Synthesis of Soluble Quinacridone Photocatalysts for the Homogeneous Oxidation of Thioethers

Abstract Evaluation of quinacridone (QA) derivatives as homogeneous metal‐free photocatalysts is here presented. QA derivatives were synthetized and systematically characterized, measuring their ground state and excited state redox potentials in dichloromethane (DCM) and 1,1,1,3,3,3‐hexafluoro‐2‐propanol (HFIP), in order to understand how structural modifications influenced their photocatalytic properties. In particular, the effect of dicyanomethylene and nitro EWG groups was investigated, in order to develop a photocatalyst capable of promoting oxidative processes in the presence of molecular oxygen. Among the analyzed derivatives, 2,9‐dinitro‐ N,N′ ‐dibutylquinacridone (DNDBQA) was the one with the highest excited state reduction potential (E red* =1.60 V in HFIP vs SCE), while N,N’ ‐dibutylquinacridone (DBQA) showed valuable excited state redox potentials (E red* =1.29 V; E ox* =−1.28 V in HFIP vs SCE), making it suitable for bimodal applications in oxidative and reductive photocatalytic processes. Afterwards, the synthetized QA derivatives were examined as photocatalysts to promote the selective aerobic oxidation of thioether to sulfoxide. Promising results in thioanisole oxidation were achieved with all the QA derivatives tested as photocatalysts, in terms of yield and selectivity. Remarkably, DBQA showed the best performances, catalyzing the reaction in only 20 minutes, using 0.5 % of the photocatalyst, and showing excellent performances in the oxidation of several thioether derivatives.