Efficient Heck Reactions Catalyzed by Palladium(0) and -(II) Complexes Bearing N-Heterocyclic Carbene and Amide Functionalities

Abstract

A palladium(0) NHC complex Pd0(LH1)2(MA) (MA = maleic anhydride) was prepared from the amide-imidazolium salt [LH1H2]Cl (H1 = NH proton; H2 NCHN proton). The X-ray diffraction studies confirmed that a η2-MA ligand and two monodentate NHC ligands with the H1 protons remaining intact are coordinated. These NH protons are involved in intra- or intermolecular hydrogen bonds stabilizing the solid-state structure. Degradation of Pd(LH1)2(MA) in air leads to the formation of the chelate complex trans-PdIIL2 and other unidentified products. Negative-ion electrospray mass spectrometry revealed some intriguing Pd(0) species, including a 14-electron [Pd0L]− species that bears only a bidentate NHC/amido ligand. The anionic amido group imparts a high electron density on a palladium center, as shown by X-ray photoelectron study. The palladium(0) precatalyst is highly efficient in catalyzing Heck reactions with activated aryl chlorides in ionic liquid. For deactivating aryl chlorides and bulky aryl bromides, cis-PdIIL2 is more effective. A range of Heck-coupled products can be prepared by Pd0(LH1)2(MA) and cis-PdIIL2. The latter complex also successfully mediates one-pot sequential Heck/Heck and Suzuki/Heck coupling reactions with 4-bromochlorobenzene as substrate.

The interaction between heteroditopic pyridine–nitrogen NHC with novel sulfur NHC ligands in palladium(0) derivatives: Synthesis and structural characterization of a bis-carbene palladium(0) olefin complex and formation in solution of an alkene–alkyne mixed intermediate as a consequence of the ligands hemilability

“…All bond distances involving the Pd(0) metal are in reasonable agreement with those in other complexes of Pd(0) with maleic anhydride [18,23]. In complex 5b the Pd1-C2 length (2.125 (3) Å) is considerably longer than the corresponding Pd1-C1 (2.060(3) Å) owing to the trans influence exerted by the carbene carbon of the ligand on the coordinated olefin, whereas the Pd-olefin distances in the case of complex 9 are similar in agreement with the findings of Lee et al [18]. Remarkably, the Pd1-C5 and Pd1-C16 bonds in the bis-carbene derivative 9 are lengthened with respect to those of the Pd-C carbene complex 5b and of those of the chelate heteroditopic complexes described elsewhere [5g,11c,13b] but compare well with those of the similar complexes described in Ref.…”

“…In both structures the palladium center is trigonal planar, as expected for zero-valent complexes of the type [PdL 0 L 00 (g 2 -alkene)]. All bond distances involving the Pd(0) metal are in reasonable agreement with those in other complexes of Pd(0) with maleic anhydride [18,23]. In complex 5b the Pd1-C2 length (2.125 (3) Å) is considerably longer than the corresponding Pd1-C1 (2.060(3) Å) owing to the trans influence exerted by the carbene carbon of the ligand on the coordinated olefin, whereas the Pd-olefin distances in the case of complex 9 are similar in agreement with the findings of Lee et al [18].…”

“…Moreover, palladium(0) complexes bearing two monocoordinating carbene and maleic anhydride or tetracyanoethene have appeared so far in the literature [12,18]. However, attempts to synthesize complexes bearing two different heteroditopic carbenes simultaneously coordinated to low valent palladium were never carried out.…”

Synthesis of palladium(0) and -(ii) complexes with chelating bis(N-heterocyclic carbene) ligands and their application in semihydrogenation

“…The bis-carbene bite angle of the complex is 87.50(9)°, a small deviation from the ideal angle dictated by the methylene bridge. 39 The palladium(0)-carbene bond distances of 3a are shorter than those of the recently reported Y-shaped [Pd 0 (bis-NHC)- 43 The CvC double bond of the maleic anhydride, which is η 2 coordinated to the metal center and almost perpendicular to the coordination plane (Fig. 2), is elongated compared to the free alkene due to π-back bonding from the palladium(0) center.…”

Revisiting Heck–Mizoroki reactions in ionic liquids

“…7 are the structure of other palladium complexes bearing NHC and amide functionalities that showed also to be effective catalysts for reactions involving aryl chlorides and electron poor alkenes. 80,81 The possibility of using NHC-Pd complexes immobilized on silica nanoparticles as catalysts in Heck-Mizoroki reactions was considered by Sen and Tandukar. 82 The synthetic strategy to obtain the immobilized NHC ligand (Scheme 20) involved after the alkylation of imidazole with decyl bromide, the quaternization with neat chloropropyl triethoxysilane.…”