Robust alkyl-bridged bis(N-heterocyclic carbene)palladium(II) complexes anchored on Merrifield's resin as active catalysts for the selective synthesis of flavones and alkynones

Highly active and efficient propylene-bridged bis(N-heterocyclic carbene)palladium(II) complexes covalently anchored on Merrifield's resin were synthesized and characterized using various physical and spectroscopic techniques. The two anchored Pd(II) complexes consist of the system: Merrifield's resin-linker-bis(NHC)Pd(II), the linkers being benzyl and benzyl-O-(CH₂)₃ for (Pd-NHC1@M) and (Pd-NHC2@M), respectively. The short linker anchored bis-benzimidazolium ligand precursor (PBBI-1@M) was synthesized via direct carbon–nitrogen alkylation of a propylene-bridged bis(benzimidazole) (PBBI-1) by Merrifield's resin chlorobenzyl group. The longer linker anchored bis-benzimidazolium ligand precursor (PBBI-2@M) was obtained in a two-step reaction involving first alkylation of (PBBI-1) with 3-chloro-1-propanol followed by a nucleophilic substitution at Merrifield's resin chlorobenzyl group. Both supported ligand precursors (PBBI-1@M and PBBI-2@M) reacted with palladium acetate to produce the two heterogeneous catalysts (Pd-NHC1@M) and (Pd-NHC2@M). ¹³C NMR palladation shift of the benzimidazole N–C–N (C2) carbon was found very similar in both the liquid NMR spectra of the homogeneous complexes and the CP/MASS spectra of the corresponding covalently anchored complexes. The catalytic activity, stability, and the recycling ability of the supported catalysts have been investigated in the carbonylative Sonogashira coupling reactions of aryl iodides with aryl alkynes and alkyl alkynes and also in the cyclocarbonylative Sonogashira coupling reactions of aryl iodides with aryl alkynes via one pot reactions. The longer linker catalyst Pd-NHC2@M demonstrated excellent catalytic activity, stability, and very high recycling ability in the two carbonylative coupling reactions. These systems exhibit the hypothesized thermodynamic stability offered by the chelate effect in addition to the strong sigma donor ability of a bis(NHC) ligand system generating electron-rich palladium centers that favor the oxidative addition step of the aryl halide.