Cobalt-Catalyzed Regioselective Synthesis of Indenamine fromo-Iodobenzaldimine and Alkyne: Intriguing Difference to the Nickel-Catalyzed Reaction

Electro-oxidative intermolecular CSP2-H amination of heteroarenes via proton-coupled electron transfer

A new electrochemical proton-coupled electron transfer method for the intermolecular CSP2-H amination of heteroarenes without oxidants, metal catalysts and external electrolytes has been developed. Various new N-containing heteroarenes were prepared in medium to high yields, and the indole-containing product could be converted into practical 2-oxindole by simple basic hydrolysis. Mechanistic investigation indicated that ester sulfonyl-substituted N-radicals could be formed by the combination of 2,6-lutidine and electrochemical oxidation, which is the key to achieve the desired chemoselectivity.

Synthesis of silyl indenes by ruthenium-catalyzed aldehyde- and acylsilane-enabled C-H alkylation/cyclization

A ruthenium-catalyzed C-H alkylation/cyclization sequence is presented to prepare silyl indenes with atom and step-economy. This domino reaction is triggered by acyl silane-directed C-H activation, and an aldehyde controlled the following enol cyclization/condensation other than β-H elimination. The protocol tolerates a broad substitution pattern, and the further synthetic elaboration of silyl indenes allows access to a diverse range of interesting indene and indanone derivatives.

Palladium Catalyzed Dicarbonylation of α-Iodo-Substituted Alkylidenecyclopropanes: Synthesis of Carbamoyl Substituted Indenones

A palladium catalyzed dicarbonylation of α-iodo-substituted ACPs for the synthesis of carbamoyl substituted indenones has been developed. Two carbonyl groups were incorporated into the product with the cleavage of the proximal C-C bond of the ACPs. A broad range of carbamoyl substituted indenones were efficiently prepared in good to excellent yields.

Rhodium(iii)-catalyzed successive C(sp2)–H and C(sp2)–C(sp2) bond activation of aryl oximes: synthetic and mechanistic studies

A rhodium(iii)-catalyzed redox-neutral reaction of aryl oximes and internal alkynes to generate novel N-(2-cyanoaryl) indanone imines.

Rhodium-Catalyzed C-H Activation-Based Construction of Axially and Centrally Chiral Indenes through Two Discrete Insertions

Reported herein is asymmetric [3+2] annulation of arylnitrones with different classes of alkynes catalyzed by chiral rhodium(III) complexes, with the nitrone acting as an electrophilic directing group. Three classes of chiral indenes/indenones have been effectively constructed, depending on the nature of the substrates. The coupling system features mild reaction conditions, excellent enantioselectivity, and high atom-economy. In particular, the coupling of N-benzylnitrones and different classes of sterically hindered alkynes afforded C-C or C-N atropochiral pentatomic biaryls with a C-centered point-chirality in excellent enantio- and diastereoselectivity (45 examples, average 95.6 % ee). These chiral center and axis are disposed in a distal fashion and they are constructed via two distinct migratory insertions that are stereo-determining and are under catalyst control.

Rhenium-Catalyzed Arylation-Acyl Cyclization between Enol Lactones and Organomagnesium Halides: Facile Synthesis of Indenones

A set of rhenium-catalyzed arylation-acyl cyclizations between (hetero)arylmagnesium halides and enol lactones through a cascade C(sp² )-C(sp² )/C(sp² )-C(sp² ) bond formation under mild reaction conditions has been developed. Indeed, a wide range of functional groups on both organomagnesium halides and enol lactones is well tolerated by the simple rhenium catalysis, thus furnishing polyfunctionalized indenones in one-pot fashion and with complete control of the regioselectivity. Moreover, this approach also provides a straightforward synthetic route to neolignan and (iso)pauciflorol F. Mechanistic studies demonstrated that the reaction involves a sequence of syn-carborhenation and intramolecular nucleophilic addition.

Synthesis of Indanones and Spiroindanones by Diastereoselective Annulation Based on a Hydrogen Autotransfer Strategy

An unprecedented nickel-catalyzed domino reductive cyclization of alkynes and o-bromoaryl aldehydes is described. The reaction features broad substrate scope and is tolerant of a variety of functional groups, providing straightforward access to biologically significant indanones and spiroindanone pyrrolidine derivatives in good yields with excellent regio- and diastereoselectivity. Preliminary mechanistic studies have shown that indanones are formed by the cyclization of o-bromoaryl aldehydes and alkynes to form indenol intermediates, followed by hydrogen autotransfer.

Transition-Metal-Catalyzed Addition/Cyclization Reactions of the C-N Multiple Bonds Containing Species

This article describes our research work for the past decade, which involves the transition-metal-catalyzed cyclization reactions of the C-N multiple bonds containing species and their synthetic applications to access various heterocyclic compounds. The concepts of reactions including four types of coupling with a subsequent cyclization are (1) the transition-metal performs as a Lewis acid to activate a nitrile and accelerate the nucleophilic addition, (2) the transition-metal-catalyzed 1,2-insertion reaction of nitrile, (3) the Cu-catalyzed C-N coupling reaction of imine, and (4) the Co-catalyzed addition/cyclization reaction of imine. These methods can be used to synthesize various N-containing aromatic heterocycles with higher efficiency, and can be applied to the synthesis of relevent natural alkaloids, their derivatives as well as biologically active compounds.

Synthesis of cyclopentaquinolinone and cyclopentapyridinone from ortho-alkynyl-N-arylaldehyde via superbase-promoted C-N, C-O and C-C bond formation

An environmentally benign, transition metal-free, superbase-mediated intramolecular annulation of o-alkynylaldehydes with primary amines forms highly functionalized amino-substituted cyclopentaquinolinones and cyclopentapyridinones via C-N, C-C, and C[double bond, length as m-dash]O bond formation. Contrary to the traditional approaches of ring closures, a different mode of annulation is disclosed. The protocol involves the in situ generations of imine intermediate followed by potassium hydroxide-promoted intramolecular cyclization and subsequent dimethyl sulfoxide induced dehydrogenation leads to the formation of N-heterocycles. X-ray crystallographic studies support the assigned structures of the amino-fused N-heterocycles.

Ruthenium-Catalyzed Brook Rearrangement Involved Domino Sequence Enabled by Acylsilane-Aldehyde Corporation

A ruthenium-catalyzed [1,2]-Brook rearrangement involved domino sequence is presented to prepare highly functionalized silyloxy indenes with atomic- and step-economy. This domino reaction is triggered by acylsilane-directed C-H activation, and the aldehyde controlled the subsequent enol cyclization/Brook Rearrangement other than β-H elimination. The protocol tolerates a broad substitution pattern, and the further synthetic elaboration of silyloxy indenes allows access to a diverse range of interesting indene and indanone derivatives.

Synthesis and biological evaluation of 5-aminoethyl benzophenanthridone derivatives as DNA topoisomerase IB inhibitors

DNA topoisomerase IB (TOP1) regulates DNA topological structure in many cellular metabolic processes and is a validated target for development of antitumor agents. Our previous study revealed that the benzophenanthridone scaffold is a novel chemotype for the discovery of TOP1 inhibitors. In this work, a series of novel 5-aminoethyl substituted benzophenanthridone derivatives have been synthesized and evaluated for TOP1 inhibition and cytotoxicity. Compound 12 exhibits the most potent TOP1 inhibition (+++) and cytotoxicity in human cancer cell lines with GI₅₀ values at nanomolar concentration range. 12 induces the cellular TOP1cc formation and DNA damage, resulting in HCT116 cell apoptosis. The pharmacokinetics, acute toxicity and antitumor efficiency in vivo of 12 were also studied.

Direct synthesis of indenes via a rhodium-catalyzed multicomponent Csp2-H annulation reaction

A highly efficient, direct and multicomponent route for the synthesis of indenes is reported herein. This process is catalyzed by a rhodium(iii) complex and conducted under very mild conditions, making the overall process atom and step-economical. DFT mechanistic studies were performed to explore the mechanism of this reaction system.

Palladium/Norbornene-Catalyzed Indenone Synthesis from Simple Aryl Iodides: Concise Syntheses of Pauciflorol F and Acredinone A

To show the synthetic utility of palladium/norbornene (Pd/NBE) cooperative catalysis, here we report concise syntheses of indenone-based natural products, pauciflorol F and acredinone A, which are enabled by direct annulation between aryl iodides and unsaturated carboxylic acid anhydrides. Compared to the previous indenone-preparation approaches, this method allows simple aryl iodides to be used as substrates with complete control of the regioselectivity. The total synthesis of acredinone A features two different Pd/NBE-catalyzed ortho acylation reactions for constructing penta-substituted arene cores, including the development of a new ortho acylation/ipso borylation.

Metal-free cascade rearrangement/radical addition/oxidative C–H annulation of propargyl alcohols with sodium sulfinates: access to 2-sulfenylindenones

A facile protocol for the construction of 2-sulfenylindenones via one-pot cascade rearrangement/radical addition/oxidative C–H cyclization of propargyl alcohols with sodium sulfinates has been developed.

Synthesis of polysubstituted 3-aminoindenes via rhodium-catalysed [3+2] cascade annulations of benzimidates with alkenes

A novel Rh-catalysed intermolecular [3+2] cascade cyclization of benzimidates and alkenes has been developed to assemble polysubstituted 3-aminoindenes, which exhibits good functional-group tolerance and excellent regioselectivity.

Discovery, Synthesis, and Evaluation of Oxynitidine Derivatives as Dual Inhibitors of DNA Topoisomerase IB (TOP1) and Tyrosyl-DNA Phosphodiesterase 1 (TDP1), and Potential Antitumor Agents

Tyrosyl-DNA phosphodiesterase 1 (TDP1) is a recently discovered enzyme repairing DNA lesions resulting from stalled topoisomerase IB (TOP1)-DNA covalent complex. Inhibiting TDP1 in conjunction with TOP1 inhibitors can boost the action of the latter. Herein, we report the discovery of the natural product oxynitidine scaffold as a novel chemotype for the development of TOP1 and TDP1 inhibitors. Three kinds of analogues, benzophenanthridinone, dihydrobenzophenanthridine, and benzophenanthridine derivatives, were synthesized and evaluated for both TOP1 and TDP1 inhibition and cytotoxicity. Analogue 19a showed high TOP1 inhibition (+++) and induced the formation of cellular TOP1cc and DNA damage, resulting in cancer cells apoptosis at nanomolar concentration range. In vivo studies indicated that 19a exhibits antitumor efficiency in HCT116 xenograft model. 41a exhibited additional TDP1 inhibition with IC₅₀ value of 7 μM and synergistic effect with camptothecin in MCF-7 cells. This work will facilitate future efforts for the discovery of natural product-based TOP1 and TDP1 inhibitors.

Impact of the Valence Charge of Transition Metals on the Cobalt- and Rhodium-Catalyzed Synthesis of Indenamines, Indenols, and Isoquinolinium Salts: A Catalytic Cycle Involving MIII/MV [M = Co, Rh] for [4 + 2] Annulation

Reaction mechanisms for the synthesis of indenamines, indenols, and isoquinolinium salts through cobalt- and rhodium-catalysis were investigated using density functional theory calculations. We found that the valence charge of transition metals dramatically influences the reaction pathways. Catalytic reactions involving lower-oxidation-state transition metals (M^I/M^III, M = Co and Rh) generally favor a [3 + 2] cyclization pathway, whereas those involving higher oxidation states (M^III/M^V) proceed through a [4 + 2] cyclization pathway. A catalytic cycle with novel M^III/M^V as a crucial species was successfully revealed for isoquinolinium salts synthesis, in which highly valent M^V was encountered not only in the [RhCp*]-catalysis but also in the [CoCp*]-catalysis.

Toward Exploring Novel Organic Materials: MP4-DFT Properties of 4-Amino-3-Iminoindene

Tautomerism links with many applications and remains an attracting feature in exploring novel systems. In this regard, properties of indene-based HNCCCN segments have not received any considerable attention. In this computational organic chemistry study, first, to calculate the proton transfer energy barrier at a reasonable cost, the study identified an accurate forth order Møller–Plesset perturbation theory-density functional theory (MP4-DFT) protocol equivalent to the outstanding pioneering benchmark calculations. The calculations illustrate that the two tautomers of the 4-amino-3-iminoindene nucleus are separated by a considerable energy barrier while featuring different molecular orbital characteristics; frontier orbital distribution, λmax, and energies, which are known basic requirements in molecular switching and logic circuit applications. The N-H/BH₂ substitution was found to have significant influence on the electronic structure of the skeleton. Similarities in the two tautomers and the boron derivative to properties of known molecular materials have been found.

Cobalt catalysis involving π components in organic synthesis

Over the last three decades, transition-metal-catalyzed organic transformations have been shown to be extremely important in organic synthesis. However, most of the successful reactions are associated with noble metals, which are generally toxic, expensive, and less abundant. Therefore, we have focused on catalysis using the abundant first-row transition metals, specifically cobalt. In this Account, we demonstrate the potential of cobalt catalysis in organic synthesis as revealed by our research. We have developed many useful catalytic systems using cobalt complexes. Overall, they can be classified into several broad types of reactions, specifically [2 + 2 + 2] and [2 + 2] cycloadditions; enyne reductive coupling; reductive [3 + 2] cycloaddition of alkynes/allenes with enones; reductive coupling of alkyl iodides with alkenes; addition of organoboronic acids to alkynes, alkenes, or aldehydes; carbocyclization of o-iodoaryl ketones/aldehydes with alkynes/electron-deficient alkenes; coupling of thiols with aryl and alkyl halides; enyne coupling; and C-H bond activation. Reactions relying on π components, specifically cycloaddition, reductive coupling, and enyne coupling, mostly afford products with excellent stereo- and regioselectivity and superior atom economy. We believe that these cobalt-catalyzed π-component coupling reactions proceed through five-membered cobaltacyclic intermediates formed by the oxidative cyclometalation of two coordinated π bonds of the substrates to the low-valent cobalt species. The high regio- and stereoselectivity of these reactions are achieved as a result of the electronic and steric effects of the π components. Mostly, electron-withdrawing groups and bulkier groups attached to the π bonds prefer to be placed near the cobalt center of the cobaltacycle. Most of these transformations proceed through low-valent cobalt complexes, which are conveniently generated in situ from air-stable Co(II) salts by Zn- or Mn-mediated reduction. Overall, we have shown these reactions to be excellent substitutes for less desirable noble-metal systems. Recent successes in cobalt-catalyzed C-H activation have especially advanced the applicability of cobalt in this field. In addition to the more common low-valent-cobalt-catalyzed C-H activation reactions, an in situ-formed cobalt(III) five-membered complex with a 1,6-enyne effectively couples with aromatic ketones and esters through ortho C-H activation, opening a new window in this research area. Interestingly, this reaction proceeds under milder reaction conditions with broad substrate scope. Furthermore, many of the reactions we have developed are highly enantioselective, including enantioselective reductive coupling of enones and alkynes, addition of organoboronic acids to aldehydes, and the cyclization of 2-iodobenzoates with aldehydes. Overall, this Account demonstrates the versatility and utility of cobalt catalysis in organic synthesis.

On the use of metal purine derivatives (M=Ir, Rh) for the selective labeling of nucleosides and nucleotides

The reactions of neutral or cationic IrIII and RhIII derivatives of phenyl purine nucleobases with unsymmetrical alkynes produce new metallacycles in a predictable manner, which allows for the incorporation of either photoactive (anthracene or pyrene) or electroactive (ferrocene) labels in the nucleotide or nucleoside moiety. The reported methodology (metalation of the purine derivative and subsequent marker insertion) could be used for the postfunctionalization and unambiguous labeling of oligonucleotides.

MeOTf-induced carboannulation of arylnitriles and aromatic alkynes: a new metal-free strategy to construct indenones

MeOTf-induced carboannulation of arylnitriles and aromatic alkynes for synthesis of indenones and their derivatives under metal-free conditions has been described.