Photo-Induced Cross-Dehydrogenative Alkylation of Heteroarenes with Alkanes under Aerobic Conditions

Merging Quinoxalin-2(1H)-ones Excitation with Cobaloxime Catalysis: C3 Alkylation of Quinoxalin-2(1H)-ones with Unactivated Alkyl Iodides and Carboxylic Acids under Light

Reported herein is a practical, economical, and efficient construction of 3-alkylated quinoxalin-2(1H)-ones with alkyl carboxylic acids and alkyl iodides by quinoxalin-2(1H)-one excitation and cobaloxime catalysis. Primary, secondary, and tertiary alkyl iodides and carboxylic acids all could be efficiently transferred into target products with excellent functional group tolerance. Mechanism studies reveal that the quinoxalin-2(1H)-one derivatives could be directly excited and yield alkyl carbon radicals from alkyl carboxylic acids and alkyl iodides with the aid of the cobaloxime complex.

Photocatalytic Synthesis of β-Keto Primary Chlorides by Selective Chlorocarbonylation of Olefins

Functionalized primary alkyl chlorides are precursors to a plethora of scaffolds but their access from chemical feedstocks remains challenging. Herein, we report a concise dual Ni/photoredox catalytic protocol for regioselective chlorocarbonylation of unactivated alkenes that enables rapid access to β-keto primary chlorides. The catalytic process features an extensive substrate scope, scalability and functional group tolerance. The Ni/photocatalytic Cl⋅ generation and subsequent cross-coupling is implicated for the process based on the control experiments and DFT study. The synthetic utility of the protocol has been further corroborated through functionalization of complex substrates and modifications of the product.

Visible-Light-Promoted Deoxygenative Alkylation of Quinoxalin-2(1H)-ones with Activated Alcohols

A one-pot strategy for deoxygenative alkylation of alcohols with quinoxalin-2(1H)-ones was developed by using xanthate salts as alcohol-activating groups for radical generation in the presence of tricyclohexylphosphine under visible-light-promoted conditions. The remarkable features of this reaction include a broad substrate scope, excellent functional group tolerance, mild conditions, and simple operation. Moreover, the synthetic utility of this reaction was validated by the success of two-step one-pot reactions, scale-up synthesis, and chemoselective radical monodeoxygenation of diols.

Metal- and light-free decarboxylative direct C-H alkylation of heteroarenes at room temperature

This study reports a metal- and light-free decarboxylative C-H alkylation of heteroarenes at room temperature. The reaction generates various primary, secondary, and tertiary alkyl radicals and functionalizes seven different privileged scaffolds widely present in bioactive molecules. During this process, one equivalent of hypervalent iodine(III) carboxylates (HICs) plays dual roles as an alkyl radical precursor and an oxidant.

Photogenerated chlorine radicals activate C(sp3)-H bonds of alkylbenzenes to access quinazolinones

An Fe-catalyzed visible-light induced condensation of alkylbenzenes with anthranilamides has been developed. Upon irradiation, the trivalent iron complex could generate chlorine radicals, which successfully abstracted the hydrogen of benzylic C-H bonds to form benzyl radicals. And these benzyl radicals were converted into oxygenated products under air conditions, which subsequently reacted with anthranilamides for the synthesis of quinazolinones.

Highly scalable photoinduced synthesis of silanols via untraversed pathway for chlorine radical (Cl•) generation

The emergence of visible light-mediated synthetic transformations has transpired as a promising approach to redefine traditional organic synthesis in a sustainable way. In this genre, transition metal-mediated photoredox catalysis has led the way and recreated a plethora of organic transformations. However, the use of photochemical energy solely to initiate the reaction is underexplored. With the direct utilization of photochemical energy herein, we have established a general and practical protocol for the synthesis of diversely functionalized organosilanols, silanediols, and polymeric siloxanol engaging a wide spectrum of hydrosilanes under ambient reaction conditions. Streamlined synthesis of bio-active silanols via late-stage functionalization underscores the importance of this sustainable protocol. Interestingly, this work also reveals photoinduced non-classical chlorine radical (Cl•) generation from a readily available chlorinated solvent under aerobic conditions. The intriguing factors of the proposed mechanism involving chlorine and silyl radicals as intermediates were supported by a series of mechanistic investigations.

Visible-Light-Driven C,N-Selective Heteroarylation of N-Fluoroalkyl Hydroxylamine Reagents with Quinoxalin-2(1H)-ones

Herein, we disclose a direct and powerful strategy for the synthesis of highly valuable α-trifluoromethylamine and N-trifluoroethylamine derivatives from a visible-light-promoted C,N-selective heteroarylation of N-trifluoroethyl hydroxylamine reagents with quinoxalin-2(1H)-ones under ambient conditions. The chemoselectivity of the process (trifluoroalkylation or N-trifluoroethylamination) can easily be dictated and modulated by a selection of N-trifluoroethyl hydroxylamine substrates. The key to success is the protecting group on the N atom of hydroxylamine reagents, which can control the process of 1,2-H shift of the in situ-generated N-trifluoroethyl radical. Remarkable features of this method include mild conditions, easy operation, high selectivity, and excellent functional group tolerability. More importantly, the trifluoroalkylated products can be readily derivatized into other interesting imidazo-fused heterocycles that would be of great potential for the exploitation of pharmaceutically relevant molecules.

Cerium Photocatalyst in Action: Structural Dynamics in the Presence of Substrate Visualized via Time-Resolved X-ray Liquidography

[Ce(III)Cl6]3-, with its earth-abundant metal element, is a promising photocatalyst facilitating carbon-halogen bond activation. Still, the structure of the reaction intermediate has yet to be explored. Here, we applied time-resolved X-ray liquidography (TRXL), which allows for direct observation of the structural details of reaction intermediates, to investigate the photocatalytic reaction of [Ce(III)Cl6]3-. Structural analysis of the TRXL data revealed that the excited state of [Ce(III)Cl6]3- has Ce-Cl bonds that are shorter than those of the ground state and that the Ce-Cl bond further contracts upon oxidation. In addition, this study represents the first application of TRXL to both photocatalyst-only and photocatalyst-and-substrate samples, providing insights into the substrate's influence on the photocatalyst's reaction dynamics. This study demonstrates the capability of TRXL in elucidating the reaction dynamics of photocatalysts under various conditions and highlights the importance of experimental determination of the structures of reaction intermediates to advance our understanding of photocatalytic mechanisms.

Metal- and Photocatalyst-Free, Visible-Light-Initiated C3 α-Aminomethylation of Quinoxalin-2(1H)-ones via Electron Donor-Acceptor Complexes

We report a metal- and photocatalyst-free C3 α-aminomethylation of quinoxalin-2(1H)-ones with N-alkyl-N-methylanilines. The reaction proceeds through the formation of a photoactivated electron donor-acceptor complex between quinoxalin-2(1H)-ones and N-alkyl-N-methylanilines. The present method provides a mild and environmentally friendly protocol that exhibits good atom economy and excellent functional group tolerance to obtain a library of biologically significant C3 α-aminomethylated quinoxalin-2(1H)-ones in good yields.

C-C bond formation via photocatalytic direct functionalization of simple alkanes

The direct functionalization of alkanes represents a very important challenge in the goal to develop more atom-efficient and clean C-C bond forming reactions. These processes, however, are hampered by the low reactivity of the aliphatic C-H bonds. Photocatalytic processes based on hydrogen atom transfer C-H bond activation strategies have become a useful tool to activate and functionalize these inert compounds. In this article, we summarize the main achievements in this field applied to the development of C-C bond forming reactions, and we discuss the key mechanistic features that enable these transformations.

Direct Access to Strained Fused Dihalo-Aziridino Quinoxalinones via C3-Alkylation Followed by Tandem Cyclization

Quinoxalinones are a privileged class of compounds, and their structural framework is found in many bioactive compounds, natural compounds, and pharmaceuticals. Quinoxalinone is a promising scaffold for different types of functionalization, and the slight modification of the quinoxalinone skeleton is known to offer a wide range of compounds for drug discovery. Owing to the importance of the quinoxalinone scaffold, we have developed a base-mediated protocol for the C3-alkylation of quinoxalinone followed by tandem cyclization to access novel types of strenuous and fused dihalo-aziridino-quinoxalinone heterocycles via the construction of C-C and C-N bonds. The protocol proved to be simple and practical to access desired fused quinoxalinone heterocycles in excellent yields (up to 98% yield). As an application, the highly functionalized fused dihalo-aziridino-quinoxalinone molecule has been further utilized for mono-dehalogenation under visible light irradiation and selective amide reduction. Moreover, the protocol has also been demonstrated on a gram scale.

Radical-mediated photoredox hydroarylation with thiosulfonate

Herein, we report a novel visible light-induced photocatalytic system that enables intramolecular hydroarylation of unactivated alkenes. Thiosulfonate compounds were found to be the key radical precursor that mediates the Minisci-type intramolecular cyclization reaction. Under the optimal reaction conditions, a wide range of pyridyquinazolinone and pyrroloquinazolinone products were obtained in moderate to good yields.

Minisci reaction of heteroarenes and unactivated C(sp3)-H alkanes via a photogenerated chlorine radical

Here, an efficient Minisci reaction of heteroarenes and unactivated C(sp³)-H alkanes was achieved using an inexpensive FeCl₃ as a photocatalyst. The photogenerated chlorine radical contributed to the HAT of C-H and subsequently initiated this reaction. Surprisingly, salt water and even seawater can act as a chlorine radical source, which provided an enlightening idea for future organic synthesis methods.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Photocatalytic Aerobic Coupling of Azaarenes and Alkanes via Nontraditional Cl• Generation

Herein, we demonstrate a nonconventional photocatalytic generation of Cl^• from a common chlorinated solvent, dichloroethane, under aerobic conditions and its successful utilization toward the cross-dehydrogenative coupling of alkanes and azaarenes via hydrogen atom transfer with Cl^•. The process is free from chloride salt, toxic oxidant, and UV light. It is applicable to a broad spectrum of substrates. The proposed mechanism involving Cl^• is supported by a series of mechanistic investigations.

Phenanthrenequinone (PQ) catalyzed cross-dehydrogenative coupling of alkanes with quinoxalin-2(1H)-ones and simple N-heteroarenes under visible light irradiation

A direct and convenient strategy to 3-alkylquinoxalin-2(1H)-ones and other alkyl N-heteroarenes via a photocatalyzed alkylation of quinoxalin-2(1H)-ones and other N-heterocycles with commercially available, low-cost alkanes under ambient conditions using phenanthrenequinone (PQ) as a photocatalyst was developed. This transformation has advantages of environment-friendly protocol, mild conditions, good functional-group tolerance, and high yields of products.

Titanium(IV) Chloride-Catalyzed Photoalkylation via C(sp3)-H Bond Activation of Alkanes

Despite the sophistication of C-H functionalization as one of the most powerful tools in organic synthesis, methodology for performing hydrogen-atom transfer of unactivated alkanes remains rather scarce. Herein, we describe chlorine radical-catalyzed C(sp³)-H photoalkylation using titanium(IV) chloride via a ligand-to-metal charge transfer process. Enabled by the unique properties of this abundant metal salt, the reaction not only effected the coupling of various alkanes with radical acceptors but also was shown to be applicable to direct photoalkylation of aromatic ketones.

Organic-acid catalysed Minisci-type arylation of heterocycles with aryl acyl peroxides

A metal-free method for the Minisci-type arylation of heterocycles with aryl acyl peroxides has been reported. This strategy enables the rapid and simple synthesis of a series of Minisci-type adducts...

Decatungstate-Photocatalyzed Direct Coupling of Inert Alkanes and Quinoxalin-2(1H)-ones with H2 Evolution

A tetrabutylammonium decatungstate (TBADT)-photocatalyzed direct coupling of inert alkanes and quinoxalin-2(1H)-ones with H2 evolution was developed at room temperature. The present transformation achieved the direct C(sp3)-H/C(sp2)-H coupling under noble metal-free,...