A General Strategy for Increasing the Air‐stability of Phosphines Including Primary Phosphines

Given recent reports of structurally diverse air‐stable primary phosphines lacking kinetic stabilization, the stability against oxidation was difficult to rationalize and attributed to the specific electronic nature of the phosphine group. This contribution shows the synthesis of a series of air‐stable primary phosphines Fc(CH2)nPH2, where n = 0,1,2,3; and Fc = ferrocenyl, and their corresponding isolable primary phosphine oxides. It was demonstrated that the ferrocene moiety exerts an antioxidant effect on the primary phosphine group, which is intermolecular, solvent dependent and increases with the electron density on the ferrocene moiety. Furthermore, we demonstrated that the presence of ferrocene in solution also inhibits the oxidation of other secondary and tertiary phosphines in air. The collected information suggests that quenching of singlet oxygen is the actual reason for the antioxidant effect which was experimentally confirmed using other established singlet oxygen quenchers as well, demonstrating a key role of singlet oxygen in the aerobic oxidation of phosphines.