Poly(benzyl ether) Dendrimers Functionalized at the Core with Palladium Bis(N-Heterocyclic Carbene) Complexes as Catalysts for the Heck Coupling Reaction

Abstract

Bis(imidazolylidene)palladium complexes 9-12 containing a sterically hindered aryl group (mesityl or 2,6-diisopropylphenyl) and a poly(benzyl ether) dendron as N-substituents of the NHC ligand are accessible up to the third generation by transmetalation of the corresponding silver complexes. Complexes 9-12 are soluble, active, and very stable catalysts under Heck reaction conditions. The NHC ligand appears to be stably coordinated to the Pd during catalysis. The catalytic activity increases with generation number, although irregularly. The palladium site is not significantly congested in the reaction solvent by the increasing size of the dendritic substituents, as corroborated by X-ray diffraction, fluorescence and DOSY-NMR spectroscopy, and MD simulation studies. This is a consequence of the conformational semiflexibility of the poly(benzyl ether) dendrons and the benzylic link between these dendrons and the N-heterocyclic ligands.

Palladium(II) Chloride Complexes of N,N′-Disubstituted Imidazole-2-thiones: Syntheses, Structures, and Catalytic Performances in Suzuki–Miyaura and Sonogashira Coupling Reactions

“…Complex 3d exhibited the highest catalytic activity of the five binuclear complexes, indicating that introducing two bulky C 6 Me 3 H 2 groups on the 2 and 5 positions of imidazole-2-thione did improve the catalytic performance. As discussed above, the longest Pd−μ-S bond and the enlarged S–Pd–S angle in 3d may favor the reductive elimination and thus increase the stability of coordinatively unsaturated active catalytic species. , Such a structure-to-activity relationship is in good agreement with previous observations , and is confirmed by our theoretical calculations described below. Complexes 3d , 4a , and 5a were thus chosen as a model system to optimize reaction conditions.…”

“…As discussed above, the longest Pd−μ-S bond and the enlarged S− Pd−S angle in 3d may favor the reductive elimination and thus increase the stability of coordinatively unsaturated active catalytic species. 93,94 Such a structure-to-activity relationship is in good agreement with previous observations 95,96 and is confirmed by our theoretical calculations described below. Complexes 3d, 4a, and 5a were thus chosen as a model system to optimize reaction conditions.…”

N-Heterocyclic Carbenes in Materials Chemistry

“…In these systems, the dendron wedges are typically anchored to either one or both of the nitrogen atoms adjacent to the carbenic carbon to form dendritic systems in which the NHC-metal center is precisely at the core. Examples of such structures with Rh, 68,296 Pd, 297,298 Au, 299,300 Ag, 61 Ir, 301 and Ru 302 as the central metal will be discussed in this section. Tsuji and co-workers described the synthesis and catalytic activity of a series of dendritic NHC−Rh(I) complexes in which the metal center was located at the core of flexible Frećhet-type poly(benzyl ether) dendrimer (Chart 11a).…”

“…297 NHC-centered dendrimers with Pd at the core were developed by de Jesuś, Flores, et al, in which the imidazolium moieties were unsymmetrically functionalized with a bulky aryl group such as 2,6-diisopropylphenyl or mesityl on one nitrogen atom and poly(benzyl ether) dendrons (193) on the other nitrogen (Chart 12). 298 These macromolecules were synthesized by transmetalation of the corresponding Ag derivatives using commercially available Pd(II) precursors. When tested as catalysts under Mizoroki−Heck reaction conditions, the dendridic catalysts did show different reaction rates depending on generation number; however, the ordering was unusual with G 2 < G 0 < G 1 < PdCl 2 < G 3 .…”

Solvent Quality Controls Macromolecular Structural Dynamics of a Dendrimeric Hydrogenase Model

“…Of the solvents used here, toluene, DMF, and THF are known to be good solvents for poly(aryl ether) dendrimers, whereas acetone is a "medium" quality solvent, and acetonitrile is classified as poor. 33,47 To the best of our knowledge, the solvent quality of chloroform for poly(aryl ether) dendrimers has not been determined experimentally. However, in a molecular dynamics study of a poly(aryl ether) dendrimer with an [FeS] 2 core, the dendritic groups were found to extend consistently in the explicit chloroform solvent, indicating that chloroform is a reasonably good solvent for poly(aryl ether) dendrons.…”