Rational Utilization of Black Phosphorus Nanosheets to Enhance Palladium-Mediated Bioorthogonal Catalytic Activity for Activation of Therapeutics

Pd-catalyzed chemistry has played a significant role in the growing subfield of bioorthogonal catalysis. However, rationally designing Pd nanocatalysts that show outstanding catalytic activity and good biocompatibility poses a great challenge. Herein, we propose an innovative strategy through exploiting black phosphorous nanosheets (BPNSs) to enhance Pd-mediated bioorthogonal catalytic activity. Firstly, the electron-donor properties of BPNSs enable in situ growth of Pd nanoparticles (PdNPs) on it. Meanwhile, due to the superb capability of reducing Pd^II, BPNSs can act as hard nucleophiles to accelerate the transmetallation in the decaging reaction process. Secondly, the lone pair electrons of BPNSs can firmly anchor PdNPs on their surface via Pd−P bonds. This design endows Pd/BP with the capability to retard tumor growth by activating prodrugs. This work proposes new insights into the design of heterogeneous transition-metal catalysts (TMCs) for bioorthogonal catalysis.     

A Polyoxometalate-Based Pathologically Activated Assay for Efficient Bioorthogonal Catalytic Selective Therapy

Since polyoxometalates (POMs) can undergo reversible multi-electron redox transformations, they have been used to modulate the electronic environment of metal nanoparticles for catalysis. Besides, POMs possess unique electronic structures and acid-responsive self-assembly ability. These properties inspired us to tackle the drawbacks of the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction in biomedical applications, such as low catalytic efficiency and unsatisfactory disease selectivity. Herein, we construct molybdenum (Mo)-based POM nanoclusters doped with Cu (Cu-POM NCs) as a highly efficient bioorthogonal catalyst, which is responsive to pathologicallyacid and H₂S for selective antibiofilm therapy. Leveraging the merits of POMs, the Cu-POM NCs exhibit biofilm-responsive self-assembly behavior, efficient CuAAC-mediated in situ synthesis of antibacterial molecules, and a NIR-II photothermal effect selectively triggered by H₂S in pathogens. The consumption of bacterial H₂S at the pathological site by Cu-POM NCs extremely decreases the number of persisterbacteria, which is conducive to the inhibition of bacterial tolerance and elimination of biofilms. Unlocked at pathological sites and endowed with NIR-II photothermal property, the constructed POM-based bioorthogonal catalytic platform provides new insights into the design of efficient and selective bioorthogonal catalysts for disease therapy.     

A DNA-Gated and Self-Protected Bioorthogonal Catalyst for Nanozyme-Assisted Safe Cancer Therapy

Transition metal catalysts (TMCs) mediated bioorthogonal uncaging catalysis has sparked increasing interest in prodrug activation. However, due to their “always-on” catalytic activity as well as the complex and catalytic-detrimental intracellular environment, the biosafety and therapeutic efficiency of TMCs are unsatisfactory. Herein, a DNA-gated and self-protected bioorthogonal catalyst has been designed by modifying nanozyme-Pd⁰ with highly programmable nucleic acid (DNA) molecules to achieve efficient intracellular drug synthesis for cancer therapy. Monolayer DNA molecules could endow the catalyst with targeting and perform as a gatekeeper to achieve selective prodrug activation within cancer cells. Meanwhile, the prepared graphitic nitrogen-doped carbon nanozyme with glutathione peroxidase (GPx) and catalase (CAT)-like activities could improve the catalytic-detrimental intracellular environment to prevent the catalyst from being inactivated and sensitize the subsequent chemotherapy. Overall, we believe that our work will promote the development of secure and efficient bioorthogonal catalytic systems and provide new insights into novel antineoplastic platforms.     

Ionic‐Liquid‐Grafted Rigid Poly(p‐Phenylene) Microspheres: Efficient Heterogeneous Media for Metal Scavenging and Catalysis

Novel guanidinium ionic liquid‐grafted rigid poly(p‐phenylene) (PPPIL) microspheres have been developed for metal scavenging and catalysis. The noble‐metal nanoparticles supported on the microspheres surface can be used as efficient heterogeneous catalysts. The combination of nanoparticles and ionic liquid fragments on the microsphere surfaces enhance the activity and durability of the catalyst. The PPPIL ? Pd⁰ catalyst has been tested in the Suzuki cross‐coupling reaction, and exhibits much higher catalytic activity than Pd catalysts supported on porous polymer matrices. The PPPIL ? Pd⁰ catalyst can be recycled at least for nine runs without any significant loss of activity. The present approach may, therefore, have potential applications in transition‐metal‐nanocatalyzed reactions.     

C−I‐Selective Cross‐Coupling Enabled by a Cationic Palladium Trimer

While there is a growing interest in harnessing synergistic effects of more than one metal in catalysis, relatively little is known beyond bimetallic systems. This report describes the straightforward access to an air‐stable Pd trimer and presents unambiguous reactivity data of its privileged capability to differentiate C?I over C?Br bonds in C?C bond formations (arylation and alkylation) of polyhalogenated arenes, which typical Pd⁰ and Pd^I‐Pd^I catalysts fail to deliver. Experimental and computational reactivity data, including the first location of a transition state for bond activation by the trimer, are presented, supporting direct trimer reactivity to be feasible.     

Mechanistic Studies on the SCS‐Pincer Palladium(II)‐Catalyzed Tandem Stannylation/Electrophilic Allylic Substitution of Allyl Chlorides with Hexamethylditin and Benzaldehydes

This paper describes a mechanistic study of the SCS‐pincer Pd^II‐catalyzed auto‐tandem reaction consisting of the stannylation of cinnamyl chloride with hexamethylditin, followed by an electrophilic allylic substitution of the primary tandem‐reaction product with 4‐nitrobenzaldehyde to yield homoallylic alcohols as the secondary tandem products. As it turned out, the anticipated stannylation product, cinnamyl trimethylstannane, is not a substrate for the second part of the tandem reaction. These studies have provided insight in the catalytic behavior of SCS‐pincer Pd^II complexes in the auto‐tandem reaction and on the formation and possible involvement of Pd⁰ species during prolonged reaction times. This has led to optimized reaction conditions in which the overall tandem reaction proceeds through SCS‐pincer Pd^II‐mediated catalysis, that is, true auto‐tandem catalysis. Accordingly, this study has provided the appropriate reaction conditions that allow the pincer catalysts to be recycled and reused.     

Chiral Catalysts for Pd0-Catalyzed Enantioselective C−H Activation

In the past few decades, processes that involve transition-metal catalysis have represented a major part of the synthetic chemist′s toolbox. Recently, the interest has shifted from the well-established cross-coupling reactions to C−H bond functionalization, thus making it a current frontier of transition-metal-catalyzed reactions. Constant progress in this field has led to the discovery of enantioselective methods to generate and control various types of stereogenic elements, thereby demonstrating its high value to generate scalemic chiral molecules. The present review is dedicated to enantioselective Pd⁰-catalyzed C−H activation, which may be considered as an evolution of Pd⁰-catalyzed cross-couplings, with a focus on the different chiral ligands and catalysts that enable these transformations.     

Palladium Catalysis for Aerobic Oxidation Systems Using Robust Metal–Organic Framework

Described here is a new and viable approach to achieve Pd catalysis for aerobic oxidation systems (AOSs) by circumventing problems associated with both the oxidation and the catalysis through an all‐in‐one strategy, employing a robust metal–organic framework (MOF). The rational assembly of a Pd^II catalyst, phenanthroline ligand, and Cu^II species (electron‐transfer mediator) into a MOF facilitates the fast regeneration of the Pd^II active species, through an enhanced electron transfer from in situ generated Pd⁰ to Cu^II, and then Cu^I to O₂, trapped in the framework, thus leading to a 10 times higher turnover number than that of the homogeneous counterpart for Pd‐catalyzed desulfitative oxidative coupling reactions. Moreover, the MOF catalyst can be reused five times without losing activity. This work provides the first exploration of using a MOF as a promising platform for the development of Pd catalysis for AOSs with high efficiency, low catalyst loading, and reusability.     

MOF-Stabilized Perfluorinated Palladium Cages Catalyze the Additive-Free Aerobic Oxidation of Aliphatic Alcohols to Acids

Extremely high electrophilic metal complexes, composed by a metal cation and very electron poor σ-donor ancillary ligands, are expected to be privileged catalysts for oxidation reactions in organic chemistry. However, their low lifetime prevents any use in catalysis. Here we show the synthesis of fluorinated pyridine-Pd²⁺ coordinate cages within the channels of an anionic tridimensional metal-organic framework (MOF), and their use as efficient metal catalysts for the aerobic oxidation of aliphatic alcohols to carboxylic acids without any additive. Mechanistic studies strongly support that the MOF-stabilized coordination cage with perfluorinated ligands unleashes the full electrophilic potential of Pd²⁺ to dehydrogenate primary alcohols, without any base, and also to activate O₂ for the radical oxidation to the aldehyde intermediate. This study opens the door to design catalytic perfluorinated complexes for challenging organic transformations, where an extremely high electrophilic metal site is required.     

Cross-linked poly-vinyl polymers versus polyureas as designed supports for catalytically active M nanoclusters: Part II. Pd/cross-linked poly-vinyl polymers versus Pd/EnCat™30NP in mild hydrogenation reactions

Innovative Pd⁰ heterogeneous catalysts were prepared upon using cross-linked, gel-type, functional acrylic polymers as the supports, along a simple route in use in our laboratories since long. The supports were obtained by polyaddition co-polymerization of N,N-dimethylacrylamide with either 2-acrylamido-2-methylpropane sulfonic acid, methacrylic acid or 4-vinylpyridine, and ethylene glycoldimethacrylate (cross-linker). The performance of these catalysts in the hydrogenation of cyclohexene, trans-methylcinnamate and 4-chloro-2-nitroanisole was compared with that of commercial Pd⁰/EnCat 30NP, produced by Reaxa. One of the catalysts (sulfonic resin as the support) behaved very well as far as activity, stability and selectivity are concerned. These results suggest that heterogeneous metal catalysts supported on polyaddition resins could be developed to become interesting materials for technical applications.     

Orthogonal Nanoparticle Catalysis with Organogermanes

Although nanoparticles are widely used as catalysts, little is known about their potential ability to trigger privileged transformations as compared to homogeneous molecular or bulk heterogeneous catalysts. We herein demonstrate (and rationalize) that nanoparticles display orthogonal reactivity to molecular catalysts in the cross‐coupling of aryl halides with aryl germanes. While the aryl germanes are unreactive in L_nPd⁰/L_nPd^II catalysis and allow selective functionalization of established coupling partners in their presence, they display superior reactivity under Pd nanoparticle conditions, outcompeting established coupling partners (such as ArBPin and ArBMIDA) and allowing air‐tolerant, base‐free, and orthogonal access to valuable and challenging biaryl motifs. As opposed to the notoriously unstable polyfluoroaryl‐ and 2‐pyridylboronic acids, the corresponding germanes are highly stable and readily coupled. Our mechanistic and computational studies provide unambiguous support of nanoparticle catalysis and suggest that owing to the electron richness of aryl germanes, they preferentially react by electrophilic aromatic substitution, and in turn are preferentially activated by the more electrophilic nanoparticles.     

Trifluoromethylthiolation of Aryl Iodides and Bromides Enabled by a Bench‐Stable and Easy‐To‐Recover Dinuclear Palladium(I) Catalyst

While palladium catalysis is ubiquitous in modern chemical research, the recovery of the active transition‐metal complex under routine laboratory applications is frequently challenging. Described herein is the concept of alternative cross‐coupling cycles with a more robust (air‐, moisture‐, and thermally‐stable) dinuclear Pd^I complex, thus avoiding the handling of sensitive Pd⁰ species or ligands. Highly efficient C? SCF₃ coupling of a range of aryl iodides and bromides was achieved, and the recovery of the Pd^I complex was accomplished via simple open‐atmosphere column chromatography. Kinetic and computational data support the feasibility of dinuclear Pd^I catalysis. A novel SCF₃‐bridged Pd^I dimer was isolated, characterized by X‐ray crystallography, and verified to be a competent catalytic intermediate.     

High‐Oxidation‐State Palladium Catalysis: New Reactivity for Organic Synthesis

Recent years have seen the rapid development of a new field of palladium catalysis in organic synthesis. This chemistry takes place outside the usually encountered Pd⁰/Pd^II cycles. It is characterized by the presence of strong oxidants, which prevent further palladium(II)‐promoted reactions at a given point of the catalytic cycle by selective metal oxidation. The resulting higher‐oxidation‐state palladium complexes have been used to develop a series of new synthetic transformations that cannnot be realized within conventional palladium catalysis. This type of catalysis by palladium in a higher oxidation state is of significant synthetic potential.     

Plasmonic photothermal catalysis for solar-to-fuel conversion: current status and prospects

Solar-to-fuel conversion through photocatalytic processes is regarded as promising technology with the potential to reduce reliance on dwindling reserves of fossil fuels and to support the sustainable development of our society. However, conventional semiconductor-based photocatalytic systems suffer from unsatisfactory reaction efficiencies due to limited light harvesting abilities. Recent pioneering work from several groups, including ours, has demonstrated that visible and infrared light can be utilized by plasmonic catalysts not only to induce local heating but also to generate energetic hot carriers for initiating surface catalytic reactions and/or modulating the reaction pathways, resulting in synergistically promoted solar-to-fuel conversion efficiencies. In this perspective, we focus primarily on plasmon-mediated catalysis for thermodynamically uphill reactions converting CO₂ and/or H₂O into value-added products. We first introduce two types of mechanism and their applications by which reactions on plasmonic nanostructures can be initiated: either by photo-induced hot carriers (plasmonic photocatalysis) or by light-excited phonons (photothermal catalysis). Then, we emphasize examples where the hot carriers and phonon modes act in concert to contribute to the reaction (plasmonic photothermal catalysis), with special attention given to the design concepts and reaction mechanisms of the catalysts. We discuss challenges and future opportunities relating to plasmonic photothermal processes, aiming to promote an understanding of underlying mechanisms and provide guidelines for the rational design and construction of plasmonic catalysts for highly efficient solar-to-fuel conversion.

Hot carrier activation and photothermal heat can be constructively coupled using plasmonic photothermal catalysts for synergistically promoted solar-to-fuel conversion efficiency.     

Efficient Catalytic Production of Hydrogen Peroxide Using Tin-containing Zeolite Fixed Palladium Nanoparticles with Oxidation Resistance

The metal surfaces tend to be oxidized in air through dissociation of the O−O bond of oxygen to reduce the performances in various fields. Although several ligand modification routes have alleviated the oxidation of bulky metal surfaces, it is still a challenge for the oxidation resistance of small-size metal nanoparticles. Herein, we fixed the small-size Pd nanoparticles in tin-contained MFI zeolite crystals, where the tin acts as an electron donor to efficiently hinder the oxidation of Pd by weakening the adsorption of molecular oxygen and suppressing the O−O cleavage. This oxidation-resistant Pd catalyst exhibited superior performance in directly synthesizing hydrogen peroxide from hydrogen and oxygen, with the productivity of hydrogen peroxide at ≈10,170 mmol g_Pd⁻¹ h⁻¹, steadily outperforming the catalysts tested previously. This work leads to the hypothesis that tin is an electron donor to realize oxidation-resistant Pd within zeolite crystals for efficient catalysis to overcome the limitation of generally supported Pd catalysts and further motivates the use of oxidation-resistant metal nanoparticles in various fields.     

Room‐Temperature Decarboxylative Couplings of α‐Oxocarboxylates with Aryl Halides by Merging Photoredox with Palladium Catalysis

Enabled by merging iridium photoredox catalysis and palladium catalysis, α‐oxocarboxylate salts can be decarboxylatively coupled with aryl halides to generate aromatic ketones and amides at room temperature. DFT calculations suggest that this reaction proceeds through a Pd⁰–Pd^II–Pd^III pathway, in which the Pd^III intermediate is responsible for reoxidizing Ir^II to complete the Ir^III–*Ir^III–Ir^II photoredox cycle.     

Reactions of Pd and Pt Complexes with Molecular Oxygen

Knowledge of exactly how metal complexes react with molecular oxygen is still limited and this has hampered efforts to develop catalysts for oxidation reactions using O₂ as the oxidant and/or oxygen‐atom source. A better understanding of the reactions of different types of metal complexes with O₂ will be of great utility in rational catalyst development. Reactions between molecular oxygen and Pd^0–II and Pt^0–IV complexes are reviewed here.     

Palladium‐Catalyzed Arylation of Olefins by Triarylphosphines via CP Bond Cleavage

C? H Arylation of olefins by triarylphosphines via C? P bond cleavage has been achieved with either Pd⁰ or Pd^II catalysts. A variety of olefins and triarylphosphines are tolerated, and we inferred that both Pd⁰ and Pd^II could function directly without pre‐oxidation or pre‐reduction.     

Merging Pd0/PdII Redox and PdII/PdII Non-redox Catalytic Cycles for the Allylarylation of Electron-Deficient Alkenes

An allylarylation of electron-deficient alkenes with aryl boronates and allylic carbonates has been developed. This method allows access to a wide variety of carbon skeletons from readily available starting materials. Mechanistic studies indicate that this reaction is enabled by a cooperative catalysis based on merging Pd⁰/Pd^II redox and Pd^II/Pd^II non-redox catalytic cycles.     

Palladium–Borane Cooperation: Evidence for an Anionic Pathway and Its Application to Catalytic Hydro‐/Deutero‐dechlorination

Metal–Lewis acid cooperation provides new opportunities in catalysis. In this work, we report a new type of palladium–borane cooperation involving anionic Pd⁰ species. The air‐stable DPB palladium complex 1 (DPB=diphosphine‐borane) was prepared and reacted with KH to give the Pd⁰ borohydride 2 , the first monomeric anionic Pd⁰ species to be structurally characterized. The boron moiety acts as an acceptor towards Pd in 1 via Pd→B interaction, but as a donor in 2 thanks to B‐H‐Pd bridging. This enables the activation of C?Cl bonds and the system is amenable to catalysis, as demonstrated by the hydro‐/deutero‐dehalogenation of a variety of (hetero)aryl chlorides (20 examples, average yield 85 %).