Regioselective Synthesis of Chromones via Cyclocarbonylative Sonogashira Coupling Catalyzed by Highly Active Bridged-Bis(N-Heterocyclic Carbene)Palladium(II) Complexes

Abstract

The one-pot regioselective and catalytic synthesis of bioactive chromones and flavones was achieved via phosphine-free cyclocarbonylative Sonogashira coupling reactions of 2-iodophenols with aryl alkynes, alkyl alkynes, and dialkynes. The reactions are catalyzed by new dibromidobis(NHC)palladium(II) complexes. The new bridged N , N ′-substituted benzimidazolium salts ( L1 , L2 , and L3 ) and their palladium complexes C1 , C2 , and C3 were designed, prepared, and fully characterized using different physical and spectroscopic techniques. The molecular structures of complexes C1 and C3 were determined by single-crystal X-ray diffraction analysis. They showed a distorted square planar geometry, where the Pd(II) ion is bonded to the carbon atoms of two cis NHC carbene ligands and two cis bromido anions. These complexes displayed a high catalytic activity in cyclocarbonylative Sonogashira coupling reactions with low catalyst loadings. The regioselectivity of these reactions was controlled by using diethylamine as the base and DMF as the solvent.

A Review of Synthetic Application of Homogeneous, Heterogenised‐Homogeneous, and Microwave‐Assisted Catalytic Carbonylation of Selected Substrates

“…In general terms, this was a cyclocarbonylative Sonogashira coupling reaction. [74] There were two products detected in the final mixture as flavone "2-phenyl-4H-chromen-4-one"(A-major product) and aurone "2-benzylidenebenzofuran-3(2H)-one" (B-Minor product) [ Scheme 19].…”

“…Regioselectivity was achieved during the coupling reactions of 2‐iodophenol with aryl alkynes, alkyl alkynes, and alkynes using dibromidobis (NHC)−Pd(II) complexes. In general terms, this was a cyclocarbonylative Sonogashira coupling reaction [74] . There were two products detected in the final mixture as flavone “2‐phenyl‐4 H ‐chromen‐4‐one“(A‐major product) and aurone “2‐benzylidenebenzofuran‐3(2 H )‐one” (B‐Minor product) [Scheme 19].…”

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

“…Following the above report, authors developed dibromidobis(NHC)palladium(II) complexes for the regioselective synthesis of chromones and flavones via one‐pot phosphine free cyclocarbonylative Sonogashira coupling of 2‐iodophenols with aryl alkynes, alkyl alkynes, and dialkynes under CO pressure (Scheme 26). [60] Higher catalytic activity was exhibited by complex 33 a due to the strong electron donating effect of the ligand and the stability of the corresponding 8‐membred ring chelate. The transformation required low catalyst loading (0.5–1 mol%) and the regioselectivity was controlled by Et 2 NH and DMF.…”

“…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.…”

Iodine-PEG as a unique combination for the metal-free synthesis of flavonoids through iodonium-triiodide ion-pair complexation

“…Synthesis of flavonoids is carried out through different methods, including Baker–Venkataraman rearrangement, 5 Claisen–Schmidt condensation, 6 Allan–Robinson reactions, 7 coupling reactions, and cyclization through chalcones, etc. Many coupling reactions have been reported involving several metal catalysts 8 however, most of these suffers with limitations such as use of toxic metal catalysts, tedious work up and formation of many side products (Scheme 1).…”