Journal of the American Chemical Society, Vol.143, No.12, 4809-4816, 2021
A Palladium Complex as an Asymmetric pi-Lewis Base Catalyst for Activating 1,3-Dienes
Here we report that palladium(0) complexes can coordinate in a eta(2) fashion to 1,3-dienes and significantly raise the energy of their highest occupied molecular orbital (HOMO) by donating the electrons from the d-orbitals to the empty antibonding molecular orbitals of double bonds (pi*) via back-bonding. Thus, the uncoordinated double bond, as a more reactive partner on the basis of the principle of vinylogy, can directly attack imines, furnishing a formal hydrodienylation reaction enantioselectively. A chemoselective cascade vinylogous addition/allylic alkylation difunctionalization process between 1,3-dienes and imines with a nucleophilic group is also compatible, by trapping in situ formed pi-allylpalladium species after initial ene addition. This pi-Lewis base catalytic mode, featuring simple eta(2)coordination, vinylogous activation, and compatibility with both conjugated neutral polyenes and electron-deficient polyenes, is elucidated by control experiments and density functional theory (DFT) calculations.