Palladium Catalyzed meta-C-H Functionalization of Masked Aromatic Aldehydes

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.

Site-Selective Ortho/Ipso C-H Difunctionalizations of Arenes using Thianthrene as a Leaving Group

Site-selective ortho/ipso C-H difunctionalizations of aromatic compounds were designed to afford polyfunctionalized arenes including challenging 1,2,3,4-tetrasubstituted ones (62 examples, up to 97 % yields). To ensure the excellent regioselectivity of the process while keeping high efficiency, an original strategy based on a "C-H thianthenation/Catellani-type reaction" sequence was developed starting from simple arenes. Non-prefunctionalized arenes were first regioselectively converted into the corresponding thianthrenium salts. Then, a palladium-catalyzed, norbornene (NBE)-mediated process allowed the synthesis of ipso-olefinated/ortho-alkylated polyfunctionalized arenes using a thianthrene as a leaving group (revisited Catellani reaction). Pleasingly, using a commercially available norbornene (NBE) and a unique catalytic system, synthetic challenges known for the Catellani reaction with aryl iodides were smoothly and successfully tackled with the "thianthrenium" approach. The protocol was robust (gram-scale reaction) and was widely applied to the two-fold functionalization of various arenes including bio-active compounds. Moreover, a panel of olefins and alkyl halides as coupling partners was suitable. Pleasingly, the "thianthrenium" strategy was successfully further applied to the incorporation of other groups at the ipso (CN/alkyl/H, aryl) and ortho (alkyl, aryl, amine, thiol) positions, showcasing the generality of the process.

Native Amino Group Directed Meta-Selective C-H Arylation of Primary Amines Via Pd/Norbornene Catalysis

The selective functionalization of remote C-H bonds in free primary amines holds significant promise for the late-stage diversification of pharmaceuticals. However, to date, the direct functionalization of the meta position of amine substrates lacking additional directing groups remains underexplored. In this Letter, we present a successful meta-C-H arylation of free primary amine derivatives using aryl iodides, resulting in synthetically valuable yields. This meta-selective C-H functionalization is achieved through a sequence involving native amino-directed Pd-catalyzed seven-membered cyclometalation, followed by the utilization of a norbornene-type transient mediator.

Vanadium pyridonates: dimerization, redox behaviour, and metal-ligand cooperativity

The synthesis, structure, and reactivity of vanadium pyridonate complexes are described. Vanadium(III) pyridonate complexes were accessed through protonolysis and reduction of a tetrakis(amido)vanadium(IV) starting material. Bis(pyridonate) vanadium(IV) precursors could be isolated depending on the amount of proteoligand added. The targeted vanadium(III) species tend to form dimers, but monomeric complexes can be achieved in the presence of neutral donors such as amines or pyridine derivatives or through the use of sterically demanding proligands. The reduction process is proposed to involve dimeric intermediates and be mediated by the amine released from protonolysis, thereby forming the corresponding imine as a byproduct. Isolated amine complexes of vanadium(III) are presented. In contrast, bis(amidate)vanadium(IV) complexes were not found to undergo a similar reduction. This work informs on design principles for the synthesis and application of new vanadium pyridonate catalysts for transformations involving dimerization and PCET for changes in oxidation state.

Aryl Halides as Halogenation Reagents in the Bromination and Iodination of Arene-Tethered Diols

Aromatic halides constitute a valuable class of building blocks that are commonly used in organic synthesis. In this study, we demonstrate usage of aryl bromides and aryl iodides in C-Br or C-I bond formation. Methyl 2-bromobenzoate and 2-nitrophenyl iodides were developed as mild and effective bromination and iodination reagents for functionalization of arene-tethered diols. This efficient cascaded catalysis can be applied to the total syntheses of natural product Mafaicheenamine A and Claulamine A.

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.

Palladium(II)-Catalyzed Selective Arylation of Tertiary C-H Bonds of Cyclobutylmethyl Ketones Using Transient Directing Groups

We report the first example of selective PdII -catalyzed tertiary C-H activation of cyclobutylmethyl ketones using a transient directing group. An electron-deficient 2-pyridone ligand was identified as the optimal external ligand to enable tertiary C-H activation. A variety of cyclobutylmethyl ketones bearing quaternary carbon centers was readily accessed without preinstalling internal directing groups in up to 81 % yield and >95 : 5 regioisomeric ratios of tertiary C-H arylation to β-methylene (β-methyl) or γ-C-H arylation.

PdII-Catalyzed Site-selective β- and γ-C(sp3)-H Arylation of Primary Aldehydes Controlled by Transient Directing Groups

Pd(II)-catalyzed site-selective β- and γ-C(sp³)-H arylation of primary aldehydes is developed by rational design of L,X-type transient directing groups (TDG). External 2-pyridone ligands are identified to be crucial for the observed reactivity. By minimizing the loading of acid additives, the ligand effect is enhanced to achieve high reactivities of the challenging primary aldehyde substrates. Site selectivity can be switched from the proximate to the relatively remote position by changing the bite angle of TDG to match the desired palladacycle size. Experimental and computational investigations support this rationale for designing TDG to potentially achieve remote site-selective C(sp³)-H functionalizations.

Ligand Enabled Pd(II)-Catalyzed γ-C(sp3)-H Lactamization of Native Amides

γ-Lactams form important structural cores of a range of medicinally relevant natural products and clinical drugs, principal examples being the new generation of immunomodulatory imide drugs (IMiDs) and the brivaracetam family. Compared to conventional multistep synthesis, an intramolecular γ-C-H amination of aliphatic amides would allow for the direct construction of valuable γ-lactam motifs from abundant amino acid precursors. Herein we report a novel 2-pyridone ligand enabled Pd(II)-catalyzed γ-C(sp³)-H lactamization of amino acid derived native amides, providing the convenient synthesis of γ-lactams, isoindolinones, and 2-imidazolidinones. C6-Substitution of the 2-pyridone ligand is crucial for the lactam formation. This protocol features the use of N-acyl amino acids, which serve as both the directing group and cyclization partner, practical and environmentally benign tert-butyl hydrogen peroxide (TBHP) as the sole bystanding oxidant, and a broad substrate scope. The utility of this protocol was demonstrated through the two-step syntheses of a lenalidomide analog and brivaracetam from readily available carboxylic acids and amino acids.

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.

Transient Directing Groups in Metal-Organic Cooperative Catalysis

The direct functionalization of C-H bonds is among the most fundamental chemical transformations in organic synthesis. However, when the innate reactivity of the substrate cannot be utilized for the functionalization of a given single C-H bond, this selective C-H bond functionalization mostly relies on the use of directing groups that allow bringing the catalyst in close proximity to the C-H bond to be activated and these directing groups need to be installed before and cleaved after the transformation, which involves two additional undesired synthetic operations. These additional steps dramatically reduce the overall impact and the attractiveness of C-H bond functionalization techniques since classical approaches based on substrate pre-functionalization are sometimes still more straightforward and appealing. During the past decade, a different approach involving both the in situ installation and removal of the directing group, which can then often be used in a catalytic manner, has emerged: the transient directing group strategy. In addition to its innovative character, this strategy has brought C-H bond functionalization to an unprecedented level of usefulness and has enabled the development of remarkably efficient processes for the direct and selective introduction of functional groups onto both aromatic and aliphatic substrates. The processes unlocked by the development of these transient directing groups will be comprehensively overviewed in this review article.

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.

C–H hetero-functionalization of arenes through palladacyclopentane-type intermediates

In this review article, we summarized recent advances in C–H hetero-functionalization of arenes through palladacyclopentane-type intermediates.

Synthesis of Chiral-Substituted 2-Aryl-ferrocenes by the Catellani Reaction

A palladium-catalyzed and norbornene-mediated methodology has been developed for the synthesis of chiral 2-aryl-ferroceneamides from chiral 2-iodo-N,N-diisopropylferrocencarboxamide, iodoarenes, and alkenes using a JohnPhos ligand and potassium carbonate as a base in dimethylformamide at 105 °C. The developed three-component coupling protocol allows the compatibility of electron-withdrawing fluoro, chloro, ester, and nitro and electron-donating methyl, methoxy, dimethoxy, benzyl ether-substituted iodo-benzenes, other iodoarenes, such as iodo-naphthalene, heteroarenes, such as iodothiophene, and terminating substrates, such as methyl, ethyl, tert-butyl acrylates, and substituted styrenes with 2-iodo-N,N-diisopropylferrocencarboxamide. Furthermore, the developed three-component Catellani method proceeded with the retention of the configuration of the planar chiral ferrocene, which depends on the role of the participating carbon-iodine bond in ferrocene. Consequently, the developed protocol enabled the formation of densely substituted chiral 2-aryl ferroceneamides, exhibiting good to excellent enantioselectivity. The conversion of an ester of the synthesized chiral 2-aryl ferroceneamides has also been carried out to further accommodate the easily expendable acid and alcohol functionalities.

Recyclable Ruthenium Catalyst for Distal meta-C-H Activation

We disclose the unprecedented hybrid-ruthenium catalysis for distal meta-C-H activation. The hybrid-ruthenium catalyst was recyclable, as was proven by various heterogeneity tests, and fully characterized with various microscopic and spectroscopic techniques, highlighting the physical and chemical stability. Thereby, the hybrid-ruthenium catalysis proved broadly applicable for meta-C-H alkylations of among others purine-based nucleosides and natural product conjugates. Additionally, its versatility was further reflected by meta-C-H activations through visible-light irradiation, as well as para-selective C-H activations.

DMAP and PivOH-promoted amination/allenization reaction

This report described the first DMAP and PivOH-promoted ortho-C-H amination and ipso-allenization reaction of iodobenzenes realized by Pd/norbornene cooperative catalysis. Based on control experiments and DFT calculations, we speculated that the three ligands have different functions and mechanism paths in the reaction.

PdII -Catalyzed Enantioselective C(sp3 )-H Arylation of Cyclobutyl Ketones Using a Chiral Transient Directing Group

The use of chiral transient directing groups (TDGs) is a promising approach for developing PdII -catalyzed enantioselective C(sp3 )-H activation reactions. However, this strategy is challenging because the stereogenic center on the TDG is often far from the C-H bond, and both TDG covalently attached to the substrate and free TDG are capable of coordinating to PdII centers, which can result in a mixture of reactive complexes. We report a PdII -catalyzed enantioselective β-C(sp3 )-H arylation reaction of aliphatic ketones using a chiral TDG. A chiral trisubstituted cyclobutane was efficiently synthesized from a mono-substituted cyclobutane through sequential C-H arylation reactions, thus demonstrating the utility of this method for accessing structurally complex products from simple starting materials. The use of an electron-deficient pyridone ligand is crucial for the observed enantioselectivity. Interestingly, employing different silver salts can reverse the enantioselectivity.

Bioaldehydes and beyond: Expanding the realm of bioderived chemicals using biogenic aldehydes as platforms

Biofuels and biochemicals derived from renewable resources are sconsidered as potential solutions for the energy crisis and associated environmental problems that human beings are facing today. However, so far the available types of bioderived chemicals are rather limited, and production efficiency is generally low. Expanding the realm of bioderived chemicals and relevant derivatives can help motivate the development of bioenergy and the general bioeconomy. Aldehydes, possessing unique reactivity, hold great promise as platform chemicals for producing a large portfolio of bioproducts. In this review, we focus on production of aldehydes from renewable bioresources and derivatization of aldehydes through chemocatalysis, biocatalysis, or de novo biosynthesis. Perspectives on combining protein engineering and cascade reactions for advanced aldehyde derivatization are also provided.

Pd-Catalyzed γ-C(sp3)-H Fluorination of Free Amines

The first example of free amine γ-C(sp³)-H fluorination is realized using 2-hydroxynicotinaldehyde as the transient directing group. A wide range of cyclohexyl and linear aliphatic amines could be fluorinated selectively at the γ-methyl and methylene positions. Electron withdrawing 3,5-disubstituted pyridone ligands were identified to facilitate this reaction. Computational studies suggest that the turnover determining step is likely the oxidative addition step for methylene fluorination, while it is likely the C-H activation step for methyl fluorination. The explicit participation of Ag results in a lower energetic span for methylene fluorination and a higher energetic span for methyl fluorination, which is consistent with the experimental observation that the addition of silver salt is desirable for methylene but not for methyl fluorination. Kinetic studies on methyl fluorination suggest that the substrate and PdL are involved in the rate-determining step, indicating that the C-H activation step may be partially rate-determining. Importantly, an energetically preferred pathway has identified an interesting pyridone-assisted bimetallic transition state for the oxidative addition step in methylene fluorination, thus uncovering a potential new role of the pyridone ligand.

Ligand-Enabled Pd(II)-Catalyzed C(sp3)-H Lactonization Using Molecular Oxygen as Oxidant

Pd(II)-catalyzed C-H lactonization of o-methyl benzoic acid substrates has been achieved using molecular oxygen as the oxidant. This finding provides a rare example of C-H oxygenation through Pd(II)/Pd(0) catalysis as well as a method to construct biologically important benzolactone scaffolds. The use of a gas mixture of 5% oxygen in nitrogen demonstrated the possibility for its application in pharmaceutical manufacturing.

Photocatalytic site-selective C-H difluoroalkylation of aromatic aldehydes

The direct photocatalyzed para-selective CAr-H difluoroalkylation of aromatic aldehyde derivatives has been accomplished using a newly explored catalytic system. In addition, when using para-substituted benzaldehydes as substrates, ortho-selective CAr-H difluoroalkylation was also accomplished. It is worth noting that all the above site-selectivity is opposite to traditional Friedel-Crafts reactions of aromatic aldehydes. The preliminary mechanistic investigations indicate that an electrophilic difluoroalkyl radical is involved in the catalytic cycle.

Palladium-catalyzed C–H activation of anisole with electron-deficient auxiliary ligands: a mechanistic investigation

In this work, the experimentally observed site-selectivity and ligand choice of Yu's meta-arylation reaction were rationalized through DFT calculations.

Chelation-directed remote meta-C-H functionalization of aromatic aldehydes and ketones

We disclose herein the development of a versatile 1,2-diol directing template for palladium-catalyzed remote meta-C-H functionalization of aromatic aldehydes and ketones. In situ-generation of acetals and ketals, as well as removal afterwards, makes the C-H bond functionalization process more straightforward and efficient. This also represents the first example of chelation-directed meta-C-H functionalization of aromatic aldehydes and ketones.

Intramolecular Remote C-H Activation via Sequential 1,4-Palladium Migration To Access Fused Polycycles

An unprecedented intramolecular remote C-H activation via sequential 1,4-palladium migration with an aromatic ring as a conveyor has been described. This reaction provides an efficient route to construct diverse polycyclic frameworks in moderate to good yield via palladium-catalyzed remote C-H activation/alkene insertion, arylation, alkenylation, and the Heck reaction. The preliminary mechanistic studies revealed that the 1,4-palladium migration process was reversible.

Palladium/Norbornene Cooperative Catalysis

Palladium/norbornene cooperative catalysis has emerged as a distinct approach to construct polyfunctionalized arenes from readily available starting materials. This Review provides a comprehensive overview of this field, including the early stoichiometric investigations, catalytic reaction developments, as well as the applications in the syntheses of bioactive compounds and polymers. The section of catalytic reactions is divided into two parts according to the reaction initiation mode: Pd(0)-initiated reactions and Pd(II)-initiated reactions.

Visible-Light-Enabled Ruthenium-Catalyzed meta-C-H Alkylation at Room Temperature

Visible-light-induced ruthenium catalysis has enabled remote C-H alkylations with excellent levels of position control under exceedingly mild conditions at room temperature. The metallaphotocatalysis occurred under exogenous-photosensitizer-free conditions and features an ample substrate scope. The robust nature of the photo-induced mild meta-C-H functionalization is reflected by the broad functional group tolerance, and the reaction can be carried out in an operationally simple manner, setting the stage for challenging secondary and tertiary meta-C-H alkylations by ruthenaphotoredox catalysis.

Cascade alkylation and deuteration with aryl iodides via Pd/norbornene catalysis: an efficient method for the synthesis of congested deuterium-labeled arenes

An efficient approach for synthesizing congested deuterium-labeled arenes via cascade alkylation and deuteration with aryl iodides has been disclosed.

Overcoming the Limitations of γ- and δ-C-H Arylation of Amines through Ligand Development

L,X-Type transient directing groups (TDGs) based on a reversible imine linkage have emerged as broadly useful tools for C-H activation of ketones and free amines. However, competitive binding interactions among multiple reaction components (TDG itself, substrate, and substrate-TDG adduct) with the palladium catalyst often lead to the formation of multiple unreactive complexes, rendering ligand development extremely challenging. Herein, we report the finding of versatile 2-pyridone ligands that addresses these problems and significantly improves the γ-methylene arylation of alkyl amines, extending the coupling partners to a wide range of medicinally important heteroaryl iodides and even previously unreactive heteroaryl bromides. The combination of an appropriate transient directing group and pyridone ligand has also enabled the δ-arylation of alkyl amines. Notably, our transient directing group design reveals the importance of matching the size of the Pd-chelation with different transient directing groups and the size of palladacycles generated from γ- and δ-C-H bonds: TDGs that coordinate with Pd(II) to form a six-membered chelate are selective toward γ-C-H bonds, whereas TDGs that coordinate with Pd(II) via a five-membered chelate tend to activate δ-C-H bonds. These findings provide an avenue for developing protecting group free and selective C-H functionalization using the transient directing group strategy.

Copper-catalyzed oxidative intermolecular 1,2-alkylarylation of styrenes with ethers and indoles

A new copper-catalyzed oxidative three-component intermolecular 1,2-alkylarylation of styrenes with ethers and indoles is disclosed.