N-Heterocyclic carbene ligands bearing poly(ethylene glycol) chains: effect of the chain length on palladium-catalyzed coupling reactions employing aryl chlorides

Abstract

N-Heterocyclic carbenes bearing poly(ethylene glycol) chains of different lengths have been designed and employed as ligands in palladium-catalyzed coupling reactions. The catalyst system having longer chains was found to be highly efficient for Suzuki-Miyaura coupling and borylation reactions employing aryl chlorides under mild reaction conditions.

An efficient Pd–NHC catalyst system in situ generated from Na2PdCl4 and PEG-functionalized imidazolium salts for Mizoroki–Heck reactions in water

“…Owing to their significant advantages, including widely commercial availability, biocompatibility, chemical and thermal stability and ease to be derived, PEGs have been widely used as phase-transfer catalysts (PTC) or in the preparation of water soluble ligands for aqueous organic reactions during the past decades [68–69]. More recently, several PEG-functionalized azolium salts have been synthesized as water soluble NHC precursors for aqueous Pd-catalyzed cross-coupling reactions [56–59 70]. Fujihara also pointed out that the flexible linear long-chain structure of PEGs could wrap and stabilize the metal center and thus significantly enhanced the catalytic efficiency [70].…”

“…More recently, several PEG-functionalized azolium salts have been synthesized as water soluble NHC precursors for aqueous Pd-catalyzed cross-coupling reactions [56–59 70]. Fujihara also pointed out that the flexible linear long-chain structure of PEGs could wrap and stabilize the metal center and thus significantly enhanced the catalytic efficiency [70]. Therefore, we chose PEG as functionalization group to prepare water soluble catalysts.…”

Poly(ethylene glycol)‐ and glucopyranoside‐substituted N‐heterocyclic carbene precursors for the synthesis of arylfluorene derivatives using efficient palladium‐catalyzed aqueous Suzuki reaction

“…Shi et al synthesized a series of short‐chain alkyl and/or PEG chain ferrocenylphospine–imidazolium salts, and they found that the catalysts bearing PEG chains showed higher catalytic activity for Pd‐catalyzed Suzuki reaction in aqueous solvents . Very recently, NHCs bearing different lengths of PEG chains were synthesized by Fujihara et al and used to promote Pd‐catalyzed Suzuki reactions, the catalysts with longer PEG chains showing better performance . We found PEG‐functionalized NHC ligands were efficient for recyclable Pd‐catalyzed Suzuki reactions in water, and recently we utilized short alkyl chain‐ and glucopyranoside‐based NHC precursors for Pd‐catalyzed Suzuki reactions, which smoothly promoted the Suzuki coupling in ethanol (95%) with a typical catalyst loading of 0.1 mol% .…”

“…Taking the hydrophilicity and flexibility of bulky poly (ethylene glycol) (PEG) chains into consideration, the incorporation of PEG chains into NHC ligands could effectively improve simultaneously the water solubility and steric hindrance of the host materials. However, up to the present, there have been only a few reports of NHC ligands containing PEG groups . The first example was reported by Hong and Grubbs who used a PEG‐decorated NHC–Ru compound for aqueous olefin metathesis .…”

Recent Advances in the Schiff Bases and N‐Heterocyclic Carbenes as Ligands in the Cross‐Coupling Reactions: A Comprehensive Review

“…Fujihara et al [177] reported NHCs bearing poly(ethylene glycol) chains of different lengths like L117 (Fig. 14) and used as ligands in Pd-catalyzed coupling reactions.…”