Potent pincer-zinc catalyzed homogeneous α-alkylation and Friedländer quinoline synthesis reaction of secondary alcohols/ketones with primary alcohols

Herein, we describe an air- and moisture-stable, homogeneous zinc catalyst stabilised using an electron deficient N^N^N pincer-type ligand. This ternary, penta-coordinated neutral molecular catalyst [Zn(N^N^N)Cl2] selectively produces α-alkylated ketone derivatives (14 examples) through a one-pot acceptorless dehydrogenative coupling (ADC) reaction between secondary and primary alcohols using the borrowing hydrogen (BH) approach in good to excellent isolated yields (up to 93%). It is worth noting that this catalyst also provides an eco-friendly route for the synthesis of quinoline derivatives (30 examples) using 2-aminobenzyl alcohols as alkylating agents via successive dehydrogenative coupling and N-annulation reactions. This cost effective, easy to synthesize and environmentally benign catalyst shows excellent stability in catalytic cycles under open-air conditions, as evident from its high turnover number (∼104), and is activated by using a catalytic amount of base under milder conditions.

ZnCl2-catalysed transfer hydrogenation of carbonyls and chemoselective reduction of the CC bond in α,β-unsaturated ketones

This manuscript describes chemoselective reduction of CC in α,β-unsaturated ketones and the transfer hydrogenation of aldehydes and ketones catalysed by ZnCl2-phosphinamino-triazolyl-pyridine (0.5 mol%) using KOH/iPrOH as a H2 source. A detailed mechanistic study using DFT calculations (B3LYP-D3/def2-TZVP) revealed the key role of metal-ligand cooperation (MLC) in the catalytic reaction demonstrating the non-innocent behaviour of the phosphine ligand.

Zn(OTf)2-catalyzed intra- and intermolecular selenofunctionalization of alkenes under mild conditions

Zn(OTf)2-catalyzed intra- and intermolecular selenofunctionalization of alkenes was achieved with electrophilic N-phenylselenophthalimide. This method provides straightforward and efficient access to various seleno-substituted heterocycles and vicinal Se heteroatom-disubstituted molecules under mild conditions. This reaction is compatible with various substrates/functional groups, and preliminary studies on the reaction mechanistic were also conducted.

Trickier than It Looks: Isomerization between Five- and Six-Coordinated Zinc in Heterometallic Li2Zn2 Molecule

This report describes the synthesis and characterization of two heterobimetallic Li-Zn coordination isomers [Li2Zn2(tbaoac)6] (tbaoac = tert-butyl acetoacetato) that have been isolated separately by the same stoichiometric reaction run in different organic solvents. The 6-coordinated zinc isomer (6-Zn) was synthesized in acetone with high yield, while the 5-coordinated one (5-Zn) was readily obtained from ethanol. The 5-Zn isomer has a low solubility in organic solvents such as alkanes and haloalkanes, while its 6-Zn counterpart exhibits a good solubility in almost all common solvents. Two isomeric molecules feature similar centrosymmetric tetranuclear cyclic assemblies, which are different in their arrangement of tbaoac ligands. While all ligands act as μ2-type in the structure of 5-Zn, the two tbaoac groups chelating Li appear as μ3-type in 6-Zn, thus providing an additional coordination for Zn ions. However, the real structural transformation between these isomers was shown to be more complex than simply making or breaking a couple of Zn-O bonds. X-ray single-crystal structure analysis, powder X-ray diffraction, multinuclear NMR, DART mass spectrometry, ICP-OES analysis, and TGA have been employed for the characterization of the isomers. The combination of powder X-ray diffraction and 1H NMR investigation revealed that 6-Zn isomer can be quantitatively transformed to 5-Zn in ethanol, while the reverse conversion instantly takes place in acetone.

Tri-coordinated zinc alkyl complexes with N^S/Se coordination of imino-phosphanamidinate chalcogenide ligands as precursors for efficient hydroboration of nitriles and esters

A series of tri-coordinated zinc alkyl complexes with the general molecular formula [κ2NE-{NHIRP(Ph)(E)N-Dipp}ZnEt] [R = Dipp (2,6-diisopropylphenyl), E = S (3a), Se (3b) and R = tBu (tert-butyl), E = S (4a), Se (4b)] bearing imino-phosphanamidinate chalcogenide ligands were prepared in good yields from the reaction between the protic imino-phosphanamidinate chalcogenide ligand [NHIRP(Ph)(E)NH-Dipp] [R = Dipp, E = S (1a), Se (1b) and R = tBu, E = S (2a), Se (2b)] and diethylzinc at room temperature. The molecular structures of all the zinc complexes were established by single-crystal X-ray diffraction analysis. In the solid state, all complexes exhibited a distorted trigonal planar geometry around the zinc ion. Metal-chalcogenide (Zn-S/Se) interactions were observed in the coordination sphere. These zinc alkyl complexes were employed as pre-catalysts in the hydroboration reaction of nitriles and esters to obtain the corresponding N,N-diborylamines and boronate esters, respectively, under ambient conditions. A wide substrate scope of nitriles and esters is presented.

Zinc amidinate-catalysed cyclization reaction of carbodiimides and alkynes. An insight into the mechanism

A synthesis of iminopyridines based on zinc has been developed. The commercially available ZnEt2 was employed as a precatalyst for this process. A mechanism has been proposed on the basis of Density Functional Theory (DFT) studies and stoichiometric reactions. The zinc amidinato intermediates underscore the critical role of zinc in this synthesis process.

Transition-Metal-Catalyzed Transformations for the Synthesis of Marine Drugs

Transition metal catalysis has contributed to the discovery of novel methodologies and the preparation of natural products, as well as new chances to increase the chemical space in drug discovery programs. In the case of marine drugs, this strategy has been used to achieve selective, sustainable and efficient transformations, which cannot be obtained otherwise. In this perspective, we aim to showcase how a variety of transition metals have provided fruitful couplings in a wide variety of marine drug-like scaffolds over the past few years, by accelerating the production of these valuable molecules.

Dinuclear Zn-Catalytic System as Brønsted Base and Lewis Acid for Enantioselectivity in Same Chiral Environment

Zinc (Zn) is a crucial element with remarkable significance in organic transformations. The profusion of harmless zinc salts in the Earth's outer layer qualifies zinc as a noteworthy contender for inexpensive and eco-friendly reagents and catalysts. Recently, widely recognized uses of organo-Zn compounds in the field of organic synthesis have undergone extensive expansion toward asymmetric transformations. The ProPhenol ligand, a member of the chiral nitrogenous-crown family, exhibits the spontaneous formation of a dual-metal complex when reacted with alkyl metal (R-M) reagents, e.g., ZnEt2. The afforded Zn complex possesses two active sites, one Lewis acid and the other Brønsted base, thereby facilitating the activation of nucleophiles and electrophiles simultaneously within the same chiral pocket. In this comprehensive analysis, we provide a thorough account of the advancement and synthetic potential of these diverse catalysts in organic synthesis, while emphasizing the reactivity and selectivities, i.e., dr and ee due to the design/structure of the ligands employed.

Transition-Metal Catalyzed Synthesis of Pyrimidines: Recent Advances, Mechanism, Scope and Future Perspectives

Pyrimidine is a pharmacologically important moiety that exhibits diverse biological activities. This review reflects the growing significance of transition metal-catalyzed reactions for the synthesis of pyrimidines (with no discussion being made on the transition metal-catalyzed functionalization of pyrimidines). The effect of different catalysts on the selectivity/yields of pyrimidines and catalyst recyclability (wherever applicable) are described, together with attempts to illustrate the role of the catalyst through mechanisms. Although several methods have been researched for synthesizing this privileged scaffold, there has been a considerable push to expand transition metal-catalyzed, sustainable, efficient and selective synthetic strategies leading to pyrimidines. The aim of the authors with this update (2017-2023) is to drive the designing of new transition metal-mediated protocols for pyrimidine synthesis.

2-Anilidomethylpyridine-Derived Three-Coordinate Zinc Hydride: The Journey Unveils Anilide Backbone's Reactive Nature

Low-coordinate heteroleptic zinc hydrides are catalytically important but rare and synthetically challenging. We herein report three-coordinate monomeric zinc hydride on a 2-anilidomethylpyridine framework (NNL). The synthetic success comes through systematically screening a few different routes from different precursors. During the process, the ligand's anilide backbone interestingly appears to be more reactive than Zn's terminal site to electrophilic Lewis and Brønsted acids. The proligand NNLH reacts with [Zn{N(SiMe3)2}2] and ZnEt2 to give [(NNL)ZnA] (A = N(SiMe3)2 (1), Et(2)). Both are inert to PhSiH3 and H2 but react with HBpin only through the internal Zn-Nanilide bond to give the borylated ligand NNLBpin (3). The reactions of 1 and 2 with Ph3EOH (E = C, Si) afford a series of divergent compounds like [(NNLH)Zn(OSiPh3)2] (4), [Zn3(OSiPh3)4Et2] (5), and [EtZn(OCPh3)] (6). But in all cases, it is invariably the Zn-Nanilide bond protonated by the -OH with equal or higher preference than the terminal Zn-N or Zn-C bonds. A DFT analysis rationalizes the origin of such a reactivity pattern. Realizing that an acid-free route might be the key, reacting [(NNL)Li] with ZnBr2 gives [(NNL)Zn(μ-Br)]2 (7), which on successively treating with KOSiPh3 and PhSiH3 gives the desired [(NNL)ZnH] (8) as a three-coordinate monomer with a terminal Zn-H bond. Estimating the ligand steric in 8 shows the openness in Zn's coordination sphere, a desired criterion for efficient catalysis. This and a positive influence of the pyridyl sidearm is reflected in 8's superior activity in hydroborating PhC(O)Me by HBpin in comparison to Jones' two-coordinate anilido zinc hydride.

Zn(II)-Catalyzed Selective N-Alkylation of Amines with Alcohols Using Redox Noninnocent Azo-Aromatic Ligand as Electron and Hydrogen Reservoir

We report a sustainable and eco-friendly approach for selective N-alkylation of various amines by alcohols, catalyzed by a well-defined Zn(II)-catalyst, Zn(L^a)Cl₂ (1a), bearing a tridentate arylazo scaffold. A total of 57 N-alkylated amines were prepared in good to excellent yields, out of which 17 examples are new. The Zn(II)-catalyst shows wide functional group tolerance, is compatible with the synthesis of dialkylated amines via double N-alkylation of diamines, and produces the precursors in high yields for the marketed drugs tripelennamine and thonzonium bromide in gram-scale reactions. Control reactions and DFT studies indicate that electron transfer events occur at the azo-chromophore throughout the catalytic process, which shuttles between neutral azo, one-electron reduced azo-anion radical, and two-electron reduced hydrazo forms acting both as electron and hydrogen reservoir, enabling the Zn(II)-catalyst for N-alkylation reaction.

A New Zn(II) Metal Hybrid Material of 5-Nitrobenzimidazolium Organic Cation (C7H6N3O2)2[ZnCl4]: Elaboration, Structure, Hirshfeld Surface, Spectroscopic, Molecular Docking Analysis, Electric and Dielectric Properties

The slow solvent evaporation approach was used to create a single crystal of (C₇H₆N₃O₂)₂[ZnCl₄] at room temperature. Our compound has been investigated by single-crystal XRD which declares that the complex crystallizes in the monoclinic crystallographic system with the P2₁/c as a space group. The molecular arrangement of the compound can be described by slightly distorted tetrahedral ZnCl₄^2- anionic entities and 5-nitrobenzimidazolium as cations, linked together by different non-covalent interaction types (H-bonds, Cl…Cl, π…π and C-H…π). Hirshfeld's surface study allows us to identify that the dominant contacts in the crystal building are H…Cl/Cl…H contacts (37.3%). FT-IR method was used to identify the different groups in (C₇H₆N₃O₂)₂[ZnCl₄]. Furthermore, impedance spectroscopy analysis in 393 ≤ T ≤ 438 K shows that the temperature dependence of DC conductivity follows Arrhenius' law. The frequency-temperature dependence of AC conductivity for the studied sample shows one region (E_a = 2.75 eV). In order to determine modes of interactions of compound with double stranded DNA, molecular docking simulations were performed at molecular level.

Regiospecific α-methylene functionalisation of tertiary amines with alkynes via Au-catalysed concerted one-proton/two-electron transfer to O2

Regioselective transformations of tertiary amines, which are ubiquitously present in natural products and drugs, are important for the development of novel medicines. In particular, the oxidative α-C-H functionalisation of tertiary amines with nucleophiles via iminium cations is a promising approach because, theoretically, there is almost no limit to the type of amine and functionalisation. However, most of the reports on oxidative α-C-H functionalisations are limited to α-methyl-selective or non-selective reactions, despite the frequent appearance of α-methylene-substituted amines in pharmaceutical fields. Herein, we develop an unusual oxidative regiospecific α-methylene functionalisation of structurally diverse tertiary amines with alkynes to synthesise various propargylic amines using a catalyst comprising Zn salts and hydroxyapatite-supported Au nanoparticles. Thorough experimental investigations suggest that the unusual α-methylene regiospecificity is probably due to a concerted one-proton/two-electron transfer from amines to O₂ on the Au nanoparticle catalyst, which paves the way to other α-methylene-specific functionalisations.

Zn-Catalyzed Regioselective and Chemoselective Reduction of Aldehydes, Ketones and Imines

An operationally convenient Zn-catalyzed synthesis of alcohols by the reduction of aldehydes, ketones, and α,β-unsaturated aldehydes/ketones is reported. It is a rare example of using mild and sustainable HBpin as a reductant for catalytic reduction of carbonyl compounds in the absence of acid or base as hydrolysis reagent. The reaction is upscalable and proceeds in high selectivity without the formation of boronate ester by-products, and tolerates sensitive functionalities, such as iodo, bromo, chloro, fluoro, nitro, trifluoromethyl, aminomethyl, alkynyl, and amide. The Zn(OAc)₂/HBpin combination has been also proved to be chemoselective for the C=N reduction of imine analogs.

Highly Stable 3D Supercuboids to 2D ZnSe Nanosheets: Formation for a High-Efficiency Catalysis System

Assembling into desired and higher-ordered superstructures makes nanocrystals superior candidates in a wide range of applications. Herein, we report a facile but robust colloidal chemistry method to obtain three-dimensional (3D) supercuboids from two-dimensional (2D) ZnSe nanosheets. It is well-defined that the formation mechanism of the supercuboids is based on the interaction among ligands on the ZnSe nanosheets. The highly stable supercuboids are composed of nanosheets with thickness of approximately ∼1.4 nm, and the spacing between two nanosheets in the supercuboid is revealed as ∼1.2 nm. Importantly, 2D nanosheets are readily regained by an exfoliation process of 3D supercuboids in a high-temperature solvothermal process. Furthermore, the Au@ZnSe heteronanosheets could be obtained by Au nanoparticles (NPs) decorated on 2D nanosheets. As a proof-of-concept application, the ZnSe nanosheets are applied in the catalysis of the aldol condensation reaction, which shows the high isolated yield of chalcone up to 80%.

Bio-Based (Chitosan-ZnO) Nanocomposite: Synthesis, Characterization, and Its Use as Recyclable, Ecofriendly Biocatalyst for Synthesis of Thiazoles Tethered Azo Groups

In recent years, nanotechnology has become a considerable research interest in the area of preparation of nanocatalysts based on naturally occurring polysaccharides. Chitosan (CS), as a naturally occurring biodegradable and biocompatible polysaccharide, is successfully utilized as an ideal template for the immobilization of metal oxide nanoparticles. In this study, zinc oxide nanoparticles have been doped within a chitosan matrix at dissimilar weight percentages (5, 10, 15, 20, and 25 wt.% CS/ZnO) and have been fabricated by using a simple solution casting method. The prepared solutions of the nanocomposites were cast in a Petri-dish and were subsequently shaped as a thin film. After that, the structural features of the nanocomposite film have been studied by measuring the FTIR, SEM, and XRD analytical tools. FTIR spectra showed the presence of some changes in the major characteristic peaks of chitosan due to interaction with ZnO nanoparticles. In addition, SEM graphs exhibited dramatic morphology changes on the chitosan surface, which is attributed to the surface adsorption of ZnO molecules. Based on the results of the investigated organic catalytic reactions, the prepared CS/ZnO nanocomposite film (20 wt.%) could be a viable an effective, recyclable, and heterogeneous base catalyst in the synthesis of thiazoles. The results showed that the nanocomposite film is chemically stable and can be collected and reused in the investigated catalytic reactions more than three times without loss of its catalytic activity.

Catalytic Hydroboration and Reductive Amination of Carbonyl Compounds by HBpin using a Zinc Promoter

In this paper, the chemoselective hydroboration of aldehydes and ketones, catalyzed by Zinc(II) complexes [ k 2 -(PyCH=NR)ZnX 2 ] [R = CPh 3 , X = Cl ( 1 ) and R = Dipp (2,6-diisoropylphenyl) and X = I ( 2 )], in the presence of pinacolborane (HBpin) in ambient temperature and solvent-free conditions, which produced corresponding boronate esters in high yield, is reported. Zinc metal complexes 1 and 2 were derived in 80-90% yield from the reaction of iminopyridine [PyCH=NR] with anhydrous zinc dichloride in dichloromethane at room temperature. The solid-state structures of both zinc complexes were confirmed using X-ray crystallography. Zinc complex 1 was also used as a competent pre-catalyst in the reductive amination of carbonyl compounds with HBpin under mild and solvent-free conditions to afford a high yield (up to 97%) of the corresponding secondary amines. The wider substrate scope of both reactions was explored. Catalytic protocols using zinc as a pre-catalyst demonstrated an atom-economic and green method with diverse substrates bearing excellent functional group tolerance. Computational studies established a plausible mechanism for catalytic hydroboration.

Nanocomposite Polymer Electrolytes for Zinc and Magnesium Batteries: From Synthetic to Biopolymers

The diversification of current forms of energy storage and the reduction of fossil fuel consumption are issues of high importance for reducing environmental pollution. Zinc and magnesium are multivalent ions suitable for the development of environmentally friendly rechargeable batteries. Nanocomposite polymer electrolytes (NCPEs) are currently being researched as part of electrochemical devices because of the advantages of dispersed fillers. This article aims to review and compile the trends of different types of the latest NCPEs. It briefly summarizes the desirable properties the electrolytes should possess to be considered for later uses. The first section is devoted to NCPEs composed of poly(vinylidene Fluoride-co-Hexafluoropropylene). The second section centers its attention on discussing the electrolytes composed of poly(ethylene oxide). The third section reviews the studies of NCPEs based on different synthetic polymers. The fourth section discusses the results of electrolytes based on biopolymers. The addition of nanofillers improves both the mechanical performance and the ionic conductivity; key points to be explored in the production of batteries. These results set an essential path for upcoming studies in the field. These attempts need to be further developed to get practical applications for industry in large-scale polymer-based electrolyte batteries.

Zinc catalysed C3-H borylation of indoles and 1,1-diboration of terminal alkynes

A low catalyst loading Zn(OTf)₂-catalysed C3-H borylation of indoles with pinacolborane was developed. This transformation represents the use of an abundant, cheap and environmentally benign zinc catalyst in catalytic direct aromatic C-H borylation and offers a simple and prompt route towards the synthesis of C3-borylated indoles. The 1,1-diboration of terminal alkynes was also achieved using the same catalytic system to produce 1,1-diborylated alkenes.

Catalytic reductive desymmetrization of malonic esters

Desymmetrization of fully substituted carbons with a pair of enantiotopic functional groups is a practical strategy for the synthesis of quaternary stereocentres, as it divides the tasks of enantioselection and C-C bond formation. The use of disubstituted malonic esters as the substrate of desymmetrization is particularly attractive, given their easy and modular preparation, as well as the high synthetic values of the chiral monoester products. Here, we report that a dinuclear zinc complex with a tetradentate ligand can selectively hydrosilylate one of the carbonyls of malonic esters to give α-quaternary β-hydroxyesters, providing a promising alternative to the desymmetric hydrolysis using carboxylesterases. The asymmetric reduction features excellent enantiocontrol that can differentiate sterically similar substituents and high chemoselectivity towards the diester motif of substrates. Together with the versatile preparation of malonic ester substrates and post-reduction derivatization, the desymmetric reduction has enabled the synthesis of a diverse array of quaternary stereocentres with distinct structural features.

Bicyclic (alkyl)(amino)carbene stabilized zinc(0) complex with singlet biradicaloid ground state

A storable bicyclic (alkyl)(amino)carbene (BICAAC) stabilized two coordinate zinc(0) complex [(BICAAC)2Zn] (2) was synthesized. DFT calculations reveal that BICAAC plays a decisive role in imparting the stability to 2. This complex activates the C(sp3)-Cl bond of trityl chloride generating the Gomberg's free radical with greater efficiency than metallic Zn powder.

A Zinc-Mediated Deprotective Annulation Approach to New Polycyclic Heterocycles

A straightforward approach to new polycyclic heterocycles, 1H-benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-ones, is presented. It is based on the ZnCl₂-promoted deprotective 6-endo-dig heterocyclization of N-Boc-2-alkynylbenzimidazoles under mild conditions (CH₂Cl₂, 40 °C for 3 h). The zinc center plays a dual role, as it promotes Boc deprotection (with formation of the tert-butyl carbocation, which can be trapped by substrates bearing a nucleophilic group) and activates the triple bond toward intramolecular nucleophilic attack by the carbamate group. The structure of representative products has been confirmed by X-ray diffraction analysis.

Zn(OTf)2-catalyzed hydroamination of ynamides with aromatic amines

The Zn(OTf)₂-catalyzed hydroamination of ynamides 2a-2l with aromatic amines 1a-1r was developed. This protocol features broad substrate scope of aromatic amines, good functional group tolerance for ynamides, and excellent regioselectivities. As a result, a variety of substituted amidine compounds 3aa-3oa, 3ab-3al and 3pa-3rk were prepared in moderate to excellent yields and with high regioselectivities.

Antioxidant and antimicrobial material by grafting of L-arginine onto enzymatic poly(gallic acid)

Microwave-mediated grafting of L-Arg onto naturally derived and stable multiradical poly(gallic acid) (PGAL) in aqueous media has been successfully achieved. This polymeric material has no adverse effect in human cells as there is no hemolytic activity upon MTT and Neutral Red assays. The analytical and computational characterization studies carried out in this study describe a helical molecular structure with random incorporation of L-Arginine pendant groups from PGAL's backbone. The antioxidant properties of the precursor polymer are preserved as proved by the elimination of stable DPPH and hydroxyl radical scavenging, as well as the FRAP and ORAC assays. Regarding the latter, the oxygen radical inhibition is enhanced compared to PGAL, which is attributed to the guanidyl moieties. PGAL-g-L-Arg displays antimicrobial activity against Gram (+) Listeria monocytogenes and Staphylococcus aureus strains with a MIC of 0.8 g/L and a bacteriostatic effect against Gram (-) Escherichia coli. Additionally, scanning electron and confocal fluorescence microscopies as well as crystal violet colorimetric assay demonstrate that the mechanism involved in the bacterial inhibition is related to the formation of porous channels on the membrane, which is discussed according to the helical secondary structure of the polymer and the amino acid guanidyl moieties interacting to bacterial membranes.

Oligo(amylene) from the reaction of fusel oil with zinc dihalide

Heating mixtures of fusel oil and zinc chloride or zinc bromide to 180 °C gave water, difusel ethers and the hydrocarbon oligo(amylene) as the major coproducts. Separation by chromatography gave oligo(amylene) in 25% yield from fusel oil. The triamylene fraction of the oligo(amylene) had a net heating value of 43.9 kJ g⁻¹ which was 3% greater than specifications for gasoline, diesel #2 and jet A-1. The cetane number of the triamylene was 31.9 so it may not be useful for diesel engines. The triamylene had a flashpoint of 38 °C, viscosity (−20 °C) of 7.85 mm² s⁻¹, density (15 °C) of 0.78 g mL⁻¹ and melting point below −78 °C which all compared well to the specifications of jet A-1.

Fusel oil was converted into the liquid hydrocarbon oligo(amylene) by zinc bromide catalysis.

Metal-Ligand Cooperation Facilitates Bond Activation and Catalytic Hydrogenation with Zinc Pincer Complexes

A series of PNP zinc pincer complexes capable of bond activation via aromatization/dearomatization metal–ligand cooperation (MLC) were prepared and characterized. Reversible heterolytic N–H and H–H bond activation by MLC is shown, in which hemilability of the phosphorus linkers plays a key role. Utilizing this zinc pincer system, base-free catalytic hydrogenation of imines and ketones is demonstrated. A detailed mechanistic study supported by computation implicates the key role of MLC in facilitating effective catalysis. This approach offers a new strategy for (de)hydrogenation and other catalytic transformations mediated by zinc and other main group metals.

Synthesis, Characterization, and Investigation of the Antimicrobial Activity of Cetylpyridinium Tetrachlorozincate

Cetylpyridinium tetrachlorozincate (referred to herein as (CP)₂ZnCl₄) was synthesized and its solid-state structure was elucidated via single-crystal X-ray diffraction (SC-XRD), revealing a stoichiometry of C₄₂H₇₆Cl₄N₂Zn with two cetylpyridinium (CP) cations per [ZnCl₄]^2- tetrahedra. Crystal structures at 100 and 298 K exhibited a zig-zag pattern with alternating alkyl chains and zinc units. The material showed potential for application as a broad-spectrum antimicrobial agent, to reduce volatile sulfur compounds (VSCs) generated by bacteria, and in the fabrication of advanced functional materials. Minimum inhibitory concentration (MIC) of (CP)₂ZnCl₄ was 60, 6, and 6 μg mL^-1 for Salmonella enterica, Staphylococcus aureus, and Streptococcus mutans, respectively. The MIC values of (CP)₂ZnCl₄ were comparable to that of pure cetylpyridinium chloride (CPC), despite the fact that approximately 16% of the bactericidal CPC is replaced with bacteriostatic ZnCl₂ in the structure. A modified layer-by-layer deposition technique was implemented to synthesize mesoporous silica (i.e., SBA-15) loaded with approximately 9.0 wt % CPC and 8.9 wt % Zn.

A competitive and highly selective 7-, 6- and 5-annulation with 1,3-migration through C-H and N-H - alkyne coupling

We demonstrated a highly competitive and selective C-C and N-C cross-coupled 7-, 6- and 5-annulation utilizing 2-ethynylanilides to afford functionalized 1H-benzo[b]azepin-2(5H)-ones, 2-quinolinones, and 3-acylindoles in high yield. ZnCl2 was found to be the smart catalyst for 7- and 5-annulation with 1,3-migration through C-H and N-H functionalization, respectively, whereas molecular iodine performed the C-H functionalized 6-annulation with a nonconventional 1,3 H-shift. The mechanism was investigated by intermediate trapping, control, and labeling experiments.

Unraveling mechanisms of the uncoordinated nucleophiles: theoretical elucidations of the cleavage of bis(p-nitrophenyl) phosphate mediated by zinc-complexes with apical nucleophiles

A theoretical approach was used to investigate the hydrolytic cleavage mechanisms of the bis(p-nitrophenyl) phosphate (BNPP-) catalyzed by Zn(ii)-complexes featuring uncoordinated nucleophiles. Ligand-based and alternative solvent-based nucleophilic attack reaction models are proposed. The pK a values of the Zn(ii)-bound water molecules or ligands in the [Zn(L n H)(η-H2O)(H2O)]2+ (n = 1, 2 and 3) complexes, as well as the dimerization tendency of the mononuclear Zn(ii)-complexes, were found to significantly influence the reaction mechanisms. The Zn(ii)-L3 complexes were found to be more favorable for the hydrolytic cleavage of the BNPP- via a ligand-based nucleophilic attack pathway. This was due to the lower pK a value for the deprotonation of the oxime ligand, the hard dimerization of the mononuclear Zn(ii)-L3 species, and the presence of an uncoordinated nucleophile. The origins of the uncoordinated reactions were systematically elucidated. The theoretical results reported here are in good agreement with experimental observations and more importantly, help to elucidate the factors that influence intermolecular nucleophilic attack reactions with coordinated/uncoordinated nucleophiles.

ZnBr2/Oxone-mediated ipso-cyclization of N-(3-phenylprop-2-yn-1-yl)aniline

In this work, a selective synthetic strategy towards 1-azaspiro[4.5]deca-3,6,9-trien-8-ones from N-tosyl-N-(prop-2-yn-1-yl)aniline is developed. The transformation proceeds smoothly in a mixed solvent including acetonitrile and water when ZnBr2 and Oxone are employed. Mechanism studies show that the reaction proceeds in a regioselective manner via a radical ipso-cyclization pathway.

Zn(OTf)2-mediated annulations of N-propargylated tetrahydrocarbolines: divergent synthesis of four distinct alkaloidal scaffolds

Experimental and computational studies gained insights into Zn(OTf)₂-mediated annulations and allowed programmable synthesis of four distinct alkaloidal scaffolds.

Intramolecular hydroarylations of N-propargylated tetrahydrocarbolines were efficiently mediated using a unique combination of Zn(OTf)₂ with t-BuOH under neutral conditions. Use of the artificial force induced reaction method in the global reaction route mapping strategy provided insights into the Zn(OTf)₂-mediated hydroarylations and the associated intriguing solvent effects of t-BuOH facilitating a protodezincation process without a Brønsted acid activator. We systematically implemented three distinct hydroarylations as well as an unanticipated α-alkenylation of a carbonyl group to obtain the four alkaloidal scaffolds 2–4, and 18. Zn(OTf)₂-mediated annulation of 1c proceeded through kinetic formation of the spiroindole 3c followed by an alkenyl shift and concomitant retro-Mannich-type fragmentation to furnish azepino[4,5-b]indole 2 framework. Substituents on substrate 1 in the vicinity of the reaction sites substantially affected the mode of the divergent annulations. Judicious choices of the substituents, solvent and reaction conditions enabled programmable divergent synthesis of the four distinct skeletons.

Zn(II)-Catalyzed Addition of Aromatic/Heteroaromatic C(sp2)-H to Azoalkenes: A Polarity-Reversed Arylation of Carbonyl Compounds

An umpolung α-(hetero)arylation strategy that involves the Michael-type reaction between electron-rich (hetero)aromatic substrates and azoalkenes is developed. The reaction proceeds under very mild conditions at room temperature and in the presence of inexpensive, nontoxic ZnCl₂ catalyst to provide access to otherwise inaccessible hydrazone structures. Subsequent hydrolysis of these latter to ketones as well as other valuable synthetic transformations to a variety of heterocyclic scaffolds demonstrate the usefulness of this protocol.