Dearomatization–Rearomatization Strategy for Reductive Cross-Coupling of Indoles with Ketones in Water

Silver-Catalyzed Single-Carbon Insertion of Indoles with Acetophenone N-Triftosylhydrazones

Here, we report a silver carbene-enabled single-carbon insertion reaction of indoles via a one-pot, two-step sequence to deliver a dearomative quaternary center quinoline scaffold in a modular fashion. Specifically, we used N-triftosylhydrazones as masked donor-donor carbene precursors that facilitate the insertion of carbon atoms bearing various functional groups to the library of functionalized quinoline. Experimental and DFT evidence support the transient presence of a cyclopropane species and removal of protecting groups.

Synthesis of N-Alkenylated Heterocycles via T3P-Promoted Condensation with Ketones

Herein, we describe a convenient protocol for the synthesis of N-alkenylated heterocycles using abundant ketone electrophiles and T3P as a water scavenger under microwave irradiation. The method can be applied to a diverse range of NH-heterocycles and ketones with good to excellent yields (up to 94%). This procedure is particularly attractive, as it is metal- and base-free, tolerates a variety of functional groups, and offers ease of product purification. The utility of the protocol was exemplified by synthesizing pharmaceutically relevant scaffolds containing the N-alkenyl motif and was further extended to a one-pot reductive amination sequence.

Palladium-Catalyzed Reaction of Indolines with Dihydropyrroles: Access to N-Alkylated Indoles

Palladium-catalyzed reaction of indolines with 1-acyl-2,3-dihydro-1H-pyrroles or 1-acyl-2,5-dihydro-1H-pyrroles in air produces N-alkylated indoles. A combination of Pd(CH3CN)2Cl2 and dppf effectively catalyzes the reaction of 1-acyl-2,3-dihydro-1H-pyrroles, and the combination of Pd(CH3CN)2Cl2 and dcypf is more effective for the reaction of 1-acyl-2,5-dihydro-1H-pyrroles. The method has a wide scope of substrates and shows good compatibility of functional groups.

Nucleophilic aromatization of monoterpenes from isoprene under nickel/iodine cascade catalysis

As a large number of organic compounds possessing two isoprene units, monoterpenes and monoterpenoids play important roles in pharmaceutical, cosmetic, agricultural, and food industries. In nature, monoterpenes are constructed from geranyl pyrophosphate (C10) via various transformations. Herein, the bulk C5 chemical-isoprene, is used for the creation of various monoterpenoids via a nucleophilic aromatization of monoterpenes under cascade catalysis of nickel and iodine. Drugs and oil mixtures from conifer and lemon can be convergently transformed to the desired monoterpenoid. Preliminary mechanistic studies are conducted to get insights about reaction pathway. Two types of cyclic monoterpenes can be respectively introduced onto two similar heterocycles via orthogonal C-H functionalization. And various hybrid terpenyl indoles are programmatically assembled from abundant C5 or C10 blocks. This work not only contributes a high chemo-, regio-, and redox-selective transformation of isoprene, but also provides a complementary approach for the creation of unnatural monoterpenoids.

Aerobic oxidative C-H phosphorylation of quinoxalines under catalyst-free conditions

We herein report a direct and efficient protocol for phosphorylation of quinoxalines, which employs aerobic oxygen as the green oxidant under catalyst-free conditions. This methodology represents one of the most environmentally friendly and easily handled protocols, providing a series of phosphorylated quinoxalines in good to excellent yields. Control experiments clearly indicated that the reaction followed a dearomatization-rearomatization strategy.

Recent Advances on Direct Functionalization of Indoles in Aqueous Media

Indoles and their derivatives have dominated a significant proportion of nitrogen-containing heterocyclic compounds and play an essential role in synthetic and medicinal chemistry, pesticides, and advanced materials. Compared with conventional synthetic strategies, direct functionalization of indoles provides straightforward access to construct diverse indole scaffolds. As we enter an era emphasizing green and sustainable chemistry, utilizing environment-friendly solvents represented by water demonstrates great potential in synthesizing valuable indole derivatives. This review aims to depict the critical aspects of aqueous-mediated indoles functionalization over the past decade and discusses the future challenges and prospects in this fast-growing field. For the convenience of readers, this review is classified into three parts according to the bonding modes (C-C, C-N, and C-S bonds), which focus on the diversity of indole derivatives, the prominent role of water in the chemical process, and the types of catalyst systems and mechanisms. We hope this review can promote the sustainable development of the direct functionalization of indoles and their derivatives and the discovery of novel and practical organic methods in aqueous phase.

Optimization of TEAD P-Site Binding Fragment Hit into In Vivo Active Lead

The dysregulated Hippo pathway and, consequently, hyperactivity of the transcriptional YAP/TAZ-TEAD complexes is associated with diseases such as cancer. Prevention of YAP/TAZ-TEAD triggered gene transcription is an attractive strategy for therapeutic intervention. The deeply buried and conserved lipidation pocket (P-site) of the TEAD transcription factors is druggable. The discovery and optimization of a P-site binding fragment (1) are described. Utilizing structure-based design, enhancement in target potency was engineered into the hit, capitalizing on the established X-ray structure of TEAD1. The efforts culminated in the optimized in vivo tool MSC-4106, which exhibited desirable potency, mouse pharmacokinetic properties, and in vivo efficacy. In close correlation to compound exposure, the time- and dose-dependent downregulation of a proximal biomarker could be shown.

Hydride transfer-initiated synthesis of 3-functionalized quinolines by deconstruction of isoquinoline derivatives

Under transition metal catalyst-free conditions, we herein present a hydride transfer-initiated construction of novel 3-(2-aminomethyl)aryl quinolines from N-isoquinolinium salts and 2-aminobenzaldehydes, proceeding with the merits of operational simplicity, high step and atom efficiency, good substrate and functional group compatibility, and mild conditions. The products are formed by reacting with the isoquinolyl motif as a two-carbon synthon along with the cleavage of its C3-N bond. Given the interesting applications of 3-aryl quinolines, the developed chemistry is anticipated to be further applied to develop new functional products.

Metal-Free Hydroxymethylation of Indole Derivatives with Formic Acid as an Alternative Way to Indirect Utilization of CO2

The selective N-alkylation of indole substrates remains an ongoing research challenge for the relative attenuated nucleophilicity toward nitrogen. Herein, we developed the hydroxymethylation of indole derivatives to afford N-alkylated indole products with formic acid. This metal-free process was promoted by the organic base 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) using phenylsilane as the reductant under mild conditions. Besides, this strategy represents an alternative way for indirect utilization of CO₂, considering the facile hydrogenation of CO₂ to produce HCOOH.

Copper-catalyzed three-component N-alkylation of quinazolinones and azoles

A Cu-catalyzed three-component N-alkylated coupling reaction of N-heteroarenes with methyl ketones and one carbon source has been developed. Using methyl ketones as alkylation reagent and DMPA (N,N’-dimethylpropionamide) as one carbon...

Photoinduced transition-metal and external photosensitizer free cross-coupling of aryl triflates with trialkyl phosphites

Photoinduced phosphonation of aryl triflates with trialkyl phosphites via a tandem single-electron-transfer, C-O bond cleavage and Arbuzov rearrangement process in the absence of transition-metal and external photosensitizer is reported herein. The protocol features good functional group compatibility and mild reaction conditions, providing various aryl phosphates in good to high yields. Furthermore, this strategy allows the late-stage phosphonation of complex and biologically active compounds.

Ruthenium(ii)-catalyzed regioselective direct C4- and C5-diamidation of indoles and mechanistic studies

A ruthenium(ii)-catalyzed regioselective direct diamidation of 3-carbonylindoles at the C4- and C5-positions using various dioxazolones is described. This novel protocol allows for the effective installation of two amide groups on the benzene ring in indole. A remarkably broad substrate scope, excellent functional group tolerance, and mild reaction conditions are notable features of this protocol. Further explorations reveal that benzo[b]thiophene-3-carboxaldehyde is a viable substrate and affords its corresponding diamidation products. The diamido indoles are further converted into various functionalized products and used as sensors for metal ion detection. Density functional theory studies are also conducted to propose a reaction mechanism and provide a detailed understanding of the regioselectivity observed in the reaction.

Ruthenium(ii)-catalyzed regioselective C4-/C5-diamidation of 3-carbonylindoles is described and a DFT study is conducted to understand the observed regioselectivity and the mechanism.

A Metal-Free Reductive N-Alkylation of Indoles with Aldehydes

A simple metal-free method has been developed for the reductive N-alkylation of indoles employing aldehydes as the alkylating agent and inexpensive Et₃SiH as the reductant. A wide range of aromatic and aliphatic aldehydes are viable substrates along with a variety of substituted indoles. In addition, the method was applied to a one-pot sequential 1,3-alkylation of a substituted indole and successfully demonstrated on a 100 mmol scale.

Unprecedented Reactivity of γ-Amino Cyclopentenone Enables Diversity-Oriented Access to Functionalized Indoles and Indole-Annulated Ring Structures

Observation of an unexpected, Lewis acid promoted displacement of latent reactive γ-amino group on cyclopentenone presented unparalleled opportunity for enone functionalization and annulations with indole derivatives, which is developed in the current study. Herein, a vast range of C3/N-indolyl enones and indole alkaloid-like compound were accessed in excellent yields (up to 99 %) and selectivity through a one-pot operation. The mechanism most likely involves an unprecedented trait of Piancatelli-type rearrangement where influence of the gem-diaryl group appeared crucial.

Nickel-catalyzed C3-alkylation of indoles with alcohols via a borrowing hydrogen strategy

An efficient Ni-catalyzed C3-alkylation of indoles with alcohols via a borrowing hydrogen pathway was achieved utilizing an N,O-donor coordinated nickel complex as the precatalyst.

Dearomatization-Rearomatization Strategy for Synthesizing Carbazoles with 2,2'-Biphenols and Ammonia by Dual C(Ar)-OH Bond Cleavages

Carbazole is an essential building block in various pharmaceuticals, agrochemicals, natural products, and materials. For future sustainability, it is highly desirable to synthesize carbazole derivatives directly from renewable resources or cheap raw materials. Phenolic compounds are a class of degradation products of lignin. On the other hand, ammonia is a very cheap industrial inorganic chemical. Herein, an efficient dearomatization-rearomatization strategy has been developed to directly cross-couple 2,2'-biphenols with ammonia by dual C(Ar)-OH bond cleavages. This strategy provides a greener pathway to synthesize valuable carbazole derivatives from phenols.

Coupling without Coupling Reactions: En Route to Developing Phenols as Sustainable Coupling Partners via Dearomatization-Rearomatization Processes

Transition-metal-catalyzed cross-coupling reactions represent one of the most straightforward and efficient protocols to assemble two different molecular motifs for the construction of carbon-carbon or carbon-heteroatom bonds. Because of their importance and wide applications in pharmaceuticals, agrochemicals, materials, etc., cross-coupling reactions have been well recognized in the 2010 Nobel Prize in chemistry. However, in the classical transition-metal-catalyzed cross-coupling reactions (e.g., the Suzuki-Miyaura, the Buchwald-Hartwig, and the Ullmann cross-coupling reactions), organohalides, which mainly stem from the nonrenewable fossil resources, are often utilized as coupling partners with halide wastes being generated after the reactions. To make cross-coupling reactions more sustainable, we initiated a general research program by employing phenols and cyclohexa(e)nones (the reduced forms of phenols) as pivotal feedstocks (coupling partners), instead of the commonly used fossil-derived organohalides, for cross-coupling reactions to build C-O, C-N, and C-C bonds. Phenols (cyclohexa(e)nones) are widely available and can be obtained from lignin biomass, highlighting their renewable and sustainable features. Moreover, water is expected to be the only stoichiometric byproduct, thus avoiding halide wastes.Notably, the cross-coupling reactions utilizing phenols/cyclohexa(e)nones are not based on the traditional transition-metal-catalyzed "oxidative-addition and reductive-elimination" mechanism, but via a novel "phenol-cyclohexanone" redox couple. This new working mechanism opens up new horizons of designing cross-coupling reactions via simple nucleophilic addition of cyclohexanones along with aromatization processes, thereby simplifying the design and avoiding laborious optimization of transition-metal precursors (e.g., Pd, Ni, Cu, etc.), as well as ligands in classical transition-metal-catalyzed cross-coupling reactions. Specifically, in this Account, we will summarize and discuss our related research work in the following three categories: "formal oxidative couplings of cyclohexa(e)nones", "formal reductive couplings of phenols", and "formal redox-neutral couplings of phenols". The successes of these research projects clearly demonstrated our initial inspirations and rational designs to develop cross-coupling reactions without the "conventional cross-coupling conditions" by pushing the reaction frontiers from initial cyclohexanones, ultimately, to the sustainable phenol targets.

Dearomatization–Rearomatization Strategy for Palladium-Catalyzed C–N Cross-Coupling Reactions

Substituted aromatic compounds play important roles in materials, biological agents, dyes, etc. Thus, the synthesis of substituted aromatic compounds has been a hot topic throughout the history of organic chemistry. Traditionally, the Friedel–Crafts reaction was a powerful tool for synthesizing substituted aromatic compounds. In recent decades, metal-catalyzed cross-coupling reactions were well developed via carbon–heteroatom bond cleavage, however, having difficulties towards some strong bonds, such as C(Ar)–OH. To overcome such challenges, newer strategies are needed. In this review, we summarize the recent efforts in the development of dearomatization–rearomatization strategy for cross-coupling reactions via C(Ar)–O bond cleavage.

1 Introduction

2 Dearomatization–Rearomatization Strategy for Cross-Coupling of Phenols

3 Dearomatization–Rearomatization Strategy for Cross-Coupling of Biphenols

4 Dearomatization–Rearomatization Strategy for Cross-Coupling of Diphenyl Ethers

5 Dearomatization–Rearomatization Strategy for Cross-Coupling of Indoles

6 Summary

Synthesis of unsymmetrically substituted triarylamines via acceptorless dehydrogenative aromatization using a Pd/C and p-toluenesulfonic acid hybrid relay catalyst

An efficient and convenient procedure for synthesizing triarylamines based on a dehydrogenative aromatization strategy has been developed. A hybrid relay catalyst comprising carbon-supported Pd (Pd/C) and p-toluenesulfonic acid (TsOH) was found to be effective for synthesizing a variety of triarylamines bearing different aryl groups starting from arylamines (diarylamines or anilines), using cyclohexanones as the arylation sources under acceptorless conditions with the release of gaseous H₂. The proposed reaction comprises the following relay steps: condensation of arylamines and cyclohexanones to produce imines or enamines, dehydrogenative aromatization of the imines or enamines over Pd nanoparticles (NPs), and elimination of H₂ from the Pd NPs. In this study, an interesting finding was obtained indicating that TsOH may promote the dehydrogenation.

An efficient and convenient procedure for synthesizing triarylamines based on a dehydrogenative aromatization strategy has been developed.

Visible-Light-Induced Metal-Free Synthesis of 2-Phosphorylated Thioflavones in Water

The introduction of phosphorus functional groups into the skeleton of thioflavones is an attractive task and of great significance. Herein, a metal-free visible-light-induced radical cascade cyclization was developed for the preparation of 2-phosphorylated thioflavones from methylthiolated alkynones and phosphine oxides. In water as a green reaction medium, a large number of such 2-phosphorylated thioflavones were prepared, catalyzed by 4CzIPN [1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene] under visible-light irradiation. These reactions could be performed at ambient temperature and feature simple operation, wide reaction scope, and recyclability of aqueous media.

Palladium-catalyzed aerobic synthesis of ortho-substituted phenols from cyclohexanones and primary alcohols

An aerobic palladium catalyzed site-specific synthesis of ortho-substituted phenols from cyclohexanones and primary alcohols via an oxidation/aldol/dehydration/aromatization process has been developed.

Construction of Spirocyclic Tetrahydro-β-carbolines via Cross-Annulation of Phenols with Tryptamines in Water

Phenols are readily available by degradation of lignin resource. Palladium-catalyzed conversion of phenols to tetrahydro-β-carboline skeletons bearing a spirocycle at the C-1 position in water is reported. Various substituted phenols are successfully cross-annulated with different tryptamines via sequential C(Ar)-O bond cleavage of phenols, C-H bond activation of tryptamines, and C-N/C-C bond formations. This method provides a new protocol of converting lignin phenols into high-value-added compounds, such as natural product Komavine.