Metal-Catalyzed Annulations through Activation and Cleavage of C−H Bonds

Rhodium(III)-Catalyzed C-H Bond Functionalization of 2-Pyridones with Alkynes: Switchable Alkenylation, Alkenylation/Directing Group Migration and Rollover Annulation

Cp*Rh(III)-catalyzed chelation-assisted direct C-H bond functionalization of 1-(2-pyridyl)-2-pyridones with internal alkynes that can be controlled to give three different products in good yields has been realized. Depending on the reaction conditions, solvents and additives, the reaction pathway can be switched between alkenylation, alkenylation/directing group migration and rollover annulation. These reaction manifolds allow divergent access to a variety of valuable C6-alkenylated 1-(2-pyridyl)-2-pyridones, (Z)-6-(1,2-diaryl-2-(pyridin-2-yl)vinyl)pyridin-2(1H)-ones and 10H-pyrido[1,2-a][1,8]naphthyridin-10-ones from the same starting materials. These protocols exhibit excellent regio- and stereoselectivity, broad substrate scope, and good tolerance of functional groups. A combination of experimental and computational approaches have been employed to uncover the key mechanistic features of these reactions.

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.

Cobalt-Catalyzed C-H Activation/Annulation of Benzamides with Fluorine-Containing Alkynes: A Route to 3- and 4-Fluoroalkylated Isoquinolinones

The C-H activation/annulation reaction of various benzamides with fluoroalkylated alkynes in the presence of a Co(acac)₂·2H₂O catalyst proceeded very smoothly to give the corresponding 3- and 4-fluoroalkylated isoquinolinones in excellent yields with approximately 70% regioselectivities. These regioisomers could be successfully separated and obtained in pure form. Major or minor regioisomers were determined as 4- or 3-fluoroalkylated isoquinolinones, respectively, based on X-ray crystallographic analyses.

β-Diazocarbonyl Compounds: Synthesis and their Rh(II)-Catalyzed 1,3 C-H Insertions

Herein, we describe the first electrophilic diazomethylation of ketone silyl enol ethers with diazomethyl-substituted hypervalent iodine reagents that gives access to unusual β-diazocarbonyl compounds. The potential of this unexplored class of diazo compounds for the development of new reactions was demonstrated by the discovery of a rare Rh-catalyzed intramolecular 1,3 C-H carbene insertion that led to complex cyclopropanes with excellent stereocontrol.

Highly Enantioselective Cobalt-Catalyzed (3+2) Cycloadditions of Alkynylidenecyclopropanes

Low-valent cobalt complexes equipped with chiral ligands can efficiently promote highly enantioselective (3+2) cycloadditions of alkyne-tethered alkylidenecyclopropanes. The annulation allows to assemble bicyclic systems containing five-membered rings in good yields and with excellent enantiomeric ratios. We also present a mechanistic discussion based on experimental and computational data, which support the involvement of Co^I /Co^III catalytic cycles.

Rhodium-Catalyzed Redox-Neutral Olefination of Aryldiazenes with Acrylate Esters via C-H Activation and Transfer Hydrogenation

Rh(III)-catalyzed redox-neutral C-H olefination of aryldiazenecarboxylates has been realized using arylate esters as the olefinating reagents. This reaction proceeds under mild and redox-neutral conditions, resulting in integration of C-H activation and transfer hydrogenation. The chemoselectivity complements that of previously reported rhodium-catalyzed coupling of the same substrates.

Kinetic Resolution of Allyltriflamides through a Pd-Catalyzed C-H Functionalization with Allenes: Asymmetric Assembly of Tetrahydropyridines

Enantioenriched, six-membered azacycles are essential structural motifs in many products of pharmaceutical or agrochemical interest. Here we report a simple and practical method for enantioselective assembly of tetrahydropyridines, which is paired to a kinetic resolution of α-branched allyltriflamides. The reaction consists of a formal (4+2) cycloaddition between the allylamine derivatives and allenes and is initiated by a palladium(II)-catalyzed C-H activation process. Both the chiral allylamide precursors and the tetrahydropyridine adducts were successfully obtained in high yields, with excellent enantioselectivity (up to 99% ee) and selectivity values of up to 127.

Rhodium-Catalyzed C4-Selective C-H Alkenylation of 2-Pyridones by Traceless Directing Group Strategy

A rhodium-catalyzed C4-selective C-H alkenylation of 3-carboxy-2-pyridones with styrenes has been developed. The carboxylic group at the C3 position works as the traceless directing group, and the corresponding C4-alkenylated 2-pyridones are obtained exclusively with concomitant decarboxylation. Unlike the reported procedures, the exclusive C4 selectivity is uniformly observed even in the presence of potentially more reactive C-H bonds at the C5 and C6 positions. By using this strategy, the multiply substituted 2-pyridone can be prepared via sequential C-H functionalization reactions.

Rh(III)-Catalyzed C-H Olefination Cascades to Divergently Construct Diverse Polyheterocycles by Tuning Manipulations of Directing Groups

Inspired by the diversity created by nature, organic chemists have been using a divergent strategy to improve the synthetic efficiency of diverse molecules. Transition-metal-catalyzed C-H functionalization has become one of the most straightforward, powerful, and atom-economical methods to construct complex scaffolds. However, C-H activation initiated divergent transformation to prepare diverse molecules is still limited. To address this challenge, we herein developed Rh(III)-catalyzed C-H olefination/annulation reaction cascades to divergently construct diverse polyheterocycles by tuning manipulations of directing groups (DGs). Up to 9 distinct scaffolds were creatively synthesized under simple conditions with good functional group tolerance, chemo-, and regioselectivity. Such a versatile strategy and its extension may encourage researchers to discover more promising manipulations of DGs for transition-metal-catalyzed C-H bond activation, making diverse available targets and materials that would have been previously out of range.

Synthesis of quaternary carbon-centered indolo[1,2-a]quinazolinones and indazolo[1,2-a]indazolones via C-H functionalization

An unprecedented Ru(ii)-catalyzed Csp2-H bond activation and annulation reaction of phenylindazolones with diaryl-substituted alkynes and dialkyl-substituted alkynes provided efficient routes for the construction of all-carbon quaternary-centered indolo[1,2-a]quinazolinones and quaternary carbon-centered indazolo[1,2-a]indazolones, respectively. The indolo[1,2-a]quinazolinones were fomed via Csp2-H activation, alkyne insertion and a 1,2-phenyl shift. Indazolo[1,2-a]indazolones were formed through a cascade reaction via the formation of exocyclic double bonds containing indolo[1,2-a]quinazolinones.

Divergent rhodium-catalyzed electrochemical vinylic C-H annulation of acrylamides with alkynes

α-Pyridones and α-pyrones are ubiquitous structural motifs found in natural products and biologically active small molecules. Here, we report an Rh-catalyzed electrochemical vinylic C-H annulation of acrylamides with alkynes, affording cyclic products in good to excellent yield. Divergent syntheses of α-pyridones and cyclic imidates are accomplished by employing N-phenyl acrylamides and N-tosyl acrylamides as substrates, respectively. Additionally, excellent regioselectivities are achieved when using unsymmetrical alkynes. This electrochemical process is environmentally benign compared to traditional transition metal-catalyzed C-H annulations because it avoids the use of stoichiometric metal oxidants. DFT calculations elucidated the reaction mechanism and origins of substituent-controlled chemoselectivity. The sequential C-H activation and alkyne insertion under rhodium catalysis leads to the seven-membered ring vinyl-rhodium intermediate. This intermediate undergoes either the classic neutral concerted reductive elimination to produce α-pyridones, or the ionic stepwise pathway to produce cyclic imidates.

Rhodium catalyzed multicomponent dehydrogenative annulation: one-step construction of isoindole derivatives

A strategy for one-pot synthesis of isoindoles is described via a catalytic multicomponent dehydrogenative annulation of diarylimines, vinyl ketones and simple amines. In the presence of a rhodium catalyst and Cu oxidant, four C-H and two N-H bonds are activated along with the formation of one new C-C and two new C-N bonds, leading to a series of isoindole derivatives in good to very high isolated yields.

A direct synthesis method towards spirocyclic indazole derivatives via Rh(iii)-catalyzed C–H activation and spiroannulation

A rhodium(iii)-catalyzed [4 + 1] spiroannulation of N-aryl phthalazine-diones (pyridazine-diones) with diazo compounds to construct spirocyclic indazole derivatives with diverse structures is described.

Rh(iii)-Catalyzed [3 + 2]/[4 + 2] annulation of acetophenone oxime ethers with 3-acetoxy-1,4-enynes involving C–H activation

A novel, synthetically simple, selective rhodium(iii)-catalyzed [3 + 2]/[4 + 2] annulation cascade reaction to construct complex azafluorenone frameworks has been developed.

C(sp3)–H Bond Functionalization of Alcohols, Ketones, Nitriles, Ethers and Amides using tert-Butyl Hydroperoxide as a Radical Initiator

The C(sp3)–H bond is found widely in organic molecules. Recently, the functionalization of C(sp3)–H bonds has developed into a powerful tool for augmenting highly functionalized frameworks in organic synthesis. Based on the results obtained in our group, the present account mainly summarizes recent progress on the functionalization of C(sp3)–H bonds of aliphatic alcohols, ketones, alkyl nitriles, and ethers with styrene or cinnamic acid using tert-butyl hydroperoxide (TBHP) as a radical initiator.

1 Introduction

2 Oxidative Coupling of Styrenes with C(sp3)–H Bonds

3 Decarboxylative Cross-Couplings of α,β-Unsaturated Carboxylic Acids with C(sp3)–H Bonds

4 Conclusions

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.

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.

Recent Advances in Construction of Polycyclic Natural Product Scaffolds via One-Pot Reactions Involving Alkyne Annulation

Polycyclic scaffolds are omnipresent in natural products and drugs, and the synthetic strategies and methods toward construction of these scaffolds are of particular importance. Compared to simple cyclic ring systems, polycyclic scaffolds have higher structure complexity and diversity, making them suitable for charting broader chemical space, yet bringing challenges for the syntheses. In this review, we surveyed progress in the past decade on synthetic methods for polycyclic natural product scaffolds, in which the key steps are one-pot reactions involving intermolecular or intramolecular alkyne annulation. Synthetic strategies of selected polycyclic carbocycles and heterocycles with at least three fused, bridged, or spiro rings are discussed with emphasis on the synthetic efficiency and product diversity. Recent examples containing newly developed synthetic concepts or toolkits such as collective and divergent total synthesis, gold catalysis, C–H functionalization, and dearomative cyclization are highlighted. Finally, several “privileged synthetic strategies” for “privileged polycyclic scaffolds” are summarized, with discussion of remained challenges and future perspectives.

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.

Ruthenium-catalyzed cascade C-H activation/annulation of N-alkoxybenzamides: reaction development and mechanistic insight

A highly selective ruthenium-catalyzed C–H activation/annulation of alkyne-tethered N-alkoxybenzamides has been developed. In this reaction, diverse products from inverse annulation can be obtained in moderate to good yields with high functional group compatibility. Insightful experimental and theoretical studies indicate that the reaction to the inverse annulation follows the Ru(ii)–Ru(iv)–Ru(ii) pathway involving N–O bond cleavage prior to alkyne insertion. This is highly different compared to the conventional mechanism of transition metal-catalyzed C–H activation/annulation with alkynes, involving alkyne insertion prior to N–O bond cleavage. Via this pathway, the in situ generated acetic acid from the N–H/C–H activation step facilitates the N–O bond cleavage to give the Ru-nitrene species. Besides the conventional mechanism forming the products via standard annulation, an alternative and novel Ru(ii)–Ru(iv)–Ru(ii) mechanism featuring N–O cleavage preceding alkyne insertion has been proposed, affording a new understanding of transition metal-catalyzed C–H activation/annulation.

A highly selective ruthenium-catalyzed C–H activation/annulation through a pathway involving N–O bond cleavage prior to alkyne insertion is developed.

Ruthenium(II)-Catalyzed Homocoupling of α-Carbonyl Sulfoxonium Ylides Under Mild Conditions: Methodology Development and Mechanistic DFT Study

A mild ruthenium(II)-catalyzed homocoupling of α-carbonyl sulfoxonium ylides was developed and the detailed mechanism was understood based on DFT calculations in the current report. The catalytic system utilizes the α-carbonyl sulfoxonium ylide as both the directing group for ortho-sp2 C-H activation and the acylmethylating reagent for C-C coupling. Various substituents are compatible in the transformation and a variety of isocoumarin derivatives were synthesized at room temperature without any protection. The theoretical results disclosed that the full catalytic cycle contains eight elementary steps, and in all the cationic Ru(II) monomer is involved as the catalytic active species. The acid additive is responsible for protonation of the ylide carbon prior to the intramolecular nucleophilic addition and C-C bond cleavage. Interestingly, the intermediacy of free acylmethylation intermediate or its enol isomer is not necessary for the transformation.

Multicomponent Synthesis of Iminocoumarins via Rhodium-Catalyzed C-H Bond Activation

We herein establish a multicomponent annulation method for the synthesis of valuable iminocoumarins using aryl thiocarbamates, internal alkynes, and sulfonamides as starting materials, which are safe and readily available. The key step is a Rh-catalyzed and sulfur-directed C-H bond activation. Preliminary mechanistic investigations suggested that the nucleophilic attack of the sulfonamide on an active iminium cation finally completes the imine segment.

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.