Ruthenium-Catalyzed Meta Sulfonation of 2-Phenylpyridines

Gold-Catalyzed Diyne-Ene Annulation for the Synthesis of Polysubstituted Benzenes through Formal [3+3] Approach with Amide as the Critical Co-Catalyst

The one-step synthesis of tetra-substituted benzenes was accomplished via gold-catalyzed diyne-ene annulation. Distinguished from prior modification methods, this novel strategy undergoes formal [3+3] cyclization, producing polysubstituted benzenes with exceptional efficiency. The critical factor enabling this transformation was the introduction of amides, which were reported for the first time in gold catalysis as covalent nucleophilic co-catalysts. This interesting protocol not only offers a new strategy to achieve functional benzenes with high efficiency, but also enlightens potential new reaction pathways within gold-catalyzed alkyne activation processes.

meta-C-H functionalization of phenylethyl and benzylic alcohol derivatives via Pd/NBE relay catalysis

The transition metal-catalyzed meta-C-H functionalization of alcohols and their hydroxylamine derivatives remains underdeveloped. Herein, we report an efficient meta-C-H arylation of both phenylethyl and benzylic alcohols and their hydroxylamine derivatives using a readily removable oxime ether directing group. Using electronically activated 2-carbomethoxynorbornene as the transient mediator and 3-trifluoromethyl-2-pyridone as the enabling ligand, this reaction features a broad substrate scope and good functional group tolerance. More importantly, with this oxime-directed meta-C-H functionalization, this method provides a dual approach for efficient access to both meta-substituted alcohols and hydroxylamines using two sets of simple deprotection conditions. This protocol leads to the efficient synthesis of bioactive compounds possessing promising reactivities for the treatment of pulmonary fibrosis.

Pd-Catalyzed Picolinamide-Directed C(sp2)-H Sulfonylation of Amino Acids/Peptides with Sodium Sulfinates

This report describes a Pd-catalyzed picolinamide-directed site-selective C(sp2)-H sulfonylation of amino acids and peptides with sodium sulfinates in moderate to good yields. Sulfonylation of levodopa and dopamine drug molecules and late-stage directed peptide sulfonylation are studied for the first time. Broad substrate scope having various functionalities, late-stage drug modifications, and various post synthetic utilities such as chalcogenation, bromination, olefination, and arylation are potential advantages.

Ligand-enabled ruthenium-catalyzed meta-C-H alkylation of (hetero)aromatic carboxylic acids

Carboxylates are ideal directing groups because they are widely available, readily cleavable and excellent linchpins for diverse follow-up reactions. However, their use in meta-selective C-H functionalizations remains a substantial unmet catalytic challenge. Herein, we report the ruthenium-catalyzed meta-C-H alkylation of aromatic carboxylic acids with various functionalized alkyl halides. A bidentate N-ligand increases the electron density at the metal center of ortho-benzoate ruthenacycles to the extent that single-electron reductions of alkyl halides can take place. The subsequent addition of alkyl radicals is exclusively directed to the position para to the CAr-Ru bond, i.e., meta to the carboxylate group. The resulting catalytic meta-C-H alkylation extends to a wide range of (hetero)aromatic carboxylic acids including benzofused five-membered ring heteroarenes but no pyridine derivatives in combination with secondary/tertiary alkyl halides, including fluorinated derivatives. It also allows site-selective C5-H alkylation of 1-naphthoic acids. The products are shown to be synthetic hubs en route to meta-alkylated aryl ketones, nitriles, amides, esters and other functionalized products.

Ruthenium-Catalyzed Meta-Selective Trifluoroisopropylation of Arenes

This work reports the mild and efficient Ru-catalyzed trifluoroisopropylation of arenes using 2-bromo-1,1,1-trifluoropropane. Various bioactive molecules, such as purine and nucleoside derivatives, were well-suited for this transformation, affording the corresponding products in moderate-to-good yields. This method provides an efficient strategy for synthesizing trifluoroisopropyl molecules for drug discovery.

DLPNO-CCSD(T) and DFT study of the acetate-assisted C-H activation of benzaldimine at [RuCl2(p-cymene)]2: the relevance of ligand exchange processes at ruthenium(II) complexes in polar protic media

To gain mechanistic insights into the acetate-assisted cyclometallations of arylimines promoted by [RuCl2(p-cymene)]2 in polar protic media, DFT geometry optimizations (with M06 and ωB97X-D3 functionals and the cc-pVDZ-PP[Ru] basis set) followed by DLPNO-CCSD(T)/CBS energy evaluations were performed using benzaldimine as a model substrate and methanol as the solvent (with CPCM or SMD models). The calculation results show that coordination of the imine to an acetate ruthenium precursor is followed by anion (chloride or acetate) dissociation as the rate-determining step of the process. H-Bonding of two explicit MeOH to the anion reduces the calculated activation energy to ca. 23 kcal mol-1, in good agreement with the experimental half-life at room temperature. Subsequent AMLA/CMD C-H activation of the intermediate cationic complexes is a faster, reversible process. Alternative reaction pathways involving neutral diacetate ruthenium complexes offer AMLA/CMD transition state structures of lower energy but are precluded due to higher energy barriers for the initial ligand exchange processes at ruthenium. Solvent assistance accelerates the final chloride/acetate exchange processes on the cycloruthenate intermediates, particularly when compression in the condensed phase is taken into consideration. The performance of six DFT functionals (with the aug-pVTZ-PP[Ru] basis set) was assessed using the DLPNO-CCSD(T)/CBS reference energies. Neutral diacetate ruthenium complexes were incorrectly predicted as being kinetically relevant when using hybrid DFT methods (PBE0-D3(BJ), M06-2X or ωB97M-V). Good agreement between the calculated barrier heights and our benchmark energy results was obtained by using double-hybrid DFT methods. PWPB95 with D3(BJ) or D4 dispersion energy corrections was found to be the most accurate (ΔG≠ MUE of ca. 1 kcal mol-1). This study may aid our understanding of and help with further experimental investigations of synthetically useful carboxylate-assisted C-H bond functionalizations involving (N,C)-cyclometallated (p-cymene)Ru(II) intermediate complexes in sustainable polar protic solvents.

Advancing Meta-Selective C-H Amination through Non-Covalent Interactions

Regioselective C-H amination of simple arenes is highly desirable, but accessing meta-sites of ubiquitous arenes has proven challenging due to the lack of both electronic and spatial preference. This study demonstrates the successful use of various privileged nitrogen-containing functionalities found in pharmaceutical compounds to direct meta-C-H amination of arenes, overcoming the long-standing requirement for a redundant directing group. The remarkable advancements in functional group accommodation for precise regiochemical control were achieved through the discovery of an unprecedented organo-initiator and the strategic utilization of non-covalent interactions. This protocol has been successfully applied in the concise synthesis and late-stage derivatization of drug molecules, which would have been otherwise challenging to achieve.

Unlocking regioselective meta-alkylation with epoxides and oxetanes via dynamic kinetic catalyst control

Regioselective arene C-H bond alkylation is a powerful tool in synthetic chemistry, yet subject to many challenges. Herein, we report the meta-C-H bond alkylation of aromatics bearing N-directing groups using (hetero)aromatic epoxides as alkylating agents. This method results in complete regioselectivity on both the arene as well as the epoxide coupling partners, cleaving exclusively the benzylic C-O bond. Oxetanes, which are normally unreactive, also participate as alkylating reagents under the reaction conditions. Our mechanistic studies reveal an unexpected reversible epoxide ring opening process undergoing catalyst-controlled regioselection, as key for the observed high regioselectivities.

An efficient and mild oxidative approach from thiols to sulfonyl derivatives with DMSO/HBr

A mild and practical method for synthesizing sulfonyl derivatives, which have a wide range of applications in pharmaceuticals, materials, and organic synthesis, was described through the oxidative functionalization of thiols with DMSO/HBr. The simple conditions, low cost and ready availability of DMSO/HBr, as well as the versatility of the transformations, make this strategy very powerful in synthesizing a variety of sulfonyl derivatives including sulfonamides, sulfonyl fluorides, sulfonyl azides, and sulfonates. Mechanistic studies revealed that DMSO served as the terminal oxidant, and HBr acted as both a nucleophile and a redox mediator to transfer the oxygen atom.

The Essence in Selectivity of Copper-Mediated Intermolecular Nucleophilic Substitution of a meta C-H Bond in 2-Methyl-N-methoxyaniline: A Theoretical Study

The detailed mechanism for NHC-Cu(I)-catalyzed intermolecular nucleophilic substitution of the C-H bonds at aniline (2-methyl-N-methoxyaniline) was studied via DFT methods to reveal the essence of the selectivity. Calculations revealed that the meta C-H functionalization proceeds via two nucleophilic attacks on the aromatic ring rather than a one-step meta C-H substitution to give the experimentally observed major product. The reaction is initiated by activation of the substrate via oxidative addition with an NHC-Cu(I) catalyst, through which an umpolung occurs at the ring. From the activated intermediate, methoxyl group transfer to benzyl forms a resting state, while a nucleophile can attack the ortho position of benzyl to form a more stable intermediate. The nucleophile group can then transfer to the meta position by a 1,2-Wagner-Meerwein rearrangement to form the final product through a proton shuttle. In contrast, other transfer processes affording ortho- or para-substituted products encounter higher activation barriers. This work investigates the relationship of product selectivity with the umpolung of the aromatic ring, as well as the priority of a nucleophilic attack at the ortho position of the aromatic, 1,2-Wagner-Meerwein rearrangement from the ortho-substituted intermediate, and proton shuttle from the meta-substituted intermediate.

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.

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.

Remote C-H Olefination of Heterocyclic Biaryls Enabled by Reversibly Bound Templates

Remote C-H functionalization of heterocyclic biaryls will be of great importance in synthesis and medicinal chemistry. Through adjusting the geometric relationship of the directing atom and target C-H bonds, two new catalytic templates have been developed to enable the functionalization of the more hindered ortho-C-H bonds of heterobiaryls bearing directing heteroatom at the meta- or para-positions, affording unprecedented site-selectivity. The use of template chaperone also overcomes product inhibition and renders the directing templates catalytic. The utility of this protocol was demonstrated by olefination of heterocyclic biaryls with various substituents, overriding conventional steric and electronic effects. These ortho-C-H olefinated heterobiaryls are sterically hindered and can often be challenging to prepare through aryl-aryl coupling reactions.

Ruthenium(II)-Catalyzed Remote C-H Sulfonylation of 2-Pyridones

Herein, a ruthenium-mediated remote C-H mono- and disulfonylation of 2-pyridones with arylsulfonyl chlorides is developed. The catalytic system consisting of a [Ru(p-cymene)Cl2]2 catalyst and KOAc additive allows 2-pyridones to undergo C3,C5-disulfonylation in 1,4-dioxane, and C5-sulfonylation when the C3-position of 2-pyridones is blocked. The successful transformation of the products and late-stage modification of estrone further highlighted the potential utility and significance of this synthetic protocol. Preliminary mechanistic studies indicated that the remote regioselectivity might be dictated via chelation-assisted ruthenation.

Transition-Metal-Catalyzed C-H Bond Activation for the Formation of C-C Bonds in Complex Molecules

Site-predictable and chemoselective C-H bond functionalization reactions offer synthetically powerful strategies for the step-economic diversification of both feedstock and fine chemicals. Many transition-metal-catalyzed methods have emerged for the selective activation and functionalization of C-H bonds. However, challenges of regio- and chemoselectivity have emerged with application to highly complex molecules bearing significant functional group density and diversity. As molecular complexity increases within molecular structures the risks of catalyst intolerance and limited applicability grow with the number of functional groups and potentially Lewis basic heteroatoms. Given the abundance of C-H bonds within highly complex and already diversified molecules such as pharmaceuticals, natural products, and materials, design and selection of reaction conditions and tolerant catalysts has proved critical for successful direct functionalization. As such, innovations within transition-metal-catalyzed C-H bond functionalization for the direct formation of carbon-carbon bonds have been discovered and developed to overcome these challenges and limitations. This review highlights progress made for the direct metal-catalyzed C-C bond forming reactions including alkylation, methylation, arylation, and olefination of C-H bonds within complex targets.

Charge-controlled Pd catalysis enables the meta-C–H activation and olefination of arenes

The regioselective C-H activation of arenes remains one of the most promising techniques for accessing highly important functionalized motifs. Such functionalizations can generally be achieved through directed and non-directed processes. The directed approach requires a covalently attached directing group (DG) on the substrate to induce reactivity and selectivity and therefore intrinsically leaves a functional group at the point of attachment within the molecule, even after the tailored DG has been removed. Conversely, non-directed methods typically suffer from regioselectivity issues, especially for unbiased substrates. Herein, we report a unique approach that employs weak charge-charge and charge-dipole interactions to enable the meta-selective activation and olefination of arenes to address these challenges in Pd catalysis. The charged moiety can easily be converted to uncharged simple arenes by hydrogenation or cross-coupling. In-depth mechanistic studies prove that the charge is responsible for the observed selectivity. We expect our studies to be generalizable and thereby enable further regioselective transformations.

Remote C-H Glycosylation by Ruthenium(II) Catalysis: Modular Assembly of meta-C-Aryl Glycosides

The prevalence of C-aryl glycosides in biologically active natural products and approved drugs has long motivated the development of efficient strategies for their selective synthesis. Cross-couplings have been frequently used, but largely relied on palladium catalyst with prefunctionalized substrates, while ruthenium-catalyzed C-aryl glycoside preparation has thus far proven elusive. Herein, we disclose a versatile ruthenium(II)-catalyzed meta-C-H glycosylation to access meta-C-aryl glycosides from readily available glycosyl halide donors. The robustness of the ruthenium catalysis was reflected by mild reaction conditions, outstanding levels of anomeric selectivity and exclusive meta-site-selectivity.

Ru(II)-Catalyzed Hydroarylation of in situ Generated 3,3,3-Trifluoro-1-propyne by C-H Bond Activation: A Facile and Practical Access to β-Trifluoromethylstyrenes

In this study, a practical and straightforward synthesis of β-(E)-trifluoromethylstyrenes by ruthenium-catalyzed C-H bond activation was developed. The readily available and inexpensive 2-bromo-3,3,3-trifluoropropene (BTP), a non-ozone depleting reagent, was used as a reservoir of 3,3,3-trifluoropropyne. With this approach, the monofunctionalization of a panel of heteroarenes was possible in a safe and scalable manner (23 examples, up to 87 % yield). Mechanistic investigations and density functional theory (DFT) calculations were also conducted to get a better understanding of the mechanism of this transformation. These studies suggested that 1) a cyclometallated ruthenium complex enabled the transformation, 2) this complex exhibited high efficiency in this transformation compared to the commercially available [RuCl2 (p-cymene)]2 and 3) the mechanism proceeded through a bis-cyclometallated ruthenium intermediate for the carboruthenation step.

Pd(ii)-catalyzed meta-C-H bromination and chlorination of aniline and benzoic acid derivatives

The classic electrophilic bromination leads to ortho- and para-bromination of anilines due to their electron-rich properties. Herein we report the development of an unprecedented Pd-catalyzed meta-C-H bromination of aniline derivatives using commercially available N-bromophthalimide (NBP), which overcomes the competing ortho/para-selectivity of electrophilic bromination of anilines. The addition of acid additives is crucial for the success of this reaction. A broad range of substrates with various substitution patterns can be tolerated in this reaction. Moreover, benzoic acid derivatives bearing complex substitution patterns are also viable with this mild bromination reaction, and meta-C-H chlorination is also feasible under similar reaction conditions. The ease of the directing group removal and subsequent diverse transformations of the brominated products demonstrate the application potential of this method and promise new opportunities for drug discovery.

Copper-catalyzed regioselective C2-H chlorination of indoles with para-toluenesulfonyl chloride

A copper-catalyzed, pyrimidine directed regioselective C-H chlorination of indoles with para-toluenesulfonyl chloride (TsCl) has been developed. The reactions proceeded smoothly in the presence of 20 mol% of Cu(OAc)₂ as the catalyst and TsCl as the chlorine source, delivering C2-chlorinated indoles with structural diversity in moderate to excellent yields. Mechanistic studies suggested that single electron transfer (SET) from Cu(II) to TsCl accompanied by the release of the p-toluenesulfonyl radical and the related Cu(III)Cl species might be involved in the reactions.

Distal Ruthenaelectro-Catalyzed meta-C-H Bromination with Aqueous HBr

While electrochemical ortho-selective C-H activations are well established, distal C-H activations continue to be underdeveloped. In contrast, we herein describe the electrochemical meta-C-H functionalization. The remote C-H bromination was accomplished in an undivided cell by RuCl3 ⋅3 H2 O with aqueous HBr. The electrohalogenation proceeded under exogenous ligand- and electrolyte-free conditions. Notably, pyrazolylarenes were meta-selectively brominated at the benzenoid moiety, rather than on the electron-rich pyrazole ring for the first time. Mechanistic studies were suggestive of an initial ruthenacycle formation, and a subsequent ligand-to-ligand hydrogen transfer (LLHT) process to liberate the brominated product.

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.

Ruthenium(II)-Catalyzed Chelation-Assisted Desulfitative Arylation of Benzo[h]quinolines with Arylsulfonyl Chlorides

Herein, a novel chelation-assisted C-H arylation reaction of benzo[h]quinoline is described. This transformation, using [RuCl₂(p-cymene)]₂ as the catalyst and cheap and easily accessible arylsulfonyl chlorides as the arylation source, featured simple reaction conditions, a broad substrate scope, and functional group tolerance. The successful application of some bioactive-molecule-based sulfonyl chlorides further highlighted the potential utility and importance of this desulfitative C-H arylation protocol.

Catalysis with cycloruthenated complexes

Cycloruthenated complexes have been studied extensively over the last few decades. Many accounts of their synthesis, characterisation, and catalytic activity in a wide variety of transformations have been reported to date. Compared with their non-cyclometallated analogues, cycloruthenated complexes may display enhanced catalytic activities in known transformations or possess entirely new reactivity. In other instances, these complexes can be chiral, and capable of catalysing stereoselective reactions. In this review, we aim to highlight the catalytic applications of cycloruthenated complexes in organic synthesis, emphasising the recent advancements in this field.

We discuss recent advances in the applications of cycloruthenated complexes in organic synthesis, comprising C–H activation, chiral-at-metal catalysis, Z-selective olefin metathesis, transfer hydrogenation, enantioselective cyclopropanations and cycloadditions.

Looking deep into C-H functionalization: the synthesis and application of cyclopentadienyl and related metal catalysts

Metal catalyzed C-H functionalization offers a versatile platform for methodology development and a wide variety of reactions now exist for the chemo- and site-selective functionalization of organic molecules. Cyclopentadienyl-metal (CpM) complexes of transition metals and their correlative analogues have found widespread application in this area, and herein we highlight several key applications of commonly used transition-metal Cp-type catalysts. In addition, an understanding of transition metal Cp-type catalyst synthesis is important, particularly where modifications to the catalyst structure are required for different applications, and a summary of this aspect is given.

Three-Component Ruthenium-Catalyzed meta-C-H Alkylation of Phenol Derivatives

Herein, we realized the multicomponent reactions of phenol derivatives via a six-membered cycloruthenated intermediate for the first time. This strategy exhibited good substrate suitability and functional group tolerance with various phenol derivatives and provided a potential synthetic drug approach. Mechanistic studies showed that a radical might be involved in this process. In addition, the meta alkylated phenol was obtained by further removal of the directing group.

Site-Selective Coupling of Remote C(sp3)−H/meta-C(sp2)−H Bonds Enabled by Ru/Photoredox Dual Catalysis and Mechanistic Studies

Construction of C(sp2)−C(sp3) bonds via regioselective coupling of C(sp2)−H/C(sp3)−H bonds is challenging due to the low reactivity and regioselectivity of C−H bonds. Here, a novel photoinduced Ru/photocatalyst-cocatalyzed regioselective cross-dehydrogenative coupling...

Regioselective direct sulfenylation of glycals using arylsulfonyl chlorides in the presence of triphenylphosphine: access to C2-thioaryl glycosides

Cheap and easily available arylsulfonyl chlorides as a sulfur source reductively couple with glycals in the presence of triphenylphosphine to afford C2-thioaryl glycosides.

Insight into the mechanism of the arylation of arenes via norbornene relay palladation through meta- to para-selectivity

A theoretical insight was shown into the origin of site-selectivity in the arylation of arenes by a norbornene relay palladation through meta- to para-selectivity.

Expeditious and Efficient ortho-Selective Trifluoromethane-sulfonylation of Arylhydroxylamines

A metal- and oxidant-free, practical and efficient method for the synthesis of highly versatile and synthetically useful ortho-trifluoromethanesulfonylated anilines from arylhydroxylamines and trifluoromethanesulfinic chloride was developed. This rapid transformation proceeded smoothly with good yields and excellent ortho-selectivity in the absence of any metals or ligands. Mechanistically, the reaction comprised a noncanonical O-trifluoromethanesulfinylation of the arylhydroxylamine, and the subsequent [2,3]-sigmatropic rearrangement to afford ortho-trifluoromethanesulfonylated aniline derivatives. The practical application of this reaction was demonstrated by further conversion into a series of functional molecules under different reaction conditions.

A directive Ni catalyst overrides conventional site selectivity in pyridine C-H alkenylation

Achieving the transition metal-catalysed pyridine C3-H alkenylation, with pyridine as the limiting reagent, has remained a long-standing challenge. Previously, we disclosed that the use of strong coordinating bidentate ligands can overcome catalyst deactivation and provide Pd-catalysed C3 alkenylation of pyridines. However, this strategy proved ineffective when using pyridine as the limiting reagent, as it required large excesses and high concentrations to achieve reasonable yields, which rendered it inapplicable to complex pyridines prevalent in bioactive molecules. Here we report that a bifunctional N-heterocyclic carbene-ligated Ni-Al catalyst can smoothly furnish C3-H alkenylation of pyridines. This method overrides the intrinsic C2 and/or C4 selectivity, and provides a series of C3-alkenylated pyridines in 43-99% yields and up to 98:2 C3 selectivity. This method not only allows a variety of pyridine and heteroarene substrates to be used as the limiting reagent, but is also effective for the late-stage C3 alkenylation of diverse complex pyridine motifs in bioactive molecules.

Recent Progress and Emerging Technologies towards a Sustainable Synthesis of Sulfones

Sulfones play a pivotal role in modern organic chemistry. They are highly versatile building blocks and find various applications as drugs, agrochemicals, or functional materials. Therefore, sustainable access to this class of molecules is of great interest. Herein, the goal was to provide a summary on recent developments in the field of sustainable sulfone synthesis. Advances and existing limitations in traditional approaches towards sulfones were reviewed on selected examples. Furthermore, novel emerging technologies for a more sustainable sulfone synthesis and future directions were discussed.

Predictable site-selective functionalization: Promoter group assisted para-halogenation of N-substituted heteroaromatics under metal-free condition

Herein, regioselective para-C-H halogenation of N-pyrimidyl (hetero)aromatics through S_EAr (electrophilic aromatic substitution) type reaction is disclosed. S_EAr type reaction has been utilized for the C5-bromination of indolines (para-selective) with N-bromosuccinimide under metal and additive-free conditions in good to excellent yields. The developed methodology is also applicable for iodination and challenging chlorination. The pyrimidyl group is identified as a reactivity tuner that also controls the regioselectivity. The present method is also applicable for selective halogenation of aniline, pyridine, indole, oxindole, pyrazole, tetrahydroquinoline, isoquinoline, and carbazole. DFT studies such as Fukui nucleophilicity and natural charge maps also support the observed p-selectivity. Post-functionalization of the title compound into the corresponding arylated, olefinated, and dihalogenated products is achieved in a one-pot, two-step fashion. Late-stage C-H bromination was also executed on drug/natural molecules (harmine, etoricoxib, clonidine, and chlorzoxazone) to demonstrate the applicability of the developed protocol.

Photocatalyzed Coupling-Cyclization of ortho-Alkynylaryl Vinylethers with Arylsulfonyl Azides

A novel visible-light-induced coupling-cyclization of ortho-alkynylaryl vinylethers with arylsulfonyl azides has been described. This transformation provided a concise approach to access C3-exocyclic C═C bond/C2-alkylsulfone-tethered benzofurans via a solvent-leveraged carbosulfonylation and [2 + 2 + 3] cyclization. Primary mechanistic studies demonstrated that THF belongs to a crucial H atom source.

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.