Friedel-Crafts Reaction of Acylsilanes: Highly Chemoselective Synthesis of 1-Hydroxy-bis(indolyl)methanes and 1-Silyl-bis(indolyl)methanes Derivatives

A novel double Friedel-Crafts reaction of acylsilanes in water is described. This strategy enables synthesis of bis(indolyl)methane derivatives with 1-hydroxy or 1-silyl substituents in moderate to high yield. Compared to the 1-silyl-bis(indolyl)methane derivatives from indole substrate, 1-hydroxy-bis(indolyl)methane derivatives were synthesized from the 5-hydroxyindole, and the hydrogen bonds in the 5-hydroxyindole play a crucial role in regulating the reaction selectivity.

Recent advances in transition-metal-catalyzed carbene insertion to C-H bonds

C-H functionalization has been emerging as a powerful method to establish carbon-carbon and carbon-heteroatom bonds. Many efforts have been devoted to transition-metal-catalyzed direct transformations of C-H bonds. Metal carbenes generated in situ from transition-metal compounds and diazo or its equivalents are usually applied as the transient reactive intermediates to furnish a catalytic cycle for new C-C and C-X bond formation. Using this strategy compounds from unactivated simple alkanes to complex molecules can be further functionalized or transformed to multi-functionalized compounds. In this area, transition-metal-catalyzed carbene insertion to C-H bonds has been paid continuous attention. Diverse catalyst design strategies, synthetic methods, and potential applications have been developed. This critical review will summarize the advance in transition-metal-catalyzed carbene insertion to C-H bonds dated up to July 2021, by the categories of C-H bonds from aliphatic C(sp³)-H, aryl (aromatic) C(sp²)-H, heteroaryl (heteroaromatic) C(sp²)-H bonds, alkenyl C(sp²)-H, and alkynyl C(sp)-H, as well as asymmetric carbene insertion to C-H bonds, and more coverage will be given to the recent work. Due to the rapid development of the C-H functionalization area, future directions in this topic are also discussed. This review will give the authors an overview of carbene insertion chemistry in C-H functionalization with focus on the catalytic systems and synthetic applications in C-C bond formation.

Metal-free, Phosphoric Acid-catalyzed Regioselective 1,6-Hydroarylation of para-Quinone Methides with Indoles in Water

An efficient, cheap and green protocol for the highly regioselective 1,6-hydroarylation of para -quinone methides ( p -QMs) with indoles at the C-3 position has been established by phosphoric acid catalysis in water under the transition-metal-free reaction conditions. A wide range of indole derivatives and para -quinone methides ( p -QMs) are compatible for the reaction, affording the corresponding 1,6-hydroarylation products with good to excellent yields. The possible mechanism of the reaction has been explored by step-by-step control experiments. The protocol is convenient for practical application, leading a safe, green and feasible way for the formation of C-3 diarylmethyl functionalized indole derivatives.

Catalytic Transformations of 2-Pyridones by Rhodium-Mediated Carbene Transfer

An enantioselective cyclopropanation reaction of N-substituted 2-pyridones with diazo compounds has been realized by using a chiral rhodium complex as the catalyst, and the corresponding chiral cyclopropanes could be formed in good yields with high enantioselectivities. Moreover, using acceptor-acceptor dimethyl 2-diazomalonate as the carbene precursor, a novel 1,4-rearrangement of a Boc group from N to C has also been discovered under rhodium catalysis.

Bis-indolylation of aldehydes and ketones using silica-supported FeCl3: molecular docking studies of bisindoles by targeting SARS-CoV-2 main protease binding sites

We report herein an operationally simple, efficient and versatile procedure for the synthesis of bis-indolylmethanes via the reaction of indoles with aldehydes or ketones in the presence of silica-supported ferric chloride under grindstone conditions. The prepared supported catalyst was characterized by SEM and EDX spectroscopy. The present protocol has several advantages such as shorter reaction time, high yield, avoidance of using harmful organic solvents during the reaction and tolerance of a wide range of functional groups. Molecular docking studies targeted toward the binding site of SARS-CoV-2 main protease (3CLpro or Mpro) enzymes were investigated with the synthesized bis-indoles. Our study revealed that some of the synthesized compounds have potentiality to inhibit the SARS-CoV-2 Mpro enzyme by interacting with key amino acid residues of the active sites via hydrophobic as well as hydrogen bonding interactions.

Gold-Catalyzed Synthesis of Diaza-hexatrienes Via Diazo Attack at Vinylgold Carbenes: An Easy Access to 1H-Pyrazolo[4,3-b]pyridine-5-ones

This work reports a gold-catalyzed stereoselective synthesis of highly substituted E-configured 2,3-diaza-1,3,5-hexatrienes using α-diazo nitriles and cyclopropene derivatives; such products arise from an atypical diazo attack of α-aryldiazo nitriles at vinylgold carbenes. For these 2,3-diaza-1,3,5-hexatrienes, we develop a novel anionic cyclization of derivatives of one family to form 1H-pyrazolo[4,3-b]pyridine-5-ones.

Multi C-H Functionalization Reactions of Carbazole Heterocycles via Gold-Catalyzed Carbene Transfer Reactions

Herein we describe a multiple C-H functionalization reaction of carbazole heterocycles with diazoalkanes. We show that gold catalysts play a distinct role in enabling a multiple C-H functionalization reaction to introduce up to six carbene fragments onto molecules containing multiple carbazole units or to link multiple carbazole units into a single molecule. A one-pot stepwise approach enables the introduction of two different carbene fragments to allow orthogonal deprotection and straightforward derivatization.

Cu/Pd-Catalyzed chemoselective synthesis of C-3 dicarbonyl indoles and bis(indolyl)alkanes from aldehydes and indoles

A novel Cu/Pd-catalyzed chemoselective synthesis of C-3 dicarbonyl indoles and bis(indolyl)alkanes from aldehydes and indoles has been achieved, which has good substrate universality and high efficacy on large-scale.

Ruthenium-catalyzed tandem annulation/arylation for the synthesis of unsymmetrical bis(heteroaryl)methanes

A new Ru-catalyzed tandem furan annulation/arylation strategy has been developed to afford unsymmetrical bis(heteroaryl)methanes by a reaction between propargyl amines and indoles. A series of bis(heteroaryl)methanes containing furan and indole as well as indole and imidazopyridine moieties have been synthesized in high yields. The reaction possibly proceeds through furan annulation followed by nucleophilic addition of indole.

Rhodium(I)-Catalyzed Coupling-Cyclization of C═O Bonds with α-Diazoketones

An unprecedented intermolecular nucleophilic attack of C═X bonds (X = O and S) on the rhodium(I)-carbenes has been developed. This transformation allows for the coupling-cyclization of aroylamides with α-diazoketones and provides concise access to 2,4,5-trisubstituted 1,3-oxazoles and 1,3-thiazoles with a broad tolerance of functional groups.

Visible light-induced aerobic oxidative cross-coupling of glycine derivatives with indoles: a facile access to 3,3′ bisindolylmethanes

A rapid synthesis of 3,3′-bisindolylmethanes is achieved via a photocatalyzed double Friedel–Crafts alkylation reaction between glycine derivatives and indoles.

Gold-catalyzed C5-alkylation of indolines and sequential oxidative aromatization: access to C5-functionalized indoles

A novel protocol towards the rapid assembly of C5-alkylated indole derivatives has been developed via gold-catalyzed regioselective functionalization and subsequent oxidative aromatization.

Photoredox Catalysis Induced Bisindolylation of Ethers/Alcohols via Sequential C-H and C-O Bond Cleavage

A visible-light-engaged 2-fold site-selective alkylation of indole derivatives with aliphatic ethers or alcohols has been accomplished for easy access to symmetric 3,3'-bisindolylmethane derivatives. The experimental data suggest a sequential photoredox catalysis induced radical addition and proton-mediated Friedel-Crafts alkylation mechanism.

Copper-catalyzed direct coupling of benzoxazin-2-ones with indoles for the synthesis of diverse 3-indolylbenzoxazin-2-ones: access to natural cephalandole A

Diverse and functionalized 3-indolyl benzo[b][1,4]oxazin-2-ones were synthesized via copper-catalyzed direct coupling of benzo[b][1,4]oxazin-2-one and indoles in air.