A new, efficient heterogeneous Pd catalyst for enantioselective allylic substitution

The enantioselective allylic substitution of (E)-1,3-diphenylallyl acetate with dimethyl malonate was studied on Pd/Al2O3 in the presence of optically active ferrocenyl phosphines as chiral modifiers. These catalyst systems offer 81–92% yield in 2–6 h, along with good enantioselectivity (45–88% ee) despite the elevated reaction temperature (60–120 °C) and the low modifier/substrate ratio (0.2 mol%). Addition of the modifiers to Pd leads to significant rate acceleration and some modifiers induce also kinetic resolution of the racemic allylic acetate substrate. The remarkably different behavior of heterogeneous and homogeneous Pd–(R)-(S)-Josiphos catalyst systems, and particularly the missing enantioselection after removal of Pd/Al2O3, indicate that the enantioselective transformation occurs at the chirally modified Pd surface.