Photocatalytic Carbosulfonylation/Cyclization of N-Homoallyl and N-Allyl Aldehyde Hydrazones toward Sulfonylated Tetrahydropyridazines and Dihydropyrazoles

Palladium-catalysed alleneamination of γ,δ-unsaturated hydrazones with propargylic acetates: access to tetrahydropyridazines bearing allenes

A highly efficient palladium-catalysed alleneamination of γ,δ-unsaturated hydrazones with propargylic acetates has been developed. This novel method was used to synthesize tetrahydropyridazines bearing allene derivatives, and did so in moderate to good yields. This protocol features a simple catalytic system, broad substrate scope, easy scale-up, and versatile transformations.

Access to C5-Sulfonylated Tetrahydropyridazines via Photoinduced Cascade Sulfonylation-Cyclization of N-Cinnamyl Aldehyde Hydrazones

Direct installation of a sulfonyl functional group into the C5-position of tetrahydropyridazine was achieved using N'-benzylidene-N-cinnamylacetohydrazide as a new building skeleton via the photocatalytic carbosulfonylation/annulation procedure. Various substituted C5-sulfonylated tetrahydropyridazines were obtained in good to excellent yields employing aryl- or alkylsulfonyl chlorides as the sulfonyl radical sources. This reaction was realized through the selective addition of sulfonyl radical to the C2 position of cinnamyl followed by the 6-endo-trig annulation to the hydrazone CH═N bond.

Recent progress in C-S bond formation via electron donor-acceptor photoactivation

Recent advancements in C-S bond formation via electron donor-acceptor (EDA) complex photoactivation have been remarkable. EDA complexes, which are composed of electron donors and acceptors, facilitate C-S bond construction under mild conditions through single-electron transfer events upon visible light irradiation. This review highlights the utilization of various sulfur-containing substrates, including diacetoxybenzenesulfonyl (DABSO), sulfonic acids, sodium sulfinates, sulfonyl chlorides, and thiophenols, in EDA-promoted sulfonylation and thiolation reactions, covering the works published since 2017 to date. These reactions offer novel, environmentally friendly pathways for the synthesis of sulfur-containing compounds.

Energy Transfer (EnT)-Mediated Stereoselective Aryl-Heterofunctionalization of Unactivated Alkynes via Radical Rearrangement

In this study, we present a novel catalyst-free energy transfer mediated radical rearrangement strategy for the aryl-heterofunctionalization of unactivated alkynes, leading to the synthesis of polyfunctional olefins with exceptional stereoselectivity. This innovative approach, driven by visible light, exemplifies green chemistry principles by eliminating the reliance on transition metals, external oxidants, and photocatalysts. The broad applicability of our method is demonstrated through the successful synthesis of a diverse array of compounds, including vinyl sulfones, vinyl selenides, and vinyl sulfides. Preliminary mechanistic insights suggest an energy transfer mechanism, highlighting the efficiency and selectivity of this novel strategy.

Access to 1-aryl-pyrazolin-5-ones via photoinduced chemoselective cyclization of N-methacrylo aldehyde hydrazones

A photocatalytic sulfamoylation/5-endo-trig cyclization of (E)-N'-arylidene-N-phenylmethacrylohydrazides with sulfamoyl chlorides was developed. The chemoselective intramolecular addition of the carbon-centered radical intermediate to the CN bond in the hydrazone motif gave the sulfamoylated pyrazolin-5-one. Besides, sulfonyl chlorides are also suitable reaction partners to access sulfonylated pyrazolin-5-ones. This approach is characterized by mild reaction conditions, broad substrates scope, excellent selectivity and the late-stage modification of drug molecules.

Organic Photoredox Catalytic Difluoroalkylation of Unactivated Olefins to Access Difluoro-Containing Tetrahydropyridazines

Herein, we disclose a readily available phenothiazine derivative as an organocatalyst, which upon excitation with 371 nm light acquires a strongly reducing power and serves to induce the radical cascade difluoromethylation/cyclization reaction of N-homoallylacetohydrazides. A variety of CF2COR-tetrahydropyridazines have been obtained in moderate to excellent yields. This catalytic platform proceeds under metal-free conditions with a wide substrate scope and broad functional group compatibility, which unlocks the new reactivity of phenothiazine derivatives and adds significant synthetic value to N-heterocycles.

Sulfonyl Radical-Induced Regioselective Cyclization of Enamide-Olefin To Form Sulfonylated 6-7-Membered Cyclic Enamines

Remarkable progress has been made in the radical cascade cyclization of heteroaryl- or aryl-tethered alkenes to construct benzene-fused frameworks via the cracking of aryl C-H bonds. In contrast, the radical cascade cyclization of linear dienes through the cracking of vinyl C-H bonds to construct nonbenzene-fused ring frameworks with endocyclic double bonds has significantly lagged behind, and major advances have largely been restricted to the generation of 5-membered heterocycles, such as pyrrolinones. Herein, we report the silver-mediated regioselective sulfonylation-cyclization of linear dienes with sodium sulfinates to form sulfonylated 6- and 7-membered cyclic enamines.

Advances in the synthesis of functionalized tetrahydropyridazines from hydrazones

The tetrahydropyridazine motif is widely present in plenty of natural products and biologically active molecules. Easily prepared from the condensation of carbonyls with hydrazines, hydrazones are versatile synthetic building blocks that are frequently used in organic synthesis. Hydrazones are also utilized in the synthesis of nitrogen-containing molecules, especially nitrogen-containing heterocycles. The presence of the CN-N unit in the product makes hydrazones ideal substrates for the synthesis of tetrahydropyridazine derivatives. Here, in this review, we summarize the recent progress in the construction of variously substituted tetrahydropyridazines from different hydrazone derivatives together with mechanism discussions.

Photocatalytic synthesis of polyfluoroalkylated dihydropyrazoles and tetrahydropyridazines

A photocatalytic trifluoromethylation/cyclization reaction of N-allyl and N-homoallyl aldehyde hydrazones with trifluoromethyl thianthrenium triflate was developed for the synthesis of trifluoromethylated dihydropyrazoles and tetrahydropyridazines. Besides, PhI(O2CCHF2)2 was employed to realize the construction of difluoromethylated dihydropyrazoles and tetrahydropyridazines. These protocols exhibit a broad substrate scope and good functional group tolerance.

Recent advances of 5-endo-trig radical cyclization: promoting strategies and applications

5-endo-trig radical cyclization has long been recognized as one of the most straightforward ways for the construction of densely functionalized five-membered rings. Nevertheless, according to Baldwin's rules, the 5-endo-trig radical cyclization is kinetically disfavored due to stereoelectronic effects and thus usually proceeds via a slow rate, which renders its application a challenging task. In recent years, with the emergence of efficient radical generation methods and effective cyclization strategies, 5-endo-trig radical cyclization has been successfully accelerated to a synthetically useful rate and has been utilized in the access of diverse five-membered carbo- and heterocyclic compounds. This review comprehensively summarizes the methodologies involving the 5-endo-trig radical cyclization process, with particular emphasis on the elucidation of the promoting strategies, which include the polar effect, geometrical constraints, spin delocalization effect, and persistent radical effect. Each of these strategies is discussed in detail, with illustrative examples from recent literature studies to highlight their practical applications and effectiveness. It is anticipated that the in-depth understanding of the 5-endo-trig radical cyclization provided by this review would inspire further innovation of this privileged reaction mode and expand its applications. Moreover, the potent ring-closure-promoting strategies revealed herein would also contribute to achieving other challenges of cyclizations with particular significance for organic synthesis.

Electron Donor-Acceptor Complex Enabled Cyclization/Sulfonylation Cascade of N-Heterocycles with Thianthrenium Salts

We report a visible-light-promoted cyclization/sulfonylation cascade of N-heterocycles with thianthrenium salts using DABSO as the SO2 surrogate. This method features excellent functional group tolerance, wide substrate scope, and late-stage elaboration of bioactive relevant molecules. Mechanistic investigations reveal that the photoactive electron donor-acceptor (EDA) complexes between thianthrenium salts and DABCO are capable of the generation of aryl radicals, which induce the following SO2 insertion by attacking DABSO, thus triggering the key radical cyclization step.

Electrochemical oxidative thioetherification of aldehyde hydrazones with thiophenols

An electrochemically promoted oxidative dehydrogenation cross-coupling reaction between aldehyde hydrazones and thiophenols is demonstrated for the first time, which resulted in a variety of (Z)-thioetherified products in moderate to excellent yields. This strategy can be carried out under an air atmosphere, featuring scalability and excellent stereoselectivity. In addition, the transformation efficiently produces readily recyclable disulfide as a by-product with high yields, which significantly reduces the environmental pollution caused by thioetherification.

Photo-induced phosphorylation/cyclization of N-homoallyl and N-allyl aldehyde hydrazones to access phosphorylated tetrahydropyridazines and dihydropyrazoles

A photocatalytic radical carbophosphorylation/cyclization of N-homoallyl aldehyde hydrazones with phosphine oxides was developed under metal-free conditions, achieving phosphorylated tetrahydropyridazines in yields up to 95%. Phosphorylated dihydropyrazoles were also constructed, by reacting N-allyl aldehyde hydrazones with phosphine oxides under the same conditions.