Pd-Catalyzed Asymmetric Dearomatization of Indoles via Decarbonylative Heck-Type Reaction of Thioesters

Palladium-Catalyzed Domino Heck/Decarboxylative Cyanomethylation of Indoles and Alkenes with Cyanoacetate Salts

A palladium-catalyzed reaction of indoles with cyanoacetate salts enables the synthesis of 2,6-disubstituted indolines via tandem dearomatization/decarboxylative cyanomethylation. Remarkably, this is the first example of indole difunctionalization at the C2 and C6 positions. Moreover, this methodology extends to the palladium-catalyzed cyclization/decarboxylative cyanomethylation of aryl halide-tethered alkenes.

Cobalt-Catalyzed Intramolecular Enantioselective Reductive Heck Reaction toward the Synthesis of Chiral 3-Trifluoromethylated Oxindoles

Herein, a cobalt-catalyzed intramolecular enantioselective reductive Heck reaction is disclosed. Starting from N-ortho-iodoaryl-2-(trifluoromethyl)acrylamides, a plethora of chiral oxindoles bearing trifluoromethylated quaternary stereogenic centers at the C3-position are achieved in moderate to good yields (up to 88% yield) and good to excellent enantioselectivities (up to 94% ee) by employing zinc/silane as reducing agent. Other than the trifluoromethyl group, a number of chiral oxindoles bearing alkyl, aryl, and ester groups at C3-position were also obtained albeit in relatively lower enantioselectivities (68-78% ee).

A Cascade C(sp3)-H Annulation Involving C(alkyl),C(alkyl)-Palladacycle Intermediates

C-H bond functionalization involving C,C-palladacycle intermediates provides a unique platform for developing novel reactions. However, the vast majority of studies have been limited to the transformations of C(aryl),C-palladacycles. In sharp contrast, catalytic reactions involving C(alkyl),C(alkyl)-palladacycles have rarely been reported. Herein, we disclose an unprecedented cascade C(sp3)-H annulation involving C(alkyl),C(alkyl)-palladacycles. In this protocol, alkene-tethered cycloalkenyl bromides undergo intramolecular Heck/C(sp3)-H activation to generate C(alkyl),C(alkyl)-palladacycles, which can be captured by α-bromoacrylic acids to afford tricyclic fused pyridinediones. In addition, this strategy can also be applied to indole-tethered cycloalkenyl bromides to construct pentacyclic fused pyridinediones via suquential Heck dearomatization/C(sp3)-H activation/decarboxylative cyclization. Notably, the removal of α-bromoacrylic acids in the reaction of alkene-tethered cycloalkenyl bromides can build an interesting tricyclic skeleton containing a four-membered ring. Preliminary mechanistic experiments indicate that five-membered C(alkyl),C(alkyl)-palladacycles serve as the key intermediates. Meanwhile, density functional theory (DFT) calculations have provided insights into the reaction pathway.

Palladium-Catalyzed Dearomative Heck/C(sp2)-H Activation/Decarboxylative Cyclization of C2-Tethered Indoles

Until now, palladium-catalyzed dearomative Heck reactions of indoles were largely limited to β-H elimination and nucleophilic capture of the transient alkyl-Pd(II) species. Herein, we disclose a novel palladium-catalyzed dearomative Heck/C(sp2)-H activation/decarboxylative cyclization of C2-tethered indoles. In this protocol, the alkyl-Pd(II) species formed by dearomatization of C2-tethered indoles is not terminated by common β-H elimination or nucleophilic capture, but rather generates C,C-palladacycle via C-H activation. The latter is intercepted by α-bromoacrylic acids to produce pentacyclic and heptacyclic fused indolines.

Copper-catalyzed asymmetric 1,2-arylboration of enamines: access to chiral borate-containing 3,3'-disubstituted isoindolinones

An enantioselective copper-catalyzed 1,2-arylboration reaction of enamines has been developed by employing (R)-xyl-BINAP as a chiral ligand. A number of chiral borate-containing 3,3'-disubstituted isoindolinones were obtained in moderate to good yields and good to excellent enantioselectivities from the reactions of N-(o-iodobenzoyl)enamines and bis(pinacolato)diboron (B2pin2) under mild reaction conditions. Synthetic transformations of the products were conducted to demonstrate the practicality of this reaction.

Palladium-Catalyzed Domino Heck/Cross-Coupling Cyclization Reaction: Diastereoselective Synthesis of Furan-Containing Indolines

Herein, a palladium-catalyzed diastereoselective dearomatization/cross-coupling cyclization reaction between N-arylacyl indoles and (E)-β-chlorovinyl ketones is reported. Through this cyclization/cycloisomerization cascade, a series of furan-containing indolines were obtained in yields up to 95%. The reaction features readily accessible starting materials, benzyl Pd(II)-catalyzed cycloisomerization of (E)-β-chlorovinyl ketones, the sequential formation of three bonds and bis-heterocycles, and excellent diastereoselectivity. More importantly, the carbene-secondary benzyl migratory insertion is proven to be a critical process in the sequential cyclizations.

Green synthesis for diverse bioactive benzo-fused spiroindolines through DBU-catalysed post-Ugi double cyclization

A metal-free protocol utilizing DBU catalysis for post-Ugi amide-ester exchange and Conia-ene double cyclization has been successfully developed, allowing the synthesis of diverse highly functionalized benzo-fused spiroindolines with anti-cancer activities under mild conditions. Remarkably, this methodology demonstrates promising prospects for green chemistry, as it allows for the preparation of the spiroindolines in water. Control experiments indicate that a crucial role of the cyclic imide, specifically ring rigidification, facilitates the subsequent Conia-ene cyclization.

Pd-Catalyzed Asymmetric Intramolecular Dearomatizing Reductive Heck Reaction of Indoles

An enantioselective Pd-catalyzed intramolecular dearomative reductive Heck reaction of N-(o-bromoaryl) indole-3-carboxamide is developed. By employing Pd(dba)2/SPINOL-based phosphoramidite as the chiral catalyst and HCO2Na as the hydride source, a series of enantioenriched spiro indolines bearing vicinal stereocenters were afforded in moderate to good yields with excellent enantioselectivities. The reductive Heck reaction of formal tetrasubstituted alkene bearing β-hydrogens is therefore realized by inhibiting β-H elimination.

Recent Advances in Palladium-Catalyzed Asymmetric Heck/Tsuji-Trost Reactions of 1,n-Dienes

Transition-metal catalyzed tandem asymmetric reactions were powerful tools to access various chiral compounds. Many strategies have been developed for the coupling of 1,n-dienes with aryl halides via a tandem Heck/Tsuji-Trost process. However, the control of regio- and stereo-chemistry remains a challenging task. This minireview details the recent advances in the field of asymmetric Heck/Tsuji-Trost reactions catalyzed by palladium complex, which have opened new opportunities and expanded our understanding in this area of research in recent years.

Development of novel transition metal-catalyzed synthetic approaches for the synthesis of a dihydrobenzofuran nucleus: a review

The synthesis of dihydrobenzofuran scaffolds bears pivotal significance in the field of medicinal chemistry and organic synthesis. These heterocyclic scaffolds hold immense prospects owing to their significant pharmaceutical applications as they are extensively employed as essential precursors for constructing complex organic frameworks. Their versatility and importance make them an interesting subject of study for researchers in the scientific community. While exploring their synthesis, researchers have unveiled various novel and efficient pathways for assembling the dihydrobenzofuran core. In the wake of extensive data being continuously reported each year, we have outlined the recent updates (post 2020) on novel methodological accomplishments employing the efficient catalytic role of several transition metals to forge dihydrobenzofuran functionalities.

Recent advances in Pd-catalyzed asymmetric cyclization reactions

Over the past few decades, there have been major developments in transition metal-catalyzed asymmetric cyclization reactions, enabling the convenient access to a wide spectrum of structurally diverse chiral carbo- and hetero-cycles, common skeletons found in fine chemicals, natural products, pharmaceuticals, agrochemicals, and materials. In particular, a plethora of enantioselective cyclization reactions have been promoted by chiral palladium catalysts owing to their outstanding features. This review aims to collect the latest advancements in enantioselective palladium-catalyzed cyclization reactions over the past eleven years, and it is organized into thirteen sections depending on the different types of transformations involved.

Nickel-Catalyzed Enantioselective Dearomative Heck-Reductive Allylic Defluorination Reaction of Indoles

Herein, we describe a nickel-catalyzed asymmetric dearomative aryl-difluoroallylation reaction of indoles with α-trifluoromethyl alkenes as an electrophilic coupling partner. The reaction proceeds via a cascade sequence involving dearomative Heck cyclization and reductive allylic defluorination. A series of gem-difluoroallyl substituted indolines are obtained in moderate to good yields (36-77% yield) with excellent enantioselectivity (up to 99% ee). The reaction features broad functional group tolerance, scaled-up synthesis, and late-stage diversification.

Divergent Acyl and Decarbonylative Liebeskind-Srogl Cross-Coupling of Thioesters by Cu-Cofactor and Pd-NHC (NHC = N-Heterocyclic Carbene) Catalysis

Transition-metal-catalyzed cross-coupling reactions of thioesters by selective acyl C(O)-S cleavage have emerged as a powerful platform for the preparation of complex molecules. Herein, we report divergent Liebeskind-Srogl cross-coupling of thioesters by Pd-NHC (NHC = N-heterocyclic carbene) catalysis. The reaction provides straightforward access to functionalized ketones by highly selective C(acyl)-S cleavage under mild conditions. Most crucially, the conditions enable direct functionalization of a range of complex pharmaceuticals decorated with a palette of sensitive functional groups, providing attractive products for medicinal chemistry programs. Furthermore, decarbonylative Liebeskind-Srogl cross-coupling by C(acyl)-S/C(aryl)-C(O) cleavage is reported. Cu metal cofactor directs the reaction pathway to acyl or decarbonylative pathway. This reactivity is applicable to complex pharmaceuticals. The reaction represents the mildest decarbonylative Suzuki cross-coupling discovered to date. The Cu-directed divergent acyl and decarbonylative cross-coupling of thioesters opens up chemical space in complex molecule synthesis.

Dearomatizing [2+2+1] Spiroannulation of Indoles with Alkynes

A palladium-catalyzed dearomatizing [2+2+1] spiroannulation of indoles with two molecular internal alkynes is developed in the presence of Cu(OAc)₂/O₂ as the oxidant, in which a domino sequence including C-H activation of indole followed by consecutive Heck reactions is involved. A range of 3,3'-spiroindolines bearing tetrasubstituted cyclopentadiene moieties and exocyclic C═C bonds at C2 are obtained in moderate to excellent yields.

Hydride Relay Exchange Mechanism for the Heterocyclic C-H Arylation of Benzofuran and Benzothiophene Catalyzed by Pd Complexes

The mechanism and regioselectivity of the heterocyclic C-H arylation of benzofuran and benzothiophene catalyzed by Pd(OAc)₂ complexes were investigated using the density functional theory (DFT) method. The Pd(0)L₂(PhI) complex (L = HOAc) is proposed to be the catalytic species. Compared to the traditional Heck-type mechanism, concerted metalation-deprotonation (CMD) mechanism, and electrophilic aromatic substitution (S_EAr) mechanism for the C-H arylation, a new hydride relay exchange mechanism was proposed for the benzoheterocyclic C-H arylation catalyzed by Pd complexes, which consists of two redox processes between Pd(II) and Pd(0) species to complete the regioselective C-H activation. The calculated results indicate that the reaction along the hydride relay exchange mechanism is more favorable than those along other mechanisms, including the traditional Heck-type mechanism and the base-assisted anti-H elimination mechanism. This agrees well with the experimental results. Meanwhile, the origin for the regioselective C-H arylation was unveiled in which the α-C-H arylation products are major for the heterocyclic C-H arylation of benzofuran, but the β-C-H arylation products are major for that of benzothiophene. This study might provide a deep mechanistic understanding on the regioselective C-H activation and arylation of benzoheterocycle compounds catalyzed by transition-metal complexes.

Dearomative Michael addition involving enals and 2-nitrobenzofurans realized under NHC-catalysis

In this manuscript, the first enantioselective dearomative Michael addition between α,β-unsaturated aldehydes and 2-nitrobenzofurans realized under N-heterocyclic carbene activation has been described. The reaction proceeds via addition of homoenolate to Michael acceptors leading to the formation of biologically important heterocycles with high yields and stereoselectivities. Their functionalization potential has been confirmed in selected, diastereoselective transformations.

Enantioselective Dearomative [3 + 2] Umpolung Annulation of N-Heteroarenes with Alkynes

Enantioselective [3 + 2] annulation of N-heteroarenes with alkynes has been developed via a cobalt-catalyzed dearomative umpolung strategy in the presence of chiral ligand and reducing reagent. A variety of electron-deficient N-heteroarenes, including quinolines, isoquinolines, quinoxaline, and pyridines, and internal or terminal alkynes are employed in this reaction, showing a broad substrate scope and good functionality tolerance. Annulation of electron-rich indoles with alkynes is also developed. This protocol provides a straightforward access to a variety of N-spiroheterocyclic molecules in excellent enantioselectivities (76 examples, up to 99% ee).

NiH-catalyzed dearomative hydroalkylation of indoles

A NiH-catalyzed intermolecular dearomative hydroalkylation of indoles has been developed, which assembles an array of structurally diverse C2-alkylated indolines with moderate to excellent yields. This protocol exhibits good functional group...

Visible-Light-Mediated Cross Dehydrogenative Coupling of Thiols with Aldehydes: Metal-Free Synthesis of Thioesters at Room Temperature

Thioesters play a crucial role in biological systems and serve as important building blocks for organic synthesis. Herein, Eosin Y and TBHP mediated photochemical cross dehydrogenative coupling (PCDC) between feedstock aldehydes and thiols has been described at room temperature to synthesize thioesters. This thioesterification protocol proceeds smoothly to give the desired products in good to excellent yields by the suitable PCDC of both alkyl/aryl- aldehydes with a variety of alkyl/aryl-thiols and generates water and ^tBuOH as green byproducts. This method is also found to be scalable with good efficiency. Mechanistic investigations reveal that under this photochemical condition, the formation of acyl radical can be achieved from aldehyde. This acyl radical was further intercepted with an intermediate disulfide, generated in situ via the dehydrogenation of thiol to give the desired thioester. Moreover, disulfides, which are relatively easier to handle, also provided good to excellent yields in the optimized reaction condition. This protocol was further extended toward the more challenging direct transformation of alcohols to thioesters.

Nickel-Catalyzed Dearomative Arylboration of Indoles: Regioselective Synthesis of C2- and C3-Borylated Indolines

Indole dearomatization is an important strategy to access indolines: a motif present in a variety of natural products and biologically active molecules. Herein, a method for transition-metal catalyzed regioselective dearomative arylboration of indoles to generate diverse indolines is presented. The method accomplishes intermolecular dearomatization of simple indoles through a migratory insertion pathway on substrates that lack activating or directing groups on the C2- or C3-positions. Synthetically useful C2- and C3-borylated indolines can be accessed through a simple change in N-protecting group in high regio- and diastereoselectivities (up to >40:1 rr and >40:1 dr) from readily available starting materials. Additionally, the origin of regioselectivity was explored experimentally and computationally to uncover the remarkable interplay between carbonyl orientation of the N-protecting group on indole, electronics of the C2-C3 π-bond, and sterics. The method enabled the first enantioselective synthesis of (-)-azamedicarpin.

Asymmetric Dearomative (3+2)-Cycloaddition Involving Nitro-Substituted Benzoheteroarenes under H-Bonding Catalysis

In our studies, the organocatalytic 1,3-dipolar cycloaddition between 2-nitrobenzofurans or 2-nitrobenzothiophene and N-2,2,2-trifluoroethyl-substituted isatin imines has been developed. The reaction has been realized by employing bifunctional organocatalysis, with the use of squaramide derivative being crucial for the stereochemical efficiency of the process. The usefulness of the cycloadducts obtained has been confirmed in selected transformations, including aromative and non-aromative removal of the nitro group.

Catalytic asymmetric dearomative [4 + 2] annulation of 2-nitrobenzofurans and 5H-thiazol-4-ones: stereoselective construction of dihydrobenzofuran-bridged polycyclic skeletons

An organocatalytic asymmetric dearomative [4 + 2] annulation of 2-nitrobenzofurans and 5H-thiazol-4-ones is developed for the construction of dihydrobenzofuran-bridged polycyclic skeletons with good results.