Suzuki Coupling Reactions in Ether-Functionalized Ionic Liquids: The Importance of Weakly Interacting Cations

N-Heterocyclic carbene-containing complexes in catalysis

The wide dissemination of catalytic protocols in academic and industrial laboratories is facilitated by the development of catalysts that are not only highly active but also user-friendly, stable to moisture, air and long term storage and easy to prepare on a large scale. Herein we describe a protocol for the Heck–Mizoroki reaction mediated by cyclopalladated N,N-dimethylbenzylamine (dmba) ligated with a N-heterocyclic carbene, 1,3-bis(mesityl)imidazol-2-ylidene (IMes), that fulfils these criteria. The precatalyst can be synthesized on ∼100 g scale by a tri-component, sequential, one-pot reaction of N,N-dimethylbenzylamine, PdCl2 and IMes·HCl in refluxing acetonitrile in air in the presence of K2CO3. This single component catalyst is stable to air, moisture and long term storage and can be conveniently dispensed as a stock solution in NMP. It mediates the Heck–Mizoroki reaction of a range of aryl- and heteroaryl bromides in reagent grade NMP at the 0.1–2 mol% range without the need for rigorous anhydrous techniques or a glovebox, and is active even in air. The catalyst is capable of achieving very high levels of catalytic activity (TON of up to 5.22 × 105) for the coupling of a deactivated arylbromide, p-bromoanisole, with tBu acrylate as a benchmark substrate pair. A wide range of aryl bromides, iodides and, for the first time with a NHC-Pd catalyst, a triflate was coupled with diverse acrylate derivatives (nitrile, tert-butyl ester and amides) and styrene derivatives. The use of excess (>2 equiv.) of the aryl bromide and tert-butyl acrylate leads to mixture of tert-butyl β,β-diarylacrylate and tert-butyl cinnamate derivatives depending on the substitution pattern of the aryl bromide. Electron rich m- and p-substituted arylbromides give the diarylated products exclusively, whereas electron-poor aryl bromides give predominantly mono-arylated products. For o-substituted aryl bromides, no doubly arylated products could be obtained under any conditions. Overall, the active catalyst (IMes-Pd) shows higher activity with electron-rich aryl halides, a marked difference compared with the more commonly used phosphane-Pd or non-ligated Pd catalysts.

N-heterocycle carbene (NHC)-ligated cyclopalladated N,N-dimethylbenzylamine: a highly active, practical and versatile catalyst for the Heck–Mizoroki reaction

The terphspan scaffold was employed to support a bis(N-heterocyclic carbene) ligand (1) that provides a m-terphenyl canopy over one side of its metal complexes. Single-crystal X-ray diffraction studies on a silver complex of {[Ag(1)]AgBr2}2 revealed an unusual tetranuclear silver core with a Ag–Ag bond distance of 3.0241(8) A with 1 as a trans-chelating ligand (C–Ag–C = 171°). A preliminary X-ray structure of pseudo-square planar [PdCl2(1)] showed a similar binding mode of 1 (C–Pd–C = 177°). High yields were obtained in Suzuki–Miyaura coupling reactions utilizing [PdCl2(1)] as the procatalyst and the results were compared with analogous complexes of trans-spanning diphosphine (2) and diphosphinite (3) complexes. The diphosphinite complex, [PdCl2(3)], decomposes to [μ-ClPd(PPh2OH)(PPh2O)]2 at room temperature.

Canopied trans-chelating bis(N-heterocyclic carbene) ligand: synthesis, structure and catalysis.

Synthesis and Structure of Dinuclear Silver(I) and Palladium(II) Complexes of 2,7-Bis(methylene)naphthalene-Bridged Bis-N-Heterocyclic Carbene Ligands

First synthesis of bidentate NHC–Pd complexes with anthracene and xanthene skeletons

Synthesis and structure of bidentate NHC-metal complexes with xanthene skeleton: The formation of cis and trans complexes

We synthesized a series of new bidentate NHC (N-heterocyclic carbene) precursors with anthracene and xanthene skeletons. The corresponding NHC–Pd complexes were prepared from these precursors. The catalytic activity of these Pd complexes was examined.

Hiroki Muto

Papers

First synthesis of bidentate NHC–Pd complexes with anthracene and xanthene skeletons

Synthesis and structure of bidentate NHC-metal complexes with xanthene skeleton: The formation of cis and trans complexes

First Synthesis of Bidentate NHC—Pd Complexes with Anthracene and Xanthene Skeletons.