Catalytic C–CN Bond Activation

Modern Strategies for Carbon Isotope Exchange

In contrast to stable and natural abundant carbon-12, the synthesis of organic molecules with carbon (radio)isotopes must be conceived and optimized in order to navigate through the hurdles of radiochemical requirements, such as high costs of the starting materials, harsh conditions and radioactive waste generation. In addition, it must initiate from the small cohort of available C-labeled building blocks. For long time, multi-step approaches have represented the sole available patterns. On the other side, the development of chemical reactions based on the reversible cleavage of C-C bonds might offer new opportunities and reshape retrosynthetic analysis in radiosynthesis. This review aims to provide a short survey on the recently emerged carbon isotope exchange technologies that provide effective opportunity for late-stage labeling. At present, such strategies have relied on the use of primary and easily accessible radiolabeled C1-building blocks, such as carbon dioxide, carbon monoxide and cyanides, while the activation principles have been based on thermal, photocatalytic, metal-catalyzed and biocatalytic processes.

Enantioselective Transfer Hydrogenative Cycloaddition Unlocks the Total Synthesis of SF2446 B3: An Aglycone of Arenimycin and SF2446 Type II Polyketide Antibiotics

The first total synthesis and structure validation of an arenimycin/SF2446 type II polyketide is described, as represented by de novo construction of SF2446 B3, the aglycone shared by this family of type II polyketides. Ruthenium-catalyzed α-ketol-benzocyclobutenone [4 + 2] cycloaddition, which occurs via successive stereoablation-stereoregeneration, affects a double dynamic kinetic asymmetric transformation wherein two racemic starting materials combine to form the congested angucycline bay region with control of regio-, diastereo-, and enantioselectivity. This work represents the first application of transfer hydrogenative cycloaddition and enantioselective intermolecular metal-catalyzed C-C bond activation in target-oriented synthesis.

Rhazinilam-leuconolam family of natural products: a half century of total synthesis

Covering: 1972 to 2021The rhazinilam family of natural products exhibits a main structure with a stereogenic quaternary carbon and a tetrahydroindolizine core imbedded within a 9-membered macrocycle, imposing axial chirality. This unique architecture combined with their taxol-like antimitotic activities have attracted various attention, especially from synthetic chemists, notably in the past decade. The present review describes the known total and formal syntheses of the members of the rhazinilam family (rhazinilam, rhazinal, leuconolam and kopsiyunnanines), according to the strategy developed.

Metal-catalysed C-Het (F, O, S, N) and C-C bond arylation

The formation of C-aryl bonds has been the focus of intensive research over the last decades for the construction of complex molecules from simple, readily available feedstocks. Traditionally, these strategies involve the coupling of organohalides (I, Br, Cl) with organometallic reagents (Mg, Zn, B, Si, Sn,…) such as Kumada-Corriu, Negishi, Suzuki-Miyaura, Hiyama and Sonogashira cross-couplings. More recently, alternative methods have provided access to these products by reactions with less reactive C-Het (F, O, S, N) and C-C bonds. Compared to traditional methods, the direct cleavage and arylation of these chemical bonds, the essential link in accessible feedstocks, has become increasingly important from the viewpoint of step-economy and functional-group compatibility. This comprehensive review aims to outline the development and advances of this topic, which was organized into (1) C-F bond arylation, (2) C-O bond arylation, (3) C-S bond arylation, (4) C-N bond arylation, and (5) C-C bond arylation. Substantial attention has been paid to the strategies and mechanistic investigations. We hope that this review can trigger chemists to discover more efficient methodologies to access arylation products by cleavage of these C-Het and C-C bonds.

Direct Synthesis of Ketones from Methyl Esters by Nickel-Catalyzed Suzuki-Miyaura Coupling

The direct conversion of alkyl esters to ketones has been hindered by the sluggish reactivity of the starting materials and the susceptibility of the product towards subsequent nucleophilic attack. We have now achieved a cross-coupling approach to this transformation using nickel, a bulky N-heterocyclic carbene ligand, and alkyl organoboron coupling partners. 65 alkyl ketones bearing diverse functional groups and heterocyclic scaffolds have been synthesized with this method. Catalyst-controlled chemoselectivity is observed for C(acyl)-O bond activation of multi-functional substrates bearing other bonds prone to cleavage by Ni, including aryl ether, aryl fluoride, and N-Ph amide functional groups. Density functional theory calculations provide mechanistic support for a Ni⁰ /Ni^II catalytic cycle and demonstrate how stabilizing non-covalent interactions between the bulky catalyst and substrate are critical for the reaction's success.

Direct Carbon Isotope Exchange of Pharmaceuticals via Reversible Decyanation

The incorporation of carbon-14 allows tracking of organic molecules and provides vital knowledge on their fate. This information is critical in pharmaceutical development, crop science, and human food safety evaluation. Herein, a transition-metal-catalyzed procedure enabling carbon isotope exchange on aromatic nitriles is described. By utilizing the radiolabeled precursor Zn([¹⁴C]CN)₂, this protocol allows the insertion of the desired carbon tag without the need for structural modifications, in a single step. By reducing synthetic costs and limiting the generation of radioactive waste, this procedure will facilitate the labeling of nitrile containing drugs and accelerate ¹⁴C-based ADME studies supporting drug development.

Cross-coupling reactions with esters, aldehydes, and alcohols

This feature article describes how diverse oxygen-containing functional groups such as esters, aldehydes, and alcohols can participate in cross-coupling reactions to prepare amides, ketones, alcohols, and beyond.

Unusual cyanide and methyl binding modes at a dicobalt macrocycle following acetonitrile C-C bond activation

This communication describes the C-C bond activation of acetonitrile and the trapping of the methyl and cyanide fragments by macrocyclic, dicobalt complexes. Both products display unique structural features as a result of the constraints imposed by the macrocycle. The bridged species [(3PDI2)Co2(μ-CN)(PMe3)2][OTf] ([Co2CN]+) exhibits atypical Co-CN-Co binding, and upon either phosphine dissociation or oxidation, the flexible ligand framework is able to switch between different binding modes of μ-cyanide. Further, the bridging methyl species [(3PDI2)Co2(μ-CH3)(PMe3)][OTf] ([Co2CH3]+) is the first structurally characterized dicobalt complex with a bridging methyl group.

Asymmetric Total Syntheses of Di- and Sesquiterpenoids by Catalytic C-C Activation of Cyclopentanones

To show the synthetic utility of the catalytic C-C activation of less strained substrates, described here are the collective and concise syntheses of the natural products (-)-microthecaline A, (-)-leubehanol, (+)-pseudopteroxazole, (+)-seco-pseudopteroxazole, pseudopterosin A-F and G-J aglycones, and (+)-heritonin. The key step in these syntheses involve a Rh-catalyzed C-C/C-H activation cascade of 3-arylcyclopentanones, which provides a rapid and enantioselective route to access the polysubstituted tetrahydronaphthalene cores presented in these natural products. Other important features include 1) the direct C-H amination of the tetralone substrate in the synthesis of (-)-microthecaline A, 2) the use of phosphoric acid to enhance efficiency and regioselectivity for problematic cyclopentanone substrates in the C-C activation reactions, and 3) the direct conversion of serrulatane into amphilectane diterpenes by an allylic cyclodehydrogenation coupling.

Formal group insertion into aryl C‒N bonds through an aromaticity destruction-reconstruction process

Given the abundance and the ready availability of anilines, the selective insertion of atoms into the aryl carbon–nitrogen bonds will be an appealing route for the synthesis of nitrogen-containing aromatic molecules. However, because aryl carbon–nitrogen bonds are particularly inert, anilines are normally activated by conversion to more reactive intermediates such as aryldiazonium salts to achieve functionalization of the aryl carbon–nitrogen bonds, but the nitrogen atom is usually not incorporated into products, instead being discarded. The selective insertion of groups into aryl carbon–nitrogen bonds remains an elusive challenge and an unmet need in reaction design. Here we show an aromaticity destruction-reconstruction process that selectively inserts a trimethylenemethane (TMM) group into the aromatic carbon–nitrogen bond of anilines concomitant with a benzylic carbon–hydrogen bond functionalization. This process provides a transformative mode for anilines, and the insertion products are versatile precursor to various nitrogen-containing aromatic molecules through simple conversions.

Activation of C–N bonds of anilines requires transformation of the amino group into a more reactive functionality. Here, the authors report an aromaticity destruction-reconstruction process that converts abundant anilines into valuable amines through group insertion into the C–N bond and benzylic C–H functionalization.

Enantioselective Ruthenium-Catalyzed Benzocyclobutenone-Ketol Cycloaddition: Merging C-C Bond Activation and Transfer Hydrogenative Coupling for Type II Polyketide Construction

The first enantioselective intermolecular metal-catalyzed cycloadditions of benzocyclobutenones via C-C bond oxidative addition are described. In the presence of a ruthenium(0) complex modified by ( R)-DM-SEGPHOS, tetralone-derived ketols and benzocyclobutenones combine to form cycloadducts with complete regio- and diastereoselectivity and high enantioselectivity. Using this method, the "bay region" substructure of the angucycline natural product arenimycin was prepared.

Ruthenium(0)-Catalyzed Cycloaddition of 1,2-Diols, Ketols, or Diones via Alcohol-Mediated Hydrogen Transfer

Merging the characteristics of transfer hydrogenation and carbonyl addition, a broad new class of ruthenium(0)-catalyzed cycloadditions has been developed. As discussed in this Minireview, fused or bridged bicyclic ring systems are accessible in a redox-independent manner in C-C bond-forming hydrogen transfer reactions of diols, α-ketols, or 1,2-diones with diverse unsaturated reactants.

Copper-catalyzed cyanation of heterocycle C–H bonds with ethyl(ethoxymethylene)cyanoacetate as a cyanating agent and its mechanism

A method for copper-catalyzed cyanation of heterocycles with ethyl(ethoxymethylene)cyanoacetate as a nontoxic and easily available cyanating agent via C–H bond activation has been developed.

The reductive decyanation reaction: an overview and recent developments

This review presents an overview of the reductive decyanation reaction with a special interest for recent developments. This transformation allows synthetic chemists to take advantages of the nitrile functional group before its removal. Mechanistic details and applications to organic synthesis are provided.

"Cut and Sew" Transformations via Transition-Metal-Catalyzed Carbon-Carbon Bond Activation

The transition metal-catalyzed "cut and sew" transformation has recently emerged as a useful strategy for preparing complex molecular structures. After oxidative addition of a transition metal into a carbon-carbon bond, the resulting two carbon termini can be both functionalized in one step via the following migratory insertion and reductive elimination with unsaturated units, such as alkenes, alkynes, allenes, CO and polar multiple bonds. Three- or four-membered rings are often employed as reaction partners due to their high ring strains. The participation of non-strained structures generally relies on cleavage of a polar carbon-CN bond or assistance of a directing group.

Diastereoselective Intramolecular Cyanoamidation with Alkenes

Reported herein is a diastereoselective intramolecular alkene cyanoamidation, wherein high d.r. values are imparted by chiral directing groups. Lactams with an α-all-carbon quaternary stereocenter are readily synthesized, which may enable access to structures frequently found in biologically active molecules and natural products.

Copper-free Sandmeyer cyanation of arenediazonium o-benzenedisulfonimides

The first example of a copper-free Sandmeyer cyanodediazoniation carried out using arenediazonium o-benzenedisulfonimide and tetrabutylammonium cyanide is reported in this paper.

Controlled Rh-Catalyzed Mono- and Double-decarbonylation of Alkynyl α-Diones To Form Conjugated Ynones and Disubstituted Alkynes

A Rh-catalyzed controlled decarbonylation of alkynyl α-diones is described. By using different ligand and solvent combinations, mono- and double-decarbonylation can be selectively achieved to give conjugated ynones and disubstituted alkynes, respectively. A fundamental study on catalytic activation of unstrained C-C bonds under nonoxidative conditions is presented.

Rh-Catalyzed Decarbonylation of Conjugated Ynones via Carbon-Alkyne Bond Activation: Reaction Scope and Mechanistic Exploration via DFT Calculations

In this full article, detailed development of a catalytic decarbonylation of conjugated monoynones to synthesize disubstituted alkynes is described. The reaction scope and limitation has been thoroughly investigated, and a broad range of functional groups including heterocycles were compatible under the catalytic conditions. Mechanistic exploration via DFT calculations has also been executed. Through the computational study, a proposed catalytic mechanism has been carefully evaluated. These efforts are expected to serve as an important exploratory study for developing catalytic alkyne-transfer reactions via carbon-alkyne bond activation.