Transition Metal-free Synthesis Research Articles

Transition Metal-Free Synthesis of Hydrazides from Amides and Amines

Transition Metal-Free Synthesis of Hydrazides from Amides and Amines

Transition Metal-Free Synthesis of 3-Acylquinolines through Formal [4+2] Annulation of Anthranils and Enaminones

A transition metal-free protocol has been developed for the synthesis of 3-acyl quinolines through aza-Michael addition and intramolecular annulation of enaminones with anthranils. Both methanesulfonic acid (MSA) and NaI play an important role in the reaction. This ring-opening/reconstruction strategy features easy operation, high yields, broad substrate scope and excellent efficiency.

Michael addition of P-nucleophiles to azoalkenes provides simple access to phosphine oxides bearing an alkylhydrazone moiety.

β-Hydrazonophosphine oxides are precursors of useful organophosphorus compounds, including phosphorylated N-heterocycles, α-aminophosphonates, and vinylphosphonates. In this work, a general transition metal-free synthesis of β-hydrazonophosphine oxides was developed. The method relies on the Michael addition of phosphine oxides R2P(O)H to reactive azoalkenes (1,2-diaza-1,3-butadienes), which are generated in situ from α-halohydrazones and Hunig's base. The reaction stereoselectively leads to Z-isomers of β-hydrazonophosphine oxides that are stabilized by intramolecular hydrogen bonding. The conversion of the products thus obtained into potential chelating ligands was showcased.

Transition metal-free synthesis of 2-aryl quinazolines via alcohol dehydrogenation

We report here a transition metal-free synthesis of quinazoline derivatives starting from 2-aminobenzyl alcohols and aryl amides via an alcohol dehydrogenation strategy promoted by potassium tertiary butoxide. The control experiments are carried out to identify the reaction intermediates and the role of the K+ ion in the reaction. The DFT calculations unveil the reaction mechanism, with special focus on the rate determining state. The present method tolerates a variety of functional groups providing easy access to diversely substituted quinazolines.

Base-mediated chalcogenoaminative annulation of 2-alkynylanilines for direct access to 3-sulfenyl/selenyl-1H-indoles.

An efficient and transition metal-free synthesis of 3-sulfenyl/selenyl-1H-indoles via a base-assisted chalcogenoaminative annulation of 2-alkynyl aniline with disulfides/diselenides is described. A series of 2-alkynylanilines were found compatible with dichalcogenides in this transformation providing 3-sulfenyl/selenyl-1H-indoles in good to excellent yields. The presented methodology has the advantages of easily available raw materials, functional group tolerance, and a wide range of substrates that provide access to 3-sulfenylindoles and 3-selenylindoles.

External oxidant-free and transition metal-free synthesis of 5-amino-1,2,4-thiadiazoles as promising antibacterials against ESKAPE pathogen strains.

A new route to 5-amino-1,2,4-thiadiazole derivatives via reaction of N-chloroamidines with isothiocyanates has been proposed. The advantages of this method are high product yields (up to 93%), the column chromatography-free workup procedure, scalability and the absence of additive oxidizing agents or transition metal catalysts. The 28 examples of 5-amino-1,2,4-thiadiazole derivatives obtaining via the proposing protocol were evaluated in vitro against ESKAPE pathogens strains (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter cloacae). It was found that compounds 5ba, 5bd, 6a, 6d and 6c have potent antibacterial activity (MIC values 0.09-1.5μgmL-1), which is superior to the activity of commercial antibiotics such as pefloxacin (MIC 4-8μgmL-1) and streptomycin (MIC 2-32μgmL-1). The additional cytotoxic assay of hit compounds on PANC-1 cell line demonstrated the low or non-cytotoxicity activity at the same level of concentrations. Thus, these 5 compounds are promising starting point for further antimicrobial drug development.

Rongalite-promoted transition metal-free synthesis of S-aryl phosphorothioates

The improved procedure for the synthesis of S-aryl phosphorothioates from dialkyl phosphites (H-phosphonates), aryl iodides and elemental sulfur deals with the application of rongalite (sodium hydroxy-methanesulfinate) as the radical promoter. The reactions proceeded smoothly to give S-aryl phosphorothioates in good yields.

Transition Metal-Free Synthesis of Halobenzo[b]furans from O-Aryl Carbamates via o-Lithiation Reactions.

A straightforward and transition metal-free one-pot protocol to synthesize halobenzo[b]furans has been developed employing simple and easily available starting materials such as O-aryl carbamates and alkynylsulfones. The fine-tuning of the different steps involved was key to achieving a successful one-pot procedure. Initially, a directed ortho-lithiation process, which uses the carbamate as the directed metalation group, was crucial in providing access to O-2-alkynylaryl N,N-diethyl carbamates by a direct alkynylation of the o-lithiated carbamate, with arylsulfonylalkynes as electrophilic reagents. Cyclization of the generated o-alkynylaryl carbamates was successfully accomplished through a strategy involving in situ carbamate alkaline hydrolysis under conventional heating or microwave irradiation, coupled with a subsequent heterocyclization step delivering the desired benzo[b]furans. A wide variety of new halobenzo[b]furans has been synthesized and their utility has been demonstrated by their further transformation.

Photocatalytic synthesis of tetra-substituted furans promoted by carbon dioxide.

We report a simple protocol for the transition metal-free, visible-light-driven conversion of 1,3-diketones to tetra-substituted furan skeleton compounds in carbon dioxide (CO2) atmosphere under mild conditions. It was found that CO2 could be incorporated at the diketone enolic OH position, which was key to enabling the cleavage of a C–O bond during the rearrangement of a cyclopropane intermediate. This method allows for the same-pot construction of two isomers of the high-value tetra-substituted furan scaffold. The synthetic scope and preliminary mechanistic investigations are presented.

Recent advances in the transition metal-free synthesis of heterocycles from α,β-unsaturated ketones

The review highlights the transition metal-free methodologies developed for synthesising various heterocyclic compounds by utilising α,β-unsaturated ketones as valuable synthons during the period of 2009 to 2020.

Stitching Triazoles to Arenes via a Transition Metal-Free Aryne Diels-Alder/1,3-Prototropic Shift/Dehydrobromination Cascade.

The 1,2,3-triazole-fused polyaromatic frameworks are traditionally obtained through transition metal-catalyzed C-H/C-X arylation of the preinstalled triazoles at a very high temperature. Herein, a transition metal-free direct synthesis via an aryne Diels-Alder/1,3-prototropic shift/dehydrobromination cascade is reported. This method gives access to triazole-fused aromatics as well as the corresponding dihydrocarbocycles under mild reaction conditions.

Iterative Exponential Growth of Oxygen-Linked Aromatic Polymers Driven by Nucleophilic Aromatic Substitution Reactions.

This work presents the first transition metal-free synthesis of oxygen-linked aromatic polymers by integrating iterative exponential polymer growth (IEG) with nucleophilic aromatic substitution (SNAr) reactions. Our approach applies methyl sulfones as the leaving groups, which eliminate the need for a transition metal catalyst, while also providing flexibility in functionality and configuration of the building blocks used. As indicated by 1) 1H-1H NOESY NMR spectroscopy, 2) single-crystal X-ray crystallography, and 3) density functional theory (DFT) calculations, the unimolecular polymers obtained are folded by nonclassical hydrogen bonds formed between the oxygens of the electron-rich aromatic rings and the positively polarized C–H bonds of the electron-poor pyrimidine functions. Our results not only introduce a transition metal-free synthetic methodology to access precision polymers but also demonstrate how interactions between relatively small, neutral aromatic units in the polymers can be utilized as new supramolecular interaction pairs to control the folding of precision macromolecules.

Asymmetric 1,4-Michael Addition in Aqueous Medium Using Hydrophobic Chiral Organocatalysts

Organic transformations exclusively in water as an environmentally friendly and safe medium have drawn significant interest in the recent years. Moreover, transition metal-free synthesis of enantiopure molecules in water will have a great deal of attention as the system will mimic the natural enzymatic reactions. In this work, a new set of proline-derived hydrophobic organocatalysts have been synthesized and utilized for asymmetric Michael reactions in water as the sole reaction medium. Among the various catalysts screened, the catalyst 1 is indeed efficient for stereoselective 1,4-conjugated Michael additions (dr: >97:3, ee up to >99.9%) resulting in high chemical yields (up to 95%) in a very short reaction time (1 h) at room temperature. This methodology provides a robust, green, and convenient protocol and can thus be an important addition to the arsenal of the asymmetric Michael addition reaction. Upon successful implementation, the present strategy also led to the formation of an optically active octahydroindole, the key component found in many natural products.

Cyanide Ion-Promoted Nucleophilic Displacement of Aromatic Nitro Groups: A Mild, One-Pot, Transition Metal-Free Synthesis of Nitrile-Containing Naphthalimide Aryl Ether Derivatives

Herein we report the novel synthesis of nitrile-containing naphthalimide aryl ether derivatives via transition metal-free cyanide ion-promoted nucleophilic displacement of nitronaphthalimide with alcohols or phenols. The transformation involves a one-pot reaction, which is operationally simple and mild and bereft of transition metal catalysts. This work provides a novel route for the preparation of diaryl ether derivatives.

Peroxide- and transition metal-free electrochemical synthesis of α,β-epoxy ketones.

A novel electrochemical method for the synthesis of α,β-epoxy ketones is reported. With KI as the redox mediator, methyl ketones reacted with aldehydes under peroxide- and transition metal-free electrolytic conditions and afforded α,β-epoxy ketones in one pot (36 examples, 52-90% yield). This safe and environmental-friendly method has a broad substrate scope and can readily provide a variety of α,β-epoxy ketones in gram-scales for evaluation of their anti-cancer activities.

Visible Light Induced Oxidation of α-Diazo Esters for the Transition Metal-Free Synthesis of α-Keto Esters

Visible Light Induced Oxidation of <i>α</i>-Diazo Esters for the Transition Metal-Free Synthesis of <i>α</i>-Keto Esters

Blue light-promoted radical sulfoximido-chalcogenization of aliphatic and aromatic alkenes

A transition metal-free synthesis of N-(arylthio/seleno)ethyl sulfoxidmines via blue LED light-promoted radical sulfoximido-chalcogenization of aliphatic and aromatic alkenes was developed.

Transition metal-free diastereospecific synthesis of (Z)-2-arylidene-2,3-dihydrobenzo[b][1,4]dioxines by reaction of (Z)-1,2-dibromo-3-aryl-2-propenes with catechols

Abstract A transition metal-free, diastereospecific reaction between substituted (Z)-1,2-dibromo-3-phenyl-2-propenes and substituted catechols using Cs2CO3 as a base at 140 °C for 18 h delivers exclusively substituted (Z)-2-arylidene-2,3-dihydrobenzo[b][1,4]dioxines in yields up to 89%. Experiments as well as quantum chemical calculations support the assumption that the one pot transformation proceeds as an intermolecular O-allylation/intramolecular O-vinylation.

Ionic liquid-immobilized proline(s) organocatalyst-catalyzed one-pot multi-component Mannich reaction under solvent-free condition

A highly efficient clean and simple methodology has been established for the one-pot Mannich reaction using ionic liquid-immobilized proline(s) organocatalyst under solvent-free conditions. The three components comprising substituted acetophenones, substituted aromatic aldehydes and substituted aromatic amines underwent Mannich reactions in the presence of 7 mol% of ionic liquid-immobilized proline(s) organocatalyst to provide β-amino carbonyl compounds in 2–3 h at room temperature with excellent yields. This methodology provides several advantages such as mild reaction conditions, short reaction time, low catalyst loading percentage, multi-component approach, transition metal-free and solvent-free synthesis. The ionic liquid-immobilized proline(s) organocatalyst was recycled and reused five times without a significant loss of its catalytic activity.

Isatin as a 2-aminobenzaldehyde surrogate: transition metal-free efficient synthesis of 2-(2'-aminophenyl)benzothiazole derivatives.

A transition metal-free, convenient, and efficient practical approach has been devised for the synthesis of substituted 2-(2'-aminophenyl)benzothiazoles via a sulfur insertion strategy using isatin derivatives as 2-aminobenzaldehyde surrogates. KI assisted one-pot operation of isatin, arylamines and elemental sulfur resulted in the formation of a C-N and two C-S bonds and cascade cleavage of the isatin ring resulting in the formation of 2-(2'-aminophenyl)benzothiazoles. The significant features of this strategy are the readily available and inexpensive starting materials, broad substrate scope, sustainable reaction conditions and high yield of products. Importantly, the strategy was found to be appropriate for gram scale synthesis (>10 g) of 2-(2'-aminophenyl)benzothiazole derivatives. Moreover, the excellent photophysical properties (ΦF up to 60%) of 2-(2'-aminophenyl)benzothiazole derivatives provide huge scope in materials science.