Chiral Bro̷nsted Acid-Catalyzed Asymmetric Friedel−Crafts Alkylation of Pyrroles with Nitroolefins

Synthesis of chiral systems featuring the pyrrole unit: a review

Synthetic strategies towards pyrroles within chiral frameworks are summarised, focussing on reports published 2010-2023. The synthesis of pyrroles featuring substituents bearing chiral centres are summarised, as are those whereby pyrroles are located within axially chiral systems courtesy of restricted bond rotation.

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.

Synthesis of C8-Aminated Pyrrolo-Phenanthridines or -Indoles via Series C(sp2 or sp3)-H Activation and Fluorescence Study

This report developed a method for the synthesis of C8-aminated pyrrolo-phenanthridines or -indoles by series ortho C(sp²)-H amination/ipso C(sp²)-H or C(sp³)-H arylation. N-benzoyloxyamines, as electrophilic amination reagents, did not undergo an electrophilic substitution reaction with the pyrrole side, but they did undergo a site-selective C-H amination reaction with the benzene side via Pd/NBE catalysis. The C8-aminated pyrrolo-phenanthridines have strong fluorescence in solution and solid state. X-ray single crystal diffraction shows that the steric hindrance of amino and ortho benzene ring may inhibit aggregation-caused quenching (ACQ).

Chiral spiro phosphoric acid-catalysed enantioselective reaction of ketenes with N-H pyrroles

In the presence of a chiral spiro phosphoric acid catalyst, the asymmetric reaction of disubstituted ketenes with N-H pyrroles occurred to afford enantioenriched C-acylated pyrroles bearing α-stereogenic carbon centres. The described reaction constitutes a rare example of a catalytic asymmetric reaction of ketenes with carbon-based nucleophiles.

Synthesis and practical applications of 2-(2-nitroalkyl)pyrroles

Functionalization of pyrroles introducing a 2-nitroalkyl moiety allows the formation of nitro-containing compounds to be used as pivotal intermediates for the synthesis of bioactive compounds. The reaction of pyrroles with nitroalkenes under the Friedel-Crafts conditions allows a direct entry to 2-(2-nitroalkyl)pyrroles. This approach can also be used for the preparation of enantioenriched derivatives exploiting asymmetric catalysis. In a complementary fashion, the Henry reaction between 2-formylpyrroles and nitroalkanes generates the corresponding nitroaldol products which upon dehydration and reduction of the intermediate 2-pyrrolylnitroethene efficiently afford 2-(2-nitroalkyl)pyrroles. This review article summarizes the most relevant procedures for the preparation of 2-(2-nitroalkyl)pyrroles during the last two decades as well as their significant practical applications.

Strong and Confined Acids Catalyze Asymmetric Intramolecular Hydroarylations of Unactivated Olefins with Indoles

In recent years, several organocatalytic asymmetric hydroarylations of activated, electron-poor olefins with activated, electron-rich arenes have been described. In contrast, only a few approaches that can handle unactivated, electronically neutral olefins have been reported and invariably require transition metal catalysts. Here we show how an efficient and highly enantioselective catalytic asymmetric intramolecular hydroarylation of aliphatic and aromatic olefins with indoles can be realized using strong and confined IDPi Brønsted acid catalysts. This unprecedented transformation is enabled by tertiary carbocation formation and establishes quaternary stereogenic centers in excellent enantioselectivity and with a broad substrate scope that includes an aliphatic iodide, an azide, and an alkyl boronate, which can be further elaborated into bioactive molecules.

Metal organic frameworks as solid catalysts for liquid-phase continuous flow reactions

This Feature Article describes the recent developments in the use of MOFs as catalysts under continuous flow conditions illustrating that these materials can meet the required stability.

Brønsted Acid-Promoted Friedel-Crafts Alkylation/Cyclization of (7-Hydroxynaphthalenyl)pyrrole or (2-Hydroxyphenyl)pyrroles with Isatins for the Construction of Pyrrolospirooxindole Derivatives

An efficient trifluoroacetic acid-catalyzed cascade Friedel-Crafts alkylation/cyclization of 1-(7-hydroxynaphthalenyl)pyrrole or 1-(2-hydroxyphenyl)pyrroles with isatins has been developed, providing practical access to a variety of biologically important pyrrole-containing spirooxindoles.

Sixteen isostructural phosphonate metal-organic frameworks with controlled Lewis acidity and chemical stability for asymmetric catalysis

Heterogeneous catalysts typically lack the specific steric control and rational electronic tuning required for precise asymmetric catalysis. Here we demonstrate that a phosphonate metal–organic framework (MOF) platform that is robust enough to accommodate up to 16 different metal clusters, allowing for systematic tuning of Lewis acidity, catalytic activity and enantioselectivity. A total of 16 chiral porous MOFs, with the framework formula [M₃L₂(solvent)₂] that have the same channel structures but different surface-isolated Lewis acid metal sites, are prepared from a single phosphono-carboxylate ligand of 1,1′-biphenol and 16 different metal ions. The phosphonate MOFs possessing tert-butyl-coated channels exhibited high thermal stability and good tolerances to boiling water, weak acid and base. The MOFs provide a versatile family of heterogeneous catalysts for asymmetric allylboration, propargylation, Friedel–Crafts alkylation and sulfoxidation with good to high enantioselectivity. In contrast, the homogeneous catalyst systems cannot catalyze the test reactions enantioselectively.

Asymmetric synthesis predominantly falls within the realm of homogeneous catalysis. Here, the authors synthesized 16 chiral metal–organic frameworks differing in the nature of the transition metal and demonstrate their excellent stability, catalytic activity and recyclability in a number of enantioselective reactions.

Enantioselective synthesis of 4,5,6,7-tetrahydroindoles via olefin cross-metathesis/intramolecular Friedel–Crafts alkylation reaction of pyrroles

A sequential catalysis involving olefin cross-metathesis/asymmetric intramolecular Friedel–Crafts alkylation of pyrrole derivatives has been developed.

Organocatalytic asymmetric reaction of indol-2-yl carbinols with enamides: synthesis of chiral 2-indole-substituted 1,1-diarylalkanes

The chiral phosphoramide-catalyzed asymmetric reaction of indol-2-yl carbinols with enamides is presented.

1,4 Addition of unprotected pyrrole onto chiral acrylamides: toward synthesis of new polypeptidic architectures

A variety of new chiral functionalized pyrroloamides have been synthesized by a simple and robust process involving Lewis acids. Unprotected pyrrole could be selectively monoalkylated or dialkylated.

Merging gold and organocatalysis: a facile asymmetric synthesis of annulated pyrroles

The combination of cinchona-alkaloid-derived primary amine and Au(I) -phosphine catalysts allowed the selective C-H functionalization of two adjacent carbon atoms of pyrroles under mild reaction conditions. This sequential dual activation provides seven-membered-ring-annulated pyrrole derivatives in excellent yields and enantioselectivities.

Stereoselective construction of all-carbon quaternary center by means of chiral phosphoric acid: highly enantioselective Friedel–Crafts reaction of indoles with β,β-disubstituted nitroalkenes

Chiral phosphoric acid catalyzed Friedel–Crafts reaction of indoles with β-alkoxycarbonyl-β-disubstituted nitroalkenes was achieved.