Palladium-Catalyzed Transformations of Alkyl C-H Bonds

Direct Photoreforming of Real-World Polylactic Acid Plastics into Highly Selective Value-Added Pyruvic Acid under Visible Light

Although polylactic acid (PLA) represents a pivotal biodegradable polymer, its biodegradability has inadvertently overshadowed the development of effective recycling techniques, leading to the potential wastage of carbon resources. The photoreforming-recycling approach for PLA exhibits significant potential in terms of concepts and methods. However, the reaction faces enormous challenges due to the limited selectivity of organic oxidation products as well as the increased costs and challenging separation of organic products associated with alkali-solution-assisted prehydrolysis. Herein, we report an alkali-free direct-photoreforming pathway for real-world PLA plastics utilizing the Pd-CdS photocatalyst under visible-light illumination, obviating the need for chemical pretreatment of PLA. The devised pathway successfully produces H2 at a rate of 49.8 μmol gcat.-1 h-1, sustained over 100 h, and exhibits remarkable selectivity toward pyruvic acid (95.9% in liquid products). Additionally, experimental findings elucidate that Pd sites not only function as a typical cocatalyst for enhancing the photocatalytic evolution of H2 but also suppress competitive side reactions (e.g., lactic acid coupling or decarboxylation), consequently augmenting the yield and selectivity of pyruvic acid and H2. This investigation provides a straightforward and sustainable direct-photoreforming route capable of simultaneously mitigating and repurposing plastic waste into valuable chemicals, thus offering a promising solution to the current environmental challenges.

Hole-Transporting Materials based on Oligo(hetero)aryls with a Naphthodithiophene Core - Succinct Synthesis by Twofold Direct C-H Olefination

This work demonstrated the first synthetic application of direct C-H olefinations in the step-saving preparation of various hole-transporting materials (HTM) for efficient perovskite solar cells (PSC). Cross-dehydrogenative couplings of naphthodithiophene (NDT) with vinyl arenes under palladium-catalysis facilely generated various new oligo(hetero)aryls with internal alkenes. Reaction conditions were optimized, which gave the product isolated yields of up to 71 % with high (E)-stereoselectivity. These readily accessible NDT core-based small molecules involving olefin as π-spacers displayed immediate power conversion efficiencies of up to 17.2 % without a device oxidation process that is required for the commercially available spiro-OMeTAD and most other existing HTMs while fabricated in corresponding PSC devices.

Photo-induced versatile aliphatic C-H functionalization via electron donor-acceptor complex

The ability to selectively introduce diverse functionality onto hydrocarbons is of substantial value in the synthesis of both small molecules and pharmaceuticals. In this endeavour, as a photocatalyst- and metal-free process, the electron donor-acceptor (EDA) strategy has not been well explored. Here we report an approach to aliphatic carbon-hydrogen bond diversification through an EDA complex constituted by HCl and SIV=O groups. As an efficient hydrogen atom transfer (HAT) reagent, chlorine radical can be produced via a proton-coupled electron transfer process in this system. Based on this unusual path, a photo-promoted versatile aliphatic C-H functionalization is developed without photo- and metal-catalysts, including thiolation, arylation, alkynylation, and allylation. This conversion has concise and ambient reaction conditions, good functional group tolerance, and substrate diversity, and provides an alternative solution for the high value-added utilization of bulk light alkanes.

Picolinamide-assisted ortho-C-H functionalization of pyrenylglycine derivatives using aryl iodides

Chemical transformations involving the pyrenylglycine motif (an unnatural amino acid) and practical methods toward it are seldom known. This work aimed at developing a method for synthesizing novel pyrenylglycine (pyrene-based glycine) unnatural amino acid derivatives. To realize this, initially, a new pyrenylglycine substrate possessing the picolinamide moiety was assembled via the Ugi multicomponent reaction. The picolinamide moiety linked to amine substrates is a well-known bidentate directing group for accomplishing the site-selective γ-C-H functionalization of amines. Subsequently, it was aimed at using a Pd(II)-catalyzed bidentate directing group-aided γ-C-H arylation strategy for generating a wide range of unprecedented examples of C(2)-H arylated pyrenylglycines. Accordingly, pyrenylglycine possessing the picolinamide moiety was subjected to Pd(II)-catalyzed C(2)-H arylation in the non-K-region to afford a library of C(2)-arylated pyrenylglycines (π-extended pyrenes). Additionally, pyrenylglycine-based small peptides were assembled using C(2)-arylated pyrenylglycines. The X-ray structure of a representative compound was obtained, which corroborated the structure of pyrenylglycine and the regioselectivity of C(2)-H arylation of the pyrene in the non-K-region.

Visible-Light-Promoted α-C(sp3)-H Amination of Ethers with Azoles and Amides

A visible-light-induced highly efficient C(sp3)-H amination of ethers with amides and azoles has been presented under mild conditions via a nitrogen- and carbon-centered radical coupling process. This protocol successfully utilizes 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and tert-butyl nitrite (TBN) as cocatalysts to deliver the aminated products of ethers under aerobic conditions. Notably, the developed reaction features the corresponding products in good yields (up to 93%) with a wide substrate scope. The mechanistic study indicates that C-N bond formation proceeds via a direct radical cross-coupling process. Preliminary biological activity analysis indicates that the resulting products have good and selective inhibitory activity on osteosarcoma (OS) cell lines and are promising for use as hits for drug discovery.

Synthesis of Naphtho[1',2':4,5]furo[3,2-b]pyridinones via Ir(III)-Catalyzed C6/C5 Dual C-H Functionalization of N-Pyridyl-2-pyridones with Diazonaphthalen-2(1H)-ones

Presented herein is an unprecedented synthesis of naphtho[1',2':4,5]furo[3,2-b]pyridinones via Ir(III)-catalyzed C6/C5 dual C-H functionalization of N-pyridyl-2-pyridones with diazonaphthalen-2(1H)-ones. This protocol forms C-C and C-O bonds in one pot in which diazonaphthalen-2(1H)-ones serve as bifunctional reagents, providing both alkyl and aryloxy sources. To the best of our knowledge, this is the first example of an Ir(III)-catalyzed synthesis of the title compounds by using diazonaphthalen-2(1H)-ones as bifunctional substrates. Notably, this method features operational simplicity, good functional group tolerance, high efficiency, and high atom economy.

Catalytic Enantioselective Primary C-H Borylation for Acyclic All-Carbon Quaternary Stereocenters

Creating a perfect catalyst to operate enzyme-like chiral recognition has been a long-sought aim. A challenging example in this context is constructing acyclic all-carbon quaternary stereogenic centers by transition metal-catalyzed enantioselective C-H activation. We now report highly enantioselective iridium-catalyzed primary C-H borylation of α-all-carbon substituted 2,2-dimethyl amides enabled by a tailor-made chiral bidentate boryl ligand (CBL). The success of the current transformation is attributed to the CBL/iridium catalyst, which has a confined chiral pocket. This protocol provides a diverse array of acyclic all-carbon quaternary stereocenters with excellent enantiocontrol and distinct structural features. Computational study reveals that steric hindrance of CBL could regulate the type of dominant orbital interaction between the catalyst and substrate, which is crucial to conferring high chiral induction.

Co(III)-Catalyzed C6-Selective C-H Activation/Pyridine Migration of 2-Pyridones with Propiolates

A versatile Co(III)-catalyzed C6-selective C-H activation/pyridine migration of 2-pyridones with available propiolates as coupling partners was demonstrated. This method features high atom economy, excellent regioselectivity, and good functional group tolerance by employing an inexpensive Co(III) catalyst under mild reaction conditions. Moreover, gram-scale synthesis and late-stage modifications of pharmaceuticals were performed to prove the effectiveness of these synthetic approaches.

Cp*Rh(III)-catalyzed regioselective cyclization of aromatic amides with allenes

A new Cp*Rh(III)-catalyzed regioselective cyclization reaction of aromatic amides with allenes is reported. The use of allenyl derivatives bearing a directing-group assistant as a reaction promoter was the key to the success of this protocol. In this catalytic system, N-(pivaloyloxy)benzamide substrates react with allenes via Rh-σ-alkenyl intermediates, while N-(pivaloyloxy) indol substrates react via Rh-π-allyl intermediates. These reactions were characterized by mild reaction conditions, a broad substrate scope, and high functional-group compatibility to yield several high-value isoquinolinone and pyrimido[1,6-a]indol-1(2H)-one skeleton-containing compounds. The synthetic applications and primary mechanisms were also investigated.

Palladium-Catalyzed Electrooxidative Double C-H Arylation

The electrochemical transition metal-catalyzed cross-dehydrogenative reaction has emerged as a promising platform to achieve a sustainable and atom-economic organic synthesis that avoids hazardous oxidants and minimizes undesired byproducts and circuitous functional group operations. However, a poor mechanistic understanding still prevents the widespread adoption of this strategy. In this regard, we herein present an electrochemical palladium-catalyzed oxidative coupling strategy to access biaryls in the absence of a stoichiometric chemical oxidant. The robust palladaelectrocatalysis considerably suppresses the occurrence of homocoupling and oxygenation, being compatible even with electron-deficient arenes. Late-stage functionalization and Boscalid precursor synthesis further highlighted the practical importance of our electrolysis. Remarkably, mechanistic studies including the evaluation of the reaction order of each component by variable time normalization analysis (VTNA) and initial rate analysis, H/D exchange experiment, kinetic isotope effect, and stoichiometric organometallic experiments provided strong support for the involvement of transmetalation between two organopalladium complexes in the turnover limiting step. Therefore, matching the concentrations or lifetimes of two distinct organopalladium intermediates is revealed to be a pivot to the success of electrooxidative catalysis. Moreover, the presence of cationic copper(II) seems to contribute to the stabilization of the palladium(0) catalyst instead of playing a role in the oxidation of the catalyst.

Visible-Light-Induced Transition-Metal-Free Redox-Neutral Carboxylation of Remote Benzylic C(sp3)-H Bonds via 1,5-Hydrogen Atom Transfer

The direct carboxylation of the benzylic C-H bonds under mild conditions is of great importance and is quite challenging. Herein, we report an approach for the carboxylation of remote benzylic C(sp3)-H bonds by integrating the redox-neutral visible-light photoredox catalysis and the nitrogen-centered 1,5-hydrogen atom transfer. The chemical transformation progresses via consecutive single electron transfer, 1,5-hydrogen atom transfer, formation of benzylic carbanion, and nucleophilic attack on the CO2 steps, thereby enabling access to the desired carboxylation products with moderate to high yields. We also endeavor to recover the CO2 groups generated in situ intramolecularly to achieve carboxylation under a nitrogen atmosphere, resulting in moderate yields of corresponding carboxylation.

Continuous-Flow Enantioselective Hydroacylations under Heterogeneous Chiral Rhodium Catalysts

Transition metal-catalyzed enantioselective C-H bond functionalizations have become efficient methods for the synthesis of complex optically active molecules. Heterogeneous catalysts for this chemistry remain largely unexplored despite the advantages they offer in terms of ease of separation and reuse of catalysts. Herein, we report the development of heterogeneous chiral Rh catalysts for continuous-flow enantioselective hydroacylations. Heterogeneous catalysts could be prepared simply by mixing supports and Rh complexes. The prepared catalysts exhibited excellent activity and enantioselectivity affording optically active ketones in quantitative yields with 99 % ee's. Under the optimized reaction conditions, a turnover number >300 was achieved without the leaching of Rh species. The catalysts exhibited a wide substrate scope and in sequential-flow reactions with other heterogeneous catalysts, the syntheses of biologically active molecules and functional materials were demonstrated.

Modern Organometallic C-H Functionalizations with Earth-Abundant Iron Catalysts: An Update

Iron-catalyzed C-H activation has recently emerged as an increasingly powerful synthetic method for the step- and atom- economical direct C-H functionalizations of otherwise inert C-H bonds. Iron's low-cost and toxicity along with its catalytic versatility have encouraged the scientific community to elect this metal for the development of new C-H activation methodologies. Within this review, we aim to present a collection of the most recent examples of iron-catalyzed C-H functionalizations with a particular emphasis on modern synthetic strategies and mechanistic aspects.

DFT Study on the Mechanism of the Palladium-Catalyzed [3 + 2] Annulation of Aromatic Amides with Maleimides via Benzylic and meta-C-H Bond Activation: Role of the External Ligand Ac-Gly-OH

The mechanism of the Ac-Gly-OH-assisted palladium-catalyzed [3 + 2] annulation of aromatic amides with maleimides is investigated using density functional theory calculations. The results show that the reaction undergoes the sequential steps of N-H bond deprotonation, first benzylic C-H bond activation, maleimide insertion, second meta-C-H bond activation, reductive elimination, and oxidation. The external ligand Ac-Gly-OH acts as the internal base for hydrogen abstraction in the first benzylic C-H bond activation. The maleimide insertion step is found to be the rate-determining step. Based on the nearly same energetic span of the two pathways to generate the enantio products, the computational results are consistent with the experimental observation that the terminal [3 + 2] annulation products are racemic when using an achiral ligand. These calculation results disclose the detailed reaction mechanism and shed light on some experimental ambiguities.

A Formal γ-C-H Functionalization of Carboxylic Acids Guided by Metal-Nitrenoids as an Unprecedented Mechanistic Motif

Harnessing the key intermediates in metal-catalyzed reactions is one of the most essential strategies in the development of selective organic transformations. The nitrogen group transfer reactivity of metal-nitrenoids to ubiquitous C-H bonds allows for diverse C-N bond formation to furnish synthetically valuable aminated products. In this study, we present an unprecedented reactivity of iridium and ruthenium nitrenoids to generate remote carbocation intermediates, which subsequently undergo nucleophile incorporation, thus developing a formal γ-C-H functionalization of carboxylic acids. Mechanistic investigations elucidated a unique singlet metal-nitrenoid reactivity to initiate an abstraction of γ-hydride to form the carbocation intermediate that eventually reacts with a broad range of carbon, nitrogen, and oxygen nucleophiles, as well as biorelevant molecules. Alternatively, the same intermediate can lead to deprotonation to afford β,γ-unsaturated amides in a less nucleophilic solvent.

The Recent Advances in Iron-Catalyzed C(sp3 )-H Functionalization

The use of iron as a core metal in catalysis has become a research topic of interest over the last few decades. The reasons are clear. Iron is the most abundant transition metal on Earth's crust and it is widely distributed across the world. It has been extracted and processed since the dawn of civilization. All these features render iron a noncontaminant, biocompatible, nontoxic, and inexpensive metal and therefore it constitutes the perfect candidate to replace noble metals (rhodium, palladium, platinum, iridium, etc.). Moreover, direct C-H functionalization is one of the most efficient strategies by which to introduce new functional groups into small organic molecules. The majority of organic compounds contain C(sp3 )-H bonds. Given the enormous importance of organic molecules in so many aspects of existence, the utilization and bioactivity of C(sp3 )-H bonds are of the utmost importance. This review sheds light on the substrate scope, selectivity, benefits, and limitations of iron catalysts for direct C(sp3 )-H bond activations. An overview of the use of iron catalysis in C(sp3 )-H activation protocols is summarized herein up to 2022.

Regiodivergent sp3 C-H Functionalization via Ni-Catalyzed Chain-Walking Reactions

The catalytic translocation of a metal catalyst along a saturated hydrocarbon side chain constitutes a powerful strategy for enabling bond-forming reactions at remote, yet previously unfunctionalized, sp3 C-H sites. In recent years, Ni-catalyzed chain-walking reactions have offered counterintuitive strategies for forging sp3 architectures that would be difficult to accomplish otherwise. Although these strategies have evolved into mature tools for advanced organic synthesis, it was only recently that chemists showed the ability to control the motion at which the catalyst "walks" throughout the alkyl chain. Specialized ligand backbones, additives and a judicious choice of noninnocent functional groups on the side chain have allowed the design of "a la carte" protocols that enable regiodivergent bond-forming scenarios at different sp3 C-H sites with distinct topological surface areas. Given the inherent interest in increasing the fraction of sp3 hybridized carbons in medicinal chemistry, Ni-catalyzed regiodivergent chain-walking reactions might expedite the access to target leads in drug discovery campaigns.

Organo-Photoredox Catalyzed C(sp3 )-H Bond Arylation of Aliphatic Amides

A C(sp3 )-H bond arylation of aliphatic amides has been achieved via organophotoredox catalysis. The reaction could be realized at room temperature with visible light source and metal-free catalyst. Quinuclidine is employed as an efficient HAT reagent and a range of aliphatic amides is employed as both substrate and solvent in the reaction. This photocatalyzed transformation provides a convenient protocol to afford a board range of N-benzyl amides.

β-C-H Allylation of Trialkylamines with Allenes Promoted by Synergistic Borane/Palladium Catalysis

Functionalization of the C(sp3 )-H bonds of trialkylamines is challenging, especially for reactions at positions other than the α position. Herein, we report a method for β-C(sp3 )-H allylation of trialkylamines. In these reactions, which involve synergistic borane/palladium catalysis, an enamine intermediate is first generated from the amine via α,β-dehydrogenation promoted by B(C6 F5 )3 and a base, and then the enamine undergoes palladium-catalyzed reaction with an allene to give the allylation product. Because the hydride and the proton resulting from the initial dehydrogenation are ultimately shuttled to the product by B(C6 F5 )3 and the palladium catalyst, respectively, these reactions show excellent atom economy. The establishment of this method paves the way for future studies of C-H functionalization of trialkylamines by means of synergistic borane/transition-metal catalysis.

Ligand-Enabled γ-C(sp3)-H Hydroxylation of Free Amines with Aqueous Hydrogen Peroxide

Selective oxidation of the γ-C-H bonds from abundant amine feedstocks via palladium catalysis is a valuable transformation in synthesis and medicinal chemistry. Despite advances on this topic in the past decade, there remain two significant limitations: C-H activation of aliphatic amines requires an exogenous directing group except for sterically hindered α-tertiary amines, and a practical catalytic system for C(sp3)-H hydroxylation using a green oxidant, such as oxygen or aqueous hydrogen peroxide, has not been developed to date. Herein, we report a ligand-enabled selective γ-C(sp3)-H hydroxylation using sustainable aqueous hydrogen peroxide (7.5-10%, w/w). Enabled by a CarboxPyridone ligand, a series of primary amines (1°), piperidines, and morpholines (2°) were hydroxylated at the γ-position with excellent monoselectivity. This method provides an avenue for the synthesis of a wide range of amines, including γ-amino alcohols, β-amino acids, and azetidines. The retention of chirality in the reaction allows rapid access to chiral amines starting from the abundant chiral amine pool.

Rhodium(III)-catalyzed three-component C(sp2)-H activation for the synthesis of amines

Rhodium-catalyzed three-component C-H bond activation of aromatics with amides and aldehydes to synthesize amines was established. The addition of copper was found to be essential to ensure the high reactivity. The mechanistic studies indicated that key intermediates formed by the transmetallization between rhodium and copper could further promote the addition between 2-(pyridin-2-yl)-phenyl-metal species and imines. A series of densely substituted amines could be conveniently prepared by this one-step, three-component procedure from commercially available substrates via C-H bond activation with water as the only by-product.

Ligand-Enabled Double γ-C(sp3 )-H Functionalization of Aliphatic Acids: One-Step Synthesis of γ-Arylated γ-Lactones

γ-methylene C(sp3 )-H functionalization of linear free carboxylic acids remains a significant challenge. Here in we report a Pd(II)-catalyzed tandem γ-arylation and γ-lactonization of aliphatic acids enabled by a L,X-type CarboxPyridone ligand. A wide range of γ-arylated γ-lactones are synthesized in a single step from aliphatic acids in moderate to good yield. Arylated lactones can readily be converted into disubstituted tetrahydrofurans, a prominent scaffold amongst bioactive molecules.

Aryl Chloride-Directed Enantioselective C(sp2)-H Borylation Enabled by Iridium Catalysis

We herein report the iridium-catalyzed enantioselective C-H borylation of aryl chlorides. A variety of prochiral biaryl compounds could be well-tolerated, affording a vast array of axially chiral biaryls with high enantioselectivities. The current method exhibits a high turnover number (TON) of 7000, which represents the highest in functional-group-directed asymmetric C-H activation. The high TON was attributed to a weak catalyst-substrate interaction that was caused by mismatched chirality between catalyst and substrate. We also demonstrated the synthetic application of the current method by C-B, ortho-C-H, and C-Cl bond functionalization, including programmed Suzuki-Miyaura coupling for the synthesis of axially chiral polyarenes.

Merging Visible Light Photocatalysis and P(III)-Directed C-H Activation by a Single Catalyst: Modular Assembly of P-Alkyne Hybrid Ligands

Metal-catalyzed C-H activation strategies provide an efficient approach for synthesis by minimizing atom, step, and redox economy. Developing milder, greener, and more effective protocols for these strategies is always highly desirable to the scientific community. In this study, the utilization of a single rhodium complex enabled the visible-light-induced late-stage C-H activation of biaryl-type phosphines with alkynyl bromides, employing inherent phosphorus atoms as directing groups. This chemistry combines P(III)-directed C-H activation with visible light photocatalysis, under exogenous photosensitizer-free conditions, offering a unique platform for ligand design and preparation. Furthermore, this study also explores the asymmetric catalysis and coordination chemistry of the resulting P-alkyne hybrid ligands with specific transition metals. Experimental results and density functional theory calculations demonstrate the mechanistic intricacies of this transformation.

2-Hydroxypyridine-based Ligands as Promoter in Ruthenium(II) Catalyzed C-H Bond Activation/Arylation Reactions

A class of 2-hydroxypyridine based ligands are explored to achieve enhanced catalytic activity for ortho-C-H bond activation/arylation reaction over [(η6 -p-cymene)RuCl2 ]2 catalyst in water. Extensive studies using a series of substituted 2-hydroxypyridine based ligands (L1-L6) inferred that 5-trifluoromethyl-2-hydroxypyridine (L6) exhibited favorable effects to enhance the catalytic activity of Ru(II) catalyst for ortho C-H bond arylation of 2-phenylpyridine by 8 folds compared to those performed without ligands. The (η6 -p-cymene)Ru - L6 system also exhibited enhanced catalytic activity for ortho C-H bond arylation of 2-phenylpyridine using a variety of aryl halides. NMR and mass investigations inferred the presence of several ligand coordinated Ru(II) species, suggesting the involvement of these species in C-H bond activation reaction. Further in concurrence with the experimental findings, the density functional theory (DFT) calculations also evidenced the prominent role of 2-hydroxypyridine based ligands in Ru(II) catalyzed C-H bond arylation of 2-phenylpyridine with lower energy barrier for the C-H activation step.

Organocatalytic C-H Functionalization of Simple Alkanes

The direct functionalization of inert C(sp3 )-H bonds to form carbon-carbon and carbon-heteroatom bonds offers vast potential for chemical synthesis and therefore receives increasing attention. At present, most successes come from strategies using metal catalysts/reagents or photo/electrochemical processes. The use of organocatalysis for this purpose remains scarce, especially when dealing with challenging C-H bonds such as those from simple alkanes. Here we disclose the first organocatalytic direct functionalization/acylation of inert C(sp3 )-H bonds of completely unfunctionalized alkanes. Our approach involves N-heterocyclic carbene catalyst-mediated carbonyl radical intermediate generation and coupling with simple alkanes (through the corresponding alkyl radical intermediates generated via a hydrogen atom transfer process). Unreactive C-H bonds are widely present in fossil fuel feedstocks, commercially important organic polymers, and complex molecules such as natural products. Our present study shall inspire a new avenue for quick functionalization of these molecules under the light- and metal-free catalytic conditions.

Axially Chiral 2-Hydroxybiaryls by Palladium-Catalyzed Enantioselective C-H Activation

This article describes the discovery and development of a palladium-catalyzed asymmetric C-H olefination of 2-hydroxybiaryls. The strategy allows a direct assembly of optically active, axially chiral 2-substituted-2'-hydroxybiaryls from readily available precursors and demonstrates that the native hydroxy unit of the substrates can work as an efficient directing group for the C-H activation. This represents a substantial advantage over other approaches that require the preinstallation of metal coordinating units. The simplicity of the approach and versatility of the products allow a practical and efficient synthesis of a broad variety of optically active binaphthyl derivatives.

Palladium-Catalyzed Enantioselective C-H Olefination to Access Planar-Chiral Cyclophanes by Dynamic Kinetic Resolution

Planar-chiral cyclophanes have received increasing attention for drug discovery and catalyst design. However, the catalytically asymmetric synthesis of planar-chiral cyclophanes has been a longstanding challenge. We describe the first Pd(II)-catalyzed enantioselective C-H olefination of prochiral cyclophanes. The low rotational barrier of less hindered benzene ring in the substrates allows the reaction to proceed through a dynamic kinetic resolution. This approach exhibits broad substrate scope, providing the planar-chiral cyclophanes in high yields (up to 99 %) with excellent enantioselectivities (up to >99 % ee). The ansa chain length scope studies reveal that the chirality of the cyclophanes arises from the bond rotation constraint of the benzene ring around the macrocycle plane, rather than the C-N axis. The C-H activation approach is also applicable to the late-stage modification of bioactive molecules and pharmaceuticals.

Palladium nanoparticles on gallium nitride as a Mott-Schottky catalyst for efficient and durable photoactivation of unactivated alkanes

The direct functionalization of inert C-H bonds has long been a "holy grail" for the chemistry world. In this report, the direct C(sp3)-N bond formation of unactivated alkanes is reported with a GaN based Mott-Schottky catalyst under photocatalytic reaction conditions. Long term stability and reaction efficiency (up to 92%) were achieved with this photocatalyst. The deposition of a Pd co-catalyst on the surface of GaN significantly enhanced the reaction efficiency. Microscopic investigation suggested a remarkable interaction in the Pd/GaN Schottky junction, giving a significant Pd/GaN depletion layer. In addition, density functional theory (DFT) calculations were performed to show the distinct performance of Pd nanoparticles at the atomic level.

Access to unsaturated bicyclic lactones by overriding conventional C(sp3)-H site selectivity

Transition metal catalysis plays a pivotal role in transforming unreactive C-H bonds. However, regioselective activation of distal aliphatic C-H bonds poses a tremendous challenge, particularly in the absence of directing templates. Activation of a methylene C-H bond in the presence of methyl C-H is underexplored. Here we show activation of a methylene C-H bond in the presence of methyl C-H bonds to form unsaturated bicyclic lactones. The protocol allows the reversal of the general selectivity in aliphatic C-H bond activation. Computational studies suggest that reversible C-H activation is followed by β-hydride elimination to generate the Pd-coordinated cycloalkene that undergoes stereoselective C-O cyclization, and subsequent β-hydride elimination to provide bicyclic unsaturated lactones. The broad generality of this reaction has been highlighted via dehydrogenative lactonization of mid to macro ring containing acids along with the C-H olefination reaction with olefin and allyl alcohol. The method substantially simplifies the synthesis of important bicyclic lactones that are important features of natural products as well as pharmacoactive molecules.

Palladium-Catalyzed Dearomatization of Benzothiophenes: Isolation and Functionalization of a Discrete Dearomatized Intermediate

A Pd-catalyzed decarboxylative dearomatization reaction of a heterocyclic substrate enables access to an uncommon reaction intermediate that rearomatizes in the presence of amine bases in a net C-H functionalization sequence. The dearomatized benzo[b]thiophene intermediate bears an exocyclic alkene that can be functionalized through cycloaddition and halogenation reactions to deliver complex heterocyclic products.

Selective Oxidative Methyl C-H Functionalization of Butylated Hydroxytoluene toward Arylimines/N-Heterocycles

A metal-free and selective oxidative methyl C-H functionalization of BHT with aniline compounds has been developed. This innovative method enables the facile and efficient synthesis of a diverse array of BHT-functionalized N-containing skeletons, including arylamines, benzoxazoles, benzothiazoles, benzimidazoles, quinazolines, and quinazolinones, all of which are challenging to access. The control experiment involving TEMP18O suggests that the radical adduct of TEMPO with the benzyl radical of BHT may serve as an intermediate.

DFT Studies on the Mechanisms of Carboamination/Diamination of Unactivated Alkenes Mediated by Pd(IV) Intermediates

Density functional theory (DFT) calculations have been employed to investigate the mechanism of carboamination and diamination of unactivated alkenes mediated by Pd(IV) intermediates. Both reactions share a common Pd(IV) intermediate, serving as the starting point for either the carboamination or the diamination pathway. The formation of this Pd(IV) intermediate encompasses a transition state that substantially impacts the turnover frequency (TOF) of catalytic cycles, with an apparent activation free-energy barrier of 26.1 kcal mol-1. Carboamination of unactivated alkenes proceeds through the coordination of a toluene molecule, C-H activation, inner reductive elimination, and the separation of the carboamination product from this intermediate, while diamination of unactivated alkenes involves the formation of the ion nucleophile, SN2 attack, and the separation of the diamination product. A comparison of the free-energy profiles for carboamination and diamination of unactivated alkenes can elucidate the origin of the chemoselectivity, and Bader's atoms in molecules (AIM) wave function analyses have been performed to analyze the contributions of the outer C-N bonding in the diamination process.

Photoelectrochemical oxidative C(sp3)-H borylation of unactivated hydrocarbons

Organoboron compounds are of high significance in organic synthesis due to the unique versatility of boryl substituents to access further modifications. The high demand for the incorporation of boryl moieties into molecular structures has witnessed significant progress, particularly in the C(sp3)-H borylation of hydrocarbons. Taking advantage of special characteristics of photo/electrochemistry, we herein describe the development of an oxidative C(sp3)-H borylation reaction under metal- and oxidant-free conditions, enabled by photoelectrochemical strategy. The reaction exhibits broad substrate scope (>57 examples), and includes the use of simple alkanes, halides, silanes, ketones, esters and nitriles as viable substrates. Notably, unconventional regioselectivity of C(sp3)-H borylation is achieved, with the coupling site of C(sp3)-H borylation selectively located in the distal methyl group. Our method is operationally simple and easily scalable, and offers a feasible approach for the one-step synthesis of high-value organoboron building blocks from simple hydrocarbons, which would provide ample opportunities for drug discovery.

Recent Advances in the Catalytic Synthesis of the Cyclopentene Core

Five-membered carbocycles are ubiquitously found in natural products, pharmaceuticals, and other classes of organic compounds. Within this category, cyclopentenes deserve special attention due to their prevalence as targets and as well as key intermediates for synthesizing more complex molecules. Herein, we offer an overview summarizing some significant recent advances in the catalytic assembly of this structural motif. A great variety of synthetic methodologies and strategies are covered, including transition metal-catalyzed or organocatalyzed processes. Both inter- and intramolecular transformations are documented. On this ground, our expertise in the application of C-H functionalization reactions oriented towards the formation of this ring and its subsequent selective functionalization is embedded.

Rhodium(III)-Catalyzed C-H/N-H Activation for Direct Synthesis of Pyrimidoindolones under Mild Conditions

Pyrimidoindolones are an important structural motif found in many natural products and are essential to the pharmaceutical and agrochemical industry. Direct synthesis of 3,4-unsubstituted pyrimidoindolones is not easily accessible. Here we report a rhodium(III)-catalyzed C-H/N-H activation and annulation approach for obtaining pyrimidoindolones from N-carbamoylindoles and vinylene carbonate. The reaction occurs at room temperature and does not require any external oxidants. A diverse spectrum of indoles were demonstrated to be viable substrates capable of producing the desired pyrimidoindolones in high yields. In addition, the reaction scope has been expanded to include pyrrole substrate. Furthermore, detailed mechanistic studies have been performed to delineate the working mode of the reaction.

Non-Covalent Interaction-Controlled Site-Selective C-H Transformations

Site-selective C-H transformations are important to obtain desired compounds as single products in a highly efficient manner. However, it is generally difficult to achieve such transformations because organic substrates contain many C-H bonds with similar reactivities. Therefore, the development of practical and efficient methods for controlling site selectivity is highly desirable. The most frequently used strategy is "directing group method". Although this method is highly effective and promotes site-selective reactions, it has several limitations. Our group recently reported other methods to achieve site-selective C-H transformations using non-covalent interactions between a substrate and a reagent or a catalyst and a substrate (non-covalent method). In this personal account, the background of site-selective C-H transformations, our reaction design to achieve site-selective C-H transformations, and recently reported reactions are explained.

Recent Advances in Photochemical Asymmetric Three-Component Reactions

Over the past decades, asymmetric photochemical synthesis has garnered significant attention for its sustainability and unique ability to generate enantio-enriched molecules through distinct reaction pathways. Photochemical asymmetric three-component reactions have demonstrated significant potential for the rapid construction of chiral compounds with molecular diversity and complexity. However, noteworthy challenges persist, including the participation of high-energy intermediates such as radical species, difficulties in precise control of stereoselectivity, and the presence of competing background and side reactions. Recent breakthroughs have led to the development of sophisticated strategies in this field. This review explores the intricate mechanisms, synthetic applications, and limitations of these methods. We anticipate that it will contribute towards advancing asymmetric catalysis, photochemical synthesis, and green chemistry.

Molecular Editing of Pyrroles via a Skeletal Recasting Strategy

Heterocyclic scaffolds are commonly found in numerous biologically active molecules, therapeutic agents, and agrochemicals. To probe chemical space around heterocycles, many powerful molecular editing strategies have been devised. Versatile C-H functionalization strategies allow for peripheral modifications of heterocyclic motifs, often being specific and taking place at multiple sites. The past few years have seen the quick emergence of exciting "single-atom skeletal editing" strategies, through one-atom deletion or addition, enabling ring contraction/expansion and structural diversification, as well as scaffold hopping. The construction of heterocycles via deconstruction of simple heterocycles is unknown. Herein, we disclose a new molecular editing method which we name the skeletal recasting strategy. Specifically, by tapping on the 1,3-dipolar property of azoalkenes, we recast simple pyrroles to fully substituted pyrroles, through a simple phosphoric acid-promoted one-pot reaction consisting of dearomative deconstruction and rearomative reconstruction steps. The reaction allows for easy access to synthetically challenging tetra-substituted pyrroles which are otherwise difficult to synthesize. Furthermore, we construct N-N axial chirality on our pyrrole products, as well as accomplish a facile synthesis of the anticancer drug, Sutent. The potential application of this method to other heterocycles has also been demonstrated.

Iso-Pentadienyl Carbonate as a Five Carbon Synthon in Manganese(I)-Catalyzed Selective Linear 1,3-Dienylation

Selective linear 1,3-dienylations are essential transformations, and numerous synthetic efforts have been documented. However, a general method enabling access to electron-rich, -poor, and biologically relevant dienyl molecules is in high demand. Hence, we report a straightforward method of manganese(I)-catalyzed C-H dienylation of arenes by using iso-pentadienyl carbonate as a five carbon synthon. This is a highly unprecedented report for selective linear 1,3-dienylation using manganese C-H activation catalysis. Our method facilitates the synthesis of varieties of dienes, including those suitable for normal or inverse electron demand Diels-Alder reactions, dienyl glycoconjugates, and unnatural amino acids. Extensive mechanistic studies, including isolation of C-H activated organo-manganese complex and isotopic analyses, have supported the proposed mechanism of this dienylation. The synthetic applicability of this method eased to deliver a 6/6/5-fused tricyclic nagilactone scaffold.

Mechanism and Selectivity of Copper-Catalyzed Bromination of Distal C(sp3)-H Bonds

Unactivated C(sp3)-H bonds are the most challenging substrate class for transition metal-catalyzed C-H halogenation. Recently, the Yu group [Liu, T.; Myers, M. C.; Yu, J. Q. Angew. Chem., Int. Ed.2017, 56 (1), 306-309] has demonstrated that a CuII/phenanthroline catalyst and BrN3, generated in situ from NBS and TMSN3 precursors, can achieve selective C-H bromination distal to a directing group. The current understanding of the mechanism of this reaction has left numerous questions unanswered. Here, we investigated the mechanism of Cu-catalyzed C(sp3)-H bromination with distal site selectivity using density functional theory calculations. We found that this reaction starts with the Br-atom transfer from BrN3 to the Cu center that occurs via a small energy barrier at the singlet-triplet state seam of crossing. In the course of this reaction, the presence of the N-H bond in the substrate is critically important and acts as a directing group for enhancing the stability of the catalyst-substrate interaction and for the recruitment of the substrate to the catalyst. The required C-centered radical substrate formation occurs via direct C-H dehydrogenation by the Cu-coordinated N3 radical, rather than via the previously proposed N-H bond dehydrogenation and then the 1,5-H transfer from the γ-(C-H) bond to the N-radical center pathway. The C-H bond activation by the azide radical is a regioselectivity-controlling step. The following bromination of the C-centered radical by the Cu-coordinated bromine completes the product formation. This reaction step is the rate-limiting step, occurs at the singlet-to-triplet state seam of the crossing point, and is exergonic.

Suzuki-Miyaura Type Regioselective C-H Arylation of Aromatic Aldehydes by a Transient Directing Strategy

Herein, we disclose a common approach for palladium-catalyzed direct coupling of the ortho-C-H bond of aromatic aldehydes with various organoboronic reagents by a transient directing strategy. In contrast to widely used cross-coupling reactions of C-H bonds with aryl halides, which generally need silver salt as a halide removal reagent, the method which used BQ/TFA as weak oxidation system for the PdII/Pd0 redox cycle is cost-effective, ecofriendly, and more aligned with green catalysis. This broadly applicable method opens up a new and efficient Suzuki-Miyaura coupling route for the direct formation of carbon-carbon bonds by C-H bond activation.

The crucial role of silver(I)-salts as additives in C-H activation reactions: overall analysis of their versatility and applicability

Transition-metal catalyzed C-H activation reactions have been proven to be useful methodologies for the assembly of synthetically meaningful molecules. This approach bears intrinsic peculiarities that are important to be studied and comprehended in order to achieve its best performance. One example is the use of additives for the in situ generation of catalytically active species. This strategy varies according to the type of additive and the nature of the pre-catalyst that is being used. Thus, silver(I)-salts have proven to play an important role, due to the resulting high reactivity derived from the pre-catalysts of the main transition metals used so far. While being powerful and versatile, the use of silver-based additives can raise concerns, since superstoichiometric amounts of silver(I)-salts are typically required. Therefore, it is crucial to first understand the role of silver(I) salts as additives, in order to wisely overcome this barrier and shift towards silver-free systems.

Conceptual advances in nucleophilic organophosphine-promoted transformations

Catalysis by trivalent nucleophilic organophosphines has emerged as an essential tool in organic synthesis. Several new organic transformations promoted by phosphines substantiate and complement the existing synthetic chemistry tools. Mere design of the substrate and reagent combinations has introduced new modes of reactivity patterns, which are otherwise difficult to achieve. These design considerations have led to the rapid build-up of complex molecular entities and laid a solid foundation to synthesise bioactive natural products and pharmaceuticals. This article presents an overview of some of the conceptual advances, including our contributions to nucleophilic organophosphine chemistry. The scope, limitations, mechanistic insights, and applications of these metal-free transformations are discussed elaborately.

Palladium-Catalyzed Direct C(sp2)-H Cyanomethylation of Arylamides using Chloroacetonitrile

In this study, we devised a palladium-catalyzed efficient and versatile method for C(sp2)-H ortho-cyanomethylation of arylamides with a broad substrate scope and moderate to excellent yields. An inexpensive and commercially available chloroacetonitrile was employed as a cyanomethylating source. This method is also compatible with the air atmosphere. Further, the synthetic feasibility of this technique is established by gram-scale synthesis and functional group transformation of the products.

Regioselective Iridium-Catalyzed C8-H Borylation of 4-Quinolones via Transient O-Borylated Quinolines

The quinolone-quinoline tautomerization is harnessed to effect the regioselective C8-borylation of biologically important 4-quinolones by using [Ir(OMe)(cod)]2 as the catalyst precursor, the silica-supported monodentate phosphine Si-SMAP as the ligand, and B2 pin2 as the boron source. Initially, O-borylation of the quinoline tautomer takes place. Critically, the newly formed 4-(pinBO)-quinolines then undergo N-directed selective Ir-catalyzed borylation at C8. Hydrolysis of the OBpin moiety on workup returns the system to the quinolone tautomer. The C8-borylated quinolines were converted to their corresponding potassium trifluoroborate (BF3 K) salts and to their C8-chlorinated quinolone derivatives. The two-step C-H borylation-chlorination reaction sequence resulted in various C8-Cl quinolones in good yields. Conversion to C8-OH-, C8-NH2 -, and C8-Ar-substituted quinolones was also feasible by using this methodology.

Charting the Evolution of Chemoenzymatic Strategies in the Syntheses of Complex Natural Products

Bolstered by recent advances in bioinformatics, genetics, and enzyme engineering, the field of chemoenzymatic synthesis has enjoyed a rapid increase in popularity and utility. This Perspective explores the integration of enzymes into multistep chemical syntheses, highlighting the unique potential of biocatalytic transformations to streamline the synthesis of complex natural products. In particular, we identify four primary conceptual approaches to chemoenzymatic synthesis and illustrate each with a number of landmark case studies. Future opportunities and challenges are also discussed.

Rhodium-Catalyzed Asymmetric C-H Functionalization Reactions

This review summarizes the advancements in rhodium-catalyzed asymmetric C-H functionalization reactions during the last two decades. Parallel to the rapidly developed palladium catalysis, rhodium catalysis has attracted extensive attention because of its unique reactivity and selectivity in asymmetric C-H functionalization reactions. In recent years, Rh-catalyzed asymmetric C-H functionalization reactions have been significantly developed in many respects, including catalyst design, reaction development, mechanistic investigation, and application in the synthesis of complex functional molecules. This review presents an explicit outline of catalysts and ligands, mechanism, the scope of coupling reagents, and applications.

1,2-Alkynyl Functionalization of Unactivated Alkenes via Diverse Radical-Triggered Functional Group Migration

We have developed a transition-metal-free radical approach for 1,2-alkynyl functionalization of unactivated alkenes through the combination of 3-exo-dig cyclization with alkynyl migration triggered by in situ-generated diverse radical precursors. This strategy provides a robust toolkit to access a variety of synthetically important α-functionalized alkynyl ketones, simultaneously installing densely functionalized carbonyl, alkynyl, and other various functional groups into the alkenes. The broad substrate scope, which includes distinctly electron-donating or electron-withdrawing alkynyl migrating groups, excellent functional group compatibility, and remarkable selectivity make this protocol practical and attractive.

Highly Enantioselective Catalytic Lactonization at Nonactivated Primary and Secondary γ-C-H Bonds

Chiral oxygenated aliphatic moieties are recurrent in biological and pharmaceutically relevant molecules and constitute one of the most versatile types of functionalities for further elaboration. Herein we report a protocol for straightforward and general access to chiral γ-lactones via enantioselective oxidation of strong nonactivated primary and secondary C(sp3)-H bonds in readily available carboxylic acids. The key enabling aspect is the use of robust sterically encumbered manganese catalysts that provide outstanding enantioselectivities (up to >99.9%) and yields (up to 96%) employing hydrogen peroxide as the oxidant. The resulting γ-lactones are of immediate interest for the preparation of inter alia natural products and recyclable polymeric materials.