N-Aminopyridinium reagents as traceless activating groups in the synthesis of N-Aryl aziridines

Dynamic Kinetic Activation of Aziridines Enables Radical-Polar Crossover (4 + 3) Cycloaddition with 1,3-Dienes

The cycloaddition of aziridines with unsaturated compounds is a valuable method for synthesizing nitrogen heterocycles. However, this process is predominantly substrate-controlled, posing significant challenges in regulating the regioselectivity of the C-N bond cleavage. In this study, we report a nickel-catalyzed dynamic kinetic activation strategy that enables catalyst-controlled activation of aziridines. Various types of aziridines, including 2-phenyl, 2-carbonyl, 2-alkyl, and disubstituted aziridines, consistently cleave their more sterically hindered C-N bonds to generate 1,3-radical anion intermediates. These intermediates participate in a highly regioselective 1,4-Heck/allylic substitution cascade with aromatic branched 1,3-dienes, resulting in a radical-polar crossover (4 + 3) cycloaddition that produces seven-membered azepine products. This approach not only complements traditional dipolar cycloaddition, in which aziridines typically act as zwitterionic 1,3-dipoles, but also introduces an unusual cycloaddition mode for 1,3-dienes. Experimental investigations and density functional theory (DFT) calculations provide insight into the reaction mechanism.

Temperature-Dependent Diastereodivergent [4 + 3] Annulation: Synthesis of Ferrocene-Fused Azepines via Rh(III) Catalysis

Herein, we disclose the first temperature-dependent diastereodivergent [4 + 3] annulation of ferrocene-p-tosylamides via C-H activation with allenes by a Rh catalyst. At room temperature, Rh-catalyzed [4 + 3] annulation selectively offered a kinetically controlled diastereomer [>20:1 diastereomeric ratio (dr)], whereas at 60 °C, a thermodynamically controlled diastereomer was obtained exclusively with >20:1 dr.

Anomeric Amide-Enabled Alkene-Arene and Alkene-Alkene Aminative Coupling

Despite the prominence of C-N bond forming cross-coupling reactions as a strategy to assemble molecular fragments, aminative coupling approaches, in which two fragments are assembled directly at the heteroatom, represents a rarely exploited retrosynthetic strategy. Herein, we report the design, synthesis, and implementation of an anomeric amide reagent capable of promoting highly regioselective aminative alkene-arene and alkene-alkene coupling reactions. This transformation follows a sequence of catalyst-free chloroamination, N-deprotection, and formal nitrene functionalization, all in one-pot. Due to the simplicity of both the protocol and the building blocks required, high-throughput experimentation (HTE) was employed, in combination with a full-scale scope, to rapidly and efficiently explore a wide range of chemical space and determine the limits of reactivity. In addition, alternative reactivity modes from the functionalized intermediates delivered by this protocol demonstrate the divergent nature of this aminative coupling strategy.

Metal-Free Aziridination of Unactivated Olefins via Transient N-Pyridinium Iminoiodinanes

We describe a metal-free aziridination of unactivated olefins to generate N-pyridinium aziridines. Subsequent cross-coupling affords N-aryl aziridines, and reductive depyridylation affords N-H aziridines. Kinetics experiments, based on a variable time normalization analysis (VTNA), indicate that aziridination proceeds via a highly electrophilic N-pyridinium iminoiodinane intermediate. These studies expand build-and-couple aziridine synthesis to unactivated olefins and introduce charge-enhanced electrophilicity into the chemistry of iminoiodinanes.

Aziridine Group Transfer via Transient N-Aziridinyl Radicals

Aziridines are the smallest nitrogen-containing heterocycles. Strain-enhanced electrophilicity renders aziridines useful synthetic intermediates and gives rise to biological activity. Classical aziridine syntheses─based on either [2 + 1] cycloadditions or intramolecular substitution chemistry─assemble aziridines from acyclic precursors. Here, we introduce N-aziridinyl radicals as a reactive intermediate that enables the transfer of intact aziridine fragments in organic synthesis. Transient N-aziridinyl radicals are generated by the reductive activation of N-pyridinium aziridines and are directly characterized by spin-trapped EPR spectroscopy. In the presence of O2, N-aziridinyl radicals are added to styrenyl olefins to afford 1,2-hydroxyaziridination products. These results establish aziridinyl radicals as new reactive intermediates in synthetic chemistry and demonstrate aziridine group transfer as a viable synthetic disconnection.

Bidirectional Electron Transfer Strategies for Anti-Markovnikov Olefin Aminofunctionalization via Arylamine Radicals

Arylamines are common structural motifs in pharmaceuticals, natural products, and materials precursors. While olefin aminofunctionalization chemistry can provide entry to arylamines, classical polar reactions typically afford Markovnikov products. Nitrogen-centered radical intermediates provide the opportunity to access anti-Markovnikov selectivity; however, anti-Markovnikov arylamination is unknown in large part due to the lack of arylamine radical precursors. Here, we introduce bidirectional electron transfer processes to generate arylamine radical intermediates from N-pyridinium arylamines: Single-electron oxidation provides arylamine radicals that engage in anti-Markovnikov olefin aminopyridylation; single-electron reduction unveils arylamine radicals that engage in anti-Markovnikov olefin aminofunctionalization. The development of bidirectional redox processes complements classical design principles for radical precursors, which typically function via a single redox manifold. Demonstration of both oxidative and reductive mechanisms to generate arylamine radicals from a common N-aminopyridinium precursor provides complementary methods to rapidly construct and diversify arylamine scaffolds from readily available radical precursors.

β-Phenethylamine Synthesis: N-Pyridinium Aziridines as Latent Dual Electrophiles

β-Phenethylamines are widely represented in biologically and pharmacologically active organic small molecules. Here, we introduce N-pyridinium aziridines as latent dual electrophiles for the synthesis of β-phenethylamines. Bromide-promoted ring opening generates β-halopyridinium amines. Selective Ni-catalyzed C-C cross-coupling between organozinc nucleophiles and the benzylic C-Br electrophile affords a diverse family of β-functionalized phenethylaminopyridinium salts, and coupling is stereoconvergent in the presence of chiral ligands. Subsequent Ni-catalyzed reductive N-N bond activation within the β-functionalized phenethylaminopyridinium salts furnishes the products of formal olefin carboamination. Other reductive N-N cleavage reactions are demonstrated to provide access to free primary amines, alkylated amines, heterocycles, and products derived from N-centered radical chemistry. The developed reaction sequence can be implemented in the context of complex molecules and natural product derivatives. Together, the described results provide a general and modular synthesis of β-phenethylamines and significantly expand the utility of N-pyridinium aziridines as linchpins in chemical synthesis.

B2(OH)4-Mediated Reductive Ring-Opening of N-Tosyl Aziridines by Nitroarenes: A Green and Regioselective Access to Vicinal Diamines

The nucleophilic ring-opening of aziridine derivatives provides an important synthetic tool for the preparation of various β-functionalized amines. Amines as nucleophiles are employed to prepare synthetically useful 1,2-diamines in the presence of various catalysts or activators. Herein, the B2(OH)4-mediated reductive ring-opening transformation of N-tosyl aziridines by nitroarenes was developed. This aqueous protocol employed nitroarenes as cheap and readily available amino sources and proceeds under external catalyst-free conditions. Control experiments and DFT calculations pointed to the in situ reduction of nitroarenes to aryl amines via N-aryl boramidic acid (E) and an SN1-type ring-opening of N-tosylaziridines by the resultant aryl amines with high regioselectivity.

Synthesis of Secondary Amines via Self-Limiting Alkylation

N-centered nucleophilicity increases upon alkylation, and thus selective partial alkylation of ammonia and primary amines can be challenging: Poor selectivity and overalkylation are often observed. Here we introduce N-aminopyridinium salts as ammonia surrogates for the synthesis of secondary amines via self-limiting alkylation chemistry. Readily available N-aryl-N-aminopyridinium salts engage in N-alkylation and in situ depyridylation to afford secondary aryl-alkyl amines without any overalkylation products. The method overcomes classical challenges in selective amine alkylation by accomplishing alkylation via transient, highly nucleophilic pyridinium ylide intermediates and can be applied in the context of complex molecular scaffolds. These findings establish N-aminopyridinium salts as ammonia synthons in synthetic chemistry and a strategy to control the extent of amine alkylation.

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.

Electrochemical Benzylic C-H Amination via N-Aminopyridinium

An electrochemical protocol for benzylic C(sp3)-H aminopyridylation via direct C-H/N-H cross-coupling of alkylarenes with N-aminopyridinium triflate has been developed. This method features excellent site-selectivity, broad substrate scope, redox reagent-free and facile scalability. The generated benzylaminopyridiniums can be readily converted to benzylamines via electroreductive N-N bond cleavage.

Copper(I)-Photocatalyzed Diastereoselective Aziridination of N-Sulfonyl Imines with Vinyl Azides: Application to Benzo[f][1,2,3]oxathiazepines Dioxides and Fused Isoxazolines

An in situ generated photoactive copper(I)-complex-catalyzed aziridination reaction of cyclic N-sulfonyl imines with α-aryl-substituted vinyl azides irradiated by blue-LEDs light is reported for the first time. This novel SET process represents a mild, sustainable, and pragmatic method for accessing synthetically resourceful sulfamidate-fused aziridines in acceptable chemical yields with excellent diastereoselectivities. Delightedly, pharmacologically attractive benzo[f][1,2,3]oxathiazepine dioxides and fused isoxazoline frameworks were achieved through our newly developed metal-free based ring-expansion techniques, highlighting the synthetic value of accessed aziridines. Finally, the possible mechanism for [2+1] aza-cyclization was presented based on the conduction of a series of control experiments.

Olefin Difunctionalization for the Synthesis of Tetrahydroisoquinoline, Morpholine, Piperazine, and Azepane

This report outlines a versatile strategy for synthesizing a diverse array of N-heterocycles. By the utilization of common olefins, this simple protocol facilitates their coupling with various bifunctional reagents. Furthermore, it can be integrated with C-H amination techniques to directly produce N-heterocycles in a multicomponent cascade coupling process. The unique bond disconnection logic employed in this process underscores its efficiency in achieving rapid simplification through cascade couplings.

Generation of Aromatic N-Heterocyclic Radicals for Functionalization of Unactivated Alkenes

Nitrogen-centered radicals (NCRs) have been widely recognized as versatile synthetic intermediates for the construction of nitrogen containing molecules of high value. As such, there has been a long-standing interest in the field of organic synthesis to develop novel nitrogen-based radicals and explore their inherent reactivity. In this study, we present the generation of aromatic N-heterocyclic radicals and their application in a novel and diverse functionalization of unactivated alkenes. Bench-stable aromatic N-heterocyclic pyridinium salts were employed as crucial NCR precursors, which enabled the efficient conversion of various unactivated alkenes into medicinally relevant alkylated N-heterocyclic amines. This approach offers an unexplored retrosynthetic disconnection for the synthesis of related molecules that commonly possess therapeutic value. Furthermore, this platform can be extended to the synthesis of densely functionalized heterocyclic amines by utilizing disulfides and diethyl bromomalonate as radical quenchers. Mechanistic investigations indicate an energy transfer (EnT) pathway involving the formation of a transient aromatic N-heterocyclic radical, radical addition to unactivated alkenes, and subsequent generation of the amination product through either hydrogen atom transfer (HAT) or radical addition processes.

On the mechanism of intermolecular nitrogen-atom transfer from a lattice-isolated diruthenium nitride intermediate

Catalyst confinement within microporous media provides the opportunity to site isolate reactive intermediates, enforce intermolecular functionalization chemistry by co-localizing reactive intermediates and substrates in molecular-scale interstices, and harness non-covalent host-guest interactions to achieve selectivities that are complementary to those accessible in solution. As part of an ongoing program to develop synthetically useful nitrogen-atom transfer (NAT) catalysts, we have demonstrated intermolecular benzylic amination of toluene at a Ru2 nitride intermediate confined within the interstices of a Ru2-based metal-organic framework (MOF), Ru3(btc)2X3 (btc = 1,3,5-benzenetricarboxylate, i.e., Ru-HKUST-1 for X = Cl). Nitride confinement within the extended MOF lattice enabled intermolecular C-H functionalization of benzylic C-H bonds in preference to nitride dimerization, which was encountered with soluble molecular analogues. Detailed study of the kinetic isotope effects (KIEs, i.e., kH/kD) of C-H amination, assayed both as intramolecular effects using partially labeled toluene and as intermolecular effects using a mixture of per-labeled and unlabeled toluene, provided evidence for restricted substrate mobility on the time scale of interstitial NAT. Analysis of these KIEs as a function of material mesoporosity provided approximate experimental values for functionalization in the absence of mass transport barriers. Here, we disclose a combined experimental and computational investigation of the mechanism of NAT from a Ru2 nitride to the C-H bond of toluene. Computed kinetic isotope effects for a H-atom abstraction (HAA)/radical rebound (RR) mechanism are in good agreement with experimental data obtained for C-H amination at the rapid diffusion limit. These results provide the first detailed analysis of the mechanism of intermolecular NAT to a C-H bond, bolster the use of KIEs as a probe of confinement effects on NAT within MOF lattices, and provide mechanistic insights unavailable by experiment because rate-determining mass transport obscured the underlying chemical kinetics.

N-Amino Pyridinium Salts in Organic Synthesis

C-N bond forming reactions hold immense significance to synthetic organic chemistry. In pursuit of efficient methods for the introduction of nitrogen in organic small molecules, myriad synthetic methods have been developed, and methods based on both nucleophilic and electrophilic aminating reagents have received sustained research effort. In response to continued challenges - the need for substrate prefunctionalization, the requirement for vestigial N-activating groups, and the need to incorporate nitrogen in ever more complex molecular settings - the development of novel aminating reagents remains a central challenge in method development. N-aminopyridinums and their derivatives have recently emerged as a class of bifunctional aminating reagents, which combine N-centered nucleophilicity with latent electrophilic or radical reactivity by virtue of the reducible N-N bond, with broad synthetic potential. Here, we summarize the synthesis and reactivity of N-aminopyridinium salts relevant to organic synthesis. The preparation and application of these reagents in photocatalyzed and metal-catalyzed transformations is discussed, showcasing the reactivity in the context of bifunctional platform and its potential for innovation in the field.

Recent advances of aminoazanium salts as amination reagents in organic synthesis

This review summarizes the utilization of aminoazaniums as amination reagents in organic synthesis, including the amination of aldehydes, boronic esters, olefins, etc.