Synthesis of Hemilabile Phosphine−Phosphine Oxide Ligands via the Highly Selective Pd-Catalyzed Mono-oxidation of Bidentate Phosphines: Scope, Limitations, and Mechanism

The first simple and efficient, one-step catalytic method has been developed for the preparation of bis-phosphine monoxides, valuable hemilabile ligands that have proven usefulness in homogeneous catalysis, synthesis, analytical chemistry, cancer and AIDS research, chemistry of materials, etc. Readily available bidentate phosphines (dppm, dppe, dppp, dppb, dppbz, dppfc, and BINAP) are selectively oxidized to the corresponding mono-oxides in 65−90% isolated yield with 1,2-dibromoethane in the presence of alkali and catalytic quantities (0.15−2 mol %) of a Pd(II) salt. This anaerobic oxidation smoothly occurs under biphasic conditions at 20−80 °C and atmospheric pressure. In certain cases, the mono-oxidation is promoted by I - . Stoichiometric studies of the novel process demonstrate that the catalytic loop involves (i) the formation of Pd(0) and a bis-phosphine monoxide (BPMO) upon reaction of a Pd(II) complex of the corresponding bis-phosphine with alkali, (ii) chelation-driven phosphine ligand exchange to displace the coordinated BPMO produced, and (iii) reoxidation of the Pd(0) complex with 1,2-dibromoethane via C−Br oxidative addition followed by β-bromide elimination.