Rhodium-catalyzed cascade oxidative annulation leading to substituted naphtho[1,8-bc]pyrans by sequential cleavage of C(sp2)-H/C(sp3)-H and C(sp2)-H/O-H bonds

CuBr2-mediated dehydrogenative [4+2] annulation of 1-naphthyl-1,3-indandiones and alkenes

Intermolecular annulation reactions of 1-naphthyl-1,3-indandiones with alkenes proceed efficiently in the presence of a copper catalyst to generate spirocarbocycle compounds. Various spirocyclic molecules bearing an all-carbon quaternary center could be obtained by this novel method with good yields, excellent regioselectivity, and good functional group tolerance. A radical mechanism is proposed based on the HRMS analysis results of control experiments.

Rh(iii)-catalyzed building up of used heterocyclic cations: facile access to white-light-emitting materials

The first example of rhodium-catalyzed nondirected C-H activation/annulation reactions for the construction of fused heterocyclic cations is reported herein with excellent regioselectivity. Deuterium-labeling experiments indicated that the C(sp3)-H bond cleavage of the N-methyl group might be the rate-limiting step during the reaction process. This protocol provides an opportunity to rapidly access highly π-conjugated fused heterocyclic cations, which opens up a new avenue for efficient screening of single-molecular white-light-emitting materials, pure red-light-emitting materials, and π-conjugated radical materials. Importantly, novel white-light-emitting materials exhibited distinct anti-Kasha dual-emission and could rapidly be fabricated into robust organic and low-cost white light-emitting diodes.

Deconstructive annulation mediated one-pot synthesis of xanthene derivatives

Direct conversion of naphthoxazines to diverse xanthene derivatives was achieved under one-pot operation through deconstructive annulation methodology. Sequential oxidative C(sp3)-O/C(sp3)-N cleavage followed by intramolecular/intermolecular annulation reaction was carried out under aerobic reaction conditions. Mechanistic analyses performed on the substrate revealed that the C(sp3)-O bond cleavage supersedes the C(sp3)-N bond scission. The in situ generated Betti base intermediate through the C(sp3)-O cleavage was successfully isolated. Based on a molecular docking investigation, the intermolecular annulated products demonstrated good α-glucosidase inhibitory properties.

Rh-catalyzed chemo-, stereo- and regioselective C-H cascade annulation of indolyloxopropanenitriles for pyranoindoles

Selective annulations of alkynes represent a powerful tool for constructing multicyclic scaffolds while installing desired substitution patterns with precision. Herein, we report a Rh-catalyzed unique annulation of indolyl oxopropanenitrile with hydroxy-alkynoates to access pyranoindole cyclic motifs, featuring enol oxygen as a rare chemoselective reactive terminal. The reaction proceeds via a five-membered oxy-rodacycle through C-H activation by a rhodium complex guided by enolic- and propargyloxy dual co-ordination to enable a regio- and stereoselective modular assembly.

Ru(II)-catalyzed synthesis of poly-substituted furans via intermolecular oxidative annulation reaction of ethyl 3-oxo-3-phenylpropanoates with aryl alkynes/heteroaryl alkynes

The oxidative annulation reaction of ethyl 3-oxo-3-phenylpropanoates with internal alkynes proceeds efficiently in the presence of a Ru(II)-catalyst, a copper oxidant and an additive such as AgSbF₆ to give poly-substituted furans, which offers a novel method for the selective construction of poly-substituted furans. The reaction has wider substrate scope with simple starting materials, and the desired tetrasubstituted furans were prepared in good to excellent yields.

Rh(III)-Catalyzed C-H Annulation of Sulfoxonium Ylides and 1,3-Diynes: A Rapid Access to Alkynyl-1-Naphthol Derivatives

An effective redox-neutral strategy to synthesize aryl/alkynyl and alkyl/alkynyl substituted 1-naphthol derivatives has been efficaciously developed by Rh(III)-catalyzed [4+2]-annulation of sulfoxonium ylides and 1,3-diynes with excellent regio- and chemoselectivity. Subsequently, the same strategy was extended to furnish various unsymmetrical binaphthol motifs in one-pot manner. Interestingly, the TMS-derived 1,3-diyne predominantly delivered the 3-alkynyl-1-naphthol via desilylation pathway. The salient features such as traceless directing group, broad substrate scope, good functional group tolerance, and operationally simple conditions made the present protocol more valuable and appealing.

Rhodium(III)-Catalyzed Triple Aryl/Alkenyl C-H Bond Activation of Aryl Enaminones to Access Naphtho[1,8-bc]pyrans

Rhodium(III)-catalyzed triple C-H bond activation of aryl enaminones was achieved to access naphtho[1,8-bc]pyrans by oxidative annulation to internal alkynes. 1-Naphthols might be formed as the only products, depending on the steric and/or electronic environment around the aroyl moiety of the aryl enaminones or the electronic impact from the alkynes. With propargyl alcohols as the masked terminal alkynes, aryl enaminones underwent rhodium(III)- or rhodium(I)-catalyzed internal alkenyl C-H bond activation to afford functionalized but-2-ene-1,4-diones. The resultant naphtho[1,8-bc]pyrans are highly fluorescent and can be further transformed by chlorination, bromination, and difluoromethylation, demonstrating potential practicability of the synthetic protocol.

Fluorogenic Protein Labeling by Generation of Fluorescent Quinoliziniums Using [Cp*RhCl2]2

Fluorogenic labeling has received considerable attention as a result of the high demand in chemical biology and synthetic biology applications. Herein, we develop a new strategy for fluorescent turn-on ligation targeting alkyne- and quinoline-linked peptides and proteins (λ_em of 515 nm and up to Φ_F of 0.20) using the [Cp*RhCl₂]₂ catalyst. The good conversion, high flexibility, broad utility, ease of use, and mild reaction conditions are great advantages to extend the rhodium-mediated turn-on fluorogenic bioconjugation for further applications.

Synthesis of Indenone Derivatives by Rh(III)-Catalyzed C-H Functionalization of Sulfoxonium Ylides with 1,3-Diynes

The transition-metal-catalyzed C-H functionalization of sulfoxonium ylides with alkynes formally participates in [4 + 2] annulations to deliver the naphthol scaffolds. In contrast, herein we disclose the first Rh(III)-catalyzed C-H activation, followed by redox-neutral [3 + 2] annulation of sulfoxonium ylides with 1,3-diynes, which delivers the alkynated indenone derivatives. This protocol features a good functional group tolerance, a broad substrate scope, moderate to excellent yields, and mild reaction conditions. The reaction mechanism was supported through ESI-HRMS by characterizing key intermediates in the catalytic cycle.

Synthesis of spiropyrans via the Rh(III)-catalyzed annulation of 3-aryl-2H-benzo[b][1,4]oxazines with diazo ketoesters

Rh(III)-Catalyzed 1 : 2 coupling of 3-aryl-2H-benzo[b][1,4]oxazines with α-diazo-β-ketoesters has been realized for the mild synthesis of spiropyrans. The reaction proceeded via twofold C-H activation followed by unusual [3+3] and [4+2] annulation with decent functional group tolerance. Moreover, a pyranoid-skeleton intermediate was isolated as a key intermediate as a result of monoalkylation and enol oxygen annulation, which offers direct mechanistic insight.

Multiple annulations of inert C(sp2)-H bonds with alkynes

Transition-metal catalyzed directing group (DG) assisted annulation of inert C-H bonds leads to the formation of complex molecular frameworks from readily accessible substrates. Thus, multiple annulation of less functionalized substrates with unsaturated species leads to the construction of structurally diverse fused poly(hetero)cycles. The directed inert C(arene)-H bond activation and the mode of TM-migration in this process could enabled obatining L-type [involves DG heteroatom, o-C(arene)-H bond, and C(arene)-H bond of aryl-motif in alkyne], Y-type [involves two heteroatoms of the DG and o-,o'-C(arene)-H bonds], and B-type [involves o-C(arene)-H bond and m-C(arene)-H bond] π-extended annulation products. The coordination preference of the DG heteroatom makes the transformation chemo- and regio-selective. This article underlines the conceptual development of unsymmetrical multiple annulation of arene C(sp²)-H bonds with alkynes, which is exceedingly appealing and highly important.

Rhodium-Catalyzed C-H Annulation of Free Anilines with Vinylene Carbonate as a Bifunctional Synthon

Chemical transformation with vinylene carbonate as an emerging synthetic unit has recently attracted considerable attention. This report is a novel conversion pattern with vinylene carbonate, in which such a vibrant reagent unprecedentedly acts as a difunctional coupling partner to complete the C-H annulation of free anilines. From commercially available substrates, this protocol leads to the rapid construction of synthetically versatile 2-methylquinoline derivatives (43 examples) with excellent functionality tolerance.

Sulfur and Nitrogen Modulated One-Pot Double Annulation of Arenes

The sulfur and nitrogen moieties of methylphenyl sulfoximine (MPS)-enabled aryl thioamides are independently involved in annulation with unactivated alkynes to construct the unusual 6,6-fused thiopyranoisoquinoline skeletons. The MPS directing group plays a vital role in making this unprecedented Ru-catalyzed one-pot double annulation of aryl thioamides with alkynes chemo- and regioselective. Both the o,o'-C-H bonds of the aryl motif are sequentially functionalized to form four bonds [C-C, C-S and C-C, C-N] in a single operation by overcoming the undisputed challenges, viz. the "S" poisoning effect on the transition-metal catalyst and the susceptibility of S to oxidation. The novel isothiochromene-1-one skeletons are successfully constructed, as the annulation is initiated with S in preference over the N motif of thioamides with alkynes. The preliminary photophysical properties of the thiopyrano-isoquinoline derivatives are also discussed.

Stereoselective rhodium-catalyzed 2-C-H 1,3-dienylation of indoles: dual functions of the directing group

A rhodium-catalyzed intermolecular highly stereoselective 1,3-dienylation at the 2-position of indoles with non-terminal allenyl carbonates has been developed by using 2-pyrimidinyl or pyridinyl as the directing group. The reaction tolerates many functional groups affording the products in decent yields under mild conditions. In addition to C-H bond activation, the directing group also played a vital role in the determination of Z-stereoselectivity for the C-H functionalization reaction with 4-aryl-2,3-allenyl carbonates, which is confirmed by the E-selectivity observed with 4-alkyl-2,3-allenyl carbonates. DFT calculations have been conducted to reveal that π-π stacking involving the directing 2-pyrimidinyl or pyridinyl group is the origin of the observed stereoselectivity. Various synthetic transformations have also been demonstrated.

Synthesis of Conjugated Polymers via Transition Metal Catalysed C-H Bond Activation

Transition metal catalysed C-H bond activation chemistry has emerged as an exciting and promising approach in organic synthesis. This allows us to synthesize a wider range of functional molecules and conjugated polymers in a more convenient and more atom economical way. The formation of C-C bonds in the construction of pi-conjugated systems, particularly for conjugated polymers, has benefited much from the advances in C-H bond activation chemistry. Compared to conventional transition-metal catalysed cross-coupling polymerization such as Suzuki and Stille cross-coupling, pre-functionalization of aromatic monomers, such as halogenation, borylation and stannylation, is no longer required for direct arylation polymerization (DArP), which involve C-H/C-X cross-coupling, and oxidative direct arylation polymerization (Ox-DArP), which involves C-H/C-H cross-coupling protocols driven by the activation of monomers' C(sp² )-H bonds. Furthermore, poly(annulation) via C-H bond activation chemistry leads to the formation of unique pi-conjugated moieties as part of the polymeric backbone. This review thus summarises advances to date in the synthesis of conjugated polymers utilizing transition metal catalysed C-H bond activation chemistry. A variety of conjugated polymers via DArP including poly(thiophene), thieno[3,4-c]pyrrole-4,6-dione)-containing, fluorenyl-containing, benzothiadiazole-containing and diketopyrrolopyrrole-containing copolymers, were summarized. Conjugated polymers obtained through Ox-DArP were outlined and compared. Furthermore, poly(annulation) using transition metal catalysed C-H bond activation chemistry was also reviewed. In the last part of this review, difficulties and perspective to make use of transition metal catalysed C-H activation polymerization to prepare conjugated polymers were discussed and commented.

Cascade Oxidative C-H Annulation of Thiophenes: Heck-Type Pathway Enables Concise Access to Thienoacenes

The pursuit of efficient synthetic route to thienoacenes represents an appealing yet challenging task in the fields of both organic synthetic chemistry and organic functional materials. In this work, we disclose a rhodium-catalyzed cascade C-H annulation of phenacyl phosphoniums with (benzo)thiophenes via a Heck-type pathway to provide a new class of planar thienoacenes, which involves the formation of three C_aryl -C_aryl bonds and one C_aryl -O bond in a single operation. The neutral S,O-heteroacenes exhibit superior stability and adopt a herringbone-like packing mode with efficient π-π stacking in the crystals, suggesting their potential in organic semiconducting materials. This work first exemplifies the superiority of cascade oxidative C-H annulation involving a Heck-type pathway in the development of concise access to heteroacenes.

Harnessing sulfur and nitrogen in the cobalt(iii)-catalyzed unsymmetrical double annulation of thioamides: probing the origin of chemo- and regio-selectivity

An unconventional cobalt(iii)-catalyzed one-pot domino double annulation of aryl thioamides with unactivated alkynes is presented. Sulfur (S), nitrogen (N), and o,o'-C-H bonds of aryl thioamides are involved in this reaction, enabling access to rare 6,6-fused thiopyrano-isoquinoline derivatives. A reverse 'S' coordination over a more conventional 'N' coordination of thioamides to the Co-catalyst specifically regulates the formation of four [C-C and C-S at first and then C-N and C-C] bonds in a single operation, a concept which is uncovered for the first time. The power of the N-masked methyl phenyl sulfoximine (MPS) directing group in this annulation sequence is established. The transformation is successfully developed, building a novel chemical space of structural diversity (56 examples). In addition, the late-stage annulation of biologically relevant motifs and drug candidates is disclosed (17 examples). The preliminary photophysical properties of thiopyrano-isoquinoline derivatives are discussed. Density functional theory (DFT) studies authenticate the participation of a unique 6π-electrocyclization of a 7-membered S-chelated cobaltacycle in the annulation process.

Electrooxidative Rhodium-Catalyzed [5+2] Annulations via C-H/O-H Activations

Electrooxidative annulations involving mild transition metal-catalyzed C-H activation have emerged as a transformative strategy for the rapid construction of five- and six-membered heterocycles. In contrast, we herein describe the first electrochemical metal-catalyzed [5+2] cycloadditions to assemble valuable seven-membered benzoxepine skeletons by C-H/O-H activation. The efficient alkyne annulation featured ample substrate scope, using electricity as the only oxidant. Mechanistic studies provided strong support for a rhodium(III/I) regime, involving a benzoxepine-coordinated rhodium(I) sandwich complex as the catalyst resting state, which was re-oxidized to rhodium(III) by anodic oxidation.

Pd-Catalyzed sp3 C-H alkoxycarbonylation of 8-methylquinolines using Mo(CO)6 as a CO surrogate

A Pd(ii)-catalyzed three-component sp³ C-H alkoxycarbonylation of 8-methylquinonlines (8-MQs) with alcohols is accomplished using the colorless crystalline Mo(CO)₆ as a CO source. The protocol is compatible with a wide range of 8-MQs and alcohols, furnishing the carbonylated adducts in moderate to good yields. The substrate scope, functional group tolerance and natural product mutation are the important practical features.

Switchable cascade C-H annulation to polycyclic pyryliums and pyridiniums: discovering mitochondria-targeting fluorescent probes

Disclosed herein is a counterion additive-switched rhodium-catalyzed cascade triple C-H annulation of 4-hydroxy-1-naphthaldehydes with alkynes, in which six chemical bonds are formed in one-pot. This reaction enables the rapid assembly of diverse polycyclic pyrylium and pyridinium fluorophores, which leads to two specific mitochondria-labeling reagents with low cytotoxicity and superior photostability.

Transition Metal-Catalyzed Intermolecular Cascade C-H Activation/Annulation Processes for the Synthesis of Polycycles

Polycycles are abundantly present in numerous advanced chemicals, functional materials, bioactive molecules and natural products. However, the strategies for the synthesis of polycycles are limited to classical reactions and transition metal-catalyzed cross-coupling reactions, requiring pre-functionalized starting materials and lengthy synthetic operations. The emergence of novel approaches shows great promise for the fields of organic/medicinal/materials chemistry. Among them, transition metal-catalyzed C-H activation followed by intermolecular annulation reactions prevail, due to their straightforward manner with high atom- and step-economy, providing rapid, concise and efficient methods for the construction of diverse polycycles. Several strategies have been developed for the synthesis of polycycles, relying on sequential multiple C-H activation/annulation, or combination of C-H activation/annulation and further interaction with a proximal group, or merger of C-H activation with a cycloaddition reaction, or in situ formation of the directing group. These are attractive, efficient, step- and atom-economic methods starting from commercially available materials. This Minireview will provide an introduction to transition metal-catalyzed C-H activation for the synthesis of polycycles, helping researchers to discover indirect connections and reveal hidden opportunities. It will also promote the discovery of novel synthetic strategies relying on C-H activation.

Rh(III)-Catalyzed Dual C-H Functionalization/Cyclization Cascade by a Removable Directing Group: A Method for Synthesis of Polycyclic Fused Pyrano[de]Isochromenes

An interesting Rh(III)-catalyzed dual C-H functionalization/cyclization cascade of azomethine imine with diazophosphonate by a removable directing group for the synthesis of highly fused pyrano[de]isochromene has been achieved. The transformation shows that the desired pyrano[de]isochromenes with two oxygen atoms on its core scaffold could be constructed with good to excellent yields (up to 86%) via a facile one-pot, multiple-step cascade reaction, along with broad generality and versatility.

Directing Group Assisted Unsymmetrical Multiple Functionalization of Arene C-H Bonds

Multiple C-H bond functionalizations promptly install diverse groups on the molecular framework and consequently fabricate complex molecular entities. This review briefly surveys the conceptual development of directing group assisted unsymmetrical multiple functionalization of arene C(sp² )-H bonds, which is exceedingly appealing and highly important.

Rhodium(III)-Catalyzed Cyclopropane C-H/C-C Activation Sequence Provides Diastereoselective Access to α-Alkoxylated γ-Lactams

A Cp*Rh(III)-catalyzed C-H/C-C bond activation sequence of cyclopropyl hydroxamates has been developed. The three-component process allows trapping of the intermediate rhodacycle with diazomalonates and an alcohol nucleophile to provide access to synthetically valuable α-alkoxylated γ-lactams with trans diastereoselectivity.

C–H Functionalization Strategies in the Naphthalene Series: Site Selections and Functional Diversity

Naphthalene is certainly not a common arene. In contrast to benzene, the bicyclic feature of naphthalene offers multiple differentiable positions and thus a broad diversity of substitution patterns. Naphthalene is a central building block for the construction of elaborated polycyclic architectures with applications in broad domains such as life and materials sciences. As a result, C–H functionalization strategies specially designed for naphthalene substrates have become essential to install valuable substituents on one or both rings towards polysubstituted naphthalenes. This short review provides a focus on uncommon substitution patterns; however, classical ortho C–H activation is not covered.

1 Introduction

2 C–H Functionalization Using a Directing Group Located at Position 1

2.1 Functionalization on the Ring Bearing the DG: 1,3-Substitution Pattern

2.2 Functionalization on the Ring Bearing the DG: 1,4-Substitution Pattern

2.3 Functionalization on the Neighboring Ring: 1,6-, 1,7- and 1,8-Substitution Patterns

3 C–H Functionalization Using a Directing Group Located at Position 2

3.1 Functionalization on the Ring Bearing the DG: 2,4- and 2,1-Substitution Patterns

3.2 Miscellaneous Substitution Patterns

4 Bis C–H Functionalization

4.1 Symmetrical Bisfunctionalization: 1,2,8-Substitution Pattern

4.2 Symmetrical Bisfunctionalization: 2,3,1-Substitution Pattern

4.2 Unsymmetrical Bisfunctionalization: 2,3,1-Substitution Pattern

4.3 Symmetrical Bisfunctionalization: 2,4,8-Substitution Pattern

5 Conclusion and Outlook

The C-H Activation/Bidirecting Group Strategy for Selective Direct Synthesis of Diverse 1,1'-Biisoquinolines

Multidentate ligands are highly important but difficult to access. Herein we disclose an atom- and step-economic synthesis of highly substituted 1,1'-biisoquinolines by a C-H activation/bididirecting group strategy. Through rational design of a bididirecting group to "N-OH + N-OAc", selective unsymmetrical diannulation with two different alkynes in a one-pot reaction has been achieved for the first time to access unsymmetrical biisoquinolines. Moreover, the resultant biisoquinolines show tunable photoluminescence and serve as aggregation-induced emission (AIE) systems.

Enantioselective Twofold C-H Annulation of Formamides and Alkynes without Built-in Chelating Groups

Twofold C-H annulation of readily available formamides and alkynes without built-in chelating groups was achieved. Ni-Al bimetallic catalysis enabled by a bulky BINOL-derived chiral secondary phosphine oxide (SPO) ligand proved to be critical for high reactivity and high selectivity. This reaction uses readily available formamides as starting materials and provides a concise synthetic pathway to a broad range of chiral ferrocenes in 40-98 % yield and 93-99 % ee.

Access to [4,3,1]-Bridged Carbocycles via Rhodium(III)-Catalyzed C-H Activation of 2-Arylindoles and Annulation with Quinone Monoacetals

Reported herein is the Rh(III)-catalyzed annulation of N-unprotected 2-arylindoles with quinone monoacetals for the straightforward synthesis of [4,3,1]-bridged carbocycles with exclusive C(3) selectivity. Mechanistic studies, particularly deuterium-labeling experiments, suggest that the coupling likely proceeds via two-fold C-H activation with two-fold migratory insertion into the enone moieties.

Ligand-Enabled Ni-Al Bimetallic Catalysis for Nonchelated Dual C-H Annulation of Arylformamides and Alkynes

A bifunctional secondary phosphine oxide (SPO) ligand-controlled method was developed for Ni-Al-catalyzed nonchelated dual C-H annulation of arylformamides with alkynes, providing a series of substituted amide-containing heterocycles in ≤97% yield. The SPO-bound bimetallic catalysis proved to be critical to the reaction efficiency.

Cu(I)-Catalyzed Ligand-Free Tandem One-Pot or Sequential Annulation via Knoevenagel Intermediates: An Entry into Multifunctional Naphthalenes, Phenanthrenes, Quinolines, and Benzo[b]carbazoles

A simple but efficient one-pot or sequential copper-catalyzed protocol using 2-bromoaldehydes and active methylene group containing substrates that affords multifunctional naphthalenes, phenanthrenes, quinolines, and benzo[b]carbazoles via Knoevenagel condensation, C-arylation, and decarboxylation, followed by aromatization, is developed. The reaction utilizes the potential of Knoevenagel intermediates and does not require any ancillary ligand. The phenanthrene products thus obtained show moderate fluorescence activity. Structural elaboration of the products to obtain dihydrobenzoquinazolines is also highlighted.

Annulation of imidazo[1,2-a]pyridines under metal-free conditions

Synthesis of benzo[a]imidazo[5,1,2-cd]indolizines from 2-arylimidazo[1,2-a]pyridines and benzyne precursors under metal-free conditions has been described.

Double annulation of ortho- and peri-C–H bonds of fused (hetero)arenes to unusual oxepino-pyridines

Direct difunctionalization of chemically distinct ortho- and peri-C–H bonds of fused hetero(arenes) is illustrated through an unusual one-pot domino {[4 + 2] & [5 + 2]} double annulation with alkynes for the first time.