Palladium(II)‐N‐Heterocyclic Carbene Complexes: Efficient Catalysts for the Direct C‐H Bond Arylation of Furans with Aryl Halides

Abstract

This paper contains the synthesis and characterization of the seven new benzimidazolium salts and their corresponding new palladium(II)‐NHC complexes with the general formula [PdX2(NHC)2], (NHC = N‐heterocyclic carbene, X = Cl or Br), and also their catalytic activity in direct C‐H bond arylation of 2‐substituted furan derivatives with aryl bromides and aryl chlorides. Under the optimal conditions, these palladium(II)‐NHC complexes showed the good catalytic performance for the direct C‐H bond arylation of 2‐substituted furans with (hetero)aryl bromides, and with readily available and inexpensive aryl chlorides. The C‐H bond arylation regioselectively produced C5‐arylated furans by using 1 mol% of the palladium(II)‐NHC catalysts in moderate to high yields.

Direct C—H Bond Activation of Benzoxazole and Benzothiazole with Aryl Bromides Catalyzed by Palladium(II)‐N‐heterocyclic Carbene Complexes

“…In the presence of 2 mol% of 2f as the catalyst at 150 °C for 8 h, the C2-arylated benzothiazole derivative namely, 2-(4-acetylphenyl)-benzothiazole was obtained in full conversion with 86% isolated yield (Table 2, entry 2). When catalyst loading was decreased from 2 mol% to 1 mol% at 150 °C for 8 h, similar yield of product was also observed ( [56,57,[60][61][62] NMR data showed that all palladium(II)-NHC complexes were cis/trans mixtures. .…”

“…[45][46][47][48] In this connection, recently, we have synthesized, characterized, and investigated palladium(II)-N-heterocyclic carbene (Pd-NHC) complexes for catalytic activity in the direct C-H bond activation of different (hetero)aromatic compounds with aryl halides. [49][50][51][52][53][54][55][56][57] The strong σ-donating but poor π-accepting ability of NHC ligands lead to the formation of many stable palladium(II)-NHC complexes. Due to activity, stability and selectivity of palladium(II)-NHC complexes, they have been widely used as highly reactive and rather selective catalysts for numerous direct arylation reactions.…”

“…[58,59] Obtained spectroscopic values for the 1a and 1b compounds are consistent with those found in the literature for benzimidazolium salts. [39,53,[55][56][57]…”

“…The spectroscopic data are similar to those found for described palladium(II)-NHC complexes in the literature. [50][51][52]56,57] All the novel compounds were prepared according to general reaction pathway depicted in Scheme 2. The analytical data are in good agreement with the compositions proposed for all the novel compounds we prepared, and are summarized in the Table 1.…”

Novel 2-hydroxyethyl substituted N-coordinate-Pd(II)(NHC) and bis(NHC)Pd(II) complexes: Synthesis, characterization and the catalytic activity in the direct arylation reaction

“…In particular, it is known that cis and trans isomeric products form depending on the experimental conditions. [29][30][31] Recently, unusually, Han et al, performed a synthesis of a new type of bis(NHC)Pd(II) complexes from 1,3disubstituted NHC precursor containing bulky substituents in DMSO. 32 This complex contains both the NHC (carbene) ligand and the N-bound (NHC precursor) ligand.…”