DFT mechanistic investigation of Wacker-type oxidation of olefins catalyzed by a Pd(ii) quinoline-2-oxazoline complex: the effect of electronic asymmetry of the ligand

Abstract

This study investigates the mechanism of Wacker-type oxidation of olefins mediated by the Pd–Quinox catalyst, which incorporates an electronically asymmetric quinoline-2-oxazoline (Quinox) ligand, utilizing density functional theory (DFT) calculations. The results demonstrate that oxidation proceeds via syn-peroxypalladation, followed by ring expansion and a 1,2-hydride shift, which is in agreement with the experimental results reported earlier. The 1,2-hydride shift is identified as the rate-limiting step. To evaluate the effects of ligand modification, a series of Pd(II) catalysts bearing Quinox ligands with various substituents on the quinoline moiety are examined. Furthermore, a series of para-substituted styrene derivatives are employed to examine the effects of substrate variation. We also investigate the effect of the electronic asymmetry of Quinox, pyridyl oxazoline (Pyrox), 2-(pyridin-2-yl)benzoxazole (PBO), and imidazolin-2-imine (AmIm) ligands on the stability of catalytic intermediates in palladium-catalyzed Wacker-type oxidation, utilizing DFT and natural bond orbital (NBO) analyses. The findings indicate that these ligands influence the overall reaction pathway by controlling the arrangement of reactants on the metal center within the catalytic intermediates. The Pyrox ligand displays behavior similar to that of Quinox, whereas the AmIm ligand exhibits a stronger σ-donating and π-accepting character compared to the structurally related β-diketiminate (BDK) ligands. A detailed structural analysis of the optimized geometries of intermediates with these ligands, along with an in-depth discussion of second-order perturbation energies associated with various donor–acceptor interactions, is presented.