Redox-neutral α-oxygenation of amines: reaction development and elucidation of the mechanism

L-Proline catalyzed multi-component synthesis of N-pyridyl-tetrahydroisoquinolines and their α-C(sp3)-H oxygenation

Herein, we report an effective multi-component synthesis that starts with readily available starting materials and accesses poly-substituted pyridine derivatives by using L-proline as a benign catalyst. This process uses cyclic amines, aldehydes, and malononitrile in a condensation reaction to produce a variety of pyridine derivatives under mild conditions. Furthermore, depending on the catalysts used, the selective synthesis of an amide and/or an aldehyde functionality is achieved through α-C(sp3)-H oxygenation of the tertiary amine moiety in the resultant pyridine derivatives. The pyridine ring's nitrogen atom plays a crucial role in accelerating C-H oxygenation at the α-position of the tertiary amine, highlighting the synthetic versatility and usefulness of this method.

A New Polycyclic Naphthoxazine Resin: Facile Synthesis, Characterization, Polymerization, and Thermal Properties of Its Corresponding Thermosets

A novel polycyclic naphthoxazine resin (NSA-thiq) is synthesized via N, O-acetal forming reaction between o-hydroxyl naphthaldehyde and 1,2,3,4-tetrahydroisoquinoline. The chemical structure of the monomer is investigated and confirmed by 1H and 13C NMR, Fourier-transform infrared (FT-IR) spectroscopy, and high-resolution mass spectrometry. Besides, the ring-opening polymerization behavior similar to ordinary benzoxazine resins is observed by differential scanning calorimetry (DSC) and in situ FT-IR analyses, leading to the formation of the phenolic cross-linked network. Notably, DSC thermograms indicate that the newly obtained polycyclic naphthoxazine resin exhibits much lower polymerization temperatures compared to many other reported benzoxazine or naphthoxazine resins. Moreover, the corresponding polybenzoxazine (poly(NSA-thiq)) shows comparable thermal stability in comparison with thermosets derived from monobenzoxazine resins. As a consequence of these unique performances, NSA-thiq is applied as a property modifier for a commercialized benzoxazine resin (BA-a). The glass transition temperature of copolymeric thermosets is enhanced without sacrificing too much thermal stability and heat resistance. Here, another series of naphthoxazine thermosetting resin is explored, which can provide more examples for constructing composites based on thermoset polymers.

Visible-light-induced redox-neutral difunctionalization of alkenes and alkynes

The twelve principles of green chemistry illuminate the pathway in the direction of sustainable and eco-friendly synthesis, marking a fundamental shift in synthetic organic chemistry paradigms. In this realm, harnessing the power of visible light for the difunctionalization of various skeletons without employing any external oxidant or reductant, specifically termed as redox-neutral difunctionalization, has attracted tremendous interest from synthetic organic chemists due to its low cost, easy availability and environmentally friendly nature in contrast to traditional metal-catalyzed difunctionalizations. This review presents an overview of recent updates on visible-light-induced redox-neutral difunctionalization reactions with literature coverage up to May 2024.

Stereoselective synthesis of 6/7/6-fused heterocycles with multiple stereocenters via an internal redox reaction/inverse electron-demand hetero-Diels-Alder reaction sequence

A highly stereoselective synthesis of fused heterocycles with multiple stereocenters via an internal redox reaction/inverse electron-demand hetero-Diels-Alder (IEDHDA) reaction sequence is described. The present reaction sequence has three interesting features: (1) complete control of two potentially competitive processes, i.e., hetero-Diels-Alder reaction and [1,5]-hydride shift; (2) one-shot construction of the complicated 6/7/6-fused heterocyclic structure having multiple stereocenters; and (3) high control of its stereoselectivity. When alkenylidene barbiturates with an allyl benzyl ether moiety were treated with a catalytic amount of Sc(OTf)3 and 2,2'-bipyridine, the internal redox reaction/IEDHDA reaction proceeded successively to afford 6/7/6-fused heterocycles in good chemical yields with good to excellent diastereoselectivities.

Divergent synthesis of multi-substituted phenanthrenes via an internal redox reaction/ring expansion sequence

We report an effective synthetic route to multi-substituted phenanthrenes via an internal redox reaction/ring expansion sequence. The interesting feature of the present system is that it allows for the divergent synthesis of the target skeleton depending on the selected Lewis acid catalyst. When benzylidene malonates with a cyclic structure at the ortho-position were treated with BF3·OEt2, three sequential processes (internal redox reaction/elimination of the alkoxy group/ring expansion) proceeded to give phenanthrene derivatives in which the alkoxycarbonyl (CO2R) group and the alkyl (R) group were in close proximity to each other, in good chemical yields. In sharp contrast, treatment with Bi(OTf)3 exclusively led to the formation of another type of phenanthrene, whose R group was positioned distal to the CO2R group.

Synthesis of Polysubstituted Naphthalenes by a Hydride Shift Mediated C-H Bond Functionalization/Aromatization Sequence

A synthetic strategy for forming multisubstituted naphthalenes based on hydride shift mediated C(sp3)-H bond functionalization was developed. This strategy consists of three successive transformations: (1) an intramolecular hydride shift mediated C(sp3)-H bond functionalization; (2) a decarboxylative fragmentation; and (3) an oxidation reaction. When benzylidene malonates having a 2-alkoxyethyl group at the ortho position were treated with a catalytic amount of Al(OTf)3, the hydride shift/cyclization reaction proceeded smoothly to afford tetralin derivatives in good chemical yields. The resulting tetralins were easily converted into naphthalenes by exposing them to modified Krapcho decarboxylation reaction conditions (LiCl, DMSO, and heating under an O2 atmosphere). The one-pot operation of these two reactions was also realized.

Silyl-Group Boosted Internal Redox Reaction: Hydride Shift from an Aliphatic Secondary Position for the Formation of Six- and Seven-Membered Carbocycles

We report a hydride shift/cyclization reaction at the aliphatic secondary position (methylene group). The key to accomplishing this reaction was the employment of benzylidene malonate having a silyl group β to the hydride donor carbon. When the corresponding malonates were treated with a catalytic amount of Al(OTf)3, the [1,5]-hydride shift from the simple aliphatic secondary position proceeded smoothly to afford silyl-group substituted tetralin derivatives in excellent chemical yields (up to 98%). This reaction system was applied to the formation of seven-membered carbocycles via the [1,6]-hydride shift mediated process.

Recent advances in bio-based polybenzoxazines as an interesting adhesive coating

Polybenzoxazine (PBz) is an excellent and highly intriguing resin for various sophisticated uses. Benzoxazines have piqued the curiosity of academics worldwide because of their peculiar properties. Nonetheless, most benzoxazine resin manufacturing and processing methods, notably bisphenol A-based benzoxazine, rely on petroleum resources. Because of the environmental consequences, bio-based benzoxazines are being researched as alternatives to petroleum-based benzoxazines. As a result of the environmental implications, bio-based benzoxazines are being developed to replace petroleum-based benzoxazines, and they are gaining traction. Bio-based polybenzoxazine, epoxy, and polysiloxane-based resins have piqued the interest of researchers in coatings, adhesives, and flame-retardant thermosets in recent years due to their anticorrosion, ecologically friendly, affordable, and low water absorption properties. As a result, numerous scientific studies and patents on polybenzoxazine continues to rise in polymer research. Based on its mechanical, thermal, and chemical characteristics, bio-based polybenzoxazine has several applications, including coatings (anticorrosion and antifouling), adhesives (highly crosslinked network, outstanding mechanical and thermal capabilities), and flame retardants (with the high charring capability). This review reports an overview of polybenzoxazine, highlighting the current advances and progress in synthesizing bio-based polybenzoxazine, their properties, and their use in coating applications.

Redox-neutral carbon-heteroatom bond formation under photoredox catalysis

Recently, visible-light-mediated photoredox catalysis has been emerging as one of the fastest growing fields in organic chemistry because of its low cost, easy availability and environmental benignness. In the past five years, a new yet challenging trend, visible-light-induced redox-neutral carbon-heteroatom bond formation reaction involving presumed radical intermediates, has been flourishing rapidly. Although mostly transition metal-based photoredox catalysts were reported, a few organophotoredox catalysts have also shown efficacy towards carbon-heteroatom bond formation reactions. This review intends to summarize the recent research progress in redox-neutral carbon-heteroatom bond formations based on active intermediate(s) involved under photoredox catalysis.

Selective oxidative β-C-H bond sulfenylation of tetrahydroisoquinolines with elemental sulfur

In this article, a convenient and efficient KIO₃-promoted oxidative sulfenylation at the β-position of tetrahydroisoquinolines and subsequent aromatization in the presence of elemental S₈ is presented. The reaction proceeds with moderate to good yields via a double C-S formation process. A wide range of structurally diverse 4-sulfenylisoquinolines/3-sulfenylpiperidine were synthesized with excellent functional group tolerance and high efficiency.

Synthesis of Polycyclic Isoindolines via α-C-H/N-H Annulation of Alicyclic Amines

Relatively unstable cyclic imines, generated in situ from their corresponding alicyclic amines via oxidation of their lithium amides with simple ketone oxidants, engage aryllithium compounds containing a leaving group on an ortho-methylene functionality to provide polycyclic isoindolines in a single operation. The scope of this transformation includes pyrrolidine, piperidine, azepane, azocane, and piperazines.

Cycloaddition of Huisgen 1,4-dipoles: synthesis and rapid epimerization of functionalized spiropyrido[2,1-b][1,3]oxazine-pyrroles and related products

1,4-Dipolar cycloaddition has emerged as a powerful tool for the synthesis of various cyclic compounds. In the present work, 1H-pyrrole-2,3-diones are proposed as new dipolarophiles for 1,4-dipolar cycloaddition. Their [4 + 2] cycloaddition with dipoles generated from dimethyl acetylenedicarboxylate and pyridine was found to proceed regioselectively affording spiro[pyrido[2,1-b][1,3]oxazine-2,3'-pyrroles] as diastereomeric mixtures which exist in rapid equilibrium in solution. It was established that this phenomenon of rapid epimerization is a characteristic of other similar spiropyrido[2,1-b][1,3]oxazines and even related spiroquinolizines, which was demonstrated by the investigation of related products of previously reported, and reproduced in this work, 1,4-dipolar cycloaddition reactions.

Divergent Access to Seven/Five-Membered Rings Based on [1,6]-Hydride Shift/Cyclization Process

We have achieved a divergent access to seven/five-membered rings based on a [1,6]-hydride shift/cyclization process from benzylidenemalonate with an o-alkoxymethyl group. Whereas Yb(OTf)₃ afforded benzoxepines (with a seven-membered ring) selectively, indanes (with a five-membered ring) were the main products when Sc(OTf)₃ was employed.

Condensation-Based Methods for the C-H Bond Functionalization of Amines

This review aims to provide a comprehensive overview of condensation-based methods for the C-H bond functionalization of amines that feature azomethine ylides as key intermediates. These transformations are typically redox-neutral and share common attributes with classic name reactions such as the Strecker, Mannich, Friedel-Crafts, Pictet-Spengler, and Kabachnik-Fields reaction, while incorporating a redox-isomerization step. This approach provides an ideal platform to rapidly transform simple starting materials into complex amines.

C-H Bond Functionalization of Amines: A Graphical Overview of Diverse Methods

This Graphical Review provides a concise overview of the manifold and mechanistically diverse methods that enable the functionalization of sp3 C-H bonds in amines and their derivatives.

α-C-H/N-H Annulation of Alicyclic Amines via Transient Imines: Preparation of Polycyclic Lactams

Polycyclic lactams are prepared in a single operation from o-toluamides and cyclic amines in a process that involves transient cyclic imines, species that are conveniently obtained in situ from the corresponding lithium amides and simple ketone oxidants. Imines thus generated, such as 1-pyrroline and 1-piperideine, engage lithiated o-toluamides in a facile annulation process. Undesired side reactions such as imine deprotonation and o-toluamide dimerization are suppressed through the judicious choice of reaction conditions.

FeCl2 Catalyzed Three-Component Reactions of Phospholes, Pyrrolidine, and Ketones (Aldehydes): Chemoselective Synthesis of 1-Phosphafulvenes

We have developed an unprecedented approach for the synthesis of transient 1-phosphafulvenes through three component reactions of phospholes. The generation of 1-phosphafulvenes was demonstrated by in situ [6 + 4] cycloaddition with 2H-phospholes and [6 + 6] self-dimerization. The [6 + 4] and [6 + 6] reaction pathway could be modulated by the starting ketones and aldehydes. The construction of 1-phosphafulvenes is illustrated by a proposed mechanism combining nucleophilic addition of phospholide to the iminium or isomerized azomethine ylide and a [1,5]-shift of phosphole.

Traceless Redox-Annulations of Alicyclic Amines

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with ortho-(nitromethyl)benzaldehyde. Benzoic acid acts as a promoter in these reactions, which involve concurrent amine α-C-H bond and N-H bond functionalization. Subsequent removal of the nitro group provides access to tetrahydroprotoberberines not accessible via typical redox-annulations. Also reported are decarboxylative annulations of ortho-(nitromethyl)benzaldehyde with proline and pipecolic acid.

N-Activated 1,3-Benzoxazine Monomer as a Key Agent in Polybenzoxazine Synthesis

A novel and successful application of ring-closing reactions of aminophenols has been proposed for the formation of a new type of 1,3-benzoxazine ionic derivatives. The optimization of the reaction and detailed computational studies have been reported for the estimation of heterocyclic ring stability and its further transformation, which is crucial in the polymerization process. The molecular structure of the obtained compounds has been fully characterized by applying X-ray analysis and spectroscopic methods. The novel benzoxazines undergo an intriguing thermal reaction leading to classical benzoxazines and chloroalkanes, which is the first step of transformation before polymerization. To gain more insights into the transformation behavior of ionic benzoxazine derivatives, the Fourier transform infrared (FT-IR) spectra of gaseous products were recorded in experiments with near simultaneous FT-IR/TGA measurements. The combination of thermogravimetry with FT-IR spectroscopy enables the quantitative and qualitative characterization of thermal transformation products and clarification of the reaction mechanism. The experimental data have been verified by applying DFT(B3LYP) and DFT(M062x) theoretical studies.

Construction of 1,3-Oxazolidines through a Three-Component [3+2] Cycloaddition of Tetrahydroisoquinolines, Aldehydes, and Ethyl Ketomalonate

A chemoselective and diastereoselective synthesis of fused oxazolidines was achieved by a three-component cascade reaction of tetrahydroisoquinolines (THIQs), α,β-unsaturated aldehydes, and diethyl 2-oxomalonate. Probably due to the reactivity difference between the aldehyde and the ketone, the reaction proceeded through the condensation of THIQs with α,β-unsaturated aldehydes and 1,3-dipolar cycloaddition of the generated azomethine ylide intermediate with 2-oxomalonate. The key features are easily available starting materials, mild reaction conditions, broad substrate scope, and high chemo- and diastereoselectivity.

Metal- and Solvent-Free Multicomponent Decarboxylative A3-Coupling for the Synthesis of Propargylamines: Experimental, Computational, and Biological Investigations

Decarboxylative A³-coupling of ortho-hydroxybenzaldehydes, secondary amines, and alkynoic acids is performed under catalyst and solvent-free conditions. The developed methodology provided a waste-free method for the synthesis of hydroxylated propargylamines which are versatile precursors for various bioactive heterocyclic scaffolds. The experimental and density functional theory studies revealed that the in situ-formed ortho-quinonoid intermediate (formed from ortho-hydroxybenzaldehyde and amine) undergoes a concerted Eschweiler-Clarke type decarboxylation with alkynoic acids. The synthesized compounds were evaluated for MAO-A, MAO-B, and AChE inhibitory activities as potential drug candidates for the treatment of various neurological disorders. Compound 4f was found to be the most potent and selective MAO-B (high selectivity over MAO-A) and AChE inhibitor in the series with IC₅₀ values of 4.27 ± 0.07 and 0.79 ± 0.03 μM, respectively.

Redox-Annulations of Cyclic Amines with ortho-Cyanomethylbenzaldehydes

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with ortho-cyanomethylbenzaldehydes. These amine α-C-H bond functionalization reactions are promoted by acetic acid. The resulting β-aminonitriles can be converted to the corresponding β-aminoalcohols in diastereoselective fashion.

Design, synthesis and characterization of structurally dynamic cyclic N,S-acetals

We report the synthesis, characterization and comparison of a series of electronically perturbed, cyclic N,S-acetals.

Redox-Annulations of Cyclic Amines with Electron-Deficient o-Tolualdehydes

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with 2-methyl-3,5-dinitrobenzaldehyde and closely related substrates. Acetic acid serves as the solvent and sole promoter of these transformations which involve dual C-H functionalization.

Highly diastereoselective synthesis of tricyclic fused-pyrans by sequential hydride shift mediated double C(sp3)-H bond functionalization

Described herein is the Brønsted acid-catalyzed double C(sp3)-H bond functionalization of alkyl phenethyl ether derivatives. In this process, a [1,5]-[1,5]-hydride shift occurred successively to afford tricyclic fused pyran derivatives in excellent chemical yields with excellent diastereoselectivities (up to >20 : 1). The key to achieve this reaction was the introduction of two methyl groups at the benzylic position, which was effective in both hydride shift processes: (1) the Thorpe-Ingold effect for the first hydride shift and (2) conformational control in the second hydride shift.

Redox-Annulations of Cyclic Amines with 2-(2-Oxoethyl)malonates

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with 2-(2-oxoethyl)malonates in the presence of catalytic amounts of benzoic acid. These reactions install a fully saturated five-membered ring and provide access to structures closely related to the natural products crispine A and harmicine.

Multicomponent Synthesis of Tetrahydroisoquinolines

A multicomponent synthesis of tetrahydroisoquinolines from carboxylic acids, alkynyl ethers, and dihydroisoquinolines is described. This process features readily available starting materials, simple experimental procedures for achievement of molecule complexity, and structural diversity. The preliminary control experiment and crossover reaction provide important insight into the reaction mechanism. The formed tetrahydroisoquinolines could be transformed to an array of compounds.

Triple Nucleophilic Attack of Nitromethane on (2-Iminoaryl)divinyl Ketones: A Domino Synthetic Strategy for Hexahydrophenanthridinones

A novel domino reaction of (2-iminoaryl)divinyl ketones with nitromethane was developed for the efficient synthesis of hexahydrophenanthridin-9(5H)-ones. The reaction proceeded smoothly from readily available starting materials under mild reaction conditions to construct three new bonds and two rings with high diastereoselectivities in good to excellent yields in a single step. A mechanism is proposed, involving a stepwise double Michael addition/aza-Henry reaction cascade, and in this transformation, nitromethane acts as a trinucleophile.