CBr4-Mediated Cross-Dehydrogenative Coupling Reaction of Amines

Synthetic Methods for Azaheterocyclic Phosphonates and Their Biological Activity: An Update 2004-2024

The increasing importance of azaheterocyclic phosphonates in the agrochemical, synthetic, and medicinal field has provoked an intense search in the development of synthetic routes for obtaining novel members of this family of compounds. This updated review covers methodologies established since 2004, focusing on the synthesis of azaheterocyclic phosphonates, of which the phosphonate moiety is directly substituted onto to the azaheterocyclic structure. Emphasizing recent advances, this review classifies newly developed synthetic approaches according to the ring size and providing information on biological activities whenever available. Furthermore, this review summarizes information on various methods for the formation of C-P bonds, examining sustainable approaches such as the Michaelis-Arbuzov reaction, the Michaelis-Becker reaction, the Pudovik reaction, the Hirao coupling, and the Kabachnik-Fields reaction. After analyzing the biological activities and applications of azaheterocyclic phosphonates investigated in recent years, a predominant focus on the evaluation of these compounds as anticancer agents is evident. Furthermore, emerging applications underline the versatility and potential of these compounds, highlighting the need for continued research on synthetic methods to expand this interesting family.

CBr4 as a Mild Oxidant-Enabled Oxidation of a sp3 C-H Bond: A Facile Synthesis of the Persistent Iminium Salts of Tetrahydroisoquinolines

Using CBr4 as a mild oxidant, the direct C-H bond oxidation of N-aryltetrahydroisoquinolines was achieved, giving a series of the corresponding iminium salts in high yields under metal- and photo-free reaction conditions. This reaction is superior in high yields and good functional group tolerance, and the late-stage derivatization showed that these iminium salts can readily be applied to the synthesis of the functionalized THIQs.

Green approach to the synthesis of α-aminophosphonate-tetrahydroisoquinoline hybrids and their anti-cholinesterase activity

A series of 19 novel α-aminophosphonate-tetrahydroisoquinoline hybrids were synthesized through a cross dehydrogenative coupling reaction between N-aryl-tetrahydroisoquinolines and dialkylphosphites, using tert-butyl hydroperoxide as oxidazing agent. This simple procedure provided products with high atom economy and moderate to high yields. In vitro cholinesterase inhibitory activity of these compounds was evaluated. All the synthesized compounds showed good to excellent selective inhibition against butyrylcholinesterase. Compound 3bc was found to be the most active derivative with an IC50 of 9 nM. Molecular modelling studies suggested that the inhibitor is located in the peripheral anionic site (PAS) of the enzyme and interacts with some residue of the catalytic anionic site. Kinetic studies revealed that 3bc acts as a non-competitive inhibitor. Predicted ADME showed good pharmacokinetics and drug-likeness properties for most hybrids. Each newly synthesized compound was characterized by IR, 1H NMR, 13C NMR, 31P NMR spectral studies and also HRMS. The results of this study suggest that α-aminophosphonate-tetrahydroisoquinoline hybrids can be promising lead compounds in the discovery of new and improved drugs for the treatment of Alzheimer's disease and related neurodegenerative disorders.

NBS-mediated bromination and dehydrogenation of tetrahydro-quinoline in one pot: scope and mechanistic study

A facile and general approach was developed for the efficient construction of functionalized bromoquinolines by the dehydrogenation of tetrahydroquinolines using NBS as the electrophile and as oxidant. The cascade transformation proceeded with good functional group tolerance under metal-free conditions with a short reaction duration. Various tetrahydroquinolines bearing either electron-rich or electron-deficient groups at different positions were successfully converted into the corresponding target products in moderate to high yields under mild conditions. It is worth noting that the obtained polybromoquinolines could further undergo classic metal-catalyzed cross-coupling reactions with good regioselectivity. The Sonagashira coupling reaction occurred regioselectively in the C-6 position of the obtained products followed by a Suzuki coupling reaction to give multifunctionalized quinolines. The mechanism indicated that electrophilic bromination/radical dehydrogenation sequences occurred in one pot.

Transition-Metal-Free Dehydrogenation of Benzyl Alcohol for C-C and C-N Bond Formation for the Synthesis of Pyrazolo[3,4-b]pyridine and Pyrazoline Derivatives

A series of cascade reactions that produce a range of functionalized aromatic heterocyclic compounds with pyrazole/pyrazoline cores have been developed. The method relies on a metal-free dehydrogenative process to produce in-situ benzaldehydes. The produced benzaldehyde was then allowed to react with some other substances, including acetophenone, pyrazole amine, and phenylhydrazine. The intermediate produced from these substrates underwent several chemical processes, including electrocyclization, the aza-Diels-Alder reaction, and the formation of intramolecular C-N bonds. These positive outcomes would open up the possibility of producing biologically active pyrazolo[3,4-b]pyridine and pyrazoline derivatives through a variety of possible reactions.

Uranyl nitrate as a recyclable homogeneous photocatalyst for selective cross-coupling of N-substituted amines and indoles

A homogeneous photocatalytic recyclable system for the selective radical-radical cross-coupling of N-substituted amines and indoles has been established. This system could conduct in water or acetonitrile, featuring the reuse of uranyl nitrate as the recyclable photocatalyst via a simple extraction. With this mild strategy in hand, good to excellent yields of cross-coupling products could be achieved even under the irradiation of sunlight, including 26 natural product derivatives and 16 natural product inspired re-engineered compounds. A radical-radical cross-coupling mechanism was newly proposed based on experimental evidence and reported literature. This strategy has been also applied to a gram scale synthesis to demonstrate its practical utility.

Visible-light-induced Csp3-H functionalization of glycine derivatives by cerium catalysis

A Ce(III)-catalyzed, visible-light-induced aerobic oxidative dehydrogenative coupling/aromatization reaction between glycine derivatives and alkenes has been developed, which provides an efficient approach for the synthesis of quinoline derivatives and post-modification of oligopeptides containing glycine residues under mild conditions without the need for external photosensitizers.

Photocatalytic Azetidine Synthesis by Aerobic Dehydrogenative [2 + 2] Cycloadditions of Amines with Alkenes

Photocatalytic dehydrogenative [2 + 2] cycloadditions between amines and alkenes were developed that allow for the stereoselective and high-yielding synthesis of functionalized azetidines. The oxidative formal Aza Paternò-Büchi reactions are induced by photoredox-catalyzed aerobic oxidation of dihydroquinoxalinones 1 as the amines, and in the presence of structurally diverse alkenes 3 intermolecular [2 + 2] cyclization to dihydro-1H-azeto[1,2-a]quinoxalin-3(4H)-ones 4 occurs. The utility of the method is illustrated by the selective conversion of amino acid derived dihydroquinoxalinones 1, including oxidation-prone lysine and tryptophan derivatives.

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.

Direct phosphorylation of benzylic C-H bonds under transition metal-free conditions forming sp3C-P bonds

Direct phosphorylation of benzylic C-H bonds was achieved in a biphasic system under transition metal-free conditions. A selective radical/radical sp3C-H/P(O)-H cross coupling was proposed, and various substituted toluenes were applicable. The transformation provided a promising method for constructing sp3C-P bonds.

The Povarov Reaction: A Versatile Method to Synthesize Tetrahydroquinolines, Quinolines and Julolidines

The multicomponent Povarov reaction represents a powerful approach for the construction of substances containing N-heterocyclic frameworks. By using the Povarov reaction, in addition to accessing tetrahydroquinolines, quinolines and julolidines in a single step, it is possible to form the following new bonds: two Csp 3–Csp 3 and one Csp 3–Nsp 3, two Csp 2–Csp 2 and one Csp 2–Nsp 2, and four Csp 3–Csp 3 and two Csp 3–Nsp 1, respectively. This short review discusses the main features of the Povarov reaction, including its mechanism, the reaction scope by employing different catalysts and substrates, as well as stereoselective versions.

1 Introduction

2 Mechanism of the Povarov Reaction

3 Tetrahydroquinolines

4 Quinolines

5 Julolidines

6 Concluding Remarks

Transition-Metal-Free Addition of Dialkyl Phosphites to Phthalazin-2-ium Bromide: Synthesis of α-Aminophosphonate Analogues

α-Aminophosphonate analogues containing a phthalazine skeleton were efficiently obtained by a new transition-metal-free addition of dialkyl phosphites to phthalazin-2-ium bromide under mild conditions. A mechanistic study using isotope labeling and radical inhibition experiment revealed that the present transformation passes through a nucleophilic addition of dialkyl phosphates, rather than an insertion of P-H to carbenes.

The Photocatalyst-Free Cross-Dehydrogenative Coupling Reaction Enabled by Visible-Light Direct Excitation of Substrate

A new photocatalyst-free strategy for the cross-dehydrogenative C-C and C-P coupling reaction has been described. This protocol provides a concise method to synthesize various 1-substituted tetrahydroisoquinoline (THIQ) derivatives enabled by visible-light direct excitation of substrates without using any photocatalyst. Moreover, a wide substrate scope demonstrated good synthetic versatility and practicality.

Environmentally Benign Synthesis of Quinoline-Spiroquinazolinones by Iron-Catalyzed Dehydrogenative [4 + 2] Cycloaddition of Secondary/Tertiary Anilines and 4-Methylene-quinazolinones

We report an efficient iron-catalyzed cross-dehydrogenative coupling [4 + 2] annulation of secondary/tertiary anilines with quinazolinones to generate quinoline-spiroquinzolinones. The reaction proceeds smoothly with a relatively broad variety of functional groups, a cheap transition metal catalyst (FeCl₃), and environmentally friendly oxidant (H₂O₂/O₂) under mild reaction conditions. Creatively, N-methylanilines are employed for the first time for the cycloaddition as both methyl and methylene sources attached to the N atom of tetrahydroquinolines.

Cyclometalated Half-Sandwich Iridium(III) Complexes: Synthesis, Structure, and Diverse Catalytic Activity in Imine Synthesis Using Air as the Oxidant

Four air-stable cyclometalated half-sandwich iridium complexes 1-4 with C,N-donor Schiff base ligands were prepared through C-H activation in moderate-to-good yields. These complexes have been well characterized, and their exact structure was elaborated on by single-crystal X-ray analysis. The iridium(III) complexes 1-4 showed good catalytic activity in the imine synthesis under open-flask conditions (air as the oxidant) from primary amine oxidative homocoupling, secondary amine dehydrogenation, and the cross-coupling reaction of amine and alcohol. Substituents bonded on the ligands of the iridium complexes displayed little effect on the catalytic efficiency. The stability and good catalytic efficiency of the iridium catalysts, mild reaction conditions, and substrate universality showed their potential application in industrial production.

Recent advances in the application of tetrabromomethane in organic synthesis

This review article covers the use of tetrabromomethane as mediator, catalyst and reagents for organic synthesis for the period from 2007 to 2020.

Visible-Light-Mediated Aerobic Tandem Dehydrogenative Povarov/Aromatization Reaction: Synthesis of Isocryptolepines

A metal‐free, photoinduced aerobic tandem amine dehydrogenation/Povarov cyclization/aromatization reaction between N‐aryl glycine esters and indoles leads to tetracyclic 11H‐indolo[3,2‐c]quinolines under mild conditions and with high yields. The reaction can be performed by using molecular iodine along with visible light, or by combining an organic photoredox catalyst with a halide anion. Mechanistic studies reveal that product formation occurs through a combination of radical‐mediated oxidation steps with an iminium ion or N‐haloiminium ion [4+2]‐cycloaddition, and the N‐heterocyclic products constitute new analogues of the antiplasmodial natural alkaloid isocryptolepine.

Visible-light-enabled aerobic oxidative Csp3-H functionalization of glycine derivatives using an organic photocatalyst: access to substituted quinoline-2-carboxylates

A practical visible-light-induced aerobic oxidative dehydrogenative coupling reaction of glycine derivatives with olefins has been developed to efficiently synthesize quinoline-2-carboxylates. This metal-free process proceeds smoothly under mild conditions and exhibits good tolerance of functional groups. Given the low cost of the catalyst and feedstock materials, the mild reaction conditions and the absence of hazardous byproducts, this protocol should find broad applications in the synthesis of quinoline-2-carboxylate derivatives.

One-electron oxidative dehydrogenative annulation and cyclization reactions

This review focuses on the recent advances in one-electron oxidation involved oxidative dehydrogenative annulations and cyclizations for the intermolecular and intramolecular construction of valuable ring structures.

Photoinduced iodine-mediated tandem dehydrogenative Povarov cyclisation/C–H oxygenation reactions

Visible light and iodine mediate dehydrogenative imine [4+2]-cycloaddition/C–H oxygenation reactions to furnish highly functionalised 3-quinolones under metal-free conditions.

Direct intramolecular double cross-dehydrogentive-coupling (CDC) cyclization of N-(2-pyridyl)amidines under metal-free conditions

A facile transition-metal-free protocol to form 2-iminoimidazo[1, 2-a]-pyridines bearing a -CHBr₂ group and an aza-quaternary carbon center at the 3 position from N-(2-pyridyl)amidines substrates, in which the new heterocyclic skeletons constructed from amidines via radical reactions or nucleophilic substitution reactions are promoted only by CBr₄ under mild conditions, is demonstrated. The reactions were realized by intramolecular CDC reaction involving C-N and C-C bond formation via the sequential C(sp³)-H bifunctionalization mode on the same carbon atom under mild conditions. Moreover, this work also provides an excellent and representative example for CBr₄ as an efficient reagent to initiate radical reactions under initiator-free conditions or to give rise to nucleophilic substitution reactions only by base.

TBHP/NH4I-Mediated Direct N-H Phosphorylation of Imines and Imidates

A direct and practical metal-free N-H phosphorylation has been achieved via the TBHP/NH₄I-mediated cross-dehydrogenative coupling (CDC) reactions between imines/imidates and P(O)H compounds. This transformation provides an efficient synthetic route to the construction of P-N bonds with good functional group compatibility, leading to the formation of N-phosphorylimines and N-phosphorylimidates in up to 95% yield (33 examples) under mild conditions.

Oxidative [4+2] Cycloaddition of α-(N-Arylamino) Carbonyls with Aryl Alkenes by Multiple C-H Functionalizations and [1,2]-Aryl Shifts

A new, general copper-catalyzed oxidative tandem [4+2] cycloaddition of α-(N-arylamino) carbonyl compounds with aryl alkenes to produce highly substituted quinolines has been developed, which allows the formation of three new C-C bonds through a sequence of multiple C-H functionalizations, annulation, and [1,2]-aryl shifts.

Visible-light-mediated de-aminative alkylation of N-arylamines with alkyl Katritzky salts

A visible-light-mediated de-aminative alkylation of N-arylamines has been developed, providing direct access to α-amino C–H functionalization from readily available Katritzky salts under mild conditions.

Iodine monobromide catalysed regioselective synthesis of 3-arylquinolines from α-aminoacetophenones and trans-β-nitrostyrenes

A simple and efficient method for regioselective synthesis of 3-arylquinolines is described from α-aminoacetophenones and trans-β-nitrostyrenes using 20 mol% iodine monobromide as a catalyst in acetonitrile solvent at 80 °C.

Oxidative cross-dehydrogenative [2 + 3] annulation of α-amino ketones with α-keto esters: concise synthesis of clausenamide analogues

A one-pot oxidative cross-dehydrogenative [2 + 3] annulation of α-amino ketones with α-keto esters at room temperature is reported.

An efficient synthesis of benzothiazole using tetrabromomethane as a halogen bond donor catalyst

A CBr₄ catalyzed simple, mild, and efficient protocol has been developed for the synthesis of 2-substituted benzothiazole from 2-aminothiophenols and N-methylthioamides under solvent free conditions.

Tandem radical cyclization of N-methacryloyl benzamides with CBr4 to construct brominated isoquinolinediones

A simple cumene (isopropylbenzene, IPB) promoted auto-oxidation involved tandem radical cyclization of N-methacryloyl benzamides using stable and easy-to-handle CBr₄ as the bromine source is described. This strategy provides an efficient and practical approach for the synthesis of bromine containing isoquinolinediones. This method also presents a new way to generate bromine radicals using a mild auto-oxidation pathway.

Radical Cation Salt-initiated Aerobic C-H Phosphorylation of N-Benzylanilines: Synthesis of α-Aminophosphonates

A radical cation salt-initiated phosphorylation of N-benzylanilines was realized through an aerobic oxidation of the sp3 C-H bond, providing a series of α-aminophosphonates in high yields. An investigation of the reaction scope revealed that this mild catalyst system is superior in good functional group tolerance and high reaction efficiency. The mechanistic study implied that the cleavage of the sp3 C-H bond was involved in the rate-determining step.

Oxidative Difluoromethylation of Tetrahydroisoquinolines Using TMSCF2SPh: Synthesis of Fluorinated Pyrrolo[2,1-a]isoquinolines and Benzo[a]quinolizidines

An efficient C1-difluoromethylation of tetrahydroisoquinolenes was achieved using TMSCF₂SPh as a difluoromethylating agent and 2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate (TEMPO⁺BF₄^-) as an oxidant. The process provides an access to a variety of C1-difluoro(phenylsulfanyl)methylated tetrahydroisoquinoline adducts in good yields. These adducts were employed as key precursors for preparing fluorinated pyrrolo[2,1-a]isoquinoline and benzo[a]quinolizidines.