ausblenden:
Schlagwörter:
C–H functionalization; alkenes; arenes; site-selectivity; stereoselectivity; late-stage fluorination; (metallo)photoredox catalysis; thianthrenation
Zusammenfassung:
Direct C–H functionalization is one of the most straightforward yet challenging strategies to quickly build up functionalities from existing organic compounds. Considering the ubiquitous existence of multiple C–H bonds in one molecule, such as the coexistence of alkyl (sp3), aryl/alkenyl (sp2) and alkynyl (sp) C–H bonds or several with the same hybridization (aryl sp2 and alkenyl sp2), functionalization with high positional selectivity is a challenge. Typically, direct C–H functionalization protocols can differentiate C–H bonds with different hybridization (sp3 vs. sp2), yet there remains challenges for site-selectivity especially when the target C–H bonds have a similar chemical environment (e.g. ortho, meta, para C–H bonds in one arene). In 2019, the Ritter group reported a highly positional-selective C–H thianthrenation of arenes with a broad substrate scope and high functional group tolerance, and the resulting aryl thianthrenium salts provide access to versatile functional groups via Pa catalysis or photoredox chemistry. The work summarized in this thesis describes one site-selective functionalization of alkenes and one two-step site-selective fluorination of arenes, both of which benefit from the site-selective C–H thianthrenation reaction.
Part I describes the development of a method for regio- and stereoselective C–H thianthrenation of alkenes. The reaction can be carried out under ambient atmosphere without exclusion of air and moisture and can be performed on a gram scale. For terminal olefins, the reaction provides alkenyl thianthrenium products with high stereoselectivity; the E isomers are often the only isolable products. For internal olefins, the double bond geometry maintains unchanged throughout the thianthrenation reaction. Terminal olefins containing structurally complex and highly functionalized moieties are functionalized well. We isolated thianthrenium dicationic bicycloadducts, which afforded the desired alkenyl thianthrenium salts upon treatment with base; most likely via an E1cBirr deprotonation mechanism. The resulting alkenyl thianthrenium salts are good alkenyl electrophiles in conventional palladium-catalyzed cross-coupling reactions such as Negishi, Sonogashira and Heck reactions and ruthenium-catalyzed (pseudo)halogenation.
Part II describes the development of a photoredox catalyzed fluorination method for aryl thianthrenium (ArTT) salts, obtained from site-selective C–H thianthrenation of arenes. The substitution of aryl C–H bonds by thianthrene creates a precursor, which readily engages in photoredox chemistry. The C–H thianthrenation reaction tolerates a variety of functional groups and was applied to the synthesis of natural product derivatives. The photoredox properties of aryl thianthrenium salts were established by cyclic voltammetry, density functional theory studies, Hammett analysis, and fluorescence quenching experiments. Subsequent fluorination of ArTT species with fluoride under copper mediation and photoredox catalysis (also termed as metallophotoredox catalysis) provides pharmaceutically relevant aryl fluorides as a single constitutional isomer in a two-step sequence from the corresponding unfunctionalized arenes.
