Functionalized nitrogen ligands for palladium catalyzed cross-coupling reactions (part I)

A palladium-catalyzed approach to allenic aromatic ethers and first total synthesis of terricollene A

A palladium-catalyzed C-O bond formation reaction between phenols and allenylic carbonates to give 2,3-allenic aromatic ethers with decent to excellent yields under mild reaction conditions has been described. A variety of synthetically useful functional groups are tolerated and the synthetic utility of this method has been demonstrated through a series of transformations of the allene moiety. By applying this reaction as the key step, the total syntheses of naturally occurring allenic aromatic ethers, eucalyptene and terricollene A (first synthesis; 4.5 g gram scale), have been accomplished.

Modelling of palladium(II) adsorption onto amine-functionalised zeolite using a generalised surface complexation approach

The many uses of palladium in medicine, catalysts and other industries make it a very important precious element. Many industries using palladium discharge process wastewaters that may release elevated concentrations of palladium into the environment. This study focused on the recovery of palladium from aqueous solutions by zeolite functionalised with spent brewer's yeast. Batch experimental results were used to calibrate a generalised surface complexation model based on coupling parameter estimation (PEST) to the PHREEQC geochemical modelling code. PHREEQC is an acronym which stands for pH, redox, equilibrium and C programming language. Calibration was based on the determination of sorption constants for the reactions of palladium with the adsorbent. The generalised amine surface groups (derived from yeast), the moles of adsorption sites and surface area were specified. The recovery of palladium was assessed as a function of solution pH, adsorbent dosage and initial concentration of palladium in the presence of other cations and anions at different concentrations. The highest recovery of palladium (>97%) was observed at pH 2 and 10 g L^-1 adsorbent dosage which, decreased with increasing solution pH. The amount of palladium removed increased in the presence of competing ions and anions. There was no significant difference (p > 0.05) between the modelled and measured data, which indicated that PHREEQC modelling code coupled with PEST can accurately determine the recovery of palladium using amine-based adsorbents when all the required information is specified. This is very useful in instances where limited experimental data is available for non-conventional and novel surfaces to make accurate predictions of sorption processes involving them.

Mimics of Pincer Ligands: An Accessible Phosphine-Free N-(Pyrimidin-2-yl)-1,2-azole-3-carboxamide Framework for Binuclear Pd(II) Complexes and High-Turnover Catalysis in Water

We report for the first time cyclic phosphine-free "head to tail" N,N,N pincer-like (pincer complexes mimicking) N-(pyrimidin-2-yl)-1,2-azole-3-carboxamide Pd(II) complexes with deprotonated amide groups as high-turnover catalysts (TON up to 10⁶, TOF up to 1.2 × 10⁷ h^-1) for cross-coupling reactions on the background of up to quantitative yields under Green Chemistry conditions. The potency of the described catalyst family representatives was demonstrated in Suzuki-Miyaura, Mizoroki-Heck, and Sonogashira reactions on industrially practical examples. Corresponding ligands could be synthesized based on readily available reagents through simple chemical transformations. Within the complex structures, a highly unusual 1,3,5,7-tetraza-2,6-dipalladocane frame could be observed.

Synthesis of Mono- and Bis-Pyrazoles Bearing Flexible p-Tolyl Ether and Rigid Xanthene Backbones, and Their Potential as Ligands in the Pd-Catalysed Suzuki–Miyaura Cross-Coupling Reaction

The present work describes the synthesis of mono- and bis-pyrazole compounds bearing flexible p-tolyl ether and rigid 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene backbones as pyrazolyl analogues of DPEphos and Xantphos ligands, respectively. The synthesis of new pyrazolyl analogues was accomplished following an Ullmann coupling protocol, and the resulting products were isolated in overall good yields. In addition, a hybrid imidazolyl–pyrazolyl analogue bearing a xanthene backbone was synthesized using the same protocol, whereas a hybrid selanyl–pyrazolyl analogue with a xanthene backbone was synthesized in a good yield employing a second C–H activation step. The symmetrical bis-pyrazolyl and the hybrid imidazolyl–pyrazolyl analogues were found to be the most active among the new ligands evaluated in the Pd-catalysed Suzuki-Miyaura cross-coupling of aryl halides with aryl boronic acids. A simple catalytic system based on Pd(OAc)2/2a was developed, which efficiently catalyses the Suzuki–Miyaura reaction of aryl halides and aryl boronic acids and provides moderate to excellent yields of the corresponding cross-coupling products.

Pd(CH3CN)2Cl2/Pipecolinic Acid as a Highly Efficient Catalytic System for Suzuki-Miyaura Cross-coupling Reaction of Bromoaryl Carboxylic Acids in Water

In this study, a convenient and highly efficient catalytic system for the Suzuki-Miyaura coupling reaction was investigated under mild conditions. A combination of Pd(CH3CN)2Cl2 and pipecolinic acid showed excellent catalytic performance and afforded high turnover numbers; turnover numbers were up to 4.9 × 105 for the coupling reaction of 4-bromobenzoic acid and tetraphenylboron sodium. The catalytic system was also effective for the indexes of 4-bromobenzoic acid, and high turnover numbers were obtained.

Phosphorus dendrimers functionalised with nitrogen ligands, for catalysis and biology

Phosphorus dendrimers (dendrimers having one phosphorus atom at each branching point) possess versatile properties, depending on the type of their terminal functions.

Evidence for a Cooperative Mechanism Involving Two Palladium(0) Centers in the Oxidative Addition of Iodoarenes

Oxidative addition of iodoarenes (ArI) to Pd⁰ ligated by 1-methyl-1H-imidazole (mim) in polar solvents leads to cationic arylpalladium(II) complexes [ArPd(mim)₃ ]⁺ . Kinetic analyses evidence that this reaction is second order with respect to the concentration of Pd⁰ , and a mechanism involving the cooperative intervention of two Pd⁰ centers has been postulated to explain this finding. This unusual behavior is also observed with other nitrogen-containing ligands and it is general for iodobenzenes substituted with electron-donating or weakly electron-withdrawing groups. In contrast, bromoarenes and electron-poor iodoarenes display first-order kinetics with respect to Pd⁰ . Theoretical calculations performed at the density functional theory (DFT) level suggest the existence of mim-ligated ArI-Pd⁰ complexes, in which the iodoarene is bound to the metal in a halogen-bond-like fashion. Coordination weakens the C-I bond and facilitates the oxidative insertion of another Pd⁰ center across this C-I bond. This conceptually novel mechanism, involving the cooperative participation of two distinct metal centers, allows a full explanation of the experimentally observed kinetics.