Synthesis and preliminary biological studies of 3-substituted indoles accessed by a palladium-catalyzed enantioselective alkene difunctionalization 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.

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.

Transition-metal-free and additive-free intermolecular hydroarylation of alkenes with indoles in hexafluoroisopropanol

Hydroarylation of alkenes is one of the most straightforward and atom-economical strategy for the construction of multi-aryl-substituted alkanes, but systematic studies have been limited to transition metal catalysis. Here we report a hexafluoroisopropanol (HFIP)-promoted hydroarylation of alkenes with indoles without the presence of transition metal catalysts or any additive. HFIP was the only reagent used in this work, and could be easily removed via evaporation, and recovered via distillation in industry settings. This reaction was shown to provide an efficient, clean and operationally simple procedure with a remarkable substrate scope and versatile transformations, delivering a variety of multi-aryl alkanes incorporating the indole motif. In preliminary studies, several of these products showed biologically activity against cells from an array of human cancer cell lines. A mechanistic study was also carried out and suggested that the quinone methide might be the key intermediate. And in contrast to the conclusions of a previous report, the current work suggested that protonation by HFIP might not be the rate-determining step.

Mechanistic study on the side arm effect in a palladium/Xu-Phos-catalyzed enantioselective alkoxyalkenylation of γ-hydroxyalkenes

Recently, the asymmetric bifunctionalization of alkenes has received much attention. However, the development of enantioselective alkoxyalkenylation has posed a considerable challenge and has lagged largely behind. Herein, we report a new palladium-catalyzed enantioselective alkoxyalkenylation reaction, using a range of primary, secondary, and tertiary γ-hydroxy-alkenes with alkenyl halides. By employing newly identified Xu-Phos (Xu8 and Xu9) with a suitable side-arm adjacent to the PCy2 motif, a series of allyl-substituted tetrahydrofurans were obtained in good yields with up to 95% ee. Besides (E)-alkenyl halides, (Z)-alkenyl halide was also examined and provided the corresponding (Z)-product as a single diastereomer, supporting a stereospecific oxidative addition and reductive elimination step. Moreover, deuterium labeling and VCD experiments were employed to determine a cis-oxypalladation mechanism. DFT calculations helped us gain deeper insight into the side-arm effect on the chiral ligand. Finally, the practicability of this method is further demonstrated through a gram-scale synthesis and versatile transformations of the products.

Diverting the 5-exo-Trig Oxypalladation to Formally 6-endo-Trig Fluorocycloetherification Product through 1,2-O/Pd(IV) Dyotropic Rearrangement

Pd-catalyzed cyclizative functionalization of γ-hydroxyalkenes affords tetrahydrofuran derivatives via a key 5-exo-trig oxypalladation step. Herein, we report a palladium(II)-catalyzed, Selectfluor-mediated formal 6-endo-trig fluorocycloetherification of γ-hydroxyalkenes for the synthesis of functionalized tetrahydropyrans. Mechanistically, an σ-alkyl-Pd(II) intermediate resulting from the 5-exo-trig oxypalladation process is isolated and characterized by X-ray crystallographic analysis. Its oxidation with Selectfluor to Pd(IV) triggers the chemoselective 1,2-O/Pd(IV) dyotropic rearrangement affording, after C-F bond-forming reductive elimination, the tetrahydropyrans with concurrent generation of a tertiary carbon-fluorine bond. The occurrence of this 1,2-positional interchange is further evidenced by trapping the rearranged quaternary C(sp³)-Pd bond by an internal nucleophile that is materialized by the development of a Pd(II)-catalyzed oxidative bis-heterocyclization of alkenes.

Brønsted Acid Catalyzed Friedel-Crafts Alkylation of Naphthols with In Situ Generated Naphthol-Derived ortho-Quinone Methides: Synthesis of Chiral and Achiral Xanthene Derivatives

We disclose herein an enantioselective protocol for the Brønsted acid catalyzed addition of naphthols to in situ generated naphthol-derived ortho-quinone methides (o-QMs) followed by intramolecular cyclization, which delivers substituted chiral xanthene derivatives, in a one-pot reaction sequence under mild conditions. This process serves to convert naphthol-derived ortho-hydroxyl benzylic alcohols into reactive naphthol-derived o-QMs using a chiral phosphoric acid (CPA) catalyst. Moreover, it is helpful in controlling the enantioselectivity of the carbon-carbon bond-forming event via hydrogen-bonding followed by intramolecular cyclization. Additionally, for the first time, we observe a Brønsted acid catalyzed C(sp²)-C(sp³) bond cleavage of naphthol-derived ortho-hydroxyl benzylic alcohols for the synthesis of achiral xanthene (sigma plane containing) derivatives in good to excellent yields.

Visible-light-induced indole synthesis via intramolecular C-N bond formation: desulfonylative C(sp2)-H functionalization

Despite significant advances made on the synthesis of indole derivatives through photochemical strategies during the past several years, the requirement of equivalent amounts of oxidants, bases or other additional additives has limited their practical applications in the synthesis of natural products and pharmaceuticals as environment-friendly processes. Herein, we report LED visible-light-induced redox neutral desulfonylative C(sp2)-H functionalization for the synthesis of N-substituted indoles with a broad scope through γ-fragmentation under mild conditions in the absence of any additional additive. The reaction mechanism paradigm has been investigated on the basis of deuterium labeling experiments, kinetic analysis, Hammett plotting analysis and DFT calculations.

Recent advances in 3-aminoindazoles as versatile synthons for the synthesis of nitrogen heterocycles

Nitrogen-based heterocycles are an important class of structural scaffolds distributed in biologically active natural products, medicinal chemistry, and agrochemicals. Hence, there is increasing interest in the development of novel synthetic strategies for the construction of these privileged structural motifs. Recently, 3-aminoindazoles have emerged as versatile synthons participating in a variety of condensation annulation, denitrogenative transannulation and rearrangement ring expansion reactions, which provide efficient synthetic routes for the formation of nitrogen heterocycles. This review systematically highlights for the first time the most recent advances in 3-aminoindazoles to provide a deep understanding of using 3-aminoindazoles as versatile synthons in organic transformations for synthetic and medicinal chemists.

Co-Mabiq Flies Solo: Light-Driven Markovnikov-Selective C- and N-Alkylation of Indoles and Indazoles without a Cocatalyst

Indoles and indazoles are common moieties in pharmaceuticals and naturally occurring bioactive compounds. The development of light-driven methods using earth-abundant transition-metal catalysts offers an attractive route for functionalization of such compounds. Herein, we report a visible-light-induced method for the C3- and N-alkylation of indoles and indazoles with styrenes, catalyzed by Co complexes based on the macrocyclic Mabiq ligand (Mabiq = 2-4:6-8-bis(3,3,4,4-tetramethyldihydropyrrolo)-10-15-(2,2'-biquinazolino)-[15]-1,3,5,8,10,14-hexaene-1,3,7,9,11,14-N₆). The photochemical behavior of two Co^III catalysts was examined: Co(Mabiq)Cl₂ and the newly synthesized Co(MabiqBr)Cl₂, which contains the Br-modified ligand. Both complexes undergo visible-light-induced homolysis that is significant to their activity but exhibit differences in reactivity. The alkylation reactions are regioselective, furnishing the alkylated indole and indazole products in a Markovnikov fashion with excellent yields of up to 96% across a broad range of substrates. Notably, in contrast to dual-transition-metal and photoredox-catalyzed cross-coupling reactions, our studies reveal that the Co complex plays a dual role─as a photosensitizer and catalytically active metal center with the Mabiq ligand offering regiocontrol.

Air-Stable PdI Dimer Enabled Remote Functionalization: Access to Fluorinated 1,1-Diaryl Alkanes with Unprecedented Speed

While remote functionalization via chain walking has the potential to enable access to molecules via novel disconnections, such processes require relatively long reaction times and can be in need of elevated temperatures. This work features a remote arylation in less than 10 min reaction time at room temperature over a distance of up to 11 carbons. The unprecedented speed is enabled by the air-stable PdI dimer [Pd(μ-I)(PCy2 t Bu)]2 , which in contrast to its Pt Bu3 counterpart does not trigger direct coupling at the initiation site, but regioconvergent and chemoselective remote functionalization to yield valuable fluorinated 1,1-diaryl alkanes. Our combined experimental and computational studies rationalize the origins of switchability, which are primarily due to differences in dispersion interactions.

Iron–iodine co-catalysis towards tandem C–N/C–C bond formation: one-pot regioselective synthesis of 2-amino-3-alkylindoles

An efficient intermolecular C2,3-H aminoalkylation of indoles with 9H-xanthenes and azoles via iron–iodine co-catalyzed tandem C–N/C–C bond formation has been developed.

Copper-catalyzed enantioselective alkene carboetherification for the synthesis of saturated six-membered cyclic ethers

The enantioselective copper-catalyzed oxidative coupling of alkenols with styrenes for the construction of dihydropyrans, isochromans, pyrans and morpholines is reported. A concise formal synthesis of a σ₁ receptor ligand using this alkene carboetherification methodology was demonstrated. Ligand, solvent and base all impact reaction efficiency. DFT transition state calculations are presented.

Rate Dependence on Inductive and Resonance Effects for the Organocatalyzed Enantioselective Conjugate Addition of Alkenyl and Alkynyl Boronic Acids to β-Indolyl Enones and β-Pyrrolyl Enones

Two key factors bear on reaction rates for the conjugate addition of alkenyl boronic acids to heteroaryl-appended enones: the proximity of inductively electron-withdrawing heteroatoms to the site of bond formation and the resonance contribution of available heteroatom lone pairs to stabilize the developing positive charge at the enone β-position. For the former, the closer the heteroatom is to the enone β-carbon, the faster the reaction. For the latter, greater resonance stabilization of the benzylic cationic charge accelerates the reaction. Thus, reaction rates are increased by the closer proximity of inductive electron-withdrawing elements, but if resonance effects are involved, then increased rates are observed with electron-donating ability. Evidence for these trends in isomeric substrates is presented, and the application of these insights has allowed for reaction conditions that provide improved reactivity with previously problematic substrates.

Arenesulfonyl indole: new precursor for diversification of C-3 functionalized indoles

Arenesulfonyl indoles, bearing a good leaving group, are effective precursors for vinylogous imine intermediates which are generated in situ under basic conditions. This intermediate can readily react with other nucleophilic reagents to obtain C-3 substituted indole derivatives. In the last few years, a plethora of exciting synthetic applications of this substrate have been exploited. The stability of arylsulfonyl-containing substrates, mild reaction conditions, and the large variety of nucleophiles involved in these procedures are the key to their success in organic synthesis.

Hydroquinine-catalyzed asymmetric 1,4-hydrophosphination of in situ generated aza-o-quinone methides with H-phosphine oxides

An organocatalytic enantioselective 1,4-addition of H-phosphine oxides to in situ generated aza-o-quinone methides has been successfully established using hydroquinine.

Box-copper catalyzed asymmetric inverse-electron-demand oxa-hetero-Diels–Alder reaction for efficient synthesis of spiro pyranyl-oxindole derivatives

A chiral Box/Cu catalyzed asymmetric IEDDA reaction between isatin-derived β,γ-unsaturated α-ketoesters and electron-rich olefins was developed, which provided chiral spiro oxindole-pyrans in excellent yields with excellent stereoselectivities.

Chiral cis-iron(ii) complexes with metal- and ligand-centered chirality for highly regio- and enantioselective alkylation of N-heteroaromatics

Iron-catalyzed highly regio- and enantioselective organic transformations with generality and broad substrate scope have profound applications in modern synthetic chemistry; an example is herein described based on cis-Fe^II complexes having metal- and ligand-centered chirality. The cis-β Fe^II(N₄) complex [Fe^II(L)(OTf)₂] (L = N,N′-bis(2,3-dihydro-1H-cyclopenta-[b]quinoline-5-yl)-N,N′-dimethylcyclohexane-1,2-diamine) is an effective chiral catalyst for highly regio- and enantioselective alkylation of N-heteroaromatics with α,β-unsaturated 2-acyl imidazoles, including asymmetric N1, C2, C3 alkylations of a broad range of indoles (34 examples) and alkylation of pyrroles and anilines (14 examples), all with high product yields (up to 98%), high enantioselectivity (up to >99% ee) and high regioselectivity. DFT calculations revealed that the “chiral-at-metal” cis-β configuration of the iron complex and a secondary π–π interaction are responsible for the high enantioselectivity.

A cis-β Fe^II complex having metal- and ligand-centered chirality catalyzes highly regio- and enantioselective alkylation of indoles (at the N1, C2, or C3 position), pyrroles and anilines with α,β-unsaturated 2-acyl imidazoles (48 examples, up to 99% ee).

Rh-Catalyzed Highly Enantioselective Synthesis of Aliphatic Sulfonyl Fluorides

Rh-catalyzed, highly enantioselective (up to 99.8% ee) synthesis of aliphatic sulfonyl fluorides was accomplished. This protocol provides a portal to a class of novel 2-aryl substituted chiral sulfonyl fluorides, which are otherwise extremely difficult to access. This asymmetric synthesis has the feature of mild conditions, excellent functional group compatibility, and wide substrate scope (51 examples) generating a wide array of structurally unique chiral β-arylated sulfonyl fluorides for sulfur(VI) fluoride exchange (SuFEx) click reaction and drug discovery.

Enantioselective benzylic C-H arylation via photoredox and nickel dual catalysis

The asymmetric cross-coupling reaction is developed as a straightforward strategy toward 1,1-diaryl alkanes, which are a key skeleton in a series of natural products and bioactive molecules in recent years. Here we report an enantioselective benzylic C(sp³)−H bond arylation via photoredox/nickel dual catalysis. Sterically hindered chiral biimidazoline ligands are designed for this asymmetric cross-coupling reaction. Readily available alkyl benzenes and aryl bromides with various functional groups tolerance can be easily and directly transferred to useful chiral 1,1-diaryl alkanes including pharmaceutical intermediates and bioactive molecules. This reaction proceeds smoothly under mild conditions without the use of external redox reagents.

Chiral 1,1-diaryl alkanes are important targets in pharmaceutical industry. Here, the authors report report a redox-neutral enantioselective benzylic C−H bond arylation via photoredox/nickel dual catalysis accessing chiral 1,1-diarylalkane compounds under mild conditions.

Palladium-Catalyzed Oxidation Reactions of Alkenes with Green Oxidants

Transition-metal-catalyzed oxidative functionalization of alkenes has emerged as a powerful and valuable tool in modern organic synthesis. Recently, many methods have been established for the assembly of C-C and C-heteroatom bonds, which provides tremendous possibility for application in biology, medicine, and materials science. However, the use of stoichiometric amounts of strong oxidants leads to poor selectivity, low atom economy, and a series of undesired waste products. By contrast, green oxidants, such as O₂ , H₂ O₂ , or tert-butyl hydroperoxide (TBHP), have bright prospects due to their attributes of mild, low cost, and great sustainability in transition-metal-catalyzed oxidation reactions. Based on the great and unique potential for the development of aerobic reactions, this review mainly highlights homogenous palladium-catalyzed green oxidations of alkenes that have been reported in recent years. These methods provide new strategies for the transformation and functionalization of alkenes; some of them have also been successfully applied to the synthesis of the core structures of drugs and natural products. Additionally, through in-depth studies of the reaction mechanisms in this field, it is believed that palladium-catalyzed green oxidation reactions of alkenes with O₂ , H₂ O₂ , or TBHP will create added value for organic synthetic chemistry.

Brønsted Acid-Catalyzed, Diastereo- and Enantioselective, Intramolecular Oxa-Diels-Alder Reaction of ortho-Quinone Methides and Unactivated Dienophiles

A stereoselective, phosphoric acid-catalyzed synthesis of dihydrochromenochromenes has been developed using transient ortho-quinone methides ( o-QMs). Three contiguous stereogenic centers were formed with excellent yields, partially as single diastereomers and with moderate to excellent enantioselectivity. This intramolecular hetero-Diels-Alder reaction features unactivated dienophiles and o-QM precursors tethered by a simple phenoxy linker and furnishes cycloadducts with a prominent structural motif found in many natural products. Through an appropriate choice of dienophile configuration and backbone substitution either exo- or endo-stereoisomers were formed selectively with up to a 96:04 enantiomeric ratio.