Mechanism of electrophilic fluorination with Pd(IV): fluoride capture and subsequent oxidative fluoride transfer
Electrophilic fluorinating reagents derived from fluoride are desirable for the synthesis of 18F-labeled molecules for positron emission tomography (PET). Here, we study the mechanism by which a Pd(IV)–complex captures fluoride and subsequently transfers it to nucleophiles. The intermediate Pd(IV)–F is formed with high rates even at the nano- to micromolar fluoride concentrations typical for radiosyntheses with 18F due to fast formation of an outer-sphere complex between fluoride and Pd(IV). The subsequent fluorine transfer from the Pd(IV)–F complex is proposed to proceed through an unusual SET/fluoride transfer/SET mechanism. The findings detailed in this manuscript provide a theoretical foundation suitable for addressing a more general approach for electrophilic fluorination with high specific activity 18F PET imaging.