A Visible-Light-Mediated Oxidative CN Bond Formation/Aromatization Cascade: Photocatalytic Preparation of N-Arylindoles

Divergent Functionalization of N-Alkyl-2-alkenylanilines: Efficient Synthesis of Substituted Indoles and Quinolines

An efficient divergent functionalization of N-alkylated ortho-alkenylanilines to substituted indoles and quinolines has been accomplished by employing rhodium-catalyzed cross-dehydrogenative coupling and silver-mediated oxidative cyclization, respectively. The developed methods tolerate various functional groups and allow the synthesis of substituted indoles and quinolines in good to excellent yield. Synthetic utility is demonstrated through conversion to an indole with antimicrobial activity and C-H bond functionalization of 2-arylquinolines. Furthermore, a plausible mechanism was proposed based on preliminary mechanistic investigations.

Synthesis of Arylamines via Aminium Radicals

Arylamines constitute the core structure of many therapeutic agents, agrochemicals, and organic materials. The development of methods for the efficient and selective construction of these structural motifs from simple building blocks is desirable but still challenging. We demonstrate that protonated electron-poor O-aryl hydroxylamines give aminium radicals in the presence of Ru(bpy)3 Cl2 . These highly electrophilic species undergo polarized radical addition to aromatic compounds in high yield and selectivity. We successfully applied this method to the late-stage modification of chiral catalyst templates, therapeutic agents, and natural products.

Photoredox Imino Functionalizations of Olefins

Shown herein is that polyfunctionalized nitrogen heterocycles can be easily prepared by a visible-light-mediated radical cascade process. This divergent strategy features the oxidative generation of iminyl radicals and subsequent cyclization/radical trapping, which allows the effective construction of highly functionalized heterocycles. The reactions proceed efficiently at room temperature, utilize an organic photocatalyst, use simple and readily available materials, and generate, in a single step, valuable building blocks that would be difficult to prepare by other methods.

Selective Oxidative Esterification from Two Different Alcohols via Photoredox Catalysis

Esters functionalities are important building blocks that are extensively used in the chemical industry and academic laboratories. Direct oxidative esterification from easy-available alcohols to esters would be a much more appealing approach, especially using O₂ as terminal oxidant. Inputting external energy by photocatalysis for dioxygen activation, a mild and simple method for ester synthesis from two different alcohols has been achieved in this work. This reaction is performed under neutral conditions using O₂ as the terminal oxidant. A variety of primary alcohols, especially long chain alcohols and secondary alcohols are tolerated in this system.

Electrocatalytic Generation of Amidyl Radicals for Olefin Hydroamidation: Use of Solvent Effects to Enable Anilide Oxidation

Oxidative generation of synthetically important amidyl radicals from N-H amides is an appealing and yet challenging task. Previous methods require a stoichiometric amount of a strong oxidant and/or a costly noble-metal catalyst. We report herein the first electrocatalytic method that employs ferrocene (Fc), a cheap organometallic reagent, as the redox catalyst to produce amidyl radicals from N-aryl amides. Based on this radical-generating method, an efficient intramolecular olefin hydroamidation reaction has been developed.

Visible-Light-Mediated Generation of Nitrogen-Centered Radicals: Metal-Free Hydroimination and Iminohydroxylation Cyclization Reactions

The formation and use of iminyl radicals in novel and divergent hydroimination and iminohydroxylation cyclization reactions has been accomplished through the design of a new class of reactive O-aryl oximes. Owing to their low reduction potentials, the inexpensive organic dye eosin Y could be used as the photocatalyst of the organocatalytic hydroimination reaction. Furthermore, reaction conditions for a unique iminohydroxylation were identified; visible-light-mediated electron transfer from novel electron donor-acceptor complexes of the oximes and Et3N was proposed as a key step of this process.

Diastereoselective Oxidative C-N/C-O and C-N/C-N Bond Formation Tandems Initiated by Visible Light: Synthesis of Fused N-Arylindolines

The synthesis of fused N-arylindolines using visible light photoredox catalysis has been developed. We previously described that photogenerated amine radical cations generate substituted indoles through an intermediate benzylic carbocation. Herein, we expand the application of this chemistry by trapping the benzylic carbocation with tethered heteronucleophiles. The reactivity of the photogenerated benzylic carbocation is explored and applied to a range of substrates with various electronic characters and ring constraints. The method described provides C2 and C3 fused indolines bearing a tetrasubstituted carbon stereocenter with greater than 99:1 diastereoselectivity in moderate to good yields.

Photocatalytic generation of N-centered hydrazonyl radicals: a strategy for hydroamination of β,γ-unsaturated hydrazones

A visible-light photocatalytic generation of N-centered hydrazonyl radicals has been accomplished for the first time. This approach allows efficient intramolecular addition of hydrazonyl radical to terminal alkenes, thus providing hydroamination and oxyamination products in good yields. Importantly, the protocol involves deprotonation of an N-H bond and photocatalytic oxidation to an N-centered radical, thus obviating the need to prepare photolabile amine precursors or the stoichiometric use of oxidizing reagents.

Direct arylation of N-heteroarenes with aryldiazonium salts by photoredox catalysis in water

A highly effective visible light-promoted "radical-type" coupling of N-heteroarenes with aryldiazonium salts in water has been developed. The reaction proceeds at room temperature with [Ru(bpy)3 ]Cl2 ⋅6 H2 O as a photosensitizer and a commercial household light bulb as a light source. Pyridine and a variety of substituted pyridines are effective substrates under these reaction conditions, and only monosubstituted products are formed with different regioselectivities. Using aqueous formic acid as solvent, an array of xanthenes, thiazole, pyrazine, and pyridazine are compatible with this new arylation approach. The broad substrate scope, mild reaction conditions, and use of water as reaction solvent make this procedure a practical and environmentally friendly method for the synthesis of compounds containing aryl-heteroaryl motifs.