Transition Metal-Catalysed, Direct and Site-Selective N1-, C2- or C3-Arylation of the Indole Nucleus: 20 Years of Improvements

Rh(iii)-catalyzed regioselective C-H activation dialkenylation/annulation cascade for rapid access to 6H-isoindolo[2,1-a]indole

6H-isoindolo[2,1-a]indoles were accessed via a Rh(iii)-catalyzed N-H free indole directed C-H activation dialkenylation/annulation cascade in moderate to excellent yields. This protocol also features: reaction procedures that are insensitive to air and moisture, excellent regioselectivity and good functional group tolerance.

Facile synthesis of indolizinoindolone, indolylepoxypyrrolooxazole, indolylpyrrolooxazolone and isoindolopyrazinoindolone heterocycles from indole and imide derivatives

Chemo-, regio- and diastereoselective coupling reactions of indole with imide derivatives leading to unique heterocyclic systems are demonstrated. Acid-induced 3-position coupling reactions of indole with cyclic imide derived lactamols followed by acid promoted 2-position cyclizations with the corresponding aldehydes are described to obtain the indolizinoindolones and benzoindolizinoindolones. Base induced 2-position coupling reactions of N-tosylindole with N-(2-iodoethyl)imides and the subsequent cyclizations provide indolylepoxypyrrolooxazole, indolylpyrrolooxazolone and indolyloxazoloisoindolone. Reductive cleavage of indolyloxazoloisoindolone to the corresponding alcohol followed by mesylation and base promoted N-cyclization affords the in situ air-oxidized pentacyclic product hydroxyisoindolopyrazinoindolone. A regioisomeric structural revision of the natural product from 1,2,5,6,7,11c-hexahydro-3H-indolizino[7,8-b]indol-3-one to 1,2,5,6,11,11b-hexahydro-3H-indolizino(8,7-b)indol-3-one is also reported in the present studies focussed on the methodologies for heterocyclic synthesis.

Tris(pentafluorophenyl)borane-Catalyzed Formal Cyanoalkylation of Indoles with Cyanohydrins

Despite the significant achievements related to the C3 functionalization of indoles, cyanoalkylation reactions continue to remain rather limited. We herein report on the formal C3 cyanoalkylation of indoles with cyanohydrins in the presence of a tris(pentafluorophenyl)borane (B(C₆F₅)₃) catalyst. It is noteworthy that cyanohydrins are used as a cyanoalkylating reagent in the present reaction, even though they are usually used as only a HCN source. Mechanistic investigations revealed the unique reactivity of the B(C₆F₅)₃ catalyst in promoting the decomposition of a cyanohydrin by a Lewis acidic activation through the coordination of the cyano group to the boron center. In addition, a catalytic three-component reaction using indoles, aldehydes as a carbon unit, and acetone cyanohydrin that avoids the discrete preparation of each aldehyde-derived cyanohydrin is also reported. The developed methods provide straightforward, highly efficient, and atom-economic access to various types of synthetically useful indole-3-acetonitrile derivatives containing α-tertiary or quaternary carbon centers.

An entry to 2-(cyclobut-1-en-1-yl)-1H-indoles through a cyclobutenylation/deprotection cascade

A transition-metal-free strategy for the synthesis of 2-(cyclobut-1-en-1-yl)-1H-indoles under mild conditions is described herein. A series of substituted 2-(cyclobut-1-en-1-yl)-1H-indoles are accessed by a one-pot cyclobutenylation/deprotection cascade from N-Boc protected indoles. Preliminary experimental and density functional theory calculations suggest that a Boc-group transfer is involved in the underlying mechanism.

Synthesis of C5-Allylindoles through an Iridium-Catalyzed Asymmetric Allylic Substitution/Oxidation Reaction Sequence of N-Alkyl Indolines

Iridium/Brønsted acid cooperative catalyzed asymmetric allylic substitution reactions at the C5 position of indolines have been reported for the first time. The highly efficient protocol allows rapid access to various C5-allylated products in good to high yields (48-97%) and enantioselectivities (82% to >99% ee) with wide functional group tolerance. The transformations allow not only the formation of C5-allylindoline derivatives but also the synthesis of C5-allylindole analogues in good yields and excellent stereoselectivities via an allylation/oxidation reaction sequence.

Copper(II)-catalysed direct C3-H esterification of indoles assisted by an N,N-bidentate auxiliary moiety

The regioselective direct C3-esterification of indoles with OXA is developed in an efficient reaction with carboxylic acids using the catalyst CuBr2 and oxidants Ag2CO3 and K2S2O8. The simple experimental procedure is proved to be broadly applicable to a range of substrates, including aromatic and aliphatic acids, and the corresponding products were obtained in good yields up to 87%. At the same time, it provides a valuable approach to produce C3-benzyl derivatives of indoles through reaction with benzyl carboxylic acid under the same reaction conditions.

Catalyst-Controlled Regioselectivity in Pd-Catalyzed Aerobic Oxidative Arylation of Indoles

Pd-catalyzed C-H arylation of heteorarenes is an important and widely studied synthetic transformation; however, the regioselectivity is often substrate-controlled. Here, we report catalyst-controlled regioselectivity in the Pd-catalyzed oxidative coupling of N-(phenylsulfonyl)indoles and aryl boronic acids using O₂ as the oxidant. Both C2- and C3-arylated indoles are obtained in good yield with >10:1 selectivity. A switch from C2 to C3 regioselectivity is achieved by including 4,5-diazafluoren-9-one or 2,2'-bipyrimidine as an ancillary ligand to a "ligand-free" Pd(OTs)₂ catalyst system. Density functional theory calculations indicate that the switch in selectivity arises from a change in the mechanism, from a C2-selective oxidative-Heck pathway to a C3-selective C-H activation/reductive elimination pathway.

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.

Benzoic Acid-Promoted C2-H Borylation of Indoles with Pinacolborane

A benzoic acid-promoted C2-H borylation of indoles with pinacolborane to afford C2-borylated indoles is developed. Preliminary mechanistic studies indicate BH₃-related borane species formed via the decomposition of pinacolborane to be the probable catalyst. This transformation provides a prompt route toward the synthesis of diverse C2-functionalized indoles.

Palladium-Catalyzed Silacyclization of (Hetero)Arenes with a Tetrasilane Reagent through Twofold C-H Activation

The use of an operationally convenient and stable silicon reagent (octamethyl-1,4-dioxacyclohexasilane, ODCS) for the selective silacyclization of (hetero)arenes via twofold C-H activation is reported. This method is compatible with N-containing heteroarenes such as indoles and carbazoles of varying complexity. The ODCS reagent can also be utilized for silacyclization of other types of substrates, including tertiary phosphines and aryl halides. A series of mechanistic experiments and density functional theory (DFT) calculations were used to investigate the preferred pathway for this twofold C-H activation process.

Copper(II)-Catalyzed Direct C-H (Hetero)arylation at the C3 Position of Indoles Assisted by a Removable N,N-Bidentate Auxiliary Moiety

The regioselective arylation of inert C3-H bonds in indoles reacting with arylboronates via effective copper-mediated catalysis with the aid of a facile and removable 2-pyridinylisopropyl (PIP) group without ligand participation is reported. This newly established method features high compatibility with diverse functional groups between coupling partners, including both indole substrates and arylboron reagents, consequentially leading to operational simplicity and providing access to generate the desired arylated products in good to excellent yields of up to 97%. Synthetically, the PIP-derived amide moiety could subsequently be readily removed under mild reaction conditions to produce useful indole carboxylic acids for further transformation.

Access to the C2 C–H olefination, alkylation and deuteration of indoles by rhodium(iii) catalysis: an opportunity for diverse syntheses

A concise approach for a diversity-oriented synthesis via regioselective C2 C–H olefination, alkylation, and deuteration of indoles by Rh(iii) catalysis is described.

NaI/TBHP-promoted reaction of indole-2-thiones with arylsulfonyl hydrazides: construction of achiral axial 3,3′-biindole-2,2′-dibenzenesulfonothioate derivatives

A new method for the synthesis of achiral axial 3,3′-biindole-2,2′-dibenzenesulfonothioate derivatives from indole-2-thiones with arylsulfonyl hydrazides promoted by NaI/TBHP is disclosed.

Pd/β-cyclodextrin-catalyzed C–H functionalization in water: a greener approach to regioselective arylation of (NH)-indoles with aryl bromides

A greener and more practical approach to the regioselective C3/C2-arylation of (NH)-indoles with (hetero)aryl bromides in water is developed via the Na2PdCl4-catalyzed and β-cyclodextrin ligand-mediated cross-coupling reactions.

The site-selectivity and mechanism of Pd-catalyzed C(sp2)-H arylation of simple arenes

Control over site-selectivity is a critical challenge for practical application of catalytic C–H functionalization reactions in organic synthesis. Despite the seminal breakthrough of the Pd-catalyzed C(sp²)–H arylation of simple arenes via a concerted metalation–deprotonation (CMD) pathway in 2006, understanding the site-selectivity of the reaction still remains elusive. Here, we have comprehensively investigated the scope, site-selectivity, and mechanism of the Pd-catalyzed direct C–H arylation reaction of simple arenes. Counterintuitively, electron-rich arenes preferably undergo meta-arylation without the need for a specifically designed directing group, whereas electron-deficient arenes bearing fluoro or cyano groups exhibit high ortho-selectivity and electron-deficient arenes bearing bulky electron-withdrawing groups favor the meta-product. Comprehensive mechanistic investigations through a combination of kinetic measurements and stoichiometric experiments using arylpalladium complexes have revealed that the Pd-based catalytic system works via a cooperative bimetallic mechanism, not the originally proposed monometallic CMD mechanism, regardless of the presence of a strongly coordinating L-type ligand. Notably, the transmetalation step, which is influenced by a potassium cation, is suggested as the selectivity-determining step.

The transmetalation step, not the C–H activation step, is suggested as the selectivity-determining step in Pd-catalyzed C–H arylation of simple arenes.

Convenient and Efficient Synthesis of Functionalized 2-Sulfenylindoles

A simple, efficient, and practical sulfenylation at the C2 position of N-tosylindoles under mild conditions was developed. The designed transformation is based on the reaction of N-tosylindoles with BuLi and S-alkyl, and S-aryl phosphorodithioates or thiotosylates to produce 2-sulfenylindoles in moderate to high yields. The presence of additional hydroxy, carboxy, or amino functionalities did not disturb the formation of products.

Regiocontrol in the oxidative Heck reaction of indole by ligand-enabled switch of the regioselectivity-determining step

Efficient control of regioselectivity is a key concern in transition-metal-catalyzed direct C-H functionalization reactions. Various strategies for regiocontrol have been established by tuning the selectivity of the C-H activation step as a common mode. Herein, we present our study on an alternative mode of regiocontrol, in which the selectivity of the C-H activation step is no longer a key concern. We found that, in a reaction where the C-H activation step exhibits a different regio-preference from the subsequent functionalization step, a ligand-enabled switch of the regioselectivity-determining step could provide efficient regiocontrol. This mode has been exemplified by the Pd(ii)-catalyzed aerobic oxidative Heck reaction of indoles, in which a ligand-controlled C3-/C2-selectivity was achieved for the first time by the development of sulfoxide-2-hydroxypyridine (SOHP) ligands.

Direct C3-Selective Arylation of N-Unsubstituted Indoles with Aryl Chlorides, Triflates, and Nonaflates Using a Palladium-Dihydroxyterphenylphosphine Catalyst

A palladium-dihydroxyterphenylphosphine (DHTP) catalyst was successfully applied to the direct C3-arylation of N-unsubstituted indoles with aryl chlorides, triflates, and nonaflates. This catalyst showed C3-selectivity, whereas catalysts with other structurally related ligands exhibited N1-selectivity. Complex formation between the lithium salts of the ligand and the indole is assumed to accelerate the arylation at the C3 position. Reactions using 3-alkylindoles afforded 3,3-disubstituted indolenines, which can be further converted to the corresponding indoline derivatives.

Tridentate Nickel(II)-Catalyzed Chemodivergent C-H Functionalization and Cyclopropanation: Regioselective and Diastereoselective Access to Substituted Aromatic Heterocycles

A Schiff-base nickel(II)-phosphene-catalyzed chemodivergent C-H functionalization and cyclopropanation of aromatic heterocycles is reported in moderate to excellent yields and very good regioselectivity and diastereoselectivity. The weak, noncovalent interaction between the phosphene ligand and Ni center facilitates the ligand dissociation, generating the electronically and coordinatively unsaturated active catalyst. The proposed mechanisms for the reported reactions are in good accord with the experimental results and theoretical calculations, providing a suitable model of stereocontrol for the cyclopropanation reaction.

Weak Coordination Enabled Switchable C4-Alkenylation and Alkylation of Indoles with Allyl Alcohols

A weak carbonyl coordination facilitated tunable reactivity between alkenylation and alkylation of indoles at the C4 C-H site is presented using readily accessible allylic alcohols in the presence of Rh catalysis by switching the additives or directing group. Exclusive site selectivity, functional group tolerance, and late-stage modifications are the important practical features.

Domino Transformations of Ene/Yne Tethered Salicylaldehyde Derivatives: Pluripotent Platforms for the Construction of High sp3 Content and Privileged Architectures

Diversity-oriented synthesis (DOS) has become a powerful synthetic tool that facilitates the construction of nature-inspired and privileged chemical space, particularly for sp³ -rich non-flat scaffolds, which are needed for phenotypic screening campaigns. These diverse compound collections led to the discovery of novel chemotypes that can modulate the protein function in underrepresented biological space. In this context, starting material-driven DOS is one of the most important tools used to build diverse compound libraries with rich stereochemical and scaffold diversity. To this end, ene/yne tethered salicylaldehyde derivatives have emerged as a pluripotent chemical platform, the products of which led to the construction of a privileged chemical space with significant biological activities. In this review, various domino transformations employing o-alkene/alkyne tethered aryl aldehyde/ketone platforms are described and discussed, with emphasis on the period from 2011 to date.

Nickel-catalyzed C-H alkylation of indoles with unactivated alkyl chlorides: evidence of a Ni(i)/Ni(iii) pathway

A mild and efficient nickel-catalyzed method for the coupling of unactivated primary and secondary alkyl chlorides with the C-H bond of indoles and pyrroles is described which demonstrates a high level of chemo and regioselectivity. The reaction tolerates numerous functionalities, such as halide, alkenyl, alkynyl, ether, thioether, furanyl, pyrrolyl, indolyl and carbazolyl groups including acyclic and cyclic alkyls under the reaction conditions. Mechanistic investigation highlights that the alkylation proceeds through a single-electron transfer (SET) process with Ni(i)-species being the active catalyst. Overall, the alkylation follows a Ni(i)/Ni(iii) pathway involving the rate-influencing two-step single-electron oxidative addition of alkyl chlorides.

Cu(II)-Mediated Cross-Dehydrogenative Coupling of Indolines with Sulfonamides, Carboxamides, and Amines

A facile and efficient Cu-mediated protocol for the cross-dehydrogenative coupling of indoline with sulfonamides, carboxamides, and anilines is reported. The reaction takes place through Cu-mediated C7-H activation via a 6-membered metallacycle to afford the amide and amine derivatives in good yields with a wide range of functional group tolerance. The importance of the protocol has been demonstrated by synthesizing the antiproliferative agent, ER-67836.

Nickel-Catalyzed C(2)-H Arylation of Indoles with Aryl Chlorides under Neat Conditions

Nickel-catalyzed regioselective C(2)-H arylation of indoles and pyrroles with aryl chlorides is achieved under neat conditions. This method allows the efficient coupling of diverse aryl chlorides employing a user-friendly and inexpensive Ni(OAc)₂/dppf catalyst system at 80 °C. Numerous functionalities, such as halides, alkyl ether, fluoro-alkyl ether, and thioether, and substituted amines, including heteroarenes like benzothiazolyl, pyrrolyl, indolyl, and carbazolyl, are well tolerated under the reaction conditions. The preliminary mechanistic study highlights a single-electron transfer (SET) pathway for the arylation reaction.

Rh(iii)-catalyzed C-7 arylation of indolines with arylsilanes via C-H activation

Site-selective synthesis of C-7 arylated indolines has been achieved via oxidative arylation of indolines with arylsilanes under Rh(iii)-catalyzed C-H activation of indolines by using CuSO4 as a co-oxidant. This transformation has been explored for a wide range of substrates under mild conditions.