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

Abstract

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.

Recent Advances in Benzimidazole Based NHC‐Metal Complex Catalysed Cross‐Coupling Reactions**

“…Bridged bis NHC carbene complexes were utilized for the carbonylative Suzuki-Miyaura coupling of aryl boronic acids with aryl halides by Ali et al (Scheme 17). [50] The stability and catalytic activity of these synthesized complexes were ascribed to the chelating effect of the bridging dicarbene ligands. The carbonylative coupling of different aryl iodides and aryl diiodides with aryl boronic acids and aryl diboronic acids were achieved with low catalyst (0.01 mol%) loading giving the desired product in excellent yield within 3 hours.…”

“…The possibility of diverse structural modifications with benzimidazole scaffolds makes it a reliable ligand in transition metal catalyzed reactions. In recent years, several multidendate ligands have been developed by anchoring two or three monodentate benzimidazole based NHCs to a pivotal centre-CH 2 [35,38,50,60,67] or a rigid framework for instance, biphenyl, [46] phenylene [66] etc. Moreover, these multidenated benzimidazole NHC ligands were found to show excellent stability towards oxygen as well as moisture, thus acting as potential candidates for cross coupling reactions.…”

Recent advances in the synthesis of 4H-chromen-4-ones (2012 − 2021)

“…High catalytic activity has been observed using bis(triphenylphosphine)palladium(II) dichloride as catalyst in the presence of a benzimidazolium ligand in dipropylamine at 130 °C191 or using triethylamine and PEG-2000 in water at room temperature. 192 Palladium(II) acetate, 193 palladium on carbon, 194 bridgedbis(N-heterocyclic carbene, NHC)palladium(II) complexes, 195 propylenebridged bis(NHC)palladium(II) complexes covalently anchored on Merrifield's resin 196 and palladium-supported amine-functionalized montmorillonite as a heterogeneous catalyst 197 were also good catalytic systems for these regioselective syntheses. An alternative approach used Mo(CO) 6 , as solid CO source, for the one-pot synthesis of 2-alkyl/2-aryl-4Hchromen-4-ones via carbonylative Sonogashira annulation reaction of 2-iodophenols with terminal alkynes promoted by a Pd-NHC catalyst and dimethylamine in DMF ate 95 °C.…”

Fabrication of thiosemicarbazone-based Pd(ii) complexes: structural elucidations, catalytic activity towards Suzuki–Miyaura coupling reaction and antitumor activity against TNBC cells

“…Bulky dialkylbiaryl and triarylphosphines continue to be the most employed ligands for cross-coupling processes, followed by N-heterocyclic carbenes (NHCs). [39][40][41][42][43][44] Several palladium complexes containing pincer-type ligands have been found to be effective catalysts for C-C bond formation reactions. However, there are some reports of heterocyclic thiosemicarbazonebased Pd(II) complexes as catalysts in the SMC reaction.…”