Hydrophilic CNC‐Pincer Palladium Complexes: A Source for Highly Efficient, Recyclable Homogeneous Catalysts in Suzuki–Miyaura Cross‐Coupling

Abstract

Dedicated to the memory of our colleague and friend Prof. Marcial Moreno-Mañas. Abstract:The Suzuki biaryl coupling of a range of electronically dissimilar arylboronic acids and aryl bromides is performed in neat water with excellent to quantitative yields by means of a new CNCpincer palladium catalyst that is soluble in water due to its para-carboxy group. Extremely high turnover numbers and frequencies combined with a remarkable robustness allow an effective catalyst reuse in sustainable conditions.

Insights into the Role of New Palladium Pincer Complexes as Robust and Recyclable Precatalysts for Suzuki–Miyaura Couplings in Neat Water

“…Treatment of readily available 2,6-dibromoisonicotinic acid 4 [10] with commercially available ethyl 4-(1H-imidazol-1-yl)butanoate 5 provided intermediate imidazolium salt 6, which was palladated with PdA C H T U N G T R E N N U N G (OAc) 2 / DMSO in a similar way to the work by Crabtree et al [11] and later by our own group, [8] providing the target complex 2 (Scheme 1). Hydrolysis under acidic conditions afforded pincer complex 3 bearing three carboxy groups (Scheme 1).…”

“…[5] In this context, water is a non-flammable, safe, and cheap solvent with a great potential, since organic products can be readily separated from the aqueous layer and many reactions proceed with better selectivity in aqueous environments. [6] Considering the experience of our group on the development of new palladium catalytic systems to perform cross-coupling reactions, [7] and taking into account the excellent results obtained upon SuzukiMiyaura biaryl coupling performed in neat water with hydrophilic palladacycle 1 synthesized in our labs, (Figure 1), [8] we focused our attention on the synthesis of new pincer palladium complexes 2 and 3 seeking improved properties ( Figure 2). The general skeleton of 1 was tuned to incorporate more hydrophilic groups which would likely increase the solubility in water, thus modifying the catalytic properties in terms of efficiency or scope of applications in different cross-coupling reactions.…”

Donor‐Functionalised N‐Heterocyclic Carbene Complexes of Group 9 and 10 Metals in Catalysis: Trends and Directions

“…The high stability of complexes of the pincer ligand type 33 (without a CH 2 spacer), was utilised in an interesting study by SanMartin, Domínguez et al A hydrophilic CNC pincer-Pd complex was synthesised and tested in the Suzuki-Myaura coupling reaction with water as the solvent. [69] Water solubility was provided by a p-carboxy group on the backbone of the pincer. The reaction conditions consisted of: 0.1 mol-% catalyst, appropriate substrate, K 2 CO 3 , in pure water as solvent, refluxed for 2 h. A range of electron-rich, electro-neutral and electron-poor bromoarenes and arylboronic acids were tested, all giving 100 % yield of product.…”

The Arrangement of Two N-Heterocyclic Carbene Moieties in Palladium Pincer Complexes Affects Their Catalytic Activity towards Suzuki–Miyaura Cross-Coupling Reactions in Water

“…3 (0.4 mmol) and 1.0 mL of water were added to a sealed tube in air. After the mixture was stirred for 30 sec at room temperature, the sealed tube was placed in a preheated oil bath (100°C) for 16 h. After cooling the mixture to room temperature, the aqueous phase was extracted with CH 2 Cl 2 three times.…”

“…Thus, water would eliminate those problems 2 and make separation of products and recovery of catalysts easy. To develop catalysts that are active in water, metal complexes with ligands having water-soluble units, such as carboxylates, 3 sulfonates, 4 and ammoniums, 4a are needed. By incorporating sugar groups as water-soluble units into N-heterocyclic carbene (NHC) ligands, water-soluble complexes for transfer hydrogenation, 5 coupling reactions, 6 and olefin methathesis 7 have been prepared.…”