionoester 1b [-OC(S)CH₃], and ester 1c [-OC(O)CH₃] 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 CH₃C(O)S^•] plays an important role in the photoreactions of 1a and 1b, an ion pair [i.e., 7-diethylamino(coumarin-4-yl)methyl cation, and CH₃CO₂^-] was the key in the photoreaction of 1c. ¹⁸O-isotope-labeling of 1c 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.