Divergent Photosensitizer Controlled Reactions of 4‐Hydroxycoumarins and Unactivated Olefins: Hydroarylation and Subsequent [2+2] Cycloaddition

Abstract

Herein, we report a photoinduced approach for hydroarylation of unactivated olefins using 4‐hydroxycoumarins as the arylating reagent. Key to the success of this reaction is the conversion of nucleophilic 4‐hydroxycoumarins into electrophilic carbon radicals via photocatalytic arene oxidation, which not only circumvents the polarity‐mismatch issue encountered under ionic conditions but also accommodates a broad substrate scope and inhibits side reactions that were previously observed. Moreover, divergent reactivity was achieved by changing the photocatalyst, enabling a subsequent [2+2] cycloaddition to deliver cyclobutane‐fused pentacyclic products that are otherwise challenging to access in high yields and with high diastereoselectivity. Mechanistic studies have elucidated the mechanism of the reactions and the origin of the divergent reactivity.

Efficient Radical‐Mediated Intermolecular α‐Alkylation Reactions of Carbonyl Compounds with Nonactivated Alkenes

“…In 2023, Ye et al also reported catalytic α-alkylation reactions of 4-hydroxycoumarins with nonactivated alkenes (Scheme 18). [73] A combination of a dicyanopyrazine-derived organophotocatalyst, DPZ (56), and 2,4,6-triisopropylbenzenethiol (TRIPSH, 57) was used, and high yields were obtained in methanol under blue-LED irradiation. A SET process proceeded between the coumarin, which contains an enol structure of 1,3dicarbonyl compounds, and the photocatalyst to form a radical intermediate for the reaction.…”