Asymmetric CC Bond-Formation Reaction with Pd: How to Favor Heterogeneous or Homogeneous Catalysis?

摘要

The enantioselective allylic alkylation of (E)-1,3-diphenylallyl acetate was studied to clarify the heterogeneous or homogeneous character of the Pd/Al₂O₃(R)-BINAP catalyst system. A combined approach was applied: the catalytic tests were completed with in situ XANES measurements to follow the oxidation state of Pd as a function of the reaction conditions. The study revealed that the oxidized Pd (after exposure to ambient air) is efficiently reduced by the solvents THF and dioxane, and by the nucleophile sodium dimethyl malonate, and thus these conditions prevent Pd leaching. The chiral modifier BINAP plays a dual role: a considerable coverage of the Pd surface by the bulky compound slows down the initial reduction of the surface oxides but BINAP itself may consume surface oxygen (through its conversion to BINAPO and BINAPO₂) and contribute to the maintenance of the active metal surface during the reaction. Carrying out the reaction under pressure in an inert gas atmosphere is important to minimize the oxygen diffusion into the reaction mixture and to avoid leaching. The (known) effect of temperature is critical as well: our catalyst system is inactive at room temperature, which is a clear deviation from the behavior of the corresponding homogeneous system. In contrast, halogenated solvents are easily dehalogenated on Pd/Al₂O₃ and thus they favor leaching of the metal and formation of soluble compounds, analogous to classical metal corrosion in the presence of halide ions. The frequently observed dissolution of Pd in the presence of halogenated substrates may be explained similarly.