Nickel-Catalyzed Addition of C-C Bonds of Amides to Strained Alkenes: The 1,2-Carboaminocarbonylation Reaction

Palladium-catalyzed intermolecular asymmetric dearomatizative arylation of indoles and benzofurans

Indoles represent one of the most robust and synthetically versatile classes of heteroaromatic compounds. However, the stereoselective conversion of planar indole rings into three-dimensional indoline skeletons bearing multiple stereogenic centers remains a persistent challenge in organic synthesis. Herein, we describe an intermolecular catalytic asymmetric dearomatization of simple indoles via a palladium-catalyzed three-component cross-coupling reaction. By using readily accessible diazonium salts and aromatic boronic acids as arylative reagents under a ligand-swap strategy, this method enables the efficient construction of 2,3-diarylated indolines. Mechanistic studies reveal that the chiral BiOx ligand governs the highly stereoselective migratory insertion of the aryl-palladium intermediate into the indole's C═C double bond with complete diastereo- and regioselectivity, whereas the achiral fumarate ligand facilitates the reductive elimination step, as corroborated by density functional theory calculations. Furthermore, this protocol is extended to the dearomative diarylation of benzofurans, affording chiral 2,3-dihydrobenzofuran derivatives with high stereocontrol.

Skeletal Rearrangements of Amides via Breaking Inert Bonds

Skeletal rearrangements of amides provide rapid access to complex nitrogen-containing scaffolds from simple readily available starting materials. While classical reactions such as the Hofmann and Curtius rearrangements have been widely utilized in organic synthesis, recent advances in amide activation strategies have brought new types of transformations and offered many new applications. This review focuses on the development of amide skeletal rearrangement reactions over the past two decades. The content is organized based on the initial bond cleavage pathways: C─N bond cleavage, C─C bond cleavage, and C═O bond activation.

Photocatalytic Pyridyl-carbamoylation of Alkenes for Accessing β-Pyridyl Amides

The β-pyridyl amide is a critical scaffold in medical discovery yet lacks efficient synthetic methods. Here, we describe, for the first time, a visible-light-induced, redox-neutral radical cross-coupling reaction involving alkenes, oxamic acids, and cyanopyridines that offers a versatile assembly of β-pyridylamides. This approach features mild reaction conditions, high step efficiency, and substrate breadth, providing a green and efficient strategy for alkene pyridyl-carbamoylation. Achieving this transformation relies on the efficient catalytic system, which adeptly avoids the competing cross-coupling of the nucleophilic carbamoyl radical with the electrophilic pyridyl radical, enabling the three-component radical tandem reaction process with high chemoselectivity.

Skeletal Modification via Activation of Relatively Unstrained C-C Bonds

ConspectusMethods that can directly modify the skeletons of complex molecules have become increasingly attractive for preparing novel analogues without the need for de novo synthesis in drug discovery processes. Among the various skeletal modification approaches, those targeting unstrained C-C bonds are particularly challenging to realize, owing to the relative inertness of these bonds toward common reagents. Compared to C-H or C-X (X: heteroatom) bonds, the activation of unstrained C-C bonds is often not thermodynamically and/or kinetically favorable. As a result, strategies relying on highly strained substrates or oxidative conditions are generally employed, which inevitably limit the scope and applications of C-C bond activation reactions. Hence, the development of redox-neutral catalytic C-C activation methods remains highly sought after for late-stage skeletal modification of complex bioactive compounds.In this Account, we summarize our recent progress in skeletal modifications through the catalytic activation of relatively unstrained C-C bonds. Enabled by transient or removable directing groups (DGs), the scope of C-C bond activation can be greatly expanded, encompassing a wide range of substrates, including ketones, amides, lactams, and biaryls. Consequently, different types of skeletal modification transformations have been developed. The major topics covered include the following: (1) Skeletal rearrangement and "cut-and-sew" transformations of cyclic ketones: we developed an aminopyridine/Rh-N-heterocyclic carbene (NHC) cooperative catalysis system that specifically targets the α-C-C bond of cyclic ketones. For substrates bearing a β-aryl substitution, the rhodacycle formed after the C-C bond activation can undergo an intramolecular C-H activation, resulting in the skeletal rearrangement from cyclopentanones/cyclohexanones to 1-tetralones/1-indanones. Additionally, the "cut-and-sew" transformations between indanones and ethylene or alkynes have been realized to offer a two-carbon ring expansion. (2) Chain homologation of linear amides and downsizing of lactams: the Rh-NHC activation system can be extended to the linear amides and lactams through preinstalling removable DGs. This approach has provided some new tools for precise amide modifications, including tunable homologation of tertiary amides via a "hook-and-slide" strategy and the downsizing transformation of lactams. (3) "Cut-and-sew" transformations of biphenols: using the preinstalled phosphinite DGs, unstrained 2,2'-biphenols can undergo split cross-coupling with various aryl iodides. When diiodide coupling partners are used, an interesting phenylene insertion into the aryl-aryl bond of biphenols can be achieved, which represents another type of "cut-and-sew" transformation.Collectively, these methods provide a reliable means to manipulate inert molecular scaffolds and offer new bond-disconnecting strategies to access useful structural motifs. The applications of these methods in the synthesis of bioactive natural products and complex analogues underscore their practical significance. Mechanistic insights gained from these studies are also discussed, which are expected to inspire future endeavors in this field.

Synthesis of α-Quaternary Amides via Cp*Co(III)-Catalyzed Sequential C-H Bond Addition to 1,3-Dienes and Isocyanates

The synthesis of sterically congested amides was accomplished via Cp*Co(III)-catalyzed sequential C-H bond addition to 1,3-dienes followed by aminocarbonylation with isocyanates, a coupling partner that had never been utilized in sequential C-H bond addition reactions. A variety of C-H bond reactants, internally substituted dienes, and aromatic isocyanates provided secondary amide products incorporating α-all-carbon quaternary centers. The conversion of the amide products to other useful compound classes was also demonstrated.

Visible light-mediated organocatalyzed 1,3-aminoacylation of cyclopropane employing N-benzoyl saccharin as bifunctional reagent

The carboamination of unsaturated molecules using bifunctional reagents is considered an attractive approach for the synthesis of nitrogen-containing compounds. However, bifunctional C-N reagents have never been employed in the carboamination of cyclopropane. In this study, we use an N-heterocyclic carbene (NHC), N-benzoyl saccharin, as a bifunctional reagent and a photoredox catalyst for a dual-catalyzed 1,3-aminoacylation of cyclopropane. NHCs play multiple roles, functioning as Lewis base catalysts to activate C-N bonds, promoting the oxidative quenching process of PC*, and acting as efficient acyl radical transfer catalysts for the formation of C-C bonds. The oxidative quenching process between the excited-state PC* and acyl NHC adduct is the key to the photooxidation generality of aryl cyclopropanes.

KOtBu-Catalyzed Synthesis of Isoindole Derivatives from N-Benzyl Amides Bearing an Ortho Alkynyl Group via 1,2-Acyl Shift

In this study, we present the catalytic conversion of benzylamides featuring an ortho alkynyl moiety into 1-acylisoindole derivatives via a 1,2-acyl shift. Remarkably, this transformation proceeds without the need for transition-metal catalysts; instead, KOtBu alone serves as the catalyst. This method enables the efficient synthesis of isoindoles from easily accessible amides with an atom economy of 100%.

Computational Exploration of 1,2-Carboamine Carbonylation Catalyzed by Nickel

Nickel-catalyzed carbonylation of alkenes is a stereoselective and regioselective method for the synthesis of amide compounds. Theoretical predictions with density functional theory calculations revealed the mechanism and origin of stereoselectivity and regioselectivity for the nickel-catalyzed carbonylation of norbornene. The carbonylation reaction proceeds through oxidative addition, migration insertion of alkenes, and subsequent reduction elimination to afford cis-carbonylation product. The C-N bond activation of amides is unfavorable because the oxidative addition ability of the C-C bond is stronger than that of the C-N bond. The determining step of stereoselectivity is the migratory insertion of the strained olefin. The structural analysis shows that steroselectivity is controlled by the steric hindrance of methyl groups to olefins and substituents to IMes in ligands.

Rhodium catalyzed tunable amide homologation through a hook-and-slide strategy

Preparation of diverse homologs from lead compounds has been a common and important practice in medicinal chemistry. However, homologation of carboxylic acid derivatives, particularly amides, remains challenging. Here we report a hook-and-slide strategy for homologation of tertiary amides with tunable lengths of the inserted carbon chain. Alkylation at the α-position of the amide (hook) is followed by highly selective branched-to-linear isomerization (slide) to effect amide migration to the end of the newly introduced alkyl chain; thus, the choice of alkylation reagent sets the homologation length. The key step involves a carbon-carbon bond activation process by a carbene-coordinated rhodium complex with assistance from a removable directing group. The approach is demonstrated for introduction of chains as long as 16 carbons and is applicable to derivatized carboxylic acids in complex bioactive molecules.

Distinguishing Competing Mechanistic Manifolds for C(acyl)-N Functionalization by a Ni/N-Heterocyclic Carbene Catalyst System

Carboxylic acid derivatives are appealing alternatives to organohalides as cross-coupling electrophiles for fine chemical synthesis due to their prevalence in biomass and bioactive small molecules as well as their ease of preparation and handling. Within this family, carboxamides comprise a versatile electrophile class for nickel-catalyzed coupling with carbon and heteroatom nucleophiles. However, even state-of-the-art C(acyl)-N functionalization and cross-coupling reactions typically require high catalyst loadings and specific substitution patterns. These challenges have proven difficult to overcome, in large part due to limited experimental mechanistic insight. In this work, we describe a detailed mechanistic case study of acylative coupling reactions catalyzed by the commonly employed Ni/SIPr catalyst system (SIPr = 1,3-bis(2,6-di-isopropylphenyl)-4,5-dihydroimidazol-2-ylidine). Stoichiometric organometallic studies, in situ spectroscopic measurements, and crossover experiments demonstrate the accessibility of Ni(0), Ni(I), and Ni(II) resting states. Although in situ precatalyst activation limits reaction efficiency, the low concentrations of active, SIPr-supported Ni(0) select for electrophile-first (closed-shell) over competing nucleophile-first (open-shell) mechanistic manifolds. We anticipate that the experimental insights into the nature and controlling features of these distinct pathways will accelerate rational improvements to cross-coupling methodologies involving pervasive carboxamide substrate motifs.

Nickel/Photoredox Dual-Catalyzed Conversion of Allyl Esters to Ketones via the Formal Deletion of Oxygen

We report herein the catalytic conversion of allylic esters into the corresponding ketones by the formal deletion of an oxygen atom. The key to the success of the reaction is the dual use of nickel and photoredox catalysts; the former mediates C-O bond activation and C-C bond formation, while the latter is responsible for deoxygenation of the acyloxy group using PPh3 as a stoichiometric reductant. Catalytic replacement of an oxygen atom of an allyl ester with a tethered alkene is also accomplished.

Transition-metal-catalyzed domino reactions of strained bicyclic alkenes

This review presents a comprehensive overview of transition-metal-catalyzed domino reactions of strained bicyclic alkenes, including both homo- and heterobicyclic alkenes. These compounds are important synthons in organic synthesis, providing an important platform for the construction of biologically/medicinally significant compounds which bear multiple stereocenters. The review has been divided according to the metal used in the reaction. An overview of the substrate scope, reaction conditions, and their potential applications in organic synthesis is discussed. A comprehensive outlook on the reactivity paradigms of homo- and heterobicyclic alkenes is discussed and should shed light on future directions for further development in this field.

Amide N-C Bond Activation: A Graphical Overview of Acyl and Decarbonylative Coupling

This Graphical Review provides an overview of amide bond activation achieved by selective oxidative addition of the N-C(O) acyl bond to transition metals and nucleophilic acyl addition, resulting in acyl and decarbonylative coupling together with key mechanistic details pertaining to amide bond distortion underlying this reactivity manifold.

Palladium-Catalyzed Difunctionalization of Norbornenes via Arylation and Alkynylation

We report the first general and practical method for the addition of aryl halides and alkynes to norbornenes with palladium catalysis. Norbornenes have been used as the unsaturated acceptors of aryl and alkynyl groups to construct saturated bridged C-C bonds. The combination of Pd(OAc)₂/PCy₃HBF₄ has been identified as the optimal system promoting difunctionalization of norbornenes via the C-X/C-H bond cleavage and highly selective C(sp³)-C(sp²)/C(sp³)-C(sp) bond formation. Broad substrate scope and excellent functional group tolerance have been achieved to show the high efficiency of this approach. Mechanism studies based on experiments and DFT have been performed to gain insights into the catalytic mechanism.

Metal-free hypervalent iodine-promoted tandem carbonyl migration and unactivated C(Ph)-C(Alkyl) bond cleavage for quinolone scaffold synthesis

An unexpected iodine(III)-mediated C(sp³)-C(sp²) bond cleavage of 3-(methylamino)-2-(2-substitutedbenzoyl)acrylates for efficient synthesis of privileged scaffold 4-quinolones was described. Notably, a wide range of alkyl groups (e.g. methyl, tert-butyl or alkyl chain) can be conveniently cleaved in this system. The detailed mechanism studies revealed that the transformation proceeded through cascade ipso-cyclization and 1,2-carbonyl migration, the smaller bond energy determined ortho C-C bond cleavage rather than C-H bond cleavage, via an enamine radical intermediate.

Nickel-catalyzed 1,4-aryl rearrangement of aryl N-benzylimidates via C-O and C-H bond cleavage

We report herein that nickel-catalyzed reaction of aryl imidates bearing an N-benzyl group results in 1,4-migration of an O-aryl group via the cleavage of C-O and C-H bonds. This protocol allows for the benzylic C-H bond arylation of benzylamine building blocks using phenols as an aryl source to form elaborate diarylmethylamine derivatives.