Regulating Efficient and Selective Single-atom Catalysts for Electrocatalytic CO2 Reduction

Anchoring transition metal (TM) atoms on suitable substrates to form single-atom catalysts (SACs) is a novel approach to constructing electrocatalysts. Graphdiyne with sp−sp² hybridized carbon atoms and uniformly distributed pores have been considered as a potential carbon material for supporting metal atoms in a variety of catalytic processes. Herein, density functional theory (DFT) calculations were performed to study the single TM atom anchoring on graphdiyne (TM₁−GDY, TM=Sc, Ti, V, Cr, Mn, Co and Cu) as the catalysts for CO₂ reduction. After anchoring metal atoms on GDY, the catalytic activity of TM₁−GDY (TM=Mn, Co and Cu) for CO₂ reduction reaction (CO₂RR) are significantly improved comparing with the pristine GDY. Among the studied TM₁−GDY, Cu₁−GDY shows excellent electrocatalytic activity for CO₂ reduction for which the product is HCOOH and the limiting potential (U_L) is −0.16 V. Mn₁−GDY and Co₁−GDY exhibit superior catalytic selectivity for CO₂ reduction to CH₄ with U_L of −0.62 and −0.34 V, respectively. The hydrogen evolution reaction (HER) by TM₁−GDY (TM=Mn, Co and Cu) occurs on carbon atoms, while the active sites of CO₂RR are the transition metal atoms . The present work is expected to provide a solid theoretical basis for CO₂ conversion into valuable hydrocarbons.     

Confined Interfacial Synthesis of Highly Crystalline and Ultrathin Graphdiyne Films and Their Applications for N2 Fixation

Graphdiyne (GDY), as a new carbon allotrope, possessing both sp- and sp²-hybridized carbon atoms, has attracted extensive attention due to great application potentials in various fields. To realize a fundamental understanding of the intrinsic properties of GDY, the controllable synthesis of ultrathin and highly crystalline GDY is necessary and challenging. Herein, a confined interfacial synthetic strategy towards highly crystalline ultrathin GDY at the water/oil/organogel interface, with greatly improved control over GDY structures, is reported. The morphology and chemical composition of GDY was characterized accordingly. After loading with gold, the as-prepared hydrophobic Au/GDY system showed excellent performance in the nitrogen reduction reaction, reaching the highest yield of 4.15 μg cm⁻² h⁻¹ with a Faraday efficiency of 1.95 %.     

Plasmonic Silver-Nanoparticle-Catalysed Hydrogen Abstraction from the C(sp3)−H Bond of the Benzylic Cα atom for Cleavage of Alkyl Aryl Ether Bonds

Selective activation of the C(sp³)−H bond is an important process in organic synthesis, where efficiently activating a specific C(sp³)−H bond without causing side reactions remains one of chemistry's great challenges. Here we report that illuminated plasmonic silver metal nanoparticles (NPs) can abstract hydrogen from the C(sp³)−H bond of the C_α atom of an alkyl aryl ether β-O-4 linkage. The intense electromagnetic near-field generated at the illuminated plasmonic NPs promotes chemisorption of the β-O-4 compound and the transfer of photo-generated hot electrons from the NPs to the adsorbed molecules leads to hydrogen abstraction and direct cleavage of the unreactive ether C_β−O bond under moderate reaction conditions (≈90 °C). The plasmon-driven process has certain exceptional features: enabling hydrogen abstraction from a specific C(sp³)−H bond, along with precise scission of the targeted C−O bond to form aromatic compounds containing unsaturated, substituted groups in excellent yields.     

Carboboration-Driven Generation of a Silylium Ion for Vinylic C−F Bond Functionalization by B(C6F5)3 Catalysis

Strong main-group Lewis acids such as silylium ions are known to effectively promote heterolytic C(sp³)−F bond cleavage. However, carrying out the C(sp²)−F bond transformation of vinylic C−F bonds has remained an unmet challenge. Herein, we describe our development of a new and simple strategy for vinylic C−F bond transformation of α-fluorostyrenes with silyl ketene acetals catalyzed by B(C₆F₅)₃ under mild conditions. Our theoretical calculations revealed that a stabilized silylium ion, which is generated from silyl ketene acetals by carboboration, cleaves the C−F bond of α-fluorostyrenes. A comparative study of α-chloro or bromostyrenes demonstrated that our reaction can be applied only to α-fluorostyrenes because the strong silicon-fluorine affinity facilitates an intramolecular interaction of silylium ions with fluorine atom to cleave the C−F bond. A broad range of α-fluorostyrenes as well as a range of silyl ketene acetals underwent this C−F bond transformation.     

Multicomponent Aromatic and Benzylic Mannich Reactions through C−H Bond Activation

Multicomponent Mannich reactions through C−H bond activation are described. These transformations allowed for the straightforward generation of densely substituted benzylic and homo-benzylic amines in good yields. The reaction involves a reaction between two transient species: an organometallic species, generated by transition-metal-catalyzed sp² or sp³ C−H bond activation and an in situ generated imine. The use of an acetal as an aldehyde surrogate was found essential for the reaction to proceed. The process could be successfully applied to Rh^III-catalyzed sp² C−H bond functionalization and extended to Cu^II-catalyzed sp³ C−H bond functionalization.     

C(spn)−X (n=1–3) Bond Activation by Palladium

We have studied the palladium-mediated activation of C(spⁿ)−X bonds (n = 1–3 and X = H, CH₃, Cl) in archetypal model substrates H₃C−CH₂−X, H₂C=CH−X and HC≡C−X by catalysts PdL_n with L_n = no ligand, Cl⁻, and (PH₃)₂, using relativistic density functional theory at ZORA-BLYP/TZ2P. The oxidative addition barrier decreases along this series, even though the strength of the bonds increases going from C(sp³)−X, to C(sp²)−X, to C(sp)−X. Activation strain and matching energy decomposition analyses reveal that the decreased oxidative addition barrier going from sp³, to sp², to sp, originates from a reduction in the destabilizing steric (Pauli) repulsion between catalyst and substrate. This is the direct consequence of the decreasing coordination number of the carbon atom in C(spⁿ)−X, which goes from four, to three, to two along this series. The associated net stabilization of the catalyst–substrate interaction dominates the trend in strain energy which indeed becomes more destabilizing along this same series as the bond becomes stronger from C(sp³)−X to C(sp)−X.     

Transition‐metal‐free Chemoselective Oxidative C−C Coupling of the sp3 C−H Bond of Oxindoles with Arenes and Addition to Alkene: Synthesis of 3‐Aryl Oxindoles,and Benzofuro‐ and Indoloindoles

A transition‐metal (TM)‐free and halogen‐free NaOt Bu‐mediated oxidative cross‐coupling between the sp³ C−H bond of oxindoles and sp² C−H bond of nitroarenes has been developed to access 3‐aryl substituted and 3,3‐aryldisubstituted oxindoles in DMSO at room temperature in a short time. Interestingly, the sp³ C−H bond of oxindoles could also react with styrene under TM‐free conditions for the practical synthesis of quaternary 3,3‐disubstituted oxindoles. The synthesized 3‐oxindoles have also been further transformed into advanced heterocycles, that is, benzofuroindoles, indoloindoles, and substituted indoles. Mechanistic experiments of the reaction suggests the formation of an anion intermediate from the sp³ C−H bond of oxindole by tert ‐butoxide base in DMSO. The addition of nitrobenzene to the in‐situ generated carbanion leads to the 3‐(nitrophenyl)oxindolyl carbanion in DMSO which is subsequently oxidized to 3‐(nitro‐aryl) oxindole by DMSO.     

Insights into the Role of Graphitic Carbon Nitride as a Photobase in Proton-Coupled Electron Transfer in (sp3)C−H Oxygenation of Oxazolidinones

Graphitic carbon nitride (g-CN) is a transition metal free semiconductor that mediates a variety of photocatalytic reactions. Although photoinduced electron transfer is often postulated in the mechanism, proton-coupled electron transfer (PCET) is a more favorable pathway for substrates possessing X−H bonds. Upon excitation of an (sp²)N-rich structure of g-CN with visible light, it behaves as a photobase—it undergoes reductive quenching accompanied by abstraction of a proton from a substrate. The results of modeling allowed us to identify active sites for PCET—the ‘triangular pockets’ on the edge facets of g-CN. We employ excited state PCET from the substrate to g-CN to selectively cleavethe endo-(sp³)C−H bond in oxazolidine-2-ones followed by trapping the radical with O₂. This reaction affords 1,3-oxazolidine-2,4-diones. Measurement of the apparent pK_a value and modeling suggest that g-CN excited state can cleave X−H bonds that are characterized by bond dissociation free energy (BDFE) ≈100 kcal mol⁻¹.     

Palladium‐Catalyzed Carbon–Fluorine and Carbon–Hydrogen Bond Alumination of Fluoroarenes and Heteroarenes

Through serendipitous discovery, a palladium bis(phosphine) complex was identified as a catalyst for the selective transformation of sp²C−F and sp²C−H bonds of fluoroarenes and heteroarenes to sp²C−Al bonds (19 examples, 1 mol % Pd loading). The carbon–fluorine bond functionalization reaction is highly selective for the formation of organoaluminium products in preference to hydrodefluorination products (selectivity=4.4:1 to 27:1). Evidence is presented for a tandem catalytic process in which hydrodefluorination is followed by sp²C−H alumination.     

Loading Copper Atoms on Graphdiyne for Highly Efficient Hydrogen Production

Graphdiyne, as a magical support, can anchor zero valence metal atoms, providing us with an opportunity to develop emerging catalysts with the maximized active sites and selectivity. Herein we report high-performance atom catalysts (ACs), Cu⁰/GDY, by anchoring Cu atoms on graphdiyne (GDY) for hydrogen evolution reaction (HER). The activity and selectivity of this catalyst are obviously superior to that of commercial 20 wt.% Pt/C, and the turnover frequency of 30.52 s⁻¹ is 18 times larger than 20 wt.% Pt/C. Density functional theory (DFT) calculations demonstrate that the strong p-d coupling induced charge compensation leads to the zero valence state of the atomic-scaled transition metal catalyst. Our results show the strong advantages of graphdiyne-anchored metal atom catalysts in the field of electrochemical catalysis and opens up a new direction in the field of electrocatalysis.     

Carbodicarbene: geminal-Bimetallic Coordination in Selective Manner

The reaction of Pd(OAc)₂ with free carbodicarbene (CDC) generates a Pd acetate trinuclear complex 1 via intramolecular C(sp³)−H bond activation at one of the CDC methyl side arms. The solid structure of 1 reveals the capability of CDC to facilitate a double dative bond with two palladium centers in geminal fashion. This is attributed to the chelating mode of CDC, which can frustrate π-conjugation within the CDC framework. Such effect maybe also amplified by ligand-ligand interaction. The formation of other gem-bimetallic Pd−Pd, Pd−Au, and Ni−Au provides further structural evidence for this proof-of-concept in selective installation. Structural analysis is supported by computational calculations based on state-of-the-art energy decomposition analysis (EDA) in conjunction with natural orbitals for chemical valence (NOCV) method.     

Shedding Light on the Diverse Reactivity of NacNacAl with N-Heterocycles

The aluminum(I) compound NacNacAl (NacNac=[ArNC(Me)CHC(Me)NAr]⁻, Ar=2,6-iPr₂C₆H₃, 1 ) shows diverse and substrate-controlled reactivity in reactions with N-heterocycles. 4-Dimethylaminopyridine (DMAP), a basic substrate in which the 4-position is blocked, induces rearrangement of NacNacAl by shifting a hydrogen atom from the methyl group of the NacNac backbone to the aluminum center. In contrast, C−H activation of the methyl group of 4-picoline takes place to produce a species with a reactive terminal methylene. Reaction of 1 with 3,5-lutidine results in the first example of an uncatalyzed, room-temperature cleavage of an sp² C−H bond (in the 4-position) by an Al^I species. Another reactivity mode was observed for quinoline, which undergoes 2,2′-coupling. Finally, the reaction of 1 with phthalazine produces the product of N−N bond cleavage.     

Nickel-Catalyzed C−I-Selective C(sp2)−C(sp3) Cross-Electrophile Coupling of Bromo(iodo)arenes with Alkyl Bromides

Despite several methodologies established for C(sp²)−I selective C(sp²)−C(sp³) bond formations, achieving arene-flanked quaternary carbons by cross-coupling of tertiary alkyl precursors with bromo(iodo)arenes in a C(sp²)−I selective manner is rare. Here we report a general Ni-catalyzed C(sp²)−I selective cross-electrophile coupling (XEC) reaction, in which, beyond 3° alkyl bromides (for constructing arene-flanked quaternary carbons), 2° and 1° alkyl bromides are also demonstrated to be viable coupling partners. Moreover, this mild XEC displays excellent C(sp²)−I selectivity and functional group compatibility. The practicality of this XEC is demonstrated in simplifying the routes to several medicinally relevant and synthetically challenging compounds. Extensive experiments show that the terpyridine-ligated Ni^I halide can exclusively activate alkyl bromides, forming a Ni^I−alkyl complex through a Zn reduction. Attendant density functional theory (DFT) calculations reveal two different pathways for the oxidative addition of the Ni^I−alkyl complex to the C(sp²)−I bond of bromo(iodo)arenes, explaining both the high C(sp²)−I selectivity and generality of our XEC.     

Boron-Copper 1,3-Rearrangement: the New Concept Behind the Boryl Migration from C(sp2) in Alkenyl Boranes to C(sp3)

Regioselective borylcupration of borylated skipped (Z)-dienes generates diborylated alkylcopper species that are involved in an intramolecular stereospecific B/Cu 1,3-rearrangement by migration of Bpin moiety from C(sp²) to C(sp³). DFT mechanistic studies showed that boryl migration occurs through the formation of 4-membered boracycle intermediate with a moderate free-energy barrier. Moreover, the use of KOMe forms stable Lewis base adducts with Bpin moieties that blocks the reaction. Subsequently to the 1,3-boron shift, the in situ electrophilic trapping allows selective C−H, C−C and C−X bonds, followed by intramolecular cross coupling giving access to cyclic functionalized alkylidenecyclohexanes or alkylidenecyclobutanes.     

Synthesis of Substituted 2‐Amino‐1,3‐oxazoles via Copper‐Catalyzed Oxidative Cyclization of Enamines and N,N‐Dialkyl Formamides

A facile synthesis of 2‐amino‐1,3‐oxazoles via Cu^I‐catalyzed oxidative cyclization of enamines and N ,N ‐dialkyl formamides has been developed. The reaction proceeds through an oxidative C−N bond formation, followed by an intramolecular C(sp²)−H bond functionalization/C−O cyclization in one pot. This protocol provides direct access to useful 2‐amino‐1,3‐oxazoles and features protecting‐group‐free nitrogen sources, readily available starting materials, a broad substrate scope and mild reaction conditions.     

Chiral Lithiated Phosphoric Triamides: Structure,Reactivity, and Salt Effects

The theoretical structure of a cyclic phosphoric triamide 3 and of its monolithiated isomers 4 – 6 was calculated by ab initio methods (Fig. 1, Tables 1 and 2). The global minimum in 4 consists of a five-membered Li−C−N−P−O chelate. The intermediates 5 and 6 are, relative to 4 , energetically unfavorable by 15 and 18 kcal mol⁻¹, respectively, due to distortion in order to accommodate the N-complexation of the Li⁺ ions. NMR Investigations (¹H, ¹³C, ³¹P, and ⁷Li) of the lithiated bicyclic phosphoric triamide 1 were performed (Tables 3 – 5). The lithium aminomethanide 2 is characterized by a sp³-hybridized anion supporting Li−C contacts. The anions exist in a monomer-dimer equilibrium in solution (Scheme 2). Trapping reactions of rac- 2 with carbonyl compounds generated the corresponding amino-alcohol derivatives with high diastereoselectivities (Scheme 3, Table 6). A rational for the stereochemical outcome is given (Fig. 4). In the presence of LiBr, a P−N bond cleavage occurred on reaction of rac- 2 with aldehydes, which allowed the synthesis of (1-hydroxylalkyl)phosphonic diamides (Scheme 5, Table 7).     

Fabrication of Multi-pore Structure Cu,N-codoped Porous Carbon-based Catalyst and its Oxygen Reduction Reaction Catalytic Performance for Microbial Fuel Cell

The development of a non-noble metal cathode ORR catalyst with low cost, high activity and high stability has become an inevitable trend in MFC. The purpose of this study is to develop an efficient and stable Cu, N-codoped porous carbons catalysts with multi-pore structure for MFC. Herein, Cu, N-codoped porous carbons materials (Cu−NC−T) with high N content and multi-pore structure were successfully developed by co-pyrolysis with MOF-199 and melamine. By contrast, Cu-doped porous carbon (Cu−C−T) without melamine was synthesized using MOF-199 as template. The results showed that Cu−NC−T possessed a rough octahedral crystal with a unique multi-mesopore structure with pore centers of 3.4 nm and 11.2 nm, respectively. Owing to high N content, abundantly exposed Cu−N_x active sites and the multi-pore structure, Cu−NC−800 had a pronounced electrochemical ORR activity in neutral solution (onset potential and limiting current density were 0.161 V and −6.256 mA ⋅ cm⁻²), which were slightly lower than 20 wt % Pt/C (0.189 V and −6.479 mA ⋅ cm⁻²). Moreover, the MFC with Cu−NC−800 showed a power density of 662.8±3.6 mW ⋅ m⁻², which was higher than that of Cu−C−800 (425.7±3.9 mW ⋅ m⁻²) and was slightly lower than that 20 wt % Pt/C (815.0±6.2 mW ⋅ m⁻²). The output voltage of MFC with Cu−NC−T had no obvious decreasing trend in 30 days, demonstrating that the Cu−NC−T had great stability.     

Controllable Si−C Bond Activation Enables Stereocontrol in the Palladium-Catalyzed [4+2] Annulation of Cyclopropenes with Benzosilacyclobutanes

A novel and unusual palladium-catalyzed [4+2] annulation of cyclopropenes with benzosilacyclobutanes is reported. This reaction occurred through chemoselective Si−C(sp²) bond activation in synergy with ring expansion/insertion of cyclopropenes to form new C(sp²)−C(sp³) and Si−C(sp³) bonds. An array of previously elusive bicyclic skeleton with high strain, silabicyclo[4.1.0]heptanes, were formed in good yields with excellent diastereoselectivity under mild conditions. An asymmetric version of the reaction with a chiral phosphoramidite ligand furnished a variety of chiral bicyclic silaheterocycle derivatives with good enantioselectivity (up to 95.5:4.5 er). Owing to the mild reaction conditions, the good stereoselectivity profile, and the ready availability of the functionalized precursors, this process constitutes a useful and straightforward strategy for the synthesis of densely functionalized silacycles.     

Closing the Cycle as It Begins: Synthesis of ortho-Iodobiaryls via Catellani Reaction

Despite the advances in the field of carbon-halogen bond formation, the straightforward catalytic access to selectively functionalized iodoaryls remains a challenge. Here, we report a one-pot synthesis of ortho-iodobiaryls from aryl iodides and bromides by palladium/norbornene catalysis. This new example of Catellani reaction features the initial cleavage of a C(sp²)−I bond, followed by the key formation of a palladacycle through ortho C−H activation, the oxidative addition of an aryl bromide and the ultimate restoration of the C(sp²)−I bond. A large variety of valuable o-iodobiaryls has been synthesized in satisfactory to good yields and their derivatization have been described too. Beyond the synthetic utility of this transformation, a DFT study provides insights on the mechanism of the key reductive elimination step, which is driven by an original transmetallation between palladium(II)-halides complexes.     

Ligand-Enabled [3+2] Annulation of Aromatic Acids with Maleimides by C(sp3)−H and C(sp2)−H Bond Activation

A palladium-catalyzed [3+2] annulation of substituted benzoic acids with maleimides leading to tricyclic heterocyclic molecules having a free carboxylic group in a high atom- and step-economical manner is described. The reaction proceeds via a dual C−H bond activation such as C(sp³)−H at the benzylic position and C(sp²)−H bond activation at the meta position of substituted aromatics. An external ligand (MPAA) is crucial for the success of present protocol. Further, the decarboxylation and esterification of the free carboxylic acid group of observed products were carried out.