The synthesis, characterization, and catalytic properties of ($\kappa ^{2}-C,N)$-palladacycles with N-heterocyclic carbene-based ancillary ligands

Abstract

Novel five-membered ( κ 2 −C, N ) -palladacyclic complexes were prepared from the reaction of the corresponding acetate-bridged palladacycle dimer with N-heterocyclic carbene (NHC) ligands in high yields. Palladacyclic complexes (3) were fully characterized by elemental analysis and 1 H and 13 C NMR spectroscopy. Palladacyclic complexes were tested as catalyst for the C-C bond forming reaction. These complexes were found to be efficient catalysts for the Suzuki-Miyaura reaction of aryl bromides.

Synthesis, characterization and catalytic activity in Suzuki–Miyaura coupling of palladacycle complexes with n‐butyl‐substituted N‐heterocyclic carbene ligands

“…Thus, the highest turnover frequency (TOF) values at 80°C were between 70 and 99 h −1 . These TOF values are higher than in the case of similar systems based on NHC‐bearing palladacycles …”

“…All the 1 H NMR spectra of 1a-c display characteristic singlets at 10.26, 10.27 and 10.14 ppm for the imidazolium protons, as reported previously in the literature for similar imidazolium salts. [6,7]…”

“…These TOF values are higher than in the case of similar systems based on NHC-bearing palladacycles. [6,7] In our previous work, [7] we synthesized imidazolium salts that bear a methyl side-chain attached to one of the nitrogen atoms of the NHC ring and their Pd-NHC complexes (5a-c, 6a-c; Scheme 4). If we compare the TOF values obtained, it appears that the catalytic activity is higher for the metal complexes in which the n-butyl side-chain is attached to one of the nitrogen atoms of the NHC ring (Table 1).…”

“…Although a wide variety of phosphorus‐, nitrogen‐, sulfur and oxygen‐derived palladacycles have been synthesized and utilized in catalytic coupling reactions, [1b] the number of palladacycles with NHCs is rather limited. Recently, we succeeded in developing a series of NHC‐ligated palladacycles, and these complexes showed good catalytic properties in the Suzuki–Miyaura cross‐coupling reaction . Inspired by these studies, we decided to explore the behaviour of palladacycles bearing n ‐butyl‐substituted NHC ligands.…”

“…Recently, we succeeded in developing a series of NHCligated palladacycles, and these complexes showed good catalytic properties in the Suzuki-Miyaura cross-coupling reaction. [6,7] Inspired by these studies, we decided to explore the behaviour of palladacycles bearing n-butylsubstituted NHC ligands. This kind of palladacycle combines the characteristics of the NHC ligands with the stability of palladacycle complexes, and may potentially be applicable to the catalysis of cross-coupling reactions.…”