Nitrene Chemistry in Organic Synthesis: Still in Its Infancy?

Intermolecular C-H Amidation of (Hetero)arenes to Produce Amides through Rhodium-Catalyzed Carbonylation of Nitrene Intermediates

Amide bond formation is one of the most important reactions in organic chemistry because of the widespread presence of amides in pharmaceuticals and biologically active compounds. Existing methods for amides synthesis are reaching their inherent limits. Described herein is a novel rhodium-catalyzed three-component reaction to synthesize amides from organic azides, carbon monoxide, and (hetero)arenes via nitrene-intermediates and direct C-H functionalization. Notably, the reaction proceeds in an intermolecular fashion with N₂ as the only by-product, and either directing groups nor additives are required. The computational and mechanistic studies show that the amides are formed via a key Rh-nitrene intermediate.

Interconversion of Phosphinyl Radical and Phosphinidene Complexes by Proton Coupled Electron Transfer

The isolable complex [Os(PHMes*)H(PNP)] (Mes*=2,4,6-t Bu3 C6 H3 ; PNP=N{CHCHPt Bu2 }2 ) exhibits high phosphinyl radical character. This compound offers access to the phosphinidene complex [Os(PMes*)H(PNP)] by P-H proton coupled electron transfer (PCET). The P-H bond dissociation energy (BDE) was determined by isothermal titration calorimetry and supporting DFT computations. The phosphinidene product exhibits electrophilic reactivity as demonstrated by intramolecular C-H activation.

Iridium-Catalyzed C-H Amination/Cyclization for Medium to Large N-Heterocycle-Fused Dihydroquinazolinones

A practical iridium-catalyzed cascade/stepwise synthesis of dihydroquinazolinones (DHQs) and bis-DHQs fused to medium to large N-heterocyclic rings is developed. The reaction undergoes benzamide-directed intermolecular C-H amination with an aldehyde-tethered alkyl azide, and then the newly installed amino group undergoes intramolecular cyclization with a remote aldehyde group present in azide and amidyl group of benzamide either intrinsically or catalyzed by phosphoric acid, facilitating the formation of bicyclic and/or tricyclic rings in a very efficient manner.

Intermolecular Radical C(sp3 )-H Amination under Iodine Catalysis

The direct amination of aliphatic C-H bonds has remained one of the most tantalizing transformations in organic chemistry. Herein, we report on a unique catalyst system, which enables the elusive intermolecular C(sp³ )-H amination. This practical synthetic strategy provides access to aminated building blocks and fosters innovative multiple C-H amination within a new approach to aminated heterocycles. The synthetic utility is demonstrated by the synthesis of four relevant pharmaceuticals.

Intermolecular, Branch-Selective, and Redox-Neutral Cp*IrIII -Catalyzed Allylic C-H Amidation

Herein, we report the redox-neutral, intermolecular, and highly branch-selective amidation of allylic C-H bonds enabled by Cp*Ir^III catalysis. A variety of readily available carboxylic acids were converted into the corresponding dioxazolones and efficiently coupled with terminal and internal olefins in high yields and selectivities. Mechanistic investigations support the formation of a nucleophilic Ir^III -allyl intermediate rather than the direct insertion of an Ir-nitrenoid species into the allylic C-H bond.

Iron-Catalyzed Intramolecular Amination of Aliphatic C-H Bonds of Sulfamate Esters with High Reactivity and Chemoselectivity

It is challenging to develop simple and low cost catalytic systems while maintaining high reactivity and selectivity. An iron-catalyzed intramolecular C-H amination of sulfamate esters using simple and cheap ligands is reported with general substrate scope (31 examples, up to 95% yield). The addition of second ligand, bipyridine, is able to accelerate the reaction and increase the yield. The ready availability of these iron catalysts provides a promising approach to selective introduction of nitrogen into hydrocarbon feedstock.

Rh(II)-Catalyzed Nitrene-Transfer [5 + 1] Cycloadditions of Aryl-Substituted Vinylcyclopropanes

Formal [5 + 1] cycloadditions between aryl-substituted vinylcyclopropanes and nitrenoid precursors are reported. The method, which employs Rh₂(esp)₂ as a catalyst, leads to the highly regioselective formation of substituted tetrahydropyridines. Preliminary mechanistic studies support a stepwise, polar mechanism enabled by the previously observed Lewis acidity of Rh-nitrenoids. Overall, this work expands the application of nitrene-transfer cycloaddition, a relatively underexplored approach to heterocycle synthesis, to the formation of six-membered rings.

Site-selective nitrenoid insertions utilizing postfunctionalized bifunctional rhodium(ii) catalysts

We report a new strategy for the preparation of dirhodium(ii) complexes with the general formula Rh2(A)4 that allows the isolation of a dirhodium tetracarboxylate complex with a free amino group available for postfunctionalization. The postfunctionalization of this complex enables the incorporation of a variety of functional groups, including double and triple bonds as well as nucleophilic moieties, thus paving the way to new classes of polymeric as well as bifunctional catalysts, and polymetallic complexes. Furthermore, we demonstrate that a urea containing dirhodium(ii) complex enables site-selective nitrenoid insertions by remote hydrogen bonding control.

Chemoselective Intramolecular Formal Insertion Reaction of Rh-Nitrenes into an Amide Bond Over C-H Insertion

The past few decades have witnessed extensive efforts to disclose the unique reactivity of metal-nitrenes, because they could be a powerful synthetic tool for introducing the amine functionality into unactivated chemical bonds. The reactivity of metal-nitrenes, however, is currently mainly confined to aziridination (an insertion into a C=C bond) and C-H amination (an insertion into a C-H bond). Nitrene insertion into an amide C-N bond, however, has not been reported so far. In this work we have developed a rhodium-catalyzed one-nitrogen insertion into amide C-N and sulfonamide S-N bonds. Experimental and theoretical analyses based on density functional theory indicate that the formal amide insertion proceeds via a rhodium-coordinated ammonium ylide formed between the nitrene and the amide nitrogen, followed by acyl group transfer concomitant with C-N bond cleavage. Mechanistic studies have allowed rationalization of the origin of the chemoselectivity observed between the C-H and amide insertion reactions. The methodology presented herein is the first example of an insertion of nitrene into amide bonds and provides facile access to unique diazacyclic systems with an N-N bond linkage.

Enantioselective intramolecular C-H amination of aliphatic azides by dual ruthenium and phosphine catalysis

By combining a chiral-at-metal ruthenium catalyst with catalytic amounts of tris(p-fluorophenyl)phosphine (both 1 mol%), the challenging catalytic enantioselective ring-closing C(sp³)-H amination of unactivated aliphatic azides has been achieved with high enantioselectivities.

The catalytic enantioselective intramolecular C(sp³)-H amination of aliphatic azides represents an efficient method for constructing chiral saturated cyclic amines which constitute a prominent structural motif in bioactive compounds. We report a dual catalytic system involving a chiral-at-metal bis(pyridyl-NHC) ruthenium complex and tris(4-fluorophenyl)phosphine (both 1 mol%), which facilitates the cyclization of aliphatic azides to chiral α-aryl pyrrolidines with enantioselectivities of up to 99% ee, including a pyrrolidine which can be converted to the anti-tumor alkaloid (R)-(+)-crispine. Mechanistically, the phosphine activates the organic azide to form an intermediate iminophosphorane and transfers the nitrene unit to the ruthenium providing an imido ruthenium intermediate which engages in the highly stereocontrolled C–H amination. This dual catalysis combines ruthenium catalysis with the Staudinger reaction and provides a novel strategy for catalyzing enantioselective C–H aminations of unactivated aliphatic azides.

Copper-catalyzed nitrene transfer/cyclization cascade to synthesize 3a-nitrogenous furoindolines and pyrroloindolines

Copper-catalyzed nitrene transfer for amination/cyclization of tryptophols and tryptamines to generate the corresponding indole alkaloids in good to excellent yields was successfully developed.

Mechanistic studies for dirhodium-catalyzed chemoselective oxidative amination of alkynyl-tethered sulfamates

Theoretical studies revealed the detailed mechanisms for dirhodium-datalyzed chemoselective oxidative amination of alkynyl-tethered sulfamates and account for the origin of product selectivity.

Computational insights into different chemoselectivities in Rh2(ii)-catalyzed N-aryl nitrene and analogous Rh2(ii)/Cu(i)-catalyzed aryl-substituted carbene involving reactions

Computational studies were carried out to investigate the mechanisms and the origins of chemoselectivities among multiple reactive sites for Rh₂(ii)-catalyzed N-aryl nitrene and analogous Rh₂(ii)/Cu(i)-catalyzed aryl carbene involving reactions.

Recent Progress on Cyclic Nitrenoid Precursors in Transition-Metal-Catalyzed Nitrene-Transfer Reactions

Nitrene-transfer reactions are powerful synthetic tools for the direct incorporation of nitrogen atoms into organic molecules. The discovery of novel nitrene-transfer reactions has been dominantly supported not only by improvements in transition-metal catalysts but also by the employment of novel precursors of nitrenoids. Since pioneering work involving the use of organic azides and iminoiodinanes as practical synthetic tools for nitrogen-containing compounds was reported, a new approach using various N-heterocycles containing strain energy or a weak bond has emerged. In this review, we briefly summarize the history of nitrene-transfer chemistry from the viewpoint of its precursors. In particular, the use of N-heterocycles such as 2H-azirines, 1,4,2-dioxazol-5-ones, 1,2,4-oxadiazol-5-ones, isoxazol-5(4H)-ones, and isoxazoles is comprehensively described, showing the recent remarkable progress in this chemistry.

Investigation of transition metal-catalyzed nitrene transfer reactions in water

Transition metal-catalyzed nitrene transfer is a powerful method for incorporating new CN bonds into relatively unfunctionalized scaffolds. In this communication, we report the first examples of site- and chemoselective CH bond amination reactions in aqueous media. The unexpected ability to employ water as the solvent in these reactions is advantageous in that it eliminates toxic solvent use and enables reactions to be run at increased concentrations with lower oxidant loadings. Using water as the reaction medium has potential to expand the scope of nitrene transfer to encompass a variety of biomolecules and highly polar substrates, as well as enable pH control over the site-selectivity of CH bond amination.

Ir(III)-Catalyzed Stereoselective Haloamidation of Alkynes Enabled by Ligand Participation

Described herein is the application of a strategy of ligand participation for the Ir-catalyzed imido transfer into alkynes. On the basis of a stoichiometric [3 + 2] cycloaddition of Cp*Ir(III)(κ²- N, O-chelate) with alkynyl dioxazolone, a catalytic haloamidation was developed for the first time by employing [Cp*IrCl₂]₂ precatalyst and NaX salts (X = Cl or Br) as practical halide sources to furnish synthetically versatile Z-(halovinyl)lactams with excellent stereoselectivity.

Iridium-Catalyzed Unreactive C(sp3)-H Amination with 2,2,2-Trichloroethoxycarbonyl Azide

An additive-assisted iridium-catalyzed directed C(sp³)-H amination with 2,2,2-trichloroethoxycarbonyl azide as an amino source is reported. Both carboxylate anions and the corresponding cations in the additives are crucial to achieve satisfactory efficiency. Sodium acetate or n-pentanoic acid can promote the amination of various primary C(sp³)-H bonds adjacent to secondary, tertiary, and quaternary carbons in ketoximes or N-aromatic heterocycles, respectively, providing a practical route to versatile β-amino ketoxime and N-heteroaryl ethanamine derivatives. The amination products can be treated as isocyanate analogues and can be converted to other useful amino functionalities. An iridacyclic compound was isolated and identified as a plausible intermediate.

An iridium(iii/iv/v) redox series featuring a terminal imido complex with triplet ground state

The iridium(iii/iv/v) imido redox series [Ir(NtBu){N(CHCHPtBu2)2}]0/+/2+ was synthesized and examined spectroscopically, magnetically, crystallographically and computationally. The monocationic iridium(iv) imide exhibits an electronic doublet ground state with considerable 'imidyl' character as a result of covalent Ir-NtBu bonding. Reduction gives the neutral imide [Ir(NtBu){N(CHCHPtBu2)2}] as the first example of an iridium complex with a triplet ground state. Its reactivity with respect to nitrene transfer to selected electrophiles (CO2) and nucleophiles (PMe3), respectively, is reported.

Selective formation of γ-lactams via C-H amidation enabled by tailored iridium catalysts

Intramolecular insertion of metal nitrenes into carbon-hydrogen bonds to form γ-lactam rings has traditionally been hindered by competing isocyanate formation. We report the application of theory and mechanism studies to optimize a class of pentamethylcyclopentadienyl iridium(III) catalysts for suppression of this competing pathway. Modulation of the stereoelectronic properties of the auxiliary bidentate ligands to be more electron-donating was suggested by density functional theory calculations to lower the C-H insertion barrier favoring the desired reaction. These catalysts transform a wide range of 1,4,2-dioxazol-5-ones, carbonylnitrene precursors easily accessible from carboxylic acids, into the corresponding γ-lactams via sp³ and sp² C-H amidation with exceptional selectivity. The power of this method was further demonstrated by the successful late-stage functionalization of amino acid derivatives and other bioactive molecules.

Manganese-catalysed benzylic C(sp3)-H amination for late-stage functionalization

Reactions that directly install nitrogen into C-H bonds of complex molecules are significant because of their potential to change the chemical and biological properties of a given compound. Although selective intramolecular C-H amination reactions are known, achieving high levels of reactivity while maintaining excellent site selectivity and functional-group tolerance remains a challenge for intermolecular C-H amination. Here, we report a manganese perchlorophthalocyanine catalyst [MnIII(ClPc)] for intermolecular benzylic C-H amination of bioactive molecules and natural products that proceeds with unprecedented levels of reactivity and site selectivity. In the presence of a Brønsted or Lewis acid, the [MnIII(ClPc)]-catalysed C-H amination demonstrates unique tolerance for tertiary amine, pyridine and benzimidazole functionalities. Mechanistic studies suggest that C-H amination likely proceeds through an electrophilic metallonitrene intermediate via a stepwise pathway where C-H cleavage is the rate-determining step of the reaction. Collectively, these mechanistic features contrast with previous base-metal-catalysed C-H aminations and provide new opportunities for tunable selectivities.

Palladium-catalyzed intermolecular amination of unactivated C(sp3)-H bonds via a cleavable directing group

Palladium-catalyzed intermolecular amination of unactivated C(sp³)-H bonds was developed. Using NFSI as both the amino source and the oxidant, this protocol operates under mild conditions with excellent terminal selectivity and a broad substrate scope. Moreover, the directing group can be easily removed to produce 1,2-amino alcohols.