Catalytic Asymmetric Arylation of 3-Indolylmethanols: Enantioselective Synthesis of 3,3′-Bis(indolyl)oxindoles with High Atom Economy

Design and Catalytic Asymmetric Synthesis of Furan-Indole Compounds Bearing both Axial and Central Chirality

In the chemistry community, catalytic asymmetric synthesis of furan-based compounds bearing both axial and central chirality has proven to be a significant but challenging issue owing to the importance and difficulty in constructing such frameworks. In this work, we have realized the first catalytic asymmetric synthesis of five-five-membered furan-based compounds bearing both axial and central chirality via organocatalytic asymmetric (2+4) annulation of achiral furan-indoles with 2,3-indolyldimethanols with uncommon regioselectivity. By this strategy, furan-indole compounds bearing both axial and central chirality were synthesized in high yields with excellent regio-, diastereo-, and enantioselectivities. Moreover, theoretical calculations were conducted to provide an in-depth understanding of the reaction pathway, activation mode, and the origin of the selectivity.

Construction of diverse spirooxindoles via a domino reaction of arylamines, but-2-ynedioates and 3-hydroxy-3-(indol-3-yl)indolin-2-ones

An iodine-promoted domino reaction of arylamines/benzylamines, dialkyl but-2-ynedioates and 3-hydroxy-3-(indol-3-yl)indolin-2-ones showed very interesting molecular diversity. The reaction in acetonitrile at 65 °C in the presence of 30% mmol I2 resulted in spiro[indoline-3,1'-pyrido[4,3-b]indoles] in satisfactory yields. When anilines without para-substituents were used in the reaction, a direct substitution of the hydroxyl group to 2-(phenylamino)maleate at the para-position of aniline gave chain products in good yields. Additionally, similar reactions with benzylamines not only gave spiro[indoline-3,1'-pyrido[4,3-b]indoles], but also afforded spiro[indoline-3,1'-pyrano[4,3-b]indol]-2-ones in lower yields. A plausible domino annulation mechanism was rationally proposed for the formation of different kinds of polycyclic compounds.

Oxygen Dependent Switchable Selectivity during Ruthenium Catalyzed Selective Synthesis of C3-Alkylated Indoles and Bis(indolyl)methanes

Herein, we report a ligand-centered redox-controlled oxygen-dependent switchable selectivity during ruthenium-catalyzed selective synthesis of C3-alkylated indoles and bis(indolyl)methanes (BIMs). A wide variety of C3-alkylated indoles and BIMs were prepared selectively in moderate to good isolated yields by coupling a wide variety of indoles and alcohols, catalyzed by a well-defined, air-stable, and easy-to-prepare Ru(II)-catalyst (1a) bearing a redox-active tridentate pincer (L1a). Catalyst 1a efficiently catalyzed the C3-alkylation of indoles under an argon atmosphere while, under an oxygen environment, exclusively producing the BIMs. A few drug molecules containing BIMs were also synthesized efficiently. 1a exhibited excellent chemoselectivity with alcohols containing internal carbon-carbon double bonds. Mechanistic investigation revealed that the coordinated azo-aromatic ligand actively participates during the catalysis. During the dehydrogenation of alcohols, the azo-moiety of the ligand stores the hydrogen removed from the alcohols and subsequently transfers the hydrogen to the alkylideneindolenine intermediate, forming the C3-alkylated indoles. While under an oxygen environment, the transfer of hydrogen from the ligand scaffold to the molecular oxygen generates H2O2, leaving no scope for hydrogenation of the alkylideneindolenine intermediate, rather than it undergoing 1,4-Michael-type addition forming the BIMs.

Asymmetric N-Alkylation of 1H-Indoles via Carbene Insertion Reaction

An intermolecular enantioselective N-alkylation reaction of 1H-indoles has been developed by cooperative rhodium and chiral phosphoric acid catalyzed N-H bond insertion reaction. N-Alkyl indoles with newly formed stereocenter adjacent to the indole nitrogen atom are produced in good yields (up to 95 %) with excellent enantioselectivities (up to >99 % ee). Importantly, both α-aryl and α-alkyl diazoacetates are tolerated, which is extremely rare in asymmetric X-H (X=N, O, S et al.) and C-H insertion reactions. With this method, only 0.1 mol % of rhodium catalyst and 2.5 mol % of chiral phosphoric acid are required to complete the conversion as well as achieve the high enantioselectivity. Computational studies reveal the cooperative relay of rhodium and chiral phosphoric acid, and the origin of the chemo and stereoselectivity.

Catalytic Enantioselective Synthesis of 2,3'-Bis(indolyl)methanes Bearing All-Carbon Quaternary Stereocenters via 2-Indole Imine Methides

An organocatalytic enantioselective formal hydroarylation of 2-vinyl indoles for the preparation of enantioenriched 2,3'-bis(indolyl)methanes bearing an all-carbon quaternary stereocenter is described. This reaction features mild conditions, low catalyst loading, excellent efficiency and enantioselectivity. The obtained products showed promising anticancer activity.

Catalyst-Switchable Divergent Synthesis of Bis(indolyl)alkanes and 3-Alkylated Indoles from Styrene Oxides

A novel and effective Brønsted acid-catalyzed chemoselective synthesis of bis(indolyl)alkanes and 3-alkyl indoles is reported. The selectivity of two significant indole derivatives is attained by allowing the same substrates to go through divergent reaction routes catalyzed by different catalysts. Furthermore, this mild approach is applicable to a wide range of substrates and has high efficacy in large-scale reactions. A plausible mechanism is provided based on the control experiments and spectroscopic studies.

Gold(I)-Catalyzed Cycloisomerization-Indole Addition Cascade: Synthesis of 3(2H)-Furanone-Incorporated Unsymmetrical 3,3'- Bis(indolyl)methanes

An unprecedented Au(I)-catalyzed domino intramolecular carbonyl-alkyne cyclization/indole addition strategy has been disclosed here. This generalized strategy enables the synthesis of 3(2H)-furanone-incorporated unsymmetrical bis(indolyl)methanes with generation of a stereocenter at the furanone junction from easily accessible indole-tethered ynediones. In addition, this present protocol could also be extended for the synthesis of a number of indolyl-(hetero)arylmethanes by employing a variety of (hetero)arenes as a nucleophile coupling partner.

NHPI-catalyzed electrochemical C–H alkylation of indoles with alcohols to access di(indolyl)methanes via radical coupling

The present indirect electrochemically mediated radical protocol outperforms the traditional Friedel–Crafts route with a broad substrate scope and functional group tolerance, as well as facile gram-scale synthesis without metal contamination.

Organocatalytic enantioselective SN1-type dehydrative nucleophilic substitution: access to bis(indolyl)methanes bearing quaternary carbon stereocenters

A highly general and straightforward approach to access chiral bis(indolyl)methanes (BIMs) bearing quaternary stereocenters has been realized via enantioconvergent dehydrative nucleophilic substitution. A broad range of 3,3′-, 3,2′- and 3,1′-BIMs were obtained under mild conditions with excellent efficiency and enantioselectivity (80 examples, up to 98% yield and >99 : 1 er). By utilizing racemic 3-indolyl tertiary alcohols as precursors of alkyl electrophiles and indoles as C–H nucleophiles, this organocatalytic strategy avoids pre-activation of substrates and produces water as the only by-product. Mechanistic studies suggest a formal S_N1-type pathway enabled by chiral phosphoric acid catalysis. The practicability of the obtained enantioenriched BIMs was further demonstrated by versatile transformation and high antimicrobial activities (3al, MIC: 1 μg mL⁻¹).

A highly general and straightforward approach to access chiral bis(indolyl)methanes (BIMs) bearing quaternary stereocenters has been realized via enantioconvergent dehydrative nucleophilic substitution.

Design, synthesis and evaluation of structurally diverse ortho-acylphenol-diindolylmethane hybrids as anticancer agents

A highly efficient synthesis of structurally diverse ortho-acylphenol–diindolylmethane hybrids 3 using carboxylic acid-activated chromones as versatile synthetic building blocks is reported here for the first time, through 1,4-nucleophilic addition and followed by a decarboxylation and pyrone ring opening reaction process.

Decarboxylative Addition of Propiolic Acids with Indoles to Synthesize Bis(indolyl)methane Derivatives with a Pd(II)/LA Catalyst

Exploring new protocols for efficient organic synthesis is crucial for pharmaceutical developments. The present work introduces a Pd(II)/LA-catalyzed (LA: Lewis acid) decarboxylative addition reaction for the synthesis of bis(indolyl)methane derivatives. The presence of Lewis acid such as Sc(OTf)₃ triggered Pd(II)-catalyzed decarboxylative addition of propiolic acids with indoles to offer the bis(indolyl)methane derivatives in moderate to good yields, whereas neither Pd(II) nor Lewis acid alone was active for this synthesis. The catalytic efficiency of Pd(OAc)₂ was highly dependent on the Lewis acidity of the added Lewis acid, that is, a stronger Lewis acid provided a higher yield of the bis(indolyl)methane derivatives. Meanwhile, this Pd(II)/LA-catalyzed decarboxylative addition reaction showed good tolerance toward versatile electron-rich or -deficient substituents on the indole skeleton and on the benzyl ring of propiolic acids. The studies on the in situ ¹H NMR kinetics of this Pd(II)/Sc(III) catalysis disclosed the formation of a transient vinyl-Pd(II)/Sc(III) intermediate generated by the pyrrole addition to the alkynyl-Pd(II)/Sc(III) species after decarboxylation, which was scarcely observed before.

FeCl3·6H2O as a Mild Catalyst for Nucleophilic Substitution of Symmetrical Bis(indoyl)methanes

In this paper, unsymmetrical bis(indolyl)methane (BIM) and 3-alkylindole derivatives are smoothly synthesized from symmetrical BIMs with a variety of nucleophiles including heteroaromatic/aromatic compounds, allylsilane and alkynylsilane. FeCl₃·6H₂O is found to be a mild and highly effective catalyst for this nucleophilic substitution reaction in which N-methyl-2-phenylindole behaves as a good leaving group in the C_sp³-C_sp² bond cleavage reaction. The operational ease, nonexpensive and environmentally friendly catalyst, mild reaction conditions, broad functional group tolerance, and scalability of this reaction strategy are advantages of the present procedure.

Brønsted Acid-Catalysed Dehydrative Substitution Reactions of Alcohols

The direct, catalytic dehydrative substitution of alcohols is a challenging, yet highly desirable process in the development of more sustainable approaches to organic chemistry. This review outlines recent advances in Brønsted acid-catalysed dehydrative substitution reactions for C-C, C-O, C-N and C-S bond formation. The wide range of processes that are now accessible using simple alcohols as the formal electrophile are highlighted, while current limitations and therefore possible future directions for research are also discussed.

Molecular diversity of the acid promoted domino reaction of 3-hydroxy-3-(indol-3-yl)indolin-2-ones and cyclic mercapto-substituted β-enamino esters

The acetic acid promoted reaction of 3-hydroxy-3-(indol-3-yl)indolin-2-ones and mercapto-substituted β-enamino esters showed very interesting molecular diversity.

Selectivity Control in Gold-Catalyzed Hydroarylation of Alkynes with Indoles: Application to Unsymmetrical Bis(indolyl)methanes

Gold-catalyzed hydroarylation of unactivated alkynes with indoles have previously been reported to proceed with double indole addition to produce symmetrical bis(indolyl)methanes (BIMs). We demonstrate for the first time that the selectivity of the gold-catalyzed reaction can be fully switched to allow for isolation of the vinylindole products instead. Furthermore, this selective reaction can be utilized to synthesize the more difficult to access unsymmetrical BIMs from readily available starting materials.

Enantioselective synthesis of mixed 3,3'-bisindoles via a phosphine-catalyzed umpolung γ-addition of 3'-indolyl-3-oxindoles to allenoates

An enantioselective umpolung γ-addition reaction of 3'-indolyl-3-oxindoles to allenoates catalyzed by amino acid-derived bifunctional phosphine catalysts has been developed. A wide range of chiral mixed 3,3'-bisindole scaffolds containing an all-carbon quaternary stereogenic center were obtained in high yields and with excellent enantioselectivities. 3,3'-Bisindoles are valuable synthetic intermediates, the employment of which led to formal total syntheses of (+)-Chimonanthine, (+)-Folicanthine and (-)-Calycanthine, as well as facile creation of useful pyrrolidinoindoline core structure.

Organocatalytic Asymmetric Synthesis of Indole-Based Chiral Heterocycles: Strategies, Reactions, and Outreach

Indole-based chiral heterocycles constitute a class of important heterocyclic compounds that are found in numerous pharmaceuticals, functional materials, and chiral catalysts or ligands. Catalytic asymmetric synthesis, for which the 2001 Nobel Prize in Chemistry was awarded, has been demonstrated to be the most efficient method for accessing chiral compounds. Therefore, the catalytic asymmetric synthesis of indole-based chiral heterocycles has attracted great interest from the scientific community. However, the strategies toward this goal are rather limited, and great challenges remain in this field, such as metal contamination in the products, the limited number of platform molecules with versatile reactivity, and the limited number of catalytic asymmetric reactions that offer high step economy, atom economy, and excellent enantiocontrol. Therefore, novel strategies for the catalytic asymmetric synthesis of indole-based chiral heterocycles are urgently needed. To achieve this goal, our group has developed a series of unique strategies, such as designing and developing versatile platform molecules and their corresponding organocatalytic asymmetric reactions to access indole-based chiral heterocycles. In this Account, we describe our efforts to address the remaining challenges in this research field. Namely, we have designed and developed vinylindoles, indolylmethanols, arylindoles and indole derivatives as versatile platform molecules for the construction of indole-based chiral heterocyclic scaffolds with structural diversity and complexity. Based on the reactivities of these platform molecules, we have designed and accomplished a series of organocatalytic asymmetric cycloaddition, cyclization, addition and dearomatization reactions with a high step economy, atom economy and excellent enantiocontrol. Using these strategies, a wide range of indole-based chiral heterocycles, including five-membered to seven-membered heterocycles, axially chiral heterocycles and tetrasubstituted heterocycles, have been synthesized with high efficiency and excellent enantioselectivity. In addition, we have investigated the properties of some indole-based chiral heterocycles, including their bioactivities and catalytic activities, and showed that these chiral heterocycles have potent anticancer activities and promising catalytic activities in asymmetric catalysis. These results help elucidate the potential applications of indole-based chiral heterocycles in drug development and chiral catalysts. The organocatalytic asymmetric synthesis of indole-based chiral heterocycles has undoubtedly become and will continue to be a hot topic in the field of asymmetric catalysis and synthesis. Our efforts, summarized in this Account, will not only open a window for the future development of innovative strategies toward organocatalytic asymmetric synthesis of indole-based chiral heterocycles but also inspire chemists worldwide to confront the remaining challenges in this field and prompt further advances.

Catalytic asymmetric 1,4-type Friedel–Crafts (hetero)arylations of 1-azadienes: the highly enantioselective syntheses of chiral hetero-triarylmethanes

Strategies for achieving the direct catalytic asymmetric syntheses of benzofuran-containing hetero-triarylmethanes using a 1,4-type Friedel–Crafts (hetero)arylation reaction were developed.

Catalytic Asymmetric Synthesis of Chiral Bis(indolyl)methanes Using a Ts-PyBidine-Nickel Complex

A chiral tosyl-substituted bis(imidazolidine)pyridine Ts-PyBidine-nickel complex was an efficient catalyst for Friedel-Crafts reaction of indoles with methylene indolinones to give bisindolylmethane compounds having differently oxidized indole units with high enantioselectivities. Alkylation of the products proceeded smoothly in a highly diastereoselective manner, providing an all-carbon quaternary carbon center without significant loss of enantiomeric excess.

Ga(OTf)3-Catalyzed Temperature-Controlled Regioselective Friedel-Crafts Alkylation of Trifluoromethylated 3-Indolylmethanols with 2-Substituted Indoles: Divergent Synthesis of Trifluoromethylated Unsymmetrical 3,3'-and 3,6'-Bis(indolyl)methanes

An unprecedented Ga(OTf)₃-catalyzed, temperature-controlled regiodivergent alkylation of 2-substituted indoles with trifluoromethylated 3-indolylmethanols is described that provides structurally diverse unsymmetrical 3,3'- and 3,6'-bis(indolyl)methanes with a CF₃-containing quaternary carbon center in good to excellent yields under mild conditions. In addition, this present protocol could be successfully extended to the synthesis of difluoromethylated 3,3'- and 3,6'-bis(indolyl)methanes with excellent efficiency.

Zn(OTf)2-catalyzed access to symmetrical and unsymmetrical bisindoles from α-keto amides

Zn(OTf)2-catalyzed synthesis of 3,3'-bisindolyl acetamides from α-keto amides is developed. Both aromatic α-keto amides substituted with electron-donating as well as -withdrawing groups and aliphatic α-keto amides are well tolerated to provide symmetrical bisindoles in moderate to excellent yields. The chemoselective bisindolylation of the keto group of α-keto amides in the presence of a simple keto functionality is successfully achieved in good yields. The transformation is further extended to the synthesis of challenging unsymmetrical bisindoles by treating indolyl α-hydroxy amides with substituted indoles. The unsymmetrical bisindoles are isolated in good to excellent yields.

3-Hydroxy-3-((3-methyl-4-nitroisoxazol-5-yl)methyl)indolin-2-one as a versatile intermediate for retro-Henry and Friedel–Crafts alkylation reactions in aqueous medium

The first example of a retro-Henry type reaction is reported using 3-hydroxy-3-((3-methyl-4-nitroisoxazol-5-yl)methyl)indolin-2-ones which are prepared via catalyst-free Henry reaction of 3,5-dimethyl-4-nitroisoxazole and isatin.

A catalytic asymmetric interrupted Nazarov-type cyclization of 2-indolylmethanols with cyclic enaminones

A chiral phosphoric acid-catalyzed asymmetric interrupted Nazarov-type cyclization of C3-alkenyl-substituted 2-indolylmethanols has been established by using cyclic enaminones as nucleophiles.

Catalytic enantioselective and regioselective substitution of 2,3-indolyldimethanols with enaminones

2,3-Indolyldimethanols as a new class of indolyldimethanols have been designed, and the first catalytic asymmetric substitution of 2,3-indolyldimethanols has been established by using cyclic enaminones as nucleophiles.