Rhodium(III)-Catalyzed C–H Activation/Annulation with Vinyl Esters as an Acetylene Equivalent

Solvent-Driven Room-Temperature Curtius Rearrangements to Access Nucleotides Bearing Substituted Fused Pyridones

The intramolecular Curtius rearrangement suffers from a high reaction temperature, low yields, tedious product isolation, and difficult scale up. This study presents a room-temperature Curtius rearrangement that can be novelly driven by the HFIP solvent, followed by light-illuminated intramolecular cyclization. Such a mild reaction allows for the preparation of various fused pyridone derivatives with diverse substituent groups that have rarely been incorporated by previous methods. The roles of HFIP and light are investigated by a set of control experiments through a combination of IR and NMR titration. Furthermore, using the substituted fused pyridones as unnatural bases, we can obtain a panel of new nucleotides.

Construction of Isoquinolone Scaffolds on DNA via Rhodium(III)-Catalyzed C-H Activation

Isoquinolone is one of the most common heterocyclic core structures in countless natural products and many bioactive compounds. Here, a highly efficient approach to synthesize isoquinolone scaffolds on DNA via rhodium(III)-catalyzed C-H activation has been described. This chemistry transformation is robust and has shown good compatibility with DNA, which is suitable for DNA-encoded library synthesis.

Photoredox-Enabled Deconstructive [5 + 1] Annulation Approach to Isoquinolones from Indanones in Water

We disclose a deconstructive [5 + 1] annulation protocol for the synthesis of isoquinolones through a nitrogen insertion into abundant indanones. This method exploits photoredox-catalyzed ring-opening of oxime esters. The reaction proceeds smoothly with water as the reaction medium and tolerates a range of functional groups on diverse thiophenols, amines, or indanones. Moreover, the representative isoquinolones exhibit promising antifungal activities.

Regiocontrol via Electronics: Insights into a Ru-Catalyzed, Cu-Mediated Site-Selective Alkylation of Isoquinolones via a C-C Bond Activation of Cyclopropanols

A site-selective C(3)/C(4)-alkylation of N-pyridylisoquinolones is achieved by employing C-C bond activation of cyclopropanols under Ru(II)-catalyzed/Cu(II)-mediated conditions. The regioisomeric ratios of the products follow directly from the electronic nature of the cyclopropanols and isoquinolones used, with electron-withdrawing groups yielding predominantly the C(3)-alkylated products, whereas the electron-donating groups primarily generate the C(4)-alkylated isomers. Density functional theory calculations and detailed mechanistic investigations suggest the simultaneous existence of the singlet and triplet pathways for the C(3)- and C(4)-product formation. Further transformations of the products evolve the utility of the methodology thereby yielding scaffolds of synthetic relevance.

Harnessing Vinyl Acetate as an Acetylene Equivalent in Redox-Neutral Cp*Co(III)-Catalyzed C-H Activation/Annulation for the Synthesis of Isoquinolones and Pyridones

We have developed Cp*Co(III)-catalyzed redox-neutral synthesis of 3,4-unsubstituted isoquinoline 1(2H)-ones at ambient temperature using N-chloroamides as a starting material. The reaction utilizes vinyl acetate as an inexpensive and benign acetylene surrogate. The N-Cl bond of the N-chlorobenzamides plays the role of an internal oxidant and hence precludes the need for an external oxidant. The reaction works with a wide range of substrates having various functional groups and a substrate containing a heterocyclic ring. Notably, the reaction is extended to the N-chloroacrylamides in which vinylic C-H activation occurs to furnish the 2-pyridone derivatives. Preliminary mechanistic studies were also conducted to shed light on the mechanism of this reaction.

Sulfoximine Assisted C-H Activation and Annulation via Vinylene Transfer: Access to Unsubstituted Benzothiazines

In this study, we report the synthesis of unsubstituted 1,2-benzothiazines through a redox-neutral Rh(III)-catalyzed C-H activation and [4+2]-annulation of S-aryl sulfoximines with vinylene carbonate. Notably, the introduction of an N-protected amino acid ligand significantly enhances the reaction rate. The key aspect of this redox-neutral process is the utilization of vinylene carbonate as an oxidizing acetylene surrogate and an efficient vinylene transfer agent. This vinylene carbonate enables the cyclization with the sulfoximine motifs, successfully forming a diverse array of 1,2-benzothiazine derivatives in moderate to good yields. Importantly, this study highlights the potential of Rh(III)-catalyzed C-H activation and [4+2]-annulation reactions for the synthesis of optically pure 1,2-benzothiazines with high enantiomeric purity.

Rhodium-Catalyzed Isoquinoline Synthesis Using Vinyl Selenone as Oxidizing Acetylene Surrogate

Isoquinoline is a privileged structure in many bioactive compounds and valuable ligands. Transition-metal-catalyzed oxidative annulation of imine derivatives has become a promising synthetic method; however, catalytic synthesis of 3,4-nonsubstituted isoquinolines by formal acetylene annulation has been scarce to date. Herein, we introduce vinyl selenone as an effective acetylene surrogate for the Rh-catalyzed annulative coupling under mild conditions. The Se fragment can be recovered as diselenide and recycled. The product can readily be converted to 1-aminoisoquinolines.

Cp*Co(III)-Catalyzed Regioselective [4 + 2]-Annulation of N-Chlorobenzamides with Vinyl Acetate/Vinyl Ketones

An efficient and straightforward strategy for the synthesis of isoquinolones through [4 + 2]-annulation of N-chlorobenzamides with vinyl acetate in the presence of CoCp*(III) catalyst in a regioselective manner is described. Furthermore, the annulation reaction was diversified by using vinyl ketones. By utilizing this strategy, biologically valuable isoquinolone derivatives were prepared in good yields. Subsequently, isoquinolone derivatives were further transformed into 1-chloroisoquinolines in the presence of POCl₃. Furthermore, mechanistic investigations such as deuterium labeling study and competition experiment were performed to support the proposed reaction mechanism.

Design, Synthesis, and Physicochemical Studies of Configurationally Stable β-Carboline Atropisomers

Axially chiral atropisomers have energetic barriers to rotation, ΔG_rot, that prevent racemization of the respective enantiomers. We used computational modeling to develop a suite of 10 bio-inspired 1-aryl-β-carbolines with varying ΔG_rot, from which a strong structure-activity relationship was observed for 2-substituted-1-naphthyl substituents. We then synthesized two of these atropisomers, 1d and 1f, by a four-step racemic synthesis and resolved the enantiomers via chiral chromatography. Racemization studies revealed experimental ΔG_rot values of 39.5 and 33.0 kcal/mol for 1d and 1f, respectively, which were consistent with our computational results. These atropisomers exhibited long half-lives, which allowed for their physicochemical characterization and stereochemical assignment via UV-vis spectroscopy, fluorescence spectroscopy, electronic circular dichroism, and vibrational circular dichroism.

Rh(III)-Catalyzed C-H Activation/Annulation of Benzohydroxamates and 2-Imidazolones: Access to Urea-Fused-Dihydroisoquinolone Scaffolds Reminiscent of Pyrrole Alkaloid Natural Products

A Rh(III)-catalyzed C-H activation/annulation with an imidazolone as alkene partner is reported to access dihydroisoquinolone-fused imidazolin-2-ones. These bicycles are reminiscent of scaffolds belonging to the pyrrole alkaloid family of natural products. This approach facilitates construction of a variety of urea-fused dihydroisoquinolone scaffolds including heterocyclic moieties.

Synthesis of Isoquinolone, 1,2-Benzothiazine, and Naphtho[1',2':4,5]imidazo[1,2-a]pyridine Derivatives via Rhodium(III)-Catalyzed (4 + 2) Annulation

In this study, we report a novel and efficient synthetic method to construct isoquinolone scaffold via the Rh(III)-catalyzed (4 + 2) annulation of benzamide with an unreported coupling reagent methyl 2-chloroacrylate. Accordingly, other valuable 1,2-benzothiazine and naphtho[1',2':4,5]imidazo[1,2-a]pyridine derivatives are also obtained through a similar synthetic protocol. Thus, our developed method is highlighted by high yield and reaction versatility.

Transition metal catalysed direct construction of 2-pyridone scaffolds through C-H bond functionalizations

Substituted 2-pyridone is one of the most frequent scaffolds among nitrogen-containing bioactive natural products, pharmaceuticals and organic materials. Besides the classical syntheses to construct this class of molecules, retrosynthetically more straightforward approaches based on transition metal catalysed C-H bond functionalizations have been explored recently. In this review, we have summarized the recent progress in the direct transition metal catalysed construction of substituted 2-pyridone scaffolds via site-selective C-H bond functionalizations.

Redox-neutral rhodium(III)-catalyzed chemo- and regiospecific [4 + 1] annulation between benzamides and alkenes for the synthesis of functionalized isoindolinones

Herein, using electron-deficient alkenes embedded with an oxidizing function/leaving group as a rare and nontraditional C₁ synthon, we have achieved the redox-neutral Rh(III)-catalyzed chemo- and regioselective [4 + 1] annulation of benzamides for the synthesis of functionalized isoindolinones. This method features broad substrate scope, good to excellent yields, excellent chemo- and regioselectivity, good tolerance of functional groups and mild external-oxidant-free conditions.

Cobalt-Catalyzed 2-(1-Methylhydrazinyl)pyridine-Assisted C-H Alkylation/Annulation: Mechanistic Insights and Rapid Access to Cyclopenta[c]isoquinolinone Derivatives

We have developed cobalt-catalyzed, bidentate 2-(1-methylhydrazinyl)pyridine (MHP)-directed C(sp²)-H alkylation/annulation of benzoic hydrazides with various alkenes. Notably, diverse cyclopenta[c]isoquinolinones and dihydroisoquinolinones were obtained via this functional group-tolerant protocol. The reaction can be performed on a gram scale while maintaining an excellent yield, and the directing group can be removed efficiently under mild conditions. Furthermore, density-functional theory (DFT) calculations provide an incisive understanding of the observed regioselectivities for different olefins.

Development and Mechanistic Studies of the Iridium-Catalyzed C-H Alkenylation of Enamides with Vinyl Acetates: A Versatile Approach for Ketone Functionalization

Ketone functionalization is a cornerstone of organic synthesis. Herein, we describe the development of an intermolecular C-H alkenylation of enamides with the feedstock chemical vinyl acetate to access diverse functionalized ketones. Enamides derived from various cyclic and acyclic ketones reacted efficiently, and a number of sensitive functional groups were tolerated. In this iridium-catalyzed transformation, two structurally and electronically similar alkenes-enamide and vinyl acetate-underwent selective cross-coupling through C-H activation. No reaction partner was used in large excess. The reaction is also pH- and redox-neutral with HOAc as the only stoichiometric by-product. Detailed experimental and computational studies revealed a reaction mechanism involving 1,2-Ir-C migratory insertion followed by syn-β-acetoxy elimination, which is different from that of previous vinyl acetate mediated C-H activation reactions. Finally, the alkenylation product can serve as a versatile intermediate to deliver a variety of structurally modified ketones.

Cobalt-Catalyzed Oxidative [4+2] Annulation of Benzamides with Dihydrofuran: A Facile Route to Tetrahydrofuro[2,3-c]isoquinolinones

A cobalt-catalyzed oxidative [4+2] annulation of benz­amides with dihydrofuran was developed for the expeditious synthesis of tetrahydrofuro[2,3-c]isoquinolinones. Commercially available and inexpensive Co(OAc)2·4H2O was used as an efficient catalyst. Various functional groups were tolerated, affording the desired products in good yields with excellent regioselectivity and good diastereoselectivity under mild conditions.

CuCl2-catalyzed inexpensive, faster and ligand/additive free synthesis of isoquinolin-1(2H)-one derivatives via the coupling-cyclization strategy: Evaluation of a new class of compounds as potential PDE4 inhibitors

In spite of possessing a wide range of pharmacological properties the anti-inflammatory activities of isoquinolin-1(2H)-ones were rarely known or explored earlier. PDE4 inhibitors on the other hand in addition to their usefulness in treating inflammatory diseases have been suggested to attenuate the cytokine storm in COVID-19 especially TNF-α. In our effort, a new class of isoquinolin-1(2H)-ones derivatives containing an aminosulfonyl moiety were designed and explored as potential inhibitors of PDE4. Accordingly, for the first time a CuCl₂-catalyzed inexpensive, faster and ligand/additive free approach has been developed for the synthesis of these predesigned isoquinolin-1(2H)-one derivatives via the coupling-cyclization strategy. Thus, the CuCl₂-catalyzed reaction of 2-iodobenzamides with appropriate terminal alkynes proceeded with high chemo and regioselectivity affording the desired compounds in 77-84% yield within 1-1.5 h. The methodology also afforded simpler isoquinolin-1(2H)-ones devoid of aminosulfonyl moiety showing a broader generality and scope of this approach. Several of the synthesized compounds especially 3c, 3k and 3s showed impressive inhibition (83-90%) of PDE4B when tested at 10 µM in vitro whereas compounds devoid of aminosulfonyl moiety was found to be less active. In spite of high inhibition showed at 10 µM these compounds did not show proper concertation dependent inhibition below 1 µM that was reflected in their IC₅₀ values e.g. 2.43 ± 0.32, 3.26 ± 0.24 and 3.63 ± 0.80 µM for 3k, 3o and 3s respectively. The anti-inflammatory potential of these compounds was indicated by their TNF-α inhibition (60-50% at 10 µM). The in silico docking studies of these molecules suggested good interactions with PDE4B and selective inhibition of PDE4B by 3k over PDE4D that was supported by in vitro assay results. These observations together with the favorable ADME and safety predicted for 3kin silico not only suggested 3k as an interesting hit molecule for further studies but also reveal the first example of isoquinolin-1(2H)-one based inhibitor of PDE4B.

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.

Effect of Transition Metals on Chemodivergent Cross-Coupling of Acrylamides with Vinyl Acetate via C-H Activation

A novel chemodivergent cross-coupling of acrylamides and vinyl acetates has been realized via metal-catalyzed vinylic C-H activation. The selective olefinic C-H vinylation and alkenylation reaction was examined with a variety of differently functionalized acrylamides. The reaction efficiently generates a range of highly synthetically valuable butadienes with good functional group tolerance in good to moderate yields. A possible catalytic reaction mechanism involving the chelation-assisted olefinic C-H activation via an acetate-assisted deprotonation pathway is proposed.

Synthesis of Isoquinolones by Sequential Suzuki Coupling of 2-Halobenzonitriles with Vinyl Boronate Followed by Cyclization

A novel, facile, and expeditious two-step synthesis of 3,4-unsubstituted isoquinolin-1(2H)-ones from a Suzuki cross-coupling between 2-halobenzonitriles and commercially available vinyl boronates followed by platinum-catalyzed nitrile hydrolysis and cyclization is described.

Vinylation of α-Aminoazoles with Triethylamine: A General Strategy to Construct Azolo[1,5-a]pyrimidines with a Nonsubstituted Ethylidene Fragment

A new general synthesis of pharmaceutically important azolo[1,5-a]pyrimidines starting from widely available 3(5)-aminoazoles, aldehydes, and triethylamine is developed. The key is to enable the vinylation reaction that allows the in situ generation of elusive acyclic enamines and the subsequent annulation reaction to occur. This direct and practical strategy is capable of constructing a range of 5,6-unsubstituted pyrazolo[1,5-a]pyrimidines and [1,2,4]triazolo[1,5-a]pyrimidines. More importantly, this protocol provides a concise synthetic route to prepare the clinically used zaleplon.

Rhodium(iii)-catalyzed annulation of enamides with sulfoxonium ylides toward isoquinolines

An efficient rhodium(iii)-catalyzed C-H activation followed by intermolecular annulation between enamides and sulfoxonium ylides has been developed. The transformation proceeds smoothly with a broad range of substrates, affording a series of isoquinoline derivatives in moderate to good yields under additive-free conditions.

Rh(iii)-Catalyzed C–H allylation/annulative Markovnikov addition with 5-methylene-1,3-dioxan-2-one: formation of isoquinolinones containing a C3 quaternary centre

Rh(iii)-Catalyzed C–H allylation/annulative Markovnikov addition reaction was disclosed, offering isoquinolinones containing a C3 quaternary centre. By using this method as the key step, the US28 inverse agonist analogs were synthesized.

Gold/Lewis acid catalyzed oxidative cyclization involving activation of nitriles

A gold-catalyzed oxidative cyclization of alkyne-nitriles using water or alcohol as the external nucleophiles has been developed. The catalytic system, featured with gold and Lewis acid dual catalysis, allows a facile synthesis of functionalized isoquinolin-1(2H)-ones and 1-alkoxy-isoquinolines with a wide structural diversity.

Supramolecular Diversity of Oxabicyclo[2.2.2]octenes Formed between Substituted 2H-Pyran-2-ones and Vinyl-Moiety-Containing Dienophiles

In Diels–Alder reactions, 2H-pyran-2-ones as dienes can yield a large variety of cycloadducts with up to four contiguous carbon stereogenic centers. Some of the potentially most useful, however difficult to prepare due to their low thermal stability, are the primary CO2-containing oxabicyclo[2.2.2]octenes, which could be formed as eight distinctive isomers (two sets of regioisomers, each of these composed of four different stereoisomers). A high-pressure synthesis of such products was recently described in a few cases where vinyl-moiety-containing dienophiles were used as synthetic equivalents of acetylene. However, structures of the primary products have been so far only rarely investigated in detail. Herein, we present seven novel single-crystal X-ray diffraction structures of such cycloadducts of both stereoisomeric forms, i.e., endo and exo. Additionally, we present a single-crystal structure of a rare case of a cyclohexadiene system stable at room temperature, obtained as a secondary product upon the retro-hetero-Diels–Alder elimination of CO2 under thermal conditions (microwave irradiation), during this elimination the symmetry is increased and out of eight initially possible isomers of the reactant, this number in the product is decreased to four. In oxabicyclo[2.2.2]octene compounds, centrosymmetric hydrogen bonding was found to be the predominant motif and diverse supramolecular patterns were observed due to rich variety of C–H⋯O and C–H⋯π interactions.

Regio- and Stereoselective Synthesis of the Core Structure of Hexahydrobenzo[c]phenanthridine Alkaloids via Redox-Neutral Cp*Rh(III)-Catalyzed C-H/N-H Annulation of Cyclic Alkenes with Benzamides

A highly stereo- and regioselective synthesis of the core skeleton of hexahydrobenzo[c]phenanthridine-type alkaloids is reported herein for the first time. A wide range of substrate scope, excellent functional group tolerance, and good to excellent yields were observed. This protocol gives very concise and efficient access to the core skeleton of chelidonine alkaloids as compared to the earlier approaches.

RhIII -Catalyzed C-H Activation of Aryl Hydroxamates for the Synthesis of Isoindolinones

RhIII -catalyzed C-H functionalization reaction yielding isoindolinones from aryl hydroxamates and ortho-substituted styrenes is reported. The reaction proceeds smoothly under mild conditions at room temperature, and tolerates a range of functional groups. Experimental and computational investigations support that the high regioselectivity observed for these substrates results from the presence of an ortho-substituent embedded in the styrene. The resulting isoindolinones are valuable building blocks for the synthesis of bioactive compounds. They provide easy access to the natural-product-like compounds, isoindolobenzazepines, in a one-pot two-step reaction. Selected isoindolinones inhibited Hedgehog (Hh)-dependent differentiation of multipotent murine mesenchymal progenitor stem cells into osteoblasts.

Cobalt(iii)-catalyzed ketone-directed C–H vinylation using vinyl acetate

Weakly coordinating, ketone-directed C–H vinylation using vinyl acetate is reported here for a wide range of aromatic ketones such as acetophenones, diaryl ketones, chromones and chalcones under cost-effective and air-stable cobalt(iii)-catalysis.

Rhodium(iii)-catalyzed synthesis of 3-trifluoromethylindanones from N-methoxybenzamides via C–H activation and Claisen/retro-Claisen reaction

An efficient synthesis of 3-trifluoromethylindanones via C–H annulation of N-methoxybenzamides with β-trifluoromethyl-α,β-unsaturated ketones under Rh(iii)-catalysis is described.

Combining the Ugi-azide multicomponent reaction and rhodium(III)-catalyzed annulation for the synthesis of tetrazole-isoquinolone/pyridone hybrids

An efficient sequence based on the Ugi-azide reaction and rhodium(III)-catalyzed intermolecular annulation has been established for the preparation of tetrazole-isoquinolone/pyridone hybrids. Several N-acylaminomethyltetrazoles were reacted with arylacetylenes to form the hybrid products in moderate to very good yields. The method relies on the capacity of the rhodium catalyst to promote C(sp2)-H activation in the presence of a suitable directing group. The Ugi-azide reaction provides broad molecular diversity and enables the introduction of the tetrazole moiety, which may further assist the catalytic reaction by coordinating the metal center. The scope of the isoquinolones is very wide and may be extended to the preparation of complex compounds having heterocyclic moieties such as pyridone, furan, thiophene and pyrrole, as well as the corresponding benzo-fused derivatives. The developed procedure is simple, reproducible and does not require inert conditions.

Reaching Green: Heterocycle Synthesis by Transition Metal-Catalyzed C-H Functionalization in Sustainable Medium

Catalytic C-H functionalization has emerged as an efficient alternative to traditional coupling reactions. However, some of these reactions depend on environmentally harmful solvents, weakening the overall green nature of these methods. As organic processes consume large amount of solvents, the use of less harmful solvents enhance the sustainability of these reactions. Herein, we present an overview of transition metal-catalyzed C-H functionalization reactions for the synthesis of heterocycles in sustainable solvents based on CHEM21 solvent selection guide.