Efficient PEPPSI-Themed Palladium N-Heterocyclic Carbene Precatalysts for the Mizoroki–Heck Reaction

Abstract

Two sets of six PEPPSI (pyridine enhanced precatalyst preparation, stabilization, and initiation) themed palladium complexes of amide-functionalized N-heterocyclic carbenes [PdBr2(NHC)(Py)], where NHC = 1-acetamido-3-R-imidazolin-2-ylidene, 1-acetamido-3-R-benzimidazolin-2-ylidene and Py = pyridine, were prepared. Solid-state structures of all these complexes were determined by single-crystal X-ray diffraction methods. All complexes under scrutiny were found to adopt a trans arrangement with square-planar geometry. These complexes showed excellent catalytic activity toward the Mizoroki–Heck cross-coupling reaction of aryl chlorides and styrene. Benzimidazole-derived complexes exhibited better catalytic activity than imidazole-based complexes. Formation of palladium nanoparticles in the reaction mixture was evidenced by dynamic light scattering and transmission electron microscopy studies and a mercury poisoning experiment, suggesting the possible involvement of palladium nanoparticles in the catalytic reactions.

Novel (N‐heterocyclic carbene)Pd(pyridine)Br2 complexes for carbonylative Sonogashira coupling reactions: Catalytic efficiency and scope for arylalkynes, alkylalkynes and dialkynes

“…In both palladium(II) complexes 3a and 3b , the NHC carbene and pyridine ligands are in trans positions and the geometry is distorted square planar. The PdC, PdBr and PdN bond distances are in the same range as those reported for analogous palladium complexes . The dihedral angles between the NHC and [PdBr 2 CN] mean planes are 89.6(2)° and 86.68(9)° for 3a and 3b , respectively, while those between the pyridine and [PdBr 2 CN] mean planes are 42.9(2)° and 53.4(1)°, respectively.…”

“…Recently, much attention has been paid to the development of N‐heterocyclic carbene (NHC) metal complexes, and notable advances have been made especially for their applications as organometallic catalysts . This is attributed to the ability of NHC ligands to provide highly active and stable transition metal catalysts for various organic transformations.…”

Suzuki–Miyaura and Hiyama coupling catalyzed by PEPPSI-type complexes with non-bulky NHC ligand

“…Air and moisture stable PEPPSI complexes have found many applications as catalysts of C C and C N cross-coupling reactions due to their ready activation under the reaction conditions [3][4][5][6][7] . In addition, PEPPSI complexes can be easily obtained and are easy to handle in contrast to other Pd-NHC precursors [8][9][10][11][12][13][14][15].…”

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

“…42 The catalyst system is capable of delivering excellent trans product yield with aryl chlorides, which are known to be less reactive (Table 1, entries 12-15). Benzimidazole-derived complexes (C4a-c) exhibited better catalytic activity than imidazole-based complexes (C3a-c).…”