Palladium-Catalyzed Intramolecular Arylative Carboxylation of Allenes with CO2 for the Construction of 3-Substituted Indole-2-carboxylic Acids

Synthesis of Substituted 1,2-Dihydroisoquinolines by Palladium-Catalyzed Cascade Cyclization-Coupling of Trisubstituted Allenamides with Arylboronic Acids

1,2-Dihydroisoquinolines are important compounds due to their biological and medicinal activities, and numerous approaches to their synthesis have been reported. Recently, we reported a facile synthesis of trisubstituted allenamides via N-acetylation followed by DBU-promoted isomerization, where various substituted allenamides were conveniently synthesized from readily available propargylamines with high efficiency. In light of this research background, we focused on the utility of this methodology for the synthesis of substituted 1,2-dihydroisoquinolines. In this study, a palladium-catalyzed cascade cyclization-coupling of trisubstituted allenamides containing a bromoaryl moiety with arylboronic acids is described. When N-acetyl diphenyl-substituted trisubstituted allenamide and phenylboronic acid were treated with 10 mol% of Pd(OAc)2, 20 mol% of P(o-tolyl)3, and 5 equivalents of NaOH in dioxane/H2O (4/1) at 80 °C, the reaction proceeded to afford a substituted 1,2-dihydroisoquinoline. The reaction proceeded via intramolecular cyclization, followed by transmetallation with the arylboronic acid of the resulting allylpalladium intermediate. A variety of highly substituted 1,2-dihydroisoquinolines were concisely obtained using this methodology because the allenamides, as reaction substrates, were prepared from readily available propargylamines in one step.

Hierarchical surface-modification of nano-Cu toward one pot H-transfer-coupling-cyclization-CO2 fixation tandem reactions

Fixation of CO2 into dihydroisobenzofuran derivatives has enormous applications in both production of natural products and antidepressant drugs, and reducing the green-house effect. However, the relatively complicated multi-step processes limit the further expansion of such a valuable CO2 conversion strategy. Herein, we hierarchically modify the surface of Cu nanoparticles (NPs) with Ag NPs and the robust metal-organic framework (MOF), ZIF-8, and report the presence of the Cu-Ag yolk-shell nanoalloy based heterogeneous catalysts, Cu@Ag and Cu@Ag@ZIF-8. The latter exhibits a crystalline "raisin bread" structure and specific synergic activity for catalyzing the tandem reactions of intra-molecular H-transfer, C-C and C-O coupling, cyclization, and carboxylation from CO2, leading to the first non-homogeneous preparation of dihydroisobenzofuran derivatives in high yield, selectivity, and recyclability under mild conditions. Theoretical calculations elucidate the tandem reaction pathway synergically catalyzed by Cu@Ag@ZIF-8, which offers insights for designing multiphase catalysts towards both organic synthesis and CO2 fixation through tandem processes in one pot.

Palladium-catalysed aryl/monofluoroalkylation of allenamides: access to fluoroalkyl indoles and isoquinolones

A palladium catalysed construction of fluoroalkyl indoles and isoquinolones through aryl/monofluoroalkylation of allenamides has been developed. Monofluoromethyl-substituted heterocycles could be accessed under mild conditions with broad functional group tolerance. In addition, indole-oxindole bisheterocyclic scaffolds bearing a fluorine atom were successfully synthesized with 3-fluoro-oxindole as the nucleophile by applying this method.

Substrate-Dependent Regioselectivity: Pd/PTC Cooperatively Catalyzed Domino Heck/Allylation of Allenamides with α-Carbon of Carbonyl Compounds

A Pd/phase-transfer catalyst cooperatively catalyzed domino Heck/allylation reaction is first reported, which represents interesting substrate-dependent regioselectivity. Under the same conditions, Ts-protected N-(2-iodophenyl)allenamides produced only linear allylation products, while Cbz, Ac, or Boc-protected N-(2-iodophenyl)allenamides and N-(2-iodobenzoyl)allenamides with various compounds generated branch allylation products with an exocylic C═C bond and two vicinal stereocenters. Up-scale syntheses and diverse fused cyclization transformations of products were then carried out. The enantioselective version for the domino process was studied.

Regiodivergent cascade cyclization/alkoxylation of allenamides via N-protecting group driven rearrangement to access indole and indoline derivatives

A mild, palladium-catalyzed domino Heck-cyclization/alkoxylation sequence of aryl halide tethered allenamides is described, providing regiodivergent indole and indoline derivatives controlled by the N-protecting group. This room temperature reaction provided a functionalizable olefinic moiety with broad substrate scope. Preliminary mechanistic studies support the rearrangement of an indoline-derived intermediate to indoles with the N-acetyl allenamides forming free (NH) indoles.

Synthesis of 1,2-diselenides via potassium persulfate-mediated diselenation of allenamides with diselenides

An efficient potassium persulfate-mediated radical addition of allenamides with diselenides was developed to create a workable route to 1,2-diselenide products. The reaction tolerates a wide spectrum of functional groups to deliver the products in good to excellent yields. Mechanistic investigations including a calculation study indicated that the radical cascade proceeds through a vinyl radical intermediate, which is formed via a selenium radical added to the terminal CC double bond of allenamides.

Pd-Catalyzed Asymmetric Three-Component Allenol Carbopalladation and Allylic Cycloaddition Cascade: A Route to Functionalized Tetrahydrofurans

The first Pd-catalyzed asymmetric three-component reaction of 2,3-allenol, aryl iodides, and 2-arylmethylenemolononitriles has been developed via an allenol carbopalladation and an allylic cycloaddition cascade. This process allows rapid access to substituted tetrahydrofurans bearing diverse functional groups in good yields with high diastereoselectivities and excellent enantioselectivities. The concise total synthesis of a lignan, (-)-2-episesaminone, has been achieved by the elaboration of a functionalized tetrahydrofuran obtained from this reaction.

Visible-Light-Enabled Carboxylation of Benzyl Alcohol Derivatives with CO2 Using a Palladium/Iridium Dual Catalyst

A highly efficient carboxylation of benzyl alcohol derivatives with CO₂ using a palladium/iridium dual catalyst under visible-light irradiation was developed. A wide range of benzyl alcohol derivatives could be employed to provide benzylic carboxylic acids in moderate to high yields. Mechanistic studies indicated that the oxidative addition of benzyl alcohol derivatives was possibly the rate-determining-step. It was also found that a switchable site-selective carboxylation between benzylic C-O and aryl C-Cl moieties could be achieved simply by changing the palladium catalyst.

Advances in Palladium-Catalyzed Carboxylation Reactions

In this short review, we highlight the advancements in the field of palladium-catalyzed carbon dioxide utilization for the synthesis of high value added organic molecules. The review is structured on the basis of the kind of substrate undergoing the Pd-catalyzed carboxylation process. Accordingly, after the introductory section, the main sections of the review will illustrate Pd-catalyzed carboxylation of olefinic substrates, acetylenic substrates, and other substrates (aryl halides and triflates).

An organophotoredox-catalyzed redox-neutral cascade involving N-(acyloxy)phthalimides and allenamides: synthesis of indoles

An organophotoredox-catalyzed radical cascade of allenamides and alkyl N-(acyloxy)phthalimides for the synthesis of indoles is documented. The method features mild and robust reaction conditions, and exhibits broad scope. The tandem process enriches the limited repertoire of alkyl NHPI ester addition on electron-rich π-bonds as well as radical chemistry involving allenamides.

Asymmetric Domino Heck/Dearomatization Reaction of β-Naphthols to Construct Indole-Terpenoid Frameworks

A palladium-catalyzed enantioselective Heck cyclization/dearomatization cascade via capturing the cyclized Heck π-allylpalladium intermediate by β-naphthols is reported, which provides a new strategy for the construction of chiral indole-terpenoid frameworks. This method affords indole-functionalized β-naphthalenone compounds bearing an all-carbon-substituted quaternary chiral center in excellent yields (up to 92%) and enantioselectivities (up to 94% ee). In addition, the utility of this method is showcased by the gram-scale syntheses and diverse transformations of the dearomatized products.

Versatile Reactivity of Metalloid-Substituted π-Allylpalladium Species

π-Allylpalladium complexes can not only serve as electrophilic allylating agents for a broad range of nucleophiles, but also nucleophilic allylating agents for electrophiles depending on their electronic environments. In contrast to these typical reactivities of π-allylpalladium complexes, silylated and borylated π-allylpalladium species show unique reactivities that can allow versatile transformations in addition to simple allylation. Herein, four different types of transformations that are in principle achieved via the inherently reactive silylated and borylated π-allylpalladium species as common intermediates are described. An appropriate selection of ligands of the silylated and borylated π-allylpalladium species can allow control over the reaction pathways.

Palladium-Catalyzed Allenamide Carbopalladation/Allylation with Active Methine Compounds

A palladium-catalyzed allenamide carbopalladation/allylation with active methine compounds has been developed. Various indoles and isoquinolinones bearing a quaternary carbon center were achieved with good efficiency, a broad substrate scope and good functional group tolerance. This reaction underwent cascade oxidative addition, carbopalladation, and allylic alkylation, and two new C-C bonds were formed in one pot.

Photoredox-Catalyzed α-Aminoalkylcarboxylation of Allenes with CO2

The photoredox-catalyzed α-aminoalkylcarboxylation of aryl allenes with CO₂ and N,N-dimethylanilines is reported for the first time (26 examples, up to 96% yield). In the case of electron-deficient allenes, good regioselectivity was observed (up to 94:6), exclusively generating kinetic products over thermodynamic products. This protocol is a novel synthetic method for highly functionalized β,γ-unsaturated γ-aminobutyric esters.

An approach to unsymmetrical 3,3′-diindolylmethanes through Pd-catalyzed cascade Heck cyclization of allenamides and o-ethynylanilines

A highly efficient synthesis of unsymmetrical 3,3′-diindolylmethanes has been developed by palladium-catalyzed cascade Heck/cyclization reaction.

Reductive dearomative arylcarboxylation of indoles with CO2 via visible-light photoredox catalysis

Catalytic reductive coupling of two electrophiles and one unsaturated bond represents an economic and efficient way to construct complex skeletons, which is dominated by transition-metal catalysis via two electron transfer. Herein, we report a strategy of visible-light photoredox-catalyzed successive single electron transfer, realizing dearomative arylcarboxylation of indoles with CO₂. This strategy avoids common side reactions in transition-metal catalysis, including ipso-carboxylation of aryl halides and β-hydride elimination. This visible-light photoredox catalysis shows high chemoselectivity, low loading of photocatalyst, mild reaction conditions (room temperature, 1 atm) and good functional group tolerance, providing great potential for the synthesis of valuable but difficultly accessible indoline-3-carboxylic acids. Mechanistic studies indicate that the benzylic radicals and anions might be generated as the key intermediates, thus providing a direction for reductive couplings with other electrophiles, including D₂O and aldehyde.

Catalytic reductive coupling of two electrophiles and one C = C bond is usually performed by two electron transfer metal catalysis. Herein, the authors show a visible light photoredox-catalyzed successive single electron transfer leading to dearomative arylcarboxylation of indoles with CO₂ and generating indoline-3-carboxylic acids.

Divergent oriented synthesis (DOS) of aza-heterocyclic amides through palladium-catalyzed ketenimination of 2-iodo-N-(propa-1,2-dien-1-yl)anilines

A palladium-catalyzed tandem reaction of N-(2-iodophenyl)-4-methyl-N-(propa-1,2-dien-1-yl)benzenesulfonamide with isocyanide is described to divergently produce aza-heterocyclic amides.

Theoretical investigation on the Cu(i)-catalyzed N-carboxamidation of indoles with isocyanates to form indole-1-carboxamides: effects of solvents

The solvents act as the hydrogen-bond acceptor to facilitate intermolecular addition, and then play the proton-shuttle to assist H1⁺-shift. The stronger electron-donating property of solvent is favorable for the present Cu(i)-catalyzed reactions.

Access to N-unprotected 2-amide-substituted indoles from Ugi adducts via palladium-catalyzed intramolecular cyclization of o-iodoanilines bearing furan rings

A variety of N-unprotected 2-amide-substituted indoles were synthesized from readily available furfural-based Ugi adducts in moderate to good yields via palladium-catalyzed intramolecular cyclization of o-iodoanilines bearing furan rings. These reactions involved a cascade sequence consisting of dearomatizing arylation, opening of the furan ring, and deprotection of the N atom.

A variety of N-unprotected 2-amide-substituted indoles were synthesized from readily available furfural-based Ugi adducts in moderate to good yields via palladium-catalyzed intramolecular cyclization of o-iodoanilines bearing furan rings.

Synthesis of (difluoromethyl)naphthalenes using the ring construction strategy: C-C bond formation on the central carbon of 1,1-difluoroallenes via Pd-catalyzed insertion

The insertion of 1,1-difluoroallenes was carried out to form a C-C bond exclusively on their central carbon. o-Bromophenyl-bearing 1,1-difluoroallenes underwent intramolecular insertion in the presence of a palladium catalyst. Regioselective C-C bond formation occurred to form a six-membered carbocycle, leading to pharmaceutically and agrochemically promising difluoromethylated naphthalenes.

Development of an Improved System for the Carboxylation of Aryl Halides through Mechanistic Studies

The nickel-catalyzed carboxylation of organic halides or pseudohalides using carbon dioxide is an emerging method to prepare synthetically valuable carboxylic acids. Here, we report a detailed mechanistic investigation of these reactions using the carboxylation of aryl halides with (PPh₃)₂Ni^IICl₂ as a model reaction. Our studies allow us to understand several general features of nickel-catalyzed carboxylation reactions. For example, we demonstrate that both a Lewis acid and halide source are beneficial for catalysis. To this end, we establish that heterogeneous Mn(0) and Zn(0) reductants are multifaceted reagents that generate noninnocent Mn(II) or Zn(II) Lewis acids upon oxidation. In a key result, a rare example of a well-defined nickel(I) aryl complex is isolated, and it is demonstrated that its reaction with carbon dioxide results in the formation of a carboxylic acid in high yield (after workup). The carbon dioxide insertion product undergoes rapid decomposition, which ca These three oxidation states correspond to the onbe circumvented by a ligand metathesis reaction with a halide source. Our studies have led to both a revised mechanism and the development of a broadly applicable strategy to improve reductive carboxylation reactions. A critical component of this strategy is that we have replaced the heterogeneous Mn(0) reductant typically used in catalysis with a well-defined homogeneous organic reductant. Through its use, we have increased the range of ancillary ligands, additives, and substrates that are compatible with the reaction. This has enabled us to perform reductive carboxylations at low catalyst loadings. Additionally, we demonstrate that reductive carboxylations of organic (pseudo)halides can be achieved in high yields in more practically useful, non-amide solvents. Our results describe a mechanistically guided strategy to improve reductive carboxylations through the use of a homogeneous organic reductant, which may be broadly translatable to a wide range of cross-electrophile coupling reactions.

Computational exploration of substrate and ligand effects in nickel-catalyzed C–Si bond carboxylation with CO2

The ring strain of substrates and steric hindrance of NHC ligands are the key factors affecting the reactivity of CO₂ insertion into sila-nickelacycles.

Pd-Catalyzed Dearomative Carboxylation of Indolylmethanol Derivatives

By using a new catalytic system (PdCl₂[P( n-Bu)₃]₂ in combination with ZnEt₂), various 3-indolylmethanol derivatives were successfully carboxylated with CO₂ (1 atm) via dearomatization of the indole nucleus, affording 3-methyleneindoline-2-carboxylates. In contrast, carboxylation of 2-indolylmethanol derivatives afforded unexpected doubly carboxylated products, which are useful synthetic precursors for biologically active compounds.

Recent Progress in the Transition Metal Catalyzed Synthesis of Indoles

Indole is the most frequently found heterocyclic core structures in pharmaceuticals, natural products, agrochemicals, dyes and fragrances. For about 150 years, chemists were absorbed in finding new and easier synthetic strategies to build this nucleus. Many books and reviews have been written, but the number of new syntheses that appear in the literature, make necessary continuous updates. This reviews aims to give a comprehensive overview on indole synthesis catalyzed by transition metals appeared in the literature in the years 2016 and 2017.

Cobalt-catalyzed nucleophilic addition of the allylic C(sp3)-H bond of simple alkenes to ketones

We herein describe a cobalt/Xantphos-catalyzed regioselective addition of simple alkenes to acetophenone derivatives, affording branched homoallylic alcohols in high yields with perfect branch selectivities. The intermediate of the reaction would be a nucleophilic allylcobalt(I) species generated via cleavage of the low reactive allylic C(sp³)–H bond of simple terminal alkenes.

PIDA/I2-Mediated α- and β-C(sp3)-H Bond Dual Functionalization of Tertiary Amines

The α,β-C(sp³)-H bond dual functionalization of tertiary amines is still a challenging task for both organic and medicinal chemists. Herein a direct, mild, metal-free, and site-specific method mediated by PIDA/I₂ was developed for α,β-C(sp³)-H bond dual functionalization of tertiary amines, and this method can provide facile access to α-keto lactams or rarely studied α,α-diiodo lactams. Moreover, this method was used for the effective syntheses of three natural products [obscurumine C (13), obscurumine O (17), and strychnocarpine (18)] and direct preparation of mimics of the in vivo metabolites of two FDA-approved drugs (imatinib and donepezil) in 36-60% overall yield. The method represents a promising protocol for the late-stage α,β-C(sp³)-H bond oxidative dual functionalization of tertiary amine-containing drugs and complex natural products.

Mechanistic Insights into the Ni-Catalyzed Reductive Carboxylation of C-O Bonds in Aromatic Esters with CO2 : Understanding Remarkable Ligand and Traceless-Directing-Group Effects

The mechanism of the Ni⁰ -catalyzed reductive carboxylation reaction of C(sp² )-O and C(sp³ )-O bonds in aromatic esters with CO₂ to access valuable carboxylic acids was comprehensively studied by using DFT calculations. Computational results revealed that this transformation was composed of several key steps: C-O bond cleavage, reductive elimination, and/or CO₂ insertion. Of these steps, C-O bond cleavage was found to be rate-determining, and it occurred through either oxidative addition to form a Ni^II intermediate, or a radical pathway that involved a bimetallic species to generate two Ni^I species through homolytic dissociation of the C-O bond. DFT calculations revealed that the oxidative addition step was preferred in the reductive carboxylation reactions of C(sp² )-O and C(sp³ )-O bonds in substrates with extended π systems. In contrast, oxidative addition was highly disfavored when traceless directing groups were involved in the reductive coupling of substrates without extended π systems. In such cases, the presence of traceless directing groups allowed for docking of a second Ni⁰ catalyst, and the reactions proceed through a bimetallic radical pathway, rather than through concerted oxidative addition, to afford two Ni^I species both kinetically and thermodynamically. These theoretical mechanistic insights into the reductive carboxylation reactions of C-O bonds were also employed to investigate several experimentally observed phenomena, including ligand-dependent reactivity and site-selectivity.