Enantioselective Synthesis of Fused Isocoumarins via Palladium-Catalyzed Annulation of Alkyne-Tethered Malononitriles

Pd-Catalyzed Tandem Reaction of [1,1'-Biaryl]-2,2'-dimethylnitriles with Arylboronic Acids: Synthesis of 7-Aryl-2H-azepin-2-ones

The palladium-catalyzed addition-cyclization cascade of [1,1'-biaryl]-2,2'-dimethylnitriles with arylboronic acids was developed, providing an efficient one-pot strategy for constructing 7-aryl-2H-azepin-2-one scaffolds. The protocol demonstrates excellent chemoselectivity, enabling the synthesis of benzo[c]pyrido[2,3-e]azepin and benzo[c]thieno[3,2-e]azepin scaffolds through the selective aryl addition of arylboronic acids to the electron-rich aryl nitrile moiety. The method demonstrates the use of easy availability starting materials, achieving high efficiency, broad applicability across diverse substrates, and excellent functional group compatibility.

Palladium(II)-Catalyzed Desymmetrization of 2-Alkynylbenzoate-Tethered Cyclohexadienones for the Synthesis of Polycyclic Compounds

Pd-catalyzed cascade cyclization of 2-alkynylbenzoate-tethered cyclohexadienone was developed for the single step synthesis of isocoumarin-fused dihydrochromenones. This highly atom economical method proceeds via 6-endo-dig cyclization of tert-butyl benzoate with a tethered alkyne. By modulating the reaction solvent and metal catalyst, the methodology was diverted for the purposeful synthesis of either of the E/Z tetra-substituted alkenes.

Stereoselective Intramolecular Oxypalladation of Cyclic 1,3-Dione-Tethered Alkynes

An unconventional, highly stereoselective, and regioselective intramolecular oxypalladation/acyloxylation cascade of alkyne-tethered cyclic 1,3-diones has been developed. This atom-economic Pd(II)-catalyzed annulation reaction is initiated by the oxypalladation of alkyne with an internal carbonyl group via 5-exo-dig cyclization instead of the conventional acetoxypalladation pathway. In this process, the carboxylic acid plays a pivotal role in the generation of an active cationic Pd-complex and the subsequent acyloxylation and proto-demetalation steps. Additionally, this method enables asymmetric cyclization using a chiral bidentate BOX ligand, achieving enantiomeric ratios of ≤93:7.

Pd(II)-catalyzed cyclization of alkyne-tethered malononitriles via nitrile insertion

Herein, we have developed a Pd(II)-catalyzed cyclization of prochiral alkyne-tethered malononitriles to access five-membered carbocycles having a nitrile-containing all-carbon quaternary center. The reaction pathway involves a trans-acetoxypalladation, nitrile group insertions into the carbon-palladium bond and sequential deacetylation followed by N-acetylation. Initial studies on asymmetric cyclization were also performed with chiral Pyox ligands.

Palladium-Catalyzed Asymmetric Acetoxylative Cyclization/Acyl Transfer Cascade of Alkyne-Tethered Malononitriles with Carboxylic Acids

Palladium-catalyzed asymmetric intermolecular trans-acetoxypalladation/desymmetric cyclization/acyl transfer cascades of alkyne-tethered malononitriles with carboxylic acids have been demonstrated. Such a sequence enables the formation of multifunctionalized nitriles bearing α-all-carbon quaternary stereocenters with a high degree of enantiocontrol with a broad substrate scope. Moreover, synthetic elaborations present these multifunctionalized molecules as promising chiral building blocks. Mechanistic studies illustrate that the cascade process proceeds via a key imine intermediate, and the desymmetric cyclization is the enantio-determining step.

Asymmetric Amplification in Oxypalladation/Malononitrile Addition Cascade Enabled by Heterochiral-Assembly of Products

An enantioselective oxypalladation/malononitrile addition cascade reaction of alkyne-tethered malononitriles was reported to synthesize enaminones bearing an all-carbon quaternary center. Using Pd(TFA)2/Pyox as a precatalyst, an array of enaminone products were obtained in moderate overall yields, with excellent er (93.5:6.5-99.5:0.5) in solution phase and nearly racemic in solid phase. The usage of 3,5-dinitrobenzoic acid as the nucleophile was proven to be crucial to this significant chiral amplification due to a heterochiral self-assembly of the products through an intermolecular multiple π-π stacking interaction. A strong positive nonlinear effect (NLE) was observed in solution phase, which was further highlighted by the access to an enantiopure enaminone (99.5:0.5 er) even using a partially enantiopure catalyst (54:46 er).

Cascade Oxypalladation/1,3-Palladium Shift to Access Cyclopentene-Fused Isocoumarins

Fused isocoumarins are frequently found in several natural products and pharmaceuticals. Herein, a cascade annulation of 2-alkynylbenzoate-tethered cyclic 1,3-diones via sequential trans-oxypalladation, carbonyl insertion, 1,3-Pd shift, and β-hydride elimination is reported. This method provides efficient access to highly diastereoselective tetracyclic cyclopentene-fused isocoumarins containing two contiguous quaternary stereocenters. A plausible reaction mechanism is proposed on the basis of mechanistic studies, including deuterium labeling experiments. Studies toward enantioselective synthesis using a chiral Bpy ligand gave encouraging initial results.

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.

Access to Cyclopentenone via Pd(0)-Catalyzed Reductive Cyclization of (Z)-5-Iodo-4-pentenenitrile

A ligand-free Pd(0)-catalyzed cyclization of (Z)-5-iodo-4-pentenenitrile has been realized with the help of HCO2H reductant, which is followed by imine hydrolysis to afford cyclopentenone derivatives in moderate to good yields. Control experiments clearly points out that the proton originally came from HCO2H plays a crucial role on protonation of the resultant imido-Pd2+ intermediate. This protocol features mild condition, operational simplicity, as well as wide functional group tolerance.

Efficient Synthesis of 1H-Benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-one Derivatives Using Ag2CO3/TFA-Catalyzed 6-endo-dig Cyclization: Reaction Scope and Mechanistic Study

A small library of 1H-benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-one derivatives was prepared in good to excellent yields, involving a Ag₂CO₃/TFA-catalyzed intramolecular oxacyclization of N-Boc-2-alkynylbenzimidazole substrates. In all experiments, the 6-endo-dig cyclization was exclusively achieved since the possible 5-exo-dig heterocycle was not observed, indicating the high regioselectivity of this process. The scope and limitations of the silver catalyzed 6-endo-dig cyclization of N-Boc-2-alkynylbenzimidazoles as substrates, bearing various substituents, were investigated. While ZnCl₂ has shown limits for alkynes with an aromatic substituent, Ag₂CO₃/TFA demonstrated its effectiveness and compatibility regardless of the nature of the starting alkyne (aliphatic, aromatic or heteroaromatic), providing a practical regioselective access to structurally diverse 1H-benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-ones in good yields. Moreover, the rationalization of oxacyclization selectivity in favor of 6-endo-dig over 5-exo-dig was explained by a complementary computational study.

DFT Study of Mechanism and Stereochemistry of Nickel-Catalyzed trans-Arylative Desymmetrizing Cyclization of Alkyne-Tethered Malononitriles

Present here is a density functional theory (DFT) study of the mechanism and origin of enantioselectivity of Ni-catalyzed desymmetric cyclization of alkyne-tethered malononitriles and aryl boronic acids. The reaction starts from transmetalation and arylnickel addition, followed by trans to cis isomerization to give cis-alkenyl nickel species. The stereodetermining step is the CN insertion, which prefers a transition state with the bystander CN group staying away from the ligand to reduce steric repulsion, and gives the final (R)-product.