Recent advances in Pd-catalyzed asymmetric cyclization reactions

Synthesis of Fused Polycyclic Indole-Indenone Scaffolds via Pd-Catalyzed Sequential Cycloaddition Reactions

A Pd-catalyzed Sonogashira coupling of iodoalkynones with propargylic ethers bearing indole-tethered unactivated alkenes, followed by an Alder-ene reaction and a Diels-Alder cycloaddition, provides access to various hexacyclic heterocycles containing a dihydropyrido[1,2-a]indole scaffold. When similar N-propargylic allylic ethers were applied to the reaction, polycyclic compounds containing pyrido[1,2-a]indole scaffolds could be obtained after treatment of the fused cyclic products with trifluoroacetic acid. The method features a broad substrate scope, high stereoselectivity, and mild reaction conditions.

Group-Directed [5+2] Cycloaddition Approach to 10π-Electron Cyclohepta[c]furans and Derivatives

We present a group-assisted protocol that enables the 1,3-dien-5-yne scaffold as a novel five-carbon synthon to accomplish a palladium-catalyzed [5+2] cycloaddition with a variety of alkynes in the presence of H2O. This method affords a straightforward approach to the synthesis of 10π-electron 6H-cyclohepta[c]furans that are versatile building blocks for constructing diverse cycloheptatriene-fused molecular architectures.

Construction of Novel Chiral NPN Ligands and the Application in Gold(I)-Catalyzed Asymmetric Cycloaddition Reactions

This paper reports a novel chiral NPN ligand derived from the modification of Ming-Phos. This series of ligands possess advantages such as easy synthesis and convenient modification. Moreover, they exhibit excellent enantioselectivity in gold(I)-catalyzed asymmetric [3 + 3] cycloaddition reactions.

Recent Advances on Epoxide- and Aziridine-Based [3+2] Annulations

[3+2] Annulations are a powerful method for the synthesis of five-membered heterocyclic compounds. The annulations have concerted cycloaddition and formal (stepwise) cycloaddition reaction pathways. In addition to the well-established O-centered and N-centered ylides, epoxides and aziridines could serve as synthetic equivalent of 1,3-dipoles for [3+2] annulation with dipolarophiles for making functionalized tetrahydrofuran, pyrrolidine, and associated compounds. This review article covers recent development on epoxide- and aziridine-based [3+2] annulation reactions. The reactions are classified based on the ring opening conditions, including acid/base catalysis, organocatalysis, and transitional-metal catalysis.

Enhanced Pyridine-Oxazoline Ligand-Enabled Pd(II)-Catalyzed Aminoacetoxylation of Alkenes for the Asymmetric Synthesis of Biaryl-Bridged 7-Membered N-Heterocycles and Atropisomers

A new class of binaphthyl unit-enhanced pyridine-oxazoline ligands was developed to promote the Pd-catalyzed enantioselective intramolecular 7-exo aminoacetoxylation of unactivated biaryl alkenes. Biaryl-bridged 7-membered N-heterocycles bearing a chiral center were obtained in good yields with excellent enantioselectivities (up to 99:1 er). Computational investigations on a series of biaryl-bridged 7-membered rings provided insights into the rotational barrier of the potentially chiral biaryl unit by the substituent effect including the heteroatom, the protecting group, and the chiral center. The kinetic resolution of racemic axially chiral biaryls via intramolecular enantioselective aminoacetoxylation of alkenes has also been achieved, affording previously inaccessible biaryl-bridged 7-membered N-heterocycles bearing both a chiral center and a chiral axis, as well as axially chiral biaryl amino alcohols.

Gold/silver-catalyzed synthesis of functionalized indoles from N-allyl-2-alkynylanilines and α-diazo compounds via 1,3-allyl migration

Gold-catalyzed synthesis of decorated indoles has been developed through carbene insertion into N-H bonds of N-allyl-2-(aryl/alkyl ethynyl)anilines using α-diazo compounds followed by cyclization and concomitant 1,3 migration of allyl fragments. The developed protocol tackles the inherited challenge of direct C3 functionalization and eliminates the need for a tertiary aniline precursor for the 1,3-migration reaction. The applicability of this transformation is showcased through the practical synthesis of analogs of small drug-like and pharmaceutically relevant molecules such as ibuprofen, estradiol, menthol, and borneol, etc. The mechanism is well supported by control experiments and isolation of the reaction intermediate.

Palladium-Catalyzed Site-Selective C-H Sulfonylation via Aryl Thianthrenium Salts to Access Diarylsulfones

We presented a highly efficient palladium-catalyzed site-selective C-H sulfonylation reaction via aryl thianthrenium salts. By utilizing readily available and cost-effective arenes along with sodium sulfinates, we achieved the C(sp2)-S cross-coupling with high efficiency, establishing a dependable method for synthesizing diarylsulfones with satisfactory yields. This method exhibits excellent tolerance toward functional groups, scalability, and the synthesis or late-stage functionalization of bioactive molecules, making it a valuable sulfonylation tool for drug modifications.

Enantioselective Heck/Tsuji-Trost reaction of flexible vinylic halides with 1,3-dienes

The enantioselective domino Heck/cross-coupling has emerged as a powerful tool in modern chemical synthesis for decades. Despite significant progress in relative rigid skeleton substrates, the implementation of asymmetric Heck/cross-coupling cascades of highly flexible haloalkene substrates remains a challenging and and long-standing goal. Here we report an efficient asymmetric domino Heck/Tsuji-Trost reaction of highly flexible vinylic halides with 1,3-dienes enabled by palladium catalysis. Specifically, the Heck insertion as stereodetermining step to form ƞ3 allyl palladium complex and in situ trapping with nucleophiles enable efficient Heck/etherification in a formal (4 + 2) cycloaddition manner. Engineering the Sadphos bearing androgynous non-C2-symmetric chiral sulfinamide phosphine ligands are vital component in achieving excellent catalytic reactivity and enantioselectivity. This strategy offers a general, modular and divergent platform for rapidly upgrading feedstock flexible vinylic halides and dienes to various value-added molecules and is expected to inspire the development of other challenging enantioselective domino Heck/cross-couplings.

Pd-Catalyzed Migratory 1,1-Cycloannulation Reaction of Alkenes

Here, we report a novel strategy for the preparation of diverse heterocycles via a Pd-catalyzed migratory 1,1-cycloannulation reaction (MCAR) of alkenes. Starting from readily available alkenyl amines and alkenyl alcohols, this approach allows the formation of a wide range of five- to seven-membered azaheterocycles and oxaheterocycles with high efficiency and good functional group tolerance. The key to the realization of this reaction is the use of 4-iodophenol or 2-iodophenol derivatives where the phenolic hydroxyl group plays a critical role in controlling the direction of migration and the ring-size of the heterocycles through the formation of a quinone methide intermediate.

Recent Developments in the Metal-Catalyzed Synthesis of Nitrogenous Heterocyclic Compounds

Metal-catalyzed cyclization reactions have become a powerful and efficient approach for the stereoselective construction of both carbocyclic and heterocyclic ring systems. Transition metal complexes, with their ability to activate and selectively functionalize organic substrates, have revolutionized various areas of synthetic chemistry. This review highlights recent advancements in metal-catalyzed cyclization reactions, especially in the synthesis of nitrogen-containing heterocycles like imidazoles, pyridines, pyrimidines, and indoles. These advancements have significantly impacted fields such as natural product synthesis, pharmaceuticals, functional materials, and organic electronics. Novel catalytic systems, ligand designs, and reaction conditions continue to expand the capabilities of these reactions, driving further the progress made in synthetic organic chemistry. This review provides a comprehensive overview of recent research.

Enantioselective Intramolecular Oxidative Aminoacetoxylation from Aryl-Substituted Alkene via Chiral Triazole-oxazoline Palladium Complexes

Here, we describe an enantioselective intramolecular oxidative aminoacetoxylation reaction using a palladium catalyst and an aryl-substituted internal alkene compound as the substrate under mild conditions in several hours. The triazole-oxazoline ligand was selected for the asymmetric catalyst. A range of enantioenriched pyrrolidine-derived vicinal amino acetate compounds were synthesized, showing yields from 47% to 84%, diastereomer ratios from 57:43 to 95:5, and enantiomer excesses from 67% to 92%.

Transition-metal-catalyzed enantioselective C-N cross-coupling

Chiral amine scaffolds are among the most important building blocks in natural products, drug molecules, and functional materials, which have prompted chemists to focus more on their synthesis. Among the accomplishments in chiral amine synthesis, transition-metal-catalyzed enantioselective C-N cross-coupling is considered one of the most efficient protocols. This approach combines traditional C(sp2)-N cross-coupling methods (such as the Buchwald-Hartwig reaction Ullmann-type reaction, and Chan-Evans-Lam reaction), aryliodonium salt chemistry and radical chemistry, providing an attractive pathway to a wide range of structurally diverse chiral amines with high enantioselectivity. This review summarizes the established protocols and offers a comprehensive outlook on the promising enantioselective C-N cross-coupling reaction.

Pd/Cu-Cocatalyzed Enantio- and Diastereodivergent Wacker-Type Dicarbofunctionalization of Unactivated Alkenes

The Wacker and Wacker-type reactions are some of the most fundamental and powerful transformations in organic chemistry for their ability to efficiently produce valuable chemicals. Remarkable progress has been achieved in asymmetric oxy/aza-Wacker-type reactions; however, asymmetric Wacker-type dicarbofunctionalization remains underdeveloped, especially for the concurrent construction of two stereocenters. Herein, we report a Pd/Cu-cocatalyzed enantio- and diastereodivergent Wacker-type dicarbofunctionalization of alkene-tethered aryl triflates with imino esters. A series of 2-indanyl motifs bearing adjacent carbon stereocenters could be easily synthesized in moderate to excellent yields and with good to excellent diastereo- and enantioselectivities (up to >20:1 dr and >99% ee). Density functional theory calculations revealed that the origin of diastereoselectivity in this Pd/Cu synergistic catalytic system is jointly determined by both the intermolecular anti-carbopalladation of alkenes and the reductive elimination processes, in accordance with the Curtin-Hammett principle.

Nickel-Catalyzed Narasaka-Heck Cyclization Carbonylation of Unsaturated Oxime Esters with Arylboronic Acids

The Narasaka-Heck reaction is one of the most straightforward methods for constructing pyrroline derivatives. Herein, we report a novel nickel-catalyzed three-component carbonylation reaction, which cleverly realizes the continuous construction of C(sp3)-N bonds and C(sp3)-C(sp2) bonds and effectively promotes the synthesis of acyl-substituted pyrroline derivatives. Furthermore, this strategy not only expands the conversion pathway of γ,δ-unsaturated oxime esters but also provides a new method for the synthesis of nitrogen-containing heterocyclic compounds.

Iridium-Catalyzed Asymmetric Hydroarylation of Unactivated Alkenes with Heterobiaryls: Simultaneous Construction of Axial and Central Chirality

There are only a few examples being reported for the simultaneous control of central chirality and axial chirality because it is more challenging. Herein, we report an iridium-catalyzed asymmetric hydroarylation of unactivated alkenes with heterobiaryls to simultaneously construct axial and central chirality. The reaction showed a broad substrate scope and delivered the products with satisfactory results. The results of the control experiments demonstrated that the FerroLANE ligand promotes the reaction to proceed along a specific modified Chalk-Harrod mechanism.

Palladium-Catalyzed Ligand-Controlled Switchable Hetero-(5 + 3)/Enantioselective [2σ+2σ] Cycloadditions of Bicyclobutanes with Vinyl Oxiranes

For nearly 60 years, significant research efforts have been focused on developing strategies for the cycloaddition of bicyclobutanes (BCBs). However, higher-order cycloaddition and catalytic asymmetric cycloaddition of BCBs have been long-standing formidable challenges. Here, we report Pd-catalyzed ligand-controlled, tunable cycloadditions for the divergent synthesis of bridged bicyclic frameworks. The dppb ligand facilitates the formal (5+3) cycloaddition of BCBs and vinyl oxiranes, yielding valuable eight-membered ethers with bridged bicyclic scaffolds in 100% regioselectivity. The Cy-DPEphos ligand promotes selective hetero-[2σ+2σ] cycloadditions to access pharmacologically important 2-oxabicyclo[3.1.1]heptane (O-BCHeps). Furthermore, the corresponding catalytic asymmetric synthesis of O-BCHeps with 94-99% ee has been achieved using chiral (S)-DTBM-Segphos, representing the first catalytic asymmetric cross-dimerization of two strained rings. The obtained O-BCHeps are promising bioisosteres for ortho-substituted benzenes.

Catalytic Asymmetric Cascade Dearomatization of Indoles via a Photoinduced Pd-Catalyzed 1,2-Bisfunctionalization of Butadienes

Photoinduced Pd-catalyzed bisfunctionalization of butadienes with a readily available organic halide and a nucleophile represents an emerging and attractive method to assemble versatile alkenes bearing various functional groups at the allylic position. However, enantiocontrol and/or diastereocontrol in the C-C or C-X bond-formation step have not been solved due to the open-shell process. Herein, we present a cascade asymmetric dearomatization reaction of indoles via photoexcited Pd-catalyzed 1,2-biscarbonfunctionalization of 1,3-butadienes, wherein asymmetric control on both the nucleophile and electrophile part is achieved for the first time in photoinduced bisfunctionalization of butadienes. This method delivers structurally novel chiral spiroindolenines bearing two contiguous stereogenic centers with high diastereomeric ratios (up to >20 : 1 dr) and good to excellent enantiomeric ratios (up to 97 : 3 er). Experimental and computational studies of the mechanism have confirmed a radical pathway involving excited-state palladium catalysis. The alignment and non-covalent interactions between the substrate and the catalyst were found to be essential for stereocontrol.

Advances in the synthesis of rearranged homoisoflavonoids

Rearranged homoisoflavonoids constitute a unique group of natural products, renowned for their structural diversity and complexity. These compounds, derived from modifications in the 3-benzylchroman skeleton, are categorized into four subclasses: brazilin, caesalpin, protosappanin, and scillascillin homoisoflavonoids. This review examines the advancements in the total synthesis of these complex structures, aiming to highlight the challenges and opportunities encountered. A comparative analysis of the strategies employed thus far to synthesize these compounds provides a comprehensive understanding of the progress in this field.

Kinetic resolution of 1-(1-alkynyl)cyclopropyl ketones via gold-catalyzed divergent (4 + 4) cycloadditions: stereoselective access to furan fused eight-membered heterocycles

Chiral eight-membered heterocycles comprise a diverse array of natural products and bioactive compounds, yet accessing them poses significant challenges. Here we report a gold-catalyzed stereoselective (4 + 4) cycloaddition as a reliable and divergent strategy, enabling readily accessible precursors (anthranils and ortho-quinone methides) to be intercepted by in situ generated gold-furyl 1,4-dipoles, delivering previously inaccessible chiral furan/pyrrole-containing eight-membered heterocycles with good results (56 examples, all >20 : 1 dr, up to 99% ee). Moreover, we achieve a remarkably efficient kinetic resolution (KR) process (s factor up to 747). The scale-up synthesis and diversified transformations of cycloadducts highlight the synthetic potential of this protocol. Computational calculations provide an in-depth understanding of the stereoselective cycloaddition process.

Asymmetric Pd/Organoboron-Catalyzed Site-Selective Carbohydrate Functionalization with Alkoxyallenes Involving Noncovalent Stereocontrol

Herein, we demonstrate the robustness of a synergistic chiral Pd/organoboron system in tackling a challenging suite of site-, regio-, enantio- and diastereoselectivity issues across a considerable palette of biologically relevant carbohydrate polyols, when prochiral alkoxyallenes were employed as electrophiles. In view of the burgeoning role of noncovalent interactions (NCIs) in stereoselective carbohydrate synthesis, our mechanistic experiments and DFT modeling of the reaction path unexpectedly revealed that NCIs such as hydrogen bonding and CH-π interactions between the resting states of the Pd-π-allyl complex and the borinate saccharide are critically involved in the stereoselectivity control. Our strategy thus illuminates the untapped potential of harnessing NCIs in the context of transition metal catalysis to tackle stereoselectivity challenges in carbohydrate functionalization.

Chiral Osmium(II)/Salox Species Enabled Enantioselective γ-C(sp3)-H Amidation: Integrated Experimental and Computational Validation For the Ligand Design and Reaction Development

Herein, multiple types of chiral Os(II) complexes have been designed to address the appealing yet challenging asymmetric C(sp3)-H functionalization, among which the Os(II)/Salox species is found to be the most efficient for precise stereocontrol in realizing the asymmetric C(sp3)-H amidation. As exemplified by the enantioenriched pyrrolidinone synthesis, such tailored Os(II)/Salox catalyst efficiently enables an intramolecular site-/enantioselective C(sp3)-H amidation in the γ-position of dioxazolone substrates, in which benzyl, propargyl and allyl groups bearing various substituted forms are well compatible, affording the corresponding chiral γ-lactam products with good er values (up to 99 : 1) and diverse functionality (>35 examples). The unique performance advantage of the developed chiral Os(II)/Salox system in terms of the catalytic energy profile and the chiral induction has been further clarified by integrated experimental and computational studies.