Skeletal metalation of lactams through a carbonyl-to-nickel-exchange logic

Visible Light-Induced Single-Atom Insertion of Indenes via Aerobic Ring Scission-Condensation-Rearomatization

In this study, we present a photocatalyzed single-atom insertion of indenes, involving an aerobic ring scission into dicarbonyl intermediates, which subsequently undergo condensation and rearomatization to efficiently synthesize isoquinoline and naphthalene derivatives. The use of an inexpensive organic dye as the photocatalyst under aerobic conditions with cheap ammonium acetate (NH4OAc) as the nitrogen source makes this method very practical and environmentally friendly to access isoquinoline. Alternatively, an intramolecular carbon-atom-insertion process, involving the Aldol reaction of the dicarbonyl intermediates, affords the naphthalenamine and naphthalen-2-ol derivatives. Mechanistic studies support that the superoxide anion radical species mediates the C═C double bond scission of indenes rather than the singlet oxygen intermediate.

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.

Indole-Quinoline Transmutation Enabled by a Formal Rhodium Carbynoid

Skeleton editing is an emerging and powerful tool in organic chemistry because it can simplify synthetic procedures towards complex molecules. Herein, we present an approach for indole-quinoline transmutation through rhodium-catalyzed single-carbon insertion into the C2(sp2)─C3(sp2) bond of an indole with an α-diazotrifluoroethyl sulfonium salt that we developed. This protocol involves a formal trifluoromethyl rhodium carbynoid (CF3C+ = Rh) resembling a trifluoromethyl cationic carbyne (CF3C+:), allowing facile access to an array of quinolines in moderate to high yields. Potential applications in the late-stage skeletal editing of pharmaceutical and natural product derivatives, preparation of adapalene analogs, scaled-up synthesis, and transformations of products are highlighted. Finally, a computational study was conducted to support the envisioned mechanism.

Direct Conversion of Aromatic Lactones into Bioisosteres by Carbonyl-to-Boranol Exchange

Bioisosteric replacement is an important strategy in drug discovery and is commonly practiced in medicinal chemistry; however, the incorporation of bioisosteres typically requires laborious multistep de novo synthesis. The direct conversion of a functional group into its corresponding bioisostere is of particular significance in evaluating structure-property relationships. Herein, we report a functional-group-exchange strategy that enables the direct conversion of aromatic lactones, a prevalent motif in bioactive molecules, into their corresponding cyclic hemiboronic acid bioisosteres. Scope evaluation and product derivatization experiments demonstrate the synthetic value and broad functional-group compatibility of this strategy, while the application of this methodology to the rapid remodeling of chromenone cores in bioactive molecules highlights its utility.

Cheminformatic Analysis of Core-Atom Transformations in Pharmaceutically Relevant Heteroaromatics

Heteroaromatics are the basis for many pharmaceuticals. The ability to modify these structures through selective core-atom transformations, or "skeletal edits", can dramatically expand the landscape for drug discovery and development. However, despite the importance of core-atom modifications, the quantitative impact of such transformations on accessible chemical space remains undefined. Here, we report a cheminformatic platform to analyze which skeletal edits would most increase access to novel chemical space. This study underscores the significance of emerging single and multiple core-atom transformations of heteroaromatics in enhancing chemical diversity, for example, at a late-stage of a drug discovery campaign. Our findings provide a quantitative framework for prioritizing core-atom modifications in heteroaromatic structural motifs, calling for the development of new methods to achieve these types of transformations.

Asymmetric dearomative single-atom skeletal editing of indoles and pyrroles

Heterocycle skeletal editing has recently emerged as a powerful tactic for achieving heterocycle-to-heterocycle transmutation without the need for multistep de novo heterocycle synthesis. However, the enantioselective skeletal editing of heteroarenes through single-atom logic remains challenging. Here we report the enantiodivergent dearomative skeletal editing of indoles and pyrroles via an asymmetric carbon-atom insertion, using trifluoromethyl N-triftosylhydrazones as carbene precursors. This strategy provides a straightforward methodology to access enantiomerically enriched six-membered N-heterocycles containing a trifluoromethylated quaternary stereocentre from planar N-heteroarenes. The synthetic utility of this enantiodivergent methodology was demonstrated by a broad evaluation of reaction scope, product derivatization and concise syntheses of drug analogues. Mechanistic studies reveal that the excellent asymmetric induction arises from the initial cyclopropanation step. The asymmetric single-atom insertion strategy is expected to have a broad impact on the field of single-atom skeletal editing and catalytic asymmetric dearomatization of aromatic compounds.

Mechanistic insights on C(acyl)-N functionalisation mediated by late transition metals

The carboxamide functional group has a privileged role in organic and biological chemistry due to its prevalence and utility across synthetic and natural products. Due to nN → π*CO delocalisation, amides and related functional groups are typically kinetically resistant to degradation. Nonetheless, over the past decade, transition metal catalysis has transformed our ability to utilise molecules featuring C(acyl)-N units as reactants. Alongside the burgeoning catalytic applications ranging from COx utilisation to small molecule synthesis, elucidation of the underlying mechanisms remains a critical ongoing effort. Herein, we aggregate and analyse current understanding of the mechanisms for C(acyl)-N functionalisation of amides and related functional groups with a focus on recent developments involving mechanisms unique to the late transition metals. Discussion is organized around three general mechanistic manifolds: redox-neutral mechanisms, 2e- redox-cycling mechanisms, and mechanisms involving 1e- redox steps. For each class, we focus on reactions that directly involve a transition metal mediator/catalyst in the C(acyl)-N cleavage step. We conclude with an outlook on the outstanding ambiguities and opportunities for innovation.

Oxoammonium-Catalyzed Oxidation of N-Substituted Amines

We report the development of oxoammonium-catalyzed oxidation of N-substituted amines via a hydride transfer mechanism. Steric and electronic tuning of catalyst led to complementary sets of conditions that can oxidize a broad scope of carbamates, sulfonamides, ureas, and amides into the corresponding imides. The reaction was further demonstrated on a 100-g scale using a continuous flow setup.

Halogencarbene-free Ciamician-Dennstedt single-atom skeletal editing

Single-atom skeletal editing is an increasingly powerful tool for scaffold hopping-based drug discovery. However, the insertion of a functionalized carbon atom into heteroarenes remains rare, especially when performed in complex chemical settings. Despite more than a century of research, Ciamician-Dennstedt (C-D) rearrangement remains limited to halocarbene precursors. Herein, we report a general methodology for the Ciamician-Dennstedt reaction using α-halogen-free carbenes generated in situ from N-triftosylhydrazones. This one-pot, two-step protocol enables the insertion of various carbenes, including those previously unexplored in C-D skeletal editing chemistry, into indoles/pyrroles scaffolds to access 3-functionalized quinolines/pyridines. Mechanistic studies reveal a pathway involving the intermediacy of a 1,4-dihydroquinoline intermediate, which could undergo oxidative aromatization or defluorinative aromatization to form different carbon-atom insertion products.

Molecular Editing of Ketones through N-Heterocyclic Carbene and Photo Dual Catalysis

The molecular editing of ketones represents an appealing strategy due to its ability to maximize the structural diversity of ketone compounds in a straightforward manner. However, developing efficient methods for the arbitrary modification of ketonic molecules, particularly those integrated within complex skeletons, remains a significant challenge. Herein, we present a unique strategy for ketone recasting that involves radical acylation of pre-functionalized ketones facilitated by N-heterocyclic carbene and photo dual catalysis. This protocol features excellent substrate tolerance and can be applied to the convergent synthesis and late-stage functionalization of structurally complex bioactive ketones. Mechanistic investigations, including experimental studies and density functional theory (DFT) calculations, shed light on the reaction mechanism and elucidate the basis of the regioselectivity.

Unimolecular Fragment Coupling: A New Bond-Forming Methodology via the Deletion of Atom(s)

Unimolecular fragment coupling (UFC) is defined as a reaction format, wherein atom(s) located in the middle of a molecule are extruded, and the remaining fragments are coupled. UFC is a potentially powerful strategy that is an alternative to transition-metal-catalyzed cross-coupling because the target chemical bond is formed in an intramolecular fashion, which is inherently beneficial for chemoselectivity and stereoselectivity issues. In this Perspective, we will present an overview of the recent advances in UFC reactions, which encompass those proceeding through the elimination of CO2, CO, SO2, isocyanates, N2, or single atoms primarily via transition metal catalysis.

Carbon-nitrogen transmutation in polycyclic arenol skeletons to access N-heteroarenes

Developing skeletal editing tools is not a trivial task, and realizing the corresponding single-atom transmutation in a ring system without altering the ring size is even more challenging. Here, we introduce a skeletal editing strategy that enables polycyclic arenols, a highly prevalent motif in bioactive molecules, to be readily converted into N-heteroarenes through carbon-nitrogen transmutation. The reaction features selective nitrogen insertion into the C-C bond of the arenol frameworks by azidative dearomatization and aryl migration, followed by ring-opening, and ring-closing (ANRORC) to achieve carbon-to-nitrogen transmutation in the aromatic framework of the arenol. Using widely available arenols as N-heteroarene precursors, this alternative approach allows the streamlined assembly of complex polycyclic heteroaromatics with broad functional group tolerance. Finally, pertinent transformations of the products, including synthesis complex biheteroarene skeletons, were conducted and exhibited significant potential in materials chemistry.

Nitrogen atom insertion into arenols to access benzazepines

Advances in site-selective molecular editing have enabled structural modification on complex molecules. However, thus far, their applications have been restricted to C-H functionalization chemistry. The modification of the underlying molecular skeleton remains limited. Here, we describe a skeletal editing approach that provides access to benzazepine structures through direct nitrogen atom insertion into arenols. Using widely available arenols as benzazepine precursors, this alternative approach allowed the streamlined assembly of benzazepines with broad functional group tolerance. Experimental mechanistic studies support a reaction pathway involving dearomatizative azidation and then aryl migration. This study further highlights the potential for carbon-nitrogen transmutation sequences through combinations with oxidative carbon atom deletion, providing an alternative for the development of N-heteroarenes and demonstrating significant potential in materials chemistry.

14N to 15N Isotopic Exchange of Nitrogen Heteroaromatics through Skeletal Editing

The selective modification of nitrogen heteroaromatics enables the development of new chemical tools and accelerates drug discovery. While methods that focus on expanding or contracting the skeletal structures of heteroaromatics are emerging, methods for the direct exchange of single core atoms remain limited. Here, we present a method for 14N → 15N isotopic exchange for several aromatic nitrogen heterocycles. This nitrogen isotope transmutation occurs through activation of the heteroaromatic substrate by triflylation of a nitrogen atom, followed by a ring-opening/ring-closure sequence mediated by 15N-aspartate to effect the isotopic exchange of the nitrogen atom. Key to the success of this transformation is the formation of an isolable 15N-succinyl intermediate, which undergoes elimination to give the isotopically labeled heterocycle. These transformations occur under mild conditions in high chemical and isotopic yields.

Ring Opening of N-Acyl Lactams Using Nickel-Catalyzed Transamidation

We successfully developed a nickel-catalyzed transamidation method for the ring opening of N-acyl lactams. The method involves a reaction between N-benzoylpyrrolidin-2-one derivatives and aniline derivatives, with Ni(PPh3)2Cl2 serving as the catalyst, 2,2'-bipyridine as the ligand, and manganese as the reducing agent. This reaction led to the formation of ring-opening-amidated products in good yields. Notably, the method exhibited excellent efficiency for producing the corresponding ring-opening transamidation products for various ring sizes, including four-, five-, six-, seven-, and eight-membered ring lactams.

One-Carbon Ring Expansion of Indoles and Pyrroles: A Straightforward Access to 3-Fluorinated Quinolines and Pyridines

3-Fluorinated quinolines and pyridines are prevalent pharmacophores, yet their synthesis is often challenging. Herein, we demonstrate that dibromofluoromethane as bromofluorocarbene source enables the one-carbon ring expansion of readily available indoles and pyrroles to structurally diverse 3-fluorinated quinolines and pyridines. This straightforward protocol requires only a short reaction time of ten minutes and can be performed under air atmosphere. Preliminary investigations reveal that this strategy can also be applied to the synthesis of other valuable azines by using different 1,1-dibromoalkanes as bromocarbene sources.