Competitive Hydrogen Atom Transfer to Oxyl- and Peroxyl Radicals in the Cu-Catalyzed Oxidative Coupling of N-Aryl Tetrahydroisoquinolines Using tert-Butyl Hydroperoxide

Photoredox Cross-Dehydrogenative Coupling of N-Aryl Glycines Mediated by Mesoporous Graphitic Carbon Nitride: An Environmentally Friendly Approach to the Synthesis of Non-Proteinogenic α-Amino Acids (NPAAs) Decorated with Indoles

Indole-decorated glycine derivatives are prepared through an environmentally benign cross-dehydrogenative coupling between N-aryl glycine analogues and indoles (yield of ≤81%). Merging heterogeneous organocatalysis and photocatalysis, C–H functionalization has been achieved by selective C-2 oxidation of N-aryl glycines to afford the electrophilic imine followed by Friedel–Crafts alkylation with indole. The sustainability of the process has been taken into account in the reaction design through the implementation of a metal-free recyclable heterogeneous photocatalyst and a green reaction medium. Scale-up of the benchmark reaction (gram scale, yield of 69%) and recycling experiments (over seven runs without a loss of efficiency) have been performed to prove the robustness of the protocol. Finally, mechanistic studies were conducted employing electron paramagnetic resonance spectroscopy to unveil the roles of the photocatalyst and oxygen in the formation of odd-electron species.

Direct Phosphonylation of N-Phenyltetrahydroisoquinolines in Microdroplets

Because of their unique properties and high biological activities, organophosphorus compounds have been used worldwide in agricultural, industrial, medicinal, and veterinary applications. Conventional strategies for direct phosphonylation suffer from the usage of stoichiometric or excessive metallic or nonmetallic catalysts and long reaction times under harsh conditions, leading to a strong desire for environment-friendly protocols for phosphonylation. A protocol for the accelerated phosphonylation of N-phenyltetrahydroisoquinolines in minutes was developed without the use of any catalyst in microdroplets. The phosphonylation process was completed (>85% yields) in 10 min at 40 °C using 0.8 equiv 2,3-dicyano-5,6-dichlorobenzoquinone as the oxidant and acetonitrile as the solvent. The microdroplet phosphonylation strategy showed good suitability to alkyl phosphites and N-phenyltetrahydroisoquinolines bearing electron-withdrawing and electron-donating substitutes, and the yields of the microdroplet reaction were much greater than those of the bulk (accelerated by two orders of magnitude from the ratio of the rate constants using the microdroplet and the bulk method). Furthermore, microdroplet phosphonylation can be scaled up to a 1-phenyl-2-dimethylphosphonite-1,2,3,4-tetrahydroisoquinoline amount of 510 mg h^-1 by spraying 0.1 mol L^-1 N-phenyltetrahydroisoquinoline at 300 μL min^-1. These figures of merit make it a promising alternative to classic organic methodologies for the synthesis of organophosphorus compounds.

Cascade Cyclization for the Synthesis of Indolo[2,1-α]isoquinoline Derivatives via Visible-Light-Induced Halogen-Atom-Transfer (XAT) and Hydrogen-Atom-Transfer (HAT)

A transition metal-free photoredox cascade cyclization is herein reported. In this protocol, sustainable visible light was used as energy source and organic light-emitting molecule Eosin Y served as efficient photocatalyst....

Cr-Catalyzed Direct ortho-Aminomethylation of Phenols

We developed a Cr-catalyzed strategy for the regioselective formation of C_sp²-C_sp³ bonds through the direct and efficient ortho-aminomethylation of N,N-dimethylanilines with phenols. The approach showed excellent site selectivity at the ortho-position of phenols and accommodated broad substrate scope and functional group compatibility for both N,N-dimethylanilines and phenols. Mechanistic studies revealed that the direct ortho-aminomethylation between N,N-dimethylanilines and phenols occurred via an ionic mechanism.

A Mn-catalyzed remote C(sp3)-H bond peroxidation triggered by radical trifluoromethylation of unactivated alkenes

A manganese-catalyzed radical relay strategy for the remote trifluoromethylation-peroxidation of unactivated alkenes is disclosed. The electrophilic CF3 group was added to the C[double bond, length as m-dash]C double bonds to afford remote C-centered radicals upon 1,5-HAT, which could be efficiently trapped by Mnn+1OOBu-t species to deliver 1,6-difunctionalized products selectively under mild conditions. t-BuOOH serves as both the oxidant and the peroxy precursor in this transformation.

Computational exploration of copper catalyzed vinylogous aerobic oxidation of unsaturated compounds

Selective oxidation is one of the most important and challenging transformations in both academic research and chemical industry. Recently, a highly selective and efficient way to synthesize biologically active γ-hydroxy-α,β-unsaturated molecules from Cu-catalyzed vinylogous aerobic oxidation of α,β- and β,γ-unsaturated compounds has been developed. However, the detailed reaction mechanism remains elusive. Herein, we report a density functional theory study on this Cu-catalyzed vinylogous aerobic oxidation of γ,γ-disubstituted α,β- and β,γ-unsaturated isomers. Our computational study unveils detailed mechanism for each elementary step, i.e. deprotonation, O2 activation, and reduction. Besides, the origin of regioselectivity, divergent reactivities of substrates as well as reducing agents, and the byproduct generation have also been investigated. Notably, the copper catalyst retains the + 2 oxidation state through the whole catalytic cycle and plays essential roles in multiple steps. These findings would provide hints on mechanistic studies and future development of transition metal-catalyzed aerobic oxidation reactions.

Electron Donor-Acceptor Complex-Initiated Photochemical Cyanation for the Preparation of α-Amino Nitriles

An electron donor-acceptor complex-initiated α-cyanation of tertiary amines has been described. The reaction protocol provides a novel method to synthesize various α-amino nitriles under mild conditions. The reaction can proceed smoothly without the presence of photocatalysts and transition metal catalysts, and either oxidants are unnecessary or O₂ is the only oxidant. The practicality of this method is showcased not only by the late-stage functionalization of natural alkaloid derivatives and pharmaceutical intermediate, but also by the applicability of a stop-flow microtubing reactor.

On the Mechanism of Cross-Dehydrogenative Couplings between N-aryl Glycinates and Indoles: A Computational Study

Despite the widespread use of cross-dehydrogenative couplings in modern organic synthesis, mechanistic studies are still rare in the literature and those applied to α-amino carbonyl compounds remain virtually unexplored. Herein, the mechanism of Co-catalyzed cross-dehydrogenative couplings of N-aryl glycinates with indoles is described. Density functional theory studies supported the formation of an imine-type intermediate as the more plausible transient electrophilic species. Likewise, key information regarding the role of the N-aryl group and free NH motif within the reaction outcome has been gained, which may set the stage for further developments in this field of expertise.

Visible Light-Driven α-Alkylation of N-Aryl tetrahydroisoquinolines Initiated by Electron Donor-Acceptor Complexes

The visible light-driven α-alkylation of N-aryl tetrahydroisoquinolines was initiated through electron donor-acceptor complex photochemistry. The reaction can proceed smoothly without the addition of any photocatalysts, transition-metal catalysts, or additional oxidants. The proposed mechanism was supported by various mechanistic studies, and the reactive open-shell alkyl radicals were generally produced from an alkylamine and underwent radical coupling for alkylating a wide range of N-aryl tetrahydroisoquinolines.

Toward Rational Understandings of α-C-H Functionalization: Energetic Studies of Representative Tertiary Amines

Functionalization of α-C–H bonds of tertiary amines to build various α-C–X bonds has become a mainstream in synthetic chemistry nowadays. However, due to lack of fundamental knowledge on α-C–H bond strength as an energetic guideline, rational exploration of new synthetic methodologies remains a far-reaching anticipation. Herein, we report a unique hydricity-based approach to establish the first integrated energetic scale covering both the homolytic and heterolytic energies of α-C–H bonds for 45 representative tertiary amines and their radical cations. As showcased from the studies on tetrahydroisoquinolines (THIQs) by virtue of their thermodynamic criteria, the feasibility and mechanisms of THIQ oxidation were deduced, which, indeed, were found to correspond well with experimental observations. This integrated scale provides a good example to relate bond energetics with mechanisms and thermodynamic reactivity of amine α-C–H functionalization and hence, may be referenced for analyzing similar structure-property problems for various substrates.

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A unique hydricity-based methodology for bond energy determination

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The first integrated α-C–H bond energy scale of tertiary amines

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Thermodynamics-based diagnosis of the feasibility/mechanism of amine oxidation

Copper-Promoted Functionalization of Organic Molecules: from Biologically Relevant Cu/O2 Model Systems to Organometallic Transformations

Copper is one of the most abundant and less toxic transition metals. Nature takes advantage of the bioavailability and rich redox chemistry of Cu to carry out oxygenase and oxidase organic transformations using O₂ (or H₂O₂) as oxidant. Inspired by the reactivity of these Cu-dependent metalloenzymes, chemists have developed synthetic protocols to functionalize organic molecules under enviormentally benign conditions. Copper also promotes other transformations usually catalyzed by 4d and 5d transition metals (Pd, Pt, Rh, etc.) such as nitrene insertions or C-C and C-heteroatom coupling reactions. In this review, we summarized the most relevant research in which copper promotes or catalyzes the functionalization of organic molecules, including biological catalysis, bioinspired model systems, and organometallic reactivity. The reaction mechanisms by which these processes take place are discussed in detail.

Selective assembly of N1- and N2-alkylated 1,2,3-triazoles via copper-catalyzed decarboxylative cycloaddition of alkynyl carboxylic acids with ethers and azidotrimethylsilane

An efficient and regiocontrolled method for the synthesis of various N1- and N2 alkylated 1,2,3-triazoles via copper-catalyzed decarboxylative cycloaddition of alkynyl carboxylic acids with ethers and azidotrimethylsilane.

Site-Selective Cu-Catalyzed Alkylation of α-Amino Acids and Peptides toward the Assembly of Quaternary Centers

The Cu^I -catalyzed selective α-alkylation of α-amino acid and peptide derivatives with 2-alkyl-1,3-dioxolanes is reported. This oxidative coupling is distinguished by its site-specificity, high diastereoselectivity, and chirality preservation and exhibits absolute chemoselectivity for N-aryl glycine motifs over other amino acid units. Collectively, the method allows for the assembly of challenging quaternary centers, as well as compounds derived from natural products of high structural complexity, which may provide ample opportunities for late-stage functionalization of peptides.

Biomimetic Oxidative Coupling Cyclization Enabling Rapid Construction of Isochromanoindolenines

Herein, we report a biomimetic oxidative coupling cyclization strategy for the highly efficient functionalization of tetrahydrocarbolines (THCs). This process enables rapid access to complex isochromanoindolenine scaffolds in moderate to excellent yields. The reaction proceeds smoothly and rapidly (complete within minutes) in an open flask. This operationally simple protocol is scalable and compatible with a wide range of functional groups. Late-stage functionalization of a pharmacologically relevant molecule is also demonstrated.

Cu-Catalyzed Cross-Dehydrogenative ortho-Aminomethylation of Phenols

A highly selective Cu^II‐catalyzed cross‐dehydrogenative ortho‐aminomethylation of phenols with aniline derivatives is described. The corresponding C(sp²)−C(sp³) coupling products were obtained in moderate to excellent yields under mild reaction conditions and with a broad substrate scope. A radical mechanism is proposed.

Visible light catalyzed Mannich reaction between tert-amines and silyl diazoenolates

The present work documents the α-C–H functionalization of tertiary amines via the visible light catalyzed Mannich reaction with silyl diazoenolates.

The Photocatalyzed Aza-Henry Reaction of N-Aryltetrahydroisoquinolines: Comprehensive Mechanism, H(•)- versus H(+)-Abstraction, and Background Reactions

The cross-dehydrogenative coupling (CDC) reaction of N-aryltetrahydroisoquinolines (THIQ) is one of the most exploited photocatalytic transformation and a test reaction for an exceptional variety of catalysts. However, its mechanism remained unclear concerning involved intermediates, reactive pathways of the amine radical cation and the influence of oxygen and the light source. Therefore, nuclear magnetic resonance (NMR), electron spin resonance (ESR) and synthetic methods were combined to provide a comprehensive picture of the reaction mechanism using Ru(bpy)3Cl2 as a photocatalyst under aerobic and anaerobic conditions. The reaction profiles and involved intermediates were monitored and analyzed by NMR spectroscopy. Several intermediates contributing to product formation were identified, the iminium ion, the hydroperoxide and dimer of THIQ, and a new ring opened intermediate, cleaved at the benzylic C-N bond. Mechanistic evidence is given that under anaerobic conditions preferentially the α-amino radical is formed by deprotonation, in contrast to the formation of iminium ions via H(•)-abstraction in the presence of oxygen. Further, the light-induced background reaction in the absence of the catalyst was studied in detail, revealing that the product formation rate is correlated to the intensity and wavelength of the light source and that oxygen is essential for an efficient conversion. The reaction rate and efficiency is comparable to previously reported photocatalytic systems, performed under aerobic conditions in combination with intense blue light sources. Thus, the multitude of reaction parameters investigated reveals the preference for hydrogen atom or proton abstraction in photoreactions and allows to assess the influence of experimental conditions on the mechanistic pathways.

Dual rhodium/copper catalysis: synthesis of benzo[b]fluorenes and 2-naphthalenylmethanones via de-diazotized cycloadditions

A novel synergistic rhodium/copper catalysis has been established, enabling de-diazotized cycloadditions toward the selective formation of benzo[b]fluorenes and 2-naphthalenylmethanones.