Aziridinyl Anions: Generation, Reactivity, and Use in Modern Synthetic Chemistry

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.

Three-Component Synthesis of Di-Keto Aziridines and Highly Functionalized Alkenes from Sulfoxonium Ylides, Nitrosoarenes, and Alkynes

A catalyst-free one-pot three-component method of sulfoxonium ylides, nitrosoarenes, and alkynes for the synthesis of highly functionalized di-keto aziridines and alkenes is described. This strategy features the catalyst-free and additive-free approach, the employment of safe, more stable, and readily accessible sulfoxonium ylides, which bear a much wider substrate scope as starting materials. In terms of terminal alkynes, a cascade reaction of nitrone formation/1,3-diploar cycloaddition/Baldwin rearrangement is involved to afford a wide variety of di-keto aziridines. However, highly functionalized alkenes could be obtained instead of di-keto aziridines through the same nitrone formation/1,3-diploar cycloaddition and another different rearrangement reaction when internal alkynes are employed as starting materials.

Recent Developments in Stereoselective Reactions of Sulfonium Ylides

This review describes advances in the literature since the mid-1990s in the area of reactions of sulfonium ylide chemistry, with particular attention paid to stereoselective examples. Although the chemistry of sulfonium ylides was first popularized and applied in a substantial way in the 1960s, there has been sustained interest in the chemistry of sulfonium ylides since then. Many new ways of exploiting sulfonium ylides in productive stereoselective methodologies have emerged, often taking advantage of advances in organocatalysis and transition metal catalysis, to access stereodefined structurally complex motifs. The development of many different chiral sulfides over the last 20–30 years has also played a role in accelerating their study in a variety of reaction settings. In general, formal cycloaddition reactions ([2 + 1] and [4 + 1]) of sulfonium ylides follow a similar mechanistic pathway: initial addition of the nucleophilic ylide carbanion to an electrophile to form a zwitterionic betaine intermediate, followed by cyclization of the zwitterionic intermediate to afford the desired three-membered cyclic product (e.g., epoxide, cyclopropane, or aziridine), five-membered monocyclic (e.g., oxazolidinone), or fused bicyclic product (e.g., benzofuran, indoline).

Cleavage via Selective Catalytic Oxidation of Lignin or Lignin Model Compounds into Functional Chemicals

Lignin, a complex aromatic polymer with different types of methoxylated phenylpropanoid connections, enables the sustainable supply of value-added chemicals and biofuels through its use as a feedstock. Despite the development of numerous methodologies that upgrade lignin to high-value chemicals such as drugs and organic synthesis intermediates, the variety of valuable products obtained from lignin is still very limited, mainly delivering hydrocarbons and oxygenates. Using selective oxidation and activation cleavage of lignin, we can obtain value-added aromatics, including phenols, aldehydes, ketones, and carboxylic acid. However, biorefineries will demand a broad spectrum of fine chemicals in the future, not just simple chemicals like aldehydes and ketones containing simple C = O groups. In particular, most n-containing aromatics, which have found important applications in materials science, agro-chemistry, and medicinal chemistry, such as amide, aniline, and nitrogen heterocyclic compounds, are obtained through n-containing reagents mediating the oxidation cleavage in lignin. This tutorial review provides updates on recent advances in different classes of chemicals from the catalytic oxidation system in lignin depolymerization, which also introduces those functionalized products through a conventional synthesis method. A comparison with traditional synthetic strategies reveals the feasibility of the lignin model and real lignin utilization. Promising applications of functionalized compounds in synthetic transformation, drugs, dyes, and textiles are also discussed.

Hypervalent-Iodine(III)-Mediated Tandem Oxidative Dearomatization/Aziridination of Phenolic Amines: Synthesis of Functionalized Unactivated Aziridines

A new hypervalent-iodine(III)-mediated tandem reaction involving oxidative dearomatization and in situ aziridination of phenolic amines is described, providing a mild and effective method for the assembly of structurally interesting and synthetically useful aziridines. Importantly, the densely functionalized aziridines resulting from this unprecedented tandem reaction offer a platform for expeditious access to architecturally diverse aza-heterocycles through transformations initiated by selective ring-opening of aziridines.

Straightforward Approach toward Multifunctionalized Aziridines via Catalyst-Free Three-Component Reactions of α-Diazoesters, Nitrosoarenes, and Alkynes

A straightforward, catalyst-free atom- and step-economical approach for the synthesis of multisubstituted 1-arylaziridine-2-carboxylates via one-pot three-component reactions of α-diazoesters, nitrosoarenes, and alkynes has been described. This method could provide facile access to a variety of multifunctionalized aziridines in up to 91% yields under mild reaction conditions and features the catalyst-free strategy and use of cheap and readily accessible starting materials.

Flow Technology for the Genesis and Use of (Highly) Reactive Organometallic Reagents

In the field of organic synthesis, the advent of flow chemistry and flow microreactor technology represented a tremendous novelty in the way of thinking and performing chemical reactions, opening the doors to poorly explored or even impossible transformations using batch methods. In this Concept article, we would like to highlight the impact of flow chemistry for exploiting highly reactive organometallic reagents, and how, alongside the well-known advantages concerning safety, scalability, and productivity, flow chemistry makes possible processes that are impossible to control by using the traditional batch approach.

First synthesis of merged hybrids phosphorylated azirino[2,1-b]benzo[e][1,3]oxazine derivatives as anticancer agents

This work describes a straightforward diastereoselective synthetic access to azirino[2,1-b]benzo[e][1,3]oxazines containing phosphorus substituents such as phosphonate or phosphine oxide, by means of nucleophilic addition of functionalized phenols to the C-N double bond of 2H-azirine derivatives. In addition, the cytotoxic effect on cell lines derived from human lung adenocarcinoma (A549) and human embryonic kidney (HEK293) was also screened. Some azirino[2,1-b]benzo[e][1,3]oxazines 4 and 6 exhibited very good activity against the A549 cell line in vitro. Furthermore, selectivity towards cancer cell (A549) over (HEK293), and non-malignant cells (MCR-5) has been detected.

Synthesis of Selenoaziridines: A Study on Stereochemical Outcomes of the Reaction of Aziridine Radicals and Anions Generated from Iodoaziridines

The synthesis of a new functional group in the form of selenyl-substituted aziridines is described. Selenoaziridines are stereoselectively prepared by functionalization of intact aziridine precursors involving radical and anionic intermediates. Radicals are generated from cis-N-Ts iodoaziridines by activation of the C-I bond using alkoxides as a source of single electrons. These form predominantly trans-substituted selenoaziridines dependent on the size of the diselenide. cis-Aziridinyllithiums generated by Li-I exchange also react with diselenides stereospecifically to form a range of cis-selenoaziridines. Proposals for the stereochemical outcome are presented.

Construction of N-Alkyl- and N-Arylaziridines from Unprotected Amines via C-H Oxidative Amination Strategy

A copper-promoted intramolecular C-H oxidative amination reaction between secondary amine (N-H) and C(sp³)-H at the benzylic position of azaarenes or α-position of ketones for the synthesis of aziridine derivatives has been developed. Moreover, a practical annulation of electron-deficient vinylarenes with an unprotected primary alkyl amine by a Yb(OTf)₃-CuI relay system has also been reported. The reactions were carried out with oxygen as the sole oxidant to give the N-alkyl- and N-arylaziridines in good yields.

Synthesis and biological evaluation of cyanoaziridine phosphine oxides and phosphonates with antiproliferative activity

This work reports an efficient diastereoselective synthetic methodology for the preparation of phosphorus substituted cyanoaziridines through the nucleophilic addition of TMSCN, as cyanide source, to the C-N double bond of 2H-azirine derivatives. The aziridine ring, in these novel cyanoaziridines, can be activated by simple N-tosylation or N-acylation. In addition, the cytotoxic effect on cell lines derived from human lung adenocarcinoma (A549) and human embryonic kidney (HEK293) was also screened. N-H and N-Substituted cyanoaziridines showed excellent activity against the A549 cell line in vitro. Moreover, selectivity towards cancer cell (A549) over (HEK293), and non-malignant cells (MCR-5) has been observed.

Atom-Economical and Metal-Free Synthesis of Multisubstituted Furans from Intramolecular Aziridine Ring Opening

Multisubstituted furans were prepared from dialkyl 2-(aziridin-2-ylmethylene)malonate and/or 1,3-dione through aziridine ring opening by the internal carbonyl oxygen with the assistance of BF3·OEt2, followed by aromatization. This synthetic method is free from any metal and is atom-economical with all of the atoms in the starting material retained in the final product.

Enantioselective magnesium-catalyzed transformations

This review updates the major progress in the field of enantioselective transformations promoted by chiral magnesium catalysts, covering the literature since 2007, illustrating the power of these green catalysts to promote many types of reactions.

Recent advances in the chemistry of metallated azetidines

The almost unexplored four-membered azetidines represent a particularly interesting class of molecules, among the family of saturated nitrogen heterocycles. This review reports recent developments in direct metal-based functionalization of the azetidine ring, focusing on the regio- and stereoselectivity of these reactions.

Three-component synthesis of highly functionalized aziridines containing a peptide side chain and their one-step transformation into β-functionalized α-ketoamides

A sequential three-component process is described, starting from 3-arylmethylene-2,5-piperazinediones and involving a one-pot sequence of reactions achieving regioselective opening of the 2,5-diketopiperazine ring and diastereoselective generation of an aziridine ring. This method allows the preparation of N-unprotected, trisubstituted aziridines bearing a peptide side chain under mild conditions. Their transformation into β-trifluoroacetamido-α-ketoamide and α,β-diketoamide frameworks was also achieved in a single step.

Exploiting structural and conformational effects for a site-selective lithiation of azetidines

Interest in molecular structures bearing four-membered heterocycles (FMHs) is growing due to the possibility to explore new regions of the chemical space and get new lead molecules. Our interest in the development of divergent synthesis of functionalized FMHs, prompted us to disclose factors affecting the reactivity of nitrogen-bearing FMHs towards metalating agents. Our investigations demonstrated that structural factors and conformational preferences need to be considered in planning a site-selective functionalization of azetidines. It will be showed how such factors could have pivotal importance in the reactivity of FMHs.

Synthesis of functionalized arylaziridines as potential antimicrobial agents

By using the Suzuki-Miyaura protocol, a simple straightforward synthesis of functionalized 2-arylaziridines has been developed. By means of this synthetic strategy from readily available ortho-, meta- and para-bromophenylaziridines and aryl- or heteroarylboronic acids, new aziridines could be obtained. The cross-coupling reactions occurred without ring opening of the three membered ring. Preliminary results on the antimicrobial activity of the heterosubstituted biaryl compounds have been also included.

Synthesis of chiral N-phosphoryl aziridines through enantioselective aziridination of alkenes with phosphoryl azide via Co(II)-based metalloradical catalysis

The Co(II) complex of a new D 2-symmetric chiral porphyrin 3,5-DiMes-QingPhyrin, [Co(P6)], can catalyze asymmetric aziridination of alkenes with bis(2,2,2-trichloroethyl)phosphoryl azide (TcepN3) as a nitrene source. This new Co(II)-based metalloradical aziridination is suitable for different aromatic olefins, producing the corresponding N-phosphorylaziridines in good to excellent yields (up to 99%) with moderate to high enantioselectivities (up to 85% ee). In addition to mild reaction conditions and generation of N2 as the only byproduct, this new metalloradical catalytic system is highlighted with a practical protocol that operates under neutral and non-oxidative conditions.

Synthesis and purification of iodoaziridines involving quantitative selection of the optimal stationary phase for chromatography

The highly diastereoselective preparation of cis-N-Ts-iodoaziridines through reaction of diiodomethyllithium with N-Ts aldimines is described. Diiodomethyllithium is prepared by the deprotonation of diiodomethane with LiHMDS, in a THF/diethyl ether mixture, at -78 °C in the dark. These conditions are essential for the stability of the LiCHI2 reagent generated. The subsequent dropwise addition of N-Ts aldimines to the preformed diiodomethyllithium solution affords an amino-diiodide intermediate, which is not isolated. Rapid warming of the reaction mixture to 0 °C promotes cyclization to afford iodoaziridines with exclusive cis-diastereoselectivity. The addition and cyclization stages of the reaction are mediated in one reaction flask by careful temperature control. Due to the sensitivity of the iodoaziridines to purification, assessment of suitable methods of purification is required. A protocol to assess the stability of sensitive compounds to stationary phases for column chromatography is described. This method is suitable to apply to new iodoaziridines, or other potentially sensitive novel compounds. Consequently this method may find application in range of synthetic projects. The procedure involves firstly the assessment of the reaction yield, prior to purification, by (1)H NMR spectroscopy with comparison to an internal standard. Portions of impure product mixture are then exposed to slurries of various stationary phases appropriate for chromatography, in a solvent system suitable as the eluent in flash chromatography. After stirring for 30 min to mimic chromatography, followed by filtering, the samples are analyzed by (1)H NMR spectroscopy. Calculated yields for each stationary phase are then compared to that initially obtained from the crude reaction mixture. The results obtained provide a quantitative assessment of the stability of the compound to the different stationary phases; hence the optimal can be selected. The choice of basic alumina, modified to activity IV, as a suitable stationary phase has allowed isolation of certain iodoaziridines in excellent yield and purity.