Catalyst-free Conditions Research Articles

Mechanistic Details of the Titanium-Mediated Polycondensation Reaction of Polyesters: A DFT Study

In this work, the mechanism of polyester polycondensation catalysed by titanium catalysts was investigated using density functional theory (DFT). Three polyester polycondensation reaction mechanisms, including the Lewis acid mechanism (M1), the coordination of the ester alkoxy oxygen mechanism (M2) and the coordination of the carboxy oxygen mechanism (M3), were investigated. Three reaction mechanisms for the polycondensation reaction of diethyl terephthalate (DET) were investigated using Ti(OEt)4 and cationic Ti(OEt)3+ as the catalyst. The results show that the polycondensation reaction of the Lewis acid mechanism exhibits similar energy barriers to the catalyst-free condition (42.6 kcal/mol vs. 47.6 kcal/mol). Mechanism M3 gives the lowest energy barrier of 17.5 kcal/mol, indicating that Ti(OEt)4 is the active centre for the polycondensation reaction. The catalytic efficiency of Ti(OEt)3+ is lower than that of Ti(OEt)4 catalysts due to its higher DET distortion energy (67.6 kcal/mol vs. 37.4 kcal/mol) by distortion–interaction analysis.

A new type of convergent paired electrochemical synthesis of sulfonamides under green and catalyst-free conditions

Our main goal in this work is to synthesize valuable sulfonamide compounds according to the principles of green chemistry and also to present a unique convergent paired mechanism for their synthesis. In this study, we introduced a new type of convergent paired electro-organic synthesis of sulfonamide derivatives via a catalyst, oxidant, halogen and amine-free method. In this research, instead of using toxic amine compounds, an innovative mechanism based on the reduction of nitro compounds and in-situ production of amine compounds was used. The mechanism of electrophile generation is the cathodic reduction of the nitro compound to the hydroxylamine compound and then the anodic oxidation of the hydroxylamine to the nitroso compound. On the other hand, the nucleophile generation mechanism involves the two-electron oxidation of sulfonyl hydrazide to related sulfinic acid at the anode surface. The reaction leading to the synthesis of sulfonamides involves a one-pot reaction of the generated nitroso compound with the produced sulfinic compound.

One-pot construction of novel trifluoromethyl dihydro-imidazo[1, 2-a]pyridine: A greener approach

A one-pot three-component method for the synthesis of 3-substituted 2- trifluoromethyl-2,3-dihydroimidazo[1,2-a]pyridin-3-yl analogues from 2-aminopyridine, substituted phenacyl bromides, and trifluoroacetaldehyde methyl hemiacetal (TFAMH) under solvent-free and the catalyst-free condition is described with good to excellent yield. This chemistry features the additive-free approach with the employment of safe, stable, and readily available starting materials with a wide substrate scope strategy.

Computational and experimental analysis of catalyst-free and expeditious synthesis of Benzo[4,5]imidazo[2,1-b]thiazole derivatives

A greener and direct approach to the efficient synthesis of fused heterocyclic ring system benzo[4,5]imidazo[2,1-b]thiazoles via C–N and C–S bond formation in catalyst-free conditions has been reported using 2,2,2-trifluoroethanol (TFE) as a reaction medium at room temperature. The salient features of this approach include an elementary and clean reaction protocol to afford the desired product in higher yields in less reaction time with sufficient purity. This prominent moiety is frequently found in many natural bioactive occurring compounds and shows potential in synthesizing diverse biologically active compounds due to their therapeutic values. The synthesis of benzo[4,5]imidazo[2,1-b]thiazoles was confirmed by spectral analysis and also by DFT studies.

Cascade Cyclization of 1,5-Diynols and (RO)2P(O)SH to Construct Benzo[b]fluorenyl S-Alkyl Phosphorothioates under Catalyst-Free Conditions.

An efficient and practical cascade cyclization of 1,5-diynols with (RO)2P(O)SH as the acid promoter and nucleophile under mild conditions was developed. A variety of highly substituted benzo[b]fluorenyl-containing S-alkyl phosphorothioates were successfully constructed in moderate to excellent yields. Furthermore, this protocol exhibited good functional group tolerance, a broad substrate scope, and potential practical applications, with water as the only byproduct. The reaction proceeded with allenyl thiophosphate as a key intermediate, followed by a Schmittel-type cyclization process to produce the target product.

Metal-Free Electrocatalytic Diacetoxylation of Alkenes.

1,2-Dioxygenation of alkenes leads to a structural motif ubiquitous in organic synthons, natural products and active pharmaceutical ingredients. Straightforward and green synthesis protocols starting from abundant raw materials are required for facile and sustainable access to these crucial moieties. Especially industrially abundant aliphatic alkenes have proven to be arduous substrates in sustainable 1,2-dioxygenation methods. Here, we report a highly efficient electrocatalytic diacetoxylation of alkenes under ambient conditions using a simple iodobenzene mediator and acetic acid as both the solvent and an atom-efficient reactant. This transition metal-free method is applicable to a wide range of alkenes, even challenging feedstock alkenes such as ethylene and propylene, with a broad functional group tolerance and excellent faradaic efficiencies up to 87 %. In addition, this protocol can be extrapolated to alkenoic acids, resulting in cyclization of the starting materials to valuable lactone derivatives. With aromatic alkenes, a competing mechanism of direct anodic oxidation exists which enables reaction under catalyst-free conditions. The synthetic method is extensively investigated with cyclic voltammetry.

Electrochemical Radical Reaction Construction of C-C Bonds: Access to 1,4-Dicarbonyl Compounds from Enol Acetates and 1,3-Diketones.

As important substrates for the construction of heterocycles, a simple and efficient approach for synthesis of 1,4-diones is highly desirable. In this work, novel and efficient electrochemical radical reactions of enol acetates and 1,3-diketones have been developed to successfully achieve 1,4-diketones under catalyst-free and oxidant-free conditions. The wide range of substrates, good group tolerance, and simple operation process make the approach have important practical value. Moreover, the obtained 1,4-diketones can be easily further transformed to pyrrole and furan derivatives.

Using Triazolobenzodiazepine as the Cyclic Imine in Various Types of Joullié-Ugi Reactions.

The triazolobenzodiazepine as a cyclic imine was employed in a variety of Joullié-Ugi reactions, and three new families of unique triazolobenzodiazepine connected to carboxamide and tetrazole products were synthesized via a three-component reaction of the cyclic imine and isocyanides with each species of a carboxylic acid/water/TMSN3 under mild conditions in high yields. Furthermore, triazolobenzodiazepine imine was used in an interesting strategy based on the modified Ugi reaction (pseudo-Joullié-Ugi reaction) of cyclic imines with an isocyanide and acetylenedicarboxylates under catalyst-free conditions for the synthesis of triazolobenzodiazepine-fused pyrroles. Mechanistic investigation reveals that triazolobenzodiazepine-fused pyrroles have been generated via a surprising route. Significantly, the use of triazolobenzodiazepine in the Joullié-Ugi, azido-Joullié-Ugi, and pseudo-Joullié-Ugi reactions of a broad scope of biological scaffolds occurred under mild, simple conditions without any catalyst.

Staightforward Synthesis of Halopyridine Aldehydes via Diaminomethylation.

A novel two-step method for formylation of fluoropyridines with silylformamidine 1 under catalyst-free conditions was developed. A series of possible 18 fluoropyridines featuring one to four fluorine atoms were subjected to the reaction with 1 existing in equilibrium with its carbenic form 1'.12 Fluoropyridines were shown to react via C-H insertion. The reaction proceeded either at β- or γ-positions affording the corresponding aminals. The more fluorine atoms in pyridines, the easier the reaction proceeded. We also hypothesized that the pyridines in which the fluorine was substituted by other halogens would react in a similar manner. To test the hypothesis, a set of 3,5-disubstituted pyridines with various combination of halogen atoms was prepared. 3,5-Difluoropyridine was taken as a compound for comparison. All the pyridines in the series also reacted likewise. In most cases, hydrolysis of the aminals afforded the corresponding aldehydes. As DFT calculations indicate, the reaction mechanism includes deprotonation of pyridine by 1' as a strong base and the following rearrangement of the formed tight ionic pair to the final product. An alternative reaction pathway involving addition of 1' to the pyridine carbon with the following hydrogen transfer via a three-membered transition state structure required much higher activation energy.

Electrochemical Cycloaddition Reactions of Alkene Radical Cations – a Route Towards Cyclopropanes and Cyclobutanes

New methods to promote single-electron transfer reactions play an increasingly important role in driving modern organic chemistry. Significant advances have been made in the field of photoredox catalysis1 and synthetic organic electrochemistry2 to expand the utility of radicals in organic synthesis.3 While the initial developments in synthetic organic electrochemistry date as far back as the early 19th century, only recently has it undergone an explosive revival and it is now an important part of synthetic organic chemistry.4 Undeniably, it provides a green alternative to known and new reactive intermediates by the promotion of organic reactions under mild conditions without the necessity for excessive amounts of hazardous and wasteful oxidants and reductants, metals4 or catalysts, making chemical synthesis highly atom economical.5 Our work describes mild and efficient electrochemical methods for [2+1] and [2+2] cycloaddition reactions of alkene radical cations. Anodic oxidation of olefins produces electrophilic alkene radical cations, which further react with nucleophilic diazo compounds in a formal [2+1] cycloaddition towards cyclopropane synthesis. This methodology is also applicable for [2+2] cycloadditions with styrene derivatives, validating this efficient electrochemical system for both cyclopropane and cyclobutane ring synthesis. The advantages of this methods include the mild, metal and catalyst free conditions, simple setup, and scalability of the procedure, as well as its potential use for industrial applications since the method requires non-expensive graphite electrodes.

Metal catalyst-free selective acetosyringone synthesis from rice straw lignin.

In recent decades, due to abundance (second most abundant natural polymer after cellulose) and sustainability, lignin has attracted much interest from different researchers to use as a raw material for producing various value-added materials such as polymer and fuel. In addition to that, the aromatic structure of lignin makes it a suitable candidate for producing platform chemicals with aromatic rings. As a result, lignin depolymerization has become an interesting process to derive different phenolic monomers like vanillin, acetosyringone, and guaiacol. Among them, due to the bioactive characteristics and efficiency of acetosyringone in plant regulatory systems, the production of acetosyringone from lignin has been presented in this work. A green and cost-effective method was developed for the selective formation of acetosyringone via depolymerization of isolated rice straw lignin (RSL) by using metal catalyst-free conditions in the biphasic medium and described. The RSL was characterized with various spectroscopic techniques such as FT-IR, solid-state 13C NMR, XPS, and TGA. The selectivity of synthesized acetosyringone during depolymerization of RSL was checked from GC-MS analysis. The molecular structure and purity of acetosyringone isolated from preparative thin layer chromatography (TLC) were confirmed with the help of 1H NMR and HRMS, respectively.

Visible-Light-Induced C-3 Difluoroalkylation of Quinoxalin-2(1H)-ones with Difluoroiodane(III) Reagents under Catalyst-Free Conditions

AbstractA visible-light-induced radical difluoroalkylation of quinoxalin-2(1H)-ones by hypervalent-iodine-based reagents was developed. To facilitate the final oxidative step, [bis(trifluoroacetoxy)iodo]benzene (PIFA) was employed as an oxidant. Moreover, a one-pot protocol for this transformation was realized by generating the difluoroalkylation reagent in situ. The readily accessible reagents and the mild reaction conditions make this reaction an alternative and practical strategy for the synthesis of C(3)-difluoroalkylated quinoxalin-2(1H)-ones.

Highly Regioselective Dichalcogenation of Alkenyl Sulfonium Salts to Access 1,1-Dichalcogenalkenes.

An unprecedented geminal olefinic dichalcogenation of alkenyl sulfonium salts with dichalcogenides ArYYAr (Y = S, Se, Te) is reported, providing various trisubstituted 1,1-dichalcogenalkenes [Ar1CH = C(YAr2)2] in a highly selective manner under mild and catalyst-free conditions. The key process involves the formation of two geminal olefinic C-Y bonds via sequential C-Y cross-coupling and C-H chalcogenation. A mechanistic rationale is further supported by control experiments and density functional theory calculations.

Synthesis of 2-Benzyl-3-(2-oxo-2-phenylethyl)isoindolin-1-one Derivatives in Water under Catalyst-Free Conditions

AbstractWe have developed a catalyst-free synthesis of a series of isoindolinone derivatives via a one-pot, three-component reaction in water using o-formylbenzoic acids, primary amines, and acetophenones as starting materials. An investigation of the influence of the solvent, the substrate molar ratio, the temperature, and the reaction time revealed that the reaction proceeded optimally in water at 70 °C over 12 hours to afford a wide range of isoindolinone derivatives in yields of up to 93%. This method has the advantages of mild reaction conditions, environmental friendliness, and a broad substrate scope.

One-pot, simple, and facile synthesis of 4-(3-benzylbenzo[d]thiazol-2(3H)-ylidene)-cyclohexa-2,5-dien-1-one derivatives via a novel three-component reaction

In this study, a series of 4-(3-benzylbenzo[ d]thiazol-2(3 H)-ylidene)-cyclohexa-2,5-dien-1-one derivatives are efficiently synthesized in excellent yields by reactions of 2-chlorobenzothiazole, benzyl bromides, and phenols in acetonitrile under reflux and catalyst-free conditions in the presence of triethylamine for 2 h. The structures of the products are characterized by NMR, IR, EI-MS, and elemental analyses.

Synthesis and Characterization of a New Series of Bis(allylic-α-aminophosphonates) under Mild Reaction Conditions.

Several bis(α-aminophosphonates) have been conveniently prepared in good yields using a straightforward multicomponent Kabachnik-Fields reaction between ethane 1,2-diamine or propane 1,3-diamine, diethylphosphite and aldehydes under catalyst-free conditions. The nucleophilic substitution reaction of bis(α-aminophosphonates) prepared and ethyl (2-bromomethyl)acrylate under mild reaction conditions afforded an original synthetic approach to a new series of bis(allylic-α-aminophosphonates).

An efficient one-pot synthesis of acenaphtho[1,2-b]indolindeneone and acenaphtho[1,2-b]indoldione derivatives

A simple method for the synthesis of acenaphtho[1,2-b]indolindenedione derivatives and acenaphtho[1,2-b]indolone derivatives via four-component reaction is described. The reaction between dimedone, arylamines, acenaphthoquinone, and 1,3-indondione in refluxing ethanol under catalyst-free conditions are produced the title compounds. The advantages of this method are: environmental friendly, short reaction time, easy work-up, and high yield.

Highly Efficient Visible Light Assisted Synthesis of Perimidines under Catalyst-free and Room Temperature Conditions

Abstract: Visible light has come to the fore in organic chemistry as a cheap and readily available energy source. Energy transfer reactions represent the future directions for the development of practical and scalable industrial reactions with several environmental benefits. A facile, economic, and visible light (CFL) induced synthesis of perimidines has been established through a cyclo-condensation reaction of 1,8-diaminonaphthalene and aryl aldehydes with excellent yields (87-100%) in a short span of time. The present method has several merits a highly efficient, greener, high atom economy, no need of external heating and other instrumental setups, no need for a catalyst, and no tedious purification process.

Visible light-induced one-pot three-component synthesis of quinazolines under catalyst-free condition

A facile and efficient strategy for visible light-induced synthesis of quinazolines has been developed via one-pot three-component reaction of 2-aminoaryl ketones, aldehydes and ammonium acetate. This method displays high efficiency and broad substrate tolerance, allowing quick accesses to quinazolines under catalyst-free condition. The results reported herein disclose an important new application of visible light-induced synthesis in the clean synthesis of valuable organic products.

Upcycling Waste Polycarbonate to Poly(carbonate imine) Vitrimers with High Thermal Properties and Unprecedented Hydrolytic Stability

The global market for polycarbonate (PC) increases steadily because of its heat resistance, toughness, and optical transparency. To achieve sustainability, the waste polycarbonate (WPC) should be recycled. This work reports a 100% atom-efficiency upcycling of WPC to a WPC-derived aldehyde (WPC-CHO). Then, poly(carbonate imine) vitrimers (PCIs) based on the WPC-CHO and two aromatic diamines were prepared. Generally, polyimines can cleave in mild acidic conditions such as 0.1 M HCl(aq) and seldom show good thermal and tensile properties. The PCIs in this work showed good thermal and tensile properties and an unprecedented resistance toward acid hydrolysis. They can survive even immersing in a 5 M H2SO4 THF/H2O solution for 6 months. This unusual resistance toward acid hydrolysis expands the application of PCIs in an acidic environment. Small pieces of PCIs can be compression-molded into a newly formed polymer film at 240 °C. This thermal reprocessability demonstrates the vitrimer characteristic of the PCIs. The PCIs can be degraded through simultaneous aminolysis of carbonate and transimination of imine by hexylamine (HA) in a catalyst-free mild condition. Based on the aminolysis and transimination, we degraded the cross-linking network in the PCI-based carbon fiber-reinforced polymer (CFRP) and recovered undamaged carbon fiber. This environmentally friendly work will attract the attention of researchers who are interested in sustainability and circularity.