Palladium‐Catalyzed Directed meta ‐Selective C−H Allylation of Arenes: Unactivated Internal Olefins as Allyl Surrogates

Pd-catalyzed regioselective activation of C(sp2)-H and C(sp3)-H bonds

Differentiating between two highly similar C-H bonds in a given molecule remains a fundamental challenge in synthetic organic chemistry. Directing group assisted strategies for the functionalisation of proximal C-H bonds has been known for the last few decades. However, distal C-H bond functionalisation is strenuous and requires distinctly specialised techniques. In this review, we summarise the advancement in Pd-catalysed distal C(sp2)-H and C(sp3)-H bond activation through various redox manifolds including Pd(0)/Pd(II), Pd(II)/Pd(IV) and Pd(II)/Pd(0). Distal C-H functionalisation, where a Pd-catalyst is directly involved in the C-H activation step, either through assistance of an external directing group or directed by an inherent functionality or functional group incorporated at the site of the Pd-C bond is covered. The purpose of this review is to portray the current state of art in Pd-catalysed distal C(sp2)-H and C(sp3)-H functionalisation reactions, their mechanism and application in the late-stage functionalisation of medicinal compounds along with highlighting its limitations, thus leaving the field open for further synthetic adjustment.

Rh(III)-Catalyzed C-H Allylation of Aromatic Ketoximes with Vinylaziridines

The Rh(III)-catalyzed reaction of aromatic ketoximes with 2-vinylaziridines affords ortho-allylation products of the phenyl rings of aromatic ketoximes in moderate to excellent yields. The reaction requires 0.5 equiv of NaOAc as a base and occurs under mild conditions. The protocol exhibits ortho-monoallylation selectivity, wide scope of substrates, and good compatibility of functional groups.

Al2 O3 -coated BiVO4 Photoanodes for Photoelectrocatalytic Regioselective C-H Activation of Aromatic Amines

Photoelectrochemistry is becoming an innovative approach to organic synthesis. Generally, the current photoelectrocatalytic organic transformations suffer from limited reaction type, low conversion efficiency and poor stability. Herein, we develop efficient and stable photoelectrode materials using metal oxide protective layer, with a focus on achieving regioselective activation of amine compounds. Notably, our photoelectrochemistry process is implemented under mild reaction conditions and does not involve any directing groups, transition metals or oxidants. The results demonstrate that beyond photocatalysis and electrocatalysis, photoelectrocatalysis exhibits high efficiency, remarkable repeatability and good functional group tolerance, highlighting its great potential for applications.

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.

Iron-Catalyzed para-Selective C-H Allylation of Aniline Derivatives

Transition-metal-catalyzed directed C-H allylation of arenes offers an efficient and straightforward approach to construct value-added allylic arenes. However, these reactions are often performed with precious transition-metal catalysts and mainly limited to ortho-C-H allylation of arenes. Herein, we disclose a novel iron-catalyzed para-C-H allylation of aniline derivatives with allyl alcohols via a chelation-induced strategy, providing various allylic arenes in good yields with excellent regio- and chemoselectivity. A simple FeCl3·6H2O is employed as a catalyst, serving a dual role in the reaction: (1) coordination with N-arylpicolinamide to alter the electronic property of the aromatic ring and (2) reaction with allyl alcohol to form allyl-Fe species.

Chemodivergent Tandem Radical Cyclization of Alkene-Substituted Quinazolinones: Rapid Access to Mono- and Di-Alkylated Ring-Fused Quinazolinones

Chemodivergent tandem radical cyclization offers exciting possibilities for the synthesis of structurally diverse cyclic compounds. Herein, we revealed a chemodivergent tandem cyclization of alkene-substituted quinazolinones under metal- and base-free conditions, this transformation is initiated by alkyl radicals produced from oxidant-induced α-C(sp3 )-H functionalization of alkyl nitriles or esters. The reaction resulted in the selective synthesis of a series of mono- and di-alkylated ring-fused quinazolinones by modulating the loading of oxidant, reaction temperature, and reaction time. Mechanistic investigations show that the mono-alkylated ring-fused quinazolinones is constructed by the key process of 1,2-hydrogen shift, whereas the di-alkylated ring-fused quinazolinones is mainly achieved through crucial steps of resonance and proton transfer. This protocol is the first example of remote second alkylation on the aromatic ring via α-C(sp3 )-H functionalization and difunctionalization achieved by association of two unsaturated bonds in radical cyclization.

Palladium mono-N-protected amino acid complexes: experimental validation of the ligand cooperation model in C-H activation

Mechanistic proposals for the C-H activation reaction enabled by mono-N-protected amino acid ligands (MPAAs) have been supported by DFT calculations. The direct experimental observation of the ligand-assisted C-H activation has not yet been reported due to the lack of well-defined isolated palladium complexes with MPAAs that can serve as models. In this work, palladium complexes bearing chelating MPAAs (NBu₄)[Pd(κ²-N,O-AcN-CHR-COO)(C₆F₅)py] (Ac = MeC(O); R = H, Me) and [Pd(κ²-N,O-MeNH-CH₂-COO)(C₆F₅)py] have been isolated and characterized. Their evolution in a solution containing toluene leads to the C-H activation of the arene and the formation of the C₆F₅-C₆H₄Me coupling products. This process takes place only for the ligands with an acyl protecting group, showing the cooperating role of this group in a complex with a chelating MPAA, therefore experimentally validating this working model. The carboxylate group is inefficient in this C-H activation.

Dual Ligand Enabled Nondirected C-H Chalcogenation of Arenes and Heteroarenes

Chalcogenide motifs are present as principal moieties in a vast array of natural products and complex molecules. Till date, the construction of these chalcogen motifs has been restricted to either the use of directing groups or the employment of a large excess of electronically activated arenes, typically employed as a cosolvent. Despite being highly effective, these methods have their own limitations in the step economy and the deployment of an excess amount of arenes. Herein, we report the evolution of a catalytic system employing arene-limited, nondirected thioarylation of arenes and heteroarenes using a complimentary dual-ligand approach. The reaction is controlled by a combination of steric and electronic factors, and the utilization of a suitable ligand enables the generation of products on a complimentary spectrum to that generated by classical methods. The combination of ligands remains imperative in the reaction protocol with theoretical calculations pointing towards a monoprotected amino acid ligand being crucial in the concerted metalation deprotonation (CMD) mechanism by a characteristic [5,6]-palladacyclic transition state, while the pyridine moiety assists in the active catalyst species formation and product release. Combined experimental and computational mechanistic investigations point toward the C-H activation step being both regio- and rate-determining. Interestingly, oxidative addition of the diphenyl disulfide substrate is found to be unlikely, and an alternative transmetalation-like mechanism involving the Pd-Ag heterometallic complex is proposed to be operative.

RhodiumIII-catalyzed remote difunctionalization of arenes assisted by a relay directing group

Rhodium-catalyzed diverse tandem twofold C-H bond activation reactions of para-olefin-tethered arenes have been realized, with unsaturated reagents such as internal alkynes, dioxazolones, and isocyanates being the coupling partner as well as a relay directing group which triggers cyclization of the para-olefin group under oxidative or redox-neutral conditions. The reaction proceeded via initial ortho-C-H activation assisted by a built-in directing group in the arene, and the ortho-incorporation of the unsaturated coupling partner simultaneously generated a relay directing group that allows sequential C-H activation at the meta-position and subsequent cyclization of the para-olefins. The overall reaction represents C-C or N-C difunctionalization of the arene with the generation of diverse 2,3-dihydrobenzofuran platforms. The catalytic system proceeded with good efficiency, simple reaction conditions, and broad substrate scope. The diverse transformations of the products demonstrated the synthetic utility of this tandem reaction.

meta-Allylation of Arenes via Ruthenium-Catalyzed Cross-Dehydrogenative Coupling

A successful example of oxidative meta-dehydrogenative allylation of arenes with alkenes has been developed using Ru(PPh₃)₃Cl₂ as a catalyst and DTBP as an oxidant. In the allylation process, pyrimidines, pyrazoles, and purines, found widely in nucleosides, were effective auxiliary groups. Gram-scale experiments took place smoothly under optimized conditions. Mechanistic studies indicated that ruthenium-catalyzed meta-dehydrogenative allylation was a free-radical process. The allylation process developed herein provides an efficient and practical strategy to prepare versatile meta-allylated arenes.

Empirical Guidelines for the Development of Remote Directing Templates through Quantitative and Experimental Analyses

The ability to differentiate and selectively activate remote C-H bonds represents a perennial challenge in the field of C-H activation. Since its first report in 2012, a now-established "directing template" (DT) approach remains demonstrably effective for the functionalization of remote C-H bonds. As selectivity is hypothesized to be principally determined by the optimal positioning of the reactive catalyst to a target C-H bond, a DT's spatial factors are particularly important toward achieving high selectivity, though a systematic study on its requisite factors remain unelucidated. Through an in-depth analysis of 119 structurally unique published remote DTs, this report summarizes the key factors that are central toward achieving high selectivity at defined aryl positions, which are experimentally corroborated through the development of new aliphatic meta and para-selective DTs for electronically unbiased arenes. These empirical rules, which summarize key distance and geometric factors, are expected to be useful tools for the future development of site-selective arene C-H activation as well as other reactions that rely on covalent/noncovalent DT-mediated remote regioselection.

Recyclable Aliphatic Nitrile-Template Enabled Remote meta-C-H Functionalization at Room Temperature

This article describes the development of a new aliphatic nitrile-template-directed remote meta-selective C-H olefin functionalization reaction of arenes. Remarkably, unlike the previous reports, this process is feasible at room temperature and enabled the formation of products with excellent regioselectivity. The present protocol encompasses a broad spectrum of substituted dihydrocinnamic acids and olefins, producing meta-C-H olefinated products (up to 96% yield). In addition, the efficacy of the present method has been showcased by the synthesis of various drug analogues (e.g., cholesterol, estrone, ibuprofen, and naproxen). Significantly, the robustness of meta-olefination was also demonstrated by gram-scale synthesis. The new nitrile-based meta-directing template, in particular, could be easily synthesized in two steps and recycled under mild conditions.

Emergence of Pyrimidine-Based meta-Directing Group: Journey from Weak to Strong Coordination in Diversifying meta-C-H Functionalization

C-H activation has emerged as a powerful transformative synthetic tool to construct complex molecular frameworks, which are ubiquitous in natural products, medicines, dyes, polymers, and many more. However, reactivity and selectivity, arising from the inertness of C-H bonds and their overabundance in organic molecules, are the two major fundamental challenges in developing various carbon-carbon (C-C) and carbon-heteroatom (C-X) bond formation reactions via C-H activation technique. Functional groups with coordinating capacity to the transition metal catalysts, profoundly known as directing groups (DGs), have shown great promise in exerting selective C-H activation, often called site-selective or regioselective transformation of a target molecule. Advent of directing group (DG)-assisted strategies not only has resolved the selectivity issues but also offers a unique solution to the rapid synthesis of complex molecules in a convenient and predictable manner. Our laboratory, in this regard, is fascinated by the prospect of DG-assisted distal C-H functionalization of arenes, in which the target C-H bond is remotely located from the existing directing group. Notably, in opposition to proximal ortho-C-H activation, which proceeded via an energetically favorable five- to seven-membered metallacycle, distal C-H activation remained a formidable challenge as it required formation of a large macrocyclic metallacycle. Therefore, designing a suitable directing template that would maintain the required distance and geometric relationship between the target C-H bond and the appended directing auxiliary in order to ensure the prolific delivery of the metal catalyst to the closest proximity of targeted distal C-H bond was the key to success. In this regard, the Yu group devised an elegant "U-shaped" template for the first time to execute distal meta-C-H activation recruiting a cyano-based directing group. Our initial effort to diversify the scope of meta-C-H functionalization using a cyano-based template led us to realize that the "cyano-based DGs" are intrinsically limited with weak coordinating ability, competitive binding mode (end-on vs side-on), and incompatibility with acidic and basic reaction conditions. In search of a robust directing auxiliary, we were intrigued by the possibility of using the strongly coordinating ability of pyrimidine and quinoline-based DGs.In this Account, we describe our journey from the weakly coordinating cyano-based DG to the strongly coordinating pyrimidine-based DG to achieve diverse meta-C-H functionalization of electronically and sterically unbiased arenes. While some of the functionalizations were achieved by finding suitable reaction conditions, others were led by mechanistic understanding. Notably, initial development in this realm was constrained with short linkers, in which the DG was attached to the arene of interest through 2-4 atoms. In later studies, we demonstrated that the selective meta-C-H activation can be attained even though the DG is 10-atoms away from the targeted arene. More importantly, a transient DG was successfully utilized to deliver meta-C-H olefination of arenes via in situ imine formation, which provided a step-economic route to meta-C-H activation.We hope that this Account will stimulate further template design and will provide a guiding platform for the future development of distal meta-C-H functionalization.

Enantioselective palladaelectro-catalyzed C-H olefinations and allylations for N-C axial chirality

Enantioselective palladaelectro-catalyzed C–H alkenylations and allylations were achieved with easily-accessible amino acids as transient directing groups. This strategy provided access to highly enantiomerically-enriched N–C axially chiral scaffolds under exceedingly mild conditions. The synthetic utility of our strategy was demonstrated by a variety of alkenes, while the versatility of our approach was reflected by atroposelective C–H allylations. Computational studies provided insights into a facile C–H activation by a seven-membered palladacycle.

Enantioselective palladaelectro-catalyzed C–H alkenylations and allylations were achieved by the means of an easily-accessible amino acid for the synthesis of N–C axially chiral indole biaryls.

Cobalt-catalyzed multisubstituted allylation of the chelation-assisted C-H bond of (hetero)arenes with cyclopropenes

Cyclopropenes are highly strained three-membered carbocycles, which offer unique reactivity in organic synthesis. Herein, Cp*CoIII-catalyzed ring-opening isomerization of cyclopropenes to cobalt vinylcarbene has been utilized for the synthesis of multisubstituted allylarenes via directing group-assisted functionalization of C-H bonds of arenes and heteroarenes. Employing this methodology, various substituents can be introduced at all three carbons of the allyl moiety with high selectivity. The important highlights are excellent functional group tolerance, multisubstituted allylation, high selectivity, gram scale synthesis, removable directing group, and synthesis of cyclopenta[b]indoles. In addition, a potential cobaltocycle intermediate was identified and a plausible mechanism is also proposed.

Toolbox for Distal C-H Bond Functionalizations in Organic Molecules

Transition metal catalyzed C-H activation has developed a contemporary approach to the omnipresent area of retrosynthetic disconnection. Scientific researchers have been tempted to take the help of this methodology to plan their synthetic discourses. This paradigm shift has helped in the development of industrial units as well, making the synthesis of natural products and pharmaceutical drugs step-economical. In the vast zone of C-H bond activation, the functionalization of proximal C-H bonds has gained utmost popularity. Unlike the activation of proximal C-H bonds, the distal C-H functionalization is more strenuous and requires distinctly specialized techniques. In this review, we have compiled various methods adopted to functionalize distal C-H bonds, mechanistic insights within each of these procedures, and the scope of the methodology. With this review, we give a complete overview of the expeditious progress the distal C-H activation has made in the field of synthetic organic chemistry while also highlighting its pitfalls, thus leaving the field open for further synthetic modifications.

Mechanistic Investigation of Palladium-Catalyzed meta-C-H Bond Activation of Arenes with a Carboxyl Directing Group

The mechanism of Pd(II)-catalyzed meta-C-H bond olefination of arenes with a carboxyl directing group (DG)-containing template has been investigated with density functional theory. The reaction includes three major steps: C-H bond activation, alkene insertion, and β-hydride elimination. The C-H activation step, which proceeds via a concerted metalation-deprotonation pathway, is found to be the rate- and regioselectivity-determining step. We proposed a mono-N-protected amino acid (MPAA)/DG-assisted C-H activation model, in which the carboxyl DG coordinates with the Pd center and delivers it to the meta-position of arene, and the bidentate dianionic MPAA acts as a base for deprotonation. There is a hydrogen bonding interaction between the carboxyl DG and the carboxylate group of MPAA. An alternative Pd(OAc)₂-catalyzed mechanism without involvement of MPAA is also operative. The template is conformationally flexible, and multiple low-energy transition-state conformations contribute to the regioselectivity.

Highly Regioselective Aromatic C-H Allylation of N-(Arylmethyl)sulfonimides with Allyl Grignard Reagents Involving Benzylic C-N Cleavage

A new pair of reaction partners has been established for the aromatic C-H functionalization of benzyl electrophiles with nucleophiles via palladium-catalyzed benzylic C-N cleavage. A range of N-(1-naphthylmethyl)sulfonimides, N-(2-thienylmethyl)sulfonimides, and N-(2-furanylmethyl)sulfonimides smoothly underwent palladium-catalyzed aromatic C-H allylation with allyl Grignard reagents at room temperature, delivering structurally diverse substituted 1-allylnaphthalenes and 2-allylheteroarenes in moderate to excellent yields with extremely high regioselectivities. Replacing the N-(arylmethyl)sulfonimide with an (arylmethyl)ammonium salt, an arylmethyl chloride, or an arylmethyl phosphate as the benzyl electrophile leads to a dramatic erosion of the regioselectivity.

Decoding Directing Groups and Their Pivotal Role in C-H Activation

Synthetic organic chemistry has witnessed a plethora of functionalization and defunctionalization strategies. In this regard, C-H functionalization has been at the forefront due to the multifarious applications in the development of simple to complex molecular architectures and holds a brilliant prospect in drug development and discovery. Despite been explored tremendously by chemists, this functionalization strategy still enjoys the employment of novel metal catalysts as well metal-free organic ligands. Moreover, the switch to photo- and electrochemistry has widened our understanding of the alternative pathways via which a reaction can proceed and these strategies have garnered prominence when applied to C-H activation. Synthetic chemists have been foraging for new directing groups and templates for the selective activation of C-H bonds from a myriad of carbon-hydrogen bonds in aromatic as well as aliphatic systems. As a matter of fact, by varying the templates and directing groups, scientists found the answer to the challenge of distal C-H bond activation which remained an obstacle for a very long time. These templates have been frequently harnessed for selectively activating C-H bonds of natural products, drugs, and macromolecules decorated with multiple C-H bonds. This itself was a challenge before the commencement of this field as functionalization of a site other than the targeted site could modify and hamper the biological activity of the pharmacophore. Total synthesis and pharmacophore development often faces the difficulty of superfluous reaction steps towards selective functionalization. This obstacle has been solved by late-stage functionalization simply by harnessing C-H bond activation. Moreover, green chemistry and metal-free reaction conditions have seen light in the past few decades due to the rising concern about environmental issues. Therefore, metal-free catalysts or the usage of non-toxic metals have been recently showcased in a number of elegant works. Also, research groups across the world are developing rational strategies for directing group free or non-directed protocols that are just guided by ligands. This review encapsulates the research works pertinent to C-H bond activation and discusses the science devoted to it at the fundamental level. This review gives the readers a broad understanding of how these strategies work, the execution of various metal catalysts, and directing groups. This not only helps a budding scientist towards the commencement of his/her research but also helps a matured mind searching out for selective functionalization. A detailed picture of this field and its progress with time has been portrayed in lucid scientific language with a motive to inculcate and educate scientific minds about this beautiful strategy with an overview of the most relevant and significant works of this era. The unique trait of this review is the detailed description and classification of various directing groups and their utility over a wide substrate scope. This allows an experimental chemist to understand the applicability of this domain and employ it over any targeted substrate.

A density functional theory study on the mechanism of the allylpalladium-catalyzed dehydrogenation of aldehydes and cyclic ketones

The results of density functional theory calculations at the APFD/SDD level are detailed herein in order to study the main steps in the α,β-dehydrogenation of aldehydes and cyclic ketones in the presence of an allylpalladium complex catalyst. The mechanism is believed to proceed via an allylpalladium enolate complex (A) in equilibrium with the carbon-bonded complex (B), followed by β-hydride elimination to yield the allylpalladium hydride coordinated to the α,β-unsaturated carbonyl (complex C). The optimized structures and detailed energy profiles of these intermediates and their corresponding transition states are presented herein. The results indicate that the intermediates and their transition states are more stable in THF solution than in the gas phase. In detail, the energy barriers for the two steps are found to be 25.22 and 11.13 kcal/mol, respectively, in THF, and 29.93 and 9.77 kcal/mol, respectively, in the gas phase.

Rhodium Catalyzed Regioselective C-H Allylation of Simple Arenes via C-C Bond Activation of Gem-difluorinated Cyclopropanes

Herein, we report a rhodium catalyzed directing-group free regioselective C-H allylation of simple arenes. Readily available gem-difluorinated cyclopropanes can be employed as highly reactive allyl surrogates via a sequence of C-C and C-F bond activation, providing allyl arene derivatives in good yields with high regioselectivity under mild conditions. The robust methodology enables facile late-stage functionalization of complex bioactive molecules. The high efficiency of this reaction is also demonstrated by the high turnover number (TON, up to 1700) of the rhodium catalyst on gram-scale experiments. Preliminary success on kinetic resolution of this transformation is achieved, providing a promising access to enantio-enriched gem-difluorinated cyclopropanes.

Thioamide-Directed Cp*Co(III)-Catalyzed C-H Allylation of Ferrocenes

Herein, the first Cp*Co(III)-catalyzed C-H allylation of ferrocene thioamides with allyl carbonates has been developed. This reaction is compatible with a wide range of functional groups, providing various allylated ferrocene derivatives in up to 90% yields. In addition, the C-H allylation protocol is also compatible with the use of vinylcyclopropanes as allylating reagents by merging C-H and C-C activation into one catalytic system. Mechanistic studies revealed that the thiocarbonyl-directing group plays a vital role in C-H activation.

Imine as a linchpin approach for meta-C-H functionalization

Despite the widespread applications of C–H functionalization, controlling site selectivity remains a significant challenge. Covalently attached directing groups (DGs) served as ancillary ligands to ensure ortho-, meta- and para-C–H functionalization over the last two decades. These covalently linked DGs necessitate two extra steps for a single C–H functionalization: introduction of DG prior to C–H activation and removal of DG post-functionalization. Here we report a temporary directing group (TDG) for meta-C–H functionalization via reversible imine formation. By overruling facile ortho-C–H bond activation by imine-N atom, a suitably designed pyrimidine-based TDG successfully delivered selective meta-C–C bond formation. Application of this temporary directing group strategy for streamlining the synthesis of complex organic molecules without any necessary pre-functionalization at the meta position has been explored.

Site-selective C–H functionalization still faces some challenges, such as the introduction and removal of an appropriate directing group. Here, the authors introduce a temporary directing group for selective meta-C–H functionalization of 2-arylbenzaldehydes via reversible imine formation.

Nickel-Catalyzed [4 + 2] Annulation of Nitriles and Benzylamines by C-H/N-H Activation

Nickel-catalyzed [4 + 2] annulation of benzylamines and nitriles via C-H/N-H bond activation, providing straightforward atom-economic access to a wide variety of multisubstituted quinazolines, is reported. Mechanistic investigation revealed that the in situ formed amidines from the coupling of benzylamines and nitriles direct the nickel catalyst to activate the ortho-C-H bond of the phenyl ring of the benzylamine.

A simple removable aliphatic nitrile template 2-cyano-2,2-di-isobutyl acetic acid for remote meta-selective C–H functionalization

The remote meta-selective C–H functionalization of arenes using first aliphatic nitrile template 2-cyano-2,2-di-isobutyl acetic acid under mild conditions is presented.

Tailored quinones support high-turnover Pd catalysts for oxidative C-H arylation with O2

Palladium(II)-catalyzed carbon-hydrogen (C-H) oxidation reactions could streamline the synthesis of pharmaceuticals, agrochemicals, and other complex organic molecules. Existing methods, however, commonly exhibit poor catalyst performance with high palladium (Pd) loading (e.g., 10 mole %) and a need for (super)stoichiometric quantities of undesirable oxidants, such as benzoquinone and silver(I) salts. The present study probes the mechanism of a representative Pd-catalyzed oxidative C-H arylation reaction and elucidates mechanistic features that undermine catalyst performance, including substrate-consuming side reactions and sequestration of the catalyst as an inactive species. Systematic tuning of the quinone cocatalyst overcomes these deleterious features. Use of 2,5-di-tert-butyl-p-benzoquinone enables efficient use of molecular oxygen as the oxidant, high reaction yields, and >1900 turnovers by the Pd catalyst.

Palladium-Catalyzed Directed Atroposelective C-H Allylation via β-H Elimination: 1,1-Disubstituted Alkenes as Allyl Surrogates

Transition-metal-catalyzed dehydrogenative C-H allylation with 1,1-disubstituted alkenes via β-H elimination remains challenging, because of the low reactivity and difficulty of controlling selectivity. Herein, the development of a Pd(II)-catalyzed directed atroposelective C-H allylation with methacrylates is described. Exclusive allylic selectivity was achieved. A vast array of axially chiral biaryl-2-amines are efficiently synthesized with excellent enantioselectivities (up to >99% enantiomeric excess).

Ni-Catalyzed Regioselective 1,2-Dialkylation of Alkenes Enabled by the Formation of Two C(sp3)-C(sp3) Bonds

We disclose a Ni-catalyzed vicinal difunctionalization of alkenes with benzyl halides and alkylzinc reagents, which produces products with two new alkyl-alkyl bonds. This alkene dialkylation is effective in combining secondary benzyl halides and secondary alkylzinc reagents with internal alkenes, which furnishes products with three contiguous all-carbon secondary stereocenters. The products can be readily elaborated to access complex tetralene, benzosuberene, and bicyclodecene cores. The reaction also features as the most efficient alkene difunctionalization process to date with catalyst loadings down to 500 ppm and the catalytic turnover number (TON) and turnover frequency (TOF) registering up to 2 × 103 and 165 h-1 at rt, respectively.

Selective Dealkenylative Functionalization of Styrenes via C-C Bond Cleavage

As a readily available feedstock, styrene with about 25 million tons of global annual production serves as an important building block and organic synthon for the synthesis of fine chemicals, polystyrene plastics, and elastomers. Thus, in the past decades, many direct transformations of this costless styrene feedstock were disclosed for the preparation of high-value chemicals, which to date, generally performed on the functionalization of styrenes through the allylic C-H bond, C(sp²)-H bond, or the C=C double bond cleavage. However, the dealkenylative functionalization of styrenes via the direct C-C single bond cleavage is so far challenging and still unknown. Herein, we report the novel and efficient C-C amination and hydroxylation reactions of styrenes for the synthesis of valuable aryl amines and phenols via the site-selective C(Ar)-C(alkenyl) single bond cleavage. This chemistry unlocks the new transformation and application of the styrene feedstock and provides an efficient protocol for the late-stage modification of substituted styrenes with the site-directed dealkenylative amination and hydroxylation.

Carboxyl-Assisted meta-Selective C-H Functionalizations of Benzylsulfonamides

A protocol of carboxyl-group-assisted, Pd(II)-catalyzed remote meta-C(sp²)-H olefination and arylation of benzylsulfonamides has been developed. It was supposed to proceed through a κ² coordination of the carboxyl group to the Pd center. These findings demonstrated the versatility of carboxyl-assisted remote meta-C-H activation strategy and might stimulate the exploration of novel reactivity and selectivity of other traditional chelating groups in different contexts.

Traceless directing groups: a novel strategy in regiodivergent C-H functionalization

The use of functional groups as internal ligands for assisting C-H functionalization, termed the chelation assisted strategy, is emerging as one of the most powerful tools for construction of C-C and C-X bonds from inert C-H bonds. However, there are various directing groups which cannot be either removed after functionalization or require some additional steps or reagents for their removal, thereby limiting the scope of structural diversity of the products, and the step and atom economy of the system. These limitations are overcome by the use of the traceless directing group (TDG) strategy wherein functionalization of the substrate and removal of the directing group can be carried out in a one pot fashion. Traceless directing groups serve as the most ideal chelation assisted strategy with a high degree of reactivity and selectivity without any requirement for additional steps for their removal. The present review overviews the use of various functional groups such as carboxylic acids, aldehydes, N-oxides, nitrones, N-nitroso amines, amides, sulfoxonium ylides and silicon tethered directing groups for assisting transition metal catalyzed C-H functionalization reactions in the last decade.

Ultrasound-Facilitated Direct meta-C-H Functionalization of Arenes: A Time-Economical Strategy under Ambient Temperature with Improved Yield and Selectivity

The horizon of ultrasound-assistance has been expanded to palladium-catalyzed distal C-H functionalization of arenes. Compared to thermal conditions, operationally simple ultrasound mediated distal C-H functionalization occurred with a shorter reaction time and enhanced reactivity of reactants to give superior yields with improved selectivity both in terms of meta:others and mono:di. A wide variety of meta-functionalizations such as olefination, alkylation, acetoxylation, allylation and cyanation were successfully carried out under ambient temperature.

Achieving Site-Selectivity for C-H Activation Processes Based on Distance and Geometry: A Carpenter's Approach

The ability to differentiate between highly similar C-H bonds in a given molecule remains a fundamental challenge in organic chemistry. In particular, the lack of sufficient steric and electronic differences between C-H bonds located distal to functional groups has prevented the development of site-selective catalysts with broad scope. An emerging approach to circumvent this obstacle is to utilize the distance between a target C-H bond and a coordinating functional group, along with the geometry of the cyclic transition state in directed C-H activation, as core molecular recognition parameters to differentiate between multiple C-H bonds. In this Perspective, we discuss the advent and recent advances of this concept. We cover a wide range of transition-metal-catalyzed, template-directed remote C-H activation reactions of alcohols, carboxylic acids, sulfonates, phosphonates, and amines. Additionally, we review eminent examples which take advantage of non-covalent interactions to achieve regiocontrol. Continued advancement of this distance- and geometry-based differentiation approach for regioselective remote C-H functionalization reactions may lead to the ultimate realization of molecular editing: the freedom to modify organic molecules at any site, in any order.

Dehydration in water: frustrated Lewis pairs directly catalyzed allylization of electron-rich arenes and allyl alcohols

A frustrated Lewis pair (FLP)-catalyzed allylation of allyl alcohols with electron-rich arenes has been developed. Interestingly, in this reaction, the electron-rich arenes and allyl alcohols are dehydrated in water. What's more, water was the sole byproduct of the reaction. In this protocol, various allyl alcohols can be converted into allyl cations and attacked by the electron-rich arenes to form aryl cation intermediates. Finally, the aryl cation intermediates are deprotonated to give the 1,3-diarylpropenes. In this protocol, indole allyl alcohols can undergo a bimolecular ring closure reaction, and structurally diverse tetrahydroindolo[3,2-b]carbazoles could be smoothly obtained. The reaction is not sensitive to oxygen and has been performed on a gram-scale.

Mechanism and Origin of MAD-Induced Ni/N-Heterocyclic Carbene-Catalyzed Regio- and Enantioselective C-H Cyclization of Pyridines with Alkenes

This work presents a DFT-based computational study on the regio- and enantioselective C-H functionalization of pyridines with alkenes at the relatively unreactive C4-position, which was successfully achieved by Shi et al. [J. Am. Chem. Soc. 2019, 141, 5628-5634] using Ni⁰ /N-heterocyclic carbene (NHC) catalysis under the assistance of an aluminum-based Lewis acid additive (2,6-tBu₂ -4-Me-C₆ H₂ O)₂ AlMe (MAD). The calculations indicate that the selective functionalization involves a three-step mechanism in which a unique H-migration assisted oxidation metalation (HMAOM) step is identified as the rate- and enantioselectivity-determining step. The newly proposed mechanism can well rationalize the experimental observation that the preferred product is the endo-type (vs. exo-type), R-configuration (vs. S-configuration) product at the C4 (vs. C2) position, and also unveil the reasons that the NHC ligand and the MAD additive can facilitate the reaction.

Dual ligand-promoted palladium-catalyzed nondirected C-H alkenylation of aryl ethers

Direct C-H functionalization of aryl ethers remains challenging owing to their low reactivity and selectivity. Herein, a novel strategy for nondirected C-H alkenylation of aryl ethers promoted by a dual ligand catalyst was demonstrated. This catalytic system readily achieved the highly efficient alkenylation of alkyl aryl ethers (anisole, phenetole, n-propyl phenyl ether, n-butyl phenyl ether and benzyl phenyl ether), cyclic aryl ethers (1,4-benzodioxan, 2,3-dihydrobenzofuran, dibenzofuran), and diphenyl oxides. Moreover, the proposed methodology was successfully employed for the late-stage modification of complex drugs containing the aryl ether motif. Interestingly, the compounds developed herein displayed fluorescent properties, which would facilitate their biological applications.

Chromium(III)-Catalyzed C(sp2)-H Alkynylation, Allylation, and Naphthalenation of Secondary Amides with Trimethylaluminum as Base

Among base metals used for C-H activation reactions, chromium(III) is rather unexplored despite its natural abundance and low toxicity. We report herein chromium(III)-catalyzed C(sp²)-H functionalization of an ortho-position of aromatic and α,β-unsaturated secondary amides using readily available AlMe₃ as a base and using bromoalkynes, allyl bromide, and 1,4-dihydro-1,4-epoxynaphthalene as electrophiles. This redox-neutral reaction taking place at 70-90 °C, requires as low as 1-2 mol % of CrCl₃ or Cr(acac)₃ as a catalyst without any added ligand, and tolerates functional groups such as aryl iodide, boronate, and thiophene groups. Stoichiometric and kinetics studies as well as kinetic isotope effects suggest that the catalytic cycle consists of a series of thermally stable but reactive intermediates bearing two molecules of the amide substrate on one chromium atom and also that one of these chromate(III) complexes takes part in the alkynylation, allylation, and naphthalenation reactions. The proposed mechanism accounts for the effective suppression of methyl group delivery from AlMe₃ for ortho-C-H methylation.