Pd-Catalyzed Diamination of 1,2,4-Triazinyl Complexant Scaffolds

Homoleptic coordination polymers and complexes of transition metals with 2-(1,2,4-1H-triazol-3-yl) pyridine and tuning towards white-light emission

Twelve coordination compounds ranging from polymers to complexes based on divalent ions of the 3d-transition metals Mn to Zn, and Cd together with the ligand 2-(1,2,4-1H-triazol-3-yl)pyridine (Hpt) were synthesised and fully characterised. The main products are homoleptic, one-dimensional coordination polymers 1∞[M(pt)2] (M = Mn-Zn, and Cd, pt = 3-(pyridin-2-yl)-1,2,4-triazolate), 1∞[Cu(pt)2]·0.5Py, besides complexes [MX2(Hpt)2] from MnCl2, FeCl2, CoCl2, CoBr2, ZnCl2, and Hpt = 2-(1,2,4-1H-triazol-3-yl)pyridine. In addition to these series, single crystalline by-products of the reactions were identified and their structures determined. The obtained products were investigated with single-crystal (SCXRD) and powder X-ray diffraction (PXRD), including temperature-dependent PXRD, physisorption experiments, UV-Vis, IR, and photoluminescence spectroscopy (PL), simultaneous thermal analysis, and elemental analysis. Based on 1∞[Zn(pt)2], it was possible to generate a white light-emitting compound by addition of Eu3+ and Tb3+. It follows the RGB concept with blue ligand-based emission of the coordination polymer and red/green emission of lanthanide ions and shows excitation dependent tuneable character of emission colour from blue to practically perfect white.

Synthesis of Fused [1,2,3]-Triazoloheteroarenes via Intramolecular Azo Annulation of N-Tosylhydrazones Catalyzed by 1,8-Diaza-bicyclo[5.4.0]undec-7-ene

The structural diversity of triazoloheteroarenes render this moiety an attractive synthon for drug discovery, C-H functionalization, and complexant design for minor actinide separations. While contemporary work has demonstrated the capacity to leverage downstream functional group interconversion of the triazolopyridine, a broadly applicable method tolerant of diverse heteroaryl constructs and pendant functionality to obtain triazoloheteroarenes remains under reported. In this work, the serendipitous discovery of a metal, azide, and oxidant free transformation of various heteroaryl N-tosylhydrazones of carbaldehydes and ketones to the corresponding [1,2,3]-triazoloheteroarene via intramolecular azo annulation using a substoichiometric amount of 1,8-diaza-bicyclo[5.4.0]undec-7-ene is described. These results substantively improve upon previous approaches offering efficient access to the described heterocycles. Discovery of reaction conditions, method optimization, complexant, pyridine, and heteroarene substrate scope, as well as relevant scale-up reactions are reported herein.

Next-Generation 3,3'-AlkoxyBTPs as Complexants for Minor Actinide Separation from Lanthanides: A Comprehensive Separations, Spectroscopic, and DFT Study

Progress toward the closure of the nuclear fuel cycle can be achieved if satisfactory separation strategies for the chemoselective speciation of the trivalent actinides from the lanthanides are realized in a nonproliferative manner. Since Kolarik's initial report on the utility of bis-1,2,4-triazinyl-2,6-pyridines (BTPs) in 1999, a perfect complexant-based, liquid-liquid separation system has yet to be realized. In this report, a comprehensive performance assessment for the separation of 241Am3+ from 154Eu3+ as a model system for spent nuclear fuel using hydrocarbon-actuated alkoxy-BTP complexants is described. These newly discovered complexants realize gains that contemporary aryl-substituted BTPs have yet to achieve, specifically: long-term stability in highly concentrated nitric acid solutions relevant to the low pH of unprocessed spent nuclear fuel, high DAm over DEu in the economical, nonpolar diluent Exxal-8, and the demonstrated capacity to complete the separation cycle with high efficiency by depositing the chelated An3+ to the aqueous layer via decomplexation of the metal-ligand complex. These soft-N-donor BTPs are hypothesized to function as bipolar complexants, effectively traversing the organic/aqueous interface for effective chelation and bound metal/ligand complex solubility. Complexant design, separation assays, spectroscopic analysis, single-crystal X-ray crystallographic data, and DFT calculations are reported.

Microwave-Assisted, Metal- and Azide-Free Synthesis of Functionalized Heteroaryl-1,2,3-triazoles via Oxidative Cyclization of N-Tosylhydrazones and Anilines

As the search for competent soft-Lewis basic complexants for separations continues to evolve toward identification of a chemoselective moiety for speciation of the minor actinides from the electronically similar lanthanides, synthetic methods must congruently evolve. Synthetic options to convergently construct unsymmetric heteroaryl donor complexants incorporating a 1,2,3-triazole from accessible starting materials for evaluation in separation assays necessitated the development of the described methodology. In this report, metal- and azide-free synthesis of diversely functionalized pyridyl-1,2,3-triazole derivatives facilitated by microwave irradiation was leveraged to prepare a novel class of tridentate ligands. The described work negates the incorporation of thermally sensitive and toxic organoazides by using N-tosylhydrazones and anilines as viable synthetic equivalents in an efficient 12 min reaction time. Adaptation to alternative synthons useful for drug discovery was also realized. Method discovery, optimization, N-tosylhydrazone and aniline substrate scope, as well as a preliminary mechanistic hypotheses supported by DFT calculations are reported herein.

General Method for the Amination of Aryl Halides with Primary and Secondary Alkyl Amines via Nickel Photocatalysis

The Buchwald-Hartwig C-N coupling reaction has been ranked as one of the 20 most frequently used reactions in medicinal chemistry. Owing to its much lower cost and higher reactivity toward less reactive aryl chlorides than palladium, the C-N coupling reaction catalyzed by Ni-based catalysts has received a great deal of attention. However, there appear to be no universal, practical Ni catalytic systems so far that could enable the coupling of electron-rich and electron-poor aryl halides with both primary and secondary alkyl amines. In this study, it is reported that a Ni(II)-bipyridine complex catalyzes efficient C-N coupling of aryl chlorides and bromides with various primary and secondary alkyl amines under direct excitation with light. Intramolecular C-N coupling is also demonstrated. The feasibility and applicability of the protocol in organic synthesis is attested by more than 200 examples.

Convergent access to bis-1,2,4-triazinyl-2,2'-bipyridines (BTBPs) and 2,2'-bipyridines via a Pd-catalyzed Ullman-type reaction

Multidentate, soft-Lewis basic, complexant scaffolds have displayed significant potential in the discrete speciation of the minor actinides from the neutron-absorbing lanthanides resident in spent nuclear fuel. Efforts to devise convergent synthetic strategies to targets of interest to improve liquid-liquid separation outcomes continue, but significant challenges to improve solubility in process-relevant diluents to effectively define meaningful structure-activity relationships remain. In the current work, a synthetic method to achieve the challenging 2,2'-bipyridine bond of the bis-1,2,4-triazinyl-2,2'-bipyridine (BTBP) complexant class leveraging a Pd-catalyzed Ullman-type coupling is reported. This convergent strategy improves upon earlier work focused on linear synthetic access to the BTBP complexant moiety. Method optimization, relevant substrate scope and application, as well as a preliminary mechanistic interrogation are reported herein.

Design, preparation and characterization of aerogel NiO–CuO–CoO/SiO2 nanocomposite as a reusable catalyst for C–N cross-coupling reaction

The aerogel nanocomposite produces using the sol–gel and supercritical drying method processes. The CO₂ supercritical drying (SCD) was taken as the most powerful process, ensuring the best properties of the product.

Synthesis of Tridentate [1,2,4] Triazinyl-Pyridin-2-yl Indole Lewis Basic Complexants via Pd-Catalyzed Suzuki-Miyaura Cross-Coupling

Full closure of the nuclear fuel cycle is predicated, in part, on defining efficient separations processes for the effective speciation of the neutron-absorbing lanthanides from the minor actinides post-PUREX. Pursuant to the aforementioned, a class of tridentate, Lewis basic procomplexants have been prepared leveraging a Pd-catalyzed Suzuki-Miyaura cross-coupling between 6-bromo-[1,2,4]-triazinylpyridine derivatives and various protected indole-boronic acids to afford functionalized 2-[6-(5,6-diphenyl-[1,2,4]triazin-3-yl)-pyridin-2-yl]-1H-indoles. A highly active catalyst/ligand system with low loadings was employed rapidly affording 26 examples in yields as high as 85%. Method optimization, substrate and indole scope, comparative analysis between coupling reagents, and a scale-up experiment are reported.

Synthesis of 1H-Pyrazol-5-yl-pyridin-2-yl-[1,2,4]triazinyl Soft-Lewis Basic Complexants via Metal and Oxidant Free [3 + 2] Dipolar Cycloaddition of Terminal Ethynyl Pyridines with Tosylhydrazides

Soft-Lewis basic complexants that facilitate chemoselective separation of the minor actinides from the lanthanides are critical to the closure of the nuclear fuel cycle. Complexants that modulate covalent orbital interactions with relevant metals of interest can facilitate desired outcomes in liquid-liquid separation, allowing for further transmutative processes that decrease issues related with storage of spent nuclear fuel from energy and weapons production. Synthesis of previously unexplored scaffolds seeks to improve performance over benchmark complexants. In the current work, an intermolecular, thermally initiated, and DBU-assisted [3 + 2] cycloaddition of 3-(6-ethynyl-pyridin-2-yl)-5,6-diphenyl-[1,2,4]triazine dipolarophiles with structurally diverse 4-methylbenzenesulfono-hydrazides afforded 21 yet-to-be reported examples in 42-68% yield and modest regioselectivity for the desired regioisomer. Preparation of requisite starting materials, method definition, dipole and dipolarophile scope, ten-fold scale-up reaction, and downstream functional group interconversion are reported herein.

Synthetic Access to Functionalized Dipolarophiles of Lewis Basic Complexant Scaffolds through Sonogashira Cross-Coupling

Soft Lewis basic complexants that facilitate selective removal of discrete ions resident in spent nuclear fuel can decrease repository volume and radiotoxicity and are of significant interest. Optimization of chelation efficacy is predicated on modular access to synthons to rapidly evaluate structure-activity relationships. The following work highlights efficient access to functionalized synthons for use as potential dipolarophiles in subsequent cycloaddition processes via Sonogashira coupling of 3-(6-bromo-pyridin-2-yl)-[1,2,4]triazine scaffolds. The 41 examples explored during method development evaluated electrophile and nucleophile diversity affording the desired coupled products in 31-96% isolated yield. Method optimization, substrate scope, a scale-up reaction, and downstream product functionalization are reported herein.

Applications of Palladium-Catalyzed C-N Cross-Coupling Reactions

Pd-catalyzed cross-coupling reactions that form C–N bonds have become useful methods to synthesize anilines and aniline derivatives, an important class of compounds throughout chemical research. A key factor in the widespread adoption of these methods has been the continued development of reliable and versatile catalysts that function under operationally simple, user-friendly conditions. This review provides an overview of Pd-catalyzed N-arylation reactions found in both basic and applied chemical research from 2008 to the present. Selected examples of C–N cross-coupling reactions between nine classes of nitrogen-based coupling partners and (pseudo)aryl halides are described for the synthesis of heterocycles, medicinally relevant compounds, natural products, organic materials, and catalysts.

Modular Approaches to Diversified Soft Lewis Basic Complexants through Suzuki-Miyaura Cross-Coupling of Bromoheteroarenes with Organotrifluoroborates

Remediation or transmutation of spent nuclear fuel obtained as a function of energy production and legacy waste remains a significant environmental concern. Substantive efforts over the last three decades have focused on the potential of soft-Lewis basic complexants for the chemoselective separation of trivalent actinides from lanthanides in biphasic solvent systems. Recent efforts in this laboratory have focused on the concept of modularity to rapidly prepare complexants and complexant scaffolds not easily accessible via traditional linear methods in a convergent manner to better understand solubility and complexation structure/activity function in process-relevant solvents. The current work describes an efficient method for the construction of diversified complexants through multi-Suzuki-Miyaura cross-coupling of bromoheteroarenes with organotrifluoroborates affording efficient access to 22 novel materials in 43-99% yield over two, three, or four cross-couplings on the same scaffold. Optimization of the catalyst/ligand system, application, and limitations are reported herein.