A key LCuiii–F species, where L is a pyridine-2,6-carboxamide ligand, mediates this hydrogen atom transfer (HAT) fluorination reaction of C–H bonds. Conditions are developed which allow LCuiii–F to be generated through electrooxidation of [LCuii–F]–, which forms in situ, and allow [LCuii–F]– to be regenerated using a metal fluoride salt. The LCuiii–F complex is shown to be most stable when dichloromethane is used as a solvent; however, only a combination of propylene carbonate and MeCN solvents along with a CsF salt supports both the generation of [LCuii–F]– and the long lifetime of LCuiii–F. To prevent the reduction of Cu species at the cathode, a sacrificial C6F6 electron acceptor is added, which is reduced in preference to Cu species and decomposes to provide additional fluoride. Additionally, an acid–base mixture of collidine and trifluoroacetic acid (TFA) collidinium is beneficial to promote the desired cathodic reactions and improve the conductivity of the reaction solution. A constant potential of 3.3 V is applied, and reticulated vitreous carbon and Pt electrodes along with a n-Bu4NClO4 electrolyte are also used.
The reaction works well for a range of aliphatic C–H bonds and interestingly, appears to favour fluorination at positions adjacent to oxygen atoms rather than at benzylic sites, in contrast to other C–H fluorination methods. This selectivity is thought to be the result of the HAT process mediated by LCuiii–F proceeding through an oxidative asynchronous pathway which is dominated by electron transfer rather than proton transfer.
This is a preview of subscription content, access via your institution
