Integrating Continuous-Flow Electrochemistry and Photochemistry for the Synthesis of Acridinium Photocatalysts Via Site-Selective C-H Alkylation

Acridinium dyes are among the most frequently studied classes of organic photocatalysts because of their favorable excited-state properties such as high reduction potential and good lifetime. However, it remains challenging to modulate their catalytic performance by a structural modification. Here, we report a two-step continuous-flow system for the synthesis of functionalized acridinium photocatalysts through a site-selective late-stage C(aryl)-H functionalization of the acridinium core. The alkylation is achieved by pumping the parent acridinium dye sequentially through a photoreactor to achieve cross-coupling with an organotrifluoroborate and an electrochemical reactor for electrocatalytic dehydrogenation. The two-step automatic system allows the introduction of a diverse range of alkyl groups at the 3-position of the acridinium dye. Subjecting 3-alkylated acridinium salts to the flow system for a second alkylation forms 3,6-disubstituted acridinium dyes.