Organocatalytic Olefin Aziridination via Iminium-Catalyzed Nitrene Transfer: Scope, Limitations, and Mechanistic Insight

Synthesis of a stable crystalline nitrene

Nitrenes are a highly reactive, yet fundamental compound class. They possess a mono-valent nitrogen atom and usually a short life span, typically in the nanosecond range. Here, we report on the synthesis of a stable nitrene by photolysis of the arylazide MSFluindN3 (1), which gave rise to the quantitative formation of the arylnitrene MSFluindN (2) (MSFluind = dispiro[fluorene-9,3'-(1',1',7',7'-tetramethyl-s-hydrindacen-4'-yl)-5',9''-fluorene]), that remains unchanged for at least 3 days when stored under argon atmosphere at room temperature. The extraordinary life span permitted the full characterization of 2 by single crystal x-ray crystallography, EPR spectroscopy and SQUID magnetometry, which supported a triplet ground state. Theoretical simulations suggest in addition to the kinetic stabilization conferred by the bulky MSFluind aryl substituent, that electron delocalization across the central aromatic ring contributes to the electron stabilization of 2.

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.

Organic Dye-Sensitized Nitrene Generation: Intermolecular Aziridination of Unactivated Alkenes

Aziridines are important structural motifs and intermediates, and several synthetic strategies for the direct aziridination of alkenes have been introduced. However, many of these strategies require an excess of activated alkene, suffer from competing side-reactions, have limited functional group tolerance, or involve precious transition metal-based catalysts. Herein, we demonstrate the direct aziridination of alkenes by combining sulfonyl azides as a triplet nitrene source with a catalytic amount of an organic dye functioning as photosensitizer. We show how the nature of the sulfonyl azide, in combination with the triplet-excited state energy of the photosensitizer, affects the aziridination yield and provide a mechanistic rationale to account for the observed dependence of the reaction yield on the nature of the organic dye and sulfonyl azide reagents. The optimized reaction conditions enable the aziridination of structurally diverse and complex alkenes, carrying various functional groups, with the alkene as the limiting reagent.

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.

Aziridination Reactivity of a Manganese(II) Complex with a Bulky Chelating Bis(Alkoxide) Ligand

Treatment of Mn(N(SiMe₃)₂)₂(THF)₂ with bulky chelating bis(alkoxide) ligand [1,1′:4′,1′′-terphenyl]-2,2′′-diylbis(diphenylmethanol) (H₂[O-terphenyl-O]^Ph) formed a seesaw manganese(II) complex Mn[O-terphenyl-O]^Ph(THF)₂, characterized by structural, spectroscopic, magnetic, and analytical methods. The reactivity of Mn[O-terphenyl-O]^Ph(THF)₂ with various nitrene precursors was investigated. No reaction was observed between Mn[O-terphenyl-O]^Ph(THF)₂ and aryl azides. In contrast, the treatment of Mn[O-terphenyl-O]^Ph(THF)₂ with iminoiodinane PhINTs (Ts = p-toluenesulfonyl) was consistent with the formation of a metal-nitrene complex. In the presence of styrene, the reaction led to the formation of aziridine. Combining varying ratios of styrene and PhINTs in different solvents with 10 mol% of Mn[O-terphenyl-O]^Ph(THF)₂ at room temperature produced 2-phenylaziridine in up to a 79% yield. Exploration of the reactivity of Mn[O-terphenyl-O]^Ph(THF)₂ with various olefins revealed (1) moderate aziridination yields for p-substituted styrenes, irrespective of the electronic nature of the substituent; (2) moderate yield for 1,1′-disubstituted α-methylstyrene; (3) no aziridination for aliphatic α-olefins; (4) complex product mixtures for the β-substituted styrenes. DFT calculations suggest that iminoiodinane is oxidatively added upon binding to Mn, and the resulting formal imido intermediate has a high-spin Mn(III) center antiferromagnetically coupled to an imidyl radical. This imidyl radical reacts with styrene to form a sextet intermediate that readily reductively eliminates the formation of a sextet Mn(II) aziridine complex.

Combining visible-light induction and copper catalysis for chemo-selective nitrene transfer for late-stage amination of natural products

Nitrene transfer chemistry is an effective strategy for introducing C-N bonds, which are ubiquitous in pharmaceuticals, agrochemicals and diverse bioactive natural products. The development of chemical methodology that can functionalize unique sites within natural products through nitrene transfer remains a challenge in the field. Herein, we developed copper catalyzed chemoselective allylic C-H amination and catalyst-free visible-light induced aziridination of alkenes through nitrene transfer. In general, both reactions tolerate a wide range of functional groups and occur with predictable regioselectivity. Furthermore, combination of these two methods enable the intermolecular chemo-selective late-stage amination of biologically active natural products, leading to C-H amination or C=C aziridination products in a tunable way. A series of control experiments indicate two-step radical processes were involved in both reaction systems.

Synthesis of Enantioenriched Primary tert-Butanesulfonimidamides via Imination-Hydrazinolysis of N'-tert-Butanesulfinyl Amidines

The first synthesis of primary tert-butanesulfonimidamides with high enantiopurity was realized by imination (or imination/N-functionalization) of enantioenriched N'-tert-butanesulfinyl amidines, followed by hydrazinolysis. N'-Sulfinyl amidines served as imination precursors during copper-catalyzed sulfonyl nitrene transfer or iodonitrene-based NH transfer. Further transformations allowed access to primary tert-butanesulfonimidamides with diverse substitutions on the nitrogen of S═N.

Mechanism of Iminium Salt-Catalyzed C(sp3)-H Amination: Factors Controlling Hydride Transfer versus H-Atom Abstraction

Carbon-nitrogen bonds are extremely prevalent in pharmaceuticals, natural products, and other biologically relevant molecules such as nucleic acids and proteins. Intermolecular amination of C(sp³)-H bonds by catalytic nitrene transfer is a promising method for forging C-N bonds. An organocatalytic approach to nitrene transfer by way of an iminium salt offers a site-selective method for C(sp³)-H amination. Understanding of this amination mechanism including the nature of the relevant intermediates and the factors controlling the mechanism of the N-H bond formation step would aid in the design of catalysts and C(sp³)-H amination methods. In this work, the mechanism of the iminium salt-catalyzed C(sp³)-H amination via nitrene transfer was elucidated computationally using quantum mechanical methods and molecular dynamics simulations. Dispersion-corrected density functional theory (DFT) calculations provide support for an open singlet biradical species in equilibrium with the lower energy triplet species. Calculations further reveal that while the singlet biradical species undergoes N-H bond formation by a hydride transfer process, the triplet species forms the N-H bond by H-atom abstraction. Molecular dynamics (MD) simulations rule out the possibility of a fast rebound of the carbon substrate following N-H bond formation. A predictive model for mode of activation and site-selectivity that is consistent with experimental observations is presented.