Regioselective C3Alkylation of Indoles for the Synthesis of Bis(indolyl)methanes and 3-Styryl Indoles

Herein, we describe an efficient transition-metal-free regioselective C3alkylation of indoles for the synthesis of bis(indolyl)methanes and 3-styryl indoles. Nitrobenzene is employed as the oxidant to oxidize the alcohols in the presence of a strong base and the reaction avoids the use of transition metals such as Ru and Mn. The protocol provides a favorable route to access biologically active compounds such as arundine, vibrindole A, and turbomycin B.

Acetoxy Group-Directed Regioselective C2 Alkenylation of Indoles via Pd-Ag Bimetallic Catalysis

Regioselective C-H functionalizations of indoles reported to date with directing groups at C3 mainly rely on functional groups that are linked to the indole via C-C bonds. However, groups that are linked to the indole core by C-X linkages are also attractive due to the possibility of further modifications of the C-X bond. Herein, we report a 3-acetoxy directing group for the regioselective C2 alkenylation of indoles via a C-H activation-based, cross-dehydrogenative, oxidative Heck-type reaction. The reaction is catalyzed by Pd(II) and Ag(I) with stoichiometric Cu(II) as the oxidant and provides the 2-alkenylated indoles in yields of 52-84%. The reaction conditions are compatible with several functional groups at different positions as well as different N-protecting groups or free NH groups on the indole core. With respect to the alkene coupling partners, the reactions are successful with acrylates, vinyl sulfates, and phosphates. Specifically designed experiments, as well as density functional theory (DFT) computational studies, reveal that a heterodinuclear [Pd(μ-OAc)3Ag] bimetallic species is the actual catalyst responsible for the C-H alkenylation. A mechanistic path involving this catalytic species was also found to be favorable over other possible pathways for explaining the observed regioselectivity through DFT studies.

Pd(II)/Lewis acid catalyzed regioselective olefination of indole with dioxygen

Transition metal ion catalyzed indole olefination through C-H activation is a convenient protocol to synthesize versatile bioactive vinylindole compounds; however, in most cases, stoichiometric amounts of oxidants were necessary to accomplish the catalytic cycle. The present study describes a Pd(II)/LA (LA: Lewis acid) catalyzed indole olefination with dioxygen as the sole oxidant. The olefination reaction with electron-rich olefins proceeded smoothly through the pyrrolyl N-carboxamide group directed remote C-H activation at the C3 position of the indole with the Pd(II)/LA catalyst, whereas Pd(II) alone was a very sluggish catalyst under identical conditions. For the electron-deficient olefins, the directing N-carboxamide group was not essential for olefination with this Pd(II)/LA catalyst, demonstrating a different olefination pathway from that of electron-rich olefins. Remarkably, ¹H NMR kinetics disclosed that olefination proceeded much faster with electron-rich olefins than with electron-deficient ones.

Visible-Light-Induced Decarboxylative Cyclization of 2-Alkenylarylisocyanides with α-Oxocarboxylic Acids: Access to 2-Acylindoles

An efficient and practical protocol for visible-light-induced decarboxylative cyclization of 2-alkenylarylisocyanides with α-oxocarboxylic acids has been developed, which afforded a broad range of 2-acylindoles in moderate to good yields. The reaction proceeds through a cascade of acyl radical addition/cyclization reactions under irradiation of an Ir³⁺ photoredox catalyst without external oxidants and features simple operation, scalability, a broad substrate scope, and good functional group tolerance.

Rhodium(III)-Catalyzed Decarboxylative Aminomethylation of Glycine Derivatives with Indoles via C-H Activation

A rhodium(III)-catalyzed decarboxylative aminomethylation of glycine derivatives with indoles bearing a pyrimidine directing group was developed via C-H activation, exclusively affording the corresponding aminomethylated products with C3 selectivity. In this synthetic protocol, easily available and resourceful arylglycines were utilized as aminomethyl functional moieties.

Synthesis of 3-alkenylindoles through regioselective C-H alkenylation of indoles by a ruthenium nanocatalyst

3-Alkenylindoles are biologically and medicinally very important compounds, and their syntheses have received considerable attention. Herein, we report the synthesis of 3-alkenylindoles via a regioselective alkenylation of indoles, catalysed by a ruthenium nanocatalyst (RuNC). The reaction tolerates several electron-withdrawing and electron-donating groups on the indole moiety. Additionally, a "robustness screen" has also been employed to demonstrate the tolerance of several functional groups relevant to medicinal chemistry. With respect to the Ru nanocatalyst, it has been demonstrated that it is recoverable and recyclable up to four cycles. Also, the catalyst acts through a heterogeneous mechanism, which has been proven by various techniques, such as ICPMS and three-phase tests. The nature of the Ru nanocatalyst surface has also been thoroughly examined by various techniques, and it has been found that the oxides on the surface are responsible for the high catalytic efficiency of the Ru nanocatalyst.

Anthracene-rhodium complexes with metal coordination at the central ring - a new class of catalysts for reductive amination

A new class of anthracene complexes with a metal coordinated at the central ring was applied in catalysis for the first time. As a result, a simple and efficient protocol for reductive amination that involves CO as a reducing agent has been developed. The rhodium complex [(cyclooctadiene)Rh(C10H4Me2(OMe)4)]+ (1 mol%) catalyses such reactions under mild conditions (40-130 °C) and produces a variety of amines in good yields (74-95%) without affecting the functional groups. The protocol is acceptable for all combinations of aldehydes (aromatic and aliphatic), ketones (aromatic and aliphatic) and amines (aromatic and aliphatic; primary and secondary).

A new four-component reaction involving the Michael addition and the Gewald reaction, leading to diverse biologically active 2-aminothiophenes

A Gewald-four component reaction has been successfully developed for the synthesis of a series of compounds containing an indole and a 2-aminothiophene moiety separated by a methylene spacer having anti-proliferative activity.

Direct C-2 acylation of indoles with toluene derivatives via Pd(ii)-catalyzed C–H activation

A simple and efficient Pd-catalyzed method for the C2-acylation of indoles is described using toluene derivatives.

A tunable copper-catalyzed multicomponent reaction towards alkaloid-inspired indole/lactam polycycles

A tunable copper(i)-catalyzed cascade multicomponent reaction strategy between readily available (Z)-3-iodoacrylic acids, terminal alkynes, and primary amines is reported, leading to a great diversity of biorelevant indole/lactam scaffolds.

Organocatalytic routes toward substituted 1,2,3-triazoles

The present feature article describes the different organocatalytic routes for the synthesis of substituted 1,2,3-triazoles. This methodology has recently gained much attention due to its many advantages like high regioselectivity, substrate scope, high yields, and access to novel molecules. The review is divided into 4 different sections based on the active intermediate of the triazole synthesis reactions. The mechanism of each approach is critically discussed and in addition, the advantages and disadvantages of each method are described with relevant examples.

Asymmetric Total Synthesis of the Indole Diterpene Alkaloid Paspaline

An enantioselective synthesis of the indole diterpenoid natural product paspaline is disclosed. Critical to this approach was the implementation of stereoselective desymmetrization reactions to assemble key stereocenters of the molecule. The design and execution of these tactics are described in detail, and a thorough analysis of observed outcomes is presented, ultimately providing the title compound in high stereopurity. This synthesis provides a novel template for preparing key stereocenters in this family of molecules, and the reactions developed en route to paspaline present a series of new synthetic disconnections in preparing steroidal natural products.

Stereoselective α-indolylation of enals via an organocatalytic formal cross-coupling with indoles

A novel organocatalytic one-pot cascade bromination–Michael addition dearomatization–nucleophilic rearrangement aromatization cascade process for the direct α-indolylation of unfunctionalized enals from readily available indoles with good yields and high E selectivity has been developed with great simplicity and practicality.

A metal-free three-component reaction for the regioselective synthesis of 1,4,5-trisubstituted 1,2,3-triazoles

A metal-free three-component reaction to synthesize 1,4,5-trisubstituted 1,2,3-triazoles from readily available building blocks, such as aldehydes, nitroalkanes, and organic azides, is described. The process is enabled by an organocatalyzed Knoevenagel condensation of the formyl group with the nitro compound, which is followed by the 1,3-dipolar cycloaddition of the azide to the activated alkene. The reaction features an excellent substrate scope, and the products are obtained with high yield and regioselectivity. This method can be utilized for the synthesis of fused triazole heterocycles and materials with several triazole moieties.

Palladium(II)-catalyzed direct alkenylation of nonaromatic enamides

A mild and efficient method for the direct alkenylation of nonaromatic enamides was achieved through a palladium(II)-catalyzed C-H functionalization. The reaction scope includes cyclic and acyclic enamides and a range of activated alkenes. This approach represents the first successful direct C(3)-functionalization of nonaromatic cyclic enamides.

Non-asymmetric organocatalysis

Asymmetric organocatalysis is now an established methodology for the preparation of chiral compounds. However, these are not the only valuable molecules which can be conveniently obtained. Organocatalytic reactions affording achiral compounds are gaining momentum, opening unexplored pathways in the synthesis of densely functionalized aromatic moieties, olefins and useful molecules such as natural substances.