Applications of thianthrene chemistry in organic synthesis

Thianthrene was first reported in 1869 and has been widely used in materials science for redox-flow batteries, polymers, supramolecular chemistry and phosphorescent materials. Despite extensive studies of the thianthrene radical cation and its reactivity since 1957, applications of thianthrene in synthetic chemistry were virtually absent from the literature until 2019. The then-discovered unusually high selectivity for thianthrenation by electrophilic aromatic substitution allowed for the synthesis of single constitutional isomers of structurally complex aryl-thianthrenium salts, which were mostly used as aryl (pseudo)halide analogues for subsequent functionalization. Since then, it has become apparent that the electronic structure of the thianthrenium substituent enables reaction chemistry that in part goes beyond what can be achieved with conventional organo(pseudo)halides. In this Review, we analyse and explain the fundamental aspects of organothianthrene chemistry, highlight the difference in reactivity to conventional organo(pseudo)halides and showcase its diverse applications. Thianthrene, long used in materials science, has recently emerged as a powerful reagent in organic synthesis. Its unique electronic structure enables access to diverse aryl, alkenyl and alkyl thianthrenium salts, which exhibit reactivity beyond conventional (pseudo)halides. This Review highlights the fundamental properties, distinctive reactivity and synthetic applications of these thianthrenium salts.