Bromoamination of α,β-unsaturated keto alkenes by palladium(ii) metalallocycles: experimental and theoretical approaches

Abstract

Mononuclear palladium(II) complexes (Pd8MOQL^1–4) were obtained by reacting 8-methyl-2-oxo-1,2-dihydroquinoline-3-carboxaldehyde-4(N)-substituted thiosemicarbazones (8MOQL^1–4) with K₂[PdCl₄]. The ligands (8MOQL^1–4) and the corresponding Pd(II) complexes (Pd8MOQL^1–4) were characterized by FT-IR, UV-visible, and ¹H NMR spectroscopic analyses. The monomeric nature of the complexes Pd8MOQL¹, Pd8MOQL² and Pd8MOQL⁴ was revealed by X-ray crystallographic analysis, and the stoichiometry of the complex Pd8MOQL³ was confirmed from its mass spectral data. The ligands coordinated with the palladium ion through quinolone oxygen, azomethine nitrogen and thiolate sulphur atoms. The catalytic potential of the synthesized complexes was revealed through the bromoamination reactions of various α,β-unsaturated keto alkenes in the presence of N-bromosuccinimide (NBS) and p-toluenesulfonamide as bromo and amino sources, respectively. Optimization studies indicated that the complex Pd8MOQL² exhibited the best catalytic efficiency among the synthesized complexes. Appreciable yield of the coupled products was obtained with minimal catalyst loading (0.05 mol%), and the obtained chiral products were confirmed through ¹H and ¹³C NMR, mass and CD spectroscopy analyses. A plausible mechanism was proposed for the formation of bromoaminated products based on the isolated α-bromo β-chloro dihalogenated keto alkane, and theoretical confirmation of the optimized intermediates (C, D1 and F1) and final product (P1) was performed through DFT studies.