Light‐Induced Tetrazole‐Quinone 1,3‐Dipolar Cycloadditions

Elucidating the N-N and C-N Bond-breaking Mechanism in the Photoinduced Formation of Nitrile Imine

In this study, we revealed the significance of chemical bonding for the photochemically induced mechanism of 2-phenyl tetrazole derivatives generating nitrile imines. The correlated electron localization function shows that the formation of imine nitrile involves two key bond events: (i) the heterolytic C-N breakage taking place in the T₁ state and (ii) the homolytic N-N rupture occurring in the T₂ excited state. In particular, a cation-radical specie results from the C-N cleavage, whereas the N-N rupture creates a biradical resonant form of imine nitrile. Additionally, we noticed that the substantial pair delocalization of the C-C-N bonded structure could play a significant role in the conversion of the biradical imine nitrile into both the propargylic and allenic forms via the T₁ →S₀ deactivation.

Benzylic C(sp3)-H Bonds Play the Dual Role of Starting Material and Oxidation Inhibitor for Hydrazides in the Electrochemical Synthesis of Hydrazones

The electrooxidation of benzylic C(sp³)-H bonds to produce hydrazones as an alternate for conventional pathways has an enormous dignity. Under the aegis of electricity, instead of hazardous metal catalysts and external oxidants, we unveil an electrochemical process for electrooxidation of various benzylic C(sp³)-H bonds in aqueous media in all pH ranges that subsequently produce hydrazones with further reactions. This electrooxidative reaction strategy provides an acceptable condition for synthesizing hydrazones with various functional groups in good efficiency and amenable to gram-scale synthesis. The electrochemical oxidation condition proves an excellent level of compatibility with super cheap electrolyte NaCl for the oxidation of benzylic C(sp³)-H position despite the highly oxidizable hydrazide group remaining intact in the reaction.

A close look into the biological and synthetic aspects of fused pyrazole derivatives

The fusion of pyrazole scaffold with other skeletons creates a class of attractive molecules, demonstrating significant biological and chemical potentiality in the development of medicinal chemistry. Over the past few decades, numerous biologically active molecules featuring fused pyrazole moieties have been excavated and synthesized, some of which represented by sildenafil have been marketed as drugs, and the biological importance together with chemical synthesis strategies of fused pyrazole compounds, including structural modification based on lead compounds, have been steadily progressing. In this review, we focused our attention on the biological importance of fused pyrazoles and highlighted recent progress in the synthesis of this framework over the past 10 years. What' s more, the limitations, challenges, and future prospects were proposed, wishing to provide references for the development of pyrazole fused frameworks in the field of medicinal chemistry. Contents.

Photo Click Reaction of Acylsilanes with Indoles

Light-mediated coupling of acylsilanes with indoles is reported. This photo click reaction occurs under mild conditions (415 nm) mostly in quantitative yield and provides stable silylated N,O-acetals via light mediated siloxycarbene generation with subsequent indole-N-H insertion. We show that this very efficient and fully atom economic coupling process can be applied to conjugate complex systems, as documented by the clicking of carbohydrates with indole alkaloids. The method is also applicable to the conjugation of polymer chains. The linking acetal moiety can be readily cleaved and it is also shown that wavelength-selective coupling and cleavage with acyl silanes bearing a second photoactive moiety is possible. This is documented by a successful polymerization/depolymerization sequence and by a polymer folding/unfolding process.

A straightforward conversion of 1,4-quinones into polycyclic pyrazoles via [3 + 2]-cycloaddition with fluorinated nitrile imines

In-situ-generated N-aryl nitrile imines derived from trifluoroacetonitrile efficiently react with polycyclic 1,4-quinones, yielding fused pyrazole derivatives as the exclusive products. The reactions proceed via the initially formed [3 + 2]-cycloadducts, which undergo spontaneous aerial oxidation to give aromatized heterocyclic products. Only for 2,3,5,6-tetramethyl-1,4-benzoquinone, the expected [3 + 2]-cycloadduct exhibited fair stability and could be isolated in moderate yield (53%). The presented method offers a straightforward access to hitherto little known trifluoromethylated polycyclic pyrazoles. All products were isolated as pale colored solids with medium-intensity absorption maxima in the range of 310-340 nm for naphthoquinone-derived products and low-intensity bands in the visible region (≈400 nm) for the anthraquinone series.

Light-Triggered Click Chemistry

The merging of click chemistry with discrete photochemical processes has led to the creation of a new class of click reactions, collectively known as photoclick chemistry. These light-triggered click reactions allow the synthesis of diverse organic structures in a rapid and precise manner under mild conditions. Because light offers unparalleled spatiotemporal control over the generation of the reactive intermediates, photoclick chemistry has become an indispensable tool for a wide range of spatially addressable applications including surface functionalization, polymer conjugation and cross-linking, and biomolecular labeling in the native cellular environment. Over the past decade, a growing number of photoclick reactions have been developed, especially those based on the 1,3-dipolar cycloadditions and Diels-Alder reactions owing to their excellent reaction kinetics, selectivity, and biocompatibility. This review summarizes the recent advances in the development of photoclick reactions and their applications in chemical biology and materials science. A particular emphasis is placed on the historical contexts and mechanistic insights into each of the selected reactions. The in-depth discussion presented here should stimulate further development of the field, including the design of new photoactivation modalities, the continuous expansion of λ-orthogonal tandem photoclick chemistry, and the innovative use of these unique tools in bioconjugation and nanomaterial synthesis.

Aldehydes as Photoremovable Directing Groups: Synthesis of Pyrazoles by a Photocatalyzed [3+2] Cycloaddition/Norrish Type Fragmentation Sequence

A straightforward methodology for the regioselective synthesis of pyrazoles has been developed by a domino sequence based on a photoclick cycloaddition followed by a photocatalyzed oxidative deformylation reaction. Distinguishing features of this protocol include an unprecedented photoredox-catalyzed Norrish type fragmentation under green-light irradiation and the use of α,β-unsaturated aldehydes as synthetic equivalents of alkynes, where the aldehyde is acting as a novel photoremovable directing group.

Microscopic mechanism of light-induced tetrazole-quinone 1,3-dipolar cycloaddition: a MS-CASPT2 theoretical investigation

Recently, experimentalists have developed a green and efficient method to synthesize pyrazole-fused quinones through light-induced tetrazole-quinone 1,3-dipole cycloadditions. However, the underlying microscopic mechanisms remain to be clarified. In this work, we have employed several electronic structure calculation methods (MS-CASPT2, CASSCF, DFT) to systematically explore the microscopic mechanism of related light-induced reactions and deactivation pathways. Upon excitation with ultraviolet light, one of the original reactants 2-(4-fluorophenyl)-5-phenyl-2H-tetrazole (FPT) reaches its S₁ excited state. After that, due to the ultrahigh energy and the small energy barrier, the FPT molecule breaks the N2–N3 and N4–C5 bonds sequentially, removing the nitrogen atom finally in the S₁ state. Combined with the cleavage of the second N4–C5 bond, the system reaches its conical intersection region and deactivates ultrafast to the ground state, generating the active intermediate ((4-fluorophenyl)diazen-1-ium-1-ylidene) (phenyl)methanide (FPNI). Subsequently, the active intermediate FPIN can react with naphthoquinone in the ground state by overcoming an energy barrier of about 5.7 kcal mol⁻¹, after which the 1-(4-fluorophenyl)-3-phenyl-1H-benzo[f]indazole-4,9(3aH, 9aH)-dione (FP2HQ) is formed. The FP2HQ can be oxidized to obtain the 1-(4-fluorophenyl)-3-phenyl-1H-benzo[f]indazole-4,9-dione (PFQ). Due to the high energy and small barrier, the entire reaction process can easily take place, which ultimately leads to the efficient reaction. Our present work not only explains the experimental mechanism in detail but can also be helpful for the future design of related photoinduced reactions with the aid of theoretical calculations.

The microscopic mechanisms of light-induced tetrazole-quinone 1,3-dipolar cycloaddition are elucidated using high level MS-CASPT2 calculations.

Hemin-Catalyzed Oxidative Phenol-Hydrazone [3+3] Cycloaddition Enables Rapid Construction of 1,3,4-Oxadiazines

Herein, we present a hemin-catalyzed oxidative phenol-hydrazone [3+3] cycloaddition that accommodates a broad spectrum of N-arylhydrazones, a class of less exploited 1,3-dipoles due to their significant Lewis basicity and weak tendency to undergo 1,2-prototropy to form azomethine imines. It renders expedient assembly of diversely functionalized 1,3,4-oxadiazines with excellent atom and step economy. Preliminary mechanistic studies point to the involvement of a one-electron oxidation pathway, which likely differs from the base-promoted aerobic oxidative scenario.