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Abstract:
Research on sulfonium salts, including thianthrenium salts, has been conducted for more than half a century, but their interaction with transition metals remained unexplored until the late 20th century. The past five years have been particularly productive in studies regarding these salts. This thesis is divided into three key sections: firstly, the synthesis of vinyl thianthrenium tetrafluoroborate and its application in vinylation of organoboron compounds is discussed. The second section introduces the application of a nickel salt as catalyst in the conversion of aryl thianthrenium salts, which enables the synthesis of aryl halides. The third section presents the C–O bond formation from aryl thianthrenium salts, enabled by a dual catalytic system involving both palladium and photoredox catalysis.
Styrenes are known to be involved in a broad array of diversifications. However, the absence of a practical electrophilic vinylating reagent presents a significant obstacle to their synthesis. Inspired by the thianthrenation method used in site-selective functionalization of arenes and alkenes, we developed the method to synthesize vinyl thianthrenium tetrafluoroborate (vinyl–TT+BF4–). The method employs inexpensive ethylene gas at atmospheric pressure for the preparation. Analogous to aryl and alkenyl thianthrenium salts, vinyl–TT+ acts as a vinyl electrophile in palladium-catalyzed Suzuki-type cross-coupling. In contrast to vinyl diphenylsulfonium triflate (vinyl–SPh2+TfO–), which suffers undesired C–S bond cleavage during oxidative addition, Vinyl–TT+ undergoes exclusive C(vinyl)–S bond cleavage.
In the second part of the thesis, a nickel-catalyzed halogenation of aryl thianthrenium salts is described. Compared to palladium, the use of nickel as catalyst in the transformation of these salts has been scarecely reported. Mechanistic study of the reaction suggests a self-sustained Ni(I)/Ni(III) catalytic pathway. Chlorinated, brominated, and iodinated arenes can be obtained by simply switching halide sources.
The last section focuses on the C–O bond formation from aryl thianthrenium salts. Both palladium- and photo-catalyst are crucial to this transformation. While the scope of aryl thianthrenium salts for this transformation is limited, the other coupling partner, carboxylic acid, has a wider range. The arylation of glutamic acid has potential applications on selective posttranslational modifications (PTM) of protein.
