Tetranuclear Palladium Complexes of Abnormal N‐Heterocyclic Carbene Ligands and their Catalytic Activities in Mizoroki‐Heck Coupling Reaction of Electron‐Rich Aryl Chlorides

Abstract

Based on the ligand scaffold of an imidazolyl/benziimidazolyl moiety and a N‐CH2C(=O)NHPh substituent, two series of ligand precursors with ortho hydroxy groups incorporated on the N‐phenyl rings were prepared. The structural fine tuning of the ligand scaffold allowed the synthesis of tetranuclear palladium complexes with abnormal N‐heterocyclic carbene (aNHC) ligands. For precursors with C2‐methyl blocking groups, pyridine‐assisted C−H bond activation led to the formation of mononuclear tridentate palladium aNHC complexes or tetranuclear complexes with tridentate CNO donors. Representative mononuclear and tetranuclearpalladium aNHC complexes were structurally characterized by X‐ray diffraction studies, revealing very short Pd−C bond distances. The tetranuclear palladium aNHC complexes were very effective in catalyzing Mizoroki‐Heck coupling reaction, and were capable of employing a range of aryl chlorides including deactivated substrates with low palladium loading of 0.2 mol%.magnified image

Palladium Complexes with Phenoxy- and Amidate-Functionalized N-Heterocyclic Carbene Ligands Based on 3-Phenylimidazo[1,5-a]pyridine: Synthesis and Catalytic Application in Mizoroki–Heck Coupling Reactions with Ortho-Substituted Aryl Chlorides

“…1.932 Å). 11 Nevertheless, these Pd−carbene bonds from amidate-functionalized aNHC ligands are short in comparison with the average Pd−carbene bond distance of 1.99(3) Å in reported Pd−aNHC complexes. 3b In the tetranuclear complex, the Pd1 and Pd2 atoms make a close contact with a nonbonding distance of 3.2122(18) Å, which is smaller than the sum of the van der Waals radii of Pd atoms (3.26 Å).…”

“…14 This nonbonding distance was, however, significantly longer than that in the reported tetranuclear complex A (3.1759(11) Å). 11 In contrast, the Pd(3) and Pd( 4) do not interact with each other, with a longer noninteracting distance of 3.275 Å.…”

“…The reaction between 4-chloroacetophenone and styrene was chosen as the benchmark reaction, and the reported reaction conditions were followed. 11 Initially, the new palladium complexes were screened (Table 1). Entries 1 and 2 showed that under the same monopalladium loading of 0.2 mol % (i.e., a catalyst loading of 0.05 mol % for 2bt since it is composed of four Pd atoms), the tridentate complex 2b delivered a quantitative yield of the coupled product, in comparison to the yield of 88% from the bidentate complex 2a.…”

Recent Advances on N‐Heterocyclic Carbene‐Palladium‐catalyzed Heck Reaction

“…Among aryl halides, aryl chloride finds more application in industries [49] since it is inexpensive and easily available. Although the activation of stronger Ar−Cl bond is a difficult task, tetramolecular palladium complex of abnormal NHC (aNHC) 25 showed good catalytic activity even in green reaction media as reported by Lee and co‐workers [50] . Highly polar molten salts were used to stabilize Pd (0) from forming anionic complex with halides.…”

Heteroleptic Palladium(II) Complexes of Thiazolinyl‐picolinamide Derived N∩N∩N Pincer Ligand: An Efficient Catalyst for Acylative Suzuki Coupling Reactions

“…[13,33] However, late transition metal amido complexes have been found to be potential catalysts for many organic transformations including transfer hydrogenation to an unsaturated substrate and coupling reactions (Figure 1). [34][35][36][37][38][39][40][41][42][43][44][45][46] The intrinsic structure of the pincer ligands allows the development of late transition metal amido complexes and to explore their catalytic activities. [47,48] Palladium-catalyzed cross-coupling reactions are important and pioneering methodology for producing the C sp 2 À C sp 2 , C sp 2 À N and C sp 2 À O in the synthesis of industrially important valueadded molecules and other natural products, bioactive compounds, polymers, and materials.…”