Recent Advances in Photoinduced Oxidative Cleavage of Alkenes.

Oxidative cleavage of alkenes leading to valuable carbonyl derivatives is a fundamental transformation in synthetic chemistry. In particular, ozonolysis is the mainstream method for the oxidative cleavage of alkenes that has been widely implemented in the synthesis of natural products and pharmaceutically relevant compounds. However, due to the toxicity and explosive nature of ozone, alternative approaches employing transition metals and enzymes in the presence of oxygen and/or strong oxidants have been developed. These protocols are often conducted under harsh reaction conditions that limit the substrate scope. Photochemical approaches can provide milder and more practical alternatives for this synthetically useful transformation. In this review, we outline recent visible-light-promoted oxidative cleavage reactions that involve photocatalytic activation of oxygen via electron transfer and energy transfer. Also, an emerging field featuring visible-light-promoted oxidative cleavage under anaerobic conditions is discussed. The methods highlighted in this review represent a transformative step toward more sustainable and efficient strategies for the oxidative cleavage of alkenes.

Magnetic yolk-shell structured periodic mesoporous organosilica supported palladium as a powerful and highly recoverable nanocatalyst for the reduction of nitrobenzenes.

A novel palladium-loaded yolk-shell structured nanomaterial with magnetite core and phenylene-based periodic mesoporous organosilica (PMO) shell (Fe3O4@YS-Ph-PMO/Pd) nanocatalyst was synthesized for the reduction of nitrobenzenes. The Fe3O4@YS-Ph-PMO/Pd was prepared through cetyltrimethylammonium bromide (CTAB) directed condensation of 1,4-bis(triethoxysilyl)benzene (BTEB) around Fe3O4@silica nanoparticles followed by treatment with palladium acetate. This nanocatalyst was characterized by using Fourier transform infrared (FT-IR) spectroscopy, thermal gravimetric analysis (TGA), low-angle and wide-angle powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) analyses. These analyses showed a magnetic nanomaterial with high chemical and thermal stability for the designed composite. The Fe3O4@YS-Ph-PMO/Pd nanocomposite was employed as a powerful and highly recoverable catalyst in the green reduction of nitroarenes in H2O at room temperature. A variety of nitroarene derivatives were applied as substrate in the presence of 0.9 mol% of Fe3O4@YS-Ph-PMO/Pd catalyst. All nitroarenes were selectively converted to their corresponding amines with high to excellent yields (92-96%) within short reaction times (10-18 min). This catalyst was recovered and reused at least 11 times without significant decrease in efficiency and stability.

Catalytic reduction of nitroarenes by palladium nanoparticles decorated silica@poly(chitosan-N-isopropylacrylamide-methacrylic acid) hybrid microgels.

Conversion of toxic nitroarenes into less toxic aryl amines, which are the most suitable precursors for different types of compounds, is done with various materials which are costly or take more time for this conversion. In this regards, a silica@poly(chitosan-N-isopropylacrylamide-methacrylic acid) Si@P(CS-NIPAM-MAA) Si@P(CNM) core-shell microgel system was synthesized through free radical precipitation polymerization (FRPP) and then fabricated with palladium nanoparticles (Pd NPs) by in situ-reduction method to form Si@Pd-P(CNM) and characterized with XRD, TEM, FTIR, SEM, and EDX. The catalytic efficiency of Si@Pd-P(CNM) hybrid microgels was studied for reduction of 4-nitroaniline (4NiA) under diverse conditions. Different nitroarenes were successfully transformed into their corresponding aryl amines with high yields using the Si@Pd-P(CNM) system as catalyst and NaBH4 as reductant. The Si@Pd-P(CNM) catalyst exhibited remarkable catalytic efficiency and recyclability as well as maintaining its catalytic effectiveness over multiple cycles.

S-scheme homojunction of 0D cubic/2D hexagonal ZnIn2S4 for efficient photocatalytic reduction of nitroarenes.

The targeted conversion of toxic nitroarenes to corresponding aminoarenes presents significant promise in simultaneously addressing environmental pollution concerns and producing value-added fine chemicals. In this study, we synthesize a 0D/2D ZnIn2S4 homojunction (CH-ZnIn2S4) by in situ growth of cubic ZnIn2S4 (C-ZnIn2S4) quantum dots onto the surface of ultrathin hexagonal ZnIn2S4 (H-ZnIn2S4) nanosheets for photocatalytic reduction of nitroarenes to aminoarenes using water as a hydrogen donor. The optimal performance of photocatalytic nitro reduction over the 0D/2D CH-ZnIn2S4 homojunction reaches 96.1% within 20 min of visible light irradiation, which is 2.45 and 1.52 times than that of C-ZnIn2S4 (39.3%) and H-ZnIn2S4 (63.3%), respectively. The improved photocatalytic performance can be attributed to the formation of a step-type S-scheme homojunction, characterized by identity chemical composition and natural lattice matching. The configuration enables continuous band bending and a low energy barrier of charge transportation, benefiting the charge transfer across the interface while maximizing their redox capabilities. Furthermore, the 2D structure of H-ZnIn2S4 nanosheets offers abundant surface sites to immobilize the 0D C-ZnIn2S4 that provides ample exposed active sites with low overpotential for HER, thereby ensuring high hydrogenation reduction activity of nitroarenes. The study is expected to inspire further interest in the reasonable design of homojunction structures for efficient and sustainable photocatalytic redox reactions.

Photoredox/Bismuth Relay Catalysis Enabling Reductive Alkylation of Nitroarenes with Aldehydes.

The effective transition metal-free photoredox/bismuth dual catalytic reductive dialkylation of nitroarenes with benzaldehydes has been reported. The nitroarene reduction through visible light-driven photoredox catalysis was integrated with subsequent reductive dialkylation of anilines under bismuth catalysis to enable the cascade reductive alkylation of nitroarenes with carbonyls. Salient features of this relay catalysis system include mild reaction conditions, no requirement for transition metal catalysts, easy handling, step-economy, and high selectivity.

Zn/Al layered double hydroxide and carboxymethyl cellulose composite beads as support for the catalytic gold nanoparticles and their applications in the reduction of nitroarenes.

Until now, many efficient catalysts have been reported that are used for the reduction of nitroarenes. However, a catalyst reusability is a challenge that is often faced in practical environment. In this report, we designed a hydrogel composite (CMC-LDH), which act as support and making it possible to address this challenge. In this research work, zinc/aluminum based layered double hydroxides (Zn/Al LDH) have been assembled with carboxymethyl cellulose (CMC) to prepare CMC/LDH hydrogel beads. The CMC/LDH hydrogel beads were prepared by the ionotropic gelation method. For CMC/LDH/Au preparation, the already prepared CMC/LDH beads were kept in gold ion (Au3+) solution, and their subsequent reduction with sodium borohydride (NaBH4). For the characterization of the prepared samples different instrumental techniques, such as Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy, and scanning electron microscopy (SEM) were adopted. For the catalytic evaluation of CMC/LDH/Au, it was utilized as a catalyst in 4-NP and 4-NA reduction reactions. The continuity of the reaction was monitored by a UV-visible spectrophotometer. Rate constant (kapp) of 0.48474 min-1 and 0.7486 min-1 were obtained for 4-NP and 4-NA reduction, respectively. The hydrogel beads were recycled and reused for up to five successive cycles without significantly changing their catalytic efficiency.

Bimetallic nanoparticles supported on Ce-BTC for highly efficient and stable reduction of nitroarenes: Towards environmental sustainability.

The development of a catalyst with a consistent and clearly defined crystal structure is crucial for establishing an efficient catalytic performance system. This study focuses on catalyzing the reduction of nitroarenes to amino-derivatives in an aquatic environment at ambient temperature, employing metallic (Au) and bimetallic (Au-Pd or Au-Ag) nanoparticles loaded on a Ce-BTC metal-organic framework using a facile sol-immobilization approach. Diverse analytical instruments, comprising SEM, TEM, XRD, FT-IR, XPS, TGA, and N2 isotherm, have been utilized to characterize the synthesized catalysts. Among the catalysts that were fabricated, Au-Pd@Ce-BTC displayed the maximum catalytic efficacy, offering a rate constant (kapp) of 0.5841 min-1, conversion percentages reaching 99.7%, and a KAF of 116.8 min-1g-1. Moreover, it exhibited remarkable recyclability over five consecutive cycles. This catalyst offers the advantages of operating under ambient reaction conditions and exhibiting tolerance to a broad range of substrates containing various functional moieties. The mechanistic understanding of nitroarene reduction and the factors contributing to the superior activity of Au-Pd/Ce-BTC are explored through spectroscopic and porosity analyses. Spectroscopic measurements indicate that the elevated Auo and Pdo/Pd2+ ratio, increased surface area, and the synergistic collaboration of the bimetallic NPs are key factors contributing to the heightened activity of Au-Pd/Ce-BTC. These findings hold significant appeal from both an industrial and academic standpoint.

Reduction-Specified Coupling Reactions of Nitroarenes by Heterogeneous Cobalt Catalysis.

The in-depth study on reduction-specified coupling reactions of the nitroarenes by heterogeneous cobalt catalysis opens a door for diversified syntheses of functional N-containing molecules. Guided by the structure-function relationship of heterogeneous materials, rational design of nano-catalysts can effectively regulate the routes of organic reactions. Precise transformation of the intermediates generated during the nitroarene reduction with a suitable nano-catalyst is a promising way to develop new tandem reactions, and to synthesize structurally novel compounds that are of difficult access with the conventional approaches.

Interfacial nanoarchitectonics of nickel phosphide supported on activated carbon for transfer hydrogenation of nitroarenes under mild conditions.

Metal phosphides are promising catalysts for hydrogenation reactions due to their unique ability to generate active hydrogen species which are essential for desired reactions. In this work, the hydrogenation potential of nickel phosphide (Ni2P) is explored for the transfer hydrogenation of aromatic nitro compounds using hydrazine hydrate as hydrogen source. The Ni2P was supported on activated carbon (AC) to facilitate highly exposed active reaction sites. The as-synthesized Ni2P-AC catalyst showed excellent catalytic potential for the hydrogenation of nitro compounds to corresponding amines with 100% conversion efficiency and resulted in excellent yields. The reaction conditions were optimized by varying different reaction parameters, such as time, temperature, solvents, catalyst amount and hydrogen sources. The developed reaction protocol is highly selective for nitro compounds having reduction susceptible functional groups like -Cl, -Br, -CHO, etc. The structure-activity relationship of the Ni2P-AC was also examined which suggested that both acidic and basic sites present in Ni2P-AC catalyst plays crucial role in hydrogenation reaction. Besides, an in-depth insight into the reaction mechanism illustrates that the reaction proceeds via N-phenyl hydroxylamine as the reaction intermediate. In addition, decent recyclability and stability of Ni2P-AC catalyst demonstrates its highly versatile nature for potential large-scale applications. The use of highly efficient Ni2P-AC catalyst for hydrogenation reactions can lead the way towards sustainable and effective industrial organic catalysis.

Synergy of Oxygen Vacancies and Base Sites for Transfer Hydrogenation of Nitroarenes on Ceria Nanorods.

CeO2 nanorod based catalysts for the base-free synthesis of azoxy-aromatics via transfer hydrogenation of nitroarenes with ethanol as hydrogen donor have been synthesized and investigated. The oxygen vacancies (Ov ) and base sites are critical for their excellent catalytic properties. The Ov , i.e., undercoordinated Ce cations, serve as the sites to activate ethanol and nitroarenes by lowering the energy barrier to transfer hydrogen from α-Csp3 -H in ethanol to the nitro group coupling it to the redox reactions between Ce3+ and Ce4+ . At the same time, the base sites catalyze the condensation step to selectively produce azoxy-aromatics. The catalytic route opens a much improved way to use non-noble metal oxides without additives for the selective functional group reduction and coupling reactions.

Effective Synthesis of 4-Quinolones by Reductive Cyclization of 2'-Nitrochalcones Using Formic Acid as a CO Surrogate.

4-Quinolones are the structural elements of many pharmaceutically active compounds. Although several approaches are known for their synthesis, the introduction of an aryl ring in position 2 is problematic with most of them. The reductive cyclization of o-nitrochalcones by pressurized CO, catalyzed by ruthenium or palladium complexes, has been previously reported to be a viable synthetic strategy for this aim, but the need for pressurized CO lines and autoclaves has prevented its widespread use. In this paper, we describe the use of the formic acid/acetic anhydride mixture as a CO surrogate, which allows us to perform the reaction in a cheap and commercially available thick-walled glass tube without adding any gaseous reagent. The obtained yields are often high and compare favorably with those previously reported by the use of pressurized CO. The procedure was applied to a three-step synthesis from commercially available and cheap reagents of the alkaloid Graveoline.

Direct Synthesis of Unprotected Indolines Through Intramolecular sp3 C-H Amination Using Nitroarenes as Aryl Nitrene Precursors.

Given the prevalence of molecules containing nitro groups in organic synthesis, innovative methods to expand the reactivity of this functional group are of interest in both industrial and academic settings. In this report, a metal-free intramolecular benzylic sp3 C-H amination is disclosed using aryl nitro compounds as aryl nitrene precursors. Organosilicon reagent N,N'-bis(trimethylsilyl)-4,4'-bipyridinylidene (Si-DHBP) served as an efficient reductant in the transformation, enabling the in situ generation of aryl nitrene species for the direct, metal-free synthesis of unprotected 2-arylindolines from the corresponding nitroarene compounds.

Analogy of the Reactions of Aromatic and Aliphatic π-Electrophiles with Nucleophiles.

The aim of this essay is to disclose the similarity of a great variety of reactions that proceed between nucleophiles and π-electrophiles-both aromatic and aliphatic. These reactions proceed via initial reversible addition, followed by a variety of transformations that are common for the adducts of both aliphatic and aromatic electrophiles. We hope that understanding of this analogy should help to expand the scope of the known reactions and inspire the search for new reactions that were overlooked.

Utilizing Nitroarenes and HCHO to Directly Construct Functional N-Heterocycles by Supported Cobalt/Amino Acid Relay Catalysis.

Due to the generation of multiple intermediates during the nitroarene reduction, precise interception of single one to develop tandem reactions involving both C-C and C-N bond formations still remains a significant challenge. Herein, the relay catalysis of a supported bifunctional cobalt catalyst with l-proline has been successfully applied to establish a bran-new reductive annulation reaction of nitroarenes and formaldehyde, which enables direct and diverse construction of both symmetrical and unsymmetrical 1,3-diaryl imidazolines. It proceeds with operational simplicity, good substrate and functionality compatibility, and excellent step and atom-efficiency. Mechanistic studies reveal that the Co-catalyst exhibits a synergistic effect on the formation of key N-hydroxy imine, and the l-proline subsequently facilitates the key C-C bond formation. The current work opens a door to develop useful transformations with nitroarenes by reduction-interrupted strategy.

Transition Metal-free Selenium-mediated Aryl Amines via Reduction of Nitroarenes.

A scalable and operationally simple on water seleno-mediated reduction of nitroarenes to the respective aryl amines with NaBH4 is described. The reaction proceeds under transition metal-free conditions and is promoted by the formation of Na2 Se, which is the effective reducing agent involved in the mechanism. This mechanistic information enabled the development of a mild NaBH4 -free protocol for the selective reduction of nitro derivatives bearing labile moieties, including nitrocarbonyl compounds. The selenium-containing aqueous phase can be successfully reused up to four reduction cycles, thus further improving the efficiency of the protocol disclosed.

Ni-Co Alloy Nanoparticles Catalyze Selective Electrochemical Coupling of Nitroarenes into Azoxybenzene Compounds in Aqueous Electrolyte.

In theory, electrocatalysts in their metallic forms should be the most stable chemical state under cathodic potentials. It is known that the highly dispersed nanoparticle (NP) types of electrocatalysts often possess higher activity than their bulk counterparts. However, facilely and controllably fabricating well-dispersed nonprecious metal NPs with superior electrocatalytic activity, selectivity, and durability is highly challenging. Here, we report a facile reductive pyrolysis approach to controllably synthesize NiCo alloy NPs confined on the tip of N-doped carbon nanotubes (N-CNTs) from a bimetal-MOF precursor. The electrocatalytic performance of the resultant NiCo@N-CNTs are evaluated by a wide spectrum of nitroarene reductive coupling reactions to produce azoxy-benzenes, a class of precious chemicals for textile, food, cosmetic, and pharmaceutical industries. The superior electrocatalytic stability, full conversion of nitroarenes, >99% selectivities, and >97% faradic efficiencies toward the targeted azoxy-benzene products are readily attainable by NiCo@N-CNTs, attributable to the alloying-induced synergetic effect. The presence of a CNT confinement effect in NiCo@N-CNTs induces high stability. This added to the metallic states of NiCo empowers NiCo@N-CNTs with excellent electrochemical stability under reductive reaction conditions. In an effort to enhance the energy utilization efficiency, we construct a NiCo@N-CNTs||Ni(OH)2/NF two-electrode electrolyzer to simultaneously reduce nitrobenzene at the cathode and 5-hydroxymethylfurfural with >99% yields for both azoxy-benzene and 2,5-furandicarboxylic acid.

Gold Nanoparticles Immobilized in Porous Aromatic Frameworks with Abundant Metal Anchoring Sites as Heterogeneous Nanocatalysts.

Porous aromatic frameworks (PAFs) with rich metal coordination sites are highly effective support materials for gold nanoparticles (AuNPs), which would not only prevent AuNPs agglomeration but also facilitate mass transfer during the catalytic process. In this work, PAF-160, -161, and -162 bearing diphosphine units are synthesized via the Friedel-Crafts alkylation reaction to act as efficient platforms for AuNPs immobilization. These PAFs possess high surface areas (up to 655 m2 g-1) together with excellent stabilities, and the different linkage lengths between P centers allow more scattered and accessible sites for gold coordination. In the resultant Au-PAFs, AuNPs with uniform sizes are stabilized dispersedly. The catalytic performances of these Au-PAFs are monitored by the reduction of 4-nitrophenol (4-NP), and all materials exhibit excellent catalytic activities on the reduction of 4-NP, especially Au-PAF-162 with the apparent rate constant (kapp) up to 0.019 s-1. Additionally, the reductions of various nitroarenes with different functional groups are explored and all Au-PAFs can convert most nitroaromatic derivatives to the corresponding arylamines with high conversions of 99%, in which the reaction mechanism is also proposed. Furthermore, a continuous catalytic device with Au-PAF-160 catalyst is explored, and Au-PAF-160 can convert 1-chloro-4-nitrobenzene, 2,6-dichoronitrobenzene and 1-chloro-2,4-dinitrobenzene into the corresponding amines in sequence in the continuous flow catalytic experiments. This work has enriched the variety of porous materials for noble metal immobilization and promotes their applications in heterogeneous catalysis.

Oxygen Vacancy-Rich Ni-CeO2 Heterojunction Catalyst for Hydrogenating Halogenated Nitroarenes with High Activity and Selectivity.

Halogenated arylamines are important intermediates for the synthesis of dyes, pesticides, herbicides, and other chemicals. One important way to prepare halogenated arylamines is catalytic hydrogenation of halogenated nitroarenes. Ni-based catalysts have been used in the hydrogenation of halogenated nitroarenes but suffer from low activity and dehalogenation side reaction. In this paper, Ni-CeO2/SiO2 heterojunction catalyst with a "raisin-bun" structure was prepared by reverse microemulsion. A built-in electric field and more oxygen vacancies were formed due to electron transfer from Ni to CeO2 as a result of their work function difference. The built-in electric field leads to the heterolytic cleavage of H2, thereby improving the hydrogenation activity. Oxygen vacancies preferentially adsorb and activate nitro groups, inhibiting the dehalogenation side reaction. Through the cooperation of built-in electric field and oxygen vacancy, synchronous enhancement of the activity and selectivity is obtained successfully. This finding provides a new view for the design of non-noble metal-based catalysts with high activity and selectivity.

Denitrative Hydroxylation of Unactivated Nitroarenes.

A one-step method for the conversion of nitroarenes into phenols under operationally simple, transition-metal-free conditions is described. This denitrative functionalization protocol provides a concise and economical alternative to conventional three-step synthetic sequences. Experimental and computational studies suggest that nitroarenes may be substituted by an electron-catalysed radical-nucleophilic substitution (SRN 1) chain mechanism.

Nano-sized and microporous palladium catalyst supported on modified chitosan/cigarette butt composite for treatment of environmental contaminants.

This study reports a versatile process for the fabrication of a microporous heterogeneous palladium nanocatalyst on a novel spherical, biodegradable, and chemically/physically resistant catalyst support consisting of chitosan (CS) and cigarette waste-derived activated carbon (CAC). The physicochemical properties of the microporous Pd-CS-CAC nanocatalyst developed were successfully determined by FTIR, XRD, FE-SEM, TEM, BET, and EDS techniques. TEM studies showed that the average particle size of the synthesized Pd NPs was about 30 nm. The catalytic prowess of microporous Pd-CS-CAC was evaluated in the reduction/decolorization of various nitroarenes (2-nitroaniline (2-NA), 4-nitroaniline (4-NA), 4-nitrophenol (4-NP), and 4-nitro-o-phenylenediamine (4-NPD)) and organic dyes (methyl red (MR), methyl orange (MO), methylene blue (MB), congo red (CR), and rhodamine B (RhB)) in an aqueous medium in the presence of NaBH4 as the reducing agent at room temperature. The catalytic activities were studied by UV-Vis absorption spectroscopy of the supernatant at regular time intervals. The short reaction times, mild reaction conditions, high efficiency (100% conversion), easy separation, and excellent chemical stability of the catalyst due to its heterogeneity and reusability are the advantages of this method. The results of the tests showed that reduction/decolorization reactions were successfully carried out within 10-140 s due to the good catalytic ability of Pd-CS-CAC. Moreover, Pd-CS-CAC was reused for 5 consecutive times with no loss of the initial shape, size, and morphology, confirming that it was a sustainable and robust nanocatalyst.