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DOI: 10.1055/a-1928-3466
Transition-Metal-Catalyzed Directing-Group-Assisted C4-H Carbon–Carbon Bond Formation of Indole
S.B and P.K.M thank (DST) for INSPIRE Senior Research Fellowship. T. Paul is thankful to the Ministry of Education (MoE) for a Prime Minister’s Research Fellowship (PMRF). B.D acknowledges the University Grant Commission (UGC) for a Senior Research Fellowship.
Abstract
C4-Functionalized indole scaffolds are ubiquitous in natural products, bioactive compounds, and pharmaceuticals. Much effort has thus been made to develop effective synthetic strategies for C4 functionalization of the indole core. Among them, chelation-assisted synthetic approaches using transition-metal catalysis for the C4-selective C–H functionalization of indole is attractive. This account highlights progress made in C4-carbon–carbon bond formation of indole using directing-group-assisted transition-metal-catalyzed C–H functionalization (up to May 2022). These studies have been performed using Ru, Rh, Pd and Ir-based catalytic systems, while attention has been focused on the use of first-row abundant catalytic systems.
1 Introduction
2 Alkylation
3 Acylation
4 Alkenylation
5 Alkynylation
6 Allylation
7 Annulation
8 Arylation
9 Conclusion and Outlook
#
Key words
indole - C4-H functionalization - transition-metal catalysis - directing group - carbon–carbon bond formationBiographical Sketches
Shubhajit Basak was born and brought up in West Bengal, India. He received his B.Sc. and M.Sc. from the University of North Bengal, India. Currently, he is pursuing his Ph.D. under the supervision of Professor T. Punniyamurthy at the Indian Institute of Technology Guwahati. His research interest is focused on the transition-metal-catalyzed directed C–H functionalization of heterocycles.
Tripti Paul was born and brought up in West Bengal, India. She received her B.Sc. and M.Sc. from the University of North Bengal, India. Presently, she is pursuing her Ph.D. under the supervision of Professor T. Punniyamurthy at the Indian Institute of Technology Guwahati with a Prime Ministers Research Fellowship. Her research work is based on the catalytic C–H functionalization of heterocycles.
Prabhat Kumar Maharana was born and brought up in Odisha, India. He completed his B.Sc. and M.Sc. from Berhampur University, India. Since 2019, he has been pursuing his Ph.D. under the supervision of Professor T. Punniyamurthy at the Indian Institute of Technology Guwahati. His research interest is cycloaddition reaction using strained heterocycles.
Bijoy Debnath was born and brought up in Assam, India. He completed his B.Sc. and M.Sc. from the University of Assam. Since 2019, he has been pursuing his Ph.D. with Professor T. Punniyamurthy at the Indian Institute of Technology Guwahati. His research interests are focused on synthetic strategies for medicinally important heterocyclic scaffolds using strained ring systems.
Tharmalingam Punniyamurthy completed his graduate studies at the Bharathidasan University and obtained his Ph.D. Chemistry from the Indian Institute of Technology Kanpur (Professor J. Iqbal). After postdoctoral studies at the North Dakota State University (Professor M. P. Sibi), Kyushu University (Professor T. Katsuki), Montpellier University II (Professor A. Vioux) and The Ecole Nationale Superieure de Chimie de Montpellier (Professor J. J. E. Moreau), he joined the Indian Institute of Technology Guwahati in 2001. He is presently Dean of Faculty Affairs at the Indian Institute of Technology and an elected fellow of The Indian Academy of Sciences, The National Academy of Sciences India, and The Royal Society of Chemistry. He is visiting Professor of Oxford University, Kyushu University, and The Scripps Research Institute, San Diego. His research interests include sustainable organic transformations.
Introduction
Indole is an important structural scaffold found in a wide range of natural products and bioactive compounds and regarded as ‘The Lord of the Rings’ of the aromatic compounds.[1] In addition, the fascinating molecular architecture of indole represents as an effective candidate for drug discovery and proclaims remarkable properties such as anticancer, antimicrobial, and antidepressant action.[2] Furthermore, this premier skeleton is a structural constituent of many marketed pharmaceuticals and is considered to be the fourth most prevailing heterocyclic scaffold.[2b] Indole is a versatile building block for the essential amino acid, tryptophan, a cell growth hormone, auxin, and animal hormones, serotonin and melatonin. Envisaging the broad applications in medicinal science, synthetic modification of the indole nucleus has thus become a frontline research goal of organic chemists.[3] In this regard, directed C–H bond functionalization using transition-metal catalysis has led to the development of robust, step- and atom-economical tools for the construction of carbon–carbon and carbon–heteroatom bonds, obviating the need for pre-functionalized substrate precursors.[4] Furthermore, the indole skeleton harbors six distinguishable C–H bonds, offering the opportunity to create molecular complexity using the C–H activation strategy. Consequently, the past decade has witnessed tremendous advances in C2 and C3-H functionalization of indole due to the inherent reactivity of the pyrrole core.[5] The Sandtorv group reviewed the expeditious development of C2 and C3 functionalization of indole.[5c] However, the functionalization of the less reactive benzenoid (C4–C7 C–H bonds) core presents a formidable challenge in catalysis when the C2 and C3 C–H sites remain vacant. In addition, C4-decorated indoles are the major backbone of ergot alkaloids including pyroclavine, lysergic acid, and is common in regenerative drugs such as CAM 187 (anticancer) and LP 261 (antimitotic agent) (Figure [1]).[6] Earlier approaches involved stoichiometric and directed electrophilic metallation using Tl(TFA)3 [7a] and Hg(OAc)2,[7b] as well as Cr(CO)6 induced C4-lithiation and electrophilic trapping to enable the C4-functionalization.[7c] Although these strategies have drawbacks due to the high toxicity of the organo-Cr, -Hg and -Tl compounds. Contextually, the radical reaction, namely Witkop cyclization, is one of the most eye-catching pathways to deliver indole-derived cyclic lactams with good C4-selectivity.[7d] Along this line, much effort has been devoted to modernizing the field by developing directing group (DG) assisted transition-metal catalysis as an alternative sustainable approach.
A variety of carbonyl based DGs, including aldehydes, ketones, carboxylic acids, amides, as well as thiols, and imine functionalities have been employed to realize C4-H functionalization (Figure [2]). The functionalization at C4-site in the presence of a vacant C2-site presents an implicit challenge as there is a competing metallation with the six-membered metallacycle at C4 rather than preferential formation of the five-membered metallacycle at the C2-position when carbonyl or oxime based DG is used at the C3-site (Figure [3]). However, in case of a thiol-based DG, the C4-H functionalization reaction proceeds through a five-membered metallacycle. In addition, installation of a suitable DG at the C3-site can keep the electron density rich at the C4-site compared to the C2-site, rendering the C4-position prone to electrophilic attack.[43b] This has led to considerable progress in C4-H functionalization, and for the development of effective synthetic approaches. The Ackermann group reviewed the progress of the C4-functionalization of indole,[8] whilst Shi, Frost and our groups covered the developments at the benzene core.[9] Considering the growing interest in the development of diverse synthetic strategies, a recent comprehensive update on the C4-H functionalization of indole would thus be valuable. This account focuses on our efforts as well as the progress of others to date on the transition-metal-catalyzed DG-assisted C4 C–H functionalization for the carbon–carbon bond formation of indole. These studies have been explored using Rh, Ru, Pd and Ir-based catalytic systems, while efforts have been made on the use of first-row catalysis. These accomplishments have been covered, based on the type of reactions, as alkylation, acylation, alkenylation, alkynylation, allylation and arylation.
# 2
Alkylation
Fluoroalkylation can modulate the electronic as well as physical properties of organic molecules that impact medicine, agriculture, and materials.[10] Thus, an efficient weak coordination assisted C4-trifluoroalkylation of N-methyl indole has been developed using reactive (1H,1H-perfluoroalkyl)mesityl-iodonium triflates as an alkylating source under Pd-catalysis (Scheme [1]A).[11]
A wide range of substitutions, including boron esters, were compatible, although substituents with coordination ability to Pd require a higher catalyst loading to get satisfactory yield. A Rh(III)-catalyzed pyrrolidine benzamide directed indole C4-H bond addition to nitroalkenes has been accomplished (Scheme [1]B).[12] In addition, diazo esters, as potential carbene precursors, were used as site-selective C4-alkylating agent under Rh-catalysis (Scheme [1]C).[13] This scalable reaction tolerates a range of symmetrical and non-symmetrical diazo malonates, except for donor/acceptor diazo compounds, which provided C4-alkenylated product due to the 1,2-hydrogen shift to carbene. By tuning the additive and reaction temperature, C2-annulated indoles were achieved. Moreover, replacement of the keto group with oxime DG was later realized (Scheme [1]D).[14] The protocol dealt with a wide variety of substrates and gave the indolopyridone core directly. The ketoxime DG in the products can be removed to produce DG-free C4 alkylated indole. Furthermore, trifluoro acetyl group directed switchable C4-H hydroarylated and Heck-type products of indole have been accomplished with maleimide under Rh(III)-catalysis (Scheme [1]E).[15] Acid additive leads to protodemetallation to furnish the hydroarylated product, whereas base additive facilitated Heck-type products. Moreover, an Ir-catalyzed thioether directed C4-alkylation has been achieved using α-carbonyl sulfoxonium ylide as a user-friendly carbene surrogate (Scheme [1]F).[16] The C4-alkylated products have been converted into C3–C4 looped seven- and nine-membered tricyclic scaffolds to showcase the synthetic potential.
Our group showed a carbonyl directed chemodivergent coupling of indoles with a staple coupling partner, allyl alcohol, which can undergo manifold reaction.[17] The C3-acetyl group assisted C4-alkylation was achieved with 20 mol% AgOTf and 2 equiv NaOPiv·H2O under Rh-catalysis (Scheme [2]). The reaction has broad substrate scope with respect to indole as well as allyl alcohol and affords β-arylated ketones in moderate to high yields. In addition, the generality of C4-alkylation was observed using C3-pivaloyl group with 20 mol% AgSbF6 and 2 equiv Ag2CO3 (Scheme [3]). Notably, the fibrate drug gemfibrozil derived indole was alkylated smoothly, showing the synthetic potential of the procedure.
Deuterium exchange study of C3-pivaloyl 1a independently in the presence or absence of 2a with D2O as a co-solvent showed significant deuterium incorporation at the C4-position (Scheme [4]A), which suggests the C–H activation step might be reversible. In addition, 53% deuterium incorporation at the α-carbon of the product [D n ]-3a indicates the formation of a Rh-oxa-π-allyl complex. A plausible mechanism has been shown in which the reaction is initiated via the formation of a cationic Rh(III)-species I followed by reversible C–H bond activation to form rhodacycle II (Scheme [4]B). The latter leads to intermediate III via enone insertion. In case of 3-acetyl indole, C4-alkylated product is formed via protodemetalation in the presence of in-situ generated PivOH from NaOPiv·H2O, while in case of pivaloyl indoles, the bulkier carbonyl group may produce a rigid metallacycle IVB, that can undergo isomerization to form oxa-π-allyl species V. Subsequent tautomerization delivers the corresponding alkylated product and the generated Rh(I) can be oxidized to active Rh(III) complex to complete the catalytic cycle.
Later, a carbonyl group directed Rh(III)-catalyzed C4-alkylation of indoles was demonstrated using allyl alcohols in the presence of Cu(OAc)2·H2O as oxidant (Scheme [5]A).[18a] The β-indolyl ketone product was effectively employed to synthesize a tricyclic moiety. Concurrently, a similar Rh-catalyzed C4-alkylation of indoles with allylic alcohols was shown (Scheme [5]B).[18b] Both electron-donating and -withdrawing groups on the indole frame were compatible, but only alkyl substituted allylic alcohols were found to be amenable. Subsequently, the reactivity of alkynes was shown with 3-pivaloyl indole under Co-catalysis (Scheme [5]C).[19] Depending upon the nature of the alkynes, two types of products, α-hydroxy ketone and α,β-unsaturated ketone, were obtained. The reaction has broad scope with respect to indole as well as alkyne. Moreover, a Pd-catalyzed directed C–H alkylation has been found to be successful with aziridine as coupling partner (Scheme [5]D).[20] The scalable synthesis of β-indolylethyl-amines was performed using AcOH as an acid additive at 80 °C with good substrate scope and functional group diversities.
# 3
Acylation
C4-Acylated indoles are versatile synthetic building blocks and serve as tubulin polymerization inhibitors.[21] A Pd-catalyzed C4-acylation of indoles with α-oxocarboxylic acids has been achieved (Scheme [6]A).[22a] The study revealed tosyl as the best protecting group, while NH-free indole provided a trace amount of the product. The reaction involves the generation of an acyl radical via K2S2O8 promoted decarboxylation of α-oxocarboxylic acids. Later, an Ir-catalyzed synthesis of 2-alkynyl-3,4-dicarbonyl indoles was achieved via indole C3-carbonyl directed C–H functionalization employing haloalkynes (Scheme [6]B).[22b] The scope of the procedure was extended to the synthesis of 2,5-dialkynyl-3,4-dicarbonyl indoles, where C4-acyl functionality served as a relay DG.
# 4
Alkenylation
Prevalence of C4-alkenylated indole motifs in both synthetic and medicinal domains has attracted a great deal of research interest in their synthesis. For example, lysergic acid, a precursor for ergoline alkaloids, is of high synthetic value.[6b] Recent advances in transition-metal-catalyzed DG-assisted C–H activation at the C4-H of indole can be employed for straightforward access to these scaffolds. An aldehyde directed Ru-catalyzed C4-alkenylation of indole has been accomplished employing acrylates as the coupling partner (Scheme [7]A).[23a] The reaction exclusively took place at the C4-site through a six-membered metallacycle. The substrate scope, DG removal, and synthetic transformation to furnish clavine alkaloid precursor are important features. A Pd(II)-catalyzed C4-alkenylation of the indole core has been accomplished by utilizing naturally occurring optically active tryptophan derivatives with alkenes (Scheme [7]B).[23b] The reaction was successfully utilized for the biomimetic synthesis of clavicipitic acid. Later, a Rh-catalyzed trifluoromethyl ketone assisted C4-alkenylation of indoles was accomplished using acrylate as the coupling partner (Scheme [7]C).[23c]
The C4- and C2-dialkenylation of indoles was achieved under Rh(III) catalysis with the aid of a carboxylic acid (Scheme [7]D).[24] In the presence of copper salt as oxidant and AgSbF6 as an additive, a broad range of acrylates and styrenes coupled to give the target product, devoid of carboxyl functionality. Moreover, the Jia group disclosed an efficient protocol for the C4-alkenylation of unprotected indoles following a Rh/Cu catalytic cycle (Scheme [7]E).[25a] The protocol supported a range of unprotected indoles and acrylates to yield the desired product. Further, the synthetic route was directly utilized for the asymmetric synthesis of indole alkaloids (–)-agroclavine and (–)-elymoclavine. Later, the C4-alkenylation using carboxylic acid DG was achieved using Rh(III) catalyst with AgOAc as an oxidant to afford the coupled product, wherein the carboxyl functionality remained intact (Scheme [7]F).[25b] Several electronically substituted indole motifs along with structurally diverse alkenes such as acrylate, acrylamide, and acrylonitrile participated efficiently to give the target product in moderate to good yields. Further, a Rh(III) catalyzed thioether directed C4-alkenylation has been performed via a five-membered metallacycle (Scheme [7]G).[25c] The reaction supports a broad spectrum of indole and alkene derivatives. In addition, the traceless thioether DG can be transformed into several functional groups, confirming the high synthetic applicability.
A Rh(III)-catalyzed, base controlled C4-alkenylation has been achieved using maleimides as an alkene source (Scheme [7]H).[26] Stoichiometric use of Ag2CO3 as base promotes an E2 elimination from the eight-membered rhodacycle to yield the oxidative Heck-type coupling product. Further, a transient DG-assisted C4-alkenylation of indoles has been carried out with vinyl iodides using Pd(II) catalysis (Scheme [7]I).[27] Glycine was chosen as the transient DG, which, in the presence of 3-formylindoles, forms the imine, subsequent coordination of imine and carboxylate to Pd(II) forms a six-membered metallacycle, which paves the way for the successful alkenylation. The potential of first-row transition-metal has also been explored for C4-alkenylation of indoles using Michael acceptors (Scheme [7]J).[28] The combination of Co(III) as catalyst and Cu(OAc)2·H2O, as an additive worked well for acrylates as coupling partner; however, in case of maleimides, Ag2CO3 stood out as the additive of choice. The method is highly sensitive to the electronic nature of the substituents on both coupling partners.
Our group reported a Rh(III)-catalyzed, additive controlled chemoselective C4-alkenylation using allylic alcohols (Scheme [8]).[29] The switch between probable alkylation and alkenylation was achieved by the fine tuning of additives. The use of Ag2CO3 favors the β-hydride elimination to afford target alkenylation product. This weak carbonyl coordination directed strategy supported a wide range of substituted indole scaffolds and carbonyl DGs to deliver the alkenylated product in good yield. Intriguingly, NH-free and fused indoles turned out to be viable substrates. In case of the allylic alcohol, both aryl and alkyl substituents at the carbinol carbon were amenable. Naturally occurring isophytol, which is a tertiary allyl alcohol that impedes β-H elimination, proceeds via β-hydroxy elimination to deliver the allylated product.
To gain insight into the reaction mechanism, H/D exchange and kinetic isotope experiments were conducted (Scheme [9]A). Irrespective of the presence or absence of 2a, considerable deuterium incorporation was observed at the C4-site of 1a, which indicates the C–H activation step might be reversible. Further, a kinetic isotope effect (KIE) of 1.48 was found, which implies the C–H bond-activation step is not the rate-determining step. A proposed mechanistic cycle is shown in which directed C–H bond activation occurs to afford rhodacycle II, which undergoes enone insertion to generate III (Scheme [9]B). Subsequent β-hydride elimination in the presence of Ag2CO3 can lead to the formation of alkenylated product along with the generation of a Rh(I) complex, which can be oxidized to regenerate Rh(III) with the assistance of Ag(I) and air to continue the catalytic cycle.
# 5
Alkynylation
Initially, C-4 selective alkynylation of indole was achieved with the aid of weakly coordinating ester[30a] and amide[30b] DGs under Ir(III)-catalysis (Scheme [10]A, B).
Later, the Miura group reported a detailed study on the Ir(III)-catalyzed alkynylation to furnish a variety of C4-alkynylated indole motifs (Scheme [10]C).[30c] A suitably positioned sulfur DG at the C3 position directs the Ir metal to activate the C4-H, which thus undergoes alkynylation in the presence of TIPS-EBX to yield the coupled product. A variety of substituted indoles possessing a range of electronic substituents were efficiently alkynylated; however, sterically congested C5 substituted indoles produced inferior results. In addition to the indole core, substitution at thioether DG and N-protecting group barely affected the product yield. To unleash the synthetic potential, the authors performed post-synthetic applications, wherein the alkyne moiety and the DG were functionalized to generate value-added products. Recently, a double alkynylation of the indole core was disclosed by the Dong group under Ir(III)-catalysis with the aid of a weakly chelating carbonyl auxiliary (Scheme [10]D).[31] Bromo alkyne coupled twice to furnish 2,4-dialkynylated indoles in good yield. The authors revealed that the specific combination of additives is crucial for the di-alkynylation. Further, they have mechanistically proved that the activity of alkynylation at the C4 site surpasses the activity at the C2 site.
# 6
Allylation
Indole derivatives with allyl or prenyl substitution at the C4-position are present as structural subunits in a variety of alkaloids, and they have immense importance in medicinal science.[6b] Our group reported a Rh(III)-catalyzed C4-allylation of indoles with a Morita–Baylis–Hilman (MBH) adduct employing a removable weak chelating acetyl DG (Scheme [11]).[32]
Other carbonyl-based formyl, pivaloyl, benzoyl and trifluoromethylacetyl DGs were unsuccessful for the desired functionalization. Indole derivatives as well as MBH adducts containing electronically diverse substituents were amenable to furnish the target C4-allylated product in moderate to excellent yields. To showcase the viability of the protocol, C4-allylation was carried out using MBH adduct derived from borneol, a naturally occurring terpene derivative. In addition, the α,β-unsaturated ester functionality at C4-position of indole can be useful for various synthetic transformations. Moreover, the allylation strategy was equally fruitful for other weak chelating substrates including α-tetralone, benzothiophene, and indoline derivatives. To understand the reaction pathway, H/D scrambling experiments were conducted either in the presence or absence of the MBH adduct (2a) (Scheme [12]A). A 89% deuterium incorporation at the C4-H bond was found with 2a, suggesting that the C–H activation is reversible. In the absence of 2a, 32% deuteration was observed. In addition, considerable deuteration (89%) observed at the C4-H bond supports the involvement of the acetate group of MBH adduct in the C–H activation and deprotonation step. Moreover, the kinetic isotope experiment revealed a k H/k D value of 1.53, indicating that the C4-H bond activation might not be a rate-limiting step. A plausible mechanism has been presented in which the reaction is initiated with a cationic Rh species I that can be reversibly cyclometalated at the C4-site of the indole substrate to deliver six-membered rhodacycle II (Scheme [12]B). Alkene coordination followed by favorable migratory insertion can lead to the intermediate IVA via III. Subsequently, intramolecular β-acetate elimination from the IVA may furnish the C4-allylated product and regenerate the active catalyst. However, H/D exchange experiment obviates the possible formation of π-allyl Rh-complex IVB from III via oxidative addition. Notably, the catalytic cycle follows a redox-neutral pathway, circumventing the need for external oxidant or base.
Recently, a regioselective C4-allylation and prenylation of indoles was accomplished utilizing pivaloyl DG under Rh(III)-catalysis (Scheme [13]).[33] 5-Methylene-1,3-dioxan-2-one and vinyl ethylene carbonate were employed to afford branched or linear allyl derivatives, while substituted allenes served as a prenylation source. Chemoselective allylation at the C4-site was achieved by altering the additive and solvent at moderate temperature. In contrast, the prenylation occurred at both C4- and C2-position using PivOH and Cu(OAc)2 ·H2O as mixed additive in 1,4-dioxane at an elevated temperature, resulting in a mixture of regioisomers.
# 7
Annulation
Cyclic indole congeners are ubiquitous scaffolds in bioactive compounds and medicinal products.[34] The recent advances in tandem indole C4-H bond-activation followed by annulation facilitated the development of a facile route to synthesize cyclic indole congeners. In this context, the You group reported the synthesis of benzo-fused indoles by reacting 3-formyl and 3-acetyl indoles with alkynes as coupling partner under Rh(III)-catalysis using AgSbF6 as additive and Ag2CO3 as the oxidant (Scheme [14]A).[35] Notably, the use of additives and oxidants were shown to be necessary for the reaction.
Inspired by this, the Wang group reported a one-pot synthesis of indole-fused carbazoles via tandem C2 and C4-H activation/alkyne annulation of 3-(1H-indol-3-yl)-3-oxopropanenitriles utilizing Rh(III)-catalysis and Cu(OAc)2·H2O as the additive (Scheme [14]B).[36] Both electron-donating and -withdrawing group substituted indoles were tolerated, although alkynes with an electron-withdrawing ester group was not a suitable substrate. Further, a Rh(III)-catalyzed trifluoromethylketone assisted indole C4-H activation and alkyne annulation has been achieved to afford benzo[e]indole skeleton (Scheme [14]C).[37] para- and meta-Substituted alkynes were well tolerated to yield the benzannulated product, while ortho-substituted alkynes could be used to produce the alkenylated derivative. Later, an enone directed synthesis of racemic as well as asymmetric 3,4-fused tricyclic indoles was accomplished via a sequential formation of carbon–carbon bond between electronically diverse substituted indoles and α‑diazomalonates under Rh(III) and Cu(II)-based catalytic systems (Scheme [14]D).[38] Synthesis of chiral tricyclic indoles was shown to proceed with the aid of squaramideamine (C1) and bifunctional thiourea amine (A1) with good yield and high ee.
# 8
Arylation
C4-Arylated indoles are prevalent in bioactive compounds and the key component of the dictyodendrin class of alkaloids.[39] Hence, the synthetic fraternity has been engaged in developing efficient approaches for the C4-arylation of indole. Using a transient DG strategy, the Yu group unveiled an elegant methodology for diverse ortho-C–H functionalizations of benzaldehydes using Pd-catalysis (Scheme [15]A).[40a] One example of C4-arylation of indole has been shown by introducing a formyl moiety as a DG and substituted aryl iodide as an arylating agent. The desired C4-arylation was achieved in 82% yield. Subsequently, a Pd-catalyzed C3-tethered removable pivaloyl directed C4-arylation of indole was accomplished with aryl iodide as the coupling partner (Scheme [15]B).[40b] The steric bulk of the DG prompted the preferential formation of C4-palladacylce, which can undergo oxidative addition with aryl iodide followed by reductive elimination to furnish the target product. Excellent regioselectivity was observed by switching the catalyst. Later, a Ru(II)-catalyzed diarylation of indole-5-caboxylic acid at the C4- and C6-positions using aryl halide was reported (Scheme [15]C).[41] The reaction proceeded well without the use of AgI and CuII additives. Further, a Pd-catalyzed pivaloyl directed diarylation of 6,7-benzindoles was achieved with aryl iodide at the C4- and C5-positions (Scheme [15]D).[42] The key to success of the protocol was the utilization of pivaloyl DG and the blocking effect at C6- and C7-positions.
The C4-arylation strategy has also been explored employing glycine as a transient DG under Pd-catalysis (Scheme [15]E).[43a] Later, an Ir(III)-catalyzed C2/C4 (hetero)arylation of indole was described via oxidative cross-dehydrogenative coupling (CDC) with an array of diverse heteroarenes (Scheme [15]F).[43b] Weak chelating ketone and ester functionalities enable the transformation by tuning of the oxidant. DFT calculations reveal that a trimolecular electrophilic substitution (SE3) is followed in the presence of PivOH. Exclusive C4-selectivity was attained by the use of Ag2O as an oxidant and pivalate as an external proton shuttle.
Elevated temperature leads to a DG-free C4-heteroarylation/decarbonylation cascade. Further, an unprecedented C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles has been achieved by employing aryl iodide as aryl surrogate (Scheme [15]G).[43c] The use of TsOH·H2O as an acid additive promotes C4-arylation via a Pd(I)–Pd(II)-catalytic system, while TFA allows the formation of C2/C4-disubstituted indole via C4-arylation followed by concomitant DG migration at the C2-site under Pd(II)-catalysis.
Our group reported a Pd(II)-catalyzed C3-tailored benzoyl (Bz) group assisted C4-arylation of indole using readily accessible arenes as coupling partner (Scheme [16]).[44] The use of K2S2O8 as an oxidant and trifluoroacetic acid (TFA) as an additive is essential to effect the arylation at moderate temperature. The salient features of the protocol involve the use of weak chelating detachable DG, arene as aryl source, CDC strategy, and late-stage synthetic transformations. Screening the effect of N-protecting group (PG) and DG revealed that benzyl (Bn) as PG and Bz as DG produced the best result. A broad range of diversely substituted indoles and arenes were well tolerated to afford the arylation in good to moderate yield. Sterically crowded and strongly electron-deficient substrates decreased the reaction efficacy, furnishing lower yield and even failed to afford the product.
H/D exchange experiments in the presence of o-xylene using CD3CO2D or CD3OD showed no deuteration at the C4 or C2-site of 1a, indicating the irreversibility of the initial C4-H bond-activation step (Scheme [17]A). In addition, intermolecular kinetic isotope experiments established a k H/k D value of 1.23, suggesting the C4-H activation step might not be rate-determining step, whereas the parallel KIE value of 2.0 indicates the arene C–H bond activation might be involved in the rate-determining step. A proposed catalytic cycle has been depicted in which the reaction proceeded through in-situ generated active Pd(TFA)2 complex I, which may undergo directed C–H activation with indole substrate to produce the six-membered palladacycle II (Scheme [17]B). Subsequently, arene coordination to the metal centre may lead to the formation of III, which may be converted into the intermediate IV via concomitant non-directed arene C(sp2)–H cleavage. The latter may undergo reductive elimination to afford the target product with the formation of Pd(0), which can be regenerated to active Pd(II) catalyst in the presence of K2S2O8 and TFA. The synthetic potential of the method was highlighted by structural elaboration with iterative C–H functionalization at the C2-site, intramolecular carbon–carbon bond formation and successful DG removal via a reverse Friedel–Crafts reaction. Further, a Pd(II)-catalyzed C4-arylation of NH-free indoles has been shown with aryl iodides employing formyl, acetyl, carboxylic acid, and ester functionalities at the C3-site as DG.[45] Using AgOAc as an oxidant and TFA as an additive, the desired arylation was achieved with excellent selectivity.
# 9
Conclusion and Outlook
Site-selective functionalization of the indole core, particularly regioselective manipulation of the C4-H bond, has emerged as an important synthetic goal for organic chemists due to the potential application of C4-substituted indoles in medicinal and material sciences. In this line, transition-metal-catalyzed DG-assisted methodology has emerged as an effective alternative, with high atom- and step-economy. Numerous methods have been designed either by introducing new catalytic systems, mutation of DG or readily accessible coupling partners. In this account, we have summarized our efforts in carbon–carbon bond-forming reactions to synthesize C4-functionalized indoles as well as some past and contemporary reports. Despite these substantial advances, there is ample scope yet to be explored, developing innovative strategies in terms of sustainability. In this context, the use of earth-abundant and cheap metal catalysts with the employment of photo- and electro-catalysis may drive an expansion of this field by generating less waste. We hope this account will provide a concise overview for synthetic experts and may be beneficial for synthetic elaboration of the indole nucleus.
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Conflict of Interest
The authors declare no conflict of interest.
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- 3c Pitts AK, O’Hara F, Snell RH, Gaunt MJ. Angew. Chem. Int. Ed. 2015; 54: 5451
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- 4b Wencel-Delord J, Glorius F. Nat. Chem. 2013; 5: 369
- 4c Gulías M, Mascareñas JL. Angew. Chem. Int. Ed. 2016; 55: 11000
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- 5a Lebrasseur N, Larrosa I. J. Am. Chem. Soc. 2008; 130: 2926
- 5b Joucla L, Djakovitch L. Adv. Synth. Catal. 2009; 351: 673
- 5c Sandtorv AH. Adv. Synth. Catal. 2015; 357: 2403
- 5d Deka B, Deb ML, Baruah PK. Top. Curr. Chem. 2020; 378: 22
- 6a Shetty RS, Lee Y, Liu B, Husain A, Joseph RW, Lu Y, Nelson D, Mihelcic J, Chao W, Moffett KK, Schumacher A, Flubacher D, Stojanovic A, Bukhtiyarova M, Williams K, Lee K.-J, Ochman AR, Saporito MS, Moore WR, Flynn GA, Dorsey BD, Springman EB, Fujimoto T, Kelly MJ. J. Med. Chem. 2011; 54: 179
- 6b Liu H, Jia Y. Nat. Prod. Rep. 2017; 34: 411
- 6c Liu H, Chen L, Yuan K, Jia Y. Angew. Chem. Int. Ed. 2019; 58: 6362
- 7a Hollins RA, Colnago LA, Salim VM, Seidl MC. J. Heterocycl. Chem. 1979; 16: 993
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- 9e Wen J, Shi Z. Acc. Chem. Res. 2021; 54: 1723
- 9f Prabagar B, Yang Y, Shi Z. Chem. Soc. Rev. 2021; 50: 11249
- 10 Muller K, Faeh C, Diederich F. Science 2007; 317: 1881
- 11 Borah AJ, Shi Z. Chem. Commun. 2017; 53: 3945
- 12 Potter TJ, Kamber DN, Mercado BQ, Ellman JA. ACS Catal. 2017; 7: 150
- 13 Chen X, Zheng G, Li Y, Song G, Li X. Org. Lett. 2017; 19: 6184
- 14 Biswas A, Samanta R. Eur. J. Org. Chem. 2018; 1426
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- 17 Pradhan S, Mishra M, De PB, Banerjee S, Punniyamurthy T. Org. Lett. 2020; 22: 1720
- 18a Sherikar MS, Devarajappa R, Prabhu KR. J. Org. Chem. 2020; 85: 5516
- 18b Pan C, Huang G, Shan Y, Li Y, Yu J.-T. Org. Biomol. Chem. 2020; 18: 3038
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- 25c Kona CN, Nishii Y, Miura M. Org. Lett. 2018; 20: 4898
- 26 Sherikar MS, Kapanaiah R, Lanke V, Prabhu KR. Chem. Commun. 2018; 54: 11200
- 27 Thrimurtulu N, Dey A, Singh A, Pal K, Maiti D, Volla CM. R. Adv. Synth. Catal. 2019; 361: 1441
- 28 Banjare SK, Nanda T, Ravikumar PC. Org. Lett. 2019; 21: 8138
- 29 Pradhan S, Mishra M, De PB, Banerjee S, Punniyamurthy T. Org. Lett. 2020; 22: 1720
- 30a Li X, Liu G, Zhu Z, Huo Y, Jiang H. J. Org. Chem. 2017; 82: 13003
- 30b Wu G, Ouyang W, Chen Q, Huo Y, Li X. Org. Chem. Front. 2019; 6: 284
- 30c Kona CN, Nishii Y, Miura M. Angew. Chem. Int. Ed. 2019; 58: 9856
- 31 Chai X.-Y, Xu H.-B, Dong L. Chem. Eur. J. 2021; 27: 13123
- 32 Pradhan S, De PB, Punniyamurthy T. Org. Lett. 2019; 21: 9898
- 33 Zhang S.-S, Liu Y.-Z, Zheng Y.-C, Xie H, Chen S.-Y, Song J.-L, Shu B. Adv. Synth. Catal. 2022; 364: 64
- 34a Mascal M, Modes KV, Durmus A. Angew. Chem. Int. Ed. 2011; 50: 4445
- 34b Huters AD, Quasdorf KW, Styduhar ED, Garg NK. J. Am. Chem. Soc. 2011; 133: 15797
- 34c Ferlin MG, Carta D, Bortolozzi R, Ghodsi R, Chimento A, Pezzi V, Moro S, Hanke N, Hartmann RW, Basso G, Viola G. J. Med. Chem. 2013; 56: 7536
- 35 Liu X, Li G, Song F, You J. Nat. Commun. 2014; 5: 5030
- 36 Zhou T, Lia B, Wang B. Chem. Commun. 2017; 53: 6343
- 37 Bettadapur KR, Kapanaiah R, Lanke V, Prabhu KR. J. Org. Chem. 2018; 83: 1810
- 38 Harada S, Yanagawa M, Nemoto T. ACS Catal. 2020; 10: 11971
- 39a Okano K, Fujiwara H, Noji T, Fukuyama T, Tokuyama H. Angew. Chem. Int. Ed. 2010; 49: 5925
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- 40a Liu X.-H, Park H, Hu J.-H, Hu Y, Zhang Q.-L, Wang B.-L, Sun B, Yeung K.-S, Zhang F.-L, Yu J.-Q. J. Am. Chem. Soc. 2017; 139: 888
- 40b Yang Y, Gao P, Zhao Y, Shi Z. Angew. Chem. Int. Ed. 2017; 56: 3966
- 41 Simonetti M, Cannas DM, Panigrahi A, Kujawa S, Kryjewski M, Xie P, Larrosa I. Chem. Eur. J. 2017; 23: 549
- 42 Li P.-G, Yang Y, Zhu S, Li H.-X, Zou L.-H. Eur. J. Org. Chem. 2019; 73
- 43a Thrimurtulu N, Dey A, Singh A, Pal K, Maiti D, Volla CM. R. Adv. Synth. Catal. 2019; 361: 1441
- 43b Chen S, Zhang M, Su R, Chen X, Feng B, Yang Y, You J. ACS Catal. 2019; 9: 6372
- 43c Cheng Y, Yu S, He Y, An G, Li G, Yang Z. Chem. Sci. 2021; 12: 3216
- 44 Basak S, Paul T, Punniyamurthy T. Org. Lett. 2022; 24: 554
- 45 Taskesenligil Y, Aslan M, Cogurcu T, Saracoglu N. J. Org. Chem. 2022; in press
Corresponding Author
Publication History
Received: 14 June 2022
Accepted after revision: 19 August 2022
Accepted Manuscript online:
19 August 2022
Article published online:
28 September 2022
© 2022. Thieme. All rights reserved
Georg Thieme Verlag KG
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-
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- 43c Cheng Y, Yu S, He Y, An G, Li G, Yang Z. Chem. Sci. 2021; 12: 3216
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