Photocatalytic C−I Bond Borylation and Phosphorylation of Diaryliodonium Salts with Excellent Atom‐Economy

Highly atom‐economic conversion of diaryliodonium salts represents an underdeveloped but highly desirable domain. Most reactions involving these species only utilize single aryl group of the diaryliodonium salts and produce an equivalent of aryl iodide as waste. Herein, by further transforming the side‐product aryl iodide, we report an overall new two‐step one‐pot strategy that allows photocatalytic C−I bond borylation of diaryliodonium salts and ensures effective conversion of both aryl groups of the diaryliodonium into the relevant arylboronic esters. Mechanistic investigations suggest that electron‐donor‐acceptor complex is formed between the photocatalyst phenothiazine and diaryliodonium salt substrate. Upon visible light irradiation, this complex produces aryl radical and aryl iodide. With addition of a base in the second step, the reducing ability of the photocatalyst is enhanced via proton‐coupled electron transfer process, thereby aryl iodide produced in the first step is reduced to yield aryl radical again. Both aryl radicals generated in the two steps react with B₂(OR)₄ to produce the corresponding monoarylation product in a quantitative yield. The strategy is also applicable for C−I bond phosphorylation of diaryliodonium salts, ensuring that both aryl moieties can be phosphorylated with high efficiency.