Stereodefined Access to Lactams via Olefin Difunctionalization: Iridium Nitrenoids as a Motif of LUMO-Controlled Dipoles

Chiral Osmium(II)/Salox Species Enabled Enantioselective γ-C(sp3)-H Amidation: Integrated Experimental and Computational Validation For the Ligand Design and Reaction Development

Herein, multiple types of chiral Os(II) complexes have been designed to address the appealing yet challenging asymmetric C(sp3)-H functionalization, among which the Os(II)/Salox species is found to be the most efficient for precise stereocontrol in realizing the asymmetric C(sp3)-H amidation. As exemplified by the enantioenriched pyrrolidinone synthesis, such tailored Os(II)/Salox catalyst efficiently enables an intramolecular site-/enantioselective C(sp3)-H amidation in the γ-position of dioxazolone substrates, in which benzyl, propargyl and allyl groups bearing various substituted forms are well compatible, affording the corresponding chiral γ-lactam products with good er values (up to 99 : 1) and diverse functionality (>35 examples). The unique performance advantage of the developed chiral Os(II)/Salox system in terms of the catalytic energy profile and the chiral induction has been further clarified by integrated experimental and computational studies.

Intramolecular Carboxyamidation of Alkyne-Tethered O-Acylhydroxamates through Formation of Fe(III)-Nitrenoids

We developed intramolecular carboxyamidations of alkyne-tethered O-acylhydroxamates followed by either thermally induced spontaneous or 4-(dimethylamino)pyridine-catalyzed O→O or O→N acyl group migration. Under iron-catalyzed conditions, the carboxyamidation products were generated in high yield from both Z-alkene and arene-tethered substrates. DFT calculations indicate that the iron-catalyzed carboxyamidation proceeds via a stepwise mechanism involving iron-imidyl radical cyclization followed by intramolecular acyloxy transfer from the iron center to the alkenyl radical center to furnish the cis-carboxyamidation product. Upon treatment with 4-(dimethylamino)pyridine, the Z-alkene-tethered carboxyamidation products underwent selective O→O acyl migration to generate 2-acyloxy-5-acyl pyrroles. Thermal O→N acyl migration occurs during carboxyamidation if the Z-alkene linker contains an alkyl or an aryl substituent at the β-position of the carbonyl group. On the other hand, the arene linker-containing compounds selectively undergo O→N acyl migration to generate N-acyl-3-acylisoindolinones, and the corresponding O→O acyl migration forming isoindole derivatives was not observed.

Iridium Acylnitrenoid-Initiated Biomimetic Cascade Cyclizations: Stereodefined Access to Polycyclic δ-Lactams

Ring-fused azacyclic compounds are important building units in the synthesis of biorelevant natural products, pharmaceutical agents, and molecular materials. Herein, we present a new approach to these condensed azacycles by a biomimetic cascade cyclization of arylalkenyl dioxazolones. This cascade reaction was found to proceed with excellent stereoselectivity and a high functional group tolerance. The substrate scope of arylalkenyl dioxazolones turned out to be highly flexible and extendable to additional terminating subunits, such as heteroaryl and alkynyl moieties. This biomimetic cyclization was elucidated to be initiated by an intramolecular transfer of the in situ generated electrophilic Ir-acylnitrenoid to the tethered olefinic double bond, leading to a key N-acylaziridine intermediate, which is in turn reacted with pendant (hetero)arenes or alkynes in a highly regio- and stereoselective manner to produce ring-fused azacyclic compounds.

Nitrene-Mediated Enantioselective Intramolecular Olefin Oxyamination to Access Chiral γ-Aminomethyl-γ-Lactones

Attaching a nitrene precursor to an intramolecular nucleophile allows for a catalytic asymmetric intramolecular oxyamination of alkenes in which the nucleophile adds in an endocyclic position and the amine in an exocyclic fashion. Using chiral-at-ruthenium catalysts, chiral γ-aminomethyl-γ-lactones containing a quaternary carbon in γ-position are provided in high yields (up to 99 %) and with excellent enantioselectivities (up to 99 % ee). DFT calculations support the possibility of both a singlet (concerted oxyamination of the alkene) and triplet pathway (stepwise oxyamination) for the formation of the predominant stereoisomer. γ-Aminomethyl-γ-lactones are versatile chiral building blocks and can be converted to other heterocycles such as δ-lactams, 2-oxazolidinones, and tetrahydrofurans.

Mechanistic snapshots of rhodium-catalyzed acylnitrene transfer reactions

Rhodium acylnitrene complexes are widely implicated in catalytic C-H amidation reactions but have eluded isolation and structural characterization. To overcome this challenge, we designed a chromophoric octahedral rhodium complex with a bidentate dioxazolone ligand, in which photoinduced metal-to-ligand charge transfer initiates catalytic C-H amidation. X-ray photocrystallographic analysis of the Rh-dioxazolone complex allowed structural elucidation of the targeted Rh-acylnitrenoid and provided firm evidence that the singlet nitrenoid species is primarily responsible for acylamino transfer reactions. We also monitored in crystallo reaction of a nucleophile with the in situ generated Rh-acylnitrenoid, providing a crystallographically traceable reaction system to capture mechanistic snapshots of nitrenoid transfer.

Cycloisomerization of Alkyne-Tethered N-Acyloxycarbamates to 2-(3H)Oxazolones through Nitrenoid-Mediated Carboxyamidation

The cycloisomerization of alkyne-tethered N-benzoyloxycarbamates to 2-(3H)oxazolones is described. Two catalytic systems are tailored for intramolecular 5-exo-alkyne carboxyamidation and concomitant alkene isomerization. PtCl₂ /CO (5 mol%, toluene, 100 °C) promotes both carboxyamidation and alkene isomerization but has a limited substrate scope. On the other hand, FeCl₃ (5 mol%, CH₃ CN, 100 °C) promotes carboxyamidation effectively but a cocatalyst is required for the exocyclic alkene isomerization. Thus, a two-step one-pot protocol has been developed for a broader reaction scope, which involves FeCl₃ -catalyzed carboxyamidation and base-induced alkene isomerization. Crossover experiments suggest that these reactions proceed mainly through a mechanism involving acylnitrenoid intermediates rather than carbenoid intermediates.

Endo-Selective Intramolecular Alkyne Hydroamidation Enabled by NiH Catalysis Incorporating Alkenylnickel Isomerization

Intramolecular alkyne hydroamidation represents a straightforward approach for the access to synthetically valuable cyclic enamides. Despite some advances made in this realm, the ability to attain a precise regiocontrol still remains challenging, especially for endo cyclization that leads to six-membered and larger azacyclic rings. Herein, we report a NiH-catalyzed intramolecular hydroamidation of alkynyl dioxazolones that allows for an excellent endo selectivity, thus affording a range of six- to eight-membered endocyclic enamides with a broad scope. Mechanistic investigations revealed that Ni(I) catalysis is operative in the current system, proceeding via regioselective syn-hydronickelation, alkenylnickel E/Z isomerization, and Ni-centered inner-sphere nitrenoid transfer. In particular, the key alkenylnickel isomerization step, which previously lacked mechanistic understandings, was found to take place through the η²-vinyl transition state. The synthetic value of this protocol was demonstrated by diastereoselective modifications of the obtained endocyclic enamides to highly functionalized δ-lactam scaffolds.

Photoinduced Transition-Metal-Free Chan-Evans-Lam-Type Coupling: Dual Photoexcitation Mode with Halide Anion Effect

Herein, we report a photoinduced transition-metal-free C(aryl)-N bond formation between 2,4,6-tri(aryl)boroxines or arylboronic acids as an aryl source and 1,4,2-dioxazol-5-ones (dioxazolones) as an amide coupling partner. Chloride anion, either generated in situ by photodissociation of chlorinated solvent molecules or added separately as an additive, was found to play a critical cooperative role, thereby giving convenient access to a wide range of synthetically versatile N-arylamides under mild photo conditions. The synthetic virtue of this transition-metal-free Chan-Evans-Lam-type coupling was demonstrated by large-scale reactions, synthesis of 15N-labeled arylamides, and applicability toward biologically relevant compounds. On the basis of mechanistic investigations, two distinctive photoexcitations are proposed to function in the current process, in which the first excitation involving chloro-boron adduct facilitates the transition-metal-free activation of dioxazolones by single electron transfer (SET), and the second one enables the otherwise-inoperative 1,2-aryl migration of the thus-formed N-chloroamido-borate adduct.

Access to β-Lactams via Iron-Catalyzed Olefin Oxyamidation Enabled by the π-Accepting Phthalocyanine Ligand

Herein, we report the development of an iron-catalyzed olefin oxyamidation by utilizing tethered dioxazolones as the nitrenoid precursor to produce valuable β-lactam scaffolds. Mechanistic studies revealed that a relatively strong π-accepting ability of the phthalocyanine ligand is critical in generating the key triplet iron-imidyl radical intermediate to enable the 4-exo-trig-lactamization with the incorporation of oxygen nucleophiles in high diastereoselectivity. This cyclization approach was readily extended to the highly efficient γ-lactam synthesis (TON > 300).

Recent progress in the radical α-C(sp3)-H functionalization of ketones

The direct use structurally simple ketones as α-ketone radical sources for α-C(sp³)-H functionalization is a sustainable and powerful approach for constructing complex and multifunctional chemical scaffolds with diverse applications. The reactions of α-ketone radicals with alkenes, alkynes, enynes, imides, and imidazo[1,2-a]pyridines have broadened the structural diversity and complexity of ketones. Through chosen illustrative examples, we outline the recent progress in the development of methods that enable the radical α-C(sp³)-H functionalization of ketones, with an emphasis on radical initiation systems and possible mechanisms of the transformations. The application of these strategies is illustrated by the synthesis of several biologically active molecules and drug molecules. Further subdivision is based on substrate type and reaction type.

Mechanism-Guided Development of Transition-Metal-Catalyzed C-N Bond-Forming Reactions Using Dioxazolones as the Versatile Amidating Source

Catalytic reactions that construct carbon-nitrogen bonds are one of central themes in both synthetic and medicinal chemistry since the obtainable nitrogen-containing motifs are commonly encountered in natural products and have also seen a growing prominence as key structural features in marketed drugs and preclinical candidates. Pd-catalyzed cross-couplings, such as Buchwald-Hartwig amination, are at the forefront of such synthetic methods in practical settings. However, they require prefunctionalized substrates such as (hetero)aryl halides that must be prepared independently, often by multiple operations. One emerging way to circumvent these preparatory steps and directly convert ubiquitous C-H bonds into valuable C-N bonds is catalytic C-H amination, which allows synthetic chemists to devise shorter and more efficient retrosynthetic schemes. The past two decades have witnessed considerable progress in expanding the repertoire of this strategy, especially by identifying effective amino group precursors. In this context, dioxazolones have experienced a dramatic resurgence in recent years as a versatile nitrogen source in combination with transition-metal catalyst systems that facilitate decarboxylation to access key metal-acylnitrenoid intermediates. In addition to their high robustness and easy accessibility from abundant carboxylic acids, the unique reactivity of the transient intermediates in the amido group transfer has led to a fruitful journey for mild and efficient C-H amidation reactions.This Account summarizes our recent contributions to the development of C-N bond-forming reactions using dioxazolones as effective nitrenoid precursors, which are categorized into two subsets according to their mechanistic differences: inner- versus outer-sphere pathways. The first section describes how we could unveil the synthetic potential of dioxazolones in the realm of the inner-sphere C-H amidation, where we demonstrated that dioxazolones serve not only as manageable alternatives to acyl azides but also as highly efficient reagents to significantly reduce the catalyst loading and temperature. Taking advantage of the mild conditions in combination with group 9 Cp*M complexes (M = Rh, Ir, Co) or isoelectronic Ru species, we have dramatically expanded the accessible synthetic scope. Mechanistic investigations revealed that the putative metal-nitrenoid species is involved as a key intermediate during catalysis, which leads to facile C-N bond formation. On the basis of the mechanistic underpinning, we have succeeded in developing novel catalytic platforms that harness the intermediacy of metal-nitrenoids to explore C-H insertion chemistry via an outer-sphere pathway. Indeed, the tailored catalysts were capable of suppressing the competitive Curtius-type decomposition, thus granting access to versatile lactam products. We have further repurposed the catalytic systems upon modification of chelating ligands and also the identity of the transition metal to achieve three goals: (i) addressing selectivity issues to control the regio-, chemo-, and enantioselectivities, (ii) developing sustainable catalysis by first-low metals, and (iii) navigating chemical space for (di)functionalization of alkenes/alkynes. Together with our own research efforts, highlighted herein are some important relevant advances by other groups. We finally conclude with a brief overview with an eye toward further developments.

RuV -Acylimido Intermediate in [RuIV (Por)Cl2 ]-Catalyzed C-N Bond Formation: Spectroscopic Characterization, Reactivity, and Catalytic Reactions

Metal-catalyzed C-N bond formation reactions via acylnitrene transfer have recently attracted much attention, but direct detection of the proposed acylnitrenoid/acylimido M(NCOR) (R=aryl or alkyl) species in these reactions poses a formidable challenge. Herein, we report on Ru(NCOR) intermediates in C-N bond formation catalyzed by [Ru^IV (Por)Cl₂ ]/N₃ COR, a catalytic method applicable to aziridine/oxazoline formation from alkenes, amination of substituted indoles, α-amino ketone formation from silyl enol ethers, amination of C(sp³ )-H bonds, and functionalization of natural products and carbohydrate derivatives (up to 99 % yield). Experimental studies, including HR-ESI-MS and EPR measurements, coupled with DFT calculations, lend evidence for the formulation of the Ru(NCOR) acylnitrenoids as a Ru^V -imido species.

Photoredox β-thiol-α-carbonylation of enones accompanied by unexpected Csp2–C(CO) bond cleavage

An olefinic difunctionalization method of enones was presented here via aerobic visible-light catalysis.

Ligand Controlled Ir-Catalyzed Regiodivergent Oxyamination of Unactivated Alkenes

An intramolecular Ir(III)-catalyzed regiodivergent oxyamination of unactivated alkenes provides valuable γ-lactams, γ-lactones and δ-lactams. The regioselectivity is controlled by the electronically tunable cyclopentadienyl Ir(III)-complexes enabling oxyamination via either 5-exo or 6-endo pathways. With respect to the mechanism, we propose a highly reactive [3.1.0] bicycle intermediate derived from Ir(V) nitrene-mediated aziridination to be a key intermediate toward the synthesis of γ-lactams.

Cp*CoIII-catalyzed formal [4+2] cycloaddition of benzamides to afford quinazolinone derivatives

A Cp*Co^III-catalyzed arene C–H bond amidation/annulation of benzamides was developed to afford quinazolinone derivatives in one-pot with high yields and broad substrate scope.