meta-Selective C–H Bond Alkylation with Secondary Alkyl Halides

Rational Development of Remote C-H Functionalization of Biphenyl: Experimental and Computational Studies

A simple and efficient nitrile-directed meta-C-H olefination, acetoxylation, and iodination of biaryl compounds is reported. Compared to the previous approach of installing a complex U-shaped template to achieve a molecular U-turn and assemble the large-sized cyclophane transition state for the remote C-H activation, a synthetically useful phenyl nitrile functional group could also direct remote meta-C-H activation. This reaction provides a useful method for the modification of biaryl compounds because the nitrile group can be readily converted to amines, acids, amides, or other heterocycles. Notably, the remote meta-selectivity of biphenylnitriles could not be expected from previous results with a macrocyclophane nitrile template. DFT computational studies show that a ligand-containing Pd-Ag heterodimeric transition state (TS) favors the desired remote meta-selectivity. Control experiments demonstrate the directing effect of the nitrile group and exclude the possibility of non-directed meta-C-H activation. Substituted 2-pyridone ligands were found to be key in assisting the cleavage of the meta-C-H bond in the concerted metalation-deprotonation (CMD) process.

Direct oxidative coupling of N-acyl pyrroles with alkenes by ruthenium(ii)-catalyzed regioselective C2-alkenylation

Ruthenium(ii)-catalyzed oxidative coupling by C2-alkenylation of N-acyl pyrroles with alkenes has been described. The acyl unit was found to be an effective chelating group for the activation of aryl C-H bonds ortho to the directing group. The alkenylation reaction of benzoyl pyrroles occurred regioselectively at the C2-position of the pyrrole ring, without touching the benzene ring. The reaction provides exclusively monosubstituted pyrroles under the optimized conditions. Disubstituted pyrroles could be obtained using higher loadings of the ruthenium(ii)-catalyst and the additives.

Unravelling the mechanism of cobalt-catalysed remote C-H nitration of 8-aminoquinolinamides and expansion of substrate scope towards 1-naphthylpicolinamide

Previously, an unexpected Co-catalysed remote C-H nitration of 8-aminoquinolinamide compounds was developed. This report provided a novel reactivity for Co which was assumed to proceed through the mechanistic pathway already known for analogous Cu-catalysed remote couplings of the same substrates. In order to shed light into this intriguing, and previously unobserved reactivity for Co, a thorough computational study has now been performed, which has allowed for a full understanding of the operative mechanism. This study demonstrates that the Co-catalysed remote coupling does not occur through the previously proposed Single Electron Transfer (SET) mechanism, but actually operates through a high-spin induced remote radical coupling mechanism, through a key intermediate with significant proportion of spin density at the 5- and 7-positions of the aminoquinoline ring. Additionally, new experimental data provides expansion of the synthetic utility of the original nitration procedure towards 1-naphthylpicolinamide which unexpectedly appears to operate via a subtly different mechanism despite having a similar chelate environment.

Palladium-catalyzed intermolecular C–H silylation initiated by aminopalladation

The intermolecular disilylation reaction of C,C-palladacycles obtained through aminopalladation has been developed.

A directing group-assisted ruthenium-catalyzed approach to access meta-nitrated phenols

meta-Selective C–H nitration of phenol derivatives was developed using a Ru-catalyzed σ-activation strategy. Cu(NO₃)₂·3H₂O was employed as the nitrating source, whereas Ru₃(CO)₁₂ was found to be the most suitable metal catalyst for the protocol.

Ruthenium-catalyzed meta-C-H bond alkylation of aryl 2-pyridyl ketones

The first example of meta-selective CAr-H bond functionalization of aryl 2-pyridyl ketones has been developed using [Ru(p-cymene)Cl2]2 as the catalyst and alkyl bromide as the coupling reagent. This development provides an efficient strategy for modifying the meta-position of aryl 2-pyridyl ketone skeletons, which are found in various functional molecules.

Ruthenium-Catalyzed ortho- and meta-H/D Exchange of Arenes

Ruthenium-catalyzed aromatic H/D exchange in [D₄]acetic acid has been developed. By using N-heteroarenes as directing groups, both ortho and meta positions are selectively deuterated with high levels of D incorporation. Moreover, this strategy provides an alternative way to achieve meta-C-H activation.

The ruthenium-catalyzed meta-selective C-H nitration of various azole ring-substituted arenes

The efficient ruthenium-catalyzed meta-selective C_Ar-H nitration of azole ring substituted arenes has been developed. In this work, Ru₃(CO)₁₂ was used as the catalyst, AgNO₂ as the nitro source, HPcy₃⁺·BF₄^- as the ligand, pivalic acid as the additive, and DCE as the solvent, and a wide spectrum of arenes bearing thiazole, pyrazolyl or removable oxazoline directing groups were tolerated in this meta-selective C_Ar-H nitration, affording the nitrated products in moderate to good yields. Moreover, this study reveals a gentler and environmentally friendly way to access meta-nitration arenes compared to the traditional process.

Transition Metal-Catalyzed Directing-Group-Assisted C-H Activation of Phenols

Strategies that exploit directing groups to control the site selectivity in the C-H activation of arenes have received much attention during the past two decades. In light of the importance of phenol derivatives in the areas of pharmaceuticals, agrochemicals, and materials science, transition-metal-catalyzed C-H activation of phenols has proven to be an extremely useful tool in organic synthesis. This Minireview summarizes the current state-of-the-art direct C-H activation of phenol derivatives under transition-metal catalysis.

meta C-H Arylation of Electron-Rich Arenes: Reversing the Conventional Site Selectivity

Controlling site selectivity of C-H activation without using a directing group remains a significant challenge. While Pd(II) catalysts modulated by a mutually repulsive pyridine-type ligand have been shown to favor the relatively electron-rich carbon centers of arenes, reversing the selectivity to favor palladation at the relatively electron-deficient positions has not been possible. Herein we report the first catalytic system that effectively performs meta C-H arylation of a variety of alkoxy aromatics including 2,3-dihydrobenzofuran and chromane with exclusive meta site selectivity, thus reversing the conventional site selectivity governed by native electronic effects. The identification of an effective ligand and modified norbornene (NBE-CO₂Me), as well as taking advantage of the statistics, are essential for achieving the exclusive meta selectivity.

Rhodium catalyzed template-assisted distal para-C-H olefination

Rhodium catalysis has been extensively used for ortho-C-H functionalization reactions, and successfully extended to meta-C-H functionalization. Its application to para-C-H activation remains an unmet challenge. Herein we disclose the first example of such a reaction, with the Rh-catalyzed para-C-H olefination of arenes. The use of a Si-linked cyanobiphenyl unit as a traceless directing group leads to highly para-selective arene-olefin couplings.

Nickel-catalyzed, ring-forming aromatic C-H alkylations with unactivated alkyl halides

The development of a nickel-catalyzed C-H alkylation of aromatic substrates with unactivated alkyl halides is described. This carbocyclization facilitates the synthesis of diverse fused ring systems from simple aromatic substrates and is an attractive alternative to traditional polar or radical-mediated ring formations. The present system uses unactivated primary and secondary alkyl bromides and chlorides, while avoiding the use of precious palladium catalysts and more reactive alkyl halides commonly used in related C-H alkylations.

Ligand and counteranion enabled regiodivergent C-H bond functionalization of naphthols with α-aryl-α-diazoesters

Here, an unprecedented ligand and counteranion-controlled and site-selectivity switchable direct C-H bond functionalization of unprotected naphthols with α-aryl-α-diazoesters was developed. In this transformation, site selectivities are realized by turning on/off the coordination between metal complexes and hydroxy groups. The preliminary mechanism revealed that the interaction between the hydroxy group and gold catalyst plays a key role in switching the site-selectivity of gold-carbene. This protocol potentially provides a novel design for C-H bond functionalization.

meta-Nitration of Arenes Bearing ortho/para Directing Group(s) Using C-H Borylation

Herein, we report the meta-nitration of arenes bearing ortho/para directing group(s) using the iridium-catalyzed C-H borylation reaction followed by a newly developed copper(II)-catalyzed transformation of the crude aryl pinacol boronate esters into the corresponding nitroarenes in a one-pot fashion. This protocol allows the synthesis of meta-nitrated arenes that are tedious to prepare or require multistep synthesis using the existing methods. The reaction tolerates a wide array of ortho/para-directing groups, such as -F, -Cl, -Br, -CH₃ , -Et, -iPr -OCH₃ , and -OCF₃ . It also provides regioselective access to the nitro derivatives of π-electron-deficient heterocycles, such as pyridine and quinoline derivatives. The application of this method is demonstrated in the late-stage modification of complex molecules and also in the gram-scale preparation of an intermediate en route to the FDA-approved drug Nilotinib. Finally, we have shown that the nitro product obtained by this strategy can also be directly converted to the aniline or hindered amine through Baran's amination protocol.

Palladium-Catalyzed Remote meta-C-H Bond Deuteration of Arenes Using a Pyridine Template

Palladium-catalyzed meta-selective C-H deuteration of a series of substrates, including phenylacetic acids, hydrocinnamic acid, benzylphosphonate, benzylsulfonate, and benzyl and phenyl ethyl alcohol ester, is developed by using a pyridine-based directing template. The template is installed into the substrate through a practical ester linkage. Under mild reaction conditions, a variety of phenylacetic acids containing alkyl, methoxyl, and halo substituents are compatible in the reaction, resulting in high levels of D-incorporation at the meta position.

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.

meta-Selective C-H Activation of Arenes at Room Temperature Using Visible Light: Dual-Function Ruthenium Catalysis

Ruthenium-catalyzed meta-C-H activation of arenes at room temperature is reported to proceed under blue-light irradiation. A variety of heteroarenes are compatible with this photochemical process, which leads to the corresponding meta C-C coupling products in good to very good yields. Initial mechanistic studies suggest a single-electron transfer process occurs between a photoexcited Ru^II -cyclometalated complex and alkyl halides, enabling meta-C-H functionalization reaction via carbon-centered radicals.

Accessing Remote meta- and para-C(sp2 )-H Bonds with Covalently Attached Directing Groups

Directing group assisted ortho-C-H activation has been known for the last few decades. In contrast, extending the same approach to achieve activation of the distal meta- and para-C-H bonds in aromatic molecules remained elusive for a long time. The main challenge is the conception of a macrocyclic transition state, which is needed to anchor the metal catalyst close to the target bond. Judicious modification of the chain length, the tether linkage, and the nature of the catalyst-coordinating donor atom has led to a number of successful studies in the last few years. This Review compiles the significant achievements made in this field of both meta- and para-selectivity using covalently attached directing groups, which are systematically classified on the basis of their mode of covalent attachment to the substrate as well as their chemical nature. This Review aims to create a more heuristic approach for recognizing the suitability of the directing groups for use in future organic transformations.

Insights into the role of noncovalent interactions in distal functionalization of the aryl C(sp2)-H bond

Burgeoning interest in distal functionalization of aryl C-H bonds led to the development of iridium-catalyzed borylation reactions. The significance and inadequate mechanistic understanding of C(sp2)-H borylations motivated us to investigate the key catalytic steps and the origin of a directing-group-free regiocontrol in the reaction between aryl amides and B2pin2 (bis(pinacolato)diboron). An Ir(iii)(ubpy)tris(boryl) complex, generated from the pre-catalyst [Ir(OMe)(cod)]2 by the action of a bipyridine-urea ligand (ubpy) and B2pin2, is considered as the most likely active catalyst. The meta C-H activation of N,N-dihexylbenzamide is energetically more favorable over the para isomer. The origin of this preference is traced to the presence of a concerted action of noncovalent interactions (NCIs), primarily between the catalyst and the substrate, in the regiocontrolling transition states (TSs). Molecular insights into such TSs revealed that the N-H···O interaction between the tethered urea moiety of the Ir-bound ubpy ligand of the catalyst and the amide carbonyl of the substrate is a critical interaction that helps orient the meta C-H bond nearer to iridium. Other NCIs such as C-H···π between the substrate and the catalyst, C-H···O involving the substrate C-H and the oxygen of the B2pin2 ligand and C-H···N between the substrate and the N atom of the Ir-bound ubpy confirm the significance of such interactions in providing the desirable differential energies between the competing TSs that form the basis of the extent of regioselectivity.

Palladium(II)-Catalyzed Remote meta-C-H Functionalization of Aromatic Tertiary Amines

Pd(II)-catalyzed remote C-H olefination of aromatic tertiary amines was achieved with high meta selectivity. With the assistance of an elaborated template, C-H functionalization of unreactive aryl tertiary amines, hindered by the p-π conjugation between the lone-pair electrons of the nitrogen atom and the phenyl ring, was realized with high meta regioselectivity via a quaternary ammonium salt assembly. The results demonstrate that apart from the distance and geometry of the template, the conformation of the arene substrate also plays a crucial role in the templated-assisted remote C-H functionalization.

Hydrogen-Bond-Controlled Formal Meta-Selective C-H Transformations and Regioselective Synthesis of Multisubstituted Aromatic Compounds

The meta-selective introduction of functional groups into aromatic substrates was successfully achieved by hydrogen-bond-controlled meta-selective C-H borylation and successive conversion of the boryl group to other functional groups. By this method a wide range of functional groups could be introduced without isolation of the borylated intermediates. The desired meta-functionalized aromatic products were obtained in a one-pot manner even on a gram scale. Regioselective synthesis of multisubstituted aromatic compounds was also achieved.

Recent Advances in C–H Bond Functionalization with Ruthenium-Based Catalysts

The past decades have witnessed rapid development in organic synthesis via catalysis, particularly the reactions through C–H bond functionalization. Transition metals such as Pd, Rh and Ru constitute a crucial catalyst in these C–H bond functionalization reactions. This process is highly attractive not only because it saves reaction time and reduces waste,but also, more importantly, it allows the reaction to be performed in a highly region specific manner. Indeed, several organic compounds could be readily accessed via C–H bond functionalization with transition metals. In the recent past, tremendous progress has been made on C–H bond functionalization via ruthenium catalysis, including less expensive but more stable ruthenium(II) catalysts. The ruthenium-catalysed C–H bond functionalization, viz. arylation, alkenylation, annulation, oxygenation, and halogenation involving C–C, C–O, C–N, and C–X bond forming reactions, has been described and presented in numerous reviews. This review discusses the recent development of C–H bond functionalization with various ruthenium-based catalysts. The first section of the review presents arylation reactions covering arylation directed by N–Heteroaryl groups, oxidative arylation, dehydrative arylation and arylation involving decarboxylative and sp3-C–H bond functionalization. Subsequently, the ruthenium-catalysed alkenylation, alkylation, allylation including oxidative alkenylation and meta-selective C–H bond alkylation has been presented. Finally, the oxidative annulation of various arenes with alkynes involving C–H/O–H or C–H/N–H bond cleavage reactions has been discussed.

Sequential Functionalization of meta-C-H and ipso-C-O Bonds of Phenols

The use of a template as a linchpin motif in directed remote C-H functionalization is a versatile yet relatively underexplored strategy. We have developed a template-directed approach to realizing one-pot sequential palladium-catalyzed meta-selective C-H olefination of phenols, and nickel-catalyzed ipso-C-O activation and arylation. Thus, this bifunctional template converts phenols to synthetically useful 1,3-disubstituted arenes.

MnCl2-Catalyzed C-H Alkylation on Azine Heterocycles

Low-valent manganese-catalyzed C-H alkylation of pyridine derivatives with both primary and challenging secondary alkyl halides was established by amide assistance. The strategy provided expedient access to alkylated pyridines with wide functional group tolerance and ample scope through weak chelation. Mechanistic studies provided strong support for a rate-determining C-H activation and a SET-type C-X scission.

Pd-Catalyzed para-selective C–H difluoromethylation of aromatic carbonyls

A novel Pd-catalyzed para-selective C–H bond difluoromethylation of diverse aromatic carbonyls was developed.

Ruthenium-catalyzed remote C5-sulfonation of N-alkyl-8-aminoquinolines

Ruthenium-catalyzed remote C–H sulfonation of 8-aminoquinolines with single C5-selectivity.