Palladium-Catalyzed Asymmetric Intramolecular Dearomative Heck Annulation of Aryl Halides to Furnish Indolines

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.

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.

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 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.

Palladium-Catalyzed Enantioselective Intramolecular Heck Dearomative Annulation of Indoles with N-Tosylhydrazones

An elegant Pd(dba)2-catalyzed enantioselective Heck dearomative annulation of indoles and N-tosylhydrazones for the straightforward assembly of structurally diverse optically active indoline scaffolds containing the quaternary carbon centers at the C2 position has been developed. The tandem protocol, which utilized a Pd(dba)2/BINOL-based phosphoramidite ligand as the catalytic system, proceeded smoothly through successive oxidative addition, intramolecular carbon palladation, migratory insertion, and β-elimination sequences, leading to the chiral indoline derivatives in moderate to excellent yields, with excellent enantioselectivities and diastereoselectivities. In addition, the synthetic practicability of the catalytic system was underlined by a scaled-up experiment and the late-stage derivatization of the products, thus highlighting the potential applications in synthetic chemistry, medicinal chemistry, and material science.

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.

Palladium-Catalyzed Intramolecular Heck Dearomative Alkenylation of Indoles with N-Tosylhydrazones

An elegant Pd-catalyzed intramolecular Heck dearomative alkenylation of aryl iodides with functionalized N-tosylhydrazones proceeded through a sequential dearomative carbopalladation, migratory insertion, and β-hydride elimination in the presence of Pd(CF₃COO)₂ (10 mol %), PPh₃ (30 mol %), and Cs₂CO₃ (2.0 equiv) in 1,4-dioxane (2.0 mL) at 120 °C for 14 h under an argon atmosphere. This cascade cycloaddition protocol provided a reliable and versatile approach to a sequence of structurally diverse indolines in moderate to good yields with good functional group compatibility. In addition, the synthetic robustness of the methodology is highlighted by a scaled-up experiment and derivatization of products via epoxidation and reduction reactions.

Enantioselective Pd-catalyzed dearomative reductive Heck and domino Heck-Suzuki reactions of 2-CF3-indoles

Highly enantioselective palladium-catalyzed dearomative reductive Heck reaction and domino Heck-Suzuki reaction of 2-CF₃-indoles have been developed. Using Pd(OAc)₂/(R)-Synphos as the catalyst and Et₃SiH as a hydride source, a variety of indolines bearing a 2-trifluoromethyl quaternary stereocenter were obtained via a dearomative reductive Heck reaction. Alternatively, using Pd(dba)₂/phosphoramidite as the catalyst and Ar₄BNa as a coupling partner, structurally diverse indolines containing two vicinal carbon stereocenters were afforded through the domino dearomative Heck-Suzuki reaction.

Palladium-Catalyzed Three-Component Dearomatization/Sulfonylation Cascade

The diastereoselective synthesis of sulfonylated indolines is reported. A palladium-catalyzed dearomative sulfination of (aza)indole-tethered aryl iodides generates reactive benzylic sulfinates. These intermediates react with electrophiles in a one-pot, two-step process to generate sulfonylated products in good yields and excellent diastereoselectivity. This three-component sequence demonstrates good scalability and can be applied toward the synthesis of sulfonamides. Additionally, further derivatizations of aryl iodide containing products furnish spiro- and alkynylated indoline products.

Palladium-Catalyzed Asymmetric Dearomative Carbonylation of Indoles

Herein, we disclose a strategy for the asymmetric dearomatization of N-arylacyl indoles via a palladium-catalyzed tandem Heck/carbonylation, leading to an array of indoline-3-carboxylates bearing vicinal C2-aza-quaternary and C3 tertiary stereocenters in high yields and excellent enantio- and diastereoselectivities. This study is an important advance in the field of asymmetric carbonylation and enantioselective dearomatization reactions.