Palladium-Catalyzed Cascade Cyclization for the Synthesis of Fused Benzo-Aza-Oxa-[5-6-5] Tetracycles

Dual Inert C-H Bond Site-Selective Activations Enabled by Pd/Norbornene-Mediated Cascade Cyclization toward Medium-Sized Polyheterocyclic Methylene Sulfoximines

A Pd/norbornene-mediated three-component modular one-step reaction facilitated by dual C-H bond activation and cascade cyclization is reported. This procedure uses norbornene as a catalyst in the Catellani-type reaction and as an alkylating building block to accomplish the dual unactivated C-H bond functionalization protocol, which results in the production of polyheterocyclic eight-membered sulfoximines with an indene-fused moiety. This mild, scalable protocol's wide substrate range makes it ideal for site-selective dual C-H functionalization at the highly chemoselective aryl sites.

Palladium catalyzed ortho-C(sp2)-H activation/cyclization of aryl amines assisted by imine and vinylacetic acid

Palladium-catalyzed directed C - H functionalization/cyclization is an effective approach for synthesizing nitrogen heterocycles. Imine, known for its ease of installation/removal, has been extensively used in the C-H activation of aldehydes, ketones, and alkylamines. Nevertheless, it has been rarely explored in the C(sp2)-H activation of aryl amines because of the generation of a strained four-membered palladacycle. Herein, an imine directed palladium catalyzed C(sp2)-H functionalization of aryl amines assisted by vinylacetic acid is established, providing access to a variety of γ-lactone fused tetrahydroquinolines under mild reaction conditions. The methodology demonstrates broad substrate scope and good functional group tolerance, representing notable advancement in organic synthesis. Mechanistic experiments are performed to clarify how the C(sp2)-H activation occurs, indicating the crucial role of vinylacetic acid. DFT calculations supports the observations, elucidating the strained four-membered ring C-H activation barrier is overcome via coordination and hydrogen bond interaction of vinylacetic acid.

Pd-catalyzed Markovnikov selective oxidative amination of 4-pentenoic acid

Pd-catalyzed regioselective amination of unactivated alkene remains a challenge and is of great interest. Herein, a palladium-catalyzed and ligand-controlled strategy for the Markovnikov selective oxidative amination of 4-pentenoic acid has been described. The protocol effectively reverses the carboxylic acid-directed anti-Markovnikov selectivity in oxidative amination of 4-pentenoic acid, successfully constructing γ-ketoamide derivatives.

Palladium-catalyzed 1,1-aminoxylation of 3-butenoic acid with 2-alkynylanilines

Herein, a palladium-catalyzed 1,1-aminoxylation of 3-butenoic acid and 2-alkynylanilines has been developed, achieving the installation of two distinct heteroatom motifs across an olefin skeleton. The strategy features a high step and atom economy and good functional group tolerance, which outlines an efficient approach for simultaneously building up γ-butylactone and indole skeletons. Notably, an external ligand, 2,9-dimethyl-1,10-phenanthroline, has been used to succeed in this protocol to effectively suppress the production of indole byproducts.

One-Pot Tandem/Spirocyclization Reaction: Synthesis of Spiro[pyridine-thiazolidine] Ring Derivatives

An efficient one-pot, three-component approach was devised to synthesize spiro[pyridine-thiazolidine] ring skeletons using thiazole salts, aldehydes, and enaminones. This method allows the assembly of structurally diverse spiroazepines through [3 + 1 + 2] tandem/spirocyclization reactions. This spirocyclization reaction offers several advantages, including transition metal-free conditions, high chemoselectivity, and the ability to construct structurally novel polycyclic compounds.

Palladium-catalyzed cascade of aza-Wacker and Povarov reactions of aryl amines and 1,6-dienes for hexahydro-cyclopenta[b]quinoline framework

Palladium catalyzed tandem reaction represents a one-pot synthetic approach to efficiently synthesize complex functionalized molecules while reducing synthetic steps, aligning with the principles of green chemistry. However, achieving a direct cascade of the aza-Wacker and Povarov reactions in one-pot synthesis presents a challenge due to substrate compatibility issues between the two reactions. In this work, we describe an aza-Wacker/Povarov reaction employing a highly electrophilic palladium catalyst, which effectively converts anilines and 1,6-dienes into hexahydro-cyclopenta[b]quinolines. The optimized conditions yield up to 79%, with a diastereoselectivity > 20:1. Substrate range testing reveals compatibility with various sensitive functional groups, and successful late-stage modifications are performed on several natural products and drug molecules, demonstrating the versatility and practicality of the method. Additionally, a preliminary investigation into the reaction mechanism suggests an aza-Wacker process followed by a Povarov process.

Palladium-Catalyzed Annulation of Tertiary Anilines with 3-Butenoic Acid via Dual C-H Bond Activation

Catalytic cyclization via dual C-H bond activation has evolved as a powerful strategy for building bi- and polycyclic molecules. Herein, a palladium-catalyzed annulation of tertiary anilines with 3-butenoic acid via N-α-C(sp3)-H and ortho-C(sp2)-H activation is described. The remarkable characteristics of this reaction include excellent diastereoselectivity, broad substrate scope, and good tolerance for some highly sensitive groups. In addition, the KIE experiment suggested that the C-H bond abscission is not the turnover-limiting step.

Redox-neutral α-functionalization of pyrrolidines: facile access to α-aryl-substituted pyrrolidines

α-Aryl-substituted pyrrolidine moiety is found in many natural alkaloids. Starting from pyrrolidine, we were able to synthesize α-aryl-substituted pyrrolidines in one step using quinone monoacetal as the oxidizing agent and DABCO as the base. We also discovered the reaction condition needed to efficiently remove the N-aryl moiety from the α-arylated product. When the above reaction was carried out without the addition of an aryl nucleophile, the reaction of pyrrolidine and quinone monoacetal in 2,2,2-trifluoroethanol afforded octahydro-dipyrroloquinoline in high yield, which has the same skeleton as that of natural product incargranine B.

Palladium Catalysis: Dependence of the Efficiency of C-N Bond Formation on Carboxylate Ligand and Metal Carboxylate or Carboxylic Acid Additive

The Pd-catalyzed inter- and intramolecular reactions of nitrogen compounds are often carried out with palladium carboxylates, sometimes in the presence of carboxylic acids or alkali metal carboxylates. This Mini-Review highlights the dependence of the reaction efficiency on the nature of the ligand and the carboxylate additives. The proposed reaction mechanisms are presented with, as far as possible, personal comments.