Visible-Light-Promoted Iron-Catalyzed N-Arylation of Dioxazolones with Arylboronic Acids

Visible-Light-Promoted Enantioselective α-Amidation of Aldehydes by Harnessing Organo-Iron Dual Catalysis

The strategic integration of organocatalysis with transition-metal catalysis to achieve otherwise unattainable stereoselective transformations may serve as a powerful synthetic tool. Herein, we present a synthetically versatile α-amidation of aldehydes by leveraging dual iron and chiral enamine catalysis in an enantioselective manner (up to >99:1 er). Experimental and computational studies have led us to propose a new mechanistic platform, wherein visible-light-promoted LMCT generates [Fe(II)Cl3-], which effectively activates dioxazolones to form an iron-acylnitrenoid radical that inserts into chiral enamine intermediates.

Cooperative Fe/Co-Catalyzed Remote Desaturation for the Synthesis of Unsaturated Amide Derivatives

Unsaturated amides represent common functional groups found in natural products and bioactive molecules and serve as versatile synthetic building blocks. Here, we report an iron(II)/cobalt(II) dual catalytic system for the syntheses of distally unsaturated amide derivatives. The transformation proceeds through an iron nitrenoid-mediated 1,5-hydrogen atom transfer (1,5-HAT) mechanism. Subsequently, the radical intermediate undergoes hydrogen atom abstraction from vicinal methylene by a cobaloxime catalyst, efficiently yielding β,γ- or γ,δ-unsaturated amide derivatives under mild conditions. The efficiency of Co-mediated HAT can be tuned by varying different auxiliaries, highlighting the generality of this protocol. Remarkably, this desaturation protocol is also amenable to practical scalability, enabling the synthesis of unsaturated carbamates and ureas, which can be readily converted into various valuable molecules.

Amidative β-Scission of Alcohols Enabled by Dual Catalysis of Photoredox Proton-Coupled Electron Transfer and Inner-Sphere Ni-Nitrenoid Transfer

The photoredox/Ni dual catalysis is an appealing strategy to enable unconventional C-heteroatom bond formation. While significant advances have been achieved using this system, intermolecular C(sp3)-N bond formation has been relatively underdeveloped due to the difficulty in C(sp3)-N reductive elimination. Herein, we present a new mechanistic approach that utilizes dioxazolones as the Ni(II)-nitrenoid precursor to capture carbon-centered radicals by merging proton-coupled electron transfer (PCET) with nickel catalysis, thus forming synthetically versatile N-alkyl amides using alcohols. Based on mechanistic investigations, the involvement of (κ2-N,O)Ni(II)-nitrenoid species was proposed to capture photoredox PCET-induced alkyl radicals, thereby playing a pivotal role to enable the C(sp3)-N bond formation.

Visible-Light-Driven Iron-Catalyzed Decarboxylative C-N Coupling Reaction of Alkyl Carboxylic Acids with NaNO2

An efficient visible-light-driven iron-catalyzed decarboxylative C-N coupling reaction of alkyl carboxylic acids with NaNO2 under mild conditions was developed. The reaction proceeds under photosensitizer-free conditions and features good to excellent yields, broad functional group tolerance, and an easy operation procedure. Preliminary mechanistic investigations showed that visible-light-driven iron catalysis not only achieved oxidative decarboxylation of alkyl carboxylic acids to alkyl radicals but also promoted the reduction of NO2- to NO, thus leading to the C-N radical coupling reaction.

Visible-Light-Induced Radical Cascade Cross-Coupling via C(sp3)-H Activation and C-N/N-O Cleavage: Feasible Access to Methylenebisamide Derivatives

Here we report facile and manipulable access to methylenebisamide derivatives via visible-light-driven radical cascade processes incorporating C(sp³)-H activation and C-N/N-O cleavage. Mechanistic studies reveal that a traditional Ir-catalyzed photoredox pathway and a novel copper-induced complex-photolysis pathway are both involved, contributing to activating the inert N-methoxyamides and rendering the valuable bisamides. This approach exhibits many advantages, including mild reaction conditions, broad scope and functional group tolerance, and competitive step economy. Given the mechanistic plenitude and operational simplicity, we believe this package deal paves a promising way for the synthesis of valuable nitrogen-containing molecules.

Iron-catalyzed dual decarboxylative coupling of α-amino acids and dioxazolones under visible-light to access amide derivatives

An iron-catalyzed decarboxylative C-N coupling of α-amino acids with dioxazolones is described herein to synthesize amide derivatives under visible-light. The desired products can be given in good to excellent yields under simple, mild, and oxidant-free conditions. This protocol provides a practical route for the transformation of α-amino acids to the corresponding amides. Computational studies were carried out to shed light on the mechanism of this reaction.

Copper(I)-Mediated Decarboxylative N-Arylation of Dioxazolones: Synthesis of N-Aryl Amides

Dioxazolones, which are potent amide precursors, have been widely explored for the formation of C–N bonds with the help of transition-metal catalysts. Here, we highlight our recent studies on the synthesis of N-aryl amides using dioxazolones and boronic acids in the presence of copper(I) chloride under mild conditions. The versatility of the developed reaction is demonstrated by its wide range of functional-group tolerances as well as the release of nontoxic carbon dioxide. Optimization screenings reveal that a fluorine additive improves the desired reactivity toward the intended transformation. The addition of triphenylphosphine results in an N-acyl iminophosphorane, suggesting the involvement of an N-acyl nitrene intermediate in this transformation.

Iron-Catalyzed Intramolecular Arene C(sp2 )-H Amidations under Mechanochemical Conditions

In a ball mill, FeBr3 -catalyzed intramolecular amidations lead to 3,4-dihydro-2(1H)-quinolinones in good to almost quantitative yields. The reactions do not require a solvent and are easy to perform. No additional ligand is needed for the iron catalyst. Both 4-substituted aryl and β-substituted dioxazolones provide products with high selectivity. Mechanistically, an electrophilic spirocyclization followed by C-C migration explains the formation of rearranged products.

Reaction of Dioxazolones with Boronic Acids: Copper-Mediated Synthesis of N-Aryl Amides via N-Acyl Nitrenes

Dioxazolones, as direct amide sources, have been used with boronic acids in the presence of copper(I) chloride to access N-aryl amides at room temperature. The versatility of the developed reaction is proven by ample scope having a wide range of functional group tolerance. The reaction optimization conditions revealed that a fluorine additive demonstrated improved reactivity toward the intended transformation. The addition of triphenylphosphine resulted in N-acyl iminophosphorane, suggesting the involvement of an N-acyl nitrene intermediate.

Photoredox/Iron Dual-Catalyzed Insertion of Acyl Nitrenes into C-H Bonds

In this work, the use of N-acyloxybenzamides as efficient acyl nitrene precursors under photoredox/iron dual catalysis is reported. The resulting acyl nitrenes could be captured by various types of C-H bonds and S- or P-containing molecules. Mechanism investigations suggested that the formation of the acyl nitrene from the N-acyloxybenzamide occurs by a photoredox process, and it is believed that in this redox process oxidative N-H bond cleavage of the N-acyloxybenzamide occurs prior to reductive N-O bond cleavage of the N-acyloxybenzamide.

Visible-Light-Induced Decarboxylation of Dioxazolones to Phosphinimidic Amides and Ureas

A visible-light-induced external catalyst-free decarboxylation of dioxazolones was realized for the bond formation of N=P and N-C bonds to access phosphinimidic amides and ureas. Various phosphinimidic amides and ureas (47 examples) were synthesized with high yields (up to 98%) by this practical strategy in the presence of the system's ppm Fe.

Photoinduced Transition-Metal-Free Chan-Evans-Lam-Type Coupling: Dual Photoexcitation Mode with Halide Anion Effect

Herein, we report a photoinduced transition-metal-free C(aryl)-N bond formation between 2,4,6-tri(aryl)boroxines or arylboronic acids as an aryl source and 1,4,2-dioxazol-5-ones (dioxazolones) as an amide coupling partner. Chloride anion, either generated in situ by photodissociation of chlorinated solvent molecules or added separately as an additive, was found to play a critical cooperative role, thereby giving convenient access to a wide range of synthetically versatile N-arylamides under mild photo conditions. The synthetic virtue of this transition-metal-free Chan-Evans-Lam-type coupling was demonstrated by large-scale reactions, synthesis of 15N-labeled arylamides, and applicability toward biologically relevant compounds. On the basis of mechanistic investigations, two distinctive photoexcitations are proposed to function in the current process, in which the first excitation involving chloro-boron adduct facilitates the transition-metal-free activation of dioxazolones by single electron transfer (SET), and the second one enables the otherwise-inoperative 1,2-aryl migration of the thus-formed N-chloroamido-borate adduct.

Access to β-Lactams via Iron-Catalyzed Olefin Oxyamidation Enabled by the π-Accepting Phthalocyanine Ligand

Herein, we report the development of an iron-catalyzed olefin oxyamidation by utilizing tethered dioxazolones as the nitrenoid precursor to produce valuable β-lactam scaffolds. Mechanistic studies revealed that a relatively strong π-accepting ability of the phthalocyanine ligand is critical in generating the key triplet iron-imidyl radical intermediate to enable the 4-exo-trig-lactamization with the incorporation of oxygen nucleophiles in high diastereoselectivity. This cyclization approach was readily extended to the highly efficient γ-lactam synthesis (TON > 300).