Room-Temperature Benzylic Alkylation of Benzylic Carbonates: Improvement of Palladium Catalyst and Mechanistic Study

The
palladium catalyst for the nucleophilic substitution of benzyl
carbonates was improved by using 1,1′-bis­(diisopropylphosphino)­ferrocene
(DiPrPF) as the ligand. The [Pd­(η3-C3H5)­(cod)]­BF4–DiPrPF catalyst allows the benzylic substitution with soft
carbanions to proceed even at 30 °C, affording the desired products
in high yields (up to 99% yield). Thermally unstable pyridylmethyl
esters are employable as the electrophilic substrates for the benzylic
alkylation with the improved catalyst. Furthermore, we investigated
the mechanism of the catalytic benzylic alkylation by means of DiPrPF ligand. The palladium(0) complex bearing DiPrPF activates the benzylic C–O bond to form the
(benzyl)­palladium­(II) intermediate at room temperature. The coordination
mode of the benzyl ligand would be equilibrium between the η1- and η3-manner. The nucleophile would preferentially
react with the η3-benzyl ligand to give the desired
product.