Iron–Phosphine Complex-Catalyzed Intramolecular C(sp3)–H Amination of Azides

Strategies and Mechanisms of First-Row Transition Metal-Regulated Radical C-H Functionalization

Radical C-H functionalization represents a useful means of streamlining synthetic routes by avoiding substrate preactivation and allowing access to target molecules in fewer steps. The first-row transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are Earth-abundant and can be employed to regulate radical C-H functionalization. The use of such metals is desirable because of the diverse interaction modes between first-row transition metal complexes and radical species including radical addition to the metal center, radical addition to the ligand of metal complexes, radical substitution of the metal complexes, single-electron transfer between radicals and metal complexes, hydrogen atom transfer between radicals and metal complexes, and noncovalent interaction between the radicals and metal complexes. Such interactions could improve the reactivity, diversity, and selectivity of radical transformations to allow for more challenging radical C-H functionalization reactions. This review examines the achievements in this promising area over the past decade, with a focus on the state-of-the-art while also discussing existing limitations and the enormous potential of high-value radical C-H functionalization regulated by these metals. The aim is to provide the reader with a detailed account of the strategies and mechanisms associated with such functionalization.

Iron-Catalyzed Denitrogenative Annulation Reactions between α-Azido Acetamides and Cyclic Ketones

We report an FeCl2-catalyzed annulation reaction between α-azido acetamides and cyclic ketones. Two types of α,β-unsaturated γ-lactam products can be obtained, depending on the reaction conditions. When α-azido acetamides were reacted with cyclohexanone, 8-amino-5,6,7,8-tetrahydro-1H-indol-2(4H)-ones were obtained when a primary amine was present in the reaction system; conducting the reaction in the presence of 2-aminobenzenesulfonic acid, on the contrary, resulted in the formation of 5,6-dihydro-1H-indol-2(4H)-ones. Cycloheptanone and cyclooctanone reacted in the same way as cyclohexanone. The reactions proceed via the intermediacy of 2-iminoacetamides, which are formed by FeCl2-facilitated dinitrogenation of α-azido acetamides. These reactions constitute a new strategy for expanding the synthetic dimensions of organic azides.

Deciphering Iron-Catalyzed C-H Amination with Organic Azides: N2 Cleavage from a Stable Organoazide Complex

Catalytic C-N bond formation by direct activation of C-H bonds offers wide synthetic potential. En route to C-H amination, complexes with organic azides are critical precursors towards the reactive nitrene intermediate. Despite their relevance, α-N coordinated organoazide complexes are scarce in general, and elusive with iron, although iron complexes are by far the most active catalysts for C-H amination with organoazides. Herein, we report the synthesis of a stable iron α-N coordinated organoazide complex from [Fe(N(SiMe3 )2 )2 ] and AdN3 (Ad=1-adamantyl) and its crystallographic, IR, NMR and zero-field 57 Fe Mössbauer spectroscopic characterization. These analyses revealed that the organoazide is in fast equilibrium between the free and coordinated state (Keq =62). Photo-crystallography experiments showed gradual dissociation of N2 , which imparted an Fe-N bond shortening and correspond to structural snapshots of the formation of an iron imido/nitrene complex. Reactivity of the organoazide complex in solution showed complete loss of N2 , and subsequent formation of a C-H aminated product via nitrene insertion into a C-H bond of the N(SiMe3 )2 ligand. Monitoring this reaction by 1 H NMR spectroscopy indicates the transient formation of the imido/nitrene intermediate, which was supported by Mössbauer spectroscopy in frozen solution.

Ligand-Promoted Iron-Catalyzed Nitrene Transfer for the Synthesis of Hydrazines and Triazanes through N-Amidation of Arylamines

Herein, we report that bulky alkylphosphines such as PtBu3 can switch the roles from actor to spectator ligands to promote the FeCl2 -catalyzed N-amidation reaction of arylamines with dioxazolones, giving hydrazides in high efficiency and chemoselectivity. Mechanistic studies indicated that the phosphine ligands could facilitate the decarboxylation of dioxazolones on the Fe center, and the hydrogen bonding interactions between the arylamines and the ligands on Fe nitrenoid intermediates might play a role in modulating the delicate interplay between the phosphine ligand, arylamine, and acyl nitrene N, favoring N-N coupling over N-P coupling. The new ligand-promoted N-amidation protocols offer a convenient way to access various challenging triazane compounds via double or sequential N-amidation of primary arylamines.

Discovery of a simple iron catalyst reveals the intimate steps of C-H amination to form C-N bonds

Formation of ubiquitous C-N bonds traditionally uses prefunctionalized carbon precursors. Recently, metal-catalyzed amination of unfunctionalized C-H bonds with azides has become an attractive and atom-economic strategy for C-N bond formation, though all catalysts contain sophisticated ligands. Here, we report Fe(HMDS)2 (HMDS = N(SiMe3)2 -) as an easy-to-prepare catalyst for intramolecular C-H amination. The catalyst shows unprecedented turnover frequencies (110 h-1 vs. 70 h-1 reported to date) and requires no additives. Amination is successful for benzylic and aliphatic C-H bonds (>80% yield) and occurs even at room temperature. The simplicity of the catalyst enabled for the first time comprehensive mechanistic investigations. Kinetic, stoichiometric, and computational studies unveiled the intimate steps of the C-H amination process, including the resting state of the catalyst and turnover-limiting N2 loss of the coordinated azide. The high reactivity of the iron imido intermediate is rationalized by its complex spin system revealing imidyl and nitrene character.

From the bottle: simple iron salts for the efficient synthesis of pyrrolidines via catalytic C-H bond amination

Commercially available iron salts FeX2 are remarkably active catalysts for pyrrolidine formation from organic azides via direct C-H bond amination. With FeI2, amination is fast and selective, (<30 min for 80% yield at 2 mol% loading), TONs up to 370 are reached with just 0.1 mol% catalyst, different functional groups are tolerated, and a variety of C-H bonds were activated.

Iron-Catalyzed Intermolecular Amination of Benzylic C(sp3)-H Bonds

A catalytic system for intermolecular benzylic C(sp³)-H amination is developed utilizing 1,2,3,4-tetrazole as a nitrene precursor via iron catalysis. This method enables direct installation of 2-aminopyridine into the benzylic and heterobenzylic position. The method selectively aminates 2° benzylic C(sp³)-H bond over the 3° and 1° benzylic C(sp³)-H bonds. Experimental studies reveal that the C(sp³)-H amination undergoes via the formation of a benzylic radical intermediate. This study reports the discovery of new method for 2-pyridine substituted benzylamine synthesis using inexpensive, biocompatible base metal catalysis that should have wide application in the context of medicinal chemistry and drug discovery.

An Iron-Mesoionic Carbene Complex for Catalytic Intramolecular C-H Amination Utilizing Organic Azides

The synthesis of N-heterocycles is of paramount importance for the pharmaceutical industry. They are often synthesized through atom economic and environmentally unfriendly methods, generating significant waste. A less explored, but greener, alternative is the synthesis through the direct intramolecular C-H amination utilizing organic azides. Few examples exist by using this method, but many are limited due to the required use of stoichiometric amounts of Boc₂O. Herein, we report a homoleptic C,O-chelating mesoionic carbene-iron complex, which is the first iron-based complex that does not require the addition of any protecting groups for this transformation and that is active also in strong donor solvents such as THF or even DMSO. The achieved turnover number is an order of magnitude higher than any other reported catalytic system. A variety of C-H bonds were activated, including benzylic, primary, secondary, and tertiary. By following the reaction over time, we determined the presence of an initiation period. Kinetic studies showed a first-order dependence on substrate concentration and half-order dependence on catalyst concentration. Intermolecular competition reactions with deuterated substrate showed no KIE, while separate reactions with deuterium-labeled substrate resulted in a KIE of 2.0. Moreover, utilizing deuterated substrate significantly decreased the initiation period of the catalysis. Preliminary mechanistic studies suggest a unique mechanism involving a dimeric iron species as the catalyst resting state.

Iron-Catalyzed 1,4-Phenyl Migration/Ring Expansion of α-Azido N-Phenyl Amides

Herein, we report a novel iron-catalyzed skeleton rearrangement of alkyl azides. Upon treatment with FeCl₂ and N-heterocyclic carbene SIPr·HCl in the presence of H₂O and Et₃N, 2-azido-N,N-diphenylamides underwent 1,4-phenyl migration and ring expansion to give azepin-2-ones in good yield. The reaction proceeds via intramolecular nitrene cycloaddition followed by C-N cleavage, water addition, and electrocyclic ring opening.

A soluble iron(II)-phthalocyanine-catalyzed intramolecular C(sp3)-H amination with alkyl azides

Herein, we describe a soluble iron(II)-phthalocyanine, [Fe^II(^tBu₄Pc)(py)₂] (Pc = phthalocyaninato(2-)), as an effective catalyst in intramolecular C(sp³)-H bond amination, with alkyl azides as the nitrogen source, to afford the amination products in moderate to excellent yields with a broad substrate scope.

Visible-Light-Driven Aryl Migration and Cyclization of α-Azido Amides

This paper reports two new visible-light-promoted radical reactions of α-azido amides. By catalysis of [Ir(ppy)₂(dtbbpy)]PF₆ with i-Pr₂NEt as the reducing agent, N-aryl α-azido tertiary amides were first converted to the corresponding aminyl radicals through reduction of the azido group; the aminyl radicals then underwent N-to-N aryl migration to give α-anilinyl-functionalized amides. α-Azido secondary amides, on the other hand, reacted with the solvent ethanol and i-Pr₂NEt to afford the imidazolinone products.

Iron-catalyzed stereoselective haloamidation of amide-tethered alkynes

In this work, by using N-methoxybenzamides as efficient acyl nitrene precursors, an iron-catalyzed formal cis-haloamidation of alkyne is reported. Without assistance of additives, the reaction worked well in the presence of 50 mol% FeCl3 or FeBr3, leading to a series of chloro/bromo-containing isoindolin-5-ones with high efficiency and wide reaction scope. In the reaction, the iron-facilitated haloamidation proceeds through a halo anion-participating concerted [3+2] cyclization to release the final products. The key intermediate ferric acyl nitrene A is generated in situ from a formal removal of MeOH.

Iron-Catalyzed Radical Activation Mechanism for Denitrogenative Rearrangement Over C(sp3 )-H Amination

An iron-catalyzed denitrogenative rearrangement of 1,2,3,4-tetrazole is developed over the competitive C(sp³ )-H amination. This catalytic rearrangement reaction follows an unprecedented metalloradical activation mechanism. Employing the developed method, a wide number of complex-N-heterocyclic product classes have been accessed. The synthetic utility of this radical activation method is showcased with the short synthesis of a bioactive molecule. Collectively, this discovery underlines the progress of radical activation strategy that should find wide application in the perspective of medicinal chemistry, drug discovery and natural product synthesis research.

Efficient Amination of Activated and Non-Activated C(sp3 )-H Bonds with a Simple Iron-Phenanthroline Catalyst

A readily available catalyst consisting of iron dichloride in combination with 1,10-phenanthroline catalyzes the ring-closing C-H amination of N-benzoyloxyurea to form imidazolidin-2-ones in high yields. The C-H amination reaction is very general and applicable to benzylic, allylic, propargylic, and completely non-activated aliphatic C(sp3 )-H bonds, and it also works for C(sp2 )-H bonds. The surprisingly simple method can be performed under open flask conditions.

Unravelling nitrene chemistry from acyclic precursors: recent advances and challenges

Recent advances in nitrene chemistry from acyclic precursors are reviewed in this paper.

Iron-catalyzed intramolecular acyl nitrene/alkyne metalation for the synthesis of pyrrolo[2,1-a]isoindol-5-ones

In this work, by using N-methoxybenzamides as efficient acyl nitrene precursors, an iron-catalyzed acyl nitrene/alkyne metalation is reported for the synthesis of pyrrolo[2,1-a]isoindol-5-ones.

Iron-Catalyzed Amination of Strong Aliphatic C(sp3)-H Bonds

A concept for intramolecular denitrogenative C(sp³)-H amination of 1,2,3,4-tetrazoles bearing unactivated primary, secondary, and tertiary C-H bonds is discovered. This catalytic amination follows an unprecedented metalloradical activation mechanism. The utility of the method is showcased with the short synthesis of a bioactive molecule. Moreover, an initial effort has been embarked on for the enantioselective C(sp³)-H amination through the catalyst design. Collectively, this study underlines the development of C(sp³)-H bond functionalization chemistry that should find wide application in the context of drug discovery and natural product synthesis.

Iron-catalysed 1,2-aryl migration of tertiary azides

1,2-Carbon to nitrogen aryl migration of α,α-diaryl tertiary azides was realized by using FeCl₂ and N-heterocyclic carbene SIPr·HCl as a catalyst.