Enantioselective allylic substitution on Pd/Al 2O 3 modified by chiral diphosphines

Abstract

The allylic substitution of ( E)-1,3-diphenylallyl acetate ( 1a) with dimethyl malonate was investigated on Pd/Al 2O 3 modified by ( R)- and ( S)-BINAP, ( S , S )-Chiraphos, ( R)-C 3-Tunephos, and ( R)-Solphos. Stable performance of Pd/Al 2O 3 required its reduction in hydrogen before the allylic substitution reaction, which was carried out under Ar. The enantioselectivity of the Pd/Al 2O 3-BINAP system [58–60% ee to ( S)- 2] was independent of the reaction temperature (60 and 120 °C), and only 300 ppm BINAP related to 1a was required. According to UV–vis analysis, only a small fraction of BINAP was adsorbed on Pd/Al 2O 3. At 120 °C, full conversion and 94% chemoselectivity were achieved in 6 h. For comparison, analogous soluble Pd–BINAP complexes were poorly efficient and afforded low ee to the opposite enantiomer of the product at 60 °C or above. Electron microscopy could detect no restructuring of Pd in Pd/Al 2O 3 during the reaction. Surprisingly, the addition of BINAP induced a significant rate acceleration (by a factor of almost 7 at 60 °C) and also improved the chemoselectivity of Pd/Al 2O 3. Among the diphosphine ligands tested, ( R)-Solphos was the most effective (67% ee). An important advantage of the Pd/Al 2O 3–BINAP system is that it can transform not only the allyl acetate 1a, but also the corresponding allyl alcohol 1b without the application of any additive.