Advances in synthesis and chemistry of aziridines

Organic photosensitized aziridination of alkenes

We report a new TADF-catalyzed aziridination of alkenes under visible light. In this protocol, a free triplet nitrene is in situ generated from the commercially available tosyl azide by energy transfer of the excited photocatalyst 4DPAIPN. Our finding enables the smooth installation of the strained aziridine ring into a remarkably wide scope of alkenes and pharmaceutical-derived olefins and natural products, as well as the synthesis of sitagliptin. This metal-free method provides a new opportunity for the late-stage modification of complex molecules or synthesis of nitrogen-containing pharmaceuticals.

A Mechanistic Study on Iron-Based Styrene Aziridination: Understanding Epoxidation via Nitrene Hydrolysis

Transition-metal-catalyzed nitrene transfer reactions are typically performed in organic solvents under inert and anhydrous conditions due to the involved air and water-sensitive nature of reactive intermediates. Overall, this study provides insights into the iron-based ([FeII(PBI)3](CF3SO3)2 (1), where PBI = 2-(2-pyridyl)benzimidazole), catalytic and stoichiometric aziridination of styrenes using PhINTs ([(N-tosylimino)iodo]benzene), highlighting the importance of reaction conditions including the effects of the solvent, co-ligands (para-substituted pyridines), and substrate substituents on the product yields, selectivity, and reaction kinetics. The aziridination reactions with 1/PhINTs showed higher conversion than epoxidation with 1/PhIO (iodosobenzene). However, the reaction with PhINTs was less selective and yielded more products, including styrene oxide, benzaldehyde, and 2-phenyl-1-tosylaziridine. Therefore, the main aim of this study was to investigate the potential role of water in the formation of oxygen-containing by-products during radical-type nitrene transfer catalysis. During the catalytic tests, a lower yield was obtained in a protic solvent (trifluoroethanol) than in acetonitrile. In the case of the catalytic oxidation of para-substituted styrenes containing electron-donating groups, higher yield, TON, and TOF were achieved than those with electron-withdrawing groups. Pseudo-first-order kinetics were observed for the stoichiometric oxidation, and the second-order rate constants (k2 = 7.16 × 10-3 M-1 s-1 in MeCN, 2.58 × 10-3 M-1 s-1 in CF3CH2OH) of the reaction were determined. The linear free energy relationships between the relative reaction rates (logkrel) and the total substituent effect (TE, 4R-PhCHCH2) parameters with slopes of 1.48 (MeCN) and 1.89 (CF3CH2OH) suggest that the stoichiometric aziridination of styrenes can be described through the formation of a radical intermediate in the rate-determining step. Styrene oxide formation during aqueous styrene aziridination most likely results from oxygen atom transfer via in situ iron oxo/oxyl radical complexes, which are formed through the hydrolysis of [FeIII(N•Ts)] under experimental conditions.

Synthesis of N-H Aziridines from Unactivated Olefins Using Hydroxylamine-O-Sulfonic Acids as Aminating Agent

Herein, we presented a practical methodology for the intermolecular aziridination of alkenes, using HOSA as the aminating agent, alongside pyridine or piperidine as the base, within HFIP solvent system. Notably, this approach showcases excellent reactivity, especially with nonactivated alkenes, and facilitates the transformation of various alkenes substrates, including mono-, di-, tri, and tetra-substituted alkenes, into aziridines with moderate to excellent yield. This method presents a promising avenue for synthesizing aziridines from a wide range of alkenes, featuring the benefits of straightforward operation, mild reaction conditions, extensive substrate compatibility, and scalability.

Aziridination via Nitrogen-Atom Transfer to Olefins from Photoexcited Azoxy-Triazenes

Herein, we report that readily accessible azoxy-triazenes can serve as nitrogen atom sources under visible light excitation for the phthalimido-protected aziridination of alkenes. This approach eliminates the need for external oxidants, precious transition metals, and photocatalysts, marking a departure from conventional methods. The versatility of this transformation extends to the selective aziridination of both activated and unactivated multisubstituted alkenes of varying electronic profiles. Notably, this process avoids the formation of competing C-H insertion products. The described protocol is operationally simple, scalable, and adaptable to photoflow conditions. Mechanistic studies support the idea that the photofragmentation of azoxy-triazenes results in the generation of a free singlet nitrene. Furthermore, a mild photoredox-catalyzed N-N cleavage of the protecting group to furnish the free aziridines is reported. Our findings contribute to the advancement of sustainable and practical methodologies for the synthesis of nitrogen-containing compounds, showcasing the potential for broader applications in synthetic chemistry.

Access to 2-Fluorinated Aziridine-2-phosphonates from α,α-Halofluorinated β-Iminophosphonates-Spectroscopic and Theoretical Studies

The efficient one-pot halofluorination of a β-enaminophosphonate/β-iminophosphonate tautomeric mixture resulting in α,α-halofluorinated β-iminophosphonates is reported. Subsequent imine reduction gave the corresponding β-aminophosphonates as a racemic mixture or with high diastereoselectivity. The proposed protocol is the first example of a synthesis of N-inactivated aziridines substituted by a fluorine and phosphonate moiety on the same carbon atom. Based on spectroscopic and theoretical studies, we determined the cis/trans geometry of the resulting fluorinated aziridine-2-phosphonate. Our procedure, involving the reduction of cis/trans-fluoroaziridine mixture 24, allows us to isolate chiral trans-aziridines 24 as well as cis-aziridines 27 that do not contain a fluorine atom. We also investigated the influence of the fluorine atom on the reactivity of aziridine through an acid-catalyzed regioselective ring-opening reaction. The results of DFT calculations, at the PCM/ωB97x-D/def2-TZVPD level of theory, are in good agreement with the experiments. The transition states of the S_N2 intramolecular cyclization of vicinal haloamines have been modeled.

Recent updates and future perspectives in aziridine synthesis and reactivity

In this review, selected recent advances in the preparation and reactivity of aziridines using modern synthetic approaches are highlighted, while comparing these new strategies with more classical approaches. This critical analysis is designed to help identify current gaps in the field and is showcasing new and exciting opportunities to move the chemistry of aziridines forward in the future.

Photocatalytic Carboxylation of C-N Bonds in Cyclic Amines with CO2 by Consecutive Visible-Light-Induced Electron Transfer

Visible-light photocatalytic carboxylation with CO₂ is highly important. However, it still remains challenging for reluctant substrates with low reduction potentials. Herein, we report a novel photocatalytic carboxylation of C-N bonds in cyclic amines with CO₂ via consecutive photo-induced electron transfer (ConPET). It is also the first photocatalytic reductive ring-opening reaction of azetidines, pyrrolidines and piperidines. This strategy is practical to transform a variety of easily available cyclic amines to valuable β-, γ-, δ- and ϵ-amino acids in moderate-to-excellent yields. Moreover, the method also features mild and transition-metal-free conditions, high selectivity, good functional-group tolerance, facile scalability and product derivations. Mechanistic studies indicate that the ConPET might be the key to generating highly reactive photocatalysts, which enable the reductive activation of cyclic amines to generate carbon radicals and carbanions as the key intermediates.

Synthesis of Enantiomerically Enriched Protected 2-Amino-, 2,3-Diamino- and 2-Amino-3-Hydroxypropylphosphonates

Simple and efficient strategies for the syntheses of enantiomerically enriched functionalized diethyl 2-amino-, 2,3-diamino- and 2-amino-3-hydroxypropylphosphonates have been developed starting from, respectively, N-protected (aziridin-2-yl)methylphosphonates, employing a regioselective aziridine ring-opening reaction with corresponding nucleophiles. Diethyl (R)- and (S)-2-(N-Boc-amino)propylphosphonates were obtained via direct regiospecific hydrogenolysis of the respective enantiomer of (R)- and (S)-N-Boc-(aziridin-2-yl)methylphosphonates. N-Boc-protected (R)- and (S)-2,3-diaminopropylphosphonates were synthesized from (R)- and (S)-N-Bn-(aziridin-2-yl)methylphosphonates via a regiospecific ring-opening reaction with neat trimethylsilyl azide and subsequent reduction of (R)- and (S)-2-(N-Boc-amino)-3-azidopropylphosphonates using triphenylphosphine. On the other hand, treatment of the corresponding (R)- and (S)-N-Bn-(aziridin-2-yl)methylphosphonates with glacial acetic acid led regiospecifically to the formation of (R)- and (S)-2-(N-Bn-amino)-3-acetoxypropylphosphonates.

Design, Synthesis, and Biological Evaluation of quinoxaline bearing tetrahydropyridine derivatives as anticancer, antioxidant, and anti-tubercular agents

BACKGROUND

Quinoxaline and Tetrahydropyridine derivatives showed various biological properties. The combination of these two scaffolds may contribute to good biological activity and may give novel and efficacious bioactive candidates.

OBJECTIVE

The present study aimed to identify bioactive agents with quinoxaline bearing tetrahydropyridine derivatives possessing anticancer, antioxidant, and anti-tubercular agents.

METHOD

A series of novel quinoxaline bearing tetrahydropyridine derivatives have been designed and synthesized in good yields. The synthetic protocol involves three-component Povarov reactions of 6-amino quinoxaline, propenyl guaethol, and substituted aldehydes using BF3•OEt2 as catalyst. The newly synthesized molecules were evaluated for their anticancer activity against four cell lines, i.e. A-549, MCF-7, PC-3, and HepG2.

RESULTS

The results from in vitro assay indicated that compound 4a proved to be as potent as the standard drug adriamycin against all cell lines with GI50 values <10 μg/ml. Compounds 4b, 4f, and 4i exhibited good cytotoxicity against A-549 cell line. All synthesized molecules were evaluated for their antioxidant activity and the results revealed that the compounds 4a, 4b, and 4i showed promising antioxidant activities against DPPH and H2O2 scavenging. In addition, the anti-mycobacterial activity of the synthesized compounds against MTB H37Rv strain was determined using MABA method. The results indicate that the compounds 4a, 4b, 4g, and 4i showed better anti-mycobacterial activity than the standard drugs pyrazinamide, ciprofloxacin and streptomycin with MIC value 1.6 μg/ml. Furthermore, molecular docking studies and ADME properties showed good pharmacokinetic profile and drug-likeness properties.

CONCLUSION

These studies showed that a series of novel quinoxaline bearing tetrahydropyridine derivatives exhibit anticancer, anti-mycobacterial, and antioxidant activities.

Non-Aziridination Approaches to 3-Arylaziridine-2-carboxylic Acid Derivatives and 3-Aryl-(aziridin-2-yl)ketones

Highly functionalized aziridines, including compounds with aromatic moieties, are attractive substrates both in synthetic and medical areas of chemistry. There is a broad and interesting set of synthetic methods for reaching these compounds. Aziridination represents the most explored tool, but there are several other more specific, less well-known, but highly promising approaches. Therefore, the current review focuses on recently described or updated ways to obtain 3-arylated aziridines via different non-aziridination-based synthetic methods, reported mainly since 2000. The presented methods belong to two main directions of synthesis, namely, cyclization of open-chain substrates and rearrangement of other heterocycles. Cyclization of open-chain substrates includes the classic Gabriel-Cromwell type cyclization of halogenated substrates with amines, base-promoted cyclization of activated aminoalcohols (or its analogues), and the oxidative cyclization of β-dicarbonyls. Rearrangements of other heterocycles are presented as the Baldwin rearrangement of 4-isoxazolines, the cycloaddition of 1.3-dipoles or dienes to 2H-azirines, and the addition of C- and N-nucleophiles to the double bond of azirines.

Expedient metal-free preparation of aryl aziridines via thermal cycloaddition reactions

A straightforward synthesis of aryl aziridines is reported from readily available azides and alkenes and using technical solvents in the presence of air. This methodology does not require any additives and the obtained compounds can be employed in ring-opening and ring-expansion reactions.

Organometallic 3d transition metal NHC complexes in oxidation catalysis

The development of processes for the selective oxidation of hydrocarbons is a major focus in catalysis research. Making this process simultaneously environmentally friendly is still challenging. 3d transition metals are...

An Easy Route to Aziridine Ketones and Carbinols

N,N-Dimethylaziridine-2-carboxamides react with organolithium reagents yielding 2-aziridinylketones. The reaction with one equivalent of organolithium compound is selective to amide carbonyl at a low (−78 °C) temperature. These ketones, in reaction with organolithium reagents, give symmetrical and unsymmetrical aziridinyl carbinols. The usage of excess phenyllithium may serve as a special N-Boc-protecting group cleavage method for acid-sensitive substrates.

Microwave-accelerated and efficient synthesis of structurally diverse N-(2,2-diphenylvinyl)-β-oxoamides

N-(2,2-Diphenylvinyl)-β-oxoamides are both the structural moiety of biologically active compounds and important synthetic intermediates. Structurally diverse N-(2,2-diphenylvinyl)-β-oxoamides are prepared efficiently from 2-diazo-1,3-dicarbonyl compounds and N-alkyl-2,2-diphenylaziridines via an electrophilic ring opening reaction under two different reaction conditions of reflux and microwave irradiation. 2-Diazo-1,3-dicarbonyl compounds undergo the Wolff rearrangement under heating to generate α-oxoketenes, which electrophilically react with N-alkylaziridines to directly produce structurally diverse N-(2,2-diphenylvinyl)-β-oxoamides in good to excellent yields under microwave irradiation. Microwave irradiation accelerates the reaction obviously and efficiently. Both 2-diazo-1,3-diketones and alkyl 2-diazo-3-oxoalkanoates work well. The reaction is catalyst-free and highly atom economical, involves only loss of nitrogen and does not require additives. The products are useful synthons for the convenient preparation of multisubstituted β-lactam derivatives.

3-Arylaziridine-2-carboxylic Acid Derivatives and (3-Arylaziridin-2-yl)ketones: The Aziridination Approaches

Aziridination reactions represent a powerful tool in aziridine synthesis. Significant progress has been achieved in this field in the last decades, whereas highly functionalized aziridines including 3-arylated aziridine-2-carbonyl compounds play an important role in both medical and synthetic chemistry. For the reasons listed, in the current review we have focused on the ways to obtain 3-arylated aziridines and on the recent advances (mainly since the year 2000) in the methodology of the synthesis of these compounds via aziridination.

UV Photochromism in Transition Metal Oxides and Hybrid Materials

Limited levels of UV exposure can be beneficial to the human body. However, the UV radiation present in the atmosphere can be damaging if levels of exposure exceed safe limits which depend on the individual the skin color. Hence, UV photochromic materials that respond to UV light by changing their color are powerful tools to sense radiation safety limits. Photochromic materials comprise either organic materials, inorganic transition metal oxides, or a hybrid combination of both. The photochromic behavior largely relies on charge transfer mechanisms and electronic band structures. These factors can be influenced by the structure and morphology, fabrication, composition, hybridization, and preparation of the photochromic materials, among others. Significant challenges are involved in realizing rapid photochromic change, which is repeatable, reversible with low fatigue, and behaving according to the desired application requirements. These challenges also relate to finding the right synergy between the photochromic materials used, the environment it is being used for, and the objectives that need to be achieved. In this review, the principles and applications of photochromic processes for transition metal oxides and hybrid materials, photocatalytic applications, and the outlook in the context of commercialized sensors in this field are presented.

Chiral Aziridine Sulfide N(sp3),S-Ligands for Metal-Catalyzed Asymmetric Reactions

A series of new bidentate N,S-ligands—aziridines containing a para-substituted phenyl sulfide group—was synthesized and evaluated in the Pd-catalyzed Tsuji–Trost reaction and addition of diethylzinc and phenylethynylzinc to benzaldehyde. A high enantiomeric ratio for the addition reactions (up to 94.2:5.8) was obtained using the aziridine ligand bearing a p-nitro phenyl sulfide group. Collected results reveal a specific electronic effect that, by the presence of particular electron-donating or electron-withdrawing groups in the PhS- moiety, influences the σ-donor–metal binding and the enantioselectivity of the catalyzed reactions.

Aziridine used as a vinylidene unit in palladium-catalyzed [2 + 2 + 1] domino annulation

A series of chromone fused methylenecyclopentanes are efficiently constructed in moderate to good yields by Pd-catalyzed [2 + 2 + 1] annulation, in which aziridine is used as a vinylidene unit by cleavage of two C–N bonds for the first time.

Kinetic Resolution of Aziridines Enabled by N-Heterocyclic Carbene/Copper Cooperative Catalysis: Carbene Dose-Controlled Chemo-Switchability

Catalytic kinetic resolution (KR) and dynamic kinetic asymmetric transformation (DyKAT) are alternative and complementary avenues to access chiral stereoisomers of both starting materials and reaction products. The development of highly efficient chiral catalytic systems for kinetically controlled processes has therefore been one of the linchpins in asymmetric synthesis. N-heterocyclic carbene (NHC)/copper cooperative catalysis has enabled highly efficient KR and DyKAT of racemic N-tosylaziridines by [3+3] annulation with isatin-derived enals, leading to highly enantioenriched N-tosylaziridine derivatives (up to >99 % ee) and a large library of spirooxindole derivatives with high structural diversity and stereoselectivity (up to >95:5 d.r., >99 % ee). Mechanistic studies suggest that the NHC can bind reversibly to the copper catalyst without compromising its catalytic activity and regulate the catalytic activity of the copper complex to switch the chemoselection between KR and DyKAT.

Computational study on the mechanism of the reaction of benzenesulfonyl azides with oxabicyclic alkenes

The reaction of benzenesulfonyl azides with oxabicyclic alkenes to form aziridines could either proceed via initial [3 + 2] cycloaddition to form triazoline intermediates which then leads to aziridines, or via initial dinitrogen cleavage of the benzenesulfonyl azide to afford a nitrene intermediate followed by addition of this nitrene species across the olefinic C-C bond of the oxabicyclic alkene. Calculations at the DFT M06-2X/6-311G(d,p) level indicate that the initial [3 + 2] cycloaddition reaction of benzenesulfonyl azide and oxabicyclic alkene has barriers of 15.0 kcal/mol (endo) and 10.3 kcal/mol (exo) and rate constants of 5.23 × 10³ s^-1 (endo) and 3.86 × 10⁶ s^-1 (exo) whereas the pathway involving initial formation of nitrene species has a high activation barrier of 39.2 kcal/mol and rate constant of 8.92 × 10^-12 s^-1, indicating that the reaction will go through the former route to form an exo triazoline intermediate. The exo triazoline can either undergo a concerted C-C, N-N bond cleavage to form a ring-opened intermediate, a reaction that has a barrier of 23.4 kcal/mol, followed by dinitrogen extrusion and C-C, C-N bond regeneration with barriers of 29.1 and 23.5 kcal/mol respectively to form endo aziridines, or it can undergo direct nitrogen extrusion to form the exo product, a reaction with a barrier of 38.3 kcal/mol. Since the rate-determining step of the former route is 9.2 kcal/mol more favored than the latter, the former route rate is favored. The rate constants of the rate-determining steps are 1.30 × 10^-5 s^-1 (endo) and 3.16 × 10^-11 s^-1 (exo), indicating that endo aziridine would be formed as the major product and this is in conformity with the experimental observations of Chen et al. (J. Org. Chem. 18:11863-11872, 2019). The position of substituents on the benzene group of the benzenesulfonyl azide affects the endo/exo diastereoselectivity.Graphical abstract.

Ammonium Salt-Catalyzed Ring-Opening of Aryl-Aziridines with β-Keto Esters

We herein report an ammonium salt-catalyzed protocol for the regioselective ring opening of aryl-aziridines with β-keto esters. The reaction gives access to a variety of highly functionalized target molecules with two consecutive stereo-genic centers and can be rendered enantioselective (up to e.r. = 91:9) by using bifunctional chiral ammonium salt catalysts.