Facile Oxidative Addition of Organic Halides to Heteroleptic and Homoleptic Pd⁰–N‐Heterocyclic Carbene Complexes

Abstract

Novel heteroleptic Pd0 complexes with an N‐heterocyclic carbene (NHC) ligand [(Me3P)Pd(NHC)] (NHC = IPr, 1; SIPr, 2) were obtained from [Pd(CH2=CHPh)(PMe3)2] and an equivalent of NHC [NHC = 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr) or 1,3‐bis(2,6‐diisopropylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene (SIPr)]. Further treatments of complexes 1 and 2 with an additional equimolar NHCafforded the corresponding bis(NHC)–Pd0 complexes [Pd(NHC)2] (NHC = IPr, 3; SIPr, 4). Complexes 1–4 readily reacted with dichloromethane or chloroform to give C–Cl oxidative addition products. In addition, the reactivity of complex 2 toward other organic halides such as bromobenzene, trans‐1,2‐dichloroethylene, and 5,5′‐dibromo‐2,2′‐bithiophene to produce the corresponding oxidative addition products was investigated. Finally, the ligand replacement of complex 1 with a chelating phosphane was examined.

An Efficient Synthetic Approach to trans‐(NHC)2Pd(R)Br Type Complexes and Their Use in Suzuki–Miyaura Cross‐Coupling Reactions

“…74–93 % yields (Scheme ). The Pd–CH 2 Ph and Pd–( o ‐OMeC 6 H 4 ) moieties in the 3a 1 – b 2 complexes were characterized by 1 H NMR and 13 C{ 1 H} NMR spectroscopy, in accordance with the reported values of the related mixed organohalidopalladium(II) analogs known in the literature , …”

“…Notably, a computational study investigating the mechanism of the aryl amination reaction also reported similar stabilization of the trans isomer of a mixed organohalido intermediate complex of the type trans ‐(NHC) 2 Pd(Ph)Cl (NHC = 1,3‐di‐ tert ‐butylimidazol‐2‐ylidene) over its cis isomer by 17.0 kcal mol –1 . Further justification of the trans isomer as the C–C cross‐coupling intermediate came from the observation that the oxidative addition of the aryl halide to the palladium(0) N‐heterocyclic carbene complexes too yielded analogous trans ‐[(NHC) 2 Pd(R)X] type complexes [NHC = 1,3,4,5‐tetramethylimidazol‐2‐ylidene, R = C 6 H 5 , X = I; R = p ‐NO 2 C 6 H 4 , X = I; NHC = 1,3‐di‐ tert ‐butylimidazol‐2‐ylidene, R = p ‐CH 3 C 6 H 4 , X = Cl; R = p ‐CO 2 MeC 6 H 4 , X = Cl; R = p ‐OMeC 6 H 4 , X = Cl; NHC = 1,3‐ bis (2,6‐diisopropylphenyl)imidazolidin‐2‐ylidene, R = CH 2 Cl, X = Cl] …”

“…As catalytic intermediates provide valuable insights about the reaction mechanism, their isolation and structural characterization are of significant interest. From this perspective, despite the fact that a large body of literature dwells on the utility of C–C cross‐coupling reactions, including that of the Suzuki–Miyaura cross‐coupling reaction, only a handful pertaining to the isolation and characterization of the key intermediates of its catalytic cycles have been reported Because of this, we have become interested in isolating and structurally characterizing the mixed organohalidopalladium(II) complexes, which are formed during the oxidative addition of an aryl halide substrate to a palladium(0) species, for their relevance in C–C cross‐coupling reactions. As we are interested in exploring the catalytic and biomedical applications of transition‐metal N‐heterocyclic carbene ligands, we chose to isolate the mixed organohalidopalladium complexes stabilized over N‐heterocyclic carbene ligands by exploring an alternate pathway involving a ligand‐substitution reaction on a palladium(II) complex by treatment with Grignard reagent, although it is commonly prepared by the oxidative addition route from a palladium(0) precursor complex .…”

“…From this perspective, despite the fact that a large body of literature dwells on the utility of C–C cross‐coupling reactions, including that of the Suzuki–Miyaura cross‐coupling reaction, only a handful pertaining to the isolation and characterization of the key intermediates of its catalytic cycles have been reported Because of this, we have become interested in isolating and structurally characterizing the mixed organohalidopalladium(II) complexes, which are formed during the oxidative addition of an aryl halide substrate to a palladium(0) species, for their relevance in C–C cross‐coupling reactions. As we are interested in exploring the catalytic and biomedical applications of transition‐metal N‐heterocyclic carbene ligands, we chose to isolate the mixed organohalidopalladium complexes stabilized over N‐heterocyclic carbene ligands by exploring an alternate pathway involving a ligand‐substitution reaction on a palladium(II) complex by treatment with Grignard reagent, although it is commonly prepared by the oxidative addition route from a palladium(0) precursor complex . To the best of our knowledge, in the case of palladium N‐heterocyclic carbene complexes, this ligand‐substitution reaction approach has not been reported for the preparation of the mixed organohalido (NHC) 2 Pd(R)(halide) type complexes, as these have thus far been solely synthesized by the conventional oxidative addition method from a (NHC) 2 Pd type palladium(0) precursor complex by reaction with an aryl halide .…”

Synthesis, Structure, and Reactivity of a Mononuclear η2‐(Ge–H)palladium(0) Complex Bearing a PGeP‐Pincer‐Type Germyl Ligand: Reactivity Differences between Silicon and Germanium

“…The geometry around the palladium is square planar, and the Pd–Ge bond [2.4230(7) Å] is shorter than those of η 2 ‐(Ge–H)Pd 0 4 and Pd‐dimer 5 . The Pd–C distance is 2.156(5) Å, which is obviously elongated compared with those of ethylpalladium(II) complexes having non‐pincer type ligands (1.977–2.148 Å) . Hazari reported isolation and structural analysis of a PSiP‐ethylpalladium(II) complex with Cy 2 P side arms, in which the Pd–C distance is 2.249(9) Å.…”