N-Heterocyclic Carbene Metal Complexes: Synthesis, Kinetics, Reactivity, and Recycling With Polymers

N-Heterocyclic carbenes (NHCs) are good ligands to most transition metals forming stable complexes. Many of the NHC-metal complexes are now widely used catalysts. However, the usage of these catalysts encounters the general problems associated with homogeneous catalysis: the purification of the catalysis reaction products is often time-consuming and generates large amounts of waste. Moreover, the toxic or expensive catalysts are difficult to be separated, recycled, and reused. Chapters II and III of this dissertation focus on addressing these problems through the development of an easier and “greener” process to improve the usage of some NHC-metal complexes. Polymer-supported catalysts and polymer-supported sequestrants were prepared and used to facilitate the separation/recycling of catalysts and the purification of products. These polymer-supported ligands, catalysts, and sequestrants showed comparable reactivity to their low molecular weight counterparts and had different solubility properties due to the nature of polymers. Using these materials with the corresponding operations provides simple methods to separate deeply colored, metal-containing by-products from the reaction mixtures. Chapter IV of this dissertation aims at solving a fundamental question about the nature of NHC-silver(I) complexes. The NHC-silver(I) complex is an important synthetic intermediate as it can be used to prepare other NHC-metal complexes through transmetallation. The carbene carbon of an NHC-silver(I) complex in 13C NMR spectra was usually reported as a doublet of doublets or as a singlet in different cases. This phenomenon was explained with a ligand exchange mechanism proposed twelve years ago. However, few reports are available in the literature about the mechanism of the NHC ligand exchange processes at silver. In order to facilitate the study of the solution behaviors of NHC-silver(I) complexes, 13C-labeled NHC-silver(I) complexes were prepared and studied using variable temperature 13C NMR spectroscopy. This study could be useful for future applications of ligand transferring from silver to other metals for the preparation of NHC-metal complexes

Combined experimental and computational investigations of the fluorosolvatochromism of chromeno[4,3-b]pyridine derivatives: Effect of the methoxy substitution

Extensive research has been conducted on the spectral properties of chromeno[4,3-b]pyridine derivatives, owing to their potential applications in sensing, optoelectronic devices, and drug discovery. This study presents a comprehensive investigation into the fluorosolvatochromism of selected chromeno[4,3-b]pyridine derivatives, with a particular emphasis on the impact of methoxy substitution. Three derivatives were synthesized and subjected to spectral analysis: chromeno[4,3-b]pyridine-3-carboxylate (I) as the parent compound, and its 7-methoxy (II) and 8-methoxy (III) substituted derivatives. The UV–Vis absorption spectra of all derivatives exhibited a broad band with a maximum absorption wavelength that remained unaffected by the surrounding medium. However, distinct fluorescence properties were observed among them. Specifically, derivative II displayed notable fluorescence, while derivatives I and III exhibited no fluorescence properties. Furthermore, derivative II exhibited a fluorescence spectrum that is significantly influenced by the polarity of the medium. To investigate the fluorosolvatochromic behavior in depth, we conducted a comprehensive analysis using various neat solvents with different polarities and hydrogen bonding capabilities. The results obtained revealed a significant positive fluorosolvatochromism, with a bathochromic shift in the fluorescence spectrum as the solvent polarity increased. To understand how specific and non-specific interactions between the solute and the solvent affected the fluorosolvatochromism of II, we employed the four empirical scales model of Catalán. The obtained results demonstrated that intramolecular charge transfer played a crucial role in the fluorescence behavior of II. To provide a molecular-level explanation for the experimental spectral properties, we utilized the DFT and TD-DFT/B3LYP/6-31 + G(d) computational methods with the IEFPCM implicit solvation approach. The spectral differences between II and III were rationalized in terms of the frontier molecular orbitals (FMOs: the HOMO and LUMO), where distinct natures were observed among the examined derivatives. This study offers valuable insights into the impact of methoxy substitution on the physical and chemical properties of chromeno[4,3-b]pyridine derivatives, specifically concerning their spectral properties as elucidated by their fluorosolvatochromic behavior.The support from Qatar University is thankfully acknowledged. We thank the Central Laboratories Unit at Qatar University for their support in compounds analysis. Major aspects of the calculations were performed using the supercomputing facility at Texas A&M University in Qatar. Open Access funding provided by the Qatar National Library.Scopu

Experimental and theoretical investigations of the effect of bis-phenylurea-based aliphatic amine derivative as an efficient green corrosion inhibitor for carbon steel in HCl solution

A novel bis-phenylurea-based aliphatic amine (BPUA) was prepared via a facile synthetic route, and evaluated as a potential green organic corrosion inhibitor for carbon steel in 1.0 M HCl solutions. NMR spectroscopy experiments confirmed the preparation of the targeted structure. The corrosion inhibitory behavior of the prospective green compound was explored experimentally by electrochemical methods and theoretically by DFT-based quantum chemical calculations. Obtained results revealed an outstanding performance of BPUA, with efficiency of 95.1% at the inhibitor concentration of 50 mg L−1 at 25 °C. The novel compound has improved the steel resistivity and noticeably reduced the corrosion rate from 33 to 1.7 mils per year. Furthermore, the adsorption study elucidates that the mechanism of the corrosion inhibition activity obeys Langmuir isotherm with mixed physisorption/chemisorption modes for BPUA derivatives on the steel surface. Calculated Gibb's free energy of the adsorption process ranges from −35 to −37 kJ mol−1.The SEM morphology analysis validates the electrochemical measurements and substantiates the corrosion-inhibiting properties of BPUA. Additionally, the eco-toxicity assessment on human epithelial MCF-10A cells proved the environmental friendliness of the BPUA derivatives. Density functional theory (DFT) calculations correlated the inhibitor's chemical structure with the corresponding inhibitory behavior. Quantum descriptors disclosed the potentiality of BPUA adsorption onto the surface through the heteroatom-based functional groups and aromatic rings.The support from Qatar University is gratefully acknowledged especially the Department of Chemistry and Earth Sciences and the Gas Processing Center. We also thank the Central Laboratories Unit, Qatar University for their support in material characterization. The primary computations were conducted using the advanced computational resources available at the supercomputing center located at Texas A&M University in Qatar. Open Access funding provided by the Qatar National Library

Synthesis of N -Heterocycles via Transition-Metal-Catalyzed Tandem Addition/Cyclization of a Nitrile

The diverse methodologies to synthesize N -heterocycles through transition-metal-catalyzed cascade addition/cyclization of a nitrile are discussed in this review. Aspects relating to three types of transition-metal-catalyzed addition of a nitrile with subsequent cyclization include (1) a transition-metal acting as a Lewis acid to accelerate the nucleophilic addition of a nitrile, (2) the late-transition-metal-catalyzed 1,2-insertion of a nitrile, and (3) an in situ generated radical by transition-metal catalysis to implement a radical addition/cyclization tandem reaction. Applications for the synthesis of natural alkaloids, their derivatives, and some bioactive compounds are also summarized herein. 1 Introduction 2 Nucleophilic Addition of a Nitrile Accelerated by a Lewis Acid 2.1 Late-Transition-Metal Catalysis 2.2 Early-Transition-Metal Catalysis 2.3 Lanthanide-Metal Catalysis 2.4 Cyclization from N -Arylnitriliums 3 Transition-Metal-Catalyzed Insertion of a Nitrile 4 Transition-Metal-Catalyzed Radical Addition of a Nitrile 5 Conclusions.This work was supported by the Ministry of Science and Technology, Taiwan (R.O.C.) (MOST 108-2113-M-032-001)

Synthesis of 5-oxo-5H-chromeno[3,4-c]pyridine-1-carbonitriles and features of their NMR spectra

Two different methods leading to 5-oxo-5H-chromeno[3,4-c]pyridine-1-carbonitriles were investigated. Reactions of 3-aminocrotonirile with substituted salicylaldehydes provided 5-oxo-5H-chromeno[3,4-c]pyridine-1-carbonitriles with the same substituent on positions 2 and 4 of the system. The reaction of 3-aminocrotonirile with variety of substituted 3-acetylcoumarins lead to 5-oxo-5H-chromeno[3,4-c]pyridine-1-carbonitriles with different substituents on positions 2 and 4. The structures of the products were confirmed by spectroscopic methods. The presence of nitrile moiety in the structures with fixed geometry caused the highly downfield shift of the aromatic proton at position 10 in 1H NMR spectrum. The electronic factor of the substituents caused variation of this downfield shift.This work was supported by Qatar University Grant (QUST‐1‐CAS‐2018‐31). The findings achieved herein are solely the responsibility of the authors. We thank Central Laboratories Unit, Qatar University for their support in compounds analysis

l-Proline-Catalyzed Three-Component Reaction of 4-Chloro-3-formylcoumarin, Sodium Sulfide, and α-Halo Ketones: A Direct Approach to Thieno[3,2- c ]coumarins

A new protocol for the synthesis of thieno[3,2-c]coumarins is disclosed. In this method, a 3-formyl-2-oxo-2H-chromene-4-thiolate anion is generated in situ by treatment of 4-chloro-3-formylcoumarin with sodium sulfide. This chromene-4-thiolate undergoes an L-prolinecatalyzed substitution/Knoevenagel cascade with various α-halo ketones to afford the desired thienocoumarins in moderate to good isolated yields. This protocol eliminates the need for stoichiometric amounts of inorganic bases and the use of foul-smelling thiols. The reaction conditions tolerate a variety of α-halo ketones.This work was supported by a Qatar University Student Grant [QUST2-CAS-2020-8] and the Medical Research Center (MRC)/Academic Health System (AHS) (Grant No. MRC-01-22-414

Database Facilitated Screening with NMR Spectroscopy Analysis for Drug Detection

© 2019 IEEE. Drug detection plays the key role in the enforcement of drug regulations. Various detection methods for them are thus developed. However, in most investigations, the most time-consuming and tedious part is not on the analysis itself but on the process to figure out what it is, i.e. to find out the possibility and to match the analytical results with the known information. Because of the limited amounts of the sample, an improper starting of the test could result in consuming of the evidence or unsuccessful analysis. Here we suggest a new process starting from NMR spectroscopy as NMR measurement is a non-destructive analysis, allowing other analysis methods after it. A spectra database screening system could help for quickly finding out the possible candidates and provide suggestions to confirm the results

Copper-Catalyzed Dual Cyclization for the Synthesis of Quinindolines

[[abstract]]A synthetic approach to quinindoline derivatives by the Cu-catalyzed dual cyclization has been developed. This catalytic reaction is a practical method for the systematic synthesis of quinindoline core structure, which contains a limited-step synthetic strategy and can tolerant a wide variety of substituents. In addition, the mechanistic study reveals that the reaction initiates from a Lewis acid accelerated addition of aniline to nitrile and provides the indole substructure, and then the subsequent Cu-catalyzed C-N coupling reaction furnishes the quinoline subunit and affords the quinindoline structure.[[notice]]補正完

Detecting Organic Nitrogen with 1H-15N HMBC Spectra

NMR spectroscopy has been the most important tool for organic chemistry research, providing detailed structure information. While 1H and 13C NMR spectra were frequently measured, 15N NMR spectra were relatively rare, even though nitrogen is commonly observed in organic molecules. This is due to the low gyromagnetic ratio and nature abundance. Usually 15N NMR spectra are observed when the sample is in very high concentration or the nitrogen is enriched with 15N isotope. HMBC is one of the 2D NMR techniques, measuring the through-bond correlations inside a molecule. 1H-15N HMBC actually collects a series of measurements of 1H NMR spectra with 15N information. Therefore, HMBC could get stronger signals than 15N signals and provide the opportunity for the indirect measurement of 15N signals

NMR Spectroscopy Database and Searching System

NMR spectroscopy is the most important analytical technology for organic compounds and plays the key role for the chemical characterizations and identifications in chemistry, pharmacy, materials science, environments, biology, and many related fields. In most cases, a NMR spectrum is compared with the known spectra to check if the sample contains a known chemical or is a new product. However, the current comparison process relayed on human beings and waste a lot of time and efforts. A new database and searching system is thus in need