Lewis-acid catalyzed organic reactions in water. The case of AlCl(3), TiCl(4), and SnCl(4) believed to be unusable in aqueous medium

Effectiveness of ozonation with zirconium and tin tetrachloride for stabilized anaerobic landfill leachate treatment

Landfill leachate can threaten the environment and human life. Therefore, this study aims to investigate the efficiency of ozone (O₃ ), O₃ with zirconium tetrachloride (O₃ /ZrCl₄ ), and O₃ with tin tetrachloride (O₃ /SnCl₄ ) in remediating the stabilized anaerobic landfill leachate (SAL) from Alor Pongsu, Perak. Hydroxyl radical (OH•) is an important oxidizing agent in the ozonation process. Its presence was tested using tert-butyl alcohol. Results showed that using ZrCl₄ and SnCl₄ in ozonation boosted the generation of hydroxyl radical, thereby enhancing the oxidation process and pollutant removal inside the sample. The O₃ /ZrCl₄ mix at chemical oxygen demand (COD) to ZrCl₄ ratio of 1:1.5, pH 8-9, and 90-min reaction time resulted in the highest reduction rates of COD and color at 91.9% and 99.6%, respectively. All results demonstrated that the optimum performance occurred at alkaline conditions (pH > 8), proving that OH radicals primarily oxidized the pollutants through an indirect reaction pathway. The biodegradability (biochemical oxygen demand/COD) ratio was also considerably improved from 0.02 (raw) to 0.37 using O₃ /ZrCl₄ , compared with using O₃ alone and using O₃ /SnCl₄ , which only recorded 0.23 and 0.28, respectively, after the treatment. The study demonstrated that O₃ /ZrCl₄ was the most efficient combination. PRACTITIONER POINTS: The O₃ /ZrCl₄ recorded the highest COD and color removals. The O₃ /ZrCl₄ combination also recorded higher OH• concentrations. The biodegradability of leachate (BOD₅ /COD ratio) improved from 0.02 to 0.37.

Direct Synthesis of Diamides from Dicarboxylic Acids with Amines Using Nb2O5 as a Lewis Acid Catalyst and Molecular Docking Studies as Anticancer Agents

Several Lewis and Bronsted acid catalysts were tested for the synthesis of some targeted diamides with anticancer activity from dicarboxylic acids and amines under the same reaction condition. Among those catalysts, Nb₂O₅ showed the highest catalytic activity to the corresponding diamides. Nb₂O₅ shows water- and base-tolerant properties for which it gives the highest yield of the synthesized products. Here, we present a novel and sustainable method for the direct synthesis of diamides with anticancer activity using a reusable heterogeneous catalyst Nb₂O₅. A molecular docking study was performed for all of the synthesized compounds with various therapeutical targets of cancer and found that the human epidermal growth factor receptor (HER2) has shown a significant dock score for our synthesized products. After obtaining the best pose from molecular docking, the complex is used for molecular dynamics study by running simulations for 10 ns. The root-mean-square deviations (RMSDs) of α carbon atoms of all systems are analyzed to detect their stability. This method is effective for the direct synthesis of diamides as anticancer agents from dicarboxylic acids and amines using Nb₂O₅ as a base-tolerant heterogeneous catalyst.

Synthesis of Azido Acids and Their Application in the Preparation of Complex Peptides

Azido acids are important synthons for the synthesis of complex peptides. As a protecting group, the azide moiety is atom-efficient, easy to install and can be reduced in the presence of many other protecting groups, making it ideal for the synthesis of branched and/or cyclic peptides. α-Azido acids are less bulky than urethane-protected counterparts and react more effectively in coupling reactions of difficult-to-form peptide and ester bonds. Azido acids can also be used to form azoles on complex intermediates. This review covers the synthesis of azido acids and their application to the total synthesis of complex peptide natural products.

1 Introduction

2 Synthesis of α-Azido Acids

2.1 From α-Amino Acids or Esters

2.2 Via α-Substitution

2.3 Via Electrophilic Azidation

2.4 Via Condensation of N-2-Azidoacetyl-4-Phenylthiazolidin- 2-Thi one Enolates with Aldehydes and Acetals

2.5 Synthesis of α,β-Unsaturated α-Azido Acids and Esters

3 Synthesis of β-Azido Acids

3.1 Preparation of Azidoalanine and 3-Azido-2-aminobutanoic Acids

3.2 General Approaches to Preparing β-Azido Acids Other Than Azi doalanine­ and AABA

4 Azido Acids in Total Synthesis

4.1 α-Azido Acids

4.2 β-Azido Acids and Azido Acids Containing an Azide on the Side Chain

5 Conclusions

Nucleophilic Transformations of Lewis Acid-Activated Disubstituted Epoxides with Catalyst-Controlled Regioselectivity

Due to their inherent ring strain and electrophilicity, epoxides are highly attractive building blocks for fundamental organic reactions. However, controlling the regioselectivity of disubstituted epoxide transformations is often particularly challenging. Most Lewis acid-mediated processes take advantage of intrinsic steric or electronic substrate bias to influence the site of nucleophilic attack. Therefore, the scope of many of these systems is frequently quite limited. Recent efforts to generate catalysts that can overcome substrate bias have expanded the synthetic utility of these well-known reactions. In this Perspective, we highlight various regioselective transformations of disubstituted epoxides, emphasizing those that have inspired the production of challenging, catalyst-controlled processes.

Mn(III)-ligand complexes as a catalyst in ligand-assisted oxidation of substituted phenols by permanganate in aqueous solution

Ligands can significantly increase the oxidation rates of phenolic compounds by MnO₄^-. This was often explained by the in situ formed Mn(III)- or Mn(X)-ligand complexes that can oxidize phenols faster than MnO₄^- can. This work discovered that Mn(III)-ligand complexes also acted as a catalyst for the oxidation of phenolic compounds by MnO₄^- (i.e., the catalytic role of Mn(III)-ligand). First, when phenol was mixed with MnO₄^- and pyrophosphate (PP, a representative ligand), Mn(III)-PP was found to form while phenol was quickly oxidized. However, the amount of phenol that was directly oxidized by Mn(III)-PP only accounted for ∼25% of phenol that was oxidized in the mixture, indicating that there were other pathways. Then, when pentachlorophenol (PCP) was used as another phenolic probe, the externally prepared Mn(III)-PP was observed to only slightly oxidize PCP, but its addition significantly accelerated PCP oxidation by MnO₄^-. The Mn(III)-PP concentration also remained unchanged during the above reaction, thus suggesting the catalyst role of Mn(III)-PP. This new pathway was further validated by successfully explaining all the experimental observations obtained so far, including the effect of pH, effects of different ligand amounts and types, product patterns, and the induction period. Finally, possible catalytic mechanisms of Mn(III)-ligand were discussed based on the experimental results.

One-pot transformation of lignocellulosic biomass into crude bio-oil with metal chlorides via hydrothermal and supercritical ethanol processing

Grape seeds were deconstructed in both hydrothermal and supercritical ethanol media with a combination of two metal chlorides (TiCl₄:MgCl₂) to produce bio-oils. The use of metal chloride additives in supercritical ethanol achieved the highest bio-oil yield of 49.2 wt% (300 °C, 30 min). Both the hydrothermal and supercritical ethanol deconstruction with the additives (TiCl₄:MgCl₂ = 4 mmol:4mmol) produced the bio-oils with a higher heating value (HHV) of 35 MJ/Kg. Gas chromatography-mass spectrometry (GC-MS) analysis of the bio-oils showed that the major products in bio-oils from the hydrothermal deconstruction were acids while the majority products in bio-oils form the supercritical ethanol deconstruction were esters. Nuclear magnetic resonance (NMR) data of the bio-oils suggested that both hydrothermal and supercritical ethanol deconstruction with metal chlorides significantly reduced the non-condensed OH and oxygenated lignin sub-units in bio-oils; while only supercritical ethanol deconstruction with metal chlorides reduced the aliphatic OH and O-alkylated structures in bio-oils.

Highly α-position regioselective ring-opening of epoxides catalyzed by halohydrin dehalogenase from Ilumatobacter coccineus: a biocatalytic approach to 2-azido-2-aryl-1-ols

Halohydrin dehalogenases are usually recognized as strict β-position regioselective enzymes in the nucleophile-mediated ring-opening of epoxides. Here we found the HheG from Ilumatobacter coccineus exhibited excellent α-position regioselectivity in the azide-mediated ring-opening of styrene oxide derivatives 1a–1k, producing the corresponding 2-azido-2-aryl-1-ols 2a–2k with the yields up to 96%.

Biocatalytic synthesis of 2-azido-2-aryl-1-ols was achieved via HheG-catalyzed α-position regioselective ring-opening of styrene oxide derivatives.

Streamlined Total Synthesis of Shishijimicin A and Its Application to the Design, Synthesis, and Biological Evaluation of Analogues thereof and Practical Syntheses of PhthNSSMe and Related Sulfenylating Reagents

Shishijimicin A is a scarce marine natural product with highly potent cytotoxicities, making it a potential payload or a lead compound for designed antibody-drug conjugates. Herein, we describe an improved total synthesis of shishijimicin A and the design, synthesis, and biological evaluation of a series of analogues. Equipped with appropriate functionalities for linker attachment, a number of these analogues exhibited extremely potent cytotoxicities for the intended purposes. The synthetic strategies and tactics developed and employed in these studies included improved preparation of previously known and new sulfenylating reagents such as PhthNSSMe and related compounds.

Mechanistic Studies of the Cu(OH)+ -Catalyzed Isomerization of Glucose into Fructose in Water

The isomerization of glucose to fructose is a crucial interim step in the processing of biomass to renewable fuels and chemicals. This study investigates the copper-catalyzed glucose-fructose isomerization in water, focusing on insights into the roles of the dissolved copper species. Depending on the pH, the thermodynamic equilibrium shifted towards one or a few copper species, namely Cu²⁺ , Cu(OH)⁺ , and Cu(OH)₂ . According to thermodynamics, the highest concentration of Cu(OH)⁺ is at pH 5.3, at which the highest fructose yield of 16 % is achieved. The obtained fructose yields strongly correlate with the concentration of Cu(OH)⁺ . A pH decrease of 2-3 units was observed during the reaction, resulting in the deactivation of the catalyst through hydrolysis in acidic media. Based on the results of the catalytic experiments, as well as spectroscopic and spectrometric studies, we propose Cu(OH)⁺ as an active Lewis-acidic species following an intramolecular 1,2-hydride shift.

From Conventional Lewis Acids to Heterogeneous Montmorillonite K10: Eco-Friendly Plant-Based Catalysts Used as Green Lewis Acids

The concept of green chemistry began in the USA in the 1990s. Since the publication of the 12 principles of this concept, many reactions in organic chemistry have been developed, and chemical products have been synthesized under environmentally friendly conditions. Lewis acid mediated synthetic transformations are by far the most numerous and best studied. However, the use of certain Lewis acids may cause risks to environmental and human health. This Review discusses the evolution of Lewis acid catalyzed reactions from a homogeneous liquid phase to the solid phase to yield the expected organic molecules under green, safe conditions. In particular, recent developments and applications of biosourced catalysts from plants are highlighted.

Synergy of Lewis and Brønsted acids on catalytic hydrothermal decomposition of carbohydrates and corncob acid hydrolysis residues to 5-hydroxymethylfurfural

5-hydroxymethylfurfural (HMF) is an important platform molecule in the synthesis of various chemicals and materials. Herein, we reported a simple and effective dehydration of glucose-based carbohydrates to HMF in a biphasic system containing cyclopentyl methyl ether as the organic phase and AlCl₃ with minute amounts of HCl as co-catalysts. The results showed that the mixed catalysts had a positive synergistic catalytic effect on glucose conversion to HMF compared with single AlCl₃ or HCl catalyst. For glucose, the highest HMF yield of 54.5% was achieved at 175 °C for 20 min. More importantly, the optimal catalytic system was so efficient that it achieved one of the highest reported yields of HMF (30.5%) directly from corncob acid hydrolysis residues. Thus, the catalytic system can become a promising route for effective utilization of biomass in future biorefineries.

Lewis Acid Assisted Diels-Alder Reaction with Regio- and Stereoselectivity: Anti-1,4-Adducts with Rigid Scaffolds and Their Application in Explosives Sensing

Unusual anti-1,4-adducts of anthracene derivatives and anti-adducts of inert arenes with rigid scaffolds have been obtained via AlCl3-assisted Diels-Alder reaction in good to excellent yields under mild conditions. Further derivation of 1,4-adducts gave π-conjugated polymers which could act as sensors of explosive species. This highly efficient synthesis method provides versatile approaches to solid-state emissive π-conjugated polymers.

SnCl4-catalyzed isomerization/dehydration of xylose and glucose to furanics in water

SnCl₄ was found to isomerize and dehydrate xylose and glucose effectively to furanics in water, with selectivities of 85% and 69%, respectively.

Pd(ii) coordinated deprotonated diphenyl phosphino amino pyridine: reactivity towards solvent, base, and acid

The reactivity and stability of P(iii)–N and P(iii)≈N bonds will be different towards various solvents, bases, and acids because of their difference in bond strength due to different N-pπ–P-dπ donor bonding.

The indium(iii) chloride catalyzed synthesis of sulfur incorporated 3-acylcoumarins; their photochromic and acetate sensing properties

The synthesis and evaluation of the photochromic properties of 3-acylcoumarins are very important as they exhibit selective sensing properties.

Cerium(III) chloride promoted highly regioselective ring opening of epoxides and aziridines using NaN(3) in acetonitrile: a facile synthesis of 1,2-azidoalcohols and 1,2-azidoamines

A convenient and efficient synthesis of 1,2-azidoalcohols and 1,2-azidoamines has been achieved by ring opening of epoxides and aziridines using cerium(III) chloride and sodium azide in acetonitrile. The reaction is highly regioselective and afforded the corresponding products in good to excellent yields under mild and neutral reaction conditions. The method is very rapid and equally compatible for both epoxides and aziridines.