Recent Achievements in the Rhodium‐Catalyzed Concise Construction of Medium N‐Heterocycles, Azepines and Azocines

Alossaimi M. Synthesis of Rh-MOF/PVA-PVP nanofibers for skin cancer and infection inhibition

Using electrospinning for nanofiber production, we can create unique materials with multiple applications in various industries, including medical bandages and wound dressings. One of the most important features of these materials and using the electrospinning technique, is the incorporation of compounds and metals into their structure. In this study, a new metal-organic framework (MOF) was synthesized from rhodium, a metal with significant biological potential, which was then used to produce new nanofibers using electrospinning technique, (Rh-MOF/PVA-PVP nanofiber) by mixing polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP). The newly synthesized nanofiber was tested against common microbial skin pathogens and cancer cells, showing significant inhibition. Specifically, an IC50 value of 19.45 μg/mL against cancer cells and MIC values ranging from 4 μg/mL to 64 μg/mL μg against skin pathogenic strains were observed. This notable inhibitory ability can be attributed to both physical characteristics (with specific surface area of 2,348 m2/g), and chemical factors, including the active compounds present in its rhodium (Rh) structure. The synthesized Rh-MOF/PVA-PVP nanofiber has the potential for use in developing bioactive bandages, and wound dressings.

Regioselective cyclocondensations with thiobarbituric acid: spirocyclic and azocine products, X-ray characterization, and antioxidant evaluation

Multicomponent cyclocondensations of 5-amino-3-methyl-1-phenyl-1H-pyrazole (AMPZ), thiobarbituric acid, and p-formaldehyde under conventional thermal heating or ultrasonic irradiation were studied. Treatment of the reaction mixture in ethanol in an ultrasonic bath for 3 h produced azocine compound 4b, while the same mixture in ethanol under reflux conditions for 15 h produced spiro compound 4a. This work encompasses intricate experimental details, X-ray diffraction measurements, and multifaceted computational analyses employing methods such as the density functional theory and Hirshfeld surface analysis. Crystallographic investigations revealed the molecular structure of the compound and clarified its interactions involving hydrogen bonds and weak intermolecular forces. This article describes the synthesis and characterization of a novel spirocyclic compound. The study also evaluated the antioxidant potential in vitro using the DPPH and ABTS methods. The results showed that these compounds showed the best free radical scavenging ability, even in very small amounts, and that even at very low concentrations, these compounds showed excellent radical scavenging potential. Surprisingly, these compounds exhibited strong (ABTS+) radical scavenging activities, mainly attributed to the HAT mechanism, indicating their potential as therapeutic agents. Facile multipurpose, three-component selective procedures for new spiroheterocycles have been proposed, presenting intriguing perspectives in the field of medicine, particularly in the field of antioxidants. The geometric values of the computationally optimized structure were calculated using the density functional theory in LC-BLYP/6-31(d), aligned with the X-ray diffraction data, reinforcing the precision of our findings.

Asymmetric Catalytic (3 + 2) Cyclization and Sequential Reaction to Construct Dihydrofuran- and Azepine-Based Spirooxindoles

The enantioselective formal (3 + 2) cyclization and sequential reaction of 2-malononitrile-substituted oxindoles with benzaldehydes and ortho-aminobenzaldehydes were achieved by chiral N,N'-dioxide/metal complex Lewis acid catalysts. This protocol supplies facile and efficient access to highly functionalized chiral dihydrofuran- and azepine-based spirooxindoles. Based on the control experiments and the deuterium labeling studies, the interconversion of (3 + 2) diastereomeric intermediates under the reaction conditions and reversible 1,5-H transfer step were disclosed.

Ruthenium(II)-Catalyzed C-H/C-H (4+2) Annulation of 2-Aryl-N-heterocycles with Vinylene Carbonate

A ruthenium(II)-catalyzed direct C-H/C-H (4+2) annulation of 2-aryl-N-heterocycles such as 2-aryl-4H-pyrido[1,2-a]pyrimidin-4-ones, 2-arylimidazo[1,2-a]pyridines, 2-aryl-2H-indazoles and 2-arylquinolin-4(1H)-ones with vinylene carbonate has been described. This one-pot cascade strategy provided the diversely substituted fused-polyheterocycles such as 7H-benzo[h]pyrido[2,1-b]quinazolin-7-ones, naphtho[1',2':4,5]imidazo[1,2-a]pyridines, indazolo[2,3-a]quinolines and benzo[c]acridin-7(12H)-ones in moderate to excellent yields. The developed protocol exhibited a broad substrate scope with good functional group tolerance and acid/base-free conditions. Based on a preliminary mechanistic investigation, a tentative mechanism of Ru(II)-catalyzed (4+2) annulation reaction has been proposed.

Rhodium(III)-Catalyzed Sequential Cyclization of Enaminones with 1,3-Dienes via C-H Activation for the Synthesis of Fluorenones

An efficient method for construction of various fluorenones has been achieved via Rh(III)-catalyzed C-H activation/[4 + 2] annulation/aromatization sequences of simple and readily available enaminones and 1,3-dienes. This protocol showed good substrate compatibility as an array of structurally and electronically diverse fluorenones prepared efficiently in moderate to good yields and preparative scale utility showing very good efficiency in the late-stage functionalization of complex valuable molecules.

Access to Functionalized Amines and Medium N-Heterocycles via Amine-Enabled Remote C-H Alkynylation

By using weakly coordinating amines, we developed remote C-H alkynylation with precise control of reactivity and regioselectivity, enabling modification of complex drugs, natural products, and materials. The readily transformable alkyne-containing amine products would facilitate expedient delivery of molecular libraries of functionalized amines and medium N-heterocycles, which are previously elusive to access. Moreover, the introduced alkyne functionality could serve as a versatile handle to expand the diversity and synthetic application of this remote C-H functionalization.

Ru(II)-Catalyzed Divergent C-H Alkynylation Cascade with Bifunctional α-Alcohol Haloalkynes

Native functionality directed the C-H activation cascade to enable rapid construction of molecular complexity, featuring step-economy and synthetic efficiency. Herein, by exploiting bifunctional α-alcohol haloalkynes, we developed Ru(II)-catalyzed carboxylic acid, amine, and amide assisted divergent C-H alkynylation and annulation cascade, affording polyfunctional heterocycles. Significantly, a bilateral aryl C-H polycyclization cascade of azobenzenes was achieved using the versatile haloalkynes.

Ru(II)-Catalyzed [4 + 2]-Annulation of 2-Alkenyl/Arylimidazoles with N-Substituted Maleimides and 1,4-Naphthoquinones: Access to Imidazo-Fused Polyheterocycles

Synthesis of imidazo-fused polyheterocyclic molecular frameworks, viz. imidazo[1,2-a]pyrrolo[3,4-e]pyridines, imidazo[2,1-a]pyrrolo[3,4-c]isoquinolines, and benzo[g]imidazo[1,2-a]quinoline-6,11-diones, has been achieved by the ruthenium(II)-catalyzed [4 + 2] C-H/N-H annulation of 2-alkenyl/2-arylimidazoles with N-substituted maleimides and 1,4-naphthoquinones. The developed protocol is operationally simple, exhibits broad substrate scope with excellent functional group tolerance, and provides the desired products in moderate to good yields. The mechanistic studies suggest that the reaction involves the formation of a C-C bond through Ru-catalyzed C(sp2)-H bond activation followed by intramolecular cyclization.

Rh(III) Catalyzed Redox-Neutral C-H Activation/[5 + 2] Annulation of Aroyl Hydrazides and Sulfoxonium Ylides: Synthesis of Benzodiazepinones

We herein report the Rh(III) catalyzed redox-neutral C-H activation/[5 + 2] annulation of aroyl hydrazides with sulfoxonium ylides as safe carbene precursors. The reaction shows excellent functional group tolerance, broad substrate scope, and scalability. We demonstrated the synthetic utility of the protocol via the synthesis of various diazepam drug analogues, late-stage functionalization of probenecid drug, and large scale synthesis. Finally, kinetic studies revealed C-H activation as the rate-determining step.

Twofold Heteroannulation Reactions Enabled by Gold(I)/Zinc(II) Catalysts: Synthesis of Amine-Substituted Diaryl[c,h][1,6]naphthyridines

A straightforward and atom-economical one-pot protocol catalyzed by gold(I) and zinc(II) for the synthesis of amine-substituted diaryl[c,h][1,6]naphthyridines from two different aromatic nitriles has been showcased. This dual-catalytic strategy is highly efficient, offering an array of tetracyclic heteroaromatic products in good to excellent yields. Furthermore, the base can efficiently catalyze the second annulation step, yielding structurally unique thiophene-fused [1,6]naphthyridines in good yields.

Gold and Palladium Relay Catalytic [4 + 4] Cycloadditions of Enynamides and γ-Methylene-δ-valerolactones: Diastereoselective Construction of Furan-Fused Azacyclooctanes

We report a highly efficient and diastereoselective gold and palladium sequential relay catalysis system for the synthesis of furan-fused eight-membered heterocycles. Employing a one-pot procedure, easily accessible enynamides undergo cyclization to generate azadienes in situ, which subsequently participate in diastereoselective formal [4 + 4] cycloadditions with γ-methylene-δ-valerolactones. This strategy enables the rapid and efficient construction of a series of furan-fused azacyclooctanes with diverse substituents in good yields (63-97%) and a high level of diastereoselectivity (7:1 → 20:1 dr).

Synthesis of 3,4,5-trisubstituted phenols via Rh(III)-catalyzed alkenyl C-H activation assisted by phosphonium cations

An efficient method for the construction of various 3,4,5-trisubstituted phenol derivatives has been achieved via the Rh(III)-catalyzed coupling of phosphonium cations with internal alkynes. This protocol shows good substrate compatibility, as an array of structurally and electronically diverse phosphonium compounds react efficiently with up to 87% yield.

Three-component synthesis of N-naphthyl pyrazoles via Rh(III)-catalyzed cascade pyrazole annulation and Satoh-Miura benzannulation

The synthesis of N-naphthyl pyrazoles has been realized by the direct three-component reactions of enaminones, aryl hydrazine hydrochlorides and internal alkynes via Rh(III) catalysis. The synthetic reactions employing simple substrates lead to simultaneous construction of dual cyclic moieties, including a pyrazole ring and a phenyl ring, via sequential formation of two C-N and three C-C bonds.

Rhodium(III)-catalyzed oxidative annulation of N-arylbenzamidines with maleimides via dual C-H activation

An oxidative annulation of N-arylbenzimidamides with maleimides has been developed for the first time using a catalytic amount of the [Cp*RhCl₂]₂ complex for the synthesis of a diverse range of 1H-benzo[4,5]imidazo[2,1-a]pyrrolo[3,4-c]isoquinoline-1,3(2H)-dione derivatives. This method is versatile and atom-economical for producing polycyclic benzo[4,5]imidazo[2,1-a]pyrrolo[3,4-c] isoquinoline-1,3(2H)-dione scaffolds in a single step.

A Lewis Acid-Promoted Michael Addition and Ring-Expansion Cascade for the Construction of Nitrogen-Containing Medium-Sized Rings

A Lewis acid-promoted annulation of azadienes and cyclobutamines was developed. This reaction proceeded through Michael addition and ring-expansion cascade, affording the corresponding nitrogen-containing medium-sized rings with a broad scope in moderate to high yields. The catalytic asymmetric version of this reaction has also been explored using a chiral base.

Rh(III)-catalyzed [4 + 1] cyclization of aryl substituted pyrazoles with cyclopropanols via C-H activation

A rhodium-catalyzed formal [4 + 1]-cyclization reaction of aryl substituted pyrazoles with cyclopropanols via C-H bond activation/cyclization processes to selectively construct a series of carbonyl functionalized pyrazolo[5,1-a]isoindoles is described. The reaction features good functional group compatibility and a broad substrate scope with respect to both cyclization components with up to 84% yields. Mechanistic studies indicated that the C-H cleavage might be the rate-determining step in this transformation.

Manganese-catalyzed hydroarylation of multiple bonds

Transition metal-catalyzed C-H activation has become a promising strategy in organic synthesis due to its improved atom-, step- and resource economy. Considering the Earth's abundance, economic benefits, and low toxicity, 3d metal catalysts for C-H activation have received a significant focus. In particular, organometallic manganese-catalyzed C-H activation has proven to be versatile and suitable for a wide range of transformations such as C-H addition to π-components, arylation, alkylation, alkynylation, amination, and many more. Among them, manganese-catalyzed C-H addition to C-C and C-heteroatom multiple bonds exhibited unique and promising reactivity to construct a wide range of complex organic molecules. In this review, we highlight the developments in the field of manganese-catalyzed hydroarylation of multiple bonds via C-H activation with a range of applications until August 2022.

Chemoselective Ru-Catalyzed Oxidative Lactamization vs Hydroamination of Alkynylamines: Insights from Experimental and Density Functional Theory Studies

The Ru-catalyzed intramolecular oxidative amidation (lactamization) of aromatic alkynylamines with 4-picoline N-oxide as an external oxidant has been developed. This chemoselective process is very efficient to achieve medium-sized ε- and ζ-lactams (seven- and eight-membered rings) but not for the formation of common δ-lactams (six-membered rings). DFT studies unveiled the capital role of the chain length between the amine and the alkyne functionalities: the longer the connector, the more favored the lactamization process vs hydroamination.

Synthesis of Chiral Diazocine Derivatives via a Copper-Catalyzed Dearomative [5+3] Cycloaddition

Copper-catalyzed [5+3] cycloaddition of N-aromatic zwitterions and enol diazoacetates produced enantio-enriched diazocine derivatives. A sterically encumbered BOX ligand and NaBArF additive played significant roles in driving the overall catalytic process via the unfavorable dearomatization to construct the desired eight-membered heterocyclic compounds. The induced stereoselectivity was preserved after further modifications of the skeleton, which demonstrates the potential applications of the developed asymmetric catalysis.

Rhodium(III)-Catalyzed Sequential Cyclization of N-Boc Hydrazones with Propargylic Monofluoroalkynes via C-H Activation/C-F Cleavage for the Synthesis of Spiro[cyclobutane-1,9'-indeno[1,2-a]indenes]

An effective rhodium(III) catalysis for the construction of valuable tetracyclic compounds is described herein. This domino process involving the C-H activation/[3 + 2] annulation/intramolecular Friedel-Crafts reaction sequences of simple and readily available N-Boc hydrazones and propargylic monofluoroalkynes afforded fused tetracyclic spiro[cyclobutane-1,9'-indeno[1,2-a]indenes] in moderate to good yields, featuring three C-C bond formation. Moreover, control experiments indicated that the C-H activation might be involved in the rate-determining step.

Modular construction of functionalized anilines via switchable C–H and N-alkylations of traceless N-nitroso anilines with olefins

Switchable C–H or N-alkylations of N-nitroso anilines with olefins.

Stereoselective synthesis of functionalized azepines via gold and palladium relay catalysis

We developed a cycloisomerization/asymmetric [4 + 3] cycloaddition cascade reaction via gold/palladium relay catalysis, furnishing enantioenriched furan-fused azepines efficiently.

Asymmetric three-component reaction of diazo compound with alcohol and seven-membered imine

A dirhodium and chiral phosphoric acid co-catalyzed asymmetric three-component reaction of diazo compound with alcohol and seven-membered imine has been developed via Mannich-type interception of transient oxonium ylide. This reaction...

Remote difunctionalization of 2H-indazoles using Koser's reagents

A new, efficient, and metal-free protocol has been developed for remote difunctionalization of unreactive C-H bonds at the benzene core of 2H-indazole by employing Koser's reagents, which act as both sulfonyloxylating and iodinating agents under ambient air. The present methodology represents facile access to C-4-sulfonyloxylated and C-7-iodinated 2H-indazole derivatives with high regioselectivity, wide functional group tolerance, and broad substrate scope in good to excellent yields. The formed 4,7 disubstituted 2H-indazoles are the precursors of various C-4,7-functionalized 2H-indazoles through simple transformations.

Rapid Construction of Hexacyclic Indolines via the Ru(II)-Catalyzed C-H Activation Initiated Cascade Cyclization of Phenidones with Enynones

A highly efficient cascade cyclization of phenidones and enynones has been developed via a Ru(II)-catalyzed C-H activation initiated indole formation/Diels-Alder reaction/iminium ion cyclization sequence, which afforded hexacyclic indolines as single diastereomer in good to excellent yields with a broad substrate scope under mild conditions. The reaction features the simultaneous generation of five new chemical bonds and four new rings in one pot, providing a rapid and concise approach toward polycyclic indoline alkaloids and their analogues.

Rh(III)-Catalyzed mild straightforward synthesis of quinoline-braced cyclophane macrocycles via migratory insertion

An efficient Rh(III)-catalyzed straightforward strategy is developed for the synthesis of quinoline braced cyclophane macrocycles via methyl (sp³) C-H functionalization. The method is mild, simple and regioselective with various ring sizes and has good functional group tolerance. The method proceeds via C8-methyl metalation, metal-carbene formation and a subsequent migratory insertion. High dilution is not necessary for this macrocyclization and the only byproduct is nitrogen. A preliminary investigation shows that the C-H metalation step is the rate-determining step.

Enantioenriched α-Vinyl 1,4-Benzodiazepines and 1,4-Benzoxazepines via Enantioselective Rhodium-Catalyzed Hydrofunctionalizations of Alkynes and Allenes

Benzofused seven-membered heterocycles such as 1,4-benzo[e]diazepines (1,4-BZDs) and 1,4-benzo[e]oxazepines (1,4-BZOs) were efficiently synthesized by Rh-catalyzed hydrofunctionalization of internal alkynes and allenes in good to excellent yields. The asymmetric hydroamination of (aminomethyl)anilines gave rise to 3-vinyl-1,4-BZDs with excellent enantioselectivities. Orthogonal N-deprotection of 1,4-BZDs allowed an easy entry to an advanced pyrrolobenzodiazepine metabolite of the V₂-receptor antagonist Lixivaptan.

Ruthenium-catalyzed coupling of α-carbonyl phosphoniums with sulfoxonium ylides via C–H activation/Wittig reaction sequences

A Ru(ii)-catalyzed coupling of various α-carbonyl phosphoniums with sulfoxonium ylides has been realized for the facile synthesis of 1-naphthols in good to excellent yields.