External oxidant-compatible phosphorus(III)-directed site-selective C-H carbonylation

Dispersion-controlled C6-selective C-H borylation of indoles

Through DFT and NCI analysis, we designed a bulky tertiary phosphine directing group, enabling an iridium-catalyzed C6-selective borylation of indoles with a simple and commercially available ligand, 1,10-phenanthroline. The directing group has dual dispersive interactions with both phenanthroline and a Bpin group, which enhance selectivity and reactivity.

Diastereoselective C(sp3)-H acetoxylation of phosphoramidites

Chiral phosphines are important ligands in asymmetric catalysis, yet their potential as directing groups for asymmetric C-H activation remains unexplored due to the oxidative nature of these reactions. We present a Pd-catalysed, P(III)-directed diastereoselective acetoxylation of phosphoramidites, with DFT calculations elucidating their unique reactivity and supporting the proposed reaction mechanism.

Site-Selective Electrophilic Trifluoromethylthiolation for the Synthesis of C5- or C7-SCF3-Substituted Indolines

We report herein an efficient and site-selective electrophilic trifluoromethylthiolation of indolines. In the absence of any catalyst or additive, C5-selective trifluoromethylthiolation could proceed at room temperature. With palladium used as the catalyst, the selectivity was reversed completely, giving C7-selecive trifluoromethylthiolated products. This reaction features good functional group tolerance, simple operation, mild conditions, and scale-up application. The potential utilities of the products were shown by further transformations.

Ru(II)-Catalyzed Skeletal Editing of Oxindole with Internal Alkyne To Synthesize C7-Alkylated Indole Derivatives

A Ru(II)-catalyzed skeletal editing of oxindole scaffolds was established to afford C7-alkyl acetate indole derivatives using internal alkyne and alkyl alcohol. The developed method is simple, efficient, and straightforward. The reaction was extended to substrates having wide chemoselective profiles. When unsymmetrical alkynes were used, promising regioselectivity was realized. A preliminary mechanistic study revealed that the reaction pathway proceeded by Ru(II)/Ag(I)-catalyzed amide cleavage and subsequent oxidative annulation.

Cobalt(II)-Catalyzed Intermolecular Aminocarbonylation of Indoles with Amines

A cobalt(II)-catalyzed C2-H carbonylation of indoles with amines toward indole-2-carboxamides has been developed. By employing Co(OAc)2·4H2O as an inexpensive catalyst and using benzene-1,3,5-triyl triformate (TFBen) as the CO surrogate, a variety of indole-2-carboxamide derivatives were produced in moderate to high yields. Additionally, several bioactive-molecule-related compounds can be applied as substrates, as well.

Rhodium(i)-catalyzed cascade C(sp2)-H bond alkylation - amidation of anilines: phosphorus as traceless directing group

We introduce a versatile Rh(i)-catalyzed cascade reaction, combining C(sp2)-H bond functionalization and amidation between N-arylphosphanamines and acrylates. This innovative approach enables the rapid synthesis of dihydroquinolinone scaffolds, a common heterocycle found in various pharmaceuticals. Notably, the presence of the phosphorus atom facilitates the aniline ortho-C(sp2)-H bond activation prior to N-P bond hydrolysis, streamlining one-pot intramolecular amidation. Moreover, we demonstrate the applicability of this reaction by synthesizing an antipsychotic drug. Detailed mechanistic investigations revealed the involvement of a Rh-H intermediate, with substrate inhibition through catalyst saturation.

Upgrading Carbazolyl-Derived Phosphine Ligands Using RhI-Catalyzed PIII-Directed C-H Bond Alkylation for Catalytic CO2-Fixation Reactions

We report an Rh(I)-catalyzed C-H bond alkylation of PhenCarPhos [N-(2-(diphenylphosphaneyl)phenyl)carbazole] and some congener phosphine ligands with alkenes. The C-H bond functionalization occurred exclusively at the C1 position of the carbazolyl unit because the trivalent phosphine acts as a directing group. This protocol provides straightforward access to a large library of C1-alkyl substituted PhenCarPhos, which outperformed common commercial or unfunctionalized phosphines and their precursors in the Pd-catalyzed carbon dioxide-fixation reactions with propargylic amines.

Palladium-Catalyzed Synthesis of Esters from Arenes through C-H Thianthrenation

The efficient palladium-catalyzed synthesis of esters from readily available arenes has been developed. These C-H bond esterifications were achieved relying on the regioselective thianthrenation to generate the aryl-TT salts, which were treated as reactive electrophilic substrates to couple with phenol formate and N-hydroxysuccinimide (NHS) formate giving access to phenol esters and NHS esters, respectively, in the absence of carbon monoxide. A wide range of functional esters could be prepared with high efficiency under this redox-neutral palladium-catalytic condition. Late-stage functionalization and investigations of synthetic applications demonstrated the potential application of the established platform and these products.

Synthesis of C7-Functionalized Indoles through an Aromaticity Destruction-Reconstruction Process

A process for the synthesis of C7-functionalized indoles using para-substituted 2-alkynylanilines as starting materials was reported. The process involves a dearomatization, an 1,2-addition by organic lithium or Grignard reagents, an aromatization-driven allylic rearrangement, and a cyclization. A variety of groups including alkyl, aryl, alkenyl, or alkynyl groups were selectively installed at the C7 site of indoles leading to the formation of 2,5,7-trisubstituted indoles.

Solvent-controlled regioselective arylation of indoles and mechanistic explorations

A new reaction for the regioselective arylation of indoles at C2 or C3 positions achieved by adjusting the solvent and with P(O)tBu2 as an auxiliary group is reported. And the experimental results and DFT confirmed the process.

BBr3 -Mediated P(III)-Directed C-H Borylation of Phosphines

Transition-metal-catalyzed C-H borylation has been widely used in the preparation of organoboron compounds. Here, we developed a general protocol on metal-free P(III)-directed C-H borylation of phosphines mediated by BBr₃ , resulting in the formation of products bearing both phosphorus and boron. The development of the metal-free strategy to mimic previous metallic processes has shown low cost, superior practicality, and environmental friendliness. Density functional theory (DFT) calculations demonstrate the preferred pathway for this metal-free directed C-H borylation process.

Iridium-Catalyzed Redox-Neutral C2 and C3 Dual C-H Functionalization of Indoles with Nitrones toward 7H-Indolo[2,3-c]quinolines

An iridium-catalyzed redox-neutral C-2 and C-3 dual C-H functionalization of indoles with nitrones has been developed, furnishing a range of 7H-indolo[2,3-c]quinolines with high efficiency and regioselectivity under mild reaction conditions. Notably, sequential multiple C-H bond cleavage and C-C bond formation constitute the key events of this process, in which nitrone serves as a building block and an oxidant. Distinct from the previous methods toward 7H-indolo[2,3-c]quinolines, this newly developed reaction features readily available substrates, operational simplicity, broad scope, good to high efficiency, and excellent regioselectivity.

Sulfur-Directed C7-Selective Alkenylation of Indoles under Rhodium Catalysis

Regioselective direct functionalization of an indole benzenoid fragment has been a significant challenge because of its inherently lower reactivity. In this report, we introduce a Rh-catalyzed C₇-selective alkenylation of indole derivatives using a new sulfur directing group N-SCy. A notable feature of this system is that the directing group is readily installed to the indoles and easily removed after the catalysis under mild conditions.

From C4 to C7: Innovative Strategies for Site-Selective Functionalization of Indole C-H Bonds

The widespread presence of hydrocarbons makes C-H functionalization an attractive alternative to traditional cross-coupling methods. As indole is an important heteroarene in a plethora of natural products and pharmaceuticals, C-H functionalization of indole moieties has emerged as one of the most important topics in this field. Due to the presence of multiple C-H bonds in indoles, site selectivity is a long-standing challenge. Much effort has been devoted to the C-H functionalization of indoles at the C3 or C2 position, while accessing the benzene core (from C4 to C7) is considerably more challenging.This Account summarizes our recent efforts toward site-selective C-H functionalization of indoles at the benzene core based on innovative strategies. A common method to solve the issue involves the development of directing groups (DGs). Our early studies establish that the installation of the N-P(O)^tBu₂ group at the N position can produce C7 and C6 arylation products using palladium and copper catalysts, respectively. The developed system can also be extended to direct arylation of indoles at the C5 and C4 positions by installing a pivaloyl group at the C3 position. Further investigation of indoles bearing N-P^tBu₂ groups shows a more diverse reactivity for C-H functionalizations at the C7 position, including arylation, olefination, acylation, alkylation, silylation, and carbonylation with different coupling partners. Compared to the P(V) DG, the P(III) group can be easily attached to the indole substrates and detached from the products. However, these attractive reactions rely mostly on precious metal catalysts with ligands; this requirement can be a significant limitation, particularly for large-scale syntheses and the necessity of removal of toxic trace metals in pharmaceutical products. We have also uncovered a general strategy for chelation-assisted aromatic C-H borylation just using simple BBr₃ under mild conditions, in which the installation of pivaloyl groups at the N1 or C3 position of indoles can selectively deliver the boron species to the unfavorable C7 or C4 positions and allow subsequent C-H borylation without any metal. This transition-metal-free strategy can be extended to synthesize C7 and C4 hydroxylated indoles by boron-mediated directed C-H hydroxylation under mild reaction conditions and with broad functional group compatibility.In this Account, we describe our contributions to this topic since 2015. These studies provide efficient and attractive methods for the divergent synthesis of valuable substituted indoles and insights into the exploration of new strategies for the site-selective C-H functionalization and directives for other important heteroarenes.

Transition metal-catalyzed C–H functionalizations of indoles

This review summarises a wide range of transformations on the indole skeleton, including arylation, alkenylation, alkynylation, acylation, nitration, borylation, and amidation, using transition-metal catalyzed C–H functionalization as the key step.