Mild Rhodium(III)-Catalyzed Direct CH Allylation of Arenes with Allyl Carbonates

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.

Synthesis of 1,4-epoxy-2-aryltetrahydro-1-benzazepines via rhodium(III)-catalyzed C-H allylation/intramolecular 1,3-dipolar cycloaddition

Herein, we report a new synthetic route to 1,4-epoxy-2-aryltetrahydro-1-benzazepine derivatives with high efficiency, namely the Rh(III)-catalyzed C-H allylation of nitrones with allyl precursors, followed by subsequent intramolecular 1,3-dipolar cycloaddition, to deliver the title compounds. This reaction is regio- and stereo-selective, generating the cis-isomer with a broad substrate scope and good functional group tolerance.

Palladium-Catalyzed Weak-Chelation-Assisted C4-Nitration of Indoles with tert-Butyl Nitrite: Formal Access to Aminated Indoles

Palladium-catalyzed weak-chelation-assisted C4-selective nitration of indoles has been accomplished employing tert-butyl nitrite in the presence of oxone under molecular oxygen at a moderate temperature. Aerobic conditions, C4-selectivity, substrate scope, conversion to valuable aminated indoles, and late-stage natural product modifications are the important practical features.

Ruthenium(II)-Catalyzed S(II)-Directed Aromatic C-H Allylation with Vinylaziridines

The ruthenium-catalyzed reaction of aryl methyl thioethers with vinylaziridines affords ortho-position mono- or bis-allylation products depending on substituents on the phenyl rings of sulfide substrates or the ratio of reactants. The reaction also features mild reaction conditions, good product yields, wide scope of substrates, good compatibility of functional groups, and the selective formation of E-configurated C-C double bonds.

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.

Experimental and Computational Studies on RuII -Catalyzed C7-Allylation of Indolines with Allyl Bromide

The selective C7-allylation of indolines with allyl bromide under ruthenium catalysis has been revealed here. Under established reaction conditions, C7-allylation of various indolines, including drug compounds, was accomplished with good selectivity and yields. Based on combined experimental and density functional theory (DFT) studies, the olefin insertion route was energetically favorable among four possible pathways. Experimental and DFT studies further revealed that the C-H activation is a reversible rate-limiting step.

Domino Sonogashira coupling/metal carbene-involved annulation enabled by Pd/Cu relay catalysis: rapid assembly of indazole-containing biheteroaryls

An efficient and novel method has been developed for the synthesis of indazole-containing biheteroaryls via a domino Sonogashira coupling/azaenyne cycloisomerization/Barton–Kellogg reaction.

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.

Recent advances in transition-metal catalyzed directed C–H functionalization with fluorinated building blocks

This review focuses on the advances in transition-metal catalyzed reactions with fluorinated building blocks via directed C–H bond activation for the construction of diverse organic molecules with an insight into the probable mechanistic pathway.

Dirhodium(ii)-catalysed cycloisomerization of azaenyne: rapid assembly of centrally and axially chiral isoindazole frameworks

Described herein is a dirhodium(ii)-catalyzed asymmetric cycloisomerization reaction of azaenyne through a cap-tether synergistic modulation strategy, which represents the first catalytic asymmetric cycloisomerization of azaenyne. This reaction is highly challenging because of its inherent strong background reaction leading to racemate formation and the high capability of coordination of the nitrogen atom resulting in catalyst deactivation. Varieties of centrally chiral isoindazole derivatives could be prepared in up to 99 : 1 d.r., 99 : 1 er and 99% yield and diverse enantiomerically enriched atropisomers bearing two five-membered heteroaryls have been accessed by using an oxidative central-to-axial chirality transfer strategy. The tethered nitrogen atom incorporated into the starting materials enabled easy late-modifications of the centrally and axially chiral products via C–H functionalizations, which further demonstrated the appealing synthetic utilities of this powerful asymmetric cyclization.

Rh(ii)-catalyzed asymmetric cycloisomerization of azaenyne through a cap-tether synergistic modulation strategy was described. Diverse centrally and axially chiral isoindazoles were prepared and directed C–H late-stage modifications were developed.

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.

Access to Branched Allylarenes via Rhodium(III)-Catalyzed C-H Allylation of (Hetero)arenes with 2-Methylidenetrimethylene Carbonate

A rhodium(III)-catalyzed C-H allylation of (hetero)arenes by using 2-methylidenetrimethylene carbonate as an efficient allylic source has been developed for the first time. Five different directing groups including oxime, N-nitroso, purine, pyridine, and pyrimidine were compatible, delivering various branched allylarenes bearing an allylic hydroxyl group in moderate to excellent yields.

Metal-catalysed C-Het (F, O, S, N) and C-C bond arylation

The formation of C-aryl bonds has been the focus of intensive research over the last decades for the construction of complex molecules from simple, readily available feedstocks. Traditionally, these strategies involve the coupling of organohalides (I, Br, Cl) with organometallic reagents (Mg, Zn, B, Si, Sn,…) such as Kumada-Corriu, Negishi, Suzuki-Miyaura, Hiyama and Sonogashira cross-couplings. More recently, alternative methods have provided access to these products by reactions with less reactive C-Het (F, O, S, N) and C-C bonds. Compared to traditional methods, the direct cleavage and arylation of these chemical bonds, the essential link in accessible feedstocks, has become increasingly important from the viewpoint of step-economy and functional-group compatibility. This comprehensive review aims to outline the development and advances of this topic, which was organized into (1) C-F bond arylation, (2) C-O bond arylation, (3) C-S bond arylation, (4) C-N bond arylation, and (5) C-C bond arylation. Substantial attention has been paid to the strategies and mechanistic investigations. We hope that this review can trigger chemists to discover more efficient methodologies to access arylation products by cleavage of these C-Het and C-C bonds.

Transition-Metal-Catalyzed Directing Group Assisted (Hetero)aryl C-H Functionalization: Construction of C-C/C-Heteroatom Bonds

Transition-metal-catalyzed C-H functionalization is one of the fascinating scientific fronts in organic synthesis for the formation of conjugated arenes and has emerged as a benchmark to revolutionize the synthetic enterprise since past decades. In this realm, chelation-guided functionalization of C-H bonds using an exogenous directing group has received considerable attention recently for the expedient regioselective construction of C-C and C-heteroatom bonds as an efficient and sustainable alternative. This article outlines our contribution towards a wide variety of transformations that have been achieved by the directed C-H functionalization through the fine tuning of catalytic systems.

Pd-catalyzed cross-electrophile Coupling/C-H alkylation reaction enabled by a mediator generated via C(sp3)-H activation

Transition-metal-catalyzed cross-electrophile C(sp²)–(sp³) coupling and C–H alkylation reactions represent two efficient methods for the incorporation of an alkyl group into aromatic rings. Herein, we report a Pd-catalyzed cascade cross-electrophile coupling and C–H alkylation reaction of 2-iodo-alkoxylarenes with alkyl chlorides. Methoxy and benzyloxy groups, which are ubiquitous functional groups and common protecting groups, were utilized as crucial mediators via primary or secondary C(sp³)–H activation. The reaction provides an innovative and convenient access for the synthesis of alkylated phenol derivatives, which are widely found in bioactive compounds and organic functional materials.

A cascade Pd-catalyzed cross-electrophile coupling and C–H alkylation reaction of 2-iodo-alkoxylarenes with alkyl chlorides has been developed by using an ortho-methoxy or benzyloxy group as a mediator via C(sp³)–H activation.

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.

Dual Role of the Rhodium(III) Catalyst in C-H Activation: [4 + 3] Annulation of Amide with Allylic Alcohols to 7-Membered Lactams

[4 + 3] annulation of primary and secondary benzamide and cinnamamide derivatives using allyl alcohol as a coupling partner catalyzed by Rh(III) is reported, where Rh(III) is playing a dual role of an oxidant and a catalyst for C-H activation. The Rh-catalyst oxidizes allyl alcohol to its carbonyl derivative, and the in situ-generated carbonyl compound reacts with benzamide in the presence of the Rh-catalyst, forming the corresponding alkylated products. Mechanistic studies show that AgSbF₆ is also playing a dual role. Apart from being a halide scavenger, AgSbF₆ catalyzes the cyclization of the alkylated product, forming the desired lactam. The current method has good synthetic application and is useful for synthesizing a few biologically active compounds that can act as the dopamine D₃ receptor ligand, including berberine-like analogues. The deuteration study and control experiments helped us to propose the mechanism.

Rh(iii)-Catalyzed tandem C(sp2)–H allylation/N-alkylation annulation of arene amides with 2-alkylidenetrimethylene carbonates

A Rh(iii)-catalyzed tandem C(sp2)–H allylation/N-alkylation annulation reaction of arene amides employing 5-methylene-1,3-dioxan-2-one as an efficient allyl source has been disclosed for the first time.

Recent advances in the synthesis of pyrido[1,2-a]indoles

This review deals with the different accesses leading to the pyrido[1,2-a]indole nucleus in the last 20 years.

Transition metal-catalyzed C–H functionalizations of indoles

This review summarises a wide range of transformations on the indole skeleton, including arylation, alkenylation, alkynylation, acylation, nitration, borylation, and amidation, using transition-metal catalyzed C–H functionalization as the key step.

Ruthenium-Catalyzed C(sp2)-H Bond Bisallylation with Imidazopyridines as Directing Groups

A Ru(II)-catalyzed bisallylation of imidazopyridines with vinylcyclopropanes or vinyl cyclic carbonate has been successfully realized. Notably, pharmacophore imidazopyridine was utilized as an intrinsic directing group, which gave access to value-added bisallylated products in high yields via double tandem C-H and C-C/C-O activation. The current methodology was featured with broad substrate scope, good functional group compatibility, and operational simplicity.

Iodane-Guided ortho C-H Allylation

A metal-free C-H allylation strategy is described to access diverse functionalized ortho-allyl-iodoarenes. The method employs hypervalent (diacetoxy)iodoarenes and proceeds through the iodane-guided "iodonio-Claisen" allyl transfer. The use of allylsilanes bearing electron-withdrawing functional groups unlocks the functionalization of a broad range of substrates, including electron-neutral and electron-poor rings. The resulting ortho-allylated iodoarenes are versatile building blocks, with examples of downstream transformation including a concise synthesis of the experimental antimitotic core of Dosabulin. DFT calculations shed additional light on the reaction mechanism, with notable aspects including the aromatic character of the transition-state structure for the [3,3] sigmatropic rearrangement, as well as the highly stereoconvergent nature of the trans-product formation.

Iridium-Catalyzed Enantioselective Intermolecular Indole C2-Allylation

The enantioselective intermolecular C2-allylation of 3-substituted indoles is reported for the first time. This directing group-free approach relies on a chiral Ir-(P, olefin) complex and Mg(ClO4 )2 Lewis acid catalyst system to promote allylic substitution, providing the C2-allylated products in typically high yields (40-99 %) and enantioselectivities (83-99 % ee) with excellent regiocontrol. Experimental studies and DFT calculations suggest that the reaction proceeds via direct C2-allylation, rather than C3-allylation followed by in situ migration. Steric congestion at the indole-C3 position and improved π-π stacking interactions have been identified as major contributors to the C2-selectivity.

Synthesis of rhodium(iii)-catalyzed isoquinoline derivatives from allyl carbonates and benzimidates with hydrogen evolution

A novel Rh(iii)-catalyzed cascade C-H activation/cyclization approach to access isoquinoline derivatives from benzimidates and available allyl carbonates with the liberation of H2 has been realized. Allyl carbonates were first used as a versatile and universal C2 synthon to synthesize this biological activity skeleton via an efficient and practical process just within 1 h.

Cp*Co(III)-Catalyzed γ-Selective C-H Allylation/Hydroamination Cascade for the Synthesis of Dihydroisoquinolines

A Cp*Co(III)-catalyzed γ-selective C-H allylation/hydroamination cascade toward the synthesis of 3,4-dihydroisoquinolines (DHIQs) has been successfully developed, starting from NH ketimines and allyl carbonates. Notably, highly efficient and γ-selective C-H allylations were accomplished using γ-substituted allyl reagents, thus overcoming the issues of poor α/γ selectivity and low reactivity of previous transition metal-catalyzed C-H allylations. The stereochemistry of allyl carbonates was a crucial factor, and synthesis of the DHIQs was achieved using (Z)-allyl carbonates.

Weak Coordination-Guided Regioselective Direct Redox-Neutral C4 Allylation of Indoles with Morita-Baylis-Hillman Adducts

A weak carbonyl coordination-guided regioselective C4 allylation of indoles is demonstrated using the versatile Morita-Baylis-Hillman adduct in the presence of Rh catalysts in a redox-neutral fashion. The substrate scope, functional group diversity, oxidant free character, mechanistic aspects, and synthetic utilities are important practical features.