Pd/Chiral Phosphoric Acid-Enabled Asymmetric Intramolecular Double C-H Activation Reaction for the Synthesis of P-Stereogenic Benzophosphole Oxides

A Pd-catalyzed intramolecular asymmetric double C-H activation reaction was reported, and a BINOL skeleton-based chiral phosphoric acid (CPA) was identified as a suitable ligand for this transformation. A broad range of P-stereogenic benzophosphole oxides were synthesized in moderate to excellent yield with high levels of enantioselectivity (≤96% ee).

Nickel(II)/Salox-Catalyzed Enantioselective C-H Functionalization

Recently, nickel catalysts have garnered considerable attention for their efficacy and versatility in asymmetric catalysis, attributed to their distinctive properties. However, the use of cost-effective and sustainable divalent nickel catalysts in C-H activation/asymmetric alkene insertion poses significant challenges due to the intricate control of stereochemistry in the transformation of the tetracoordinate C-Ni(II) intermediate. Herein, we report a Ni(II)-catalyzed enantioselective C-H/N-H annulation with oxabicyclic alkenes. This protocol offers straightforward access to chiral [2,2,1]-bridged bicyclic compounds bearing four consecutive stereocenters with high enantioselectivity (up to 96% ee). The development of a sterically hindered chiral salicyloxazoline (Salox) ligand, TMS-Salox, is key to the success of this protocol. Mechanistic investigations unveiled that a chiral Ni(III)-metalacyclic intermediate was formed through the in situ oxidation of achiral organometallic Ni(II) species and coordination of the Salox ligand. This process led to the creation of a tailored chiral pocket that guides the approach of alkenes, thereby influencing and determining the stereochemistry.

Enantioselective Construction of Tetrahydroindole Skeletons by Rh-Catalyzed [2+2+2] Cycloaddition of Homopropargyl Enamides with Alkynes

We have developed the Rh-catalyzed enantioselective [2+2+2] cycloaddition of homopropargyl enamides (tosylamide-tethered 1,6-enynes) with alkynes to construct tetrahydroindole skeletons found in natural alkaloids and pharmaceuticals. This cycloaddition proceeds at room temperature in high yields and regio- and enantioselectivity with a broad substrate scope. The preparative scale reaction followed by substituent conversion on the nitrogen atom and the diastereoselective [4+2] cycloaddition with singlet O2 affords hexahydroindole-diols bearing three stereogenic centers and variable substituents on the nitrogen. Mechanistic studies have revealed that the substituents of the enynes change the ratio of intramolecular and intermolecular rhodacycle formation when using terminal alkynes, varying the ee values of the cycloadducts.

Computational Insights into Palladium-Catalyzed Site-Selective Anilide and Benzamide-Type [3+2] Annulation via Double C-H Bond Activation

The mechanism of palladium-catalyzed annulation reactions of benzamide- and anilide-type aromatic systems with maleimides is investigated using density functional theory. Double C-H bond activation is key to forming the desired annulation product. The first C-H bond activation for anilide- and amide-type ligands can occur at the ortho and benzylic C-H bonds, while the second C-H activation occurs at the meta carbon of the aromatic rings. For the anilide-type system, ortho and benzylic C-H bond activations occur via four- and five-membered palladacycles, respectively. In contrast, for the benzamide-type system, ortho and benzylic C-H bond activations occur via five- and six-membered palladacycles, respectively. The energy span model suggests that the initial C-H bond activation step at the benzylic position determines the turnover frequency for both anilide- and benzamide-type systems. Energy decomposition analysis and distortion-interaction/activation-strain analyses are employed to understand the electronic and steric factors controlling the turnover frequency-determining transition state.

Pd(II)-catalyzed cyclization of 2-methyl aromatic ketones with maleimides through weak chelation assisted dual C-H activation

A palladium-catalyzed dual C-H functionalization of substituted aromatic ketones and ester with maleimides leading to tricyclic heterocyclic molecules with good to excellent yield is reported. In this protocol, weak chelation of the carbonyl groups has been successfully utilized for the selective activation of the ortho-methyl C(sp3)-H bond instead of the ortho-C(sp2)-H bond in the presence of an external bidentate ligand Ac-Ile-OH. The reaction proceeds through two-fold C-H activation to generate a five-membered cyclic ring. The first C-H activation takes place selectively at the benzylic position followed by a second C-H bond activation at the meta position. The protocol demonstrates compatibility among diverse substituted aromatic ketones and ester as well as various substituted maleimides. Further derivatization of the tricyclic ketone to an alcohol exhibits the synthetic applicability of the protocol. Also, a plausible reaction mechanism has been proposed.

Cobalt-Catalyzed Domino Transformations via Enantioselective C-H Activation/Nucleophilic [3 + 2] Annulation toward Chiral Bridged Bicycles

Selective synthesis of chiral bridged (hetero)bicyclic scaffolds via asymmetric C-H activation constitutes substantial challenges due to the multiple reactivities of strained bicyclic structures. Herein, we develop the domino transformations through an unprecedented cobalt-catalyzed enantioselective C-H activation/nucleophilic [3 + 2] annulation with symmetrical bicyclic alkenes. The methods offer straightforward access to a wide range of chiral molecules bearing [2.2.1]-bridged bicyclic cores with four and five consecutive stereocenters in a single step. Two elaborate salicyloxazoline (Salox) ligands were synthesized based on the rational design and mechanistic understanding. The well-defined chiral pockets generated from asymmetric coordination around the trivalent cobalt catalyst direct the orientation of bicyclic alkenes, leading to excellent enantioselectivity.

Rhodium-Catalyzed Diastereoselective C-H Activation/[4 + 2] Annulation of α,β-Unsaturated Amides with Bicyclic Alkenes

Herein, we report a rare example of rhodium-catalyzed C-H activation/[4 + 2] annulation of alkenyl amides with bicyclic alkenes under mild and green conditions. The reactivity of the rhodium catalyst in this study differed from that observed in cobalt-catalyzed C-H activation/[3 + 2] annulation between vinylic amides and bicyclic alkenes. In addition, the reaction was performed in EtOH at room temperature, which also displayed excellent diastereoselectivity, good functional group tolerance, and air compatibility. A series of novel bridged-ring skeletons were obtained in good to excellent yields. Scale-up experiments were carried out with 1 or 0.75 mol % rhodium catalyst, affording the desired bridged-ring skeleton in excellent yields.

Rhodium-Catalyzed Asymmetric C-H Functionalization Reactions

This review summarizes the advancements in rhodium-catalyzed asymmetric C-H functionalization reactions during the last two decades. Parallel to the rapidly developed palladium catalysis, rhodium catalysis has attracted extensive attention because of its unique reactivity and selectivity in asymmetric C-H functionalization reactions. In recent years, Rh-catalyzed asymmetric C-H functionalization reactions have been significantly developed in many respects, including catalyst design, reaction development, mechanistic investigation, and application in the synthesis of complex functional molecules. This review presents an explicit outline of catalysts and ligands, mechanism, the scope of coupling reagents, and applications.

Rhodium-catalyzed annulative approach to N-N axially chiral biaryls via C-H activation and dynamic kinetic transformation

N-N axially chiral biaryls represent a rarely explored class of atropisomeric compounds. We hereby report rhodium-catalyzed enantioselective [4 + 2] oxidative annulation of internal alkynes with benzamides bearing two classes of N-N directing groups. The coupling occurs under mild conditions via NH and CH annulation through the dynamic kinetic transformation of the directing group and is highly enantioselective with good functional tolerance. Computational studies of a coupling system at the DFT level has been conducted, and the alkyne insertion was identified as the enantio-determining as well as the turnover-limiting step.

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.

Palladium-Catalyzed [3 + 2] Annulation of Aromatic Amides with Maleimides through Dual C-H Activation

A palladium-catalyzed [3 + 2] annulation of substituted aromatic amides with maleimides providing tricyclic heterocyclic molecules in good to moderate yields through weak carbonyl chelation is reported. The reaction proceeds via a dual C-H bond activation where the first C-H activation takes place selectively at the benzylic position followed by a second C-H bond activation at the meta position to afford a five-membered cyclic ring. An external ligand Ac-Gly-OH has been used to succeed in this protocol. A plausible reaction mechanism has been proposed for the [3 + 2] annulation reaction.

Cobalt(III)-catalyzed asymmetric ring-opening of 7-oxabenzonorbornadienes via indole C-H functionalization

Asymmetric ring-opening of 7-oxabenzonorbornadienes is achieved via Co-catalyzed indole C-H functionalization. The utilization of chiral Co-catalyst consisting of a binaphthyl-derived trisubstituted cyclopentadienyl ligand resulted in high yields (up to 99%) and excellent enantioselectivity (>99% ee) for the target products with tolerance for diverse functional groups. Opposite diastereoselectivities are obtained with chiral Co-catalyst or Cp*CoI₂CO. Combined experimental and computational studies suggest β-oxygen elimination being the selectivity-determining step of the reaction. Meanwhile, the reactions of 7-azabenzonorbornadiene could also be executed in a diastereodivergent manner.

Rhodium-catalyzed enantioselective C-H alkynylation of sulfoxides in diverse patterns: desymmetrization, kinetic resolution, and parallel kinetic resolution

Rhodium-catalyzed enantioselective C-H alkynylation of achiral and racemic sulfoxides is disclosed with alkynyl bromide as the alkynylating reagent. A wide range of chiral sulfoxides have been constructed in good yield and excellent enantioselectivity (up to 99% ee, s-factor up to > 500) via desymmetrization, kinetic resolution, and parallel kinetic resolution under mild reaction conditions. The high enantioselectivity was rendered by the chiral cyclopentadienyl rhodium(iii) catalyst paired with a chiral carboxamide additive. The interactions between the chiral catalyst, the sulfoxide, and the chiral carboxylic amide during the C-H bond cleavage offer the asymmetric induction, which is validated by DFT calculations. The chiral carboxamide functions as a base to promote C-H activation and offers an additional chiral environment during the C-H cleavage.

RhodiumIII-catalyzed remote difunctionalization of arenes assisted by a relay directing group

Rhodium-catalyzed diverse tandem twofold C-H bond activation reactions of para-olefin-tethered arenes have been realized, with unsaturated reagents such as internal alkynes, dioxazolones, and isocyanates being the coupling partner as well as a relay directing group which triggers cyclization of the para-olefin group under oxidative or redox-neutral conditions. The reaction proceeded via initial ortho-C-H activation assisted by a built-in directing group in the arene, and the ortho-incorporation of the unsaturated coupling partner simultaneously generated a relay directing group that allows sequential C-H activation at the meta-position and subsequent cyclization of the para-olefins. The overall reaction represents C-C or N-C difunctionalization of the arene with the generation of diverse 2,3-dihydrobenzofuran platforms. The catalytic system proceeded with good efficiency, simple reaction conditions, and broad substrate scope. The diverse transformations of the products demonstrated the synthetic utility of this tandem reaction.

Rhodium(III)-Catalyzed Asymmetric 1,2-Carboamidation of Alkenes Enables Access to Chiral 2,3-Dihydro-3-benzofuranmethanamides

Through the initial screening and further rational design of chiral cyclopentadienyl ligands, a chiral rhodium-catalyzed enantioselective 1,2-carboamidation of aromatic tethered alkenes was developed, enabling the asymmetric preparation of various chiral 2,3-dihydro-3-benzofuranmethanamides with an enantioenriched all-carbon quaternary center at the β position of amide. This robust transformation has a broad functional group tolerance, excellent enantioselectivities (up to 98.5:1.5 er), and a mild reaction conditions, releasing CO₂ as the single byproduct.

Cobalt-catalyzed highly diastereoselective [3 + 2] carboannulation reactions: facile access to substituted indane derivatives

Efficient oxidative [3 + 2] annulation reaction involving aryl hydrazones and heterobicyclic alkenes has been realized with inexpensive and earth-abundant cobalt salts under aerobic conditions. The reaction proceeds via directing-group-assisted C-H activation and exo-selective migratory insertion, followed by the intramolecular nucleophilic attack of the alkylcobalt(III) species onto the imine with high anti-diastereoselectivity to provide complex indane derivatives. The generation of three contiguous stereogenic centers within the indanyl unit and the avoidance of the use of stoichiometric amounts of metal oxidants make this transformation more valuable and appealing.

Benzocarbazole Synthesis via Visible-Light-Accelerated Rh(III)-Catalyzed C-H Annulation of Aromatic Amines with Bicyclic Alkenes

A visible-light-accelerated Rh(III)-catalyzed C-H annulation of aromatic amines with bicyclic alkenes for the synthesis of benzocarbazole derivatives was developed. In this approach, with the cooperation of rhodium catalysis and visible-light irradiation, various aromatic amines reacted with oxabicyclic alkenes and azabicyclic alkenes smoothly at room temperature, delivering the corresponding bridged oxa or aza tetrahydro benzocarbazoles in good to excellent yields. Moreover, a series of benzo[b]carbazoles were synthesized conveniently through further aromatization in one pot. The potential of this method was demonstrated via directing-group removal, derivatization, a scale-up reaction, and fluorescence investigations.

Rhodium-Catalyzed and Chiral Zinc Carboxylate-Assisted Allenylation of Benzamides via Kinetic Resolution

Enantioenriched allenes are important building blocks. While they have been accessed by other coupling methodologies, enantioenriched allenes have been rarely obtained via C-H activation. In this work, kinetic resolution of tertiary propargyl alcohols as an allenylating reagent has been realized via rhodium(III)-catalyzed C-H allenylation of benzamides. The reaction proceeded efficiently under mild conditions, and both the allenylated products and the propargyl alcohols were obtained in high enantioselectivities with an s-factor of up to 139. The resolution results from bias of the two propargylic substituents and is assisted by a chiral zinc carboxylate additive.

Twofold C-H Activation-Based Enantio- and Diastereoselective C-H Arylation Using Diarylacetylenes as Rare Arylating Reagents

C-H bond activation has been established as an attractive strategy to access axially chiral biaryls, and the most straightforward method is direct C-H arylation of arenes. However, the arylating source has been limited to several classes of reactive and bulky reagents. Reported herein is rhodium-catalyzed 1:2 coupling of diarylphosphinic amides and diarylacetylenes for enantio- and diastereoselective construction of biaryls with both central and axial chirality. This twofold C-H activation reaction stays contrast to the previously explored Miura-Satoh type 1:2 coupling of arenes and alkynes in terms of chemoselectivity and proceeded under mild conditions with the alkyne acting as a rare arylating reagent. Both C-H activation events are stereo-determining and are under catalyst control, with the 2^nd C-H activation being diastereo-determining in a remote fashion. Analysis of the stereochemistry of the major and side products suggests moderate enantioselectivity of the initial C-H activation-desymmetrization process. However, the minor (R) rhodium vinyl intermediate is consumed more readily in undesired protonolysis, eventually resulting in high enantio- and diastereoselectivity of the major product.

Rhodium-Catalyzed C-H Activation-Based Construction of Axially and Centrally Chiral Indenes through Two Discrete Insertions

Reported herein is asymmetric [3+2] annulation of arylnitrones with different classes of alkynes catalyzed by chiral rhodium(III) complexes, with the nitrone acting as an electrophilic directing group. Three classes of chiral indenes/indenones have been effectively constructed, depending on the nature of the substrates. The coupling system features mild reaction conditions, excellent enantioselectivity, and high atom-economy. In particular, the coupling of N-benzylnitrones and different classes of sterically hindered alkynes afforded C-C or C-N atropochiral pentatomic biaryls with a C-centered point-chirality in excellent enantio- and diastereoselectivity (45 examples, average 95.6 % ee). These chiral center and axis are disposed in a distal fashion and they are constructed via two distinct migratory insertions that are stereo-determining and are under catalyst control.

Oxygen-Linked Cyclopentadienyl Rhodium(III) Complexes-Catalyzed Asymmetric C-H Arylation of Benzo[h]quinolines with 1-Diazonaphthoquinones

Chiral cyclopentadienyl rhodium (CpRh) complex-catalyzed asymmetric C-H functionalization reactions have witnessed a significant progress in organic synthesis. In sharp contrast, the reported chiral Cp ligands are limited to C-linked Cp and are often synthetically challenging. To address these issues, we have developed a novel class of tunable chiral cyclopentadienyl ligands bearing oxygen linkers, which were efficient catalysts for C-H arylation of benzo[h]quinolines with 1-diazonaphthoquinones, affording axially chiral heterobiaryls in excellent yields and enantioselectivity (up to 99 % yield, 98.5:1.5 er). Mechanistic studies suggest that the reaction is likely to proceed by electrophilic C-H activation, and followed by coupling of the cyclometalated rhodium(III) complex with 1-diazonaphthoquinones.

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.

Recent Advances in Transition-Metal-Catalyzed C–H Functionalization Reactions Involving Aza/Oxabicyclic Alkenes

Bicyclic alkenes, including oxa- and azabicyclic alkenes, readily undergo activation with facial selectivity in the presence of transition-metal complexes. This is due to the intrinsic angle strain on the carbon–carbon double bonds in such unsymmetrical bicyclic systems. During the past decades considerable progress has been made in the area of ring opening of bicyclic strained rings by employing the concept of C–H activation. This short review comprehensively compiles the various C–H bond activation assisted reactions of oxa- and azabicyclic alkenes, viz., ring-opening reactions, hydroarylation, and annulation reactions.

1 Introduction

2 Reactions of Heterobicyclic Ring Systems

2.1 Ring-Opening Reactions of Oxa- and Azabenzonorbornadienes

2.1.1 Reactions Using 7-Oxabenzonorbornadienes

2.1.2 Reactions Using 7-Azabenzonorbornadienes

2.2 Hydroarylation Reactions

2.3 Annulation Reactions

2.4 Other Reactions

3 Conclusion

Visible-Light-Induced Formation of Thiavinyl 1,3-Dipoles: A Metal-Free [3+2] Oxidative Cyclization with Alkynes as Easy Access to Thiophenes

A visible-light-induced [3+2] oxidative cyclization of various alkynes with easily available ketene dithioacetals as the previously unknown thiavinyl 1,3-dipoles in the presence of an acridine photosensitizer is reported. A series of multisubstituted thiophenes were achieved regioselectively in ≤98% yields under very mild metal-free conditions without other additives. This reaction could tolerate a wide range of substrates and achieve good efficiency in large-scale syntheses. The reaction mechanism and their applications are described in detail to reveal the reactivity of the new 1,3-dipoles and the selectivity of the reactions.

Three-Component 1,2-Carboamidation of Bridged Bicyclic Alkenes via RhIII-Catalyzed Addition of C-H Bonds and Amidating Reagents

A three-component method is described for the preparation of syn-1,2-disubstituted bridged bicyclic compounds. The reaction was demonstrated for readily available aromatic and heteroaromatic C-H bond substrates with tertiary and secondary amide, lactam, pyrazole, and triazole directing groups and a variety of bridged bicyclic alkenes, including norbornene, benzonorbornadiene, oxygen- and nitrogen-bridged analogs, and an unsaturated tropinone. A broad dioxazolone scope was also observed. The use of a chiral Cp-derived RhIII catalyst enables asymmetric synthesis of products.

Mechanochemical Solvent-Free Catalytic C-H Methylation

The mechanochemical, solvent-free, highly regioselective, rhodium-catalyzed C-H methylation of (hetero)arenes is reported. The reaction shows excellent functional-group compatibility and is demonstrated to work for the late-stage C-H methylation of biologically active compounds. The method requires no external heating and benefits from considerably shorter reaction times than previous solution-based C-H methylation protocols. Additionally, the mechanochemical approach is shown to enable the efficient synthesis of organometallic complexes that are difficult to generate conventionally.

Recent advances in the Rh-catalyzed cascade arene C-H bond activation/annulation toward diverse heterocyclic compounds

The Rh-catalyzed C-H bond activation/annulation provides a new strategy for the synthesis of new frameworks. In this review, we summarize the recent research on the Rh-catalyzed cascade arene C-H bond activation/annulation toward diverse heterocyclic compounds. The application, scope, limitations and mechanism of these transformations are also discussed.

Palladium-Catalyzed Site-Selective [3+2] Annulation via Benzylic and meta C-H Bond Activation

The palladium-catalyzed [3+2] annulation of aromatic amides with maleimides via the activation of ortho benzylic C-H and meta C-H bonds is reported. Carboxamide and anilide type substrates that contain a 2-methylthiophenyl group both participate in this [3+2] annulation, indicating that the presence of a 2-methylthiophenyl directing group is a key for the success of the reaction. The first C-H bond activation at the benzylic C-H bond is followed by a second C-H bond activation at the meta C-H bond to give five-membered cyclic products. The cleavage of these C-H bonds is irreversible.

Rh(iii)-Catalyzed switchable C–H monoalkenylation and dialkenylation of 2-(1H-pyrazol-1-yl)pyridine with alkenes via rollover cyclometalation

We have demonstrated Rh(iii)-catalyzed switchable C–H monoalkenylation and dialkenylation of 2-(1H-pyrazol-1-yl)pyridine with alkenes via rollover cyclometalation.

Catalytic asymmetric transformations of oxa- and azabicyclic alkenes

This review provides an overview of the fundamental concepts and recent developments in a wide range of enantioselective transformations involving oxa- and azabicyclic alkenes.

Pd(ii)-Catalyzed enantioconvergent twofold C–H annulation to access atropisomeric aldehydes: a platform for diversity-oriented-synthesis

The first Pd(ii)-catalyzed atroposelective dual C–H annulative strategy for diverse synthesis of functionalized axially chiral biaryls was developed.