Pd(II)-Catalyzed Asymmetric Oxidative Annulation of N-Alkoxyheteroaryl Amides and 1,3-Dienes

Copper-catalyzed enantioselective three-component radical 1,4-perfluoroalkylamination of 1,3-dienes

Catalytic enantioselective three-component aminative difunctionalization of readily available 1,3-dienes offers a straightforward methodology to fast access significant and complex chiral allylic amines. Nevertheless, compared to the widely studied two-component reactions, the three-component reactions, especially using anilines-very common bulk feedstock chemicals as aminating reagents are underdeveloped. More importantly, the limited examples of enantioselective three-component aminative difunctionalization of 1,3-dienes with anilines only showed 1,2-selectivity; and the corresponding 1,4-regioselectivity remains unknown. Here, we report a copper-catalyzed enantioselective radical three-component 1,4-perfluoroalkylamination of 1,3-dienes with anilines and perfluoroalkyl reagents, efficiently providing an array of valuable perfluoroalkylated chiral allylic amines in good to excellent yields with excellent enantioselectivity. Mechanistic investigations, including controlled experiments and DFT studies, elucidate the origination of the regioselectivity and enantioselectivity, and suggest a radical reaction pathway involving an asymmetric cross-coupling between allylic radical and copper-stabilized nitrogen radical species to construct C-N bond enantioselectively.

Enantioselective Synthesis of Hydrindanes via Palladium-Catalyzed Asymmetric Desymmetrization of Cyclohexadiene Derivatives

We herein disclose a strategy for the asymmetric desymmetrization of cyclohexadiene derivatives via a palladium-catalyzed Heck and tandem Heck/Tsuji-Trost allylic alkoxylation reaction. By employing DCE as the solvent, we obtained a variety of chiral hydrindanes containing an all-carbon quaternary carbon center and a tertiary carbon chiral center in good yields with excellent enantioselectivities. With alcohols as the solvent, the valuable chiral hydrindanes with one quaternary stereocenter and two tertiary centers were constructed with a high level of enantioinduction.

Dynamic Kinetic Activation of Aziridines Enables Radical-Polar Crossover (4 + 3) Cycloaddition with 1,3-Dienes

The cycloaddition of aziridines with unsaturated compounds is a valuable method for synthesizing nitrogen heterocycles. However, this process is predominantly substrate-controlled, posing significant challenges in regulating the regioselectivity of the C-N bond cleavage. In this study, we report a nickel-catalyzed dynamic kinetic activation strategy that enables catalyst-controlled activation of aziridines. Various types of aziridines, including 2-phenyl, 2-carbonyl, 2-alkyl, and disubstituted aziridines, consistently cleave their more sterically hindered C-N bonds to generate 1,3-radical anion intermediates. These intermediates participate in a highly regioselective 1,4-Heck/allylic substitution cascade with aromatic branched 1,3-dienes, resulting in a radical-polar crossover (4 + 3) cycloaddition that produces seven-membered azepine products. This approach not only complements traditional dipolar cycloaddition, in which aziridines typically act as zwitterionic 1,3-dipoles, but also introduces an unusual cycloaddition mode for 1,3-dienes. Experimental investigations and density functional theory (DFT) calculations provide insight into the reaction mechanism.

Photoredox Catalyzed Tandem Denitrogenative [4 + 2] Annulation of 1,2,3-Benzotriazin-4(3H)-ones with Terminal Olefins

The dihydroisoquinolones skeleton is ubiquitous in natural products and biological molecules. Reported strategies for constructing dihydroisoquinolones usually require noble metal catalysts or stoichiometric oxidants, which limit their wide applications. Herein, we developed a photoredox catalyzed tandem denitrogenative [4 + 2] annulation reaction of 1,2,3-benzotriazin-4(3H)-ones with terminal olefins. A variety of dihydroisoquinolones can be accessed in moderate to excellent yield. This protocol features high atom-economy, mild reaction conditions, and is external oxidant-free, enabling the synthesis of various substituted dihydroisoquinolones.

Cobalt-Catalyzed Synthesis of Alkenyl Heterocycles via Regioselective Intramolecular 1,4-Hydrofunctionalization of Dienes

We report a novel cobalt-catalyzed intramolecular 1,4-hydrofunctionalization of dienes. The reaction proceeds under mild conditions and is amenable to N- and O-nucleophiles. The protocol exhibits exclusive regioselectivity, yielding a number of different alkenyl heterocycles, including but not limited to dihydroisobenzofurans, isochromanes, tetrahydrofurans, morpholines, lactones, and isoindolines. Experimental studies were performed to offer some insight into the different mechanistic pathways and to rationalize the regio- and stereoselectivities of the reaction.

Assembly of 2-Substituted Tetrahydroquinolines from ortho-Methylbenzenesulfamides and Dienes, Using a C(sp3)-H Activation/Annulation Sequence

1,2,3,4-Tetrahydroquinolines (THQs) are essential structural cores in many natural products and pharmaceutical drugs. Especially relevant are those presenting substitutions at position 2, yet practical methods for their one-step assembly from acyclic precursors are very scarce. Herein, we present a straightforward approach to assembling these skeletons from ortho-methylanilines using a palladium-catalyzed C(sp3)-H activation/formal cycloaddition sequence. Key for the success of the approach is the use of dienes as partners, since they lead to stable π-allyl palladium intermediates that prevent β-hydride elimination processes and allow installation of versatile alkenyl handles at position 2. Moreover, installing a perfluorobenzenesulfonyl substituent at the amine not only facilitates the C-H activation but also allows for an easy recovery of the free amine.

Pd-Catalyzed Enantioselective Three-Component Carboamination of 1,3-Cyclohexadiene

Asymmetric Pd-catalyzed three-component carboamination reactions of dienes to construct chiral cyclohexenylamines, which are of great importance in many fields of chemistry, have remained largely unexplored. Here, we demonstrate a highly enantio- and regioselective Pd/Ming-Phos-catalyzed carboamination reactions of 1,3-cyclohexadiene with readily available aryl iodides and anilines for facile access to diverse valuable chiral cyclohexenylamines. The process shows excellent functional group tolerance, easy scalability, and mild conditions. Moreover, mechanistic studies suggest that this reaction has a first-order dependence on the concentration of the palladium catalyst and aniline.

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.

Synthesis of Chiral Diarylmethylamides via Catalytic Asymmetric Aza-Michael Addition of Amides to ortho-Quinomethanes

Chiral diarylmethylamides are a privileged skeleton in many bioactive molecules. However, the enantioselective synthesis of such molecules remains a long-standing challenge in organic synthesis. Herein, we report a chiral bifunctional squaramide catalyzed asymmetric aza-Michael addition of amides to in situ generated ortho-quinomethanes, affording enantioenriched diarylmethylamides in good yields with excellent enantioselectivities. This work not only provides a new strategy for the construction of the diarylmethylamides but also represents the practicability of amides as nitrogen-nucleophiles in asymmetric organocatalysis.

Palladium-Catalyzed Regiodivergent Three-Component Alkenylamination of 1,3-Dienes with Alkyl and Aryl Amines

We report a palladium-catalyzed method for 4,3- or 4,1-selective alkenylamination of terminal dienes. Three-component couplings proceed with alkenyl triflates and several amines, giving vicinal carboamination with a Xantphos-supported catalyst and distal difunctionalization with a phosphoramidite ligand. A number of constitutionally different disubstituted dienes also participate in regiodivergent carboaminations. Experimental evidence indicates that selectivity in the Xantphos reactions is largely influenced by the substrate, whereas the phosphoramidite-promoted process is catalyst controlled, orchestrated by a key π-stacking interaction among the ligand, solvent, and substrate.

The Construction of Highly Substituted Piperidines via Dearomative Functionalization Reaction

Nitrogen heterocycles play a vital role in pharmaceuticals and natural products, with the six-membered aromatic and aliphatic architectures being commonly used. While synthetic methods for aromatic N-heterocycles are well-established, the synthesis of their aliphatic functionalized analogues, particularly piperidine derivatives, poses a significant challenge. In that regard, we propose a stepwise dearomative functionalization reaction for the construction of highly decorated piperidine derivatives with diverse functional handles. We also discuss challenges related to site-selectivity, regio- and diastereoselectivity, and provide insights into the reaction mechanism through mechanistic studies and density functional theory computations.

Nucleophilic aromatization of monoterpenes from isoprene under nickel/iodine cascade catalysis

As a large number of organic compounds possessing two isoprene units, monoterpenes and monoterpenoids play important roles in pharmaceutical, cosmetic, agricultural, and food industries. In nature, monoterpenes are constructed from geranyl pyrophosphate (C10) via various transformations. Herein, the bulk C5 chemical-isoprene, is used for the creation of various monoterpenoids via a nucleophilic aromatization of monoterpenes under cascade catalysis of nickel and iodine. Drugs and oil mixtures from conifer and lemon can be convergently transformed to the desired monoterpenoid. Preliminary mechanistic studies are conducted to get insights about reaction pathway. Two types of cyclic monoterpenes can be respectively introduced onto two similar heterocycles via orthogonal C-H functionalization. And various hybrid terpenyl indoles are programmatically assembled from abundant C5 or C10 blocks. This work not only contributes a high chemo-, regio-, and redox-selective transformation of isoprene, but also provides a complementary approach for the creation of unnatural monoterpenoids.

Palladium-Catalyzed C-H Allylation/Annulation Reaction of Amides and Allylic Alcohols: Regioselective Construction of Vinyl-Substituted 3,4-Dihydroisoquinolones

A palladium-catalyzed highly regioselective C-H allylation/annulation reaction of N-sulfonyl amides with secondary or tertiary allylic alcohols has been developed to construct 3,4-dihydroisoquinolones bearing a synthetically valuable vinyl substituent. This cascade cyclization approach of allylic alcohols involving C-H allylation has not been reported previously. The commercially available allylic alcohol substrates, the only by-product of water, and the used terminal oxidant of O₂ provide environmentally benign advantages.

Copper-Catalyzed, N-Directed Distal C(sp3)-H Functionalization toward Azepanes

We herein report a copper-catalyzed formal [5 + 2] aza-annulation of N-fluorosulfonamides and 1,3-dienes/1,3-enynes for synthesis of structurally diverse alkene/alkyne-containing azepanes. The reaction features selective functionalization of distal unactivated C(sp³)-H bonds and a broad substrate scope, thus allowing the late-stage modification of pharmaceuticals and natural products. A radical mechanism involving 1,5-hydrogen atom transfer of N-radicals, facile coupling of alkyl radicals with 1,3-dienes/1,3-enynes, and the construction of azepane motifs via C-N bond formation is proposed.

Recent Advances in Palladium-Catalyzed Oxidative Couplings in the Synthesis/Functionalization of Cyclic Scaffolds Using Molecular Oxygen as the Sole Oxidant

Over the past years, Pd(II)-catalyzed oxidative couplings have enabled the construction of molecular scaffolds with high structural diversity via C–C, C–N and C–O bond-forming reactions. In contrast to the use of stoichiometric amounts of more common oxidants, such as metal salts (Cu and Ag) and benzoquinone derivatives, the use of molecular oxygen for the direct or indirect regeneration of Pd(II) species presents itself as a more viable alternative in terms of economy and sustainability. In this review, we describe recent advances on the development of Pd-catalyzed oxidative cyclizations/functionalizations, where molecular oxygen plays a pivotal role as the sole stoichiometric oxidant.

1 Introduction

2 Oxidative C–C and C–Nu Coupling

2.1 Intramolecular Oxidative C–Nu Heterocyclization Reactions

2.1.1 C–H Activation

2.1.2 Wacker/Aza-Wacker-Type Cyclization

2.1.3 Tandem Wacker/Aza-Wacker and Cyclization/Cross-Coupling Reactions

2.2 Intermolecular Oxidative C–Nu Heterocoupling Reactions

2.3 Intramolecular Oxidative (C–C) Carbocyclization Reactions

2.4 Intermolecular Oxidative C–C Coupling Reactions

2.4.1 Cyclization Reactions

2.4.2 Cross-Coupling Reactions

2.4.3 Homo-Coupling Reactions

3 Aerobic Dehydrogenative Coupling/Functionalization

4 Oxidative C–H Functionalization

5 Summary

Cobalt(III)-Catalyzed Regio- and Chemoselective [4 + 2]-Annulation of N-Chlorobenzamides/Acrylamides with 1,3-Dienes at Room Temperature

Co(III)-catalyzed regio- and chemoselective redox-neutral C-H annulation of arylamides/acrylamides with 1,3-dienes is described. The present annulation reaction was well suited with a less-reactive 1,3-butadiene. By employing this protocol, pharmaceutically important 3,4-dihydroisoquinolinones were synthesized in good yields. Furthermore, the prepared 3,4-dihydroisoquinolinones were converted into highly important oxirane derivatives in good yields. A plausible mechanistic cycle is proposed and supported by a competition experiment and kinetic isotopic effect (KIE) studies.

Rh(III)-Catalyzed Tandem [4+2] Annulation To Construct Functional Dihydroisoquinolinones

A highly efficient Rh(III)-catalyzed tandem [4+2] annulation to construct functional dihydroisoquinolinone derivatives with an alkenyl side chain by insertion into an N–O bond as an internal oxidation process has been achieved. A wide range of 1,3-dienes as the coupling partners makes this simple methodology even more useful.

Chiral pyridine oxazoline and 1,2,4-triazine oxazoline ligands incorporating electron-withdrawing substituents and their application in the Cu-catalyzed enantioselective nitroaldol reaction

Eight pyridine-containing and four 1,2,4-triazine-containing chiral oxazoline ligands incorporating electron-withdrawing substituents have been synthesized by two-step route including Buchwald–Hartwig amination. Enantio-inducing activity of the ligands has been assessed in the copper-catalyzed asymmetric nitroaldol reactions and the influence of the electron-withdrawing substituents on the ligands' activity has been investigated.

Graphical abstract

Construction of aziridine, azetidine, indole and quinoline-like heterocycles via Pd-mediated C-H activation/annulation strategies

N-Heterocycles can be found in natural products and drug molecules and are indispensable components in the area of organic synthesis, medicinal chemistry and materials science. The construction of these N-containing heterocycles by traditional methods usually requires the preparation of reactive intermediates. In the past decades, with the rapid growth of transition metal catalysed coupling reactions, syntheses of heterocycles from precursors with inert chemical bonds have become a challenge. More recently, in the field of transition metal associated C-H direct functionalization, efficient methods have been developed for the syntheses of N-heterocyclic compounds such as aziridines, azetidines, indoles and quinolines under the click type of reaction mode. In this review, representative synthetic methodologies developed in the recent 10 years for the preparation of this small class of N-heterocycles via the Pd-catalysed C-H activation and C-N bond formation pathway are discussed. We hope this article will provide new insights from the strategies highlighted into future molecular design, synthesis and applications in medical and materials sciences.

Highly Regioselective and Ligand-Controlled Diastereodivergent Aminomethylative Annulation of Dienyl Alcohols Enabled by Hydrogen-Bonding Assisting Effect

A ligand-controlled palladium-catalyzed highly regioselective and diastereodivergent aminomethylative annulation of dienyl alcohols with aminals has been established, which allows for producing either cis- or trans-disubstituted isochromans in good yields with...

Recent advances in transition metal-catalyzed heteroannulative difunctionalization of alkenes via C-H activation for the synthesis of heterocycles

Heterocyclic compounds are the fundamental structural motifs distributed in natural products, pharmaceuticals and biologically active compounds. Thus, there is increasing interest in the development of novel synthetic strategies for the...

Synthesis and structures of three new pyridine-containing oxazoline ligands of complexes for asymmetric catalysis

Three new chiral pyridine-containing oxazoline derivatives with fluorine and perfluoromethyl groups, namely, 2-({2-[(4S)-4-phenyl-4,5-dihydro-1,3-oxazol-2-yl]phenyl}amino)-5-(trifluoromethyl)pyridine, C₂₁H₁₆F₃N₃O, 2-({5-fluoro-2-[(4S)-4-isopropyl-4,5-dihydro-1,3-oxazol-2-yl]phenyl}amino)-5-(trifluoromethyl)pyridine, C₁₈H₁₇F₄N₃O, and 2-({2-[(3aR,8aS)-8,8a-dihydro-3aH-indeno[1,2-d]oxazol-2-yl]phenyl}amino)-5-(trifluoromethyl)pyridine, C₂₂H₁₆F₃N₃O, as chiral ligands in metal-catalysed asymmetric reactions, were synthesized and characterized by spectral and X-ray diffraction methods. The conformation of the molecules is influenced by strong N-H...N hydrogen bonding and weak C-H...X (X = O and N) interactions. There are no intermolecular hydrogen bonds in the crystal structures of the analysed compounds. Hirshfeld surface analysis showed that the H...H contacts constitute a high percentage of the intermolecular interactions. The conformational analysis was performed by theoretical calculations using the density functional theory (DFT) method. The mechanism of complex formation in terms of the electron-withdrawing effect of the substituents on the oxazoline ring and the ligand conformation is discussed.

Recent advances in palladium-catalyzed (hetero)annulation of C[double bond, length as m-dash]C bonds with ambiphilic organo(pseudo)halides

Palladium has proven to be effective in catalyzing the (hetero)annulation of C[double bond, length as m-dash]C bonds with ambiphilic organo(pseudo)halides. Through the employment of appropriate ambiphilic coupling partners, efficient annulation of a variety of allenes, 1,3-dienes, strained alkenes, styrenes, and other C[double bond, length as m-dash]C bond variants can be achieved to provide direct access to numerous useful hetero- and carbocyclic scaffolds. In this Feature Article, we summarize palladium-catalyzed (hetero)annulation methods reported since 2005 (spanning just over 15 years) and discuss outstanding challenges in this area of study.

Rh(III)-Catalyzed C-H Activation/[3 + 2] Annulation of N-Phenoxyacetamides via Carbooxygenation of 1,3-Dienes

A unique Rh(III)-catalyzed C-H activation/[3 + 2] annulation of N-phenoxyacetamides has been developed for the construction of dihydrobenzofurans via carbooxygenation of 1,3-dienes. This transformation features a redox-neutral process with specific chemoselectivity, good substrate/functional group compatibility, and profound synthetic potentials. A preliminary exploration to realize their asymmetric synthesis have been also successfully demonstrated, which further strengthens the practicality of this approach.

Catalyst- and Substituent-Controlled Regio- and Stereoselective Synthesis of Indolyl Acrylates by Lewis-Acid-Catalyzed Direct Functionalization of 3-Formylindoles with Diazo Esters

A facile and efficient In(OTf)₃- and BF₃·OEt₂-catalyzed direct transformation of 3-formylindoles with diazo esters has been developed for synthesizing diverse and functionalized indolyl acrylates. This one-pot protocol furnishes various (Z)-α-hydroxy-β-indolyl acrylates, (E)-β-(2-alkoxy-2-oxoethoxy)-α-indolyl acrylates, and (Z)-3-hydroxy-2-indolyl acrylates by a catalyst- and substituent-controlled, regio- and stereoselective cascade reaction. The protocol has several advantages, including low loading of the catalyst, mild reaction conditions, broad scope, and high functional group tolerance. The synthesized compounds can be further converted into diversely functionalized materials.

Catalytic C-H Functionalization of Unreactive Furan Cores in Bio-Derived Platform Chemicals

C-H functionalization is one of the most convenient and powerful tools in the arsenal of modern chemistry, deservedly nominated as the "Holy Grail" of organic synthesis. A frequent disadvantage of this method is the need for harsh reaction conditions to carry out transformations of inert C-H bonds, which limits the possibility of its use for modifying less stable substrates. Biomass-derived furan platform chemicals, which have a relatively unstable aromatic furan core and highly reactive side chain substituents, are extremely promising and valuable organic molecules that are currently widely used in a variety of research and industrial fields. The high sensitivity of furan derivatives to acids, strong oxidants, and high temperatures significantly limits the use of classical methods of C-H functionalization for their modification. New methods of catalytic functionalization of non-reactive furan cores are urgently required to obtain a new generation of materials with controlled properties and potentially bioactive substances.

Vinylogous Elimination/C-H Functionalization/Allylation Cascade Reaction of Allenoate Adducts: Synthesis of Ring-Fused Dihydropyridinones

A palladium-catalyzed cascade reaction of β'-allenoate adducts with aryl/heteroaryl carboxamides through a vinylogous elimination/C-H functionalization/intramolecular allylation reaction sequence has been developed with high Z stereoselectivity. Various ring-fused dihydropyridinones bearing an α,β-unsaturated ester substituent are obtained. It is the first example of application of the allenoate adducts to C-H functionalization annulations as practical precursors of hard-to-get functionalized electron-deficient 1,3-butadienes. Using air as the terminal oxidant also shows a great advantage in environmental friendliness.

Palladium-catalyzed dearomative 1,4-difunctionalization of naphthalenes

A highly diastereoselective dearomatization of naphthalenes via a Pd-catalyzed 1,4-difunctionalization reaction is described. In the presence of a commercially available palladium precursor and ligand, intramolecular dearomative Heck-type insertion provides π-allylpalladium intermediates which are readily captured by a series of nucleophiles in excellent yields (up to 99%). This reaction features mild conditions, broad substrate scope, and useful transformations of the products.

Photoinduced Palladium-Catalyzed Carbofunctionalization of Conjugated Dienes Proceeding via Radical-Polar Crossover Scenario: 1,2-Aminoalkylation and Beyond

A photoinduced palladium-catalyzed 1,2-carbofunctionalization of conjugated dienes has been developed. This mild modular approach, which does not require employment of exogeneous photosensitizers and external oxidants, allows for efficient and highly regio- and stereoselective synthesis of a broad range of allylic amines from readily available 1,3-dienes, alkyl iodides, and amines. Employment of O- and C-nucleophiles toward oxyalkylation and dialkylation products was also demonstrated. A putative π-allyl palladium radical-polar crossover path is proposed as a key event in this three-component coupling process. The utility of this protocol is highlighted by its application for derivatization of several amine-containing drugs.

Asymmetric Three-Component Heck Arylation/Amination of Nonconjugated Cyclodienes

Substituted cyclohexylamines are becoming increasingly important in drug discovery. Asymmetric Heck insertion/amination of nonconjugated cyclodienes proceeds to give 5-aryl cyclohexenylamines with good enantioselectivity and exclusive trans configurations. Primary and secondary anilines, indoline, and benzylamines are suitable amines. The weakly donating diphosphite Kelliphite forms a deep unsymmetrical pocket, which is essential for stereoselective anti attack of amines.

Asymmetric synthesis of allylic compounds via hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes

This review article provides an overview of progress in asymmetric synthesis of allylic compounds via hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes.