A Metal-Free Cycloaddition of α-Diazoacetates with Amino Acid-Derived NHPI Esters for the Facile Synthesis of 1,2,4-Triazoles.

1,2,4-Triazoles are privileged scaffolds for many pharmaceuticals, and methods for structurally diverse compound libraries are of current interest. Here we report an efficient coupling of α-diazoacetates with amino acid-derived alkyl N-hydroxy phthalimide esters, under metal-free conditions involving 1,8-diazabicyclo(5.4.0)undec-7-ene as the base, with which highly functionalized 1,2,4-triazoles can be obtained in excellent yields with remarkable functional group tolerance. Preliminary studies revealed that 1,2,4-triazole 3a exhibits potent inhibition of tyrosinase activities in melanoma B16F10 cell lines, demonstrating promising skin-whitening properties.

High-performance heterometallic photocatalysts afforded by polyoxometalate synthons for efficient H2 production.

MoS2-based materials have emerged as photoelectric semiconductors characterized by a narrow band gap, high capacity for absorbing visible light, and reduced H2 adsorption energy comparable to Pt. These attributes render them appealing for application in photocatalytic hydrogen production. Despite these advantages, the widespread adoption of MoS2-based materials remains hindered by challenges associated with limited exposure to active sites and suboptimal catalytic hydrogen production efficiency. To address these issues, we have designed and synthesized a new class of highly dispersed bimetallic/trimetallic sulfide materials. This was achieved by developing polyoxometalate synthons containing Ni-Mo elements, which were subsequently reacted with thiourea and CdS. The resulting Ni3S2-MoS2 and Ni3S2-MoS2-CdS materials achieve photocatalytic hydrogen production rates of 2770 and 2873 µmol g-1h-1, respectively. Notably, the rate of 2873 µmol g-1h-1 for Ni3S2-MoS2-CdS surpassed triple (3.23 times) the performance of CdS and nearly sextuple (5.77 times) that of single MoS2. These materials outperformed the majority of MoS2-based photocatalysts. Overall, this study introduces a straightforward methodology for synthesizing bimetallic/trimetallic sulfides with enhanced photocatalytic H2 evolution performance. Our findings underscore the potential of transition metal sulfide semiconductors in the realm of photocatalysis and pave the way for the development of more sustainable energy production systems.

Nitrogen doped 1 T/2H mixed phase MoS2/CuS heterostructure nanosheets for enhanced peroxidase activity.

Heteroatom doping and phase engineering are effective ways to promote the catalytic activity of nanoenzymes. Nitrogen-doped 1 T/2H mixed phase MoS2/CuS heterostructure nanosheets N-1 T/2H-MoS2/CuS are prepared by a simple hydrothermal approach using polyoxometalate (POM)-based metal-organic frameworks (MOFs) (NENU-5) as a precursor and urea as nitrogen doping reagent. The XPS spectroscopy (XPS) and Raman spectrum of N-1 T/2H-MoS2/CuS prove the successful N-doping. NENU-5 was used as the template to prepare 1 T/2H-MoS2/CuS with high content of 1 T phase by optimizing the reaction time. The use of urea as nitrogen dopant added to 1 T/2H-MoS2/CuS, resulted in N-1 T/2H-MoS2/CuS with an increase in the content of the 1 T phase from 80 % to 84 % and higher number of defects. N-1 T/2H-MoS2/CuS shows higher peroxidase activity than 1 T/2H-MoS2/CuS and a catalytic efficiency (Kcat/Km) for H2O2 twice as high as that of 1 T/2H-MoS2/CuS. The enhanced catalytic activity has probably been attributed to several reasons: (i) the insertion of urea during the hydrothermal process in the S-Mo-S layer of MoS2, causing an increase in the interlayer spacing and in 1 T phase content, (ii) the replacement of S atoms in MoS2 by N atoms from the urea decomposition, resulting in more defects and more active sites. As far as we know, N-1 T/2H-MoS2/CuS nanosheets have the lowest detection limit (0.16 µm) for the colorimetric detection of hydroquinone among molybdenum disulfide-based catalysts. This study affords a new approach for the fabrication of high-performance nanoenzyme catalysts.

One-Step Synthesis of Hollow CoS2 Spheres Derived from Polyoxometalate-Based Metal-Organic Frameworks with Peroxidase-like Activity.

In this work, hollow CoS2 particles were prepared by a one-step sulfurization strategy using polyoxometalate-based metal-organic frameworks as the precursor. The morphology and structure of CoS2 have been monitored by scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray powder diffraction. The mechanism for the formation of CoS2 is discussed. The reaction time and sulfur content are found to be important factors that affect the morphology and pure phase formation of CoS2, and a hollow semioctahedral morphology of CoS2 with open voids was obtained when the sulfur source was twice as large as the precursor and the reaction time was 24 h. The CoS2 (24 h) particles show an excellent peroxidase-like activity for the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue oxidized (oxTMB) by hydrogen peroxide. The polyoxometalate used as a precursor helps to stabilize oxTMB during catalytic oxidation, forming a stable curve platform for at least 8 min. Additionally, the colorimetric detection of hydroquinone is developed with a low detection limit of 0.42 µM. This research provides a new strategy to design hollow materials with high peroxidase-mimicking activity.

Highly Efficient Photocatalysts: Polyoxometalate Synthons Enable Tailored CdS-MoS2 Morphologies and Enhanced H2 Evolution.

The development of photocatalysts toward highly efficient H2 evolution reactions is a feasible strategy to achieve the effective conversion of solar energy and meet the increasing demand for new energy. To this end, we prepared two different CdS-MoS2 photocatalysts with unique morphologies ranging from hexagonal prisms to tetragonal nanotubes by carefully tuning polyoxometalate synthons. These two photocatalysts, namely, CdS-MoS2-1 and CdS-MoS2-2, both exhibited remarkable photocatalytic efficiency in H2 generation, among which CdS-MoS2-2 showed superior performance. In fact, the best catalytic hydrogen desorption rate of CdS-MoS2-2 is as high as 1815.5 µmol g-1 h-1. Such performance is superior to twice that of single CdS and almost four times that of pure MoS2. This obvious enhancement can be accredited to the highly open nanotube morphology and highly dispersed heterometallic composition of CdS-MoS2-2, which represents an excellent example of the highest noble-metal-free H2 evolution photocatalysts reported so far. Taken together, these findings suggest that the development of highly dispersed heterometallic catalysts is an auspicious route to realize highly efficient conversion of solar energy and that CdS-MoS2-2 represents a major advance in this field.

Cu-Co Dual-Atom Catalysts Supported on Hierarchical USY Zeolites for an Efficient Cross-Dehydrogenative C(sp2)-N Coupling Reaction.

A cross-coupling reaction via the dehydrogenative route over heterogeneous solid atomic catalysts offers practical solutions toward an economical and sustainable elaboration of simple organic substrates. The current utilization of this technology is, however, hampered by limited molecular definition of many solid catalysts. Here, we report the development of Cu-M dual-atom catalysts (where M = Co, Ni, Cu, and Zn) supported on a hierarchical USY zeolite to mediate efficient dehydrogenative cross-coupling of unprotected phenols with amine partners. Over 80% isolated yields have been attained over Cu-Co-USY, which shows much superior reactivity when compared with our Cu1 and other Cu-M analogues. This amination reaction has hence involved simple and non-forceful reaction condition requirements. The superior reactivity can be attributed to (1) the specifically designed bimetallic Cu-Co active sites within the micropore for "co-adsorption-co-activation" of the reaction substrates and (2) the facile intracrystalline (meso/micropore) diffusion of the heterocyclic organic substrates. This study offers critical insights into the engineering of next-generation solid atomic catalysts with complex reaction steps.

Cu-Catalyzed Cross-Electrophilic Coupling of α-Diazoesters with O-Benzoyl Hydroxylamines for the Synthesis of Unnatural N-Alkyl α-Amino Acid Derivatives.

We describe a Cu-catalyzed cross-electrophilic coupling reaction for synthesizing α-amino acid derivatives from α-diazoesters with O-benzoyl hydroxylamines with Cu(OAc)2 as the catalyst and polymethylhydrosilane (PMHS) as the hydride reagent. Excellent functional group compatibilities were demonstrated. With ethyl 2-diazo-3-oxobutanoate as the precursor, a Cu-acetoacetate complex has been characterized by ESI-MS analysis. Results from the radical trap experiments are consistent with the intermediacy of nitrogen-centered radicals. This strategy offers a simple and inexpensive synthesis of α-amino acid derivatives.

Transition Metal-Catalyzed Regioselective Direct C-H Amidation: Interplay between Inner- and Outer-Sphere Pathways for Nitrene Cross-Coupling Reactions.

Catalytic C-N bond cross-coupling reactions have been a subject of fundamental importance in synthetic organic and medicinal chemistry because amides and amines are ubiquitous motifs in natural products, functional materials, and pharmaceuticals. Since the pioneering works of Breslow and Mansuy on the metalloporphyrin-catalyzed direct hydrocarbon amidation using sulfonyliminoiodinane reagents, substantial development has been achieved toward practical and selective amination protocols. Notably, Du Bois's group developed the dirhodium(II,II) carboxylate catalytic system for direct C(sp3)-H amidations via Rh-sulfonyl nitrene intermediates. Yet, this protocol suffers from competitive alkene aziridination and is limited to electron-rich tertiary and ethereal C-H bonds; analogous direct amidation of arenes remained ineffective.This Account discusses our early effort to explore cyclopalladated complexes for ortho-selective C(aryl)-H amidations. While Buchwald-Hartwig amination cannot be directly applied to arenes, effective amidation of the 2-arylpyridines occurred when an external oxidant such as K2S2O8 was employed. Preliminary studies suggested that the amidation may proceed through reactive Pd-nitrene intermediates. Aiming to develop more diversified amidation protocols, we employed nosyloxycarbamates as nitrene precursors for the Pd-catalyzed ortho-amidation of N-pivalanilides. Likewise, we developed the ortho-selective amidation of benzoic acids to produce anthranilic acids, which are versatile precursors for many medicinally valuable heterocycles. In an attempt to expand the C(aryl)-N coupling reactions to amines, we studied the d6 piano-stool Cp*Rh(III) systems [Cp* = pentamethylcyclopentadienyl]. Our work established a sound reaction platform based on the electrophilic aminating reagents including N-chloroamines, hydroxyamides, and N-carboxyhydrazides for effective C(aryl)-N bond formation in aryl-metal complexes.Building upon the metal-nitrene reaction platform, we moved forward to examine γ-lactam synthesis by intramolecular carbonyl nitrene C(sp3)-H insertion. Noted that carbonyl nitrenes are prone to undergo Curtius-type rearrangement to form isocyanate; we found that the π-basic Ru(II) center effectively decomposes dioxazolones to afford the carbonyl nitrene for regioselective γ-C(sp3)-H insertion. With chiral diphenylethylenediamines (dpen) as ligands bearing electron-withdrawing arylsulfonyl substituents, the [(p-cymene)Ru(dpen)] complex catalyzed the decomposition of the dioxazolones to afford chiral γ-lactams by formal carbonyl nitrene C(sp3)-H insertion. Enantioselective nitrene insertion to allylic and propargylic C(sp3)-H bonds was also achieved with remarkable tolerance to the C═C and C≡C bonds. Notably, the selectivity of the [(p-cymene)Ru] system switched to C(aryl)-H bonds to give dihydroquinolinones when l-proline was employed as ligand. Recently, we aimed to address the regiocontrolled amidation of unactivated methylene C-H bonds using NiH catalyst. While tertiary and benzyl C-H bonds can be differentiated by their bond dissociation energies and steric properties, methylene groups making up the hydrocarbon skeleton display similar electronic and steric properties. In this context, we exploited the five-membered nickelacycle formation to terminate the NiH-mediated chain-walk isomerization, and the nickelacycle reacted with dioxazolones to furnish the C(sp3)-N bond at the γ-methylene position.This Account summarizes our contribution to the development of C-N bond cross-coupling reactions via C-H activation. By exploiting the inner-sphere and outer-sphere reaction pathways, we successfully developed regioselective protocols that target C(sp3)-H and C(aryl)-H bonds. The mechanistic underpinning of the selectivity of different C-H bonds and related studies on the affiliated catalytic systems will be discussed.

NiH-catalyzed anti-Markovnikov hydroamidation of unactivated alkenes with 1,4,2-dioxazol-5-ones for the direct synthesis of N-alkyl amides.

The addition of a nitrogen-based functional group to alkenes via a direct catalytic method is an attractive way of synthesizing value-added amides. The regioselective hydroamidation of unactivated alkenes is considered one of the easiest ways to achieve this goal. Herein, we report the NiH-catalyzed anti-Markovnikov intermolecular hydroamidation of unactivated alkenes enabled by using 2,9-dibutylphenathroline (diBuphen) as the ligand. This protocol provides a platform for the direct synthesis of over 90 structurally diverse N-alkyl amides using dioxazolones, which can be easily derived from abundant carboxylic acid feedstocks. This method succeeds for both terminal and internal unactivated alkenes and some natural products. Mechanistic studies including DFT calculations reveal an initial reversible insertion/elimination of the [NiH] to the alkene, followed by the irreversible amidation to furnish the N-alkyl amides. By crossover experiments and deuterium labeling studies, the observed anti-Markovnikov regioselectivities are suggested to be controlled by the sterical environment of the coupling reaction.

Thioether-Directed NiH-Catalyzed Remote γ-C(sp3)-H Hydroamidation of Alkenes by 1,4,2-Dioxazol-5-ones.

A NiH-catalyzed thioether-directed cyclometalation strategy is developed to enable remote methylene C-H bond amidation of unactivated alkenes. Due to the preference for five-membered nickelacycle formation, the chain-walking isomerization initiated by the NiH insertion to an alkene can be terminated at the γ-methylene site remote from the alkene moiety. By employing 2,9-dibutyl-1,10-phenanthroline (L4) as the ligand and dioxazolones as the reagent, the amidation occurs at the γ-C(sp3)-H bonds to afford the amide products in up to 90% yield (>40 examples) with remarkable regioselectivity (up to 24:1 rr).

Ruthenium-Catalyzed Intramolecular Arene C(sp2)-H Amidation for Synthesis of 3,4-Dihydroquinolin-2(1H)-ones.

We report the [Ru(p-cymene)(l-proline)Cl] ([Ru1])-catalyzed cyclization of 1,4,2-dioxazol-5-ones to form dihydroquinoline-2-ones in excellent yields with excellent regioselectivity via a formal intramolecular arene C(sp2)-H amidation. The reactions of the 2- and 4-substituted aryl dioxazolones proceeds initially through spirolactamization via electrophilic amidation at the arene site, which is para or ortho to the substituent. A Hammett correlation study showed that the spirolactamization is likely to occur by electrophilic nitrenoid attack at the arene, which is characterized by a negative ρ value of -0.73.

2,2-difluorovinyl benzoates for diverse synthesis of gem-difluoroenol ethers by Ni-catalyzed cross-coupling reactions.

gem-Difluoroalkene is a bioisostere of carbonyl group for improving bioavailability of drug candidates. Herein we develop structurally diverse 2,2-difluorovinyl benzoates (BzO-DFs) as versatile building blocks for modular synthesis of gem-difluoroenol ethers (44 examples) and gem-difluoroalkenes (2 examples) by Ni-catalyzed cross coupling reactions. Diverse BzO-DFs derivatives bearing sensitive functional groups (e.g., C = C, TMS, strained carbocycles) are readily prepared from their bromodifluoroacetates and bromodifluoroketones precursors using metallic zinc as reductant. With Ni(COD)2 and dppf [1,1'-bis(diphenylphosphino)ferrocene] as catalyst, reactions of BzO-DFs with arylboronic acids and arylmagnesium/alkylzinc reagents afforded the desired gem-difluoroenol ethers and gem-difluoroalkenes in good yields. The Ni-catalyzed coupling reactions features highly regioselective C(vinyl)-O(benzoate) bond activation of the BzO-DFs. Results from control experiments and DFT calculations are consistent with a mechanism involving initial oxidative addition of the BzO-DFs by the Ni(0) complex. By virtue of diversity of the BzO-DFs and excellent functional group tolerance, this method is amenable to late-stage functionalization of multifunctionalized bioactive molecules.

Photoredox Decarboxylative C(sp3)-N Coupling of α-Diazoacetates with Alkyl N-Hydroxyphthalimide Esters for Diversified Synthesis of Functionalized N-Alkyl Hydrazones.

Herein we report a metal-free photocatalytic coupling reaction for the synthesis of structurally and functionally diverse N-alkyl hydrazones from α-diazoacetates and N-alkyl hydroxyphthalimide esters. By employing Rose Bengal as a photocatalyst with yellow LEDs irradiation, over 60 N-alkyl hydrazones were synthesized. Fluorescence quenching analysis and deuterium incorporation experiments reveal that Hantzsch ester serves as both an electron donor and proton source for the reaction. This strategy offers a simple retrosynthetic disconnection for conventionally inaccessible C(sp3)-rich N-alkyl hydrazones.

Ruthenium(II)-Catalyzed Enantioselective γ-Lactams Formation by Intramolecular C-H Amidation of 1,4,2-Dioxazol-5-ones.

We report the Ru-catalyzed enantioselective annulation of 1,4,2-dioxazol-5-ones to furnish γ-lactams in up to 97% yield and 98% ee via intramolecular carbonylnitrene C-H insertion. By employing chiral diphenylethylene diamine (dpen) as ligands bearing electron-withdrawing arylsulfonyl substituents, the reactions occur with remarkable chemo- and enantioselectivities; the competing Curtius-type rearrangement was largely suppressed. Enantioselective nitrene insertion to allylic/propargylic C-H bonds was also achieved with remarkable tolerance to the C═C and C≡C bonds.

[RhIII(Cp*)]-catalyzed arylfluorination of α-diazoketoesters for facile synthesis of α-aryl-α-fluoroketoesters.

Here we describe the Cp*Rh(iii)-catalyzed cascade arylfluorination reactions of α-diazoketoesters with arylboronic acids and N-fluorobenzenesulfonimide for one-pot C(sp3)-C(aryl) and C(sp3)-F bond formation. The arylfluorination reaction can be accomplished with remarkable chemo- and regioselectivity. Our mechanistic investigation showed that the Rh-catalyzed arylfluorination of diazoacetates occurred by (1) transmetalation of arylboronic acids to form an arylrhodium(iii) complex, (2) coupling of diazomalonates with the arylrhodium(iii) complex to form carbene-rhodium, (3) migratory carbene insertion to form a diketonato-rhodium(iii) complex - probably via rearrangement of the putative σ-alkylrhodium(iii) complex, and (4) electrophilic fluorination of the diketonato-rhodium to form the α-aryl-α-fluoromalonates.

Pd(II)-Catalyzed Direct ortho-C-H Acylation of Aromatic Ketones by Oxidative Decarboxylation of α-Oxocarboxylic Acids.

A Pd-catalyzed decarboxylative acylation of aromatic ketones with α-oxocarboxylic acids was developed, and 1,2-diacylbenzenes were formed in up to 90% yield with excellent ortho-selectivity. This work demonstrates the first successful attempt to direct C-H acylation of aromatic ketones without the need for prederivatization to imines. The acylation reaction was inhibited by radical scavengers such as TEMPO, and 2,2,6,6-tetramethylpiperidin-1-yl benzoate, the adduct of TEMPO and a benzoyl radical, has been isolated and characterized. This finding is compatible with the intermediacy of acyl radicals. A mechanism involving the reaction of the palladacyclic complexes of aryl ketones with acyl radicals is proposed.

Cp*Rh(iii)-catalyzed electrophilic amination of arylboronic acids with azo compounds for synthesis of arylhydrazides.

A [Cp*Rh(iii)]-catalyzed electrophilic amination of arylboronic acids with diethyl azodicarboxylate (DEAD) was developed, and arylhydrazides were produced in excellent yields and selectivity. The analogous amination with the arylazocarboxylates afforded the corresponding N,N-diarylhydrazides. The electrophilic amination of arylboronic acids with azocarboxylates proceeds readily under mild conditions with excellent functional group tolerance. Up to 99% yields were obtained. Preliminary mechanistic studies revealed that prior formation of an arylrhodium(iii) intermediate for the azo coupling reaction can be ruled out.

Cp*Rh(III)-Catalyzed Cross-Coupling of Alkyltrifluoroborate with α-Diazomalonates for C(sp(3))-C(sp(3)) Bond Formation.

A Cp*Rh(III)-catalyzed cross-coupling of alkyltrifluoroborate with α-diazomalonates was developed; the C(sp(3))-C(sp(3)) bond coupled products were formed in up to 97% yields. The reaction tolerates some useful functional groups, including ketone, ester, amide, ether, sulfonyl, and thiophene. Electrospray ionization mass spectrometry (ESI-MS) analysis revealed the formation of a distinct molecular species corresponding to σ-alkylrhodium(III) complexes. The successful diazo coupling reaction may be attributed to coordination of the amide group that promotes stability of the alkylrhodium(III) complex through the formation of a five-membered metallacycle.

[Rh(III)(Cp*)]-catalyzed cascade arylation and chlorination of α-diazocarbonyl compounds with arylboronic acids and N-chlorosuccinimide for facile synthesis of α-aryl-α-chloro carbonyl compounds.

A Rh(III)-catalyzed cascade arylation and chlorination of α-diazocarbonyl compounds with arylboronic acids and N-chlorosuccinimide was achieved. The reaction exhibits excellent functional group tolerance on the organoboron and the diazo reagents; the functionalized α-aryl-α-chlorocarbonyl compounds were obtained in up to 86% yields. The cascade reaction should involve migratory carbene insertion of arylrhodium(III) to form some reactive rhodium(III)-diketonate complexes. Its subsequent reaction with N-chlorosuccinmide afforded the α-chlorocarbonyl products.

Rhodium(III)-catalyzed formal oxidative [4 + 1] cycloaddition of benzohydroxamic acids and α-diazoesters. A facile synthesis of functionalized benzolactams.

A Rh(III)-catalyzed oxidative [4 + 1] cycloaddition of benzohydroxamic acids and α-diazoesters is achieved to afford benzolactams in up to 93% yields. With the N-OAc amido moiety as a directing group, the ortho-C-H is selectively functionalized and the catalytic reaction exhibits excellent tolerance to different functional substituents. A notable rhodacyclic complex is isolated and structurally characterized, suggesting that C-H/N-H cyclometallation is a key step in the catalytic cycle.