A Single Electron Transfer (SET) Approach to C–H Amidation of Hydrazones via Visible-Light Photoredox Catalysis

Visible-light-driven net-1,2-hydrogen atom transfer of amidyl radicals to access β-amido ketone derivatives

Hydrogen atom transfer (HAT) processes provide an important strategy for selective C-H functionalization. Compared with the popularity of 1,5-HAT processes, however, net-1,2-HAT reactions have been reported less frequently. Herein, we report a unique visible-light-mediated net-1,2-HAT of amidyl radicals for the synthesis of β-amido ketone derivatives. Single-electron transfer (SET) to N-aryloxy amides generates nitrogen-centered radicals (N˙), which undergo a rare net-1,2-HAT to form carbon-centered radicals (C˙). The C-centered radicals are then captured by silyl enol ethers on the way to β-amido ketones. A series of β-amido ketone derivatives (33 examples, up to 97% yield) were prepared with good functional group tolerance demonstrating the synthetic utility of this method. Mechanistic studies, including EPR, radical trapping experiments, deuterium labeling and KIE measurements, suggest an intramolecular radical net-1,2-HAT pathway.

Antifungal and anti-biofilm effects of hydrazone derivatives on Candida spp

Worldwide, invasive candidiasis are a burden for the health system due to difficulties to manage patients, to the increasing of the resistance of the current therapeutics and the emergence of naturally resistant species of Candida. In this context, the development of innovative antifungal drugs is urgently needed. During invasive candidiasis, yeast is submitted to many stresses (oxidative, thermic, osmotic) in the human host. In order to resist in this context, yeast develops different strategy, especially the biosynthesis of trehalose. Starting from the 3D structural data of TPS2, an enzyme implicated in trehalose biosynthesis, we identified hydrazone as an interesting scaffold to design new antifungal drugs. Interestingly, our hydrazone derivatives which demonstrate antifungal and anti-biofilm effects on Candida spp., are non-toxic in in vitro and in vivo models (Galleria mellonella).

C-H Trifluoromethylthiolation of aldehyde hydrazones

The selective C-H trifluoromethylthiolation of aldehyde hydrazones afforded interesting fluorinated building blocks, which could be used as a synthetic platform. Starting from readily available (hetero)aromatic and aliphatic hydrazones, the formation of a C-SCF3 bond was achieved under oxidative and mild reaction conditions in the presence of the readily available AgSCF3 salt via a one-pot sequential process (28 examples, up to 91% yield). Mechanistic investigations revealed that AgSCF3 was the active species in the transformation.

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.

Construction of Trisubstituted Hydrazones via Base-Mediated Cascade Condensation N-Alkylation

A practical base-promoted tandem condensation N-alkylation reaction for the formation of trisubstituted hydrazones has been developed employing aldehydes and hydrazines with alkyl halides. Crucially, this reaction successfully overcomes chemoselectivity problems, allowing for the reaction of multiple components in a one-pot manner. Halo- and heterofunctional groups, as well as free hydroxyl and amino groups, are tolerated in this transformation to produce a wide range of trisubstituted hydrazones in good to excellent yields.

N-O Bond Activation by Energy Transfer Photocatalysis

A radical shift toward energy transfer photocatalysis from electron transfer photocatalysis under visible-light photoirradiation is often due to the greener prospects of atom and process economy. Recent advances in energy transfer photocatalysis embrace unique strategies for direct small-molecule activation and sometimes extraordinary chemical bond formation in the absence of additional/sacrificial reagents. Selective energy transfer photocatalysis requires careful selection of substrates and photocatalysts for a perfect match with respect to their triplet energies while having incompatible redox potentials to prevent competitive electron transfer pathways. Substrates containing labile N-O bonds are potential targets for generating reactive key intermediates via photocatalysis to access a variety of functionalized molecules. Typically, the differential electron densities of N and O heteroatoms have been exploited for generation of either N- or O-centered radical intermediates from the functionalized substrates by the electron transfer pathway. However, the latest developments involve direct N-O bond homolysis via energy transfer to generate both N- and O-centered radicals for their subsequent utilization in diverse organic transformations, also in the absence of sacrificial redox reagents. In this Account, we highlight our key contributions in the field of N-O bond activation via energy transfer photocatalysis to generate reactive radical intermediates, with coverage of useful mechanistic insights. More specifically, well-designed N-O bond-containing substrates such as 1,2,4-oxadiazolines, oxime esters, N-indolyl carbonates, and N-enoxybenzotriazoles were successfully utilized in versatile transformations involving selective energy transfer over electron transfer from photocatalysts with high triplet state energy. Direct access to reactive N-, O-, and C-centered (if decarboxylation follows) radical intermediates was achieved for diverse cross-couplings and rearrangement processes. In particular, a variety of open-shell nitrogen reactive intermediates, including N(sp²) and N(sp³) radicals and nitrenes, have been utilized. Notably, diversified transformations of identical substrates have been achieved through careful control of the reaction conditions. 1,2,4-Oxadiazolines were converted into spiro-azolactams through iminyl intermediates in the presence of ¹O₂, benzimidazoles, or sulfoximines with external sulfoxide reagent through triplet nitrene intermediates under inert conditions. Besides, oxime esters underwent either intramolecular C(sp³)-N radical-radical coupling or intermolecular C(sp³)-N radical-radical coupling by a combined energy transfer-hydrogen atom transfer strategy. Furthermore, a series of electrochemical and photophysical experiments as well as computational studies were performed to substantiate the proposed selective energy-transfer-driven reaction pathways. We hope that this Account will serve as a guide for the rational design of selective energy transfer processes through the activation of further labile chemical bonds.

Electrochemical Oxidative C(sp2)-H Amination of Aldehyde Hydrazones with Azoles

A general and highly efficient method for the electrochemical C(sp²)-H amination of aldehyde hydrazones with azoles has been developed. This reaction proceeds under exogenous metal-, catalyst-, and oxidant-free conditions to provide aminated hydrazone derivatives in good to excellent yields. This strategy applies to both aromatic and aliphatic aldehyde hydrazones and tolerates a broad range of functional groups.

Organocatalytic Oxidative C-H Amination of Aldehyde Hydrazones with Azoles at Ambient Temperature

An efficient, metal-free, and direct oxidative amination of aldehyde-derived hydrazones with azoles has been developed using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone as an organocatalyst at ambient temperature. This protocol provides a wide range of aminated hydrazone derivatives in a step and atom economical fashion. The reaction possibly follows a radical mechanism.

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.

Bromine-radical-induced Csp2–H difluoroalkylation of quinoxalinones and hydrazones through visible-light-promoted Csp3–Br bond homolysis

Bromine radicals derived from photo-induced Csp3–Br bond homolysis can mediate H abstraction/imine radical formation from quinoxalinones and hydrazones, which in turn quench the in situ-generated difluoroalkyl radicals to furnish the products.

Copper-catalyzed selective oxidation of hydrazones through C(sp3)-H functionalization

A simple and mild protocol for copper-catalyzed oxidation of hydrazones at the α-position has been reported. Various substrates are compatible, providing the corresponding products in moderate to good yields. This strategy provides an efficient and convenient solution for the synthesis of carbonyl hydrazone. A free radical pathway mechanism is suggested for the transformation.

5-(Diarylimino)- and 5-(sulfoximido)dibenzothiophenium triflates: syntheses and applications as electrophilic aminating reagents

The one-pot synthesis of well-defined 5-(diarylimino) and 5-(sulfoximido)dibenzothiophenium triflates, respectively from diarylimines or sulfoximines, is reported and the structures of a series of these compounds are elucidated by X-ray crystallography. In analogy to their hypervalent I(iii) analogues, the iminoyl and sulfoximidoyl groups of these compounds can be selectively transferred to organic substrates. Specifically, the uncatalyzed imination of thiols or sulfinates proceeds with good yields, while under the mild reaction conditions offered by visible light photoredox catalysis, the radical amination of hydrazones or the sulfoximidation of benzylic, allylic and propargylic C-H bonds takes place satisfactorily.

C(sp2) – H functionalization of aldimines and related compounds: advances and prospects

This is the first systematic review of the most relevant approaches to direct C(sp2)–H bond functionalization of azomethine derivatives. The scope of the applicability of various transformations is analyzed. The review assesses prospects of the application of this functionalization strategy in the multistep synthesis of valuable compounds for use in medicinal chemistry, materials science and related areas.

The bibliography includes 124 references.

Photocatalyzed intermolecular amination for the synthesis of hydrazonamides

A photocatalyzed intermolecular amination strategy for the synthesis of hydrazonamides is reported by a multi-component reaction of β-ketonitriles with N,N-disubstituted hydrazines.

Synthesis of Polysubstituted Pyrroles via Silver-Catalyzed Oxidative Radical Addition of Cyclopropanols to Imines

A silver-catalyzed formal [3 + 2] cycloaddition reaction, with cyclopropanols as a C3 subunit and imines as a two-atom subunit, is developed. The reaction takes place under mild conditions and produces a broad array of polysubstituted pyrroles in medium to high yields. It represents the first example of oxidative radical addition to imines, thus offering a new choice for the direct C-H functionalization of imines.

Photoinitiated Three-Component α-Perfluoroalkyl-β-heteroarylation of Unactivated Alkenes via Electron Catalysis

A visible-light-initiated α-perfluoroalkyl-β-heteroarylation of various alkenes with perfluoroalkyl iodides and quinoxalin-2(1 H)-ones is presented. This three-component radical cascade reaction allows an efficient synthesis of a range of perfluoroalkyl containing quinoxalin-2(1 H)-one derivatives in moderate to excellent yields under mild conditions. Reactions proceed via acidic aminyl radicals that are readily deprotonated to give the corresponding radical anions able to sustain the radical chain as single electron transfer reducing reagents. Hence, the overall cascade classifies as an electron-catalyzed process.

Exploration of C-H Transformations of Aldehyde Hydrazones: Radical Strategies and Beyond

The chemistry of hydrazones has gained great momentum due to their involvement throughout the evolution of organic synthesis. Herein, we discuss the tremendous developments in both the methodology and application of hydrazones. Hydrazones can be recognized not only as synthetic equivalents to aldehydes and ketones but also as versatile synthetic building blocks. Consequently, they can participate in a range of practical synthetic transformations. Furthermore, hydrazone derivatives display a broad array of biological activities and have been widely applied as pharmaceuticals. Owing to the weak directing group effect of simple aldehydes and ketones in C-H bond functionalizations, the C-H bond functionalizations of hydrazones that have been developed in the past five years represent a significant step forward. These novel transformations open a new door to a broader library of functionalized and complex small molecules. Moreover, a wide range of biologically important N-heterocycles (dihydropyrazoles, pyrazoles, indazoles, cinnolines, etc.) can be efficiently synthesized in an atom- and step-economical manner through single, double, or triple C-H bond functionalizations of hydrazones. Both radical C-H functionalizations and transition-metal-catalyzed directing-group strategies have enhanced the synthetic utility of hydrazones in the chemical community because these strategies solve the long-standing challenge of C-H functionalizations adjacent to aldehydes and ketones. We began this study based on our ongoing interest in visible-light photoredox catalysis. Visible-light photoredox catalysis has become a powerful tool in contemporary synthetic chemistry due to its remarkable advantages in sustainability and use of radical chemistry. By exploiting a photoredox-catalyzed aminyl radical polar crossover (ARPC) strategy, we successfully achieved visible-light-induced C(sp²)-H difluoroalkylation, trifluoromethylation, and perfluoroalkylation of aldehyde-derived hydrazones. This intriguing result was later applied in the C(sp²)-H amination of hydrazones and a cascade cyclization reaction for the synthesis of polycyclic compounds. Encouraged by this redox-neutral C-H functionalization of aldehyde hydrazones, we extended the oxidative C-H/P-H cross-coupling method, which represents a novel and efficient method for the synthesis of α-iminophosphine oxides. Furthermore, an elegant [3 + 2] cycloaddition of azides and aldehyde hydrazones for the synthesis of functionalized tetrazoles was advantageously developed during our investigation of the oxidative C(sp²)-H azidation of aldehyde hydrazones with TMSN₃. The sequential C(sp²)-H/C(sp³)-H bond functionalization of aldehyde-derived hydrazones with simple 2,2-dibromo-1,3-dicarbonyls was achieved by employing relay photoredox catalysis, and it provides a novel method of accessing bioactive fused dihydropyrazole derivatives. The notable feature of this approach was further reflected in the formal [4 + 1] annulation of aldehyde-derived N-tetrahydroisoquinoline hydrazones with 2-bromo-1,3-dicarbonyls. To complement these radical C-H functionalization strategies, we recently applied a directing-group strategy in the Rh-catalyzed C(aldehyde)-H functionalization of aldehyde-derived hydrazones for the synthesis of distinctive and bioactive 1H-indazole scaffolds. In summary, this Account presents recent contributions to the exploration, development, mechanistic insights, and synthetic applications of C-H bond functionalizations of aldehyde hydrazones.

Visible light catalyzed synthesis of quinolines from (aza)-Morita–Baylis–Hillman adducts

A mild and efficient protocol for the synthesis of quinoline scaffolds from (aza)-MBH adducts under visible light catalysis has been established.

C(sp2)–H functionalization of aldehyde-derived hydrazones via a radical process

This review is focused on the recent advances in the C(sp²)–H functionalization of aldehyde-derived hydrazones via a radical process. Diverse substituted hydrazones including N-heterocycles are afforded under mild conditions with excellent selectivities. In general, an aminyl radical as the key intermediate is involved during the reaction process.

Domino-Fluorination-Protodefluorination Enables Decarboxylative Cross-Coupling of α-Oxocarboxylic Acids with Styrene via Photoredox Catalysis

Domino-fluorination-protodefluorination decarboxylative cross-coupling of α-keto acids with styrene has been developed via photoredox catalysis. The critical part of this strategy is the formation of the carbon-fluorine (C-F) bond by the capture of a carbon-centered radical intermediate, which will overcome side reactions during the styrene radical functionalization process. Experimental studies have provided evidence indicating a domino-fluorination-protodefluorination pathway with α-keto acid initiating the photoredox cycle. The present catalytic protocol also affords a novel approach for the construction of α,β-unsaturated ketones under mild conditions.

Continuous Flow α-Arylation of N,N-Dialkylhydrazones under Visible-Light Photoredox Catalysis

The first direct α-arylation of aldehyde-derived N,N-dialkylhydrazones with electron deficient aryl and heteroaryl cyanides under visible-light photoredox catalysis has been developed. Structurally complex α,α'-diaryl-N,N-cycloalkylhydrazones were obtained in moderate yields by repetition of the direct arylation protocol. A continuous-flow procedure for the preparation of α-aryl-N,N-dialkylhydrazones on a multigram scale has also been established.

[3+2] Cycloaddition of azide with aldehyde hydrazone through an aminyl radical-polar crossover strategy

A novel approach to obtain functionalized tetrazoles through an aminyl radical-power crossover strategy is reported.

Synthesis of cyclohexylidenehydrazine-fused polycyclics via a photocatalytic radical cascade reaction of 2-ethynylaldehyde hydrazones

A general and efficient visible-light photoredox-catalyzed cascade annulation of 2-ethynylaldehyde hydrazones with α-bromo-carbonyls for the synthesis of various cyclohexylidenehydrazine-fused polycyclic compounds is described.