Phenalenyl‐Based Organozinc Catalysts for Intramolecular Hydroamination Reactions: A Combined Catalytic,Kinetic, and Mechanistic Investigation of the Catalytic Cycle

Herein, we report the synthesis and characterization of two organozinc complexes that contain symmetrical phenalenyl (PLY)‐based N,N‐ligands. The reactions of phenalenyl‐based ligands with ZnMe₂ led to the formation of organozinc complexes [N(Me),N(Me)‐PLY]ZnMe ( 1 ) and [N(iPr),N(iPr)‐PLY]ZnMe ( 2 ) under the evolution of methane. Both complexes ( 1 and 2 ) were characterized by NMR spectroscopy and elemental analysis. The solid‐state structures of complexes 1 and 2 were determined by single‐crystal X‐ray crystallography. Complexes 1 and 2 were used as catalysts for the intramolecular hydroamination of unactivated primary and secondary aminoalkenes. A combined approach of NMR spectroscopy and DFT calculations was utilized to obtain better insight into the mechanistic features of the zinc‐catalyzed hydroamination reactions. The progress of the catalysis for primary and secondary aminoalkene substrates with catalyst 2 was investigated by detailed kinetic studies, including kinetic isotope effect measurements. These results suggested pseudo‐first‐order kinetics for both primary and secondary aminoalkene activation processes. Eyring and Arrhenius analyses for the cyclization of a model secondary aminoalkene substrate afforded ΔH^≠=11.3 kcal mol^?1, ΔS^≠=?35.75 cal K^?1 mol^?1, and E_a=11.68 kcal mol^?1. Complex 2 exhibited much‐higher catalytic activity than complex 1 under identical reaction conditions. The in situ NMR experiments supported the formation of a catalytically active zinc cation and the DFT calculations showed that more active catalyst 2 generated a more stable cation. The stability of the catalytically active zinc cation was further supported by an in situ recycling procedure, thereby confirming the retention of catalytic activity of compound 2 for successive catalytic cycles. The DFT calculations showed that the preferred pathway for the zinc‐catalyzed hydroamination reactions is alkene activation rather than the alternative amine‐activation pathway. A detailed investigation with DFT methods emphasized that the remarkably higher catalytic efficiency of catalyst 2 originated from its superior stability and the facile formation of its cation compared to that derived from catalyst 1 .     

A naphthalene exciplex based Al3+ selective on-type fluorescent probe for living cells at the physiological pH range: experimental and computational studies

2-((Naphthalen-6-yl)methylthio)ethanol (HL) was prepared by one pot synthesis using 2-mercaptoethanol and 2-bromomethylnaphthalene. It was found to be a highly selective fluorescent sensor for Al(3+) in the physiological pH (pH 7.0-8.0). It could sense Al(3+) bound to cells through fluorescence microscopy. Metal ions like Mn(2+), Fe(3+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Ag(+), Cd(2+), Hg(2+), Cr(3+) and Pb(2+) did not interfere. No interference was also observed with anions like Cl(-), Br(-), F(-), SO(4)(2-), NO(3)(-), CO(3)(2-), HPO(4)(2-) and SCN(-). Experimentally observed structural and spectroscopic features of HL and its Al(3+) complex have been substantiated by computational calculations using density functional theory (DFT) and time dependent density functional theory (TDDFT).     

Charge transfer interaction of 4-acetamidophenol (paracetamol) with 2,3-dichloro-1,4-naphthoquinone: a study in aqueous ethanol medium by UV-vis spectroscopic and DFT methods

4-Acetamidophenol (paracetamol) is shown to form charge transfer complex with 2,3-dichloro1,4-naphthoquinone in aqueous ethanol media exhibiting the unusual 2:1 (paracetamol:quinone) stoichiometry. The complexation enthalpy and entropy have been estimated from the formation constant (K) determined spectrophotometrically at five different temperatures. In aqueous ethanol mixtures of varying composition K increases with increasing dielectric constant of the medium. This has been rationalized by calculating the electronic charge distribution in paracetamol molecule and its conjugate base at the DFT/B3LYP/6-31++G(d,p) level. The theoretically calculated vertical ionization potential of paracetamol also agrees with reported experimental value.     

Self-assembled monolayers of Aβ peptides on Au electrodes: an artificial platform for probing the reactivity of redox active metals and cofactors relevant to Alzheimer's disease

The water-soluble hydrophilic part of human Aβ peptide has been extended to include a C-terminal cysteine residue. Utilizing the thiol functionality of this cysteine residue, self-assembled monolayers (SAM) of these peptides are formed on Au electrodes. Atomic force microscopy imaging confirms formation of small Aβ aggregates on the surface of the electrode. These aggregates bind redox active metals like Cu and cofactors like heme, both of which are proposed to generate toxic partially reduced oxygen species (PROS) and play a vital role in Alzheimer's disease. The spectroscopic and electrochemical properties of these Cu and heme bound Aβ SAM are similar to those reported for the soluble Cu and heme bound Aβ peptide. Experiments performed on these Aβ-SAM electrodes clearly demonstrate that (1) heme bound Aβ is kinetically more competent in reducing O(2) than Cu bound Aβ, (2) under physiological conditions the reduced Cu site produces twice as much PROS (measured in situ) than the reduced heme site, and (3) chelators like clioquinol remove Cu from these aggregates, while drugs like methylene blue inhibit O(2) reactivity of the heme cofactor. This artificial construct provides a very easy platform for investigating potential drugs affecting aggregation of human Aβ peptides and PROS generation by its complexes with redox active metals and cofactors.     

Remodeling of the Fibrillation Pathway of α-Synuclein by Interaction with Antimicrobial Peptide LL-III

Liquid-liquid phase separation (LLPS) has emerged as a key mechanism for intracellular organization, and many recent studies have provided important insights into the role of LLPS in cell biology. There is also evidence that LLPS is associated with a variety of medical conditions, including neurodegenerative disorders. Pathological aggregation of α-synuclein, which is causally linked to Parkinson's disease, can proceed via droplet condensation, which then gradually transitions to the amyloid state. We show that the antimicrobial peptide LL-III is able to interact with both monomers and condensates of α-synuclein, leading to stabilization of the droplet and preventing conversion to the fibrillar state. The anti-aggregation activity of LL-III was also confirmed in a cellular model. We anticipate that studying the interaction of antimicrobial-type peptides with liquid condensates such as α-synuclein will contribute to the understanding of disease mechanisms (that arise in such condensates) and may also open up exciting new avenues for intervention.     

Quantification of piperine in different varieties of Piper nigrum by a validated high-performance thin-layer chromatography‒densitometry method

JPC – Journal of Planar Chromatography – Modern TLC - Piperine is the most important alkaloid molecule present in Piper nigrum. In this study, a simple, rapid, quantitative...     

Construction of unsymmetrical bis-urea macrocyclic host for neutral molecule and chloride-ion binding

Construction of synthetic macrocyclic host that can bind with neutral molecules and anions has potential applications in supramolecular chemistry. Herein, we have designed and synthesized blue light emitting an unsymmetrical neutral bis-urea macrocyclic host. This macrocycle can bind with neutral DMF molecule (1:1) as well as Cl⁻ ion (1:1) through noncovalent interactions. X-Ray crystal structure, ¹H NMR titrations with Job's Plot, HRMS with isotropic distribution pattern, FT-IR, and density functional theory analysis revealed the binding of bis-urea macrocyclic host with the guest molecule.     

Radon mapping in groundwater and indoor environs of Budgam,Jammu and Kashmir

Journal of Radioanalytical and Nuclear Chemistry - Radiation exposure, an inescapable share of our everyday life, primarily arises from terrestrial or cosmic sources. A small fragment of the total...