Metal-Free Amino(hetero)arylation and Aminosulfonylation of Alkenes Enabled by Photoinduced Energy Transfer

Aminosulfonylation of Rhodium Carbene via Ylide Formation and 1,4-Sulfonyl Rearrangement

We report here the use of pyridin-2-yl benzenesulfonates as sulfonylation reagents in a difunctionalization reaction based on oxy-pyridinium ylide chemistry, providing an effective protocol for the installation of both a sulfonyl group and a pyridone moiety into one molecule. Density functional theory (DFT) calculations disclose that the reaction process might proceed through sequential metal-bound ylide formation, keto-enol tautomerism, and the migratory rearrangement of the sulfonyl group.

Photocatalytic synthesis of β-amino acid derivatives from alkenes with alkyl formates

The aminocarbonylation of alkenes is an efficient approach to synthesize important β-amino acid motifs. However, simple and convenient methods are still rare. Herein, we present a novel visible-light-mediated controllable three-component radical relay coupling of alkenes, alkyl formates and oxime esters. By the combination of hydrogen atom transfer and energy transfer processes, a series of β-amino esters could be obtained smoothly in one step under mild conditions. We expect that the approach can complement current methodologies for the synthesis of β-amino esters.

Dearomative hydroamination of heteroarenes catalyzed by the phenolate photocatalyst

Dearomative functionalization of heteroarenes offers an attractive and sustainable approach for the rapid construction of complex 3D heterocyclic scaffolds from planar structures. Despite progress in this field, dearomative amination of heteroarenes via a radical anion intermediate remains a challenge. Here, we report a photoredox-catalyzed dearomative hydroamination of heteroarenes with hydrazodiformates under mild and transition-metal-free reaction conditions. Various benzofurans and benzothiophenes can efficiently participate in this transformation. A series of mechanistic experiments revealed that heteroaryl radical anions are the crucial intermediates, generated through photo-induced electron transfer between the excited phenolate photocatalyst and heteroarenes.

Photocatalytic Direct Para-Selective C-H Amination of Benzyl Alcohols: Selectivity Independent of Side Substituents

Aminoarenes are important molecules for broad applications in nearly all modern industries that involve chemicals. Direct and site-selective C-H bond amination of arenes provides the most efficient and convenient method to prepare aminoarenes. A main challenge is to selectively install the amino group (or other functional groups) to the distal para-carbon of arenes (especially multi-substituted arenes) during the C-H bond functionalization events. Herein, we address this problem by designing a new strategy via a sequential radical dearomatization/radical amination/rearomatization process for para-selective amination of benzyl alcohols. The para-selectivity of our reaction is completely independent of the electronic and steric properties of the other substituents of the arene substrates. Aminoarenes with many substituents (up to full substitution) and diverse substitution patterns, including those difficult to synthesize previously, could be readily prepared using our protocols. Further exploration of the current strategy shall lead to other challenging C-H functionalization of arenes.

Synthesis of β-Silyl Amines via Merging Photoinduced Energy and Hydrogen Atom Transfer in Flow

The development of efficient methods for synthesizing β-silyl amines has long been a significant goal in organic synthesis. Previous methods mainly relied on the use of prefunctionalized substrates or special reagents. Herein, we present a visible-light-promoted synthesis approach for β-silyl amines, utilizing a combination of photoinduced energy and hydrogen atom transfer processes. Using flow chemistry technology, a variety of valuable skeletons, including β-silyl amines and α-amino esters, can be produced from readily available feedstocks such as hydrosilanes and simple alkanes. Moreover, the strategy's full-process fluidized production capability highlights its potential for industrial-scale manufacturing. Mechanistic studies revealed that oxime esters can act as radical precursors as well as hydrogen atom transfer reagents.

Photocatalyzed Imino-Difluoromethylation of Alkenes with Bifunctional Oxime Esters

Herein, we report a simple and versatile difluoromethylene-imide reaction in which a series of olefins can undergo a difluoromethylenimine reaction under photocatalytic conditions through an energy transfer (EnT) process. The reaction has mild conditions and a wide range of applicability. We successfully synthesized 27 molecules containing difluoromethylene units, featuring easily accessible starting materials and operational simplicity.

Synthesis of 2H-imidazoles via copper-catalyzed homo/cross-coupling of oxime acetates

A facile and practical protocol to construct 2H-imidazoles by applying an oxime acetate block as the sole component via oxidative homo/cross-coupling catalyzed by Cu(I) was developed. This strategy provides a straightforward method to produce a series of substituted 2H-imidazoles in moderate to excellent yields. The transformation process is straightforward to operate and is considered as a readily available catalytic system exhibiting good substrate compatibility, eliminating the necessity for pre-functionalization of azides or the use of additives.

Photoinduced Asymmetric Alkene Aminohetarylation with Chiral Sulfoximine Reagents

Given the pivotal role of β-(het)arylethylamine moiety in bioactive molecules, the direct amino(het)arylation of alkenes occupies a privileged position in the construction of (het)arylethylamine derivatives. Herein we devise chiral sulfoximines as novel bifunctional reagents which exhibit remarkable efficiency in the challenging asymmetric alkene aminohetarylation reaction, particularly in terms of reactivity and stereo-control. The chiral reagents can be conveniently accessed in gram scale, and efficiently generate N-centered radicals under mild photochemical conditions. The transformation proceeds through enantioselective 1,4-hetaryl migration, ensuring precise chirality transfer from sulfur- to carbon-centers, rendering wide applicability to both aromatic and aliphatic alkenes. Furthermore, the method is straightforward to operate and does not require transition metals or photosensitizers, making it an attractive and practical option.

Photoinduced EnT-mediated sulfonamidylimination of alkenes and (hetero)arenes with iminophenylacetic acid oxime esters

A photoinduced EnT-mediated generation of sulfonamidyl radicals has been accomplished using rationally designed iminophenylacetic acid oxime ester reagents under metal-free conditions. This approach offers a mild, regio- and diastereoselective synthesis of N-sulfonyl diamines via diamination of alkenes and (hetero)arenes.

Photochemical dearomative skeletal modifications of heteroaromatics

Dearomatization has emerged as a powerful tool for rapid construction of 3D molecular architectures from simple, abundant, and planar (hetero)arenes. The field has evolved beyond simple dearomatization driven by new synthetic technology development. With the renaissance of photocatalysis and expansion of the activation mode, the last few years have witnessed impressive developments in innovative photochemical dearomatization methodologies, enabling skeletal modifications of dearomatized structures. They offer truly efficient and useful tools for facile construction of highly complex structures, which are viable for natural product synthesis and drug discovery. In this review, we aim to provide a mechanistically insightful overview on these innovations based on the degree of skeletal alteration, categorized into dearomative functionalization and skeletal editing, and to highlight their synthetic utilities.

CuCl2/FeCl3 Bimetallic Photocatalyst for Sustainable Ethylene Production from Ethanol via Recoverable Redox Cycles

Photocatalytic conversions of ethanol to valuable chemicals are significant organic synthesis reactions. Herein, we developed a CuCl2/FeCl3 bimetallic photocatalyst for sustainable dehydration of ethanol to ethylene by recoverable redox cycles. The selectivity of ethylene was 98.3% for CuCl2/FeCl3, which is much higher than that of CuCl2 (34.5%) and FeCl3 (86.5%). Due to the ligand-to-metal charge transfer (LMCT) process involved in generating the liquid products, the CuCl2/FeCl3 catalyst will be reduced to CuCl/FeCl2. Oxygen (O2) is required for the recovery of CuCl2/FeCl3 to avoid exhaustion. The soluble Fe3+/Fe2+ redox species deliver catalyst regeneration properties more efficiently than single metal couples, making a series of redox reactions (Cu2+/Cu+, Fe3+/Fe2+, and O2/ethanol couples) recyclable with synergistic effects. A flow reactor was designed to facilitate the continuous production of ethylene. The understanding of bimetallic synergism and consecutive reactions promotes the industrial application process of photocatalytic organic reactions.

Difunctionalization of gem-difluoroalkenes for amination and heteroarylation via metal-free photocatalysis

gem-Difluoroalkenes are widely used building blocks in fluorine chemistry. Herein, a metal-free photocatalytic amination and heteroarylation method of gem-difluoroalkenes with heteroaryl carboxylic acid oxime esters as substrates is reported. This environmentally benign reaction proceeds via radical-radical cross-coupling in energy-transfer-mediated photocatalysis and can be used in the rapid construction of heteroaryl difluoroethylamine scaffolds and late-stage modification of complex pharmaceutical structures.

γ-Amino Alcohols via Energy Transfer Enabled Brook Rearrangement

In the long-standing quest to synthesize fundamental building blocks with key functional group motifs, photochemistry in the recent past has comprehensively established its attractiveness. Amino alcohols are not only functionally diverse but are ubiquitous in the biologically active realm of compounds. We developed bench-stable bifunctional reagents that could then access the sparsely reported γ-amino alcohols directly from feedstock alkenes through energy transfer (EnT) photocatalysis. A designed 1,3-linkage across alkenes is made possible by the intervention of a radical Brook rearrangement that takes place downstream to the EnT-mediated homolysis of our reagent(s). A combination of experimental mechanistic investigations and detailed computational studies (DFT) indicates a radical chain propagated reaction pathway.

Photosensitized 1,2-Difunctionalization of Alkenes to Access β-Amino Sulfonamides

A metal-free photosensitized 1,2-imino-sulfamoylation of olefins by employing a tailor-made sulfamoyl carbamate as the difunctionalization reagent has been established. This protocol exhibits versatility across a broad substrate scope, including aryl and aliphatic alkenes, leading to the synthesis of diverse β-imino sulfonamides in moderate to good yields. This method is characterized by its metal-free reaction system, mild reaction conditions, excellent regioselectivity, and high atom economy, serving as a promising platform for the preparation of β-amino sulfonamide-containing molecules, particularly in the context of drug discovery.

Visible-Light-Induced Decarboxylative Aminosulfonylation of (Hetero)aryl Carboxylic Oxime Esters

Sulfonamides are important structures in pharmaceuticals, agrochemicals, and organocatalysts, yet the rapid and benign synthesis of these compounds is still a great challenge. Herein we report a photoinduced method for synthesizing sulfonamides from (hetero)aryl carboxylic acid oxime esters. This reaction proceeds via one-pot cascade radical-radical cross-coupling by energy-transfer-mediated photocatalysis. A wide substrate scope including (hetero)aryl substrates and late-stage modification of pharmaceutical molecular entities reveal its generality.

Photocatalyzed Amination of Alkyl Halides to Access Primary Amines

We demonstrate that oxime ester derivatives can be used as both a halogen atom transfer (XAT) agent and an imine source under photocatalytic conditions, allowing the radical amination of alkyl halides, resulting in the formation of a broad scope of imines. Hydrolysis of the latter gives direct access to the corresponding primary amines. Mechanistically, the reaction is believed to proceed through the formation of aryl radical intermediates, which are responsible for the activation of alkyl halides via XAT.

Photoinduced Direct Electron Transfer between Quinoxalin-2(1H)-ones and Alkyl Carboxylic Acids for C-H Alkylation

The progress of efficient and sustainable approaches for decarboxylative coupling reactions is synthetically appealing due to the structural diversity, lack of toxicity, and widespread commercial accessibility of carboxylic acids. However, the decarboxylation reaction still encounters challenges related to the utilization of oxidants, catalysts, and prefunctionalization conditions. We report herein a mild method that facilitates direct electron transfer between alkyl carboxylic acids and excited-state substrates for C-H alkylation of quinoxalin-2(1H)-ones without the involvement of any catalyst or additive.

Regioselective Synthesis of Unsymmetrical Vicinal Diamines via Azidoimination of Alkenes with TMSN3 and Ketimines

2-Azidoimines are versatile precursors to value-added vicinal unsymmetrical diamines, which are among the most common motifs in biologically active compounds. Herein, we report their operationally simple synthesis through a highly regioselective intermolecular azidoamination of olefins under metal-free conditions. The approach proceeded through azide and iminyl, two differentiated N-centered radicals. The synthetic potential of the protocols was further established via the condensation/amination sequential cascade and chemoselective, orthogonal transformations to access vicinal primary diamines.

Three-Component Photochemical 1,2,5-Trifunctionalizations of Alkenes toward Densely Functionalized Lynchpins

Radical remote 1,n-difunctionalization reactions (n > 2) of alkenes are powerful tools to efficiently introduce functional groups with selected distances into target molecules. Among these reactions, 1,5-difunctionalizations are an important subclass, leading to sought-after scaffolds, but typically suffer from tailored starting materials and strict limitations for the formed functional group in 2-position. Seeking to address these issues and to make radical 1,5-difunctionalizations of alkenes more applicable, we report a novel three-component 1,2,5-trifunctionalization reaction between imine-based bifunctional reagents and two distinct alkenes, driven by visible light energy transfer-catalysis. Key to achieving this selective one-step installation of three different functional groups via the choreographed formation of four bonds was the utilization of a 1,2-boron shift and the rigorous capitalization of radical polarities and stabilities. Thorough mechanistic studies were carried out, and the synthetic utility of the obtained products was demonstrated by various downstream modifications. Notably, in addition to the functionalization of individual functional groups, their interplay gave rise to a unique array of cyclic products.

Three-Component Aminoheteroarylation of Alkenes via Photoinduced EDA Complex Activation

A catalyst-free approach for the multicomponent aminoheteroarylation reaction of alkenes with N-aminopyridinium salts and heteroarenes is herein described. The reaction shows good functional group tolerance and allows the generation of valuable β-heteroarylethylamines in satisfying yields. In this transformation, N-aminopyridinium salts and heteroarenes are utilized to generate electron donor-acceptor complexes, which undergo a single-electron transfer process upon light irradiation to form key amidyl radicals and heteroaryl radical cations. The amidyl radical is subsequently captured by alkenes, followed by a Minisci-type reaction to yield the desired β-heteroarylamines as products.

The Merger of Halogen Atom Transfer (XAT) and Energy Transfer Catalysis (EnT) for the Modular 1,2-Iminylalkylation of Diazenes

The 1,2-iminylalkylation of diazenes using alkyl iodides in combination with an O-benzoyl oxime is reported. In this transformation, O-benzoyl oxime acted as a radical precursor and XAT mediator. In addition to common alkyl iodides, other alkyl iodides such as iodomethane, iodomethane-d3, trifluoroiodomethane, ethyl difluoroiodoacetate, and iodoalkanes containing unprotected hydroxyl and amide groups can also serve as C-radical precursors in the 1,2-iminylalkylation with electrophilic diazenes as radical acceptors.

Photoinduced Difunctionalization of Diazenes Enabled by N-N Radical Coupling

In this study, a metal-free difunctionalization strategy for diazenes was developed using a range of bifunctionalization reagents. This strategy involves a unique N(sp3)-N(sp2) radical coupling between the hydrazine radical and the imine radical. More than 30 triazane core motifs were constructed by installing imines and various functional groups, including alkyl, phenyl, cyanoalkyl, and sulfonyl groups, on both ends of the nitrogen-nitrogen bond of diazenes in an efficient manner.