Copper-Catalyzed Electrochemical CâH Amination of Arenes with

Article Cite This: J. Am. Chem. Soc. 2018, 140, 11487−11494

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Copper-Catalyzed Electrochemical C−H Amination of Arenes with Secondary Amines Qi-Liang Yang,†,‡ Xiang-Yang Wang,† Jia-Yan Lu,† Li-Pu Zhang,† Ping Fang,† and Tian-Sheng Mei*,† †

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State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China ‡ Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China S Supporting Information *

ABSTRACT: Electrochemical oxidation represents an environmentally friendly solution to conventional methods that require caustic stoichiometric chemical oxidants. However, C−H functionalizations merging transition-metal catalysis and electrochemical techniques are, to date, largely confined to the use of precious metals and divided cells. Herein, we report the first examples of copper-catalyzed electrochemical C−H aminations of arenes at room temperature using undivided electrochemical cells, thereby providing a practical solution for the construction of arylamines. The use of n-Bu4NI as a redox mediator is crucial for this transformation. On the basis of mechanistic studies including kinetic profiles, isotope effects, cyclic voltammetric analyses, and radical inhibition experiments, the reaction appears to proceed via a single-electron-transfer (SET) process, and a high valent Cu(III) species is likely involved. These findings provide a new avenue for transition-metal-catalyzed electrochemical C−H functionalization reactions using redox mediators.

1. INTRODUCTION Construction of carbon−nitrogen (C−N) bond is of great importance in organic synthesis since arylamines and heteroarylamines are essential structural units in many natural products and pharmaceuticals.1 Transition-metal-catalyzed amination of aryl halides or surrogates has been developed as a reliable tool for the construction of C(sp2)−N bonds.2 Alternatively, transition-metal-catalyzed direct amination of arene C−H bonds has emerged as a promising tool to construct C(sp2)−N bonds since it avoids costly prefunctionalization of the arene substrates.3 In particular, Cu-catalyzed intermolecular C−H amination of arenes has received special attention because copper is an inexpensive, earth-abundant, and nontoxic metal.4 In 2006, the Yu and Chatani groups independently developed aerobic copper-mediated C−H aminations of 2-phenylpyridine with tosylamine and aniline, respectively (Scheme 1a).5 Inspired by these seminal works, several other protocols for copper-mediated C−H amination of 2-phenylpyridine derivatives have been developed.6 In 2013, Daugulis and co-workers developed catalytic C−H amination using the removable bidentate aminoquinoline (AQ) directing group at elevated temperature (110 °C) (Scheme 1b).7 This transformation requires the use of a silver salt as a cocatalyst and N-methylmorpholine N-oxide (NMO) as a terminal oxidant. The same group more recently developed a Cucatalyzed C−H amination of benzoic acid derivatives using oxygen8 as the terminal oxidant, although 30% copper catalyst is required to achieve satisfactory yield at elevated temperature.9 In 2014, the Carretero and Chen groups independently © 2018 American Chemical Society

described a Cu-catalyzed picolinamide (PA)-directed C−H amination of aniline derivatives using a stoichiometric amount of PhI(OAc)2 as chemical oxidant, which generates PhI as a byproduct.10 Nevertheless, catalytic aminations using practical and environmentally benign oxidants that operate at ambient temperature are still lacking. The past decade has witnessed a renaissance in electrochemical organic synthesis.11−13 In particular, methods that use electric current as an oxidant for C−H functionalization reactions that proceed by transition-metal catalysis has received special attention, as this approach offers an alternative to conventional methods that require caustic stoichiometric chemical oxidants, and represents an environmentally friendly method.14 Inspired by seminal work from Amatore, Jutand, and Kakiuchi,15 our group has developed Pd-catalyzed electrochemical C(sp3)−H oxygenation and C(sp2)−H oxygenation, acylation, and alkylation reactions.16 Despite major progress in the field, C−H functionalizations using organometallic electrochemistry are largely restricted to the use of precious metal catalysts using divided electrochemical cells.17 Recently, Ackermman and Lei groups independently developed Co-catalyzed electrochemical C−H aminations of arenes with secondary amines.18 To the best of our knowledge, Cucatalyzed electrochemical C−H amination is unknown.19 Recently, much evidence has been obtained for the involvement of copper(III) species in copper-mediated Received: July 13, 2018 Published: August 30, 2018 11487

DOI: 10.1021/jacs.8b07380 J. Am. Chem. Soc. 2018, 140, 11487−11494

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Journal of the American Chemical Society Table 1. Reaction Optimization with Substrate 1aa

Scheme 1. Copper-Catalyzed C−H Amination

entry

variation from standard conditionsa

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

none Cu(OAc)2 H2O instead of Cu (OTf)2 CuCl2 instead of Cu (OTf)2 NaOPiv H2O instead of KOPiv DMF instead of CH3CN 1.5 mA instead of 3.0 mA (24 h) 1.5 mA instead of 3.0 mA (48 h) 6.0 mA instead of 3.0 mA (12 h) KI instead of n-Bu4NI n-Bu4NBr instead of n-Bu4NI no electric current no Cu(OAc)2 no KOPiv no n-Bu4NI 1 equiv of Cu(OTf)2, no electric current

92 (86)c 72 68 67 17 74 85 80 44 nr nr nr nr nr nr

a

Standard conditions: 1a (0.2 mmol), 2a (4.0 equiv), Cu(OTf)2 (10 mol %), KOPiv (2.0 equiv), n-Bu4NI (50 mol %), and CH3CN (2 mL), in an undivided cell with two platinum electrodes (each 1.5 × 1.0 cm2), 27 °C, 3.0 mA, 24 h. bThe yield was determined by 1H NMR using CH2Br2 as an internal standard. cIsolated yield in parentheses.

oxidative C−H functionalization.20 We hypothesized that copper(III) species could be generated by anodic oxidation21 and promote C−H amination of arenes under mild conditions by taking advantage of the high electrophilicity of Cu(III) intermediates.22 However, there are a few challenges associated with such a transformation: (1) overoxidation could be problematic since the resulting amination product has lower oxidation potential than the substrate; (2) functional group tolerance could be an issue since the Cu3+/Cu2+ couple normally has a high redox potential; (3) catalyst deactivation could be a problem since both alkyl amines and the resulting amination products could interact with the copper catalyst. We envisioned that the use of a redox mediator23 could be beneficial since the electrical potential required to oxidize Cu(II) species would be diminished. Thus, the electrolysis could be carried out under milder conditions and thereby exhibit improved functional group tolerance (Scheme 1c). In addition, with use of a mediator, the kinetic inhibition of the heterogeneous electron transfer between electrode and catalyst is irrelevant, resulting in overall reaction acceleration.24 Herein, we report copper-catalyzed electrochemical C−H amination of arenes and alkylamines using n-Bu4NI as a redox mediator in an undivided cell at room temperature (Scheme 1d).

1). Replacing Cu(OTf)2 with Cu(OAc)2·H2O or CuCl2 led to decreased yields (entries 2 and 3). Various bases were examined and KOPiv afforded the best yield (entry 4). Evaluating different solvents revealed that CH3CN was optimal (entry 5). Increasing or decreasing electric current resulted in lower yields (entries 6−8). The yield decreased significantly when n-Bu4NI was replaced with KI (entry 9). Finally, control experiments indicated that Cu(OTf)2, n-Bu4NI, KOPiv, and electrical current are all essential for this reaction (entries 10− 14). It is worth noting the stoichiometric Cu(OTf)2 did not give any desired amination product in the absence of electrical current under otherwise identical conditions (entry 15). This observation is distinct from many Cu(II)-mediated coupling reactions, which can be carried out under anaerobic conditions by employing Cu(II) as a stoichiometric reagent.25 This indicates that a high valent copper(III) species is likely involved in our electrochemical C−H amination (vide infra).26 An evaluation of directing groups confirmed the superiority of the NHCO-2-Py group (Scheme 2). Simple benzamide (1b), phenolic ester (1c), and N-methylated picolinamide (1d) did not engage in the C−H amination reaction, pointing to the critical roles of both the amide and ortho-pyridyl N atoms. In addition, various substituents on the picolinamide pyridine ring enabled the reaction without improving the yield (1e−1m). Versatility. With the optimized reaction conditions in hand, the scope of ortho-picolinamide-bearing arenes was investigated to test the generality and limitations of the reaction. As shown in Scheme 3, arenes substituted with a variety of functional groups such as alkyl, ether, thioether, halogen, trifluoromethyl, ester, hydroxyl, acetoxy, silyl, alkene, and alkyne groups were well-tolerated under the standard reaction conditions (5a−5y). In general, substrates with electron-rich (Me and OMe) and moderately electron-

2. RESULTS AND DISCUSSION Optimization Studies. Initially, we chose N-phenylpicolinamide (1a) and morpholine (2a) as reaction partners, and probed various reaction conditions for the envisioned electrochemical C−H amination in an undivided cell (Table 1, and see Tables S1−S9 in the Supporting Information). After extensive optimization, we found that 86% isolated yield of the desired product (3a) could be obtained under constant-current electrolysis at 3.0 mA in the presence of 10 mol % Cu(OTf)2, 50 mol % of n-Bu4NI (redox mediator),24 and 2 equiv of KOPiv in MeCN at room temperature for 24 h (Table 1, entry 11488


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Journal of the American Chemical Society Scheme 2. Evaluation of Directing Groupa

Scheme 3. Evaluation of N-Arylpicolinamide Scopea

a

Reaction conditions: 1 (0.2 mmol), 2a (4.0 equiv), Cu(OTf)2 (10 mol %), KOPiv (2.0 equiv), n-Bu4NI (50 mol %), and CH3CN (2 mL), in an undivided cell with two platinum electrodes (each 1.5 × 1.0 cm2), 27 °C, 3.0 mA, 24 h. Isolated yields are reported.

deficient (F, Cl, Br, and I) substituents at the para position reacted particularly well (typically 70−93% yield). Strongly electron-withdrawing groups, such as CF3 and CO2Me, proved compatible but afforded generally lower yields as a result of lower conversion (5n, 5o, 5x, and 5y). This amination proved sensitive toward steric encumbrance near the targeted C−H bond: the less hindered ortho position was preferentially aminated in the case of substrates bearing a meta substituent (5u−5w), while the 1-naphthyl derivative was aminated at C2 (5x, 42% yield). The presence of a substituent in the ortho position reduced reactivity (5y), presumably because of increased steric encumbrance. In addition, the structures of 5p and 5s were unambiguously confirmed by X-ray analysis. To our delight, this amination protocol could also be applied to a range of biologically and synthetically important pyridine substrates. As shown in Scheme 4, pyridines substituted with a variety of functional groups such as halogen, ether, alkyl, trifluoromethyl, and ester groups were well tolerated under the standard reaction conditions, affording the aminated products in good yields (7a−7l). To our surprise, amination takes place predominately at the ortho position adjacent to nitrogen (7a− 7k).27 The structures of 7d, 7g, and 7l were unambiguously confirmed by X-ray analysis. Encouraged by the feasibility of copper-catalyzed electrochemical amination using substituted anilines as coupling partners, we moved to examine the reactivity of a series of secondary amines (Scheme 5). To our satisfaction, a wide range of six-membered cyclic secondary amines bearing various functional groups, including ether, ester, nitrile, and hydroxyl groups are tolerated (8a−8i). Even thiomorpholine is a viable amine, providing the corresponding arylamine in 74% yield (8b). In addition, protected piperazines could also be readily introduced into the products (8j and 8k). The structure of 8k was confirmed by X-ray analysis. Unfortunately, linear secondary amines and primary amines were ineffective under current reaction conditions. The scalability of this copper-catalyzed electrochemical C− H amination was evaluated using a reaction containing 6.0 mmol of substrate (Scheme 6). Specifically, the reaction between 1a and 2a furnished the desired product in 83% yield,

a

Reaction conditions: 4 (0.2 mmol), 2a (4.0 equiv), Cu(OTf)2 (10 mol %), KOPiv (2.0 equiv), n-Bu4NI (50 mol %), and CH3CN (2 mL), in an undivided cell with two platinum electrodes (each 1.5 × 1.0 cm2), 27 °C, 3.0 mA, 24 h. Isolated yields are reported. b60 °C.

which showcases the preparative utility of this electrochemical copper-catalyzed C−H amination. Mechanistic Studies. Both organometallic C−H functionalization mechanism and single-electron-transfer (SET) mechanism have been previously invoked for copper-catalyzed C−H oxidation reactions.5,28 To gain further insight into this electrochemical C−H amination system, initial rates were measured for substrates with electronically varied substituents on the aniline ring, and faster rates were observed with substrates bearing electron-donating groups. A Hammett plot using σmeta parameters reveals a negative slope (ρ = −1.0; 11489


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Journal of the American Chemical Society Scheme 4. Evaluation of Pyridine Substratesa

Scheme 6. Gram-Scale Experiment

a

Figure 1. Hammett plot of relative initial rates.

Reaction conditions: 6 (0.2 mmol), 2a (4.0 equiv), Cu(OTf)2 (10 mol %), KOPiv (2.0 equiv), n-Bu4NI (50 mol %), and CH3CN (2 mL), in an undivided cell with two platinum electrodes (each 1.5 × 1.0 cm2), 27 °C, 3.0 mA, 24 h. Isolated yields are reported.

Scheme 7. Kinetic Isotope Effect Studies

Scheme 5. Evaluation of Aminesa

a

Reaction conditions: 1a (0.2 mmol), 2 (4.0 equiv), Cu(OTf)2 (10 mol %), KOPiv (2.0 equiv), n-Bu4NI (50 mol %), and CH3CN (2 mL) in an undivided cell with two platinum electrodes (each 1.5 × 1.0 cm2), 27 °C, 3.0 mA, 24 h. Isolated yield.

limiting step of the C−H amination, the kinetic order of each reaction component was established by studying the initial rates. Substrate 1a exhibited approximately first-order rate dependence (Figure S3). Meanwhile, morpholine, Cu(OTf)2, and n-Bu4NI exhibited approximately first-order rate dependence but with saturation at higher concentration (Figures S4− S6, see Supporting Information for experimental details). This suggests that electron transfer between iodine radical and copper complex is involved in the rate-determining step (vide infra).

Figure 1), consistent with an SET mechanism favoring electron-rich substrates.29 In addition, a kinetic isotope effect (KIE) value of 1.0 was observed by comparing the global reaction rates of 1a versus 1a−d5 (Scheme 7). This indicates that the putative C−H cleavage of anilines is not involved in the rate-determining step of the catalytic cycle.30 The observed KIE is also consistent with an SET mechanism.5 To further probe the turnover11490


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Journal of the American Chemical Society

ligands.26f We observed that the onset potential of Cu(II) complex oxidation shifted from 2.42 to 1.75 V in the presence of substrate 1a (curve c, Figure 2B), which serves as evidence that the substrate 1a is coordinated to the Cu(II). Interestingly, the addition of morpholine 2a further decreases the onset potential of Cu(II) complex oxidation to 1.51 V (curve d, Figure 2B), which is lower than the onset potential for oxidation of 1a (2.06 V) and 2a (1.61 V). Thus, the redox mediator preferentially oxidizes Cu(II) rather than substrate 1a or morpholine 2a. In addition, catalytic current was observed when n-Bu4NI was mixed with Cu(II) catalyst, substrate 1a, and morpholine 2a, indicating that the Cu(II) complex is oxidized to Cu(III) species by iodine radical in the presence of morpholine 2a (curve e, Figure 2C). Similarly, catalytic current was observed when extra morpholine 2a was added into solution of copper(II) complex, suggesting that C−H amination takes place smoothly in the Cu(III) catalyst center (Figure 2D). This also indicates that Cu(III) can promote C− H amination in the absence of n-Bu4NI. Indeed, moderate yield (53%) was obtained under controlled potential electrolysis in the absence of n-Bu4NI (Scheme 9). The low yield is due to the partial decomposition of aminated product under high oxidation potential (E = 2.0 V vs Ag/AgI). In contrast, satisfactory yield (78%) was achieved under significantly lower oxidation potential (E = 0.8 V vs Ag/AgI) in the presence of n-Bu4NI (Scheme 9).

Radical trapping experiments were conducted by adding a radical scavenger to the reaction. The addition of 2,2,6,6tetramethylpiperidine-1-oxyl radical (TEMPO) completely inhibited the reaction (Scheme 8). This observation is also consistent with an SET mechanism. Scheme 8. Radical Trapping Experiments

Next, we probed the electrochemical C−H amination mechanism by means of a series of cyclic voltammetric analyses: individual components (A), copper complex (B), copper complex with n-Bu4NI (C), copper complex with morpholine (D) (Figure 2). Iodide anion (I−), from n-Bu4NI, in a 0.1 M solution of n-Bu4NPF6 in MeCN exhibits oxidation peaks at 0.90 and 1.25 V versus Ag/AgI (curve a, Figure 2A), both of which are significantly lower than the onset potential for oxidation of copper catalyst (curve b, 2.42 V), substrate 1a (curve c, 2.06 V), morpholine 2a (curve d, 1.61 V), and product 3a (curve d, 1.44 V). It is known that the potential of Cu3+/Cu2+ couple is sensitive to changes in the nature of the

Figure 2. Cyclic voltammograms recorded on a Pt electrode (area = 0.03 cm2). (A): (a) MeCN containing 0.1 M n-Bu4NPF6 and 5 mM n-Bu4NI; (b) MeCN containing 0.1 M n-Bu4NPF6, 5 mM Cu(OTf)2, and 10 mM KOPiv; (c) MeCN containing 0.1 M n-Bu4NPF6 and 5 mM 1a; (d) MeCN containing 0.1 M n-Bu4NPF6 and 5 mM 2a; (e) MeCN containing 0.1 M n-Bu4NPF6 and 5 mM 3a. (B): (a) MeCN containing 0.1 M n-Bu4NPF6; (b) solution (a) after addition of 5 mM Cu(OTf)2 and 10 mM KOPiv; (c) solution (b) after addition of 5 mM 1a; (d) solution (c) after addition of 5 mM 2a; (e) solution (b) after addition of 5 mM 2a. (C): (a) MeCN containing 0.1 M n-Bu4NPF6 and 10 mM n-Bu4NI; (b) solution (a) after addition of 5 mM Cu(OTf)2; (c) solution (b) after addition of 5 mM 1a; (d) solution (c) after addition of 10 mM KOPiv; (e) solution (c) after addition of 10 mM 2a. (D): (a) MeCN containing 0.1 M n-Bu4NPF6, 5 mM 2a, 5 mM Cu(OTf)2, 10 mM KOPiv, and 5 mM 1a; (b) solution (a) after addition of 5 mM 2a. 11491


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yields. Further efforts to develop more transition-metalcatalyzed electrochemical C−H functionalization reactions assisted by redox mediator are currently underway in our laboratory.

Scheme 9. Controlled Potential Electrolysis

4. EXPERIMENTAL SECTION The general procedure for C−H amination via electrochemical oxidation took place as follows: The electrocatalysis was carried out in an undivided cell equipped with two platinum electrodes (1.0 × 1.5 cm2). Picolinamide 1 (0.2 mmol), morpholine (69.7 mg, 0.8 mmol), Cu(OTf)2 (7.3 mg, 0.02 mmol), KOPiv (56 mg, 0.4 mmol), n-Bu4NI (36.9 mg, 0.1 mmol), and MeCN (2 mL) were added to the electrochemical cell. Electrocatalysis was performed at 27 °C with a constant current of 3 mA maintained for 24 h (Q = 259.2 C, 13.4 F mol−1). After the reaction, the mixture was concentrated in vacuo. The resulting residue was purified by silica gel flash chromatography to give the amination product.

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On the basis of our experimental results, a plausible mechanism is presented for the Cu(II)-catalyzed electrochemical C−H amination (Scheme 10). Initially, the Cu(II)

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b07380. Experimental procedures and compound characterization data (PDF) Crystallographic data changed to X-ray data for compound 5p (CCDC 1835604) (CIF) Crystallographic data changed to X-ray data for compound 5s (CCDC 1838205) (CIF) Crystallographic data changed to X-ray data for compound 7d (CCDC 1861816) (CIF) Crystallographic data changed to X-ray data for compound 7g (CCDC 1861814) (CIF) Crystallographic data changed to X-ray data for compound 7l (CCDC 1853043) (CIF) Crystallographic data changed to X-ray data for compound 8k (CCDC 1838206) (CIF)

Scheme 10. Proposed Catalytic Cycle

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Ping Fang: 0000-0002-3421-2613 Tian-Sheng Mei: 0000-0002-4985-1071

catalyst coordinates with an amine and substrate 1a to generate copper(II) complex A, which is oxidized by iodine radical to form Cu(III) species B. The formation of Cu(III) is ratedetermining step in the catalytic cycle based on kinetic studies. Next, complex B undergoes single-electron transfer (SET) to deliver intermediate C. Intramolecular amine transfer to the radical-cation intermediate followed by another SET event affords species D, which release the aminated product 3a and generate a Cu(I) species. Upon anodic oxidation, the Cu(II) catalyst is regenerated, thereby completing the catalytic cycle.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB20000000), “1000-Youth Talents Plan”, NSF of China (Grant 21572245, 21772222, 21772220), and S&TCSM of Shanghai (Grant 17JC1401200, 18JC1415600). We thank Professor Jin-Quan Yu (The Scripps Research Institute, La Jolla, CA) for helpful discussions about the mechanism.

3. CONCLUSION In summary, we have demonstrated the first example of copper-catalyzed electrochemical C−H amination of arenes. The reaction employs a readily available and inexpensive redox mediator, n-Bu4NI, and can be carried out in an undivided cell at 27 °C. The protocol is operationally simple and robust, avoiding the use of exogenous stoichiometric chemical oxidants and exhibiting a broad substrate scope for both aniline and alkylamine components with good to excellent

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REFERENCES

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