Umpolung of Methylenephosphonium Ions in Their Manganese Half-Sandwich Complexes and Application to the Synthesis of Chiral Phosphorus-Containing Ligand Scaffolds

Half-sandwich manganese methylenephosphonium complexes [Cp(CO)(2)Mn((2)-R2PC(H)Ph)]BF4 were obtained in high yield through a straightforward reaction sequence involving a classical Fischer-type manganese complex and a secondary phosphine as key starting materials. The addition of various nucleophiles (Nu) to these species took place regioselectively at the double-bonded carbon center of the coordinated methylenephosphonium ligand R2P+C(H)Ph to produce the corresponding chiral phosphine complexes [Cp(CO)(2)Mn((1)-R2PC(H)(Ph)Nu)], from which the phosphines were ultimately recovered as free entities upon simple irradiation with visible light. The synthetic potential of this umpolung approach is illustrated herein by the preparation of novel chiral pincer-type phosphine-NHC-phosphine ligand architectures.