Photochemical reactions: a powerful tool for the functionalisation of aromatic compounds

The Friedel–Crafts reaction, one of the earliest examples of Lewis acid-promoted transformations, has long been a key method for forming C–C bonds. Traditionally, it requires a catalyst to proceed, including Lewis acids such as AlCl₃, FeCl₃, BF₃, ZnCl₂, and TiCl₄; Brønsted acids like HF, H₂SO₄, and H₃PO₄; as well as acidic oxide catalysts such as silica–alumina and cation-exchange resins. However, due to the harsh and often environmentally unfriendly conditions associated with these classical methods, alternative strategies have been actively explored. In this context, photocatalysis has emerged as a powerful and sustainable approach for C–C bond formation, enabling the generation of reactive radical intermediates under mild conditions using visible light. In photocatalysis, there are many methods to generate carbon, oxygen, or nitrogen radicals. In recent years, the use of thianthrenium salts1 has emerged as a powerful method to obtain these compounds. This work focuses on the development of a metal-free alkylation method for aromatic rings through the formation of electron donor–acceptor (EDA) complexes2, using photochemical activation and carboxylates. Thianthrenium salts are employed to establish tight ion-pair interactions with carboxylates, leading to the formation of the desired products. The approach enables efficient alkylation even in the presence of electron-withdrawing groups, which typically reduce the reactivity of the aromatic ring toward electrophilic substitution. Various sulfur-based compounds, such as thianthrene, dibenzothiophene, phenoxathiine, and tetrafluorothianthrene, are explored for this purpose.