Catalytic, asymmetric carbon-nitrogen bond formation using metal nitrenoids: from metal-ligand complexes via metalloporphyrins to enzymes

Catalytic Metalloradical System for Radical 1,6-C(sp3)-H Amination with Concurrent Control of Site-, Chemo-, and Enantio-selectivity

A catalytic radical process has been developed via metalloradical catalysis (MRC) for 1,6-C(sp3)-H amination with concurrent control of site-, chemo-, and enantioselectivity. Supported by an optimal D2-symmetric chiral amidoporphyrin ligand, the Co(II)-based metalloradical system effectively catalyzes chemoselective amination of propargylic, allylic, and benzylic C-H bonds at 1,6- over 1,5-positions of alkoxysulfonyl azides, achieving high enantioselectivity. This Co(II)-catalyzed process, which operates at room temperature, is applicable to a broad range of alkoxysulfonyl azides with a high tolerance of functional groups, enabling the efficient construction of six-membered sulfamidates in high yields with excellent enantioselectivities. Comprehensive experimental investigations, complemented by computational studies, elucidate the stepwise radical mechanism underlying this transformation. The resulting six-membered cyclic sulfamidates from the enantioselective radical process can undergo stereospecific ring-opening reactions with various nucleophiles, affording γ-functionalized α-chiral amines in high yields while retaining the original enantiopurity. Since alkoxysulfonyl azides are readily synthesized from widely available alcohols through a nucleophilic azide transfer, this union of the radical and ionic processes constitutes a versatile 1,3-difunctionalization of alcohols.

Nitrene-mediated aminative N-N-N coupling: facile access to triazene 1-oxides

Significant progress has been made in metal-catalyzed cross-coupling reactions over the past few decades. However, the development of innovative aminative coupling strategies remains highly desirable. Herein, we report a nitrene-mediated aminative N-N-N coupling reaction that leverages an anomeric amide as a key reagent to bridge amines with nitrosoarenes. This strategy enables the in situ generation of an aminonitrene intermediate, which is efficiently intercepted by nitrosoarenes, providing a direct, mild, and highly efficient route to triazene 1-oxides. Mechanistic investigations reveal that the N-substituents of the amine play a crucial role in modulating the reactivity of the aminonitrene intermediate. Complementary computational studies further indicate that aminonitrene acts as a nucleophile, while nitrosobenzene serves as an electrophile. Notably, aminonitrene-nitrosoarene coupling is significantly favored due to a substantial reduction in distortion energy, effectively outcompeting the nitrene dimerization pathway.

Catalytic Amino Group Transfer Reactions Mediated by Photoinduced Nitrene Formation from Rhodium-Hydroxamates

Herein, we report a photocatalytic platform to access transient nitrenoids by designing photo-responsive neutral rhodium-hydroxamate complexes. Combined experimental and computational mechanistic studies, including electron paramagnetic resonance (EPR) and mass spectrometric analysis, suggest that electrophilic Fischer-type Rh-acylnitrenoid intermediates could be generated via photoactivation of corresponding Rh-hydroxamates via N-O bond homolysis and redox event. Moreover, catalytic acylnitrenoid transfer was explored toward the amidation of various hydrocarbons, amines, and alcohols to furnish new N-C, N-N, and N-O bonds.

Design Approaches That Utilize Ionic Interactions to Control Selectivity in Transition Metal Catalysis

The attractive force between two oppositely charged ions can constitute a powerful design tool in selective catalysis. Enzymes make extensive use of ionic interactions alongside a variety of other noncovalent interactions; recent years have seen synthetic chemists begin to seriously explore these interactions in catalyst designs that also incorporate a reactive transition metal. In isolation, a single ionic interaction exhibits low directionality, but in many successful systems they exist alongside additional interactions which can provide a high degree of organization at the selectivity-determining transition state. Even in situations with a single key interaction, low directionality is not always detrimental, and can even be advantageous, conferring generality to a single catalyst. This Review explores design approaches that utilize ionic interactions to control selectivity in transition metal catalysis. It is divided into two halves: in the first, the ionic interaction occurs in the outer sphere of the metal complex, using a ligand which is charged or bound to an anion; in the second, the metal bears a formal charge, and the ionic interaction is with an associated counterion.

Skeletal Editing via Transition-Metal-Catalyzed Nitrene Insertion

Metal-nitrenes are valuable reactive intermediates for synthesis and are widely used to construct biologically relevant scaffolds, complexes and functionalized molecules. The ring expansion of cyclic molecules via single-nitrogen-atom insertion via nitrene or metal-nitrenoid intermediates has emerged as a promising modern strategy for driving advantageous nitrogen-rich compound synthesis. In recent years, the catalytic insertion of a single nitrogen atom into carbocycles, leading to N-heterocycles, has become an important focus of modern synthetic approaches with applications in medicinal chemistry, materials science, and industry. Catalytic single-nitrogen-atom insertions have been increasing in prominence in modern organic synthesis due to their capability to construct high-value added nitrogen-containing heterocycles from simple feedstocks. In this review, we will discuss the rapidly growing field of skeletal editing via single-nitrogen-atom insertion using transition metal catalysis to access nitrogen-containing heterocycles, with a focus on nitrogen insertion across a wide spectrum of carbocycles.

Iron-catalysed intramolecular C(sp3)-H amination of alkyl azides

Iron-catalysed intramolecular C(sp3)-H amination of alkyl azides (N3R, R = alkyl) via the iron-alkylnitrene/alkylimido (Fe(NR)) intermediate, is an appealing synthetic approach for the synthesis of various N-heterocycles. This approach provides a direct atom-economy strategy for constructing C(sp3)-N bonds, with nitrogen gas as the only by-product and iron is a biocompatible, cheap, and earth-abundant metal. However, C(sp3)-H amination with alkyl azides is challenging because alkyl nitrenes readily undergo 1,2-hydride migration to imines. This article summarizes recent major advances in this field in terms of catalyst design, substrate scope expansion, stereoselectivity control, understanding of key reaction intermediates, and applications in the synthesis of complex natural products and pharmaceuticals.

Taming Tethered Nitreniums for Alkene Functionalization Reactions

We present the first examples of amino-trifluoroacetoxylations of alkenes using N-alkoxy carbamate tethers. Hypervalent iodine oxidants mediate this transformation, providing a "green" alternative to existing intramolecular amino-hydroxylation protocols which use toxic metals such as osmium. In all cases examined, the reaction is regioselective and stereospecific, with the geometry of the starting alkene controlling the diastereomeric outcome. By analogy to prior art and from our own observations, we posit that a transient nitrenium species serves as a key intermediate.

Biocatalytic Synthesis of α-Amino Esters via Nitrene C-H Insertion

α-Amino esters are precursors to noncanonical amino acids used in developing small-molecule therapeutics, biologics, and tools in chemical biology. α-C-H amination of abundant and inexpensive carboxylic acid esters through nitrene transfer presents a direct approach to α-amino esters. Methods for nitrene-mediated amination of the protic α-C-H bonds in carboxylic acid esters, however, are underdeveloped. This gap arises because hydrogen atom abstraction (HAA) of protic C-H bonds by electrophilic metal-nitrenoids is slow: metal-nitrenoids preferentially react with polarity-matched, hydridic C-H bonds, even when weaker protic C-H bonds are present. This study describes the discovery and evolution of highly stable protoglobin nitrene transferases that catalyze the enantioselective intermolecular amination of the α-C-H bonds in carboxylic acid esters. We developed a high-throughput assay to evaluate the activity and enantioselectivity of mutant enzymes together with their sequences using the Every Variant Sequencing (evSeq) method. The assay enabled the identification of enantiodivergent enzymes that function at ambient conditions in Escherichia coli whole cells and whose activities can be enhanced by directed evolution for the amination of a range of substrates.

Skeletal Editing by Hypervalent Iodine Mediated Nitrogen Insertion

Hypervalent iodine reagents are versatile and readily accessible reagents that have been extensively applied in contemporary synthesis in modern organic chemistry. Among them, iodonitrene (ArI=NR), is a powerful reactive species, widely used for a single-nitrogen-atom insertion reaction, and skeletal editing to construct N-heterocycles. Skeletal editing with reactive iodonitrene components has recently emerged as an exciting approach in modern chemical transformation. These reagents have been extensively used to produce biologically relevant heterocycles and functionalized molecular architectures. Recently, the insertion of a nitrogen-atom into hydrocarbons to generate N-heterocyclic compounds using hypervalent iodine reagents has been a significant focus in the field of molecular editing reactions. In this review, we discuss the rapidly emerging field of nitrene insertion, including skeletal editing and nitrogen insertion, using hypervalent iodine reagents to access nitrogen-containing heterocycles, and the current mechanistic understanding of these processes.

Iron Corrole-Catalyzed Intramolecular Amination Reactions of Alkyl Azides. Spectroscopic Characterization and Reactivity of [FeV(Cor)(NAd)]

As nitrogen analogues of iron-oxo species, high-valent iron-imido species have attracted great interest in the past decades. FeV-alkylimido species are generally considered to be key reaction intermediates in Fe(III)-catalyzed C(sp3)─H bond aminations of alkyl azides but remain underexplored. Here, it is reported that iron-corrole (Cor) complexes can catalyze a wide range of intramolecular C─H amination reactions of alkyl azides to afford a variety of 5-, 6- and 7-membered N-heterocycles, including alkaloids and natural product derivatives, with up to 3880 turnover numbers (TONs) and excellent diastereoselectivity (>99:1 d.r.). Mechanistic studies including density functional theory (DFT) calculations and intermolecular hydrogen atom abstraction (HAA) reactions reveal key reactive FeV-alkylimido intermediates. The [FeV(Cor)(NAd)] (Ad = adamantyl) complex is independently prepared and characterized through electron paramagnetic resonance (EPR), resonance Raman (rR) measurement, and X-ray photoelectron spectroscopy (XPS). This complex is reactive toward HAA reactions with kinetic isotope effects (KIEs) similar to [Fe(Cor)]-catalyzed intramolecular C─H amination of alkyl azides.

Intermolecular Enantioselective Amination Reactions Mediated by Visible Light and a Chiral Iron Porphyrin Complex

In the presence of 1 mol % of a chiral iron porphyrin catalyst, various 3-arylmethyl-substituted 2-quinolones and 2-pyridones underwent an enantioselective amination reaction (20 examples; 93-99 % ee). The substrates were used as the limiting reagents, and fluorinated aryl azides (1.5 equivalents) served as nitrene precursors. The reaction is triggered by visible light which allows a facile dediazotation at ambient temperature. The selectivity of the reaction is governed by a two-point hydrogen bond interaction between the ligand of the iron catalyst and the substrate. Hydrogen bonding directs the amination to a specific hydrogen atom within the substrate that is displaced by the nitrogen substituent either in a concerted fashion or by a rebound mechanism.

Enantioselective Nitrene Transfer to Hydrocinnamyl Alcohols and Allylic Alcohols Enabled by Systematic Exploration of the Structure of Ion-Paired Rhodium Catalysts

This work describes highly enantioselective nitrene transfer to hydrocinnamyl alcohols (benzylic C-H amination) and allylic alcohols (aziridination) using ion-paired Rh (II,II) complexes based on anionic variants of Du Bois' esp ligand that are associated with cinchona alkaloid-derived chiral cations. Directed by a substrate hydroxyl group, our previous work with these complexes had not been able to achieve high enantioselectivity on these most useful short-chain compounds, and we overcame this challenge through a combination of catalyst design and modified conditions. A hypothesis that modulation of the linker between the anionic sulfonate group and the central arene spacer might provide a better fit for shorter chain length substrates led to the development of a new biaryl-containing scaffold, which has allowed a broad scope for both substrate classes to be realized for the first time. Furthermore, we describe a systematic structural "knockout" study on the cinchona alkaloid-derived chiral cation to elucidate which features are crucial for high enantioinduction. De novo synthesis of modified scaffolds led to the surprising finding that for high ee the quinoline nitrogen of the alkaloid is crucial, although its location within the heterocycle could be varied, even leading to a superior catalyst. The free hydroxyl is also crucial and should possess the naturally occurring diastereomeric configuration of the alkaloid. These findings underline the privileged nature of the cinchona alkaloid scaffold and provide insight into how these cations might be used in other catalysis contexts.

Iron-Catalyzed Asymmetric Imidation of Sulfides via Sterically Biased Nitrene Transfer

Transition-metal-catalyzed enantioselective nitrene transfer to sulfides has emerged as one of the most powerful strategies for rapid construction of enantioenriched sulfimides. However, achieving stereocontrol over highly active earth-abundant transition-metal nitrenoid intermediates remains a formidable challenge compared with precious metals. Herein, we disclose a chiral iron(II)/N,N'-dioxide-catalyzed enantioselective imidation of dialkyl and alkyl aryl sulfides using iminoiodinanes as nitrene precursors. A series of chiral sulfimides were obtained in moderate-to-good yields with high enantioselectivities (56 examples, up to 99% yield, 98:2 e.r.). The utility of this methodology was demonstrated by late-stage modification of complex molecules and synthesis of the chiral insecticide sulfoxaflor and the intermediates of related bioactive compounds. Based on experimental studies and theoretical calculations, a water-bonded high-spin iron nitrenoid species was identified as the key intermediate. The observed stereoselectivity was original from the steric repulsion between the amide unit of the ligand in the chiral cave and the bulky substituent of sulfides. Additionally, dioxazolones proved to be suitable acylnitrene precursors in the presence of an iron(III)/N,N'-dioxide complex, resulting in the formation of enantioselectivity-reversed sulfimides (14 examples, up to 81% yield, 97:3 e.r.).

Tertiary Amides as Directing Groups for Enantioselective C-H Amination using Ion-Paired Rhodium Complexes

Enantioselective C-H amination at a benzylic methylene is a vital disconnection towards chiral benzylamines. Here we disclose that butyric and valeric acid-derived tertiary amides can undergo highly enantioselective benzylic amination using an achiral anionic Rh complex that is ion-paired with a Cinchona alkaloid-derived chiral cation. A broad scope of compounds can be aminated encompassing numerous arene substitutions, amides, and two different chain lengths. Excellent tolerance of ortho substituents was observed, which has not been achieved before in asymmetric intermolecular C-H amination with Rh. We speculate that the tertiary amide group of the substrate engages in hydrogen bonding interactions directly with the chiral cation, enabling a high level of organisation at the transition state for C-H amination. This is in contrast with our previous work where a substrate bearing a hydrogen bond donor was required. Control experiments led to the discovery that methyl ethers also function as proficient directing groups under the optimised conditions, potentially also acting as hydrogen bond acceptors. This finding has the promise to dramatically expand the applicability of our ion-paired chiral catalysts.