3d Transition Metals for C-H Activation

Mechanistic insights into C(sp2)-H activation in 1-Phenyl-4-vinyl-1H-1,2,3-triazole derivatives: a theoretical study with palladium acetate catalyst

CONTEXT

The activation of C-H bonds is a fundamental process in synthetic organic chemistry, which enables their replacement by highly reactive functional groups. Coordination compounds serve as effective catalysts for this purpose, as they facilitate chemical transformations by interacting with C-H bonds. A comprehensive understanding of the mechanism of activation of this type of bond lays the foundation for the development of efficient protocols for cross-coupling reactions. We explored the activation of C(sp2)-H bonds in 1-Phenyl-4-vinyl-1H-1,2,3-triazole derivatives with CH3, OCH3, and NO2 substituents in the para position of the phenyl ring, using palladium acetate as catalyst. The studied reaction is the first step for subsequent conjugation of the triazoles with naphthoquinones in a Heck-type reaction to create a C-C bond. The basic nitrogen atoms of the 1,2,3-triazole coordinate preferentially with the cationic palladium center to form an activated species. A concerted proton transfer from the terminal vinyl carbon to one of the acetate ligands with low activation energy is the main step for the C(sp2)-H activation. This study offers significant mechanistic insights for enhancing the effectiveness of C(sp2)-H activation protocols in organic synthesis.

METHODS

All calculations were performed using the Gaussian 09 software package and density functional theory (DFT). The structures of all reaction path components were fully optimized using the CAM-B3LYP functional with the Def2-SVP basis set. The optimized geometries were analyzed by computing the second-order Hessian matrix to confirm that the corresponding minimum or transition state was located. To account for solvent effects, the Polarizable Continuum Model of the Integral Equation Formalism (IEFPCM) with water as the solvent was used.

Rhodium-catalyzed three-component C(sp3)/C(sp2)-H activation enabled by a two-fold directing group strategy

Rh-catalyzed three-component C(sp3)/C(sp2)-H activation has been achieved through a two-directing group strategy. This protocol provides a convenient and efficient pathway for the construction of diverse 8-alkyl quinoline derivatives in one-pot. Furthermore, mechanistic studies revealed that the first C-H amidation was significantly faster than the sequential C-H alkylation.

The Unique Role of the Second Coordination Sphere to Unlock and Control Catalysis in Nonheme Fe(II)/2-Oxoglutarate Histone Demethylase KDM2A

Nonheme Fe(II) and 2-oxoglutarate (2OG)-dependent histone lysine demethylases 2A (KDM2A) catalyze the demethylation of the mono- or dimethylated lysine 36 residue in the histone H3 peptide (H3K36me1/me2), which plays a crucial role in epigenetic regulation and can be involved in many cancers. Although the overall catalytic mechanism of KDMs has been studied, how KDM2 catalysis takes place in contrast to other KDMs remains unknown. Understanding such differences is vital for enzyme redesign and can help in enzyme-selective drug design. Herein, we employed molecular dynamics (MD) and combined quantum mechanics/molecular mechanics (QM/MM) to explore the complete catalytic mechanism of KDM2A, including dioxygen diffusion and binding, dioxygen activation, and substrate oxidation. Our study demonstrates that the catalysis of KDM2A is controlled by the conformational change of the second coordination sphere (SCS), specifically by a change in the orientation of Y222, which unlocks the 2OG rearrangement from off-line to in-line mode. The study demonstrates that the variant Y222A makes the 2OG rearrangement more favorable. Furthermore, the study reveals that it is the size of H3K36me3 that prevents the 2OG rearrangement, thus rendering the enzyme inactivity with trimethylated lysine. Calculations show that the SCS and long-range interacting residues that stabilize the HAT transition state in KDM2A differ from those in KDM4A, KDM7B, and KDM6A, thus providing the basics for the enzyme-selective redesign and modulation of KDM2A without influencing other KDMs.

Iron-Catalyzed Allylic C(sp3)-H Silylation: Spin-Crossover-Efficiency-Determined Chemoselectivity

The nuanced role of spin effects remains a critical gap in designing proficient open-shell catalysts. This study elucidates an iron-catalyzed allylic C(sp3)-H silylation/alkyne hydrosilylation reaction, in which the spin state of the open-shell iron catalyst dictates the reaction kinetics and pathway. Specifically, spin crossover led to alkyne hydrosilylation, whereas spin conservation resulted in a novel allylic C(sp3)-H silylation reaction. This chemoselectivity, governed by the spin-crossover efficiency, reveals an unexpected dimension in spin effects and a first in the realm of transition-metal-catalyzed in situ silylation of allylic C(sp3)-H bonds, which had been previously inhibited by the heightened reactivity of alkenes in hydrosilylation reactions. Furthermore, this spin crossover can either accelerate or hinder the reaction at different stages within a single catalytic reaction, a phenomenon scarcely documented. Moreover, we identify a substrate-assisted C-H activation mechanism, a departure from known ligand-assisted processes, offering a fresh perspective on C-H activation strategies.

Electrochemical Allylic C(sp3)-H Isothiocyanation via [3,3]-Sigmatropic Rearrangement

The direct allylic C(sp3)-H functionalization provides a straightforward protocol for the synthesis of valuable molecules. We report herein the first chemo- and site-selective method for allylic C(sp3)-H isothiocyanation of various internal alkenes under mild electrochemical conditions. This method exhibits broad functional group tolerance and excellent selectivity and can be applied for late-stage isothiocyanation of bioactive molecules. Combined experimental and computational studies indicate that the reaction proceeds via an unexpected [3,3]-sigmatropic rearrangement.

RhIII-Catalyzed Direct Heteroarylation of Unactivated C(sp3)-H with N-Heteroaryl Boronates

Disclosed herein is a rhodium(III)-catalyzed direct heteroarylation reaction between unactivated aliphatic C(sp3)-H bonds in 2-alkylpyridines and heteroaryl organoboron reagents. This catalytic protocol is compatible with various heterocyclic boronates containing ortho- and meta-pyridine, pyrazoles, furan, and quinoline with strong coordination capability. The achievement of this methodology provides an efficient route to build new C(sp3)-heteroaryl bonds.

Merging Cobalt-Catalyzed C-H Activation with the Mannich Reaction: A Modular Approach to α-Substituted N-Sulfonyl Amines

A three-component synthesis of α-substituted N-sulfonyl amines from aryl aldehydes, primary sulfonamides, and (hetero)arenes is described. This transformation enables a straightforward and modular synthesis of highly substituted sulfonamide scaffolds in good yields. The direct functionalization of C(sp2)-H bonds via cobalt-catalyzed C-H-activation offers an appealing and atom-economical alternative to classical methods for the synthesis of α-arylated amines such as the Petasis or Mannich-type reactions.

Cr-Catalyzed Intramolecular Arylative Cross-Coupling of Unactivated C-H Bonds with C-Halide Bonds

We report here a chromium-catalyzed intramolecular arylation of unactivated C-H bonds with C-halide bonds under mild conditions. This reaction was enabled by using a low-cost CrCl2 salt as the precatalyst in combination with allylmagnesium bromide and E/Z-mixed 1-halo-2-styrylarenes as substates, providing a strategy for the construction of functionalized phenanthrene compounds without using external ligands.

Divergent Synthesis of Cyclopropanated Tetrahydroquinolines by Tandem Functionalization of Quinoline Derivatives

Here, reported is a new method for divergent synthesis of functionalized tetrahydroquinolines (THQs), featuring a biomedically interesting azabicyclo[4.1.0]heptane core, proceeding with mild conditions, good substrate and functionality tolerance, and operational simplicity. Mechanistic studies suggest that the products are formed via carbonucleophilic 1,4-addition-induced dearomatization of quinolinium salts and intramolecular cyclopropanation with α-halo ketones followed by α-nucleophilic addition with different nucleophiles. The present work lays a foundation to access new N-heterocycles via the dearomative tandem functionalization of azaarenes.

Photoinduced C-H arylation of 1,3-azoles via copper/photoredox dual catalysis

The visible light-induced C-H arylation of azoles has been accomplished by dual-catalytic system with the aid of an inexpensive ligand-free copper(i)-catalyst in combination with a suitable photoredox catalyst. An organic photoredox catalyst, 10-phenylphenothiazine (PTH), was identified as effective, cost-efficient and environmentally-benign alternative to commonly-used, expensive Ir(iii)-based complexes. The method proved applicable for the C-H arylation of various azole derivatives, including oxazoles, benzoxazoles, thiazoles, benzothiazoles as well as more challenging imidazoles and benzimidazoles. Moreover, the derivatization of complex molecules and the gram scale synthesis of the natural product balsoxin reflected the synthetic utility of the developed strategy. Mechanistic studies were indicative of a single electron transfer-based (SET) mechanism with an aryl radical as key intermediate.

Catalyst Design for Rh-Catalyzed Arene and Alkane C-H Borylation: The NHC Affects the Induction Period, and Indenyl is Superior to Cp

In order to establish design criteria for Rh C-H borylation catalysts, analogues of the successful catalyst [Rh(Ind)(SIDipp)(COE)] (Ind = η5-indenyl, SIDipp = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene, and COE = cis-cyclooctene) were synthesized by changing the indenyl and carbene ligands. [RhCp(SIDipp)(COE)] (1) formed alongside the C-C activated, cyclometalated byproduct [RhCp(κ2CAr,Ccarbene-SIDipp')(iPr)] (rac-2; SIDipp' = 1-(6-isopropylphenyl)-3-(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene). Computational modeling of COE dissociation showed that both C-C and C-H activation of the SIDipp aryl group is thermally attainable and reversible under experimental conditions, with the C-C activation products being the more thermodynamically stable species. Oxidative addition of 1 with SiH(OEt)3 gave the Rh silyl hydride [RhCp(H){Si(OEt)3}(SIDipp)] (rac-3). [Rh(Ind)(IDipp)(COE)] (4; IDipp = 1,3-bis(2,6-diisopropylphenyl)-imidazole-2-ylidene), the carbonyl analogue [Rh(Ind)(IDipp)(CO)] (5; νCO = 1940 cm-1, cf. 1944 cm-1 for [Rh(Ind)(SIDipp)(COE)]), and [Rh(Ind)(IMe4)(COE)] (6; IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene) were also characterized, but attempts to synthesize Rh carbene complexes with fluorenyl or 1,2,3,4-tetrahydrofluorenyl ligands were not successful. For the catalytic C-H borylation of benzene using B2pin2, 1 was inactive at 80 °C, and [Rh(Ind)(SIDipp)(COE)] was superior to all other complexes tested due to the shortest induction period. However, the addition of HBpin to precatalyst 4 eliminated the induction period. Catalytic n-alkane C-H borylation using [Rh(Ind)(NHC)(COE)] gave yields of up to 21% alkylBpin, but [RhCp*(C2H4)2] was the better catalyst.

Breaking the Base Barrier: Cu(II)-Mediated C-H Heteroarylation of o-Carboranes with Base-Sensitive Heteroaryl Halides

While cage C-arylation reactions using strong bases are among the most frequently used transformations in carborane chemistry, there has been no general solution to allow for the use of weak bases in the reaction. Moreover, base-metal-promoted C-H heteroarylation with base-sensitive heteroaryl halides remained elusive. Herein, copper-mediated cage C-H (hetero)arylation has been achieved without the need for strong bases, leading to the facile synthesis of a wide range of C-(hetero)arylated carboranes in good to excellent yields with a broad substrate scope and good functional group compatibility.

Palladium Catalysis Enabled Sequential C(sp3)-H/C-C Activation: Access to Vinyl γ-Lactams

A Pd(II)-catalyzed tandem reaction of aliphatic amides with vinylcyclopropanes (VCPs) was accomplished by merging C(sp3)-H and C-C activation. The reaction of VCP revealed alkenylation/cyclization, followed by ring opening via C-C cleavage, delivering vinyl γ-lactams with (E)-selectivity. The role of ligands, site-selectivity, functional group diversity, mechanistic insight, and synthetic utilities are important practical features.

Mechanochemical C-H Arylation and Alkylation of Indoles Using 3 d Transition Metal and Zero-Valent Magnesium

Although the 3 d transition-metal catalyzed C-H functionalization have been extensively employed to promote the formation of valuable carbon-carbon bonds, the persistent problems, including the use of sensitive Grignard reagents and the rigorous operations (solvent-drying, inert gas protection, metal pre-activation and RMgX addition rate control), still leave great room for further development of sustainable methodologies. Herein, we report a mechanochemical technology toward in-situ preparation of highly sensitive organomagnesium reagents, and thus building two general 3 d transition-metal catalytic platforms that enables regioselective arylation and alkylation of indoles with a wide variety of halides (including those containing post transformable functionalities and heteroaromatic rings). This mechanochemical strategy also brings unique reactivity and high step-economy in producing functionalized N-free indole products.

Mechanistic Insights into Rare-Earth-Catalyzed Alternating Copolymerization through C-H Polyaddition of Functionalized Organic Compounds to Unconjugated Dienes

Density functional theory (DFT) calculations have been conducted to elucidate the detailed mechanisms of yttrium-catalyzed C-H polyaddition of 1,4-dimethoxybenzene (DMB) to 1,4-divinylbenzene (DVB). It was computationally determined that DMB not only serves as a substrate but also performs a crucial role as a ligand, stabilizing the catalytically active species and promoting alkene insertion. Side pathways involving Cβ-H activation and C═C continuous insertion were excluded due to steric and electronic factors, respectively, explaining why the reaction occurred efficiently and selectively to give perfectly alternating DMB-DVB polymers. Interestingly, the theoretical prediction of the reactivity of N,N-dimethyl-1,4-phenylenediamine and 2,2'-biethyl-4,4'-bipyridine reveals significant differences in the coordination effects of these substrates, leading to distinct mechanisms, primarily influenced by their steric effects. These findings shed new light on the previously overlooked role of substrate ligand effects in rare-earth-catalyzed step-growth copolymerization reactions.

Palladium-Catalyzed Weak Chelation-Assisted Site-Selective C-H Arylation of N-Aryl Pyridones via 2-fold C-H Activation

Palladium-catalyzed weak chelation-assisted oxidative cross-dehydrogenative coupling of arenes has been accomplished. The use of medicinally important pyridones as the intrinsic directing group, regioselectivity, 2-fold C-H activation, and late-stage modification of bioactive compounds are the important practical features.

Development of novel transition metal-catalyzed synthetic approaches for the synthesis of a dihydrobenzofuran nucleus: a review

The synthesis of dihydrobenzofuran scaffolds bears pivotal significance in the field of medicinal chemistry and organic synthesis. These heterocyclic scaffolds hold immense prospects owing to their significant pharmaceutical applications as they are extensively employed as essential precursors for constructing complex organic frameworks. Their versatility and importance make them an interesting subject of study for researchers in the scientific community. While exploring their synthesis, researchers have unveiled various novel and efficient pathways for assembling the dihydrobenzofuran core. In the wake of extensive data being continuously reported each year, we have outlined the recent updates (post 2020) on novel methodological accomplishments employing the efficient catalytic role of several transition metals to forge dihydrobenzofuran functionalities.

Masked Reactivity of Hydrated Clusters of Monovalent Manganese Ions: Water Insertion versus Nitrous Oxide Activation-A Density Functional Theory Investigation

Previous mass spectrometric (MS) studies demonstrated that singly charged hydration clusters of manganese ions [Mn(H2O)n]+ were, on one hand, highly reactive toward intracluster water insertion but, on the other hand, inert toward nitrous oxide activation. This contrast in reactivity has been rationalized by our present theoretical investigation for the interconversion between the pristine Mn(I) monovalent form as a monatomic ion in [MnI(H2O)n]+ and the oxidized Mn(III) trivalent form as a hydride-hydroxide in [HMnIIIOH(H2O)n-1], as well as their reactivity toward nitrous oxide activation. Our theoretical interpretations are supported with quantum chemical calculations based on density functional theory (DFT), performed systematically for the cluster-size range of n = 1 - 12. Our DFT results show that water insertion is kinetically and thermodynamically favorable for n ≥ 8, suggesting [HMnIIIOH(H2O)n-1]+ is the predominant form, as observed in previous MS experiments. While [MnI(H2O)n]+ is capable of N2O reduction, the process of which is highly exothermic, similar reactions are unfavorable with [HMnIIIOH(H2O)n-1]+, which can only form weakly bound adducts with N2O. This work demonstrates the masking effect of water molecules over the high reactivity of the hydrated Mn(I) center and sheds light on the potential roles of water in transition metal systems.

Site specific redox properties in ligand differentiated di-nickel complexes inspired by the acetyl CoA synthase active site

Bimetallic transition metal complexes with site-specific redox properties offer a versatile platform for understanding electron polarization, intramolecular electron transfer processes, and customizing electronic and magnetic properties that might impact reactivity and catalyst design. Inspired by the dissymmetric nickel sites in the Acetyl CoA Synthase (ACS) Active Site, three new bimetallic Ni(N2S2)-Ni(S2C2R2) complexes based on Ni(N2S2) metalloligand donor synthons, Nid, in mimicry of the nickel site distal to the redox-active iron sulfur cluster of ACS, and nickel dithiolene receiver units, designated as Nip, the nickel proximal to the 4Fe4S cluster, were combined to explore the influence of ligand environment on electronic structure and redox properties of each unit. The combination of synthons gave a matrix of three S-bridged dinickel complexes, characterized by X-ray crystallography, and appropriate spectroscopies. Computational modeling is connected to the electronic characteristics of the nickel donor and receiver units. This study demonstrated the intricacies of identifying sites of electrochemical redox processes, within multi-metallic systems containing non-innocent ligands.

Synthesis, Characterization, and the Effect of Lewis Bases on the Nuclearity of Iron Alkoxide Complexes

Inspired by the potential of alkoxides as weak-field ligands and their ability to bridge, we report herein a series of high-spin iron complexes supported by a bis-alkoxide framework PhDbf. A diiron complex [Fe2(PhDbf)2] (1a) is obtained upon metalation of the ligand, whereas addition of substituted pyridines affords five-coordinate mononuclear iron complexes [(R-Py)2Fe(PhDbf)] (2a-4a, R = H, p-tBu, p-CF3). The potential for nuclearity control of the metal complexes via auxiliary ligands is highlighted by the formation of asymmetric diiron species [(p-CF3-Py)Fe2(PhDbf)2] (5a) and [(m-CF3-Py)Fe2(PhDbf)2] (6a) with trifluoromethyl substituted pyridines, while electron-rich pyridines only produced monomeric species. Electronic properties analysis via UV-vis, electron paramagnetic resonance, 57Fe Mössbauer spectroscopy, and time-dependent density functional theory, along with redox capabilities of these complexes are reported to illustrate the effect of nuclearity on reactivity and the potential of these complexes to access higher oxidation states relevant in oxidative chemistry. Species 1a-5a, [(THF)2Fe(PhDbf)][PF6] (7), [PyFe(PhDbf)Cl] (2b), and [Py2Fe(PhDbf)][PF6] (2c) were characterized via SCXRD. Indirect evidence for the formation of dimeric Fe(III) species (1b, 5b, and 6b) is discussed.

Fluorescent coumarin-alkynes for labeling of amino acids and peptides via manganese(I)-catalyzed C-H alkenylation

The late-stage fluorescent labeling of structurally complex peptides bears immense potential for molecular imaging. Herein, we report on a manganese(I)-catalyzed peptide C-H alkenylation under exceedingly mild conditions with natural fluorophores as coumarin- and chromone-derivatives. The robustness and efficiency of the manganese(I) catalysis regime was reflected by a broad functional group tolerance and low catalyst loading in a resource- and atom-economical fashion.

Sulfur in Waste-Free Sustainable Synthesis: Advancing Carbon-Carbon Coupling Techniques

This review explores the pivotal role of sulfur in advancing sustainable carbon-carbon (C-C) coupling reactions. The unique electronic properties of sulfur, as a soft Lewis base with significant mesomeric effect make it an excellent candidate for initiating radical transformations, directing C-H-activation, and facilitating cycloaddition and C-S bond dissociation reactions. These attributes are crucial for developing waste-free methodologies in green chemistry. Our mini-review is focused on existing sulfur-directed C-C coupling techniques, emphasizing their sustainability and comparing state-of-the-art methods with traditional approaches. The review highlights the importance of this research in addressing current challenges in organic synthesis and catalysis. The innovative use of sulfur in photocatalytic, electrochemical and metal-catalyzed processes not only exemplifies significant advancements in the field but also opens new avenues for environmentally friendly chemical processes. By focusing on atom economy and waste minimization, the analysis provides broad appeal and potential for future developments in sustainable organic chemistry.

Recent Advances in the Nickel-Catalyzed Alkylation of C-H Bonds

Functionalization of C-H bonds has emerged as a powerful strategy for converting inert, nonfunctional C-H bonds into their reactive counterparts. A wide range of C-H bond functionalization reactions has become possible by the catalysis of metals, typically from the second row of transition metals. First-row transition metals can also catalyze C-H functionalization, and they have the merits of greater earth-abundance, lower cost and better environmental friendliness in comparison to their second-row counterparts. C-H bond alkylation is a particularly important C-H functionalization reaction due to its chemical significance and its applications in natural product synthesis. This review covers Ni-catalyzed C-H bond alkylation reactions using alkyl halides and olefins as alkyl sources.

Palladium-Catalyzed C-H Olefination of Imidazo[1,2a] pyridine Carboxamide in Aqueous Ethanol under Oxygen

The advancement of sustainable chemistry and changes in the economy are strongly intertwined. Reaction time, cost savings, moderate temperatures, and generation of the fewest byproducts are frequently achieved by using catalytic processes. Herein, we report the C-H olefination of imidazo[1,2a] pyridine carboxamides with various acrylates in the presence of Pd (OAc)2 with O2 as the oxidant in aqueous ethanol rather than using non-ecofriendly solvents. The C-H activation features most user-friendly reaction conditions, excellent yield as well as plenty substrate scope and applicable for C-H deuteriation of the corresponding heteroarenes with D2O. Experimental mechanistic studies indicate that C-H activation step succeeded after formation of tetra coordinated square planer Pd-substrate adduct.

Ru/O2-Catalyzed Oxidative C-H Activation/Alkyne Annulation Using Quinoline-Functionalized NHC as a Directing and Functionalizable Group

The ruthenium/O2-catalyzed oxidative annulation reaction of imidazo[1,5-a]quinolin-2-ium salts with alkynes via N-heterocyclic carbene-directed C-H activation to obtain π-conjugated fused imidazo[1,5-a]quinolin-2-ium derivatives is reported. Molecular oxygen has been explored as an economic and clean oxidant and an alternative to metal oxidants. The current protocol exhibits a wide range of substrate scope including bioactive (±)-α-tocopherol derivatives. Moreover, most of the annulated products show strong fluorescence properties, indicating their potential for making new light-emitting materials.

Iron-Catalyzed C-H Oxygenation Using Perchlorate Enabled by Secondary Sphere Hydrogen Bonds

Perchlorate (ClO4-) is a groundwater pollutant that is challenging to remediate. We report a strategy to use Fe(II) tris(2-pyridylmethyl)amine (TPA) complexes featuring appended aniline hydrogen bonds (H-bonds) to promote ClO4- reduction. These complexes facilitate oxygen atom transfer from ClO4- to PPh3 and C-H oxygenation reactions of organic substrates. Catalytic reactions using 15 mol % afforded excellent yields for oxygenation of anthracene and cyclic alkyl aromatics, and this methodology tolerates aryl halides as well as heterocycles containing either O, S, or N.

Ruthenium(II)-Catalyzed Late-Stage Incorporation of N-Aryl Triazoles and Tetrazoles with Sulfonium Salts via C-H Activation

The late-stage functionalization of active pharmaceutical ingredients is a key challenge in medicinal chemistry. Furthermore, N-aryl triazoles and tetrazoles are important structural motifs with the potential to boost the activity of diverse drug molecules. Using easily accessible dibenzothiophenium salts for the ruthenium-catalyzed C-H arylation, these scaffolds were introduced into a variety of bioactive compounds. Our methodology uses cost-efficient ruthenium, KOAc as a mild base and gives access to a plethora of highly decorated triazole and tetrazole containing drug derivatives.

Regio- and Diastereoselective Annulation of α,β-Unsaturated Aldimines with Alkenes via Allylic C(sp3)-H Activation by Rare-Earth Catalysts

The [3 + 2] or [4 + 2] annulation of α,β-unsaturated aldimines with alkenes via β'- or γ-allylic C(sp3)-H activation is, in principle, an atom-efficient route for the synthesis of five- or six-membered-ring cycloalkylamines, which are important structural motifs in numerous natural products, bioactive molecules, and pharmaceuticals. However, such a transformation has remained undeveloped to date probably due to the lack of suitable catalysts. We report herein for the first time the regio- and diastereoselective [3 + 2] and [4 + 2] annulations of α,β-unsaturated imines with alkenes via allylic C(sp3)-H activation by half-sandwich rare-earth catalysts having different metal ion sizes. The reaction of α-methyl-substituted α,β-unsaturated aldimines with alkenes by a C5Me4SiMe3-ligated scandium catalyst took place in a trans-diastereoselective [3 + 2] annulation fashion via C(sp3)-H activation at the α-methyl group (β'-position), exclusively affording alkylidene-functionalized cyclopentylamines with excellent trans-diastereoselectivity. In contrast, the reaction of β-methyl-substituted α,β-unsaturated aldimines with alkenes by a C5Me5-ligated cerium catalyst proceeded in a cis-diastereoselective [4 + 2] annulation fashion via γ-allylic C(sp3)-H activation, selectively yielding multisubstituted 2-cyclohexenylamines with excellent cis-diastereoselectivity. The mechanistic details of these transformations have been elucidated by deuterium-labeling experiments, kinetic isotope effect studies, and the isolation and transformations of key reaction intermediates. This work offers an efficient and selective protocol for the synthesis of a new family of cycloalkylamine derivatives, featuring 100% atom efficiency, high regio- and diastereoselectivity, broad substrate scope, and an unprecedented reaction mechanism.

Electronic Structure Design of Transition Metal-Based Catalysts for Electrochemical Carbon Dioxide Reduction

With the increasingly serious greenhouse effect, the electrochemical carbon dioxide reduction reaction (CO2RR) has garnered widespread attention as it is capable of leveraging renewable energy to convert CO2 into value-added chemicals and fuels. However, the performance of CO2RR can hardly meet expectations because of the diverse intermediates and complicated reaction processes, necessitating the exploitation of highly efficient catalysts. In recent years, with advanced characterization technologies and theoretical simulations, the exploration of catalytic mechanisms has gradually deepened into the electronic structure of catalysts and their interactions with intermediates, which serve as a bridge to facilitate the deeper comprehension of structure-performance relationships. Transition metal-based catalysts (TMCs), extensively applied in electrochemical CO2RR, demonstrate substantial potential for further electronic structure modulation, given their abundance of d electrons. Herein, we discuss the representative feasible strategies to modulate the electronic structure of catalysts, including doping, vacancy, alloying, heterostructure, strain, and phase engineering. These approaches profoundly alter the inherent properties of TMCs and their interaction with intermediates, thereby greatly affecting the reaction rate and pathway of CO2RR. It is believed that the rational electronic structure design and modulation can fundamentally provide viable directions and strategies for the development of advanced catalysts toward efficient electrochemical conversion of CO2 and many other small molecules.

Transition Metal-Free Catalytic C-H Zincation and Alumination

C-H metalation is the most efficient method to prepare aryl-zinc and -aluminium complexes that are ubiquitous nucleophiles. Virtually all C-H metalation routes to form Al/Zn organometallics require stoichiometric, strong Brønsted bases with no base-catalyzed reactions reported. Herein we present a catalytic in amine/ammonium salt (Et3N/[(Et3N)H]+) C-H metalation process to form aryl-zinc and aryl-aluminium complexes. Key to this approach is coupling an endergonic C-H metalation step with a sufficiently exergonic dehydrocoupling step between the ammonium salt by-product of C-H metalation ([(Et3N)H]+) and a Zn-H or Al-Me containing complex. This step, forming H2/MeH, makes the overall cycle exergonic while generating more of the reactive metal electrophile. Mechanistic studies supported by DFT calculations revealed metal-specific dehydrocoupling pathways, with the divergent reactivity due to the different metal valency (which impacts the accessibility of amine-free cationic metal complexes) and steric environment. Notably, dehydrocoupling in the zinc system proceeds through a ligand-mediated pathway involving protonation of the β-diketiminate Cγ position. Given this process is applicable to two disparate metals (Zn and Al), other main group metals and ligand sets are expected to be amenable to this transition metal-free, catalytic C-H metalation.

Rhodium-catalysed additive-free carbonylation of benzamides with diethyl dicarbonate as a carbonyl source

Phthalimides are prevalent in numerous pharmaceuticals, prompting various phthalimide syntheses through C-H activation. Nevertheless, the necessity for stoichiometric additives limits their practicality and versatility. Herein, we introduced diethyl dicarbonate as a carbonyl source for an additive-free carbonylation of benzamides. This transformation signifies an operationally simple and CO-free phthalimide synthesis.

Trialkoxysilane-Induced Iridium-Catalyzed para-Selective C-H Bond Borylation of Arenes

An ideal approach for the construction of aryl boron compounds is to selectively replace a C-H bond in arenes with a C-B bond, and controlling regioselectivity is one of the most challenging aspects of these transformations. Herein, we report an iridium-catalyzed trialkoxysilane protecting group-assisted regioselective C-H borylation of arenes, including derivatives of benzaldehydes, acetophenones, benzoic acids, benzyl alcohols, phenols, aryl silanes, benzyl silanes, and multi-functionalized aromatic rings are all well tolerated and gave the para -selective C-H borylation products in a short time without the requirement of inert gases atmosphere. The site-selective C-H borylation can be adjustable by installing the developed trialkoxysilane protecting group on different functional groups on one aromatic ring. Importantly, the preparation process of the trialkoxychlorosilane is efficient and scalable. Mechanistic and computational studies reveal that the steric hindrance of the trialkoxysilane protecting group plays a key role in dictating the para-selectivity.

Chemical and Redox Noninnocence of Pentane-2,4-dione Bis(S-methylisothiosemicarbazone) in Cobalt Complexes and Their Application in Wacker-Type Oxidation

Cobalt complexes with multiproton- and multielectron-responsive ligands are of interest for challenging catalytic transformations. The chemical and redox noninnocence of pentane-2,4-dione bis(S-methylisothiosemicarbazone) (PBIT) in a series of cobalt complexes has been studied by a range of methods, including spectroscopy [UV-vis, NMR, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS)], cyclic voltammetry, X-ray diffraction, and density functional theory (DFT) calculations. Two complexes [CoIII(H2LSMe)I]I and [CoIII(LSMe)I2] were found to act as precatalysts in a Wacker-type oxidation of olefins using phenylsilane, the role of which was elucidated through isotopic labeling. Insights into the mechanism of the catalytic transformation as well as the substrate scope of this selective reaction are described, and the essential role of phenylsilane and the noninnocence of PBIT are disclosed. Among the several relevant species characterized was an unprecedented Co(III) complex with a dianionic diradical PBIT ligand ([CoIII(LSMe••)I]).

Engineered Anchor Peptide LCI with a Cobalt Cofactor Enhances Oxidation Efficiency of Polystyrene Microparticles

A typical component of polymer waste is polystyrene (PS) used in numerous applications, but degraded only slowly in the environment due to its hydrophobic properties. To increase the reactivity of polystyrene, polar groups need to be introduced. Here, biohybrid catalysts based on the engineered anchor peptide LCI_F16C are presented, which are capable of attaching to polystyrene microparticles and hydroxylating benzylic C-H bonds in polystyrene microparticles using commercially available oxone as oxidant. LCI peptides achieve a dense surface coverage of PS through monolayer formation within minutes in aqueous solutions at ambient temperature. The catalytically active cobalt cofactor Co-L1 or Co-L2 with a modified NNNN macrocyclic TACD ligand (TACD=1,4,7,10-tetraazacyclododecane) is covalently bound to the anchor peptide LCI through a maleimide linker. Compared to the free cofactors, a 12- to 15-fold improvement in catalytic activity using biohybrid catalysts based on LCI_F16C was observed.

Cobalt-Catalyzed Enantio- and Regioselective C(sp3 )-H Alkenylation of Thioamides

An enantioselective cobalt-catalyzed C(sp3 )-H alkenylation of thioamides with but-2-ynoate ester coupling partners employing thioamide directing groups is presented. The method is operationally simple and requires only mild reaction conditions, while providing alkenylated products as single regioisomers in excellent yields (up to 85 %) and high enantiomeric excess [up to 91 : 9 enantiomeric ratio (er), or up to >99 : 1 er after a single recrystallization]. Diverse downstream derivatizations of the products are demonstrated, delivering a range of enantioenriched constructs. Extensive computational studies using density functional theory provide insight into the detailed reaction mechanism, origin of enantiocontrol, and the unusual regioselectivity of the alkenylation reaction.

Heterogeneous Photocatalytic Strategies for C(sp3 )-H Activation

Activation of ubiquitous C(sp3 )-H bonds is extremely attractive but remains a great challenge. Heterogeneous photocatalysis offers a promising and sustainable approach for C(sp3 )-H activation and has been fast developing in the past decade. This Minireview focuses on mechanism and strategies for heterogeneous photocatalytic C(sp3 )-H activation. After introducing mechanistic insights, heterogeneous photocatalytic strategies for C(sp3 )-H activation including precise design of active sites, regulation of reactive radical species, improving charge separation and reactor innovations are discussed. In addition, recent advances in C(sp3 )-H activation of hydrocarbons, alcohols, ethers, amines and amides by heterogeneous photocatalysis are summarized. Lastly, challenges and opportunities are outlined to encourage more efforts for the development of this exciting and promising field.

Divergent Synthesis of Multi-Substituted Aminotetralins via [4+2] Annulation of Aldimines with Alkenes by Rare-Earth-Catalyzed Benzylic C(sp3 )-H Activation

The search for efficient and selective methods for the divergent synthesis of multi-substituted aminotetralins is of much interest and importance. We report herein for the first time the diastereoselective [4+2] annulation of 2-methyl aromatic aldimines with alkenes via benzylic C(sp3 )-H activation by half-sandwich rare-earth catalysts, which constitutes an efficient route for the divergent synthesis of both trans and cis diastereoisomers of multi-substituted 1-aminotetralin derivatives from readily accessible aldimines and alkenes. The use of a scandium catalyst bearing a sterically demanding cyclopentadienyl ligand such as C5 Me4 SiMe3 or C5 Me5 exclusively afforded the trans-selective annulation products in the reaction of aldimines with styrenes and aliphatic alkenes. In contrast, the analogous yttrium catalyst, whose metal ion size is larger than that of scandium, yielded the cis-selective annulation products. This protocol features 100 % atom-efficiency, excellent diastereoselectivity, broad substrate scope, and good functional group compatibility. The reaction mechanisms have been elucidated by kinetic isotope effect (KIE) experiments and the isolation and transformations of some key reaction intermediates.

Enantioselective C-H bond functionalization under Co(III)-catalysis

While progress in enantioselective C-H functionalization has been accomplished by employing 4d and 5d transition metal-based catalysts, the rapid depletion of these metals in the earth's crust poses a serious threat to making these protocols sustainable. On the other hand, because of their unique reactivity, low toxicity, and high earth abundance, newer strategies utilizing affordable 3d transition metals have come to the forefront. Among the first-row transition metals, high-valent cobalt has recently attracted a lot of attention for catalytic C-H functionalization with mono and bidentate directing groups. This approach was extended for asymmetric catalysis due to a fairly thorough knowledge of its catalytic cycles. Four major themes have been investigated as a result of this insight: (1) rational design of a chiral Cp#Co(III)-catalyst, (2) chiral carboxylic acid with achiral Cp*Co(III)-catalysts using monodentate directing groups, (3) cobalt/salox-based systems, and (4) cobalt/chiral phosphoric acid-based hybrid systems with bidentate directing groups. Herein, we highlight the recent developments in high-valent cobalt-catalyzed enantioselective C-H functionalization up to October 2023, with the strong belief that the current state-of-the-art can attract considerable interest in the synthetic community, encouraging discoveries in the evolving landscape of asymmetric catalysis.

Copper-catalyzed synthesis of primary amides through reductive N-O cleavage of dioxazolones

A new method for the synthesis of primary amides is developed, in which dioxazolones are treated with a copper catalyst under mild reaction conditions. A broad scope of dioxazolones is exhibited as well as dioxazolones containing biologically active structural motifs. These robust and mild reaction conditions allow the transformation of dioxazolones to primary amides, in which sensitive functional groups such as hydroxyl, aldehyde, trialkylsilyl, and unsaturated carbon units are tolerated with excellent chemoselectivity.

Exploring Tunable Properties, Solvent-Modulated Dynamics, and Novel C(sp3)-H Activation Mechanisms in Electron Donor-Acceptor Complexes

Electron donor-acceptor (EDA) complex photochemistry has emerged as a vibrant area in visible-light-mediated synthetic radical chemistry. However, theoretical insights into the reaction mechanisms remain limited. Our study investigates the influence of solvent polarity and halogen atom types on radical reaction pathways in EDA complexes. We demonstrate that solvent polarity modulates the charge transfer and spatial arrangement within EDA complexes, thereby influencing their stability and reaction kinetics. Iodide ions play a crucial role in facilitating free radical generation and stabilizing reaction intermediates. Different halogen atom types exhibit distinct effects on radical reactions. Variations in radical concentration and solvent environment further affect the pathway selectivity. Additionally, light conditions influence the free radical generation and pathway selectivity. Our findings enhance the understanding of EDA complex photochemistry and radical reactions, offering insights for organic synthesis and photochemistry applications.

Cu(II)-Mediated Sulfonylation of (Hetero)arenes with TosMIC Using Monodentate Directing Groups

Monodentate chelation-assisted direct ortho-C-H sulfonylation of (hetero)arenes using TosMIC as the novel sulfonylating reagent has been developed. A broad range of substrates, including indolines, indoles, 2-phenylpyridines, and others were well tolerated to afford the corresponding products in moderate to good yields. Mechanistic studies revealed that the sulfonyl radical might be involved. Inspired by the above discovery, preliminary para-C-H sulfonylation of naphthalene substrate was also successfully realized. The current protocol featured with cheap metal catalysis, good functional group compatibility, and operational convenience.

Ru-Catalyzed and Selenium-Directed Selective Formation of ortho- and Dialkenylated Selanes, Mixed Organoselenoethers, and Isoselenochromenes

Herein, the Ru-catalyzed chemo- and regioselective formation of four novel organoselenium compounds is established. Mono- and dialkenylated selanes were formed by the Se-directed o-C-H activation of benzyl(phenyl)selanes with alkynes. Unprecedented debenzylative/dearylative hydroselenations of alkynes were performed by slightly varying the amount of catalyst and temperature. Catalyst-driven direct formation of novel isoselenochromenes is also recorded. Altogether, 45 new organoseleno compounds were synthesized in good amounts with varieties of alkynes and selanes. This is the first report of its kind to deal with the synthesis of novel, challenging, and unusual organoseleno compounds in one reaction.

Pd-catalysed C-H alkynylation of benzophospholes

A palladium-catalysed C2-H alkynylation of benzophospholes with alkynyl bromides has been developed to afford the corresponding phosphole-alkyne conjugations in good to high yields. The C-C triple bond as well as terminal alkyne C-H bond in the obtained products is a good synthetic handle for further manipulations, thus giving the versatile π-conjugated benzophosphole derivatives. The optoelectronic properties of the newly synthesized conjugated benzophospholes are also described.

Electrochemical C-H/C-C Bond Oxygenation: A Potential Technology for Plastic Depolymerization

Herein, we provide eco-friendly and safely operated electrocatalytic methods for the selective oxidation directly or with water, air, light, metal catalyst or other mediators serving as the only oxygen supply. Heavy metals, stoichiometric chemical oxidants, or harsh conditions were drawbacks of earlier oxidative cleavage techniques. It has recently come to light that a crucial stage in the deconstruction of plastic waste and the utilization of biomass is the selective activation of inert C(sp3 )-C/H(sp3 ) bonds, which continues to be a significant obstacle in the chemical upcycling of resistant polyolefin waste. An appealing alternative to chemical oxidations using oxygen and catalysts is direct or indirect electrochemical conversion. An essential transition in the chemical and pharmaceutical industries is the electrochemical oxidation of C-H/C-C bonds. In this review, we discuss cutting-edge approaches to chemically recycle commercial plastics and feasible C-C/C-H bonds oxygenation routes for industrial scale-up.

Construction of an Angular Tricyclic Benzofuran Skeleton Using the C-H Activation Strategy

A protocol for the construction of an angular tricyclic benzofuran skeleton based on the C-H activation strategy has been established. Different phthalide lactones on this skeleton can be easily assembled with various side chains by using C-H activation with aldehydes and subsequent reduction. This skeleton provides a versatile and crucial motif for the total synthesis of naturally occurring angular tricyclic benzofurans and their derivatives. Based on this protocol, the improved total syntheses of daldinin A and annullatin D were achieved in yields of 17.3 and 7.6%, respectively.

Zirconium and hafnium catalyzed C-C single bond hydroboration

Selective cleavage and subsequent functionalization of C-C single bonds present a fundamental challenge in synthetic organic chemistry. Traditionally, the activation of C-C single bonds has been achieved using stoichiometric transition-metal complexes. Recently, examples of catalytic processes were developed in which use is made of precious metals. However, the use of inexpensive and Earth-abundant group IV metals for catalytic C-C single-bond cleavage is largely underdeveloped. Herein, the zirconium-catalyzed C-C single-bond cleavage and subsequent hydroboration reactions is realized using Cp2ZrCl2 as a catalytic system. A series of structures of various γ-boronated amines are readily obtained, which are otherwise difficult to obtain. Mechanistic studies disclose the formation of a N-ZrIV species, and then a β-carbon elimination route is responsible for C-C single bond activation. Besides zirconium, hafnium exhibits a similar performance for this transformation.

Deracemization of Atropisomeric Biaryls Enabled by Copper Catalysis

Atropisomeric biaryls have found crucial applications in versatile chiral catalysts as well as in ligands for transition metals. Herein, we have developed an efficient crystallization-induced deracemization (CID) method to access chiral biaryls from their racemates with a chiral ammonium salt under copper catalysis including BINOL, NOBIN, and BINAM derivatives. After being significantly accelerated by its bidentate diamine ligand, the copper catalyst exhibits high efficiency and selectivity in racemizing biaryl skeletons, and the cocrystal complex would be enantioselectively formed together with chiral ammonium salt, which on acid-quenching would directly deliver chiral biaryl without further chromatographic purification. This CID process is easily scalable, and the chiral ammonium salt was nicely recoverable. Ligand effect studies showed that bulky alkyl substitution was an indispensable element to ensure efficient racemization, which probably proceeds via a radical-cation intermediate and further allows axial rotation by forming a delocalized radical.

Synthesis of anti-depressant molecules via metal-catalyzed reactions: a review

Depression is one of the most mutilating conditions in the world today. It has been difficult to make advancements toward better, more effective therapies since the introduction of antidepressant medicines in the late 1950s. One important field of medicinal chemistry is the synthesis of antidepressant molecules through metal-catalyzed procedures. The important role that different transition metals, including iron, nickel, ruthenium, and others, serve as catalysts in the synthesis of antidepressants is examined in this review. Key structural motifs included in antidepressant drugs such as tricyclic antidepressants (TCAs), selective serotonin reuptake inhibitors (SSRIs), and others can be synthesized in a variety of effective ways using metal-catalyzed steps. This review examines current developments in the catalytic synthesis of antidepressants and their potential application over the previous thirteen years.

Sequential ortho-/meta-C-H functionalizations of N-tosyl-benzamides for the synthesis of polyfunctionalized arenes

Selective one-pot sequential ortho-/meta-C-H functionalizations provided highly desirable polyfunctionalized arenes. Starting from readily available carboxylic acid derivatives, the concomitant formation of C-O and C-halogen bonds was achieved under mild reaction conditions (12 examples, up to 75% yield). The utility of the products was illustrated with post-functionalization reactions and Metiglinid synthesis.

Unveiling the Detailed Mechanism and Origins of Chemo-, Regio-, and Stereoselectivity of Rare-Earth Catalyzed Alternating Copolymerization of Polar and Nonpolar Olefins

The direct copolymerization of polar and nonpolar olefins is of great interest and significance, as it is the most atom-economical and straightforward strategy for the synthesis of functional polyolefin materials. Despite considerable efforts, the precise control of monomer-sequence and their regio- and stereochemistry is full of challenges, and the related mechanistic origins are still in their infancy to date. Herein, the mechanistic studies on the model reaction of Sc-catalyzed co-syndiospecific alternating copolymerization of anisylpropylene (AP) and styrene were performed by DFT calculations. The results suggest that the subtle balance between electronic and steric factors plays an important role during monomer insertions, and a new amino-dissociated mechanism was proposed for AP insertion at chain initiation. AP insertion follows the 2,1-si-insertion pattern, which is mainly controlled by steric factors caused by the restricted MeO···Sc interaction. As for styrene insertion, it prefers the 2,1-re-insertion manner and its regio- and stereoselectivities are influenced by steric repulsions between the inserting styrene and the polymer chain or the ligand. More interestingly, it is found that the alternating monomer-sequence is mainly determined by the "steric matching" principle, which is quantitatively expressed by the buried volume of the metal center of the preinserted species. The concept of steric pocket has been successfully applied to explain the different performances of several catalysts and other alternating copolymerization reactions. The insightful mechanistic findings and the quantitative steric pocket model present here are expected to promote rational design of new rare-earth catalysts for developing regio-, stereo-, and sequence-controlled copolymerization of specific polar and nonpolar olefins.