Unsymmetrical Pincer N-Heterocyclic Carbene–Nitrogen–Phosphine Chelated Palladium(II) Complexes: Synthesis, Structure, and Reactivity in Direct Csp²–H Arylation of Benzoxazoles

Abstract

An unsymmetrical pincer N-heterocyclic carbene–nitrogen–phosphine (CNP) and its palladium complexes PdCl2(κ2-CP) (4) and [PdCl­(κ3-CNP)]­PF6 (5·PF 6 ) were synthesized. NMR spectra disclosed that the transformation of complex 4 structure occurred in the solution. Further NMR experimental and single crystal structure analysis of complex 4 provided unequivocal and structural evidence for the formation of complex [PdCl­(κ3-CNP)]Cl (5·Cl) in the solution of complex 4. The catalytic performance of palladium complexes 4 and 5·PF 6 was investigated with the direct Csp2–H arylation of benzoxazoles with aryl bromides. Notably, aryl bromides could give up to 97% arylation products in the presence of 0.5% complex 4. For ortho-substituted substrates, the steric hindrance had a significant impact, but product yields could be improved remarkably by extending reaction time. This result implied the catalytic active species in this system was stable and kept in a long lifetime. The flexible backbone and unsymmetrical pincer structure with a flank N-heterocyclic carbine should be a feasible strategy to realize an efficient catalytic transformation. This simple catalyst system first realized the direct arylation of aryl bromides with a catalyst loading as low as 0.25% without excessive base and copper as assistant catalyst.

Transition Metal Complexes Supported by N‐Heterocyclic Carbene‐Based Pincer Platforms: Synthesis, Reactivity and Applications

“…Li′s group reported that chloride salt 92 can be metalated using Ag 2 O and then undergo transmetalation yielding species 93 (Scheme 32). [48] To promote the coordination of the central N donor, KPF 6 was employed to abstract the chlorido ligand giving complex 95 . The salt metathesis reaction can also be done before the palladation.…”

Palladium‐Based Catalysts Supported by Unsymmetrical XYC–1 Type Pincer Ligands: C5 Arylation of Imidazoles and Synthesis of Octinoxate Utilizing the Mizoroki–Heck Reaction

“…It might be existing palladacycles ( C1 – C4 ) are not real catalysts. In situ formation of palladium nanoparticles (Pd NPs) has also been recognized in many cases during the catalytic process . Thus reactions of 4‐bromobenzaldehyde with 1‐methyl‐1H‐ imidazole in DMA catalyzed with C2 under optimum conditions have been examined in detail to explain the nature of black particles produced in situ during the catalytic pathway.…”

“…Assembling of observations from XPS, TEM, Hg poisoning test and literature, a probable mechanistic pathway can be sketched out (shown in Scheme ). Palladium complexes ( C1 – C4 ) were the pre‐catalysts which form the palladium(0) species or palladium(0) clusters in the equilibrium under the reaction conditions (A' ⇄ A'') , . Due to the in‐situ formation of Pd nanoparticles during the arylation of imidazoles high surface area of Pd(0) or Pd(0) cluster shown excellent catalytic activity.…”

A decennary update on applications of metal nanoparticles (MNPs) in the synthesis of nitrogen- and oxygen-containing heterocyclic scaffolds

“…The C-H activation of heterocycles such as indoles, benzazoles, and adenosine can be easily achieved using PdNPs, which are considered active species rather than moribund species. 282 The Csp 2 -H activation of benzo[d]oxazoles (264b) was reported by Li et al to be catalyzed by pincer NHC-nitrogen-phosphinechelated Pd(II)complexes (NHC-PdNPs) (Scheme 153) using lithium tert-butoxide as a base in dimethoxyethane (DME) as the solvent at 90 C. 283 This protocol was performed using an ultra-low amount of catalyst without the requirement of Cu Li et al reported the catalytic use of a palladium diamine complex supported on GO (Pd-DI@GO) for the direct C-H activation of thiazole (266a) with bromoarenes (67c, Scheme 155) for the synthesis of substituted thiazoles (266b). 285 The same catalyst was also explored for the Suzuki coupling of bromoarenes with substituted phenyl boronic acids in moderate to excellent yields.…”

“…PdNPs (dichloro[bis{1-(dicyclohexylphosphanyl)piperidine}] palladium) obtained by the treatment of dichloro(1,5-cyclooctadiene)palladium(II) with 1-(dicyclohexylphosphanyl)piperidine was reported by Frech et al as a catalyst for the cyanation of bromo compounds (67c) using potassium ferrocyanide 312 The catalytic performance of these NPs was reported to be superior compared to Pd salts for the cyanation of aryl and heteroaryl bromides. The PdNP-supported nanosilica triazine dendritic polymer (PdNPs-nSTDP)-catalyzed Sonogashira cross-coupling of 2,6dibromopyridine or 2,4,6-trichloromopyrimidine (6l) with substituted phenyl acetylenes (3f) was achieved successfully for the green synthesis of V-or star-shaped di-or trialkynylaromatics (283), respectively (Scheme 174) under aqueous conditions at rt using N,N-diisopropyl ethylamine (DIPEA) as the base. 313 The same catalyst was also reported for the Suzuki-Miyaura cross-coupling, Heck coupling 314 and C-S coupling.…”