Replacement of Stoichiometric DDQ with a Low Potential o-Quinone Catalyst Enabling Aerobic Dehydrogenation of Tertiary Indolines in Pharmaceutical Intermediates

Metal-ligand cooperativity in chemical electrosynthesis

Electrochemistry has been an increasingly useful tool for organic synthesis, as it can selectively generate reactive intermediates under mild conditions using an applied potential. Concurrently, synergistic activity of a metal and a ligand has been used in thermal catalysis and electrocatalytic renewable fuel generation for substrate selectivity and improved catalyst activity. Combining these synthetic strategies is an attractive approach for mild, selective, and sustainable electrosynthesis. This perspective discusses examples of metal-ligand synergistic catalysis in electrochemical applications in organic and organometallic synthesis. The range of reactions and ligand design principles illustrates many opportunities for further discovery in this area and the potential for far-reaching synthetic benefits.

Optimizing the Synthetic Potential of O2: Implications of Overpotential in Homogeneous Aerobic Oxidation Catalysis

Molecular oxygen is the quintessential oxidant for organic chemical synthesis, but many challenges continue to limit its utility and breadth of applications. Extensive historical research has focused on overcoming kinetic challenges presented by the ground-state triplet electronic structure of O2 and the various reactivity and selectivity challenges associated with reactive oxygen species derived from O2 reduction. This Perspective will analyze thermodynamic principles underlying catalytic aerobic oxidation reactions, borrowing concepts from the study of the oxygen reduction reaction (ORR) in fuel cells. This analysis is especially important for "oxidase"-type liquid-phase catalytic aerobic oxidation reactions, which proceed by a mechanism that couples two sequential redox half-reactions: (1) substrate oxidation and (2) oxygen reduction, typically affording H2O2 or H2O. The catalysts for these reactions feature redox potentials that lie between the potentials associated with the substrate oxidation and oxygen reduction reactions, and changes in the catalyst potential lead to variations in effective overpotentials for the two half reactions. Catalysts that operate at low ORR overpotential retain a more thermodynamic driving force for the substrate oxidation step, enabling O2 to be used in more challenging oxidations. While catalysts that operate at high ORR overpotential have less driving force available for substrate oxidation, they often exhibit different or improved chemoselectivity relative to the high-potential catalysts. The concepts are elaborated in a series of case studies to highlight their implications for chemical synthesis. Examples include comparisons of (a) NOx/oxoammonium and Cu/nitroxyl catalysts, (b) high-potential quinones and amine oxidase biomimetic quinones, and (c) Pd aerobic oxidation catalysts with or without NOx cocatalysts. In addition, we show how the reductive activation of O2 provides a means to access potentials not accessible with conventional oxidase-type mechanisms. Overall, this analysis highlights the central role of catalyst overpotential in guiding the development of aerobic oxidation reactions.

Bio-inspired lanthanum-ortho-quinone catalysis for aerobic alcohol oxidation: semi-quinone anionic radical as redox ligand

Oxidation reactions are fundamental transformations in organic synthesis and chemical industry. With oxygen or air as terminal oxidant, aerobic oxidation catalysis provides the most sustainable and economic oxidation processes. Most aerobic oxidation catalysis employs redox metal as its active center. While nature provides non-redox metal strategy as in pyrroloquinoline quinone (PQQ)-dependent methanol dehydrogenases (MDH), such an effective chemical version is unknown. Inspired by the recently discovered rare earth metal-dependent enzyme Ln-MDH, here we show that an open-shell semi-quinone anionic radical species in complexing with lanthanum could serve as a very efficient aerobic oxidation catalyst under ambient conditions. In this catalyst, the lanthanum(III) ion serves only as a Lewis acid promoter and the redox process occurs exclusively on the semiquinone ligand. The catalysis is initiated by 1e^--reduction of lanthanum-activated ortho-quinone to a semiquinone-lanthanum complex La(SQ^-.)₂, which undergoes a coupled O-H/C-H (PCHT: proton coupled hydride transfer) dehydrogenation for aerobic oxidation of alcohols with up to 330 h⁻¹ TOF.

A decade ago the first rare-earth-metal dependent enzyme was discovered, in which a non-redox lanthanide ion is central in the active site of a methanol dehydrogenase. Inspired by this discovery, here the authors show that an open-shell semi-quinone anionic radical species, complexed with lanthanum, could serve as a very efficient aerobic oxidation catalyst under ambient conditions.

Syntheses of 1H-Indoles, Quinolines, and 6-Membered Aromatic N-Heterocycle-Fused Scaffolds via Palladium(II)-Catalyzed Aerobic Dehydrogenation under Alkoxide-Free Conditions

Aromatic N-heterocycle-fused scaffolds such as indoles and quinolines are important core structures found in various bioactive natural products and synthetic compounds. Recently, various dehydrogenation methods with the help of alkoxides, known to significantly promote dihydro- or tetrahydro-heterocycles to be oxidized, were developed for the heterocycle synthesis. However, these approaches are sometimes unsuitable due to resulting undesired side reactions such as reductive dehalogenation. Herein, expedient syntheses of 1H-indoles, quinolines, and 6-membered N-heterocycle-fused scaffolds from their hydrogenated forms through palladium(II)-catalyzed aerobic dehydrogenation under alkoxide-free conditions are reported. A total of 48 compounds were successfully synthesized with a wide range of functional groups including halogens (up to 99% yield). These methodologies provide facile routes for various privileged structures possessing aromatic N-heterocycles without the help of alkoxides, in highly efficient manners.

Iron(II)-Catalyzed Aerobic Biomimetic Oxidation of N-Heterocycles

Herein, an iron(II)-catalyzed biomimetic oxidation of N-heterocycles under aerobic conditions is described. The dehydrogenation process, involving several electron-transfer steps, is inspired by oxidations occurring in the respiratory chain. An environmentally friendly and inexpensive iron catalyst together with a hydroquinone/cobalt Schiff base hybrid catalyst as electron-transfer mediator were used for the substrate-selective dehydrogenation reaction of various N-heterocycles. The method shows a broad substrate scope and delivers important heterocycles in good-to-excellent yields.

Efficient Aerobic Oxidation of Organic Molecules by Multistep Electron Transfer

This Minireview presents recent important homogenous aerobic oxidative reactions which are assisted by electron transfer mediators (ETMs). Compared with direct oxidation by molecular oxygen (O2 ), the use of a coupled catalyst system with ETMs leads to a lower overall energy barrier via stepwise electron transfer. This cooperative catalytic process significantly facilitates the transport of electrons from the reduced form of the substrate-selective redox catalyst (SSRCred ) to O2 , thereby increasing the efficiency of the aerobic oxidation. In this Minireview, we have summarized the advances accomplished in recent years in transition-metal-catalyzed as well as metal-free aerobic oxidations of organic molecules in the presence of ETMs. In addition, the recent progress of photochemical and electrochemical oxidative functionalization using ETMs and O2 as the terminal oxidant is also highlighted. Furthermore, the mechanisms of these transformations are showcased.

Organic Electrosynthesis Towards Sustainability: Fundamentals and Greener Methodologies

The adoption of new measures that preserve our environment, on which our survival depends, is a necessity. Electro-organic processes are sustainable per se, by producing the activation of a substrate by electron transfer at normal pressure and room temperature. In the recent years, a highly crescent number of works on organic electrosynthesis are available. Novel strategies at the electrode are being developed enabling the construction of a great variety of complex organic molecules. However, the possibility of being scaled-up is mandatory in terms of sustainability. Thus, some electrochemical methodologies have demonstrated to report the best results in reducing pollution and saving energy. In this personal account, these methods have been compiled, being organized as follows: • Direct discharge electrosynthesis • Paired electrochemical reactions. and • Organic transformations utilizing electrocatalysis (in absence of heavy metals). Selected protocols are herein presented and discussed with representative recent examples. Final perspectives and reflections are also considered.

Magnetic nanocatalysts as multifunctional platforms in cancer therapy through the synthesis of anticancer drugs and facilitated Fenton reaction

Background

Heterocyclic compounds have always been used as a core portion in the development of anticancer drugs. However, there is a pressing need for developing inexpensive and simple alternatives to high-cost and complex chemical agents-based catalysts for large-scale production of heterocyclic compounds. Also, development of some smart platforms for cancer treatment based on nanoparticles (NPs) which facilitate Fenton reaction have been widely explored by different scientists. Magnetic NPs not only can serve as catalysts in the synthesis of heterocyclic compounds with potential anticancer properties, but also are widely used as smart agents in targeting cancer cells and inducing Fenton reactions.

Aim of Review

Therefore, in this review we aim to present an updated summary of the reports related to the main clinical or basic application and research progress of magnetic NPs in cancer as well as their application in the synthesis of heterocyclic compounds as potential anticancer drugs. Afterwards, specific tumor microenvironment (TME)-responsive magnetic nanocatalysts for cancer treatment through triggering Fenton-like reactions were surveyed. Finally, some ignored factors in the design of magnetic nanocatalysts- triggered Fenton-like reaction, challenges and future perspective of magnetic nanocatalysts-assisted synthesis of heterocyclic compounds and selective cancer therapy were discussed.Key Scientific Concepts of Review:This review may pave the way for well-organized translation of magnetic nanocatalysts in cancer therapy from the bench to the bedside.

Air oxidized activated carbon catalyst for aerobic oxidative aromatizations of N-heterocycles

Air oxidized activated carbon offers a robust, efficient, metal-free and recyclable catalyst for aromatizations of N-heterocycles, O2 being the terminal oxidant.

Aerobic primary and secondary amine oxidation cascade by a copper amine oxidase inspired catalyst

A CAO inspired catalyst catalyzed the cascade aerobic oxidation of primary and secondary amines for the synthesis of quinazolin-4(3H)-one core in high yields. Like the natural CAOs, a copper ion improves the o-quinone cofactor's catalytic activity.

Pd-Catalyzed Aerobic Oxidative Coupling of Thiophenes: Synergistic Benefits of Phenanthroline Dione and a Cu Cocatalyst

Substituted bithiophenes are prominent fragments in functional organic materials, and they are ideally prepared via direct oxidative C-H/C-H coupling. Here, we report a novel PdII catalyst system, employing 1,10-phenanthroline-5,6-dione (phd) as the ancillary ligand, that enables aerobic oxidative homocoupling of 2-bromothiophenes and other related heterocycles. These observations represent the first use of phd to support Pd-catalyzed aerobic oxidation. The reaction also benefits from a Cu(OAc)2 cocatalyst, and mechanistic studies show that Cu promotes C-C coupling, implicating a role for CuII different from its conventional contribution to reoxidation of the Pd catalyst.

1,2,4,5-Tetrakis(tetramethylguanidino)-3,6-diethynyl-benzenes: Fluorescent Probes, Redox-Active Ligands and Strong Organic Electron Donors

In this work, the change of reactivity induced by the introduction of two para-ethynyl substituents (CCSi(iPr)3 or CCH) to the organic electron-donor 1,2,4,5-tetrakis(tetramethylguanidino)-benzene is evaluated. The redox-properties and redox-state dependent fluorescence are evaluated, and dinuclear CuI and CuII complexes synthesized. The Lewis-acidic B(C6 F5 )3 substitutes the proton of the ethynyl -CCH groups to give new anionic -CCB(C6 F5 )3 - substituents, leading eventually to a novel dianionic strong electron donor in its diprotonated form. Its two-electron oxidation with dioxygen in the presence of a copper catalyst yields the first redox-active guanidine that is neutral (instead of cationic) in its oxidized form.

Ir-Catalyzed Reversible Acceptorless Dehydrogenation/Hydrogenation of N-Substituted and Unsubstituted Heterocycles Enabled by a Polymer-Cross-Linking Bisphosphine

The polystyrene-cross-linking bisphosphine ligand PS-DPPBz was effective for the Ir-catalyzed reversible acceptorless dehydrogenation/hydrogenation of N-heterocycles. Notably, this protocol is applicable to the dehydrogenation of N-substituted indoline derivatives with various N-substituents with different electronic and steric natures. A reaction pathway involving oxidative addition of an N-adjacent C(sp³)-H bond to a bisphosphine-coordinated Ir(I) center is proposed for the dehydrogenation of N-substituted substrates.

Sustainable Palladium-Catalyzed Tsuji-Trost Reactions Enabled by Aqueous Micellar Catalysis

Palladium-catalyzed allylic substitution, or "Tsuji-Trost" reactions, can be run under micellar catalysis conditions featuring not only chemistry in water but also numerous combinations of reaction partners that require low levels of palladium, typically on the order of 1000 ppm (0.1 mol %). These couplings are further characterized by especially mild conditions, leading to a number of cases not previously reported in an aqueous micellar medium. Inclusion of diverse nucleophiles, such as N-H heterocycles, alcohols, dicarbonyl compounds, and sulfonamides is described. Intramolecular cyclizations further illustrate the broad utility of this process. In addition to recycling studies, a multigram scale example is reported, indicative of the prospects for scale up.

Electrochemical Total Synthesis of Pyrrolophenanthridone Alkaloids: Controlling the Anodically Initiated Electron Transfer Process

Electrochemical intramolecular C(sp²)-H cross-coupling and dehydrogenative indole synthesis were developed. Both reactions were initiated by anodic oxidation of the same electron-rich indoline moiety, but the product selectivity was controlled by different electron-transfer processes. Intramolecular cross-coupling was achieved by the generation of a strong electrophilic radical cation intermediate in the MeNO₂-HFIP-LiClO₄ system. Indole formation was accomplished through benzylic oxidation and continuous deprotonation. We applied these reactions to the total synthesis of natural pyrrolophenanthridone alkaloids.

Aerobic catalytic systems inspired by copper amine oxidases

The goal of sustainable development has been accepted as a common policy in current society. In response to this challenge, the development of green processes which utilize environmentally benign oxidants, reduce chemical waste and handling costs, is highly desirable. Given the widespread importance of imines as pivotal synthetic intermediates and essential pharmacophores in numerous biologically active compounds, various catalytic methods allowing the aerobic oxidation of amines to imines have been developed. Recently, noticeable progress has arisen from the discovery of various quinone-based catalytic systems, inspired by copper amine oxidase enzymes (CuAOs), which are able to reproduce the selectivity of CuAOs for primary amines and even to expand the amine substrates scope. However, the need for synthesizing these catalysts prior use adversely affects the economics as well as the eco-friendly nature of the method. To surpass these drawbacks, the “second-order” biomimicry idea has been recently advanced to describe a system in which in situ modification of pre-catalyst components affords the active biomimetic catalyst. This minireview especially covers our recent contribution to the design of bioinspired quinone-based catalysts for the aerobic oxidation of amines to imines which has culminated in a dual bioinspired protocol as an example of “second-order” biomimicry.

Redox-Active Guanidines in Proton-Coupled Electron-Transfer Reactions: Real Alternatives to Benzoquinones?

Guanidino-functionalized aromatics (GFAs) are readily available, stable organic redox-active compounds. In this work we apply one particular GFA compound, 1,2,4,5-tetrakis(tetramethylguanidino)benzene, in its oxidized form in a variety of oxidation/oxidative coupling reactions to demonstrate the scope of its proton-coupled electron transfer (PCET) reactivity. Addition of an excess of acid boosts its oxidation power, enabling the oxidative coupling of substrates with redox potentials of at least +0.77 V vs. Fc⁺ /Fc. The green recyclability by catalytic re-oxidation with dioxygen is also shown. Finally, a direct comparison indicates that GFAs are real alternatives to toxic halo- or cyano-substituted benzoquinones.