Double C–H Functionalization of Indoles via Three-Component

Letter pubs.acs.org/acscatalysis

Double C−H Functionalization of Indoles via Three-Component Reactions/CuCl2‑Catalyzed Aerobic Dehydrogenative Coupling for the Synthesis of Polyfunctional Cyclopenta[b]indoles Liqin Jiang, Weifeng Jin, and Wenhao Hu* Shanghai Engineering Research Center of Molecular Therapeutics and New Drug Development, School of Chemistry and Molecular Engineering, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, China S Supporting Information *

ABSTRACT: A sequential Rh2(OAc)4-catalyzed multicomponent reaction and CuCl2-catalyzed postcyclization process is developed to build polyfunctional cyclopenta[b]indoles in good yields with high diastereoselectivities in an atom- and step-economic fashion. The key discovery in this process is the CuCl2-catalyzed intramolecular aerobic dehydrogenative Csp2−Csp2 cross-coupling of indole C2 with an enol functionality. Mechanistic studies including X-ray photoelectron spectroscopy analysis suggest that the catalytic cycle involves Cu(II) and Cu(I). This coupling reaction represents a unique example of aerobic Cu-catalyzed direct coupling of indoles with enols under mild conditions. KEYWORDS: dehydrogenative coupling, aerobic CuCl2 catalysis, multicomponent reaction, C−H functionalization, indole, enol

C

Scheme 1. Dehydrogenative Coupling Reactions between Indoles and α-Carbonyls and Sequential Rh2(OAc)4Catalyzed Three-Component Reactions and CuCl2-Catalyzed Aerobic Intramolecular Direct Coupling of Indoles with Enols

atalytic C−H functionalization of indoles is an efficient approach to make valuable indole derivatives.1 Among them oxidative C−H/C−H cross-coupling involving indoles without the need for preactivation of C−H functionalities demonstrates high synthetic value due to high atom-economy.2 Toward this goal, precious transition-metal complexes have been extensively investigated and used as catalysts for the reactions.2,3 For example, palladium complexes have been developed as efficient catalysts for the oxidative coupling of indoles with C(sp2)−H including arenes.2,3a−c In contrast, few successful examples have been reported by using inexpensive metal catalysts such as copper salts for the oxidative cross-coupling of indoles with C(sp2)−H.4,5 Inexpensive metal catalyzed direct couplings of indoles with C(sp3)−H bond, which is not adjacent to a nitrogen or an oxygen atom, including α-carbonyls, are less reported. Baran reported the direct coupling α-carbonyls with indoles, in which carbonyl compounds (1.0 equiv) and indoles (2.0 equiv) required 1.5 equiv of copper(II) 2-ethylhexanoate to promote the coupling.6 Both plausible mechanisms for the coupling reactions involved reduction of the copper species to copper(0) (Scheme 1a).6d Stoichiometric I27 as well as a large excess of FeCl38 have also been used to promote such kind of reactions. For intramolecular coupling to build indole-fused ring system, Kanai reported a manganese(III)-catalyzed intramolecular dehydrogenative coupling of indoles with malonates to give indole-fused 6-membered rings at reaction temperature of 130 °C in good yields. But unfortunately, this method was inefficient to build indole-fused 5-membered rings due to the proposed radical addition ringclosure step involving 5-endo-trig, a Baldwin-forbidden manner (Scheme 1b).9 © XXXX American Chemical Society

To create structural complexity, multistep, energy-, and wasteintensive syntheses are often required. The combination of multicomponent reactions, which build the precursors efficiently, with dehydrogenative couplings for the synthesis of multifunctional products from multireagents in one pot via two transitionmetal catalysts in an atom- and step-economic way are rare. Herein, we report a sequential catalytic process10 for efficient Received: July 12, 2016 Revised: August 10, 2016

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ACS Catalysis Table 1. Optimization of Reaction Conditions for Rh2(OAc)4-Catalyzed Three-Component Reactiona

entry 1 2 3 4 5 6 7 8 9 10

solvent THF THF THF CH2Cl2 toluene EA acetone dioxane EA EA

temp/°C rt rt 40 rt rt rt rt rt 0 °C−rt 0 °C−rt

time/h

yield /%b

4+5 anti-(4a + 5a) 4a 4a 4a 4a 4a 4a 4a 4a 5a

12 12 12 12 12 12 12 24 12 12

drc (anti/syn)

d

89:11 89:11 78:12 82:18 90:10 89:11 89:11 90:10 89:11 100:0

60 67 77 82 75 83 83 73 91 71e

a

General conditions: to a suspension of 1 mol % Rh2(OAc)4, 1.6 equiv of 1a, 1 equiv of 3a and 4 Å M.S. in solvent was added a solution of 1.6 equiv of 2a in solvent over 1 h via a syringe pump. bIsolated yield. cDetermined by 1H NMR analysis of the crude reaction mixture. dPurification by column chromatography on H-silica gel. eStaying the reaction residue in the H-silica gel column overnight, then proceeding chromatograph purification and recrystallization.

product (S*, S*, R*)-6 in high yield as one isomer when copper chloride or copper bromide were used as oxidizing reagents in the presence of 4 Å molecular sieves (Table 2, entries 8, 9).

synthesis of polyfunctional cyclopenta[b]indole derivatives through double C−H functionalization of indoles under mild conditions from three simple starting materials (Scheme 1c). The key discovery in this process is CuCl2-catalyzed intramolecular dehydrogenative coupling of indole C2 with an enol functionality under aerobic condition in air. Recently, our group reported novel multicomponent reactions based on trapping of zwitterionic intermediates with electrophiles.11 When conjugated α-keto ester 3a was examined to trap zwitterionic intermediate derived from N-methylindole 1a and methyl phenyldiazoacetate 2a catalyzed by Rh2(OAc)4, a mixture of three-component products (4:1) were obtained in 60% yield after column chromatography on H-silica gel (Table 1, entry 1). Out of expectation, the major product was 1,4 addition enol form anti-4a, and the minor one was the corresponding keto form anti5a. Further examination demonstrated that only enol form 4a presented in the crude reaction mixture, which could be purified as the only form by flash column chromatography using 200− 300 mesh silica gel, or could almost totally convert to corresponding keto form 5a by processing with H-silica gel (Supporting Information). It is noteworthy that similar enol products derived from addition of indoles to conjugated α-keto esters could not be isolated in a pure form previously.12 After the reaction conditions were optimized (Table 1, entries 2−9), 4a was isolated in 91% yield with 89:11 dr (entry 9), and corresponding keto form anti-5a was obtained in 71% yield (entry 10, SP). A cyclopenta[b]indole13 skeleton has been found in a large number of indole alkaloids.14 Compounds containing such a unit exhibit a wide range of biological activities.14c−f We envision that direct dehydrogenative cyclization of the three-component product 4 or 5 would allow us to efficiently build polyfunctional derivative of such an important unit. We began to explore direct dehydrogenative cyclization of 4 or 5 by employing a variety of oxidants, including copper(II) 2-ethylhexanoate, Cu(OTf)2, Cu(acac)2, Cu(OAc)2, CuBr2, CuCl2, CuI, Pd(OAc)2, FeCl3, AgOTf, I2, and Ce(NH4)2(NO3)6. Although no coupling product was obtained with the use of pure anti-5, we were pleased to find that enol form anti-4 did give the desired coupling

Table 2. Optimization of Reaction Conditions for Intramolecular Direct Coupling of C2 Carbon of Indoles with Enol form anti-4aa

entry

oxidants

yield/%b

drc (anti/syn)

1 2 4 5 6 7 8 9 10 11 12

copper(II) 2-ethylhexanoate Cu(OAc)2 Cu(acac)2 CuI I2 FeCl3 CuBr2 CuCl2 CuCl CuCl2 CuCl2

trace 0 20:1 >20:1 >20:1 >20:1

a

General conditions: a suspension of anti-4a (0.2 mmol), oxidant (0.2 mmol), 100 mg 4 Å M.S. in 1.5 mL of ethyl acetate was stirred for 1 h at ambient temperature. bIsolated yields of 6a. cDetermined by 1H NMR analysis of the crude reaction mixture. dIn the absence of 4 Å M.S. e0.1 equiv of CuCl2.

Copper(II) 2-ethylhexanoate, which was used in the intermolecular coupling indole C3 with α-carbonyls,6 was ineffective in the current reaction (Table 2, entry 1). Representative results are summarized in Table 2. When decreasing CuCl2 to a catalytic amount of 10 mol %, it was encouraging to isolate (S*, S*, R*)6a in 36% yield (entry 12), indicating CuCl2 could be used as a catalyst in the current transformation. 6147

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ACS Catalysis

Employing a catalytic amount of CuCl2 (0.1 equiv) gave 6a in 47% yield with 91:9 dr (Table 3, entry 2). After reaction condition optimization (entries 3−7), we were able to increase product yield to 72% at higher substrate concentration by using 10 mol % CuCl2 as the catalyst (entry 7) With the optimized reaction conditions in hand, we explored the substrate scope of the multicomponent sequential catalytic process. As revealed in Table 4, the reactions are tolerant to a broad array of β,γ-unsaturated α-keto esters. It is noteworthy that the keto esters could be extended to aromatic heterocyclic ones. For example, 3h bearing a thienyl functionality proceeded smoothly to give the corresponding product 6h in 70% yield with 88:12 dr (Table 4, entry 8). A range of diazoesters and indoles were selected to further expand the substrate scope. Both electrodonating and electro-withdrawing aryl diazoesters gave the corresponding cyclopenta[b]indole products 6 in moderate to good yields (Table 4, entries 11−15). Ethyl diazoacetate15 gave none of the desired cyclopenta[b]indole product. Substituted Nmethylindoles furnished the process smoothly to give corresponding products in good yields with high diastereoselectivities (Table 4, entries 16−20). The relative stereochemistry of the major isomer of 6a was determined to be (S*, S*, R*) by singlecrystal X-ray analysis (as shown in Supporting Information), and those of other products were tentatively assigned by analogy. To gain insight into the mechanism of the transformations, we performed a series of control experiments. Control reaction excluded the possibility that the three-component product 4a was formed from the C−H insertion product 11 via a stepwise reaction pathway (Supporting Information). For the dehydrogenative coupling reaction, under strictly oxygen-free conditions, anti-4a was subjected to 1 equiv of CuCl2 to give product 6a in

To simplify the process and also to avoid the possible tautomerization of 4a to 5a during separation, we attempted to explore a sequential catalytic process by combining the above two transformations in “one pot”. After the three-component reaction of 1a, 2a, and 3a was completed, 1 equiv of CuCl2 was added to the crude reaction mixture to promote the subsequent coupling transformation. The desired cyclopenta[b]indole 6a was obtained in 74% yield with 90:10 dr (Table 3, entry 1). Table 3. Optimization of the One-Pot Reaction Conditions for Polyfunctional Cyclopenta[b]indole Derivativesa

entry

temp/°C

1 2 3 4 5 6 7

rt rt 40 0 rt rt rt

concentration/ (mol/L) 0.07 0.07 0.07 0.07 0.13 0.20 1.85

yield /%b d

74 47 51 45 54 57 72

drc 90:10 91:9 90:10 91:9 88:12 90:10 91:9

a General conditions: the three-component reaction of 1a, 2a, and 3a was completed in the optimized conditions, then 0.1 equiv of CuCl2 was added to the reaction suspention and stirred at room temperature for another 1 h. bIsolated yields. cThe ratio of (S*, S*, R*) and (R*, S*, R*) was determined by 1H NMR analysis of the crude reaction mixture. d1.0 equiv of CuCl2.

Table 4. Substrate Scope of the Sequential Multi-Component Reactions/Postcyclizationa

entry

1/R1

2/X

3/R2

6/yield /%b

drc

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 2a/p-MeO C6H4 3a/p-MeC6H4 5a/p-BrC6H4 6a/p-FC6H4 7a/ m-MeO C6H4 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5

2a/H 2a/H 2a/H 2a/H 2a/H 2a/H 2a/H 2a/H 2a/H 2a/H 2a/H 2a/H 2a/H 2a/H 2a/H 2b/4-Cl 2c/4-F 2d/4- MeO 2e/5- Me 2f/6 -Me

3a/C6H5 3b/p-NO2C6H4 3c/ p-FC6H4 3d/ p-ClC6H4 3e/ p-BrC6H4 3f/p-MeO C6H4 3g/p-MeC6H4 3h/thienyl 3i/m-BrC6H4 3j/ m-MeC6H4 3a/C6H5 3a/C6H5 3a/C6H5 3a/C6H5 3a/C6H5 3a/C6H5 3a/C6H5 3a/C6H5 3a/C6H5 3a/C6H5

6a/72 6b/67 6c/67 6d/62 6e/58 6f/67 6g/50 6h/70 6i/47 6j/56 6k/61 6l/54 6m/46 6n/56 6o/71 6p/65 6q/69 6r/60 6s61 6t/46

91:9 90:10 89:11 89:11 89:11 83:17 83:17 88:12 88:12 88:12 90:10 88:12 90:10 91:9 90:10 88:12 91:9 88:12 91:9 92:8

a General reaction conditions: same as in Table 3, entry 7. bIsolated yields. cThe ratio of (S*, S*, R*) and (R*, S*, R*) was determined by 1H NMR analysis of the crude reaction mixture.

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ACS Catalysis Scheme 2. Plausible Reaction Mechanism

with high diastereoselectivity. Both enol form and keto form of the coupling products were obtained, respectively, allowing us to explore the difference in reactivity of the both forms. CuCl2catalyzed intramolecular direct dehydrogenative Csp2−Csp2 cross-coupling of the enol form was successfully disclosed. This is a rare example for the direct observation of the enol reactivity from the pure enol form. The coupling reaction represents the first example of Cu-catalyzed aerobic direct coupling of indoles with enols under mild conditions. XPS results and control experiment study suggest the catalytic cycle of Cu(II) to Cu(I). α-Chloro carbonyl intermediate generated by Cu(II)-catalyzed single-electron transfer oxidation was proposed as a key intermediate. Development of asymmetric variants of the current process is under investigation in our laboratory.

50% conversion, while no 6a was observed when employing 1 equiv of CuCl instead of CuCl2 under otherwise the same reaction conditions. After anti-4a was subjected to 1 equiv of CuCl2 for 1 h in ethyl acetate under 4 Å molecular sieves, the suspension was filtered, and the solid was studied the oxidation state of copper by X-ray photoelectron spectroscopy (XPS). The binding energy (BE) peak at ∼934 eV is assigned to Cu2+.16 The BE peak at ∼931.5 eV suggests the presence of Cu+ or Cu0 species.16 Because Cu 2p3/2 XPS cannot differentiate between Cu+ or Cu0, Auger Cu LMM spectra were used to confirm the presence of Cu+ at BE ∼ 571 eV, and no Cu0 presented in this system because there is no BE peak at ∼568 eV in the Auger Cu LMM spectra (Cu 2p spectra, Cu LMM spectra, and Cu 2p3/2 spectra were shown in Supporting Information).17 These control experiments and XPS results indicate that CuCl2 converts to CuCl during the catalytic cycle. In addition, anti-4a under 1 equiv of Cu(OAc)2 or LiCl alone gave none of 6a, but 76% yield of 6a was obtained when combining them together as reagents, demonstrating that chloride is a critical counterion in the copper(II)-catalyzed reaction. On the basis of the experimental data, a plausible mechanism was proposed in Scheme 2. Intermolecular trapping of zwitterionic intermediates IIIa or IIIb, which was formed in situ from indoles and diazoacetates initiated by Rh(OAc)2, with β,γ-unsaturated α-keto ester 3 through a Michael-type nucleophilic addition gives IV, following a “delayed proton transfer” resulted in the three-component enol-type products 4. In the presence of copper(II) chloride, the enol 4 generates the α-chloro carbonyl VI in situ along with two molecules of copper(I) chloride and 1 equiv of HCl via a copper enolate V. Then VI undergoes intramolecular Friedel−Crafts alkylation to deliver the desired cyclopenta[b]indole product 6 under 4 Å molecular sieves. In the presence of HCl, the copper(I) chloride is oxidized to regenerate copper(II) chloride catalyst under air condition. MacMillan reported a CuBr2 catalyzed carbonyl− amine coupling reaction, in which α-bromo carbonyl species were proposed as a key intermediate by CuBr2 promoted singleelectron transfer oxidation of a proposed cupper enolate from a ketone substrate.18 In conclusion, a novel multicomponent/sequential catalytic process was developed to construct polyfunctional cyclopenta[b]indole derivatives from simple starting materials in an atomand step-economic fashion. Rhodium-catalyzed trapping of zwitterionic intermediates by β,γ-unsaturated α-keto esters produce three-component coupling products in good yields

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01946. X-ray crystallographic data for anti-4a (CIF) X-ray crystallographic data for anti-5a (CIF) X-ray crystallographic data for (S*, S*, R*)-6a (CIF) Experimental procedures, XPS spectra, and compound characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the Natural Science Foundation of China (21332003), and L.J. acknowledges sponsorship from Science and Technology Commission of Shanghai Municipality (15ZR1410500). We thank Prof. Xiaobin Fan, GuoFeng Zhao for help on XPS data analysis.

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REFERENCES

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