C–H Bond Phosphorylation Using Iron Photocatalysis

Tertiary phosphines can be useful as reactants, ligands, or organocatalysts in, e.g., organic synthesis and coordination chemistry. However, the synthesis of unsymmetrical tertiary phosphines, in particular, can still be challenging. Existing methods for this can have drawbacks such as a limited scope, low yields, and inconvenient operation. New approaches to the preparation of these species are, thus, interesting research targets. Radical phosphorylation reactions have shown promise for this type of transformation.

Feng-Cheng Jia, Wuhan Institute of Technology, China, Xiao-Qiang Hu, South-Central Minzu University, Wuhan, China, and colleagues have used an FeCl₃-mediated ligand-to-metal charge transfer (LMCT) process for a radical phosphorylation in the presence of industrial phosphine sources (i.e., Ph₂PCl or PCl₃) to achieve C(sp³)–H bond phosphorylations (general reaction pictured). The team reacted a variety of compounds that feature C(sp³)–H bonds with Ph₂PCl in the presence of FeCl₃ under irradiation by 370 nm LED lights, using CH₃CN as the solvent. To stabilize the resulting air-sensitive tertiary phosphines, they added borane after the phosphorylation reaction to form phosphine-borane complexes, which were isolated. PCl₃ could also be used as a phosphine source to give dialkyl-substituted phosphines, which was followed by workup with H₂O₂ to obtain phosphine oxides.

The researchers propose a mechanism for the formation of tertiary phosphines that involves the excitation of the FeCl₃ catalyst under LED irradiation, followed by the creation of a chlorine radical and an Fe(II) species via a photoinduced LMCT process. Then, a hydrogen-atom transfer (HAT) between the chlorine radical and a C(sp³)–H bond generates an alkyl radical and HCl. The catalyst is regenerated by a single electron transfer (SET) between a chlorophosphine and the Fe(II) intermediate, which gives a diphenylphosphinyl radical. Then, the diphenylphosphinyl radical can undergo a self-coupling to give a biphosphine compound, which reacts with the alkyl radical to give the desired product. Alternatively, the alkyl radical and the diphenylphosphinyl radical could undergo a coupling reaction to give the product.