A N‐Heterocyclic Carbene‐Palladacycle with Constrained Aliphatic Linker: Synthesis, Characterization and Its Catalytic Application towards Suzuki‐Miyaura Cross‐Coupling

Abstract

The Suzuki-Miyaura coupling of various (hetero) aryl chlorides with aryl boronic acids was accomplished by N-heterocyclic carbene-palladacycles (NHCÀ Pd cycle ) in the aqueous phase. Intriguingly, the NHCÀ Pd cycle with a constrained aliphatic linker between the NHC ligand and the palladacycle moieties exhibited higher catalytic efficiency than the one without constrained linker under the identical reaction conditions, due to the fixed spatial structure around the metal catalytic center. The present outcome may offer a new strategy for the catalyst design.Carbon-carbon (CÀ C) coupling reactions play a vital role in organic synthesis, pharmaceuticals, material science, and fine chemical industry. [1] Owing to the availability and safety of organoboron compounds and mild reaction conditions, the Suzuki-Miyaura coupling is regarded as one of the most widely appreciated methods for CÀ C coupling, especially for aryl-aryl bond construction. [2] In the past decades, N-heterocyclic carbene (NHC) ligands were considered as one of the viable candidates to realize Suzuki-Miyaura coupling due to their strong σ-donating and weak π-accepting ability, [3] which could remarkably facilitate the oxidation addition between the transition metals and aryl halides. [3b,4] On the other hand, the palladacycles (Pd cycle ) constituted privileged catalysts for the CÀ C and C-heteroatom formation reactions. [5] Combined both advantages, we and other groups have designed a series of NHCÀ Pd cycle complexes, revealing high activity in Suzuki-Miyaura reactions, decarboxylation reactions and Buchwald-Hartwig aminations. [6] Notwithstanding these advances, it is still difficult to achieve satisfactory performance using NHCÀ Pd cycle derived from imidazolium salts, especially with inert (hetero)aryl chlorides (Scheme 1a). [7] Recently, by using π-extended sterically bulky acenaphthoimidazolyidene, we successfully developed an NHCÀ Pd cycle 2, which did exhibit high catalytic activity and excellent substrate scopes toward the CÀ N and CÀ C coupling reactions with (hetero)aryl chlorides in toluene (Scheme 1b). [8] Despite these advances, it is still urgent to design efficient catalysts for Suzuki-Miyaura coupling reactions, especially, under sustainable reaction conditions like aqueous solvents, low reaction temperatures and low catalyst loadings from readily available imidazolium salts. [9] Herein, we synthesize a robust NHCÀ Pd cycle complex 3 a with constrained aliphatic linker between the [a] B.

A Second-Generation Palladacycle Architecture Bearing a N-Heterocyclic Carbene and Its Catalytic Behavior in Buchwald–Hartwig Amination Catalysis

“…Therefore, researchers have focused on this interesting field of palladacycles and intensively explored their potential applications [12,30]. Today, many examples of palladacycles exist that show high catalytic activity in Suzuki-Miyaura [31][32][33][34][35][36][37][38][39][40][41][42], Sonogashira [41][42][43], Heck [44][45][46], Buchwald-Hartwig [47][48][49][50] coupling reactions, and carbonylation reactions [51][52][53][54][55].…”

The Core‐Shell Magnetic Mesoporous Microspheres Immobilized NHC‐Palladacycles: An Efficient and Recyclable Catalyst for Suzuki‐Miyaura Cross‐Coupling of Pharmaceutical Synthesis

“…In our previous study, we found that the NHCÀ Pd cycle with a constrained aliphatic linker between NHC ligand and palladacycle moiety has higher catalytic efficiency than those without a constrained linker. [12] Thus, we proposed the fabrication of the core-shell magnetic mesoporous microspheres immobilized Nheterocyclic carbene-palladacycles catalyst with constrained aliphatic linkers (Fe 3 O 4 @mSiO 2 @NHCÀ Pd), achieving high catalyst loading, outstanding catalytic activity and multiple reusability without a significant decrease in catalytic performance even after 12 cycles for the Suzuki-Miyaura cross-coupling reaction of aryl chlorides and aryl boronic acids.…”