Synthesis, characterization and evaluation of bulky bis(pyrazolyl)palladium complexes in Suzuki-Miyaura cross-coupling reactions

Catalytic system having an organotellurium ligand on graphene oxide: immobilization of Pd(0) nanoparticles and application in heterogeneous catalysis of cross-coupling reactions

First heterogeneous catalytic system, having a covalently linked hybrid bidentate organotellurium ligand [i.e., PhTe-CH2-CH2-NH2] on the surface of graphene oxide, has been synthesized with immobilized and stabilized Pd(0) nanoparticles. To the best of our knowledge, it is the first such catalytic system in which a heterogenized organotellurium ligand has been used. It has been well-characterized using different physicochemical characterization techniques viz. P-XRD, XPS, HR-TEM, EELS, FE-SEM, EDX, TGA, BET surface area analysis, FT-IR spectroscopy, and Raman spectroscopy. The Pd content of the final system has been quantified using ICP-OES. Its applications have been explored in Suzuki-Miyaura C-C cross coupling and C-O cross coupling reactions. Hot filtration experiments corroborate the heterogeneous nature of the catalysis. It is recyclable for up to five reaction cycles in Suzuki-Miyaura and C-O cross coupling with marginal loss in performance. It also catalyzes the reactions of chloroarenes such as chlorobenzene, 4-chloroaniline, 1-chloro-4-nitrobenzene, 4-chloroacetophenone, 4-chlorobenzophenone for Suzuki coupling, and 1-chloro-4-nitrobenzene, 4-chlorobenzonitrile, chlorobenzene, and 4-chlorotoluene for C-O coupling. P-XRD, FE-SEM, and EDX study reveals that the catalytic system retains its structural originality and functionality after recycling.

Facile Synthesis and Characterization of Palladium@Carbon Catalyst for the Suzuki-Miyaura and Mizoroki-Heck Coupling Reactions

Palladium-based carbon catalysts (Pd/C) can be potentially applied as an efficient catalyst for Suzuki–Miyaura and Mizoroki–Heck coupling reactions. Herein, a variety of catalysts of palladium on activated carbon were prepared by varying the content of ‘Pd’ via an in situ reduction method, using hydrogen as a reducing agent. The as-prepared catalysts (0.5 wt % Pd/C, 1 wt % Pd/C, 2 wt % Pd/C and 3 wt % Pd/C) were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and Brunauer–Emmett–Teller (BET) analyses. The catalysts were tested as a coupling catalyst for Suzuki–Miyaura coupling reactions involving aryl halides and phenyl boronic acid. The optimization of the catalyst by varying the palladium content on the activated carbon yielded Pd/C catalysts with very high catalytic activity for Suzuki reactions of aryl halides and a Mizoroki–Heck cross-coupling reaction of 4-bromoanisol and acrylic acid in an aqueous medium. A high ‘Pd’ content and uniform ‘Pd’ impregnation significantly affected the activity of the catalysts. The catalytic activity of 3% Pd/C was found to make it a more efficient catalyst when compared with the other synthesized Pd/C catalysts. Furthermore, the catalyst reusability was also tested for Suzuki reactions by repeatedly performing the same reaction using the recovered catalyst. The 3% Pd/C catalyst displayed better reusability even after several reactions.

A tryptophan-based copper(ii) coordination polymer: catalytic activity towards Suzuki–Miyaura cross-coupling reactions

An unusual tryptophan coordination mode with copper(ii) wherein l-tryp is coordinated through the carboxylate and amine groups. A heterogeneous catalyst for the Suzuki cross-coupling reaction with ∼98% yield under normal reaction conditions.

Synthesis, structures, and catalytic efficiency in ring opening polymerization of rac-lactide with tridentate vs. bidentate cobalt(ii), zinc(ii), and cadmium(ii) complexes containing N-substituted N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)amine ligands

A series of Co(ii), Zn(ii), and Cd(ii) complexes supported by 1-(3,5-dimethyl-1H-pyrazol-1-yl)-N-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-N-(furan-2-ylmethyl)methanamine (L_A) and N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-4-isopropylaniline (L_B) were synthesized. The direct chelation of CoCl₂·6H₂O, ZnCl₂, and CdBr₂·4H₂O by the ligands produced [L_nMX₂] (L_n = L_A or L_B; M = Zn or Co, with X = Cl; M = Cd, with X = Br) complexes in high yields. Structural studies revealed that [L_BCoCl₂] and [L_BZnCl₂] adopted distorted tetrahedral geometries, as L_B coordinated the metal centers in a bidentate fashion, while L_A coordinated the metal centers in a tridentate fashion through the nitrogen atoms of the pyrazole and amine moieties, so that [L_ACoCl₂] and [L_AZnCl₂] exhibited trigonal bipyramidal geometries and [L_ACdBr₂] a square pyramidal geometry. [L_BCdBr₂] has two Cd-containing structures per unit cell, whereby one Cd center adopted a distorted tetrahedral geometry and the other exhibited square bipyramidal geometry. The in situ-generated alkyl derivatives of the synthesized complexes were assessed in the ring-opening polymerization of rac-lactide. Heterotactic polylactides (PLAs) were furnished with all complexes. The [L_BZnCl₂]/MeLi system produced PLA with a superior heterotactic bias (P_r up to 0.94) at −25 °C. PLAs with wide-ranging polydispersity indices (1.16–2.23) and low molecular weights were produced in all cases, irrespective of the specific M(II) center and ancillary ligand utilized.

Co(ii), Zn(ii), and Cd(ii) complexes supported by bis-pyrazolyl ligands were applied to the ring-opening polymerization of rac-lactide to produce heterotactic polylactides (PLAs) with superior heterotactic bias i.e. P_r up to 0.94 at −25 °C.

Synthesis and characterization of new 3,3`-bipyrazole-4,4`-dicarboxylic acid derivatives and some of their palladium(II) complexes as pre-catalyst for Suzuki coupling reaction in water

The 1,3-dipolar cycloaddition reaction of bis-hydrazonyl chlorides with methyl propiolate afforded dimethyl 1,1¢-aryl-3,3`-bipyrazole-4,4`-dicarboxylates (5a,b). Heating the later compound 5a with a mixture of HCl/AcOH gave 3,3`-bipyrazole-5,5`-dicarboxylic acid derivative 6. Treatment of the hydrazonoyl chloride (1a) and 3,3`-bipyrazole-5,5`-dicarboxylic acid (6) with palladium(II) chloride gave the corresponding Pd-complexes 7 and 8, respectively. The catalytic activity of the prepared Pd-complexes was examined in the Suzuki cross-coupling reaction of phenylboronic acid with activated and deactivated aryl(hetaryl) bromides. The catalyst system provides very good to excellent yields, 85-94%. The structures of the obtained products were established from their elemental analysis, spectral data, XPS, EDX, and single crystal X-ray crystallography. Crystal data for C10H7N2O2 (6): triclinic, space group P-1 (no. 2), a = 3.9956(10) Å, b = 9.8917(18) Å, c = 10.810(3) Å, α = 94.167(15)°, β = 94.979(19)°, γ = 98.953(15)°, V = 418.83(16) Å3, Z = 2, T = 296.(2) K, μ(Cu Kα) = 0.887 mm-1, Dcalc = 1.484 g/cm3, 5469 reflections measured (11.72° ≤ 2Θ ≤ 133.24°), 1420 unique (Rint = 0.0633, Rsigma = 7.24%) which were used in all calculations. The final R1 was 0.1055 (>2sigma(I)) and wR2 was 0.3620 (all data).

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.

Iodine-Catalyzed Regioselective Oxidative Cyclization of Aldehyde Hydrazones with Electron-Deficient Olefins for the Synthesis of Mefenpyr-Diethyl

A regioselective synthesis of polysubstituted dihydropyrazoles and pyrazoles through an iodine-catalyzed oxidative cyclization strategy of aldehyde hydrazones with electron-deficient olefins is described. The protocol adopts very mild reaction conditions and provides desirable yields. The reaction is supposed to proceed via a cascade C-H functionalization, C-N bond formation, and oxidation sequential processes. The overall simplicity and regioselectivity of the catalytic system make this approach a valuable and step-economical tool to construct a C-C bond for the synthesis of Mefenpyr-Diethyl.

Water-soluble SNS cationic palladium(II) complexes and their Suzuki-Miyaura cross-coupling reactions in aqueous medium

Unlike their SCS analogues, SNS pincer complexes are poorly studied for their use in coupling reactions. Accordingly, a series of water soluble cationic Pd(II) SNS pincer complexes have been successfully synthesised and investigated in detail for their catalytic activity in Suzuki-Miyaura coupling reactions. By using only 0.5 mol % loading of the complexes, the coupling of inactivated aryl bromides and activated aryl chlorides with various boronic acids in water was achieved in excellent yields and the catalysts were found to be reusable for three cycles without a significant loss of activity. The investigation of the mechanism of the reaction revealed that a Pd(II) to Pd(IV) route is the more likely pathway which was further supported by computational studies.