Visible‐Light‐Induced Controlled Oxidation of N ‐Substituted 1,2,3,4‐Tetrahydroisoquinolines for the Synthesis of 3,4‐Dihydroisoquinolin‐1(2 H )‐ones and Isoquinolin‐1(2 H )‐ones

Harnessing photocatalytic activity of mesoporous graphitic carbon nitride decorated by copper single-atom catalysts for oxidative dehydrogenation of N-heterocycles

This work describes the application of Cu single-atom catalysts (SACs) for photocatalytic oxidative dehydrogenation of N-heterocyclic amines to the respective N-heteroaromatics through environmentally benign and sustainable pathways. The mesoporous graphitic carbon nitride (mpg-C3N4), prepared by the one-step pyrolysis method, possesses a lightweight material with a high surface area (95 m2 g-1) and an average pore diameter (3.6 nm). A simple microwave-assisted preparation method was employed to decorate Cu single-atom over mpg-C3N4 support. The Cu single-atom decorated on mpg-C3N4 support (Cu@mpg-C3N4) is characterized by various characterization techniques, including XRD, UV-visible spectrophotometry, HRTEM, HAADF-STEM with elemental mapping, AC-STEM, ICP-OES, XANES, EXAFS, and BET surface area. These characterization studies confirmed that the Cu@mpg-C3N4 catalyst exhibited high surface area, mesoporous nature, medium band gap, and low metal loading. The as-synthesized and well-characterized Cu@mpg-C3N4 single-atom photocatalyst is then evaluated for its efficacy in converting N-heterocycles into corresponding N-heteroaromatic compounds with excellent conversion and selectivity (>99 %). This transformation is achieved using water as a green solvent and a 30 W white light as a visible light source, demonstrating the catalyst's potential for sustainable and environmentally benign reactions.

Photoredox Catalyzed Tandem Denitrogenative [4 + 2] Annulation of 1,2,3-Benzotriazin-4(3H)-ones with Terminal Olefins

The dihydroisoquinolones skeleton is ubiquitous in natural products and biological molecules. Reported strategies for constructing dihydroisoquinolones usually require noble metal catalysts or stoichiometric oxidants, which limit their wide applications. Herein, we developed a photoredox catalyzed tandem denitrogenative [4 + 2] annulation reaction of 1,2,3-benzotriazin-4(3H)-ones with terminal olefins. A variety of dihydroisoquinolones can be accessed in moderate to excellent yield. This protocol features high atom-economy, mild reaction conditions, and is external oxidant-free, enabling the synthesis of various substituted dihydroisoquinolones.

Efficient and scalable synthesis of 3,4-dihydroisoquinolin-1(2H)-ones by benzylic oxidation of tetrahydroisoquinoline derivatives using cerium ammonium nitrate (CAN)

An efficient and scalable method for the synthesis of 3,4-dihydroisoquinolin-1(2H)-ones through benzylic oxidation of tetrahydroisoquinoline derivatives using a catalytic amount of cerium ammonium nitrate (CAN) and a stoichiometric amount of NaBrO3 as oxidants was developed. The reaction is significantly influenced by the substituent groups on the phenyl ring. While electron-withdrawing groups on the phenyl ring can lower the reactivities of the substrates, electron-donating groups on the phenyl ring can dramatically promote the oxidation rate.

Palladium-catalyzed denitrogenation/vinylation of benzotriazinones with vinylene carbonate

Herein, a novel Pd-catalyzed denitrogenation/vinylation of benzotriazinones using vinylene carbonate as the vinylation reagent is reported. This transformation demonstrates an unprecedented skeletal editing approach, effectively converting NN to CC fragments in situ and synthesizing a collection of isoquinolinones with broad-spectrum functional group tolerance. Moreover, the quite concise reaction system and late-stage modification of bioactive molecules comprehensively underscore the practical potential of this protocol.

Electrochemical Lactonization Enabled by Unusual Shono-Type Oxidation from Functionalized Benzoic Acids

A novel method for electrochemical lactonization via C(sp3)-H functionalization was developed. This metal- and oxidant-free strategy enabled the efficient synthesis of various lactones. Gram-scale reaction and derivatization of the lactone product demonstrated the synthetic utility of this methodology. Mechanistic studies using control experiments and CV curves elucidated the proposed intramolecular HAT and the oxidative cyclization pathway. An unusual Shono-type oxidation was realized through this electrochemical approach, proceeding without a traditional nucleophilic addition process.

Diversely functionalized isoquinolines and their core-embedded heterocyclic frameworks: a privileged scaffold for medicinal chemistry

Isoquinoline-enrooted organic small-molecules represent a challenging molecular target in the organic synthesis arsenal attributed to their structural diversity and therapeutic importance. Into the bargain, isoquinolines are significant structural frameworks in modern medicinal chemistry and drug development. Consequently, synthetic organic and medicinal chemists have been intensely interested in efficient synthetic tactics for the sustainable construction of isoquinoline frameworks and their derivatives in enantiopure or racemic forms. This review accentuates an overview of the literature on the modern synthetic approaches exploited in synthesising isoquinolines and their core embedded heterocyclic skeletons from 2021 to 2022. In detail, the methodologies and inspected pharmacological studies for the array of diversely functionalized isoquinolines or their core-embedded heterocyclic/carbocyclic structures involving the introduction of substituents at C-1, C-3, and C-4 carbon and N-2 atom, bond constructions at the C1-N2 atom and C3-N2 atom, and structural scaffolding within isoquinoline compounds have been reviewed. This intensive study highlights the need for and relevance of relatively unexplored bioisosterism employing isoquinoline-based small-molecules in drug design.

A facile synthesis of phthalimides from o-phthalaldehyde and amines via tandem cyclocondensation and α-C-H oxidation by an electrochemical oxygen reduction reaction

Electrochemical synthesis of phthalimides from o-phthalaldehyde and amines via tandem cyclocondensation and α-C-H oxygenation of isoindolinone was achieved. The α-C-H oxidation proceeded with molecular oxygen via an oxygen reduction reaction (ORR) on the cathode under electrochemical conditions. The synthetic utility of this protocol was successfully demonstrated by employing gram-scale synthesis and obtaining bioactive molecules such as thalidomide and 2-(2,6-diisopropylphenyl)-5-hydroxyisoindoline-1,3-dione. Mechanistic studies and control experiments indicate that molecular oxygen provides oxygen atoms for the reaction.

Oxidative α-Functionalization of 1,2,3,4-Tetrahydroisoquinolines Catalyzed by a Magnetically Recoverable Copper Nanocatalyst. Application in the Aza-Henry Reaction and the Synthesis of 3,4-Dihydroisoquinolones

The oxidative α-functionalization of 2-aryl-1,2,3,4-tetrahydroisoquinolines (THIQs) promoted by a versatile heterogeneous nanocatalyst consisting of copper nanoparticles immobilized on silica-coated maghemite (CuNPs/MagSilica) has been accomplished. The methodology was successfully applied in the cross-dehydrogenative coupling (CDC) reaction of N-aryl THIQs and other tertiary amines with nitromethane as a pro-nucleophile (aza-Henry reaction) and the α-oxidation of THIQs with O₂ as a green oxidant. Phosphite, alkyne, or indole derivatives were also shown to be suitable candidates for their use as pro-nucleophiles in the CDC reaction with THIQs. The catalyst, with very low copper loading (0.4-1.0 mol % Cu), could be easily recovered by means of an external magnet and reused in four cycles without significant loss of activity.

Iron(III)-catalyzed α-cyanation and carbonylation with 2-pyridylacetonitrile: divergent synthesis of α-amino nitriles and tetrahydroisoquinolinones

Iron-catalyzed oxidative synthesis of N-aryl-substituted tetrahydroisoquinolines (THIQs) toward tetrahydroisoquinoline-based derivatives is reported. A wide range of α-amino nitriles and tetrahydroisoquinolinones are synthesized in moderate to good yields. This approach involves a new organic nitrile source, a cheap iron catalyst under an oxygen atmosphere, and temperature-controlled divergent synthesis and features complete selectivity and operational simplicity.

Transannular Approach to 2,3-Dihydropyrrolo[1,2-b]isoquinolin-5(1H)-ones through Brønsted Acid-Catalyzed Amidohalogenation

A transannular approach has been developed for the construction of pyrrolo[1,2-b]isoquinolinones starting from benzo-fused nine-membered enelactams. This process takes place in the presence of a halogenating agent and under Brønsted acid catalysis and proceeds via a transannular amidohalogenation, followed by elimination. The reaction has been found to be wide in scope, enabling the formation of a variety of tricyclic products in good overall yield, regardless of the substitution pattern in the initial lactam substrate. The reaction has also been applied to the total synthesis of a reported topoisomerase I inhibitor and to the formal synthesis of rosettacin. Further extension of this methodology allows the preparation of 10-iodopyrrolo[1,2-b]isoquinolinones by using an excess of halogenating agent and these compounds can be further manipulated through standard Suzuki coupling chemistry into a variety of 10-aryl-substituted pyrrolo[1,2-b]isoquinolinones.

Acetic Acid-Catalyzed Regioselective C(sp2)-H Bond Functionalization of Indolizines: Concomitant Involvement of Synthetic and Theoretical Studies

An atom economical and environmentally benign protocol has been developed for the regioselective C(sp²)-H bond functionalization of indolizines. The acetic acid-catalyzed cross-coupling reaction proceeds under metal-free conditions, producing a wide range of synthetically useful indolizine derivatives. The present protocol showed good functional group tolerance and broad substrate scope in good to excellent yields. Quantum mechanical investigation using density functional theory (DFT) has played a crucial role in understanding that acetic acid is the key player in determining the actual pathway as the catalyst and its ultrafast nature. Different pathways involving inter- and intramolecular proton transfer, with or without acetic acid, were investigated. Calculated results revealed that a proton shuttle mechanism is involved for the least energetic, most favorable acetic acid-catalyzed pathway. Furthermore, regioselectivity has also been explained theoretically.

Lewis acid triggered N-alkylation of sulfoximines through nucleophilic ring-opening of donor-acceptor cyclopropanes: synthesis of γ-sulfoximino malonic diesters

Scandium triflate (Sc(OTf)₃) catalyzed, mild, and regioselective ring-opening reaction of donor-acceptor (D-A) cyclopropanes has been developed using sulfoximines for the synthesis of γ-sulfoximino malonic diesters. This protocol allows the synthesis of different N-alkyl sulfoximines in good to excellent yields (up to 94%) with broad functional group tolerance. In this process, N-H and C-C bonds are cleaved to form new C-N and C-H bonds. The feasibility of this method is supported by a gram-scale reaction and synthetic elaboration of the obtained product.

Click-based conjugated microporous polymers as efficient heterogeneous photocatalysts for organic transformations

Click-based conjugated microporous polymers were found to be highly efficient photocatalysts for the Ugi reaction and α-oxidation of N-substituted tetrahydroisoquinolines.

Stereoselective synthesis of fused-, spiro- and bridged heterocycles via cyclization of isoquinolinium salts: A recent update

Isoquinoline and its derivatives are ubiquitous in natural alkaloids, synthetic materials and pharmaceuticals with broad spectrum of biological activities. In particular, isoquinolinium salts are important in organic synthesis because they...

α-Oxygenation of N-aryl/alkyl heterocyclic compounds via ruthenium photocatalysis

We herein report ruthenium(iii) photocatalyzed oxidation of N-aryl tertiary amines to the corresponding amides.