Visible Light Mediated Photocatalytic N-Radical Cascade Reactivity of γ,δ-Unsaturated N-Arylsulfonylhydrazones: A General Approach to Structurally Diverse Tetrahydropyridazines

Recent Advances in Dearomative Partial Reduction of Benzenoid Arenes

Dearomative partial reduction is an extraordinary approach for transforming benzenoid arenes and has been well-known for many decades, as exemplified by the dehydrogenation of Birch reduction and the hydroarylation of Crich addition. Despite its remarkable importance in synthesis, this field has experienced slow progress over the last half-century. However, a revival has been observed with the recent introduction of electrochemical and photochemical methods. In this Minireview, we summarize the recent advancements in dearomative partial reduction of benzenoid arenes, including dihydrogenation, hydroalkylation, arylation, alkenylation, amination, borylation and others. Further, the intriguing utilization of dearomative partial reduction in the synthesis of natural products is also emphasized. It is anticipated that this Minireview will stimulate further progress in arene dearomative transformations.

Photoredox-catalyzed alkylarylation of activated alkenes via a ring-opening/Truce-Smiles rearrangement cascade

A photoredox-catalyzed alkylarylation of activated alkenes via a radical C-C bond cleavage/Truce-Smiles rearrangement cascade is developed. The protocol features mild and redox-neutral conditions, broad substrate scope and excellent functional group compatibility, providing a facile and efficient approach to the long-chain distal keto-amides with all-carbon quaternary centers at the alpha position.

Chalcone-Based Synthesis of Tetrahydropyridazines via Cloke-Wilson-Type Rearrangement-Involved Tandem Reaction between Cyclopropyl Ketones and Hydrazines

A facile and efficient approach for the synthesis of multisubstituted tetrahydropyridazines starting from cyclopropyl ketones and hydrazines has been developed. The transformation is chalcone-based and takes place via a Cloke-Wilson-type rearrangement-involved tandem reaction catalyzed by TfOH in HFIP.

PhI(OAc)2-mediated aminoacyloxylation of β,γ-unsaturated hydrazones using Togni reagent II as an acyloxyl precursor

A metal-free protocol for the direct construction of C(sp2)-N and C-O bonds via a PhI(OAc)2-mediated dehydrogenative aminoacyloxylation of β,γ-unsaturated hydrazones with Togni reagent II is reported. Initiated by the carboxyl-containing species generated in situ from Togni reagent II, this method offers a new solution for regioselective functionalization at a remote site on β,γ-unsaturated hydrazones, thus providing a straightforward method for the synthesis of acyloxyl-substituted pyridazines. This reaction features a broad substrate scope and mild conditions.

Inexpensive and bench stable diarylmethylium tetrafluoroborates as organocatalysts in the light mediated hydrosulfonylation of unactivated alkenes

In this paper, we present the synthetic potential of diarylmethylium tetrafluoroborates as catalysts for the visible light promoted hydrosulfonylation of unactivated alkenes. For the first time, these salts, which are bench stable and easily preparable on a multi-gram scale, were employed as organocatalysts. Interestingly, a catalyst loading of only 1 mol% allowed sulfone products to be efficiently obtained from good-to-excellent yields with high functional-group tolerance and scalability up to 15 mmol of alkene. The mechanistic study, both experimental and computational, presented here, revealed an alternative mechanism for the formation of the key sulfonyl radical. Indeed, the photoactive species was proved not to be the diarylcarbenium salt itself, but two intermediates, a stable S-C adduct and an ion couple, that were formed after its interaction with sodium benzenesulfinate. Upon absorbing light, the ion couple could reach an excited state with a charge-transfer character which gave the fundamental sulfonyl radical. A PCET (proton-coupled electron transfer) closes the catalytic cycle reforming the diarylcarbenium salt.

SO2-Extrusive 1,4-(Het)Aryl Migration: Synthesis of α-Aryl Amides and Related Reactions

(Het)aryl migration has emerged as a key synthetic tool and has particularly been exploited for the synthesis of α-aryl amides. This method overcomes the existing α-arylation methods, which are not always compatible with the introduction of (het)aryl groups possessing bulky or electrophilic substituents. This review focuses on SO2-extrusive (het)aryl migration in the frame of α-aryl amide synthesis. Anion- and radical-mediated transformations are reported, including the synthesis of polycyclic compounds through cascade reactions.

1 Introduction

2 Anionic Aryl Migration

3 Radical Aryl Migration

4 Conclusion

Recent Advances in Transition-Metal-Catalyzed Reactions of N-Tosylhydrazones

N-Tosylhydrazones are highly versatile precursors for in situ carbene formation and are frequently used in metal-catalyzed cross-coupling reactions. Due to their many applications in organic synthesis, including C–C, C–O, C–N, and C–S bond formation, N-tosylhydrazones have recently received much interest. They can be simply synthesized by reacting an aldehyde or ketone with N-tosylhydrazine to produce a solid N-tosylhydrazone, which is a ‘green’ precursor of diazo compounds. Using a suitable metal catalyst, N-tosylhydrazones show versatile substrate scope for the synthesis of substituted diaminopyrroles, chromenopyrazoles, alkenylpyrazoles, benzofuran thioethers, tetrahydropyridazines, sulfur-containing heterocycles, and benzofurans with potent biological activities and even regioselective N-functionalization reactions. Metal-catalyzed reactions of N-tosylhydrazones for the construction of bioactive heterocycles are still highly in demand. Hence, this review focuses on the recent synthetic application of N-tosylhydrazones influenced by different transition metals with notable features like simple workup procedures, gram-scale synthesis, broad substrate scope, multicomponent processes, cyclization, and carbon–heteroatom bond formation.

1 Introduction

2 Applications of N-Tosylhydrazones

3 Conclusion

Visible-light induced dearomatization reactions

Dearomatization reactions provide rapid access to structurally complex three-dimensional molecules from simple aromatic compounds. Plenty of reports have demonstrated their utilities in the synthesis of natural products, medicinal chemistry, and materials science in the last decades. Recently, visible-light mediated photocatalysis has emerged as a powerful tool to promote many kinds of transformations. The dearomatization reactions induced by visible-light have also made significant progress during the past several years. This review provides an overview of visible-light induced dearomatization reactions classified based on the manner in which aromaticity is disrupted.

Strategies to Generate Nitrogen-centered Radicals That May Rely on Photoredox Catalysis: Development in Reaction Methodology and Applications in Organic Synthesis

For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N-X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals.

Inverse Electron-Demand Aza-Diels-Alder Reaction of Cyclic Enamides with 1,2-Diaza-1,3-dienes in Situ Generated from α-Halogeno Hydrazones: Access to Fused Polycyclic Tetrahydropyridazine Derivatives

An efficient inverse electron-demand aza-Diels-Alder reaction of cyclic enamides and 1,2-diaza-1,3-dienes, which could be readily formed in situ from α-halogeno hydrazones and a base, has been successfully developed. With the developed approach, a wide range of fused polycyclic tetrahydropyridazines were smoothly obtained in up to 99% yield under benign reaction conditions. This reaction concept was also extended to acyclic enamide substrates for accessing 1,4,5,6-tetrahydropyridazines. A gram-scale experiment and further derivatizations of the polycyclic tetrahydropyridazine products were also conducted to verify the practicability of the methodology.