I2O5 promoted iodosulfenylation of indoles under metal-free conditions

A mild, efficient, and metal-free strategy to construct 2-iodo-3-arylthioindoles was developed via I₂O₅-promoted iodosulfenylation of indoles utilizing simple, cheap and easily available thiophenols as sulfur sources. It is the first time that the iodothiolation difunctionalization of indoles at C-2 and C-3 positions was accomplished with the formation of 2-iodo-3-arylthioindoles in moderate to good yields.     

A simple method for the direct arylation of indoles

The scope and limitations are described for a powerful new method to access indoles bearing a quaternary center at C-3 using easily accessible bisaryl λ³-iodanes and a cheap organic base.     

A general and practical method of alkynyl indole and benzofuran synthesis via tandem Cu- and Pd-catalyzed cross-couplings

A new tandem coupling approach to synthesize 2-alkynyl indoles and benzofurans is described. This reaction utilizes easily accessible gem-dibromovinyl substrates and terminal alkynes and proceeds via Pd/C- and CuI-catalyzed tandem Ullman/Sonogashira couplings.     

Preparation of 3-arylmethylindoles as selective COX-2 inhibitors

The 3-arylmethylation of indoles using TMSOTf/Et₃SiH with a wide variety of substituted benzaldehydes has been accomplished. Under these mild Lewis acid mediated reductive conditions, it was demonstrated that indoles bearing both 6-MeSO₂ and 2-methyl substituents could be 3-arylmethylated in good to excellent yields to afford the corresponding 3-arylmethyl indoles, effective as selective COX-2 inhibitors. In addition, the viability of this method for the reductive alkylation of indoles by ketones was demonstrated and shown to be C-3 regioselective. For indoles bearing both a 6-MeSO₂ and 2-cyano substituent where this indole reductive alkylation methodology was unsuccessful, an unprecedented Pd(0) mediated arylorganozinc coupling with the requisite substituted 3-methylcarbonatomethylindole proved successful in affording the desired 2-cyano-6-MeSO₂-3-arylmethylindoles effective as selective COX-2 inhibitors.     

Utilization of flow chemistry in catalysis: New avenues for the selective synthesis of Bis(indolyl)methanes

Flow chemistry enables the preparation of bis(indolyl)methanes from various indoles and structurally divergent aldehydes using Sc(OTf)₃ catalysis. The reaction is regioselective for C-3 functionalization of the indoles, occurring over short reaction times allowing for rapid investigation of scope with straightforward work up facilitating product isolation.     

One-pot synthesis of 2-substituted thieno[3,2-b]indoles from 3-aminothiophene-2-carboxylates through in situ generated 3-aminothiophenes

A convenient protocol for one-pot synthesis of thieno[3,2-b]indoles, bearing aromatic, thien-2-yl or styryl fragments at C-2 position, from easily accessible 5-substituted 3-aminothiophene-2-carboxylates using the Fischer indolization reaction, was developed during this study. Two main steps of this approach are the saponification of the starting 3-aminoesters with sodium hydroxide and next treatment of the crude 3-aminoacids sodium salts with arylhydrazines in glacial acetic acid solution. The latter step includes in situ decarboxylation of the freed 3-aminothiophene-2-caboxylic acids to the 3-aminothiophenes and their acid promoted reaction with arylhydrazines to initially form arylhydrazones of 5-substituted thiophene-3(2H)-ones, which smoothly cause indolization to afford the desired thieno[3,2-b]indoles.     

Ferric chloride-promoted direct trifluoromethylselenolation of nitrogen-containing heterocyclic compounds by Se-(trifluoromethyl) 4-methylbenzenesulfonoselenoate in water

The ferric chloride-promoted direct trifluoromethylselenolation of indoles and other nitrogen-containing heterocyclic compounds in water was performed by employing TsSeCF₃ as a trifluoromethylselenolation reagent. The shelf-stable and readily accessible reagents, along with the environmentally friendly and mild reaction conditions make this method an effective alternative for the trifluoromethylselenolation of electron-rich aromatics.     

A General and Scalable Synthesis of Polysubstituted Indoles

A consecutive 2-step synthesis of N-unprotected polysubstituted indoles bearing an electron-withdrawing group at the C-3 position from readily available nitroarenes is reported. The protocol is based on the [3,3]-sigmatropic rearrangement of N-oxyenamines generated by the DABCO-catalyzed reaction of N-arylhydroxylamines and conjugated terminal alkynes, and delivers indoles endowed with a wide array of substitution patterns and topologies.     

Thorpe-Ingold effect in copper(II)-catalyzed formal hydroalkoxylation-hydroarylation reaction of alkynols with indoles

The use of Cu(OTf)₂ as a catalyst for tandem hydroalkoxylation-hydroarylation reaction of alkynes tethered with hydroxyl group is reported. The reaction proceeds at 60 °C or even at room temperature with 5 mol % catalyst loading and produces C-3-substituted indoles in good to high yields. The method was shown to be applicable to a broad range of indoles, containing electron-withdrawing and electron-donating substituents, and alkynol substrates bearing sterically demanding substituents in the tether. Interestingly, it was found that Thorpe-Ingold effect is operating for this cyclization reaction. Easy availability and low cost of Cu(OTf)₂ make this method attractive and amenable for large-scale synthesis compared to known literature methods.     

Novel cyclometallated Pd(II) and Pt(II) complexes with indole derivatives and their use as catalysts in Heck reaction

Several palladacycle and platinacycle complexes have been prepared from easily available or naturally occurring indole derivatives, such as gramine and related compounds. Dimeric complexes were obtained with Pd(OAc)₂, while Pt(DMSO)₂Cl₂ mainly afforded monomeric structures. A notable feature of these reactions was the formation of new M-C bonds between Pd or Pt and C-2 and C-3 of the indole ring. With ligands like 2-(2′-pyridyl)-1H-indoles, N-N metallacycles were generated instead: in fact new C-M bonds with the C-3 position could only form if N-substituted indoles were used. The reactivity of Pd dimeric complexes with PPh₃, sym-collidine and DMAP was explored to obtain monomeric complexes. Three such compounds were prepared, one of which was characterized by X-ray diffraction. Metathetical reactions were carried out to effect a ligand exchange replacing OAc with halide ions, with the aim to synthesize μ-Cl and μ-Br bridged structures. Turning to the synthesis of hetaryl complexes, functionalization of the C-2 position on the indole ring was achieved. These complexes were prepared by substitution reactions starting from gramine and/or its alkylammonium salts.     

Indirect regioselective heteroarylation of indoles through a Friedel-Crafts reaction with (E)-1,4-diaryl-2-buten-1,4-diones

A two-step synthesis of 3-heteroaryl indoles has been developed. The first step of the sequence involves a Friedel-Crafts alkylation of indoles with 1,4-diaryl-2-buten-1,4-diones to give the corresponding indoles bearing a 1,4-dicarbonyl moiety. The reaction is catalyzed by InCl₃ and takes place with good yields. Cyclization of the diones under different Paal-Knorr conditions allows to prepare indoles substituted at the C3 position with 3-furanyl, 3-pyrrolyl- and 3-thienyl moieties.     

Cp*IrCl2]2-catalyzed indirect functionalization of alcohols: novel strategies for the synthesis of substituted indoles

We report novel iridium(III)-catalyzed reactions that afford substituted indoles via the indirect functionalization of alcohols via C-3 selective alkylation of indoles with alcohols and a one-pot cascade strategy from amino- or nitro-phenyl ethyl alcohols, which incorporates oxidative cyclization and C-3 alkylation.     

Iodine-catalyzed allylation and propargylation of indoles with allylic and propargylic acetates

A mild and efficient allylation/propargylation of indoles has been developed with high regioselectivity and excellent yields. In the presence of catalytic molecular iodine, various indoles could react with allylic/propargylic acetates smoothly at room temperature to exclusively provide C-3 alkylated products.     

One‐Pot Aminoethylation of Indoles/Pyrroles with Alkynes and Sulfonyl Azides

A general and efficient one‐pot aminoethylation of substituted indoles/pyrroles was accomplished for the synthesis of various tryptamine derivatives employing a combination of alkynes and sulfonyl azides as readily accessible aminoethylating agents. The reaction features a successful integration of copper‐catalyzed alkyne and azide cycloaddition to N‐sulfonyl‐1,2,3‐triazole, rhodium‐catalyzed selective insertion of α‐iminocarbenes onto the C3?H bond of indoles, and reduction of the resultant enamides to tryptamine derivatives employing either NaCNBH₃ or palladium catalyst, in one‐pot. The reaction also showed excellent functional‐group tolerance and allowed the synthesis of various substituted tryptamines in good to excellent yield. This transformation constitutes a one‐pot formal regioselective functionalization of terminal alkynes. Utility of the synthesized tryptamine was further demonstrated in the synthesis of dihydro‐β‐carboline and tryptoline.     

Propargylamine (secondary) as a building block in indole synthesis involving ultrasound assisted Pd/Cu-catalyzed coupling-cyclization method: Unexpected formation of (pyrazole)imine derivatives

Propargylamine (secondary) has been explored as a building block in synthesizing indoles via an ultrasound assisted Pd/Cu-catalyzed coupling-cyclization method. Indoles containing a pyrazole moiety at C-2 attached via the –CH₂NH– linker (designed as potential anti-tubercular agents) were synthesized first and then generality/scope of the methodology was expanded by synthesizing other indoles. Unexpected formation of imine side products in first cases helped in synthesizing related (pyrazole)imines via a Cu catalyzed ultrasound assisted aerobic oxidation of precursor amines.     

C-2/c-3 annulation and C-2 alkylation of indoles with 2-alkoxycyclopropanoate esters

The annulation reaction between various indoles and 2-alkoxycyclopropanoate esters is reported. Both high efficiency and complete stereochemical control were observed in some cases with this annulation process. A single stereocenter on the cyclopropane controls the diastereoselective formation of up to four new stereocenters. A different reaction course was observed with 3-substituted indole substrates, and an intervening C-3 to C-2-migration process arose that gives synthetically useful C-2 alkylation indole products.     

FeCl3 as Lewis acid catalyzed one-pot three-component aza-Friedel-Crafts reactions of indoles, aldehydes, and tertiary aromatic amines

A new strategy for the synthesis of 3-diarylmethyl indoles was developed through FeCl₃ as Lewis acid catalyzed three-component aza-Friedel-Crafts reactions of indoles, aldehydes, and tertiary aromatic amines in one-pot. The reactions generated the corresponding 3-diarylmethyl indoles in good yields under mild reaction conditions by using less expensive, readily available, and environmentally benign iron catalyst. It is important to note that the key feature of this reaction is operational simplicity.     

Selective reduction and functionalization of diethyl 1-alkyl-1H-indole-2,3-dicarboxylates

A convenient and highly selective reduction of easily accessible indole-2,3-dicarboxylates is described. Ten different 1-alkyl-2-formyl-1H-indole-3-carboxylates are obtained in high yield and represent interesting building blocks for novel indoles.     

Co(III)/Zn(II)-catalyzed dearomatization of indoles and coupling with carbenes from ene-yne ketones via intramolecular cyclopropanation

A straightforward and efficient protocol for dearomatizing indoles is described. The reaction, catalyzed by an inexpensive Co(III)/Zn(II) catalyst, starts from easily accessible N-pyrimidinyl indoles and ene-yne ketones. Mild reaction conditions, high diastereoselectivity, a broad substrate scope, effective functional group tolerance, and reasonable to remarkable yields were observed.     

The synthesis of C-3β functionalized indoles via a hydroboration/Suzuki-Miyaura coupling sequence

A method for the functionalization of C-3β of vinyl indoles is described. The procedure involves a hydroboration, followed by a Suzuki-Miyaura cross-coupling with the intermediate alkyl borane. Triflates, bromides, and iodides are suitable coupling partners, allowing access to a variety of elaborated indole compounds.