The Development of Bulky Palladium NHC Complexes for the Most‐Challenging Cross‐Coupling Reactions

Abstract

Palladium-catalyzed cross-coupling reactions enable organic chemists to form C-C bonds in targeted positions and under mild conditions. Although phosphine ligands have been intensively researched, in the search for even better cross-coupling catalysts attention has recently turned to the use of N-heterocyclic carbene (NHC) ligands, which form a strong bond to the palladium center. PEPPSI (pyridine-enhanced precatalyst preparation, stabilization, and initiation) palladium precatalysts with bulky NHC ligands have established themselves as successful alternatives to palladium phosphine complexes. This Review shows the success of these species in Suzuki-Miyaura, Negishi, and Stille-Migita cross-couplings as well as in amination and sulfination reactions.

Graphitized nanocarbon-supported metal catalysts: synthesis, properties, and applications in heterogeneous catalysis

“…because of their ability to construct carbon-carbon bonds [169,170]. In 2010, Richard Heck, Ei-ichi Negishi, and Akira Suzuki jointly received the Nobel Prize for Chemistry for their roles in discovering and developing highly practical method for palladium-catalyzed cross-coupling reactions [171][172][173]. The typically accepted methods for coupling reactions are performed under homogeneous conditions that employ organic ligand-stabilized homogeneous complexes.…”

Cross coupling reactions catalyzed by (NHC)Pd(II) complexes

“…Formation of palladium nanoparticles in the reaction mixture was confirmed by dynamic light scattering and transmission electron microscopy studies and a mercury poisoning experiment. The complexes C5, bearing benzimidazole and pyridine groups have been proved to be a highly efficient catalyst for the coupling reaction of aryl halides with various substituted acrylates under mild conditions in excellent yields (Table 1, entries [16][17][18][19][20][21][22]. 43 Electron-deficient aryl bromides gave a slightly higher yield than electron-rich ones under the optimized conditions.…”

“…The six-and seven-membered Pd-PEPPSI-type complexes C29a and C29b have been employed in Suzuki coupling of aryl bromide and chloride substrates (Table 4, entries [19][20][21][22]. 67 A series of dimetallic complexes (C30 and C31), bridged by bis-imidazolylidenes, with different spacers (phenylene and biphenylene) and the related monometallic complexes (C32 and C33) were screened in the Suzuki coupling between aryl halides and arylboronic acids.…”

“…Their steric bulk enables stabilization of a low-valent active intermediate and favors reductive elimination, while the strong σ -donor character facilitates the oxidative addition of aryl halides. Complex C96 was proven to be superior to its [Pd(IPr*)(acac)Cl] congener (Table 14, entries [18][19][20][21]. 135 In this context, (IPent Cl )PdCl 2 (o-Picoline) (C97)…”

“…Suzuki coupling reactions carried out using immobilized Pd-NHC catalysts. The palladium catalyst C85 based on modified halloysite nanotubes displayed good activity, allowing the synthesis of several biphenyl compounds in high yield working with only 0.1 mol% palladium loading (Table 12, entries [14][15][16][17][18][19] Applications of a polymer supported air-stable palladium NHC complex with a spacer (catalyst C90,…”