Asymmetric Friedel-Crafts Reaction Research Articles

Asymmetric Catalytic Friedel-Crafts Reactions of Unactivated Arenes.

Since its discovery more than a century ago, the Friedel-Crafts reaction has manifested itself as a powerful method for the introduction of carbon substituents to arenes. Despite its potential generality, the scope of the reaction is intrinsically limited by the arene's nucleophilicity, which has previously restrained the applicability of asymmetric variants to activated substrates. To overcome this fundamental limitation, we report herein an asymmetric Friedel-Crafts reaction of unactivated, purely hydrocarbon arenes, alkoxybenzenes, and heteroarenes with N,O-acetals to give enantioenriched arylglycine esters. Highly regio- and stereoselective C-C bond formation was achieved using strong and confined Brønsted acid organocatalysts, enabling the first asymmetric catalytic Friedel-Crafts reaction of simple alkylbenzenes.

Recent Progress in the Construction of Chiral 3-Substituted Indoles by Asymmetric Friedel-Crafts Reactions

Recent Progress in the Construction of Chiral 3-Substituted Indoles by Asymmetric Friedel-Crafts Reactions

The recent advances in the contribution of chiral triarylmethanes and tetraarylmethanes with organocatalysts

Chiral triarylmethane skeleton is an important structural unit of many known compounds that are widely applied in organic functional materials and pharmaceuticals. Thus, the efficient construction of this class of compounds has attracted intensive attention from chemists. There are two main difficulties in synthesizing this type of compound: (i) the steric resistance of molecular structures would make it hard to be constructed; (ii) there are three similar aryl groups on the stereocenter, which is difficult to achieve stereo-identification. At present, the most common strategy is to introduce the third aryl group into a diarylmethane framework through the asymmetric Friedel-Crafts reaction or addition reaction of electron-rich arenes, so as to construct chiral triarylmethanes. In this review, we summarized the recent developments in the construction of various chiral triarylmethanes and chiral tetraarylmethanes from easily accessible compounds under organocatalytic conditions. This article describes based on the types of electrophilic reagents, mainly including quinone methides, indole imine methides, and azadienes. At the same time, we also emphasize the mechanism of each representative reaction, which might enlighten the future development of this field.

Asymmetric Friedel-Crafts reaction of unsaturated carbonyl-tethered heteroarenes via vinylogous activation of Pd0-π-Lewis base catalysis.

The alkyne group can undergo facile transformations under palladium catalysis, such as hydropalladation, Wacker reaction, etc. Here we demonstrate that a chiral Pd0 complex can chemoselectively dihapto-coordinate to the alkyne moiety of 2-indolyl propiolates, and raise the Highest Occupied Molecular Orbital (HOMO)-energy ofthe deactivated heteroarenes via π-Lewis base catalysis. As a result, asymmetric C3-selective Friedel-Crafts addition to activated alkenes occurs, finally affording [3 + 2] or [3 + 4] annulation products with high enantioselectivity and exclusive E-selectivity. Moreover, this π-Lewis base vinylogous HOMO-activation strategy can be extended to remote Friedel-Crafts reaction of diverse five-membered heteroarenes tethered to a 2-enone or 2-acrylate motif with imines or 1-azadienes, and excellent enantiocontrol is generally achieved for the multifunctional adducts, which can be effectively converted to diverse frameworks with higher molecular complexity. In addition, NMR and density functional theory calculation studies are conducted to elucidate the catalytic mechanism.

Recent Advances in the Catalytic Asymmetric Friedel-Crafts Reactions of Indoles.

Functionalized chiral indole derivatives are privileged and versatile organic frameworks encountered in numerous pharmaceutically active agents and biologically active natural products. The catalytic asymmetric Friedel–Crafts reaction of indoles, catalyzed by chiral metal complexes or chiral organocatalysts, is one of the most powerful and atom-economical approaches to access optically active indole derivatives. Consequently, a wide range of electrophilic partners including α,β-unsaturated ketones, esters, amides, imines, β,γ-unsaturated α-keto- and α-ketiminoesters, ketimines, nitroalkenes, and many others have been successfully employed to achieve a plethora of functionalized chiral indole moieties. In particular, strategies for C–H functionalization in the phenyl of indoles require incorporation of a directing or blocking group in the phenyl or azole ring of indole. The discovery of chiral catalysts which can control enantiodiscrimination has gained a great deal of attention in recent years. This review will provide an updated account on the application of the asymmetric Friedel–Crafts reaction of indoles in the synthesis of diverse chiral indole derivatives, covering the timeframe from 2011 to today.

Synthesis of Non-canonical Amino Acids and Peptide Containing Them for Establishment of the Template for Drug Discovery.

Non-canonical amino acid derivatives are an attractive scaffold for novel drug candidates. Among the methods used to prepare this motif, the asymmetric Mannich-type reaction of α-imino carboxylic acid derivatives is a preeminent strategy because a wide variety of non-canonical amino acids can be accessed by changing only the nucleophile. Preparing the common substrate is difficult, however, which makes this method problematic. We developed a convenient method for synthesizing common substrates using MnO2-mediated oxidation of stable precursors. Peptides bearing non-canonical amino acids are another attractive synthetic target. We propose a new approach for synthesizing non-canonical amino acid-containing peptides by directly applying various organic reactions to peptidic substrates. Using hydrophobic anchor-supported peptides, we directly applied ring-closing metathesis and asymmetric Friedel-Crafts reactions to peptidic substrates. We also developed a novel recyclable organocatalyst according to the nature of the hydrophobic anchor tagged compound.

Light-Mediated Chiral Phosphate Catalysis for Asymmetric Dicarbofunctionalization of Enamides

A light-mediated asymmetric dicarbofunctionalization of enamides with carboxylic-acid-derived redox-active esters (RAEs) and indoles has been established by using chiral lithium phosphate catalysis in the presence or absence of photoredox catalyst. This reaction features mild reaction conditions and broad substrate scopes, delivering a wide range of highly functionalized chiral amine derivatives. Mechanistic studies suggest that chiral lithium phosphate can serve as a pocket to accelerate the aggregation of enamide and RAE through hydrogen-bonding and coordination interaction, enabling the formation of a charge-transfer complex (CTC). Either enamide or CTC can be excited by direct irradiation or Ru(II)-mediated photosensitization to furnish chiral iminium intermediates for the asymmetric Friedel–Crafts reaction of indole.

Guanosine-Based Self-Assembly as an Enantioselective Catalyst Scaffold

A self-assembled G-quadruplex formed by guanosine and borate as the chiral scaffold was used to catalyze the asymmetric Friedel-Crafts reaction in water. Catalysis, depending on the self-assembly of guanosine and borate into a fibrillar structure in the presence of Cu2+ ions, can be modulated by the assembly concentration, temperature, and amount of Cu2+ ions. Detailed spectral experiments proved that the guanosine-based assembly in solution was responsible for the enantioselective catalysis, rather than small-molecule species. Some of the similar G-quartet assemblies were unable to promote the asymmetric reaction, implying unique properties of the current system, including excellent lifetime stability and supramolecular chiral structures. This work provided the first example of the self-assembled G-quadruplex achieving enantioselective catalysis and some perspective to better understand the design of nucleoside-based self-assemblies for an enantioselective reaction. In view of guanosine as a building block, these findings may be applied to discuss the prebiotic chiral catalyst preceded ribozymes.

An organocatalytic asymmetric Friedel-Crafts reaction of 2-substituted indoles with aldehydes: enantioselective synthesis of α-hydroxyl ketones by low loading of chiral phosphoric acid.

Hydroxyl alkylation of indoles by Friedel-Crafts reaction with a carbonyl compound is a useful strategy. However, the reaction was restricted to ketones due to the easy formation of a bisindole byproduct. Therefore, hydroxyl alkylation of an aldehyde with indole is confronted with great challenges. Here, we report an efficient strategy for asymmetric hydroxyl alkylation of 2-substituted indoles with aldehydes under 0.1 mol% chiral phosphoric acid. A series of α-hydroxyl ketones were obtained in high yields (up to 99%) and good enantioselectivities (up to 97%).

Enantioselective Friedel-Crafts Reaction of Indoles with N-Tosylimines Employing Takemoto's Organocatalyst

The asymmetric Friedel-Crafts reaction of indoles with imines provides a direct approach for the enantioselective construction of 3-indolyl methanamine derivatives. In this paper, a series of thiourea/ urea-tertiary amine derivatives have been used to catalyze the asymmetric F-C reaction of indoles with N-tosylimines. The effect of solvent, temperature and catalyst loading amount were investigated. the optimized conditions were determined to be toluene as the solvent with a 5 mol% loading of Takemoto's catalyst 1c at rt. A wide range of substituted indoles and imines were evaluated to explore the scope and general applicability of this reaction. The enantioselectivities were obviously affected by the position of the substituents on the phenyl ring of imines. 4- electron-donating substituted imines appeared to favor higher enantioselectivities. Moreover, indoles containing either electron-donating groups or electron-withdrawing groups were then investigated for the reaction with imines. In all cases, high yields and excellent enantioselectivities could be achieved. Of all the different substrates, the reaction of the 5-OMe substituted indole with 4-chloride substituted imine afforded an optimal yield and enantiomeric excess. Under the screened optimal condition, optically active 3-indolyl-methanamines were obtained in good yields (70-90%) with high enantioselectivities (up to 98%ee). The catalyst type and the substrate scope were broadened in this methodology.

Design of Phosphinic Acid Catalysts with the Closest Stereogenicity at the α-Position: Synthesis and Application of α-Stereogenic Perfluoroalkyl Phosphinic Acid Catalysts.

Chiral C2-symmetric phosphinic acids were designed based on sterically demanding and helical chiral perfluoroalkyl groups at the closest α-position advancing asymmetric reaction environment and catalytic activity. The perfluoroalkyl catalysts, [(CF3)2F2] and [(C2F5)2F2] phosphinic acids, were synthesized via a stereoselective addition/cyclization sequence of methyl phosphinate and deoxofluorination. These new classes of Brønsted acid catalysts were applied to an asymmetric Friedel-Crafts reaction to give up to 89% yield and 82% R-enantioselectivity, which is higher than those obtained with the parent phosphoric acid (42% and 55.5% S).

Merging Visible-Light Photoredox and Chiral Phosphate Catalysis for Asymmetric Friedel-Crafts Reaction with in Situ Generation of N-Acyl Imines.

In the presence of visible-light photoredox and chiral phosphate catalysts, a novel asymmetric Friedel-Crafts reaction of indoles and readily accessible α-amino acid derived redox-active esters is established to afford enantioenriched 1-indolyl-1-alkylamine derivatives in moderate to high yields and with high levels of enantioselectivities. This method not only shows a mild and efficient alternative for the in situ generation of N-acyl imines but also indicates the feasibility of a multicatalyst system for asymmetric photoredox catalysis.

Enantioselective α-Aminophosphonate Functionalization of Indole Ring through an Organocatalyzed Friedel-Crafts Reaction.

Chiral phosphoric acids efficiently catalyze the asymmetric Friedel-Crafts reaction of several indoles with α-iminophosphonates to afford enantioenriched hybrid α-aminophosphonate functionalized indole derivatives.

Revisiting the CuII-Catalyzed Asymmetric Friedel-Crafts Reaction of Indole with Trifluoropyruvate.

A CuII complex of bisamidine ligand L S catalyzes the Friedel-Crafts (FC) reaction of indole with trifluoropyruvate with high generality, yielding the highly enantiomerically enriched FC adduct. Electron-rich indoles have high reactivity due to a dual activation mechanism showing second-order kinetics for CuII, whereas indoles with a soft substituent at C(4) proceed at a rate that is three orders of magnitude lower via a coordination mechanism, which reverses the sense of enantioselectivity.

Protonated Bis-1,2,3-triazole as an Anion-Binding Chiral Brønsted Acid for Catalytic Asymmetric Friedel–Crafts Reaction of Indoles with Imines

Abstract The complex of chiral C2-symmetric bis-1,2,3-triazole and an achiral Brønsted acid is introduced as a new class of chiral acid catalysts. The catalytic activity and stereocontrolling ability of this chiral Brønsted acid can be tuned simply by changing the achiral acid. The synthetic relevance of this salient feature is demonstrated in the asymmetric Friedel–Crafts reaction of indoles with N-sulfonyl imines.

Recent Advance in Catalytic Asymmetric Friedel-Crafts Reactions

Recent Advance in Catalytic Asymmetric Friedel-Crafts Reactions

A chiral multidentate salan-supported heterobimetallic catalyst for asymmetric Friedel-Crafts reaction

A new hydroxymethyl-pendant salan (3) containing a chiral backbone and an extra O4 cavity has been prepared through a facile one-pot procedure. The ligand has been characterized by ESI-MS, elemental analysis and 2D NMR techniques. To continue our previous work on the catalytically asymmetric F-C reactions using multimetallic complexes involving salan-type ligands, we investigated the applicability of 3 for both asymmetric Henry and F-C reactions, in comparison with previously reported results using salans 1 and 2. It was revealed that although 3 was not a suitable ligand for homometallic copper-catalyzed Henry reaction, it was effective for the hetereobimetallic Cu/Zn-catalyzed asymmetric F-C reaction, affording desired alkylated product with good yield and moderate enantiomeric excess (ee), after screening of various catalyst combinations.

Asymmetric Friedel–Crafts/Michael Reaction of Indoles and Pyrroles with Coumarin-3-carbonylates

Asymmetric Friedel–Crafts reactions of indoles or pyrroles with coumarin-3-carboxylates catalyzed by Cu(OTf)2 complexes with chiral bis(oxazoline) ligands have been developed. The highest ee (up to 82%) was achieved for indole with a nitro derivative of coumarin-3-carboxylate­.

Recent Advances in C-C and C-N Bond Forming Reactions Catalysed by Polystyrene-Supported Copper Complexes.

This present mini-review covers recently published results on Cu(I) and Cu(II) complexes immobilized on polystyrene carriers, which are used as heterogeneous, eco-friendly reusable catalysts applied for carbon–carbon and carbon–nitrogen forming reactions. Recent advances and trends in this area are demonstrated in the examples of oxidative homocoupling of terminal alkynes, the synthesis of propargylamines, nitroaldolization reactions, azide alkyne cycloaddition, N-arylation of nitrogen containing compounds, aza-Michael additions, asymmetric Friedel–Crafts reactions, asymmetric Mukaiyama aldol reactions, and asymmetric 1,3-dipolar cycloaddition of azomethine ylides. The type of polystyrene matrix used for the immobilization of complexes is discussed in this paper, and particularly, the efficiency of the catalysts from the point of view of the overall reaction yield, and possible enantioselectivity and potential reusing, is reviewed.

Chemo- and Regioselective Asymmetric Friedel–Crafts Reaction of Furans and Thiophenes with α,β-Unsaturated Aldehydes through Dual Activation

A highly chemo- and regioselective Friedel–Crafts alkylation reaction of furans and thiophenes has been developed, which relies on the activation from the remote conjugated Mukaiyama silyl enol ether motif. Excellent enantioslectivity is generally obtained in reactions with α,β-unsaturated aldehydes under the well-established iminium ion catalysis of a chiral secondary amine.