EPR-electrochemical monitoring of P–C coupling: Towards one-step electrochemical phosphorylation of acridine

A mild, efficient electrochemical approach for the site-selective direct C(sp2)–H phosphonation of acridine with triorganyl phosphites has been developed. Catalyst-, external oxidant- and metal-free joint electrosynthesis leads to selective C9-phosphorylation in good yields. The mechanism of these one-pot oxidative transformations was investigated by voltammetry and electron paramagnetic resonance (EPR) spectroscopy methods. The reaction intermediates, such as dihydroacridine dialkylphosphonates, have been isolated, proven, and investigated by voltammetry, X-ray diffraction, etc. The magnetic resonance parameters of intermediates, the radical cations of triorganyl phosphites captured by a spin trap PBN, acridine, as well as phosphorylated acridine derivatives, have been determined. The EPR and voltammetry data made it possible to reconsider the reaction pathways of 9-phosphono-10-hydroacridanes and 9-diethylphosphono-10-methylacridane and convert them into target derivatives of phosphorylated acridine in electrochemical oxidation reaction.