Unveiling the Mechanism, Origin of Stereoselectivity, and Ligand-Dependent Reactivity in the Pd(II)-Catalyzed Unbiased Methylene C(sp3)-H Alkenylation-Aza-Wacker Cyclization Reaction.

Abstract

The mechanism, origin of stereoselectivity, and ligand-dependent reactivity of Pd(II)-catalyzed methylene C(sp3)-H alkenylation-aza-Wacker cyclization to form (E)-β-stereogenic γ-lactam have been comprehensively studied by density functional theory (DFT) calculations. The calculated results reveal that the methylene C-H activation assisted by K2CO3 via the concerted metalation-deprotonation mechanism is found to be the most preferred pathway, where the enantioselectivity is distinguished by the orientation of the methyl group of a substrate relative to a chiral ligand. However, the stereochemistry of the olefin moiety in the generated product is mainly determined by the oxidative addition step, where the coulombic interaction and dispersion effect differentiate the energy difference of diastereomeric transition states. In terms of the agostic interaction nature of "three-center two-electron" transition states, the discrepancy of reactivities caused by different Pd catalysts is attributed to the electron induction effect of substituents on the chiral ligands. In other words, the use of an electron-withdrawing group (e.g., -CN) in place of an electron-donating group (e.g., -OMe) enhances the oxidation state of the Pd atom and lowers vacant d orbitals of the palladium atom of the catalyst and in turn facilitates a larger amount of σ-electronic-charge injection into an empty 3d shell of the palladium center. Thus, the higher catalytic activity of the Pd catalyst with ligands substituted by an electron-withdrawing group is anticipated.