Manganese-catalyzed selective oxidation of aliphatic C-H groups and secondary alcohols to ketones with hydrogen peroxide

A manganese-based catalyst system for general oxidation of unactivated olefins, alkanes, and alcohols

Non-noble metal-based catalyst systems consisting of inexpensive manganese salts, picolinic acid and various heterocycles enable epoxidation of the challenging (terminal) unactivated olefins, selective C-H oxidation of unactivated alkanes, and O-H oxidation of secondary alcohols with aqueous hydrogen peroxide. In the presence of the in situ generated optimal manganese catalyst, epoxides are generated with up to 81% yield from alkenes and ketone products with up to 51% yield from unactivated alkanes. This convenient protocol allows the formation of the desired products under ambient conditions (room temperature, 1 bar) by employing only a slight excess of hydrogen peroxide with 2,3-butadione as a sub-stoichiometric additive.

Oxygen-Free Csp3-H Oxidation of Pyridin-2-yl-methanes to Pyridin-2-yl-methanones with Water by Copper Catalysis

Aromatic ketones are important pharmaceutical intermediates, especially the pyridin-2-yl-methanone motifs. Thus, synthetic methods for these compounds have gained extensive attention in the last few years. Transition metals catalyze the oxidation of Csp3-H for the synthesis of aromatic ketones, which is arresting. Here, we describe an efficient copper-catalyzed synthesis of pyridin-2-yl-methanones from pyridin-2-yl-methanes through a direct Csp3-H oxidation approach with water under mild conditions. Pyridin-2-yl-methanes with aromatic rings, such as substituted benzene, thiophene, thiazole, pyridine, and triazine, undergo the reaction well to obtain the corresponding products in moderate to good yields. Several controlled experiments are operated for the mechanism exploration, indicating that water participates in the oxidation process, and it is the single oxygen source in this transformation. The current work provides new insights for water-involving oxidation reactions.

Organo-photocatalytic C-H bond oxidation: an operationally simple and scalable method to prepare ketones with ambient air

Oxidative C-H functionalization with O₂ is a sustainable strategy to convert feedstock-like chemicals into valuable products. Nevertheless, eco-friendly O₂-utilizing chemical processes, which are scalable yet operationally simple, are challenging to develop. Here, we report our efforts, via organo-photocatalysis, in devising such protocols for catalytic C-H bond oxidation of alcohols and alkylbenzenes to ketones using ambient air as the oxidant. The protocols employed tetrabutylammonium anthraquinone-2-sulfonate as the organic photocatalyst which is readily available from a scalable ion exchange of inexpensive salts and is easy to separate from neutral organic products. Cobalt(ii) acetylacetonate was found to be greatly instrumental to oxidation of alcohols and therefore was included as an additive in evaluating the alcohol scope. The protocols employed a nontoxic solvent, could accommodate a variety of functional groups, and were readily scaled to 500 mmol scale in a simple batch setting using round-bottom flasks and ambient air. A preliminary mechanistic study of C-H bond oxidation of alcohols supported the validity of one possible mechanistic pathway, nested in a more complex network of potential pathways, in which the anthraquinone form - the oxidized form - of the photocatalyst activates alcohols and the anthrahydroquinone form - the relevant reduced form of the photocatalyst - activates O₂. A detailed mechanism, which reflected such a pathway and was consistent with previously accepted mechanisms, was proposed to account for formation of ketones from aerobic C-H bond oxidation of both alcohols and alkylbenzenes.

Continuous Flow Epoxidation of Alkenes Using a Homogeneous Manganese Catalyst with Peracetic Acid

Epoxidation of alkenes is a valuable transformation in the synthesis of fine chemicals. Described herein are the design and development of a continuous flow process for carrying out the epoxidation of alkenes with a homogeneous manganese catalyst at metal loadings as low as 0.05 mol%. In this process, peracetic acid is generated in situ and telescoped directly into the epoxidation reaction, thus reducing the risks associated with its handling and storage, which often limit its use at scale. This flow process lessens the safety hazards associated with both the exothermicity of this epoxidation reaction and the use of the highly reactive peracetic acid. Controlling the speciation of manganese/2-picolinic acid mixtures by varying the ligand:manganese ratio was key to the success of the reaction. This continuous flow process offers an inexpensive, sustainable, and scalable route to epoxides.

A Common Active Intermediate in the Oxidation of Alkenes, Alcohols and Alkanes with H2O2 and a Mn(II)/Pyridin-2-Carboxylato Catalyst

The mechanism and the reactive species involved in the oxidation of alkenes, and alcohols with H2O2, catalysed by an in situ prepared mixture of a MnII salt, pyridine-2-carboxylic acid and a ketone is elucidated using substrate competition experiments, kinetic isotope effect (KIE) measurements, and atom tracking with 18O labelling. The data indicate that a single reactive species engages in the oxidation of both alkenes and alcohols. The primary KIE in the oxidation of benzyl alcohols is ca. 3.5 and shows the reactive species to be selective despite a zero order dependence on substrate concentration, and the high turnover frequencies (up to 30 s-1) observed. Selective 18O labelling identifies the origin of the oxygen atoms transferred to the substrate during oxidation, and is consistent with a highly reactive, e. g., [MnV(O)(OH)] or [MnV(O)2], species rather than an alkylperoxy or hydroperoxy species.

Heterolytic O-O Bond Cleavage Upon Single Electron Transfer to a Nonheme Fe(III)-OOH Complex

The one-electron reduction of the nonheme iron(III)-hydroperoxo complex, [FeIII (OOH)(L5 2 )]2+ (L5 2 =N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine), carried out at -70 °C results in the release of dioxygen and in the formation of [FeII (OH)(L5 2 )]+ following a bimolecular process. This reaction can be performed either with cobaltocene as chemical reductant, or electrochemically. These experimental observations are consistent with the disproportionation of the hydroperoxo group in the putative FeII (OOH) intermediate generated upon reduction of the FeIII (OOH) starting complex. One plausible mechanistic scenario is that this disproportionation reaction follows an O-O heterolytic cleavage pathway via a FeIV -oxo species.

A Four-Component Reaction for the Synthesis of Thienopyrrolediones under Transition Metal Free Conditions

A three-starting-material four-component reaction strategy is described to construct thienopyrrolediones (TPDs) from the simplest raw materials, elemental sulfur, aldehydes, and β-ketoamides, under transition metal free conditions. Compared with traditional multistep reaction sequences, this process is simple, efficient, environmentally friendly, and atom-economic and has laid the foundation for further development of an easily synthesized TPD unit.

Journey of anthraquinones as anticancer agents - a systematic review of recent literature

Anthraquinones are privileged chemical scaffolds that have been used for centuries in various therapeutic applications. The anthraquinone moiety forms the core of various anticancer agents. However, the emergence of drug-resistant cancers warrants the development of new anticancer agents. The research endeavours towards new anthraquinone-based compounds are increasing rapidly in recent years. They are used as a core chemical template to achieve structural modifications, resulting in the development of new anthraquinone-based compounds as promising anticancer agents. Mechanistically, most of the anthraquinone-based compounds inhibit cancer progression by targeting essential cellular proteins. Herein, we review new anthraquinone analogues that have been developed in recent years as anticancer agents. This includes a systematic review of the recent literature (2005-2021) on anthraquinone-based compounds in cell-based models and key target proteins such as kinases, topoisomerases, telomerases, matrix metalloproteinases and G-quadruplexes involved in the viability of cancer cells. In addition to this, the developments in PEG-based delivery of anthraquinones and the toxicity aspects of anthraquinone derivatives are also discussed. The review dispenses a compact background knowledge to understanding anthraquinones for future research on the expansion of anticancer therapeutics.

Off-line analysis in the manganese catalysed epoxidation of ethylene-propylene-diene rubber (EPDM) with hydrogen peroxide

The epoxidation of ethylene-propylene-diene rubber (EPDM) with 5-ethylidene-2-norbornene (ENB) as the diene to epoxidized EPDM (eEPDM) creates additional routes to cross-linking and reactive blending, as well as increasing the polarity and thereby the adhesion to polar materials, e.g., mineral fillers such as silica. The low solubility of apolar, high molecular weight polymers in the polar solvents constrains the catalytic method for epoxidation that can be applied. Here we have applied an in situ prepared catalyst comprising a manganese(ii) salt, sodium picolinate and a ketone to the epoxidation of EPDM rubber with hydrogen peroxide (H₂O₂) as the oxidant in a solvent mixture, that balances the need for polymer and catalyst/oxidant miscibility and solubility. Specifically, a mixture of cyclohexane and cyclohexanone is used, where cyclohexanone functions as a co-solvent as well as the ketone reagent. Reaction progress was monitored off-line through a combination of Raman and ATR-FTIR spectroscopies, which revealed that the reaction profile and the dependence on the composition of the catalyst are similar to those observed with low molar mass alkene substrates, under similar reaction conditions. The combination of spectroscopies offers a reliable method for off-line reaction monitoring of both the extent of the conversion of unsaturation (Raman) and the extent of epoxidation (FTIR) as well as determining side reactions, such as epoxide ring opening and further, aerobic oxidation. The epoxidation of EPDM described, in contrast to currently available methods, uses a non-scarce manganese catalyst and H₂O₂, and avoids side reactions, such as those that can occur with peracids.

Epoxidation of ethylene-propylene-diene rubber (EPDM), based on 5-ethylidene-2-norbornene, to epoxidized EPDM (eEPDM) opens routes to cross-linking and reactive blending, with increased polarity aiding adhesion to polar materials such as silica.

Anthraquinone: a promising scaffold for the discovery and development of therapeutic agents in cancer therapy

Cancer, characterized by uncontrolled malignant neoplasm, is a leading cause of death in both advanced and emerging countries. Although, ample drugs are accessible in the market to intervene with tumor progression, none are totally effective and safe. Natural anthraquinone (AQ) equivalents such as emodin, aloe-emodin, alchemix and many synthetic analogs extend their antitumor activity on different targets including telomerase, topoisomerases, kinases, matrix metalloproteinases, DNA and different phases of cell lines. Nano drug delivery strategies are advanced tools which deliver drugs into tumor cells with minimum drug leakage to normal cells. This review delineates the way AQ derivatives are binding on these targets by abolishing tumor cells to produce anticancer activity and purview of nanoformulations related to AQ analogs.

Hydrogen Peroxide as a Green Oxidant for the Selective Catalytic Oxidation of Benzylic and Heterocyclic Alcohols in Different Media: An Overview

Among a plethora of known and established oxidant in organic chemistry, hydrogen peroxide stands in a special position. It is commercially and inexpensively available, highly effective, selective, and more importantly it is compatible with current environmental concerns, dictated by principles of green chemistry. Several chemicals or their intermediates that are important in our daily life such as pharmaceuticals, flavors, fragrances, etc. are products of oxidation of alcohols. In this review, we introduce hydrogen peroxide as an effective, selective, green and privileged oxidant for the catalyzed oxidation of primary and secondary benzylic and heterocyclic alcohols to corresponding carbonyl compounds in different media such as aqueous media, under solvent-free conditions, various organic solvent, and dual-phase system.

Towards Mechanistic Understanding of Liquid-Phase Cinnamyl Alcohol Oxidation with tert-Butyl Hydroperoxide over Noble-Metal-Free LaCo1-x Fex O3 Perovskites

Noble-metal-free perovskite oxides are promising and well-known catalysts for high-temperature gas-phase oxidation reactions, but their application in selective oxidation reactions in the liquid phase has rarely been studied. We report the liquid-phase oxidation of cinnamyl alcohol over spray-flame synthesized LaCo_1-x Fe_x O₃ perovskite nanoparticles with tert-butyl hydroperoxide (TBHP) as the oxidizing agent under mild reaction conditions. The catalysts were characterized by XRD, BET, EDS and elemental analysis. LaCo_0.8 Fe_0.2 O₃ showed the best catalytic properties indicating a synergistic effect between cobalt and iron. The catalysts were found to be stable against metal leaching as proven by hot filtration, and the observed slight deactivation is presumably due to segregation as determined by EDS. Kinetic studies revealed an apparent activation energy of 63.6 kJ mol^-1 . Combining kinetic findings with TBHP decomposition as well as control experiments revealed a complex reaction network.

Origins of Catalyst Inhibition in the Manganese-Catalysed Oxidation of Lignin Model Compounds with H2 O2

The upgrading of complex bio-renewable feedstock, such as lignocellulose, through depolymerisation benefits from the selective reactions at key functional groups. Applying homogeneous catalysts developed for selective organic oxidative transformations to complex feedstock such as lignin is challenged by the presence of interfering components. The selection of appropriate model compounds is essential in applying new catalytic systems and identifying such interferences. Here, it was shown by using as an example the oxidation of a model substrate containing a β-O-4 linkage with H2 O2 and an in situ-prepared manganese-based catalyst, capable of efficient oxidation of benzylic alcohols, that interference from compounds liberated during the reaction can prevent its application to lignocellulose depolymerisation.

Oxidation of Vicinal Diols to α-Hydroxy Ketones with H2O2 and a Simple Manganese Catalyst

α-Hydroxy ketones are valuable synthons in organic chemistry. Here we show that oxidation of vic-diols to α-hydroxy ketones with H₂O₂ can be achieved with an in situ prepared catalyst based on manganese salts and pyridine-2-carboxylic acid. Furthermore the same catalyst is effective in alkene epoxidation, and it is shown that alkene oxidation with the Mn^II catalyst and H₂O₂ followed by Lewis acid ring opening of the epoxide and subsequent oxidation of the alkene to α-hydroxy ketones can be achieved under mild (ambient) conditions.

Chelating bis-N-heterocyclic carbene complexes of iron(ii) containing bipyridyl ligands as catalyst precursors for oxidation of alcohols

Chelating bis-N-heterocyclic carbene (bis-NHC) complexes of iron(ii) containing pyridyl ligands have been prepared by the reaction of [FeCl2L] [L = bipy (1), phen (2)] with [LiN(SiMe3)2] and a bis(imidazolium) salt. The [Fe(bis-NHC)L(I)2] complexes were active pre-catalysts in the oxidation of 1-phenylethanol with tert-butyl hydroperoxide in neat conditions, affording a quantitative yield of acetophenone in 4.5 h. The catalyst could be reused up to six cycles giving a turnover number (TON) of 1500. Various secondary alcohols, both aromatic and aliphatic were selectivity oxidised to the corresponding ketones in excellent yields. Compound 1 is stable in acetonitrile solution for ca. 4 h, although after 16 h, it evolves to a mixture of [Fe(bis-NHC)(bipy)2]I2 (3), [Fe(bipy)3](2+) and bis-imidazolium salt. The molecular structure of 3 has been determined by X-ray diffraction studies.

Alcohol oxidation with H2O2 catalyzed by a cheap and promptly available imine based iron complex

A cheap and easily available catalyst for alcohol oxidation with unexpected selectivity features.

Synergistic H4NI-AcOH Catalyzed Oxidation of the Csp(3)-H Bonds of Benzylpyridines with Molecular Oxygen

The oxidation of benzylpyridines forming benzoylpyridines was achieved based on a synergistic H4NI-AcOH catalyst and molecular oxygen in high yield under solvent-free conditions. This is the first nonmetallic catalytic system for this oxidation transformation using molecular oxygen as the oxidant. The catalytic system has a wide scope of substrates and excellent chemoselectivity, and this procedure can also be scaled up. The study of a preliminary reaction mechanism demonstrated that the oxidation of the Csp(3)-H bonds of benzylpyridines was promoted by the pyridinium salts formed by AcOH and benzylpyridines. The synergistic effect of H4NI-AcOH was also demonstrated by control experiments.

Aerobic oxidation at benzylic positions catalyzed by a simple Pd(OAc)2/bis-triazole system

An extremely active palladium catalyst system for the aerobic oxidation of benzyl alcohols and benzylic C–H oxidation is described.