Manganese(II)/Picolinic Acid Catalyst System for Epoxidation of Olefins

Reimagining Dearomatization: Arenophile-Mediated Single-Atom Insertions and π-Extensions

ConspectusDearomatization of simple aromatics serves as one of the most direct strategies for converting abundant chemical feedstocks into three-dimensional value-added products. Among such transformations, cycloadditions between arenes and alkenes have historically offered effective means to construct complex polycyclic architectures. However, traditionally harsh conditions, such as high-energy UV light irradiation, have greatly limited the scope of this transformation. Nevertheless, recent progress has led to the development of visible-light-promoted dearomative photocycloadditions with expanded scope capable of preparing complex bicyclic structures.A fundamentally distinct approach to dearomative photocycloadditions involves the visible-light activation of arenophiles, which undergo para-photocycloaddition with various aromatic compounds to produce arene-arenophile cycloadducts. While only transiently stable and subject to retro-cycloaddition, further functionalization of the photocycloadducts has allowed for the development of a wide collection of dearomatization methodologies that access products orthogonal to existing chemical and biological processes. Central to this strategy was the observation that arene-arenophile photocycloaddition reveals a π-system that can be functionalized through traditional olefin chemistry. Coupled with subsequent [4 + 2]-cycloreversion of the arenophile, this process acts to effectively isolate a single π-system from an aromatic ring. We have developed several transformations that bias this methodology to perform dearomative single-atom insertion and π-extension reactions to prepare unique products that cannot be prepared easily through traditional means.Through the application of a dearomative epoxidation, we were able to develop a method for the epoxidation of arenes and pyridines to arene-oxides and pyridine-oxides, respectively. Notably, when this arenophile chemistry is applied to polycyclic arenes, photocycloaddition reveals a π-system transposed from the site of native olefinic reactivity, enabling unique site-selectivity for dearomative functionalization. As a result, we were able to perform a single-atom insertion of oxygen into polycyclic (aza)arenes to prepare 3-benzoxepines. When applying this strategy in the context of cyclopropanations, we were able to accomplish a dearomative cyclopropanation of polycyclic (aza)arenes which yield benzocycloheptatrienes upon cycloreversion. Notably, while the Buchner ring expansion is a powerful method for the direct single-atom insertion of carbon into arenes, the corresponding cyclopropanation of polycyclic arenes does not yield ring-expanded products. Furthermore, this strategy could be utilized for the synthesis of novel nanographenes through the development of an M-region annulative π-extension (M-APEX) reaction. Traditionally, methods for π-extension rely on the native reactivity of polycyclic aromatics at the K- and bay-region. However, photocycloaddition of polycyclic aromatics with arenophiles acts as a strategy to activate the M-region for further reactivity. As a result, arenophile-mediated dearomative diarylation, followed by cycloreversion, could deliver π-extended nanographenes with exclusive M-region site selectivity.

Highly Position- and Enantioselective Catalytic Epoxidation of Polyolefins Mediated by a Chiral Mn Complex, Including a One-Step Conversion of Squalene to the (S)-2,3-Epoxide, a Precursor of Natural Steroids and Terpenoids

Reported herein is the synthesis of a novel chiral dicarboxylic ligand for Mn(II) and the application of the Mn complex to the highly enantio- and position-selective epoxidation of C═C under mild conditions, even with polyolefinic substrates. A stereomechanistic basis for asymmetric induction is suggested.

Spectroscopic Properties and Reactivity of a MnIII-Hydroperoxo Complex that is Stable at Room Temperature

Manganese catalysts that activate hydrogen peroxide have seen increased use in organic transformations, such as olefin epoxidation and alkane C-H bond oxidation. Proposed mechanisms for these catalysts involve the formation and activation of MnIII-hydroperoxo intermediates. Examples of well-defined MnIII-hydroperoxo complexes are rare, and the properties of these species are often inferred from MnIII-alkylperoxo analogues. In this study, we show that the reaction of the MnIII-hydroxo complex [MnIII(OH)(6Medpaq)]+ (1) with hydrogen peroxide and acid results in the formation of a dark-green MnIII-hydroperoxo species [MnIII(OOH)(6Medpaq)]+ (2). The formulation of 2 is based on electronic absorption, 1H NMR, IR, and ESI-MS data. The thermal decay of 2 follows a first order process, and variable-temperature kinetic studies of the decay of 2 yielded activation parameters that could be compared with those of a MnIII-alkylperoxo analogue. Complex 2 reacts with the hydrogen-atom donor TEMPOH two-fold faster than the MnIII-hydroxo complex 1. Complex 2 also oxidizes PPh3, and this MnIII-hydroperoxo species is 600-fold more reactive with this substrate than its MnIII-alkylperoxo analogue [MnIII(OOtBu)(6Medpaq)]+. DFT and time-dependent (TD) DFT computations are used to compare the electronic structure of 2 with similar MnIII-hydroperoxo and MnIII-alkylperoxo complexes.

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.

Oxidative Dearomatization of Pyridines

Dearomatization of pyridines is a well-established synthetic approach to access piperidines. Although remarkably powerful, existing dearomatization processes have been limited to the hydrogenation or addition of carbon-based nucleophiles to activated pyridiniums. Here, we show that arenophile-mediated dearomatizations can be applied to pyridines to directly introduce heteroatom functionalities without prior substrate activation. The arenophile platform in combination with olefin oxidation chemistry provides access to dihydropyridine cis-diols and epoxides. These previously elusive compounds are now readily accessible and can be used for the downstream preparation of diversely functionalized piperidines.

Picolinic Acid-Mediated Catalysis of Mn(II) for Peracetic Acid Oxidation Processes: Formation of High-Valent Mn Species

Metal-based advanced oxidation processes (AOPs) with peracetic acid (PAA) have been extensively studied to degrade micropollutants (MPs) in wastewater. Mn(II) is a commonly used homogeneous metal catalyst for oxidant activation, but it performs poorly with PAA. This study identifies that the biodegradable chelating ligand picolinic acid (PICA) can significantly mediate Mn(II) activation of PAA for accelerated MP degradation. Results show that, while Mn(II) alone has minimal reactivity toward PAA, the presence of PICA accelerates PAA loss by Mn(II). The PAA-Mn(II)-PICA system removes various MPs (methylene blue, bisphenol A, naproxen, sulfamethoxazole, carbamazepine, and trimethoprim) rapidly at neutral pH, achieving >60% removal within 10 min in clean and wastewater matrices. Coexistent H2O2 and acetic acid in PAA play a negligible role in rapid MP degradation. In-depth evaluation with scavengers and probe compounds (tert-butyl alcohol, methanol, methyl phenyl sulfoxide, and methyl phenyl sulfone) suggested that high-valent Mn species (Mn(V)) is a likely main reactive species leading to rapid MP degradation, whereas soluble Mn(III)-PICA and radicals (CH3C(O)O• and CH3C(O)OO•) are minor reactive species. This study broadens the mechanistic understanding of metal-based AOPs using PAA in combination with chelating agents and indicates the PAA-Mn(II)-PICA system as a novel AOP for wastewater treatment.

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.

Characterization of glycerophospholipids at multiple isomer levels via Mn(II)-catalyzed epoxidation

Characterization of glycerophospholipid isomers is of significant importance as they play different roles in physiological and pathological processes. In this work, we present a novel and bifunctional derivatization method utilizing Mn(II)-catalyzed epoxidation to simultaneously identify carbon-carbon double bond (CC bond)- and stereonumbering (sn)-positional isomers of phosphatidylcholine. Mn(II) coordinates with picolinic acid and catalyzes epoxidation of unsaturated lipids by peracetic acid. Collision-induced dissociation (CID) of the epoxides generates diagnostic ions that can be used to locate CC bond positions. Meanwhile, CID of Mn(II) ion-lipid complexes produces characteristic ions for determination of sn positions. This bifunctional derivatization takes place in seconds, and the diagnostic ions produced in CID are clear and easy to interpret. Moreover, relative quantification of CC bond-and sn-positional isomers was achieved. The capability of this method in identifying lipids at multiple isomer levels was shown using lipid standards and lipid extracts from complex biological samples.

Late-Stage Intermolecular Allylic C-H Amination

Allylic amination enables late-stage functionalization of natural products where allylic C-H bonds are abundant and introduction of nitrogen may alter biological profiles. Despite advances, intermolecular allylic amination remains a challenging problem due to reactivity and selectivity issues that often mandate excess substrate, furnish product mixtures, and render important classes of olefins (for example, functionalized cyclic) not viable substrates. Here we report that a sustainable manganese perchlorophthalocyanine catalyst, [MnIII(ClPc)], achieves selective, preparative intermolecular allylic C-H amination of 32 cyclic and linear compounds, including ones housing basic amines and competing sites for allylic, ethereal, and benzylic amination. Mechanistic studies support that the high selectivity of [MnIII(ClPc)] may be attributed to its electrophilic, bulky nature and stepwise amination mechanism. Late-stage amination is demonstrated on five distinct classes of natural products, generally with >20:1 site-, regio-, and diastereoselectivity.

Robust Superamphiphilic Membrane with a Closed-Loop Life Cycle

Oil-spill remediation is an international environmental challenge, and superamphiphilic membranes, as a promising solution, have recently drawn lots of attention. However, the robustness of the conventional membrane design is less satisfying under severe conditions during practical applications. Additionally, it is unavoidable for the membranes to face a series of foulants in their practical working environment, for example, algae and sand. These foulants will block the membrane, which leads to a new economic and environmental problem in terms of waste management at the end of their life. To address the aforementioned challenges, a new generation of superamphiphilic vitrimer epoxy resin membranes (SAVER) to separate oil and water efficiently is reported. Similar to classical epoxy resins, SAVER shows strong mechanical robustness and sustains exposure to aqua regia and sodium hydroxide solutions. Furthermore, the blocked membrane can be easily recovered when contaminated with mixed foulants by using dynamic transesterification reactions in the polymer network. The ease with which biobased SAVER can be manufactured, used, recycled, and re-used without losing value points to new directions in designing a closed-loop superamphiphilic membrane life cycle.

Chemical Equivalent of Arene Monooxygenases: Dearomative Synthesis of Arene Oxides and Oxepines

Direct epoxidation of aromatic nuclei by cytochrome P450 monooxygenases is one of the major metabolic pathways of arenes in eukaryotes. The resulting arene oxides serve as versatile precursors to phenols, oxepines, or trans-dihydrodiol-based metabolites. Although such compounds have an important biological and chemical relevance, the lack of methods for their production has hampered access to their utility. Herein, we report a general arenophile-based strategy for the dearomative synthesis of arene oxides. The mildness of this method permits access to sensitive monocyclic arene oxides without any noticeable decomposition to phenols. Moreover, this method enables direct conversion of polycyclic arenes and heteroarenes into the corresponding oxepines. Finally, these studies provided direct connection between simple aromatic precursors and complex small organic molecules via arene oxides and oxepines.

Epoxide containing molecules: A good or a bad drug design approach

Functional group modification is one of the main strategies used in drug discovery and development. Despite the controversy of being identified for many years as a biologically hazardous functional group, the introduction of an epoxide function in a structural backbone is still one of the possible modifications being implemented in drug design. In this manner, it is our intention to prove with this work that epoxides can have significant interest in medicinal chemistry, not only as anticancer agents, but also as important drugs for other pathologies. Thus, this revision paper aims to highlight the biological activity and the proposed mechanisms of action of several epoxide-containing molecules either in preclinical studies or in clinical development or even in clinical use. An overview of the chemistry of epoxides is also reported. Some of the conclusions are that effectively most of the epoxide-containing molecules referred in this work were being studied or are in the market as anticancer drugs. However, some of them in preclinical studies, were also associated with other different activities such as anti-malarial, anti-arthritic, insecticidal, antithrombotic, and selective inhibitory activity of FXIII-A (a transglutaminase). As for the epoxide-containing molecules in clinical trials, some of them are being tested for obesity and schizophrenia. Finally, drugs containing epoxide groups already in the market are mostly used for the treatment of different types of cancer, such as breast cancer and multiple myeloma. Other diseases for which the referred drugs are being used include heart failure, infections and gastrointestinal disturbs. In summary, epoxides can be a suitable option in drug design, particularly in the design of anticancer agents, and deserve to be better explored. However, and despite the promising results, it is imperative to explore the mechanisms of action of these compounds in order to have a better picture of their efficiency and safety.

SO2F2-Mediated Epoxidation of Olefins with Hydrogen Peroxide

An inexpensive, mild, and highly efficient epoxidation protocol has been developed involving bubbling SO₂F₂ gas into a solution of olefin, 30% aqueous hydrogen peroxide, and 4 N aqueous potassium carbonate in 1,4-dioxane at room temperature for 1 h with the formation of the corresponding epoxides in good to excellent yields. The novel SO₂F₂/H₂O₂/K₂CO₃ epoxidizing system is suitable to a variety of olefinic substrates including electron-rich and electron-deficient ones.

A molecular electron density theory study of the mechanism, chemo- and stereoselectivity of the epoxidation reaction of R-carvone with peracetic acid

The epoxidation reaction of R-carvone 8 with peracetic acid 9 has been studied within the molecular electron density theory at the B3LYP/6-311(d,p) computational level. The chemo- and stereoisomeric reaction paths involving the two C-C double bonds of R-carvone 8 have been studied. DFT calculations account for the high chemoselectivity involving the C-C double bond of the isopropenyl group and the low diastereoselectivity, in complete agreement with the experimental outcomes. The Baeyer-Villiger reaction involving the carbonyl group of R-carvone 8 has also been analysed. A bonding evolution theory analysis of the epoxidation reaction shows the complexity of the bonding changes taking place along this reaction. Formation of the oxirane ring takes place asynchronously at the end of the reaction by attack of anionic oxygen on the two carbons of the isopropenyl C-C double bond.

Remarkable increase in the rate of the catalytic epoxidation of electron deficient styrenes through the addition of Sc(OTf)3 to the MnTMTACN catalyst

Sc(OTf)₃ produces a remarkable enhancement in the activity of the MnTMTACN catalyst in the epoxidation of electron deficient styrenes.

Bu4NI/tBuOOH catalyzed, α-regioselective cross-dehydrogenative coupling of BODIPY with allylic alkenes and ethers

A Bu₄NI/tBuOOH-catalyzed, highly regioselective cross-dehydrogenative coupling (CDC) of the α-C–H bond(s) of the BODIPY core has been developed.

Concise Total Syntheses of (+)-Haplocidine and (+)-Haplocine via Late-Stage Oxidation of (+)-Fendleridine Derivatives

We report the first total syntheses of (+)-haplocidine and its N1-amide congener (+)-haplocine. Our concise synthesis of these alkaloids required the development of a late-stage and highly selective C-H oxidation of complex aspidosperma alkaloid derivatives. A versatile, amide-directed ortho-acetoxylation of indoline amides enabled our implementation of a unified strategy for late-stage diversification of hexacyclic C19-hemiaminal ether structures via oxidation of the corresponding pentacyclic C19-iminium ions. An electrophilic amide activation of a readily available C21-oxygenated lactam, followed by transannular cyclization and in situ trapping of a transiently formed C19-iminium ion, expediently provided access to hexacyclic C19-hemiaminal ether alkaloids (+)-fendleridine, (+)-acetylaspidoalbidine, and (+)-propionylaspidoalbidine. A highly effective enzymatic resolution of a non-β-branched primary alcohol (E = 22) allowed rapid preparation of both enantiomeric forms of a C21-oxygenated precursor for synthesis of these aspidosperma alkaloids. Our synthetic strategy provides succinct access to hexacyclic aspidosperma derivatives, including the antiproliferative alkaloid (+)-haplocidine.