Directed C-H Functionalization of C3-Aldehyde, Ketone, and Acid/Ester-Substituted Free (NH) Indoles with Iodoarenes via a Palladium Catalyst System

Pd/NBE-Catalyzed One-Pot Modular Synthesis of Tetrahydro-γ-carbolines

Tetrahydro-γ-carbolines are especially outstanding fused heterocyclic ring systems possessing significant biological activities in the central nervous system. Here, using commercially available NBE derivatives (NBEs), we report an efficient protocol for the one-pot modular synthesis of 4-substituted tetrahydro-γ-carbolines via Catellani/aza-Michael addition cascade from easily available 3-iodo-1-tosyl-1H-indole, aziridines and olefins. This approach exhibits a wide substrate scope, good yields, scalability, and potential extension toward the synthesis of Mebhydroline analogues.

Cobalt(II)-Catalyzed Selective C2-H Heck Reaction of Native (N-H) Indoles Enabled by Salicylaldehyde Ligand

Direct functionalization of native (N-H) indoles via C-H activation remains a challenge. Herein, we report a salicylaldehyde-promoted cobalt-catalyzed selective C2-H Heck reaction of native (N-H) indoles with both active and unactivated olefins in the presence of free N-H bonds. A series of structurally diverse C2-alkenylated native (N-H) indoles including natural product and drug derivatives were prepared directly and effectively without additional preprotection and deprotection procedures.

Catalyst-controlled directing group translocation in the site selective C-H functionalization of 3-carboxamide indoles and metallocarbenes

Complementary methods toward the selective functionalization of indole and oxindole frameworks employing an alternative strategy in heteroaryl C-H functionalizations are presented herein. This work focuses on a catalyst-controlled, site selective C-H activation/functionalization of 3-acyl indoles, wherein an amide serves as a robust and versatile directing group capable of undergoing concomitant 1,2-acyl translocation/C-H functionalization in the presence of a RhI/AgI co-catalysts to provide the cross-coupled adducts in high yields. In contrast, the use of IrIII/AgI catalysts subverted the 1,2-acyl migration to afford the corresponding C2-functionalized products in good to excellent yields. A notable feature of the catalyst systems was the exceptional level of site selectivity observed in which the corresponding C-H functionalized indoles were obtained exclusively. Mechanistic experiments indicate a concerted 1,2-acyl migration step and indole metallation occurring through an electrophilic aromatic substitution process.

Rhodium(III)-Catalyzed Regioselective C4 Alkylation of Indoles with Nitroalkenes

The Rh(III)-catalyzed indole C4-H bond addition to nitroalkenes is disclosed under mild and redox-neutral reaction conditions, offering straightforward access to various 4-(2-nitroalkyl)indoles (34 examples) with excellent chemo- and regioselectivity. Furthermore, late-stage diversifications and mechanistic studies were also performed.

A Metal-Free Direct Decarboxylative Fluoroacylation of Indole Carboxylic Acids with Fluorinated Acids

A straightforward preparation of diversified fluorinated indol-3-yl ketones was developed by the direct decarboxylative fluoroacylation of indole carboxylic acids. The reaction could be performed on a gram scale under net conditions. Neither a metal catalyst nor an additive was employed. This methodology featured simple reaction conditions, high efficiency, exclusive selectivity, a broad substrate scope, and easy operation, which allowed it to meet the green chemistry requirement of the modern pharmaceutical industry. Control experiments confirmed that a radical process might be involved in the tandem decarboxylative fluoroacylation sequence.

A macrocycle-based new organometallic nano-vessel towards sustainable C2-selective arylation of free indole in water

C2-selectivity of unsubstituted indole over facile C3-substitution is attempted by utilizing the π-cavity of a nano-vessel made up of a palladium complex of an amino-ether heteroditopic macrocycle. Functional group tolerance (cyano, nitro, halo, ester, etc.), a broad substrate scope and outstanding selectivities with excellent yields (80-93%) of the desired products have been achieved in 12 h by maintaining all sustainable conditions like aqueous medium, recyclable catalyst, one-pot reaction, no external additives, mild temperature, etc. Interestingly, we observed that electron-deficient indole derivatives underwent the present transformation with marginally superior reactivity in comparison with electron-rich indole derivatives. This approach establishes a green pathway for selective C-C coupling employing a π-cavitand as a nano-reactor.

Directed regioselective arylation of imidazo[1,2-a]pyridine-3-carboxamides using Rh(III) catalysis

In contrast to previously reported free-radical pathways to functionalize imidazo[1,2-a]pyridines at the C-5 centre, directing group approaches are rare. Herein, we demonstrate a rhodium(III) catalyzed efficient and regioselective strategy for directed C-5 functionalization of imidazo[1,2-a]pyridines using N-methoxyamide as a directing group. This methodology facilitates directed arylation without the necessity for pre-functionalization. It also allows for gram-scale synthesis and post-functionalization.

Pd(II)-catalyzed hydroarylations/hydroalkenylations of terminal alkynes: regioselective synthesis of allylic, homoallylic, and 1,3-diene systems

A Pd-catalyzed regioselective hydroarylation of terminal alkynes containing a heteroatom has been developed via carbopalladation for the synthesis of allylic ethers, amines, and homoallylic alcohols. Moreover, hydroalkenylation of alkynes produces a variety of stereodefined 1,4-dienes with high regioselectivity. The important features of the present protocol are that it is highly regioselective, operationally rapid, and scalable with a huge substrate scope using only 3 mol% of PdCl2(PPh3)2 catalyst in the presence of a mild base KOAc.

Acetoxy Group-Directed Regioselective C2 Alkenylation of Indoles via Pd-Ag Bimetallic Catalysis

Regioselective C-H functionalizations of indoles reported to date with directing groups at C3 mainly rely on functional groups that are linked to the indole via C-C bonds. However, groups that are linked to the indole core by C-X linkages are also attractive due to the possibility of further modifications of the C-X bond. Herein, we report a 3-acetoxy directing group for the regioselective C2 alkenylation of indoles via a C-H activation-based, cross-dehydrogenative, oxidative Heck-type reaction. The reaction is catalyzed by Pd(II) and Ag(I) with stoichiometric Cu(II) as the oxidant and provides the 2-alkenylated indoles in yields of 52-84%. The reaction conditions are compatible with several functional groups at different positions as well as different N-protecting groups or free NH groups on the indole core. With respect to the alkene coupling partners, the reactions are successful with acrylates, vinyl sulfates, and phosphates. Specifically designed experiments, as well as density functional theory (DFT) computational studies, reveal that a heterodinuclear [Pd(μ-OAc)3Ag] bimetallic species is the actual catalyst responsible for the C-H alkenylation. A mechanistic path involving this catalytic species was also found to be favorable over other possible pathways for explaining the observed regioselectivity through DFT studies.

Engineering Saccharomyces cerevisiae for the de novo Production of Halogenated Tryptophan and Tryptamine Derivatives

The indole scaffold is a recurring structure in multiple bioactive heterocycles and natural products. Substituted indoles like the amino acid tryptophan serve as a precursor for a wide range of natural products with pharmaceutical or agrochemical applications. Inspired by the versatility of these compounds, medicinal chemists have for decades exploited indole as a core structure in the drug discovery process. With the aim of tuning the properties of lead drug candidates, regioselective halogenation of the indole scaffold is a common strategy. However, chemical halogenation is generally expensive, has a poor atom economy, lacks regioselectivity, and generates hazardous waste streams. As an alternative, in this work we engineer the industrial workhorse Saccharomyces cerevisiae for the de novo production of halogenated tryptophan and tryptamine derivatives. Functional expression of bacterial tryptophan halogenases together with a partner flavin reductase and a tryptophan decarboxylase resulted in the production of halogenated tryptophan and tryptamine with chlorine or bromine. Furthermore, by combining tryptophan halogenases, production of di-halogenated molecules was also achieved. Overall, this works paves the road for the production of new-to-nature halogenated natural products in yeast.