Visible‐Light‐Driven Photocatalyst‐ and Additive‐Free Cross‐Coupling of β‐Ketothioamides with α‐Diazo 1,3‐Diketones: Access to Highly Functionalized Thiazolines

Reaction of Donor-Acceptor α-Diazo Esters and N,N-Dimethylformamide/γ-Lactams To Generate α-Amino-α-aroylethanoates

Herein, we present a metal-free, concise, and efficient protocol for the intermolecular reaction of amides (lactams) with donor-acceptor α-diazo esters to afford the functionalized α-amino-α-aroylethanoates. N-Methyl-2-pyrrolidone (NMP) and N,N-dimethylformamide (DMF) are employed as both reagents and solvents, allowing for the incorporation of all units into the products. The reaction is processed by the ester group migration and compatible with a broad range of substrates up to 50 examples.

Recent Advances of C-S Coupling Reaction of (Hetero)Arenes by C-H Functionalization

Organic sulfur compounds encompass a vast and diverse variety of species that possess unique biological activity due to the presence of sulfur atoms or sulfur-containing functional groups. These compounds are widely present in natural products, pharmaceuticals, agricultural chemicals, and functional materials. In recent years, numerous sulfur-containing compounds such as thiols, thioethers, disulfides, thiourea, dimethyl sulfoxide, sulfonates and their derivatives, as well as sulfur-containing inorganic compounds, have been utilized as coupling agents to synthesize (hetero)aryl sulfides via C-H Functionalization. These novel transformations provide effective methods for constructing C-S bond of (hetero)arenes, while also expanding the scope of (hetero)aryl sulfides with the potential biological activity. Therefore, the synthesis of aryl sulfides through C-H bond functionalization has attracted widespread attention. This review mainly focuses on the construction of (hetero)aryl sulfides via C-H bond functionalization since 2015. We hope this review offers a useful conceptual overview and inspires further advancements in the efficient construction of C-S bonds.

Spiroannulations of β-Ketothioamides with Bromoenals via Selective C-Michael Addition and S-Michael Addition-Triggered Cascade Reactions

A spiroannulation reaction of β-ketothioamides with aromatic β-bromoenals and aromatic α-bromoenals via selective C-Michael addition and S-Michael addition-triggered cascade reactions has been developed. This protocol provides a novel and rapid approach for the synthesis of substituted spirothiopyran and spirothiophene derivatives under mild conditions with moderate to good yields and a broad substrate scope.

Visible Light-Induced Reactions of Diazo Compounds and Their Precursors

In recent years, visible light-induced reactions of diazo compounds have attracted increasing attention in organic synthesis, leading to improvement of existing reactions, as well as to the discovery of unprecedented transformations. Thus, photochemical or photocatalytic generation of both carbenes and radicals provide milder tools toward these key intermediates for many valuable transformations. However, the vast majority of the transformations represent new reactivity modes of diazo compounds, which are achieved by the photochemical decomposition of diazo compounds and photoredox catalysis. In particular, the use of a redox-active photocatalysts opens the avenue to a plethora of radical reactions. The application of these methods to diazo compounds led to discovery of transformations inaccessible by the classical reactivity associated with carbenes and metal carbenes. In most cases, diazo compounds act as radical sources but can also serve as radical acceptors. Importantly, the described processes operate under mild, practical conditions. This Review describes this subfield of diazo compound chemistry, particularly focusing on recent advancements.

PIDA-promoted metal-free [3 + 2] heteroannulation of β-ketothioamides with 4-hydroxy coumarins: chemo-/regioselective access to furo[3,2-c]chromen-4-ones at room temperature

Herein, we report a viable protocol to access furo[3,2-c]chromen-4-ones by engaging easily accessible 4-hydroxy coumarins as a three-atom CCO unit and thioamides as a C2 coupling partner, mediated by phenyliodine(III) diacetate (PIDA) at room temperature in a highly efficient and pot-/step-economical manner. This strategy not only avoids potential toxicity and tiresome workup conditions, but also features operational simplicity, a broad substrate scope, good functional group tolerance, high yields, easy scalability and exclusive selectivity. A mechanistic study has shown that this metal-free reaction is triggered by PIDA via activation of the β-carbon of 4-hydroxy coumarin, followed by a nucleophilic addition/intramolecular cyclization/dethiohydration cascade. High-resolution mass spectra (HRMS) study confirms the key intermediates involved during the course of the reaction, elucidating the reaction pathways, and demonstrates the excellent regio- and chemoselectivity of this approach.

Visible photons as ideal reagents for the activation of coloured organic compounds

In recent decades, the traceless nature of visible photons has been exploited for the development of efficient synthetic strategies for the photoconversion of colourless compounds, namely, photocatalysis, chromophore activation, and the formation of an electron donor/acceptor (EDA) complex. However, the use of photoreactive coloured organic compounds is the optimal strategy to boost visible photons as ideal reagents in synthetic protocols. In view of such premises, the present review aims to provide its readership with a collection of recent photochemical strategies facilitated via direct light absorption by coloured molecules. The protocols have been classified and presented according to the nature of the intermediate/excited state achieved during the transformation.

Advances in heterocycle synthesis through photochemical carbene transfer reactions

Heterocyclic skeletons are commonly found in various bioactive molecules and pharmaceutical compounds, making them crucial in areas such as medicinal chemistry, materials science, and the realm of natural product synthesis. In recent years, the rapid advancements of visible light methodologies in organic synthesis have shown promising potential for the development of light-induced carbene transfer reactions. This is particularly significant as most organic molecules do not absorb visible light. Free carbene, known for its high activity, is frequently utilized for insertion reactions or cyclopropanation reactions. This review focuses on the photochemical strategy for the construction of heterocyclic skeletons, specifically highlighting the methods that employ visible light-promoted carbene transfer reactions.

Aroyl-S,N-Ketene Acetals: Luminous Renaissance of a Class of Heterocyclic Compounds

Aroyl-S,N-ketene acetals represent a peculiar class of heterocyclic merocyanines, compounds bearing pronounced and rather short dipoles with great push-pull characteristics that define their rich properties. They are accessible via a wide array of synthetic concepts and procedures, ranging from addition-elimination and condensation procedures up to rearrangement and metal-mediated reactions. With our work from 2020, aroyl-S,N-ketene acetals have been identified as powerful and promising dyes with pronounced and vastly tunable solid-state emission and aggregation-induced emission properties. One characteristic trademark of this class of dye molecules is the level of control that could be exerted, and which was thoroughly explored. Based on these results, the field was opened to extend the system to bi- and multichromophoric systems by the full toolkit of synthetic organic chemistry thus giving access to even more exciting properties and manifolded substance libraries capitalizing on the AIE properties. This review aims at outlining the reaction-based principles that allow for a swift and facile access to aroyl-S,N-ketene acetals, their methodical and structural evolution and the plethora of fluorescence and aggregation properties.

Access to thiazoline and spiro[indoline-3,3'-thiophene] scaffolds via a formal [3 + 2] annulation reaction of crotonate-derived sulfur ylides and β-ketothioamides

A formal [3 + 2] annulation reaction of crotonate-derived sulfur ylides and β-ketothioamides (KTAs) was successfully developed to produce good-to-excellent yields of thiazoline and spiro[indoline-3,3'-thiophene] scaffolds. This transformation is a powerful tool for the synthesis of thiazoline and spiro[indoline-3,3'-thiophene] scaffolds due to its mild reaction conditions, easily accessible starting materials, and broad substrate scope. A large-scale reaction was carried out to ensure the practical applicability of this methodology. Finally, the plausible mechanistic pathway of the developed methodology was investigated.

Access to Functionalized Thiazolothiadiazoles via the Chemoselective Cascade Heteroannulation of Thioamides with Hypervalent Iodine Reagents

We describe herein a chemo- and regioselective cascade annulation between β-ketothioamides and diazo-substituted hypervalent iodine reagents under transition-metal-free and base-free conditions at room temperature. Thus, a divergent construction of fused-heterocyclic scaffold thiazolothiadiazoles has been achieved with the advantages of operational simplicity, scalability, broad substrate compatibility, and mild reaction conditions. This one-pot strategy not only avoids potential toxicity but also broadens the arsenal of synthetic methods to obtain fused N,S-heterocyclic frameworks.

Synergistic Visible Light and Pd-Catalyzed C-H Alkylation of 1-Naphthylamines with α-Diazoesters

The combination of visible light irradiation and Pd-catalysis has been practically employed for the C-H alkylation reactions of naphthylamines and α-diazo esters, leading to the synthesis of α-naphthyl functionalized acetates via C-C bond construction under mild reaction conditions and under solvent-free conditions. The light irradiation has been proven to play a pivotal role in the reactions, probably by promoting the generation of active carbene species from α-diazo esters.

Synthesis of pyrrole disulfides via umpolung of β-ketothioamides

A Na₂CO₃-promoted reaction of β-ketothioamides (KTAs) and cyanoacetates was developed for the synthesis of pyrrole disulfides using air as a green oxidant. This protocol features a broad substrate scope and mild reaction conditions. Preliminary mechanistic studies indicate that the reaction involves a tandem unusual umpolung of KTAs, N-cyclization, tautomerization and oxidative coupling process.

Metal-Free One-Pot Annulative Coupling of 2-Hydroxybenzaldehydes with β-Ketothioamides: Access to Diverse 2-Arylimino-2H-Chromenes

A concise and practical one-pot sustainable approach for expedient synthesis of 2-arylimino-2H-chromenes by two-component cascade [4 + 2] annulative coupling of easily available 2-hydroxybenzaldehydes with β-ketothioamides has been developed in good yields for the first time. Remarkably, metal- and additive-free conditions, use of simple K₂CO₃ as a mild base, open atmosphere, exclusive regioselectivity, step/atom economy, nonhazardous reagents, and easy purification are added characteristics to the strategy. This annulative protocol will not only provide an efficient method to access diverse chromene scaffolds, but also enrich the research domain of β-ketothioamides.

Electrochemical Synthesis of 1,2,3-Thiadiazoles from α-Phenylhydrazones

We have developed an electrochemical approach for the synthesis of fully substituted 1,2,3-thiadiazoles from α-phenylhydrazones at room temperature, which is very challenging and complementary to the conventional thermal reactions. The key step involves anodic oxidation of phenylhydrazone derivatives at a constant current followed by N,S-heterocyclization. The protocol is remarkable in that it is free of a base and free of an external oxidant and can be converted to a gram scale for postsynthetic drug development with functional thiadiazoles. Most importantly, the electrochemical transformation reflected efficient electro-oxidation with an operationally friendly easy procedure with ample functional molecules. Cyclic voltammograms support the mechanism of this electro-oxidative cyclization process.

Divergent Synthesis of Aziridine and Imidazolidine Frameworks under Blue LED Irradiation

We develop a visible light-promoted divergent cycloaddition of α-diazo esters with hexahydro-1,3,5-triazines, leading to a series of aziridine and imidazolidine frameworks in average good yield, by simply changing the reaction media used. It is noteworthy that the reaction occurs under sole visible light irradiation without the need for exogenous photoredox catalysts. More significantly, a reasonable reaction mechanism was proposed on the basis of the control experiments and density functional theory calculation results.

Photo-oxidative Ruthenium(II)-Catalyzed Formal [3 + 2] Heterocyclization of Thioamides to Thiadiazoles

An operationally simple and sustainable one-pot photo-oxidative formal [3 + 2] heterocyclization of β-ketothioamides with aryldiazonium salts catalyzed by Ru(bpy)₃Cl₂ has been realized to provide 2,4-disubstituted 5-imino-1,2,3-thiadiazoles in good to high yields under mild reaction conditions for the first time. The reaction proceeded via an α-phenylhydrazone adduct of thioamides leading to 1,2,3-thiadiazoles via N-S bond formation at room temperature. Notably, the products possess Z-stereochemistry with regard to the exocyclic C═N double bond at the 5-position of the ring.

Blue Light-Promoted Formal [4+1]-Annulation of Diazoacetates with o-Aminoacetophenones: Synthesis of Polysubstituted Indolines and Computational Study

A blue light-promoted formal [4+1]-annulation of diazoacetates with o-aminoacetophenones has been reported, which provides an environmentally friendly method for the synthesis of polysubstituted indoline derivatives in moderate to good yields with excellent diastereoselectivities. Detailed mechanistic studies through density functional theory calculations reveal that the (E)-enol species is the key intermediate in this transformation, and the excellent diastereoselectivity is enabled via H-bonding in the intramolecular Aldol-type addition.

Chemo- and regioselective synthesis of polysubstituted 2-aminothiophenes by the cyclization of gem-dibromo or gem-dichloroalkenes with β-keto tertiary thioamides

A facile and practical method for the synthesis of 2,3,4-trisubstituted 2-aminothiophenes by the cyclization of gem-dibromoalkenes or gem-dichloroalkenes with β-keto tertiary thioamides has been developed. The cyclization reaction proceeded chemoselectively and regioselectively under metal-catalyst-free conditions, providing various structurally diverse 2,3,4-trisubstituted N,N'-dialkyl 2-aminothiophenes in good to excellent yields.