Enantioselective Friedel–Crafts Alkylation Reaction of Indoles with α-Trifluoromethylated β-Nitrostyrenes Catalyzed by Chiral BINOL Metal Phosphate

Diastereo-divergent synthesis of chiral hindered ethers via a synergistic calcium(II)/gold(I) catalyzed cascade hydration/1,4-addition reaction

Hindered ethers are ubiquitous in natural products and bioactive molecules. However, developing an efficient method for the stereocontrolled synthesis of all stereoisomers of chiral hindered ethers is highly desirable but challenging. Here we show a strategy that utilizes in situ-generated water as a nucleophile in an asymmetric cascade reaction involving two highly reactive intermediates, 3-furyl methyl cations and ortho-quinone methides (o-QMs), to synthesize chiral hindered ethers. The Ca(II)/Au(I) synergistic catalytic system enables the control of diastereoselectivity and enantioselectivity by selecting suitable chiral phosphine ligands in this cascade hydration/1,4-addition reaction, affording all four stereoisomers of a diverse range of chiral tetra-aryl substituted ethers with high diastereoselectivities (up to >20/1) and enantioselectivities (up to 95% ee). This work provides an example of chiral Ca(II)/Au(I) bimetallic catalytic system controlling two stereogenic centers via a cascade reaction in a single operation.

Enantioselective synthesis of 3-(N-indolyl)quinolines containing axial and central chiralities

Quinoline and indole are important core structures in biologically active compounds and materials. Atropisomeric biaryls consisting of quinoline and indole are a unique class of axially chiral molecules. We report herein enantioselective synthesis of 3-(N-indolyl)quinolines having both C-N axial chirality and carbon central chirality by a photoredox Minisci-type addition reaction catalyzed by a chiral lithium phosphate/Ir-photoredox complex. The catalytic system enabled access to a unique class of 3-(N-indolyl)quinolines with high chemo-, regio-, and stereoselectivities in good yields through the appropriate choice of an acid catalyst and a photocatalyst. This is the first example of the synthesis of 3-(N-indolyl)quinoline atropisomers in a highly enantioselective manner.

Ni-Catalyzed Reductive Dibenzylation of Trifluoromethylalkenes for CF3-Containing All-Carbon Quaternary Center Construction

A Ni-catalyzed reductive dicarbofunctionalization of α-CF3 styrenes with benzyl bromides has been accomplished. This transformation obviates the commonly facile β-F elimination effectively and enables the creation of CF3-substituted all-carbon quaternary centers of pharmaceutical interests. Preliminary mechanistic studies suggest a pathway consisting of benzyl radical addition and subsequent nickel-mediated benzylation of the resulting α-CF3-embedded tertiary C radical.

Synthesis of 2-(2-Nitroalkyl)indoles by Rhodium(III)-Catalyzed C-H Alkylation

The Rh(III)-catalyzed addition of the indole C2-H bond to nitroalkenes under an ambient atmosphere is disclosed, providing direct access to a wide range of 2-(2-nitroalkyl)indoles (33 examples) with excellent chemo- and regioselectivity. In addition, pyrrole derivatives also successfully participated in this Friedel-Crafts alkylation reaction. Representative nitroalkane products could be converted into structurally diverse and valuable indole derivatives. Furthermore, a series of control experiments were conducted, and a plausible mechanism was proposed.

Direct Nitration of Vinylcycles with Copper Nitrate

A direct nitration of vinylcyclopropanes is disclosed with Cu(NO₃)₂ and KI in a regio- and stereoselective manner to afford nitroalkenes efficiently, where the cyclopropane skeleton was retained. The given method could be extended to other vinylcycles as well as biomolecule derivatives with wide substrate scope, good functionality tolerance, and efficient synthesis modularity. Further transformations illustrated the obtained products as versatile building blocks in organic synthesis. The proposed ionic pathway could account for the untouched small ring and the effect of KI during the reaction.

Asymmetric Rubottom-Type Oxidation Catalyzed by Chiral Calcium Phosphates

A highly efficient catalytic asymmetric Rubottom-type oxidation is described. Using meta-chloroperbenzoic acid (m-CPBA) as the oxidant and chiral calcium phosphate as the catalyst, the facile transformation enables direct hydroxylation of N-Boc oxindoles and β-ketoesters in high yields (up to 99 %) and in a highly enantioenriched fashion (up to >99 % ee). The application of the established method was demonstrated by the synthesis of a pharmaceutically important 3-hydroxyoxindole with excellent enantiocontrol.

Highly Active La(III)-Based Metal-Organic Framework as a Heterogeneous Lewis Acid Catalyst for Friedel-Crafts Alkylation

In this study, a novel La(III)-based two-dimensional (2D) metal-organic framework, [La_2/3 (qptca)_1/2 ] (referred to as SLX-2), from LaCl₃ and 1,1' : 4',1'' : 4'',1''' : 4''',1''''-quinquephenyl]-2,2'',2'''',5''-tetracarboxylic acid (H₄ qptca) was synthesized by conventional solvothermal method and thoroughly characterized by using X-ray single-crystal diffraction, powder X-ray diffraction, and thermogravimetric analyses. The 2D SLX-2 features a unique lanthanum center exposed to the skeleton and was used as an efficient Lewis acid catalyst for the Friedel-Crafts alkylation of indole and pyrrole with β-nitrostyrene along with a wide substrate scope, giving the desired products in good-to-high yields under the optimal reaction conditions. Furthermore, the catalyst was used for twenty cycles, with nearly no effect on its activity, and the reaction was heterogeneous in nature. Moreover, compared to the previous hydrogen-bond-donating MOF catalysts for such alkylation reactions, SLX-2 showed an excellent stability toward harsh acidic and basic environment, and gave comparable catalytic activities.

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.

Enantioselective Friedel-Crafts Alkylation Reaction of Pyrroles with N-Unprotected Alkynyl Trifluoromethyl Ketimines

Developed herein is an enantioselective Friedel-Crafts alkylation reaction of N-unprotected alkynyl trifluoromethyl ketimines with pyrroles catalyzed by chiral phosphoric acid to furnish chiral primary α-trifluoromethyl-α-(2-pyrrolyl)propargylamines with high enantioselectivity. Transformation of the alkynyl group of the adducts afforded optically active α-trifluoromethylated amines bearing various substituents such as alkyl, alkenyl, enyne, and triazole without loss of optical purity.

Chiral Calcium Phosphate Catalyzed Enantioselective Synthesis of All-Carbon Quaternary Center by Friedel–Crafts Alkylation Reaction of Pyrroles and Trifluoromethylated Nitrostyrenes

A chiral calcium phosphate-catalyzed enantioselective Friedel–Crafts alkylation reaction of pyrroles and α-trifluoromethyl-β-nitrostyrenes has been developed. Pyrroles bearing an all-carbon quaternary center at 2-position were obtained with high to excellent enantioselectivity. Use of 4,7-dihydroindole as a nucleophile and subsequent oxidation resulted in the formation a 2-substituted indole derivative with excellent enantioselectivity. A 3,4-dihydropyrrolo[1,2-c]pyrimidine derivative was successfully synthesized by reduction followed by benzoylation of the product without loss of enantioselectivity. Application to a 1 mmol scale reaction was successfully achieved.

HFIP as Protonation Reagent and Solvent for Regioselective Alkylation of Indoles with All-Carbon Centers

The regio- and chemoselective construction of indole bearing an all-carbon center at the C3-position, a versatile bioactive building block, by C(sp²)-C(sp³) formation with olefins has been achieved through utilization of hexafluoroisopropanol (HFIP) as the protonation reagent and solvent. The catalytic reactions are operationally simple and green compared with previous reports utilizing elaborated olefins and catalysts. This protocol allows for alkylation of a variety of substituted indoles with diverse of styrene type alkenes in excellent yields and with high selectivity. Application of this protocol to the synthesis of drug was pursued and with an improved yield in contrast to previous art. Catalytic kinetics and deuterium-labeling experiments suggest that the rate-determining step involves the protonation of olefin by HFIP to generate carbocation, followed by electrophilic addition to indole derivative.

Exploiting the Chiral Ligands of Bis(imidazolinyl)- and Bis(oxazolinyl)thiophenes-Synthesis and Application in Cu-Catalyzed Friedel-Crafts Asymmetric Alkylation

Five new C₂-symmetric chiral ligands of 2,5-bis(imidazolinyl)thiophene (L1–L3) and 2,5-bis(oxazolinyl)thiophene (L4 and L5) were synthesized from thiophene-2,5-dicarboxylic acid (1) with enantiopure amino alcohols (4a–c) in excellent optical purity and chemical yield. The utility of these new chiral ligands for Friedel–Crafts asymmetric alkylation was explored. Subsequently, the optimized tridentate ligand L5 and Cu(OTf)₂ catalyst (15 mol%) in toluene for 48 h promoted Friedel–Crafts asymmetric alkylation in moderate to good yields (up to 76%) and with good enantioselectivity (up to 81% ee). The bis(oxazolinyl)thiophene ligands were more potent than bis(imidazolinyl)thiophene analogues for the asymmetric induction of the Friedel–Crafts asymmetric alkylation.

Calcium-mediated C(sp3)-H Activation and Alkylation of Alkylpyridines

Main-group metal calcium-mediated alkylpyridine benzylic C(sp³)-H activation and functionalization have been achieved. The reaction of a calcium hydride complex [{(^DIPPnacnac)CaH(thf)}₂] (^DIPPnacnac = CH{(CMe)(2,6-ⁱPr₂-C₆H₃N)}₂) with two equivalents of 2,6-lutidine rapidly yields a monomeric calcium alkyl complex with the release of dihydrogen. A hydride/carbon-bridged binuclear calcium complex [{(^DIPPnacnac)Ca}₂(μ-H){2-Me-6-(μ-CH₂)-Py}(thf)] is obtained from an equimolar treatment of calcium hydride and 2,6-lutidine that is readily converted into mono- or binuclear calcium alkyl complexes upon subsequent addition of 2,6-lutidine. DFT calculations and kinetic studies are conducted to determine their reaction profiles. More significantly, this calcium hydride complex catalyzes regioselective benzylic C-H bond addition of alkylpyridines to a variety of alkenes, affording linear or branched alkylated pyridine derivatives.

Chiral Calcium Phosphate Catalyzed Enantioselective Amination of 3-Aryl-2-benzofuranones

A 4-tert-butyl-phenyl substituted (R)-[H₈]-BINOL chiral calcium phosphate catalyzed enantioselective amination of 3-aryl-2-benzofuranones with dibenzyl azodicarboxylate is described. The catalyst loading of the reaction is 1 mol %. This transformation is facile and has a high degree atom economy, which gave products with good yields and high enantioselectivities (79% to 99%). This reaction has excellent ee and a broad substrate scope with mild reaction conditions.

B(C6 F5 )3 -Catalyzed Tandem Friedel-Crafts and C-H/C-O Coupling Reactions of Dialkylanilines

Tandem Friedel-Crafts (FC) and C-H/C-O coupling reactions catalyzed by tris(pentafluorophenyl) borane (B(C₆ F₅ )₃ ) were achieved without using any other additive in the absence of solvent. This process can be used for the reactions between a series of dialkylanilines and vinyl ethers with good isolated yields of bis(4-dialkylaminophenyl) compounds. Based on combined theoretical and experimental studies, the possible reaction mechanism was proposed. B(C₆ F₅ )₃ can activate the C=C and C-O bond for FC and C-H/C-O coupling reactions respectively. The FC reaction is slow, which is followed by a fast C-H/C-O coupling.

Imine Amidation Catalyzed by a Chiral VAPOL Calcium Phosphate

Over the last 16 years, chiral phosphoric acids (CPAs) have been shown to be excellent asymmetric catalysts, very effectively used in constructing chiral molecules with high enantiocontrol. In 2010, Ishihara et al. discovered that chiral metal phosphate complexes (or salts) could be found in substantial quantities, as contaminates, in some reported CPA-catalyzed reactions (Hatano, M.; Moriyama, K.; Maki, T.; Ishihara, K. Angew. Chem., Int. Ed. 2010, 49, 3823-3826). These metal phosphates were shown to actually catalyze the reactions in addition to CPAs. In this work, we have investigated in depth a reaction first reported to be catalyzed by CPAs based on a vaulted bis-phenanthrol (VAPOL) backbone. We have found that VAPOL metal phosphates were, in fact, superior catalysts for this reaction. Upon optimization, a wide substrate scope, low catalyst loading, and mild conditions could provide intermolecular imine amidation reactions producing chiral N,N'-aminal products in high yields and with excellent ee values (up to >99% yield, >99% ee).

Synthesis of Chiral Bis(3-indolyl)methanes Bearing a Trifluoromethylated All-Carbon Quaternary Stereocenter via Nickel-Catalyzed Asymmetric Friedel-Crafts Alkylation Reaction

Bis(3-indolyl)methanes are well-known natural products with a broad range of important biological functions including cancer cell growth inhibition and antimicrobial activity. Incorporation of a trifluoromethyl group is known to have a profound effect on the parent compound's biological activities. Here, an efficient method for the synthesis of chiral trifluoromethylated bis(3-indolyl)methanes via a catalytic asymmetric Friedel-Crafts (F-C) alkylation reaction has been established. Both enantiomers of the catalysis products can be obtained by tuning the chiral substituents of the catalyst. With 5 mol % of the Ni(II)/(imidazoline-oxazoline) complex as the catalyst, the F-C reaction of indoles with β-CF₃-β-(3-indolyl)nitroalkenes proceeded well to afford a series of chiral bis(3-indolyl)methanes bearing a trifluoromethylated all-carbon quaternary stereocenter in generally good yields with excellent enantioselectivities (up to 98% yield and 94% ee). Furthermore, by interchanging the indole moieties of the two reactants, indole vs β-CF₃-β-(3-indolyl)nitroalkene in the F-C reaction, both enantiomers of a given trifluoromethylated bis(3-indolyl)methane were obtained with high enantioselectivities (89-94% ee) upon removal of the indole N-protecting group in the F-C products. The current work represents the first general catalytic enantioselective approach to the important class of trifluoromethylated bis(3-indolyl)methanes.

Cross-dehydrogenative coupling enables enantioselective access to CF3-substituted all-carbon quaternary stereocenters

A cross-dehydrogenative coupling strategy for enantioselective access to acyclic CF3-substituted all-carbon quaternary stereocenters has been established. By using catalytic DDQ with MnO2 as an inexpensive terminal oxidant, asymmetric cross coupling of racemic δ-CF3-substituted phenols with indoles proceeded smoothly, providing CF3-bearing all-carbon quaternary stereocenters with excellent chemo- and enantioselectivities. The generality of the strategy is further demonstrated by efficient construction of all-carbon quaternary stereocenters bearing other polyfluoroalkyl and perfluoroalkyl groups such as CF2Cl, C2F5, and C3F7.

Copper-catalyzed mild desulfonylation of vinyl sulfonyl molecules

The first Cu-catalyzed chemical selective desulfonylation of vinyl sulfonyl molecules to olefins was developed using B₂pin₂–water as the reductant.