Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Electrochemical N‑Formylation of Amines via Decarboxylation of Glyoxylic Acid Dian-Zhao Lin and Jing-Mei Huang* Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China S Supporting Information *
ABSTRACT: A new method for the synthesis of formamides has been developed through electrochemical decarboxylative N-formylation of amines with glyoxylic acid. This protocol provides an efficient approach to formamides with a broad range of functional group tolerance under ambient conditions.
F
glyoxylic acid is still unknown. Inspired by our previous studies of electrochemical synthesis,19 herein, we report the first highly efficient N-formylation of amines by electrochemical decarboxylative C−N bond formation with glyoxylic acid as a formyl source. To start our investigation, N-methylaniline (1a) and glyoxylic acid (2) were chosen as model substrates. Under constant-current at 5 mA in an undivided cell, the reaction of 1a and 2 with 10 mol % Cu(OAc)2·2H2O and 20 mol % NiCl2· 6H2O in the presence of cesium carbonate (1.5 equiv) gave 95% formylated product 3a (Table 1, entry 1). The yield decreased when the reaction was performed in the absence of Cu(OAc)2·2H2O or NiCl2·6H2O (Table 1, entries 2 and 3). When NiCl2·6H2O was replaced with some other Lewis acids, such as InCl3 and BiCl3, comparable activities were shown. The results implied that the nickel salt might act as a Lewis acid in this transformation (see Supporting Information (SI) for details). CuCl gave an inferior result to Cu(OAc)2·2H2O (Table 1, entry 4).20 When the reaction was carried out in the absence of cesium carbonate, only a trace of product was obtained (Table 1, entry 5). Both NaOAc and DBU led to a lower yield (Table 1, entries 6 and 7). Next, the effect of electrolyte was explored. Sodium perchlorate was found to be optimal among the electrolytes tested (Table 1, entries 1, 8− 10).20 The choice of solvent also had substantial impact on the result of this reaction. Use of N,N-dimethylformamide resulted in a low yield, while acetonitrile had proven ineffective (Table 1, entries 11 and 12).20 The electrode material was also tested. When graphite rod was used as anode or cathode, the reaction efficiency decreased (Table 1, entries 13 and 14). Nickel foam could serve as cathode with a reduced productivity (Table 1, entry 15). Increase or decrease of the electric current caused lower yields (Table 1, entries 16 and 17). Predictably, no desired product could be detected without an electric current (Table 1, entry 18). It is worth noting that only 1 equiv of
ormamides are one group of valuable intermediates in organic synthesis and pharmaceutical chemistry.1 The synthesis of many N-heterocycles, such as imidazole, isocyanides, as well as formamidines, is achieved via formamides as intermediates.2 Various pharmaceutically important compounds, for instance, leucovorin,3 formoterol4 and orlistat,5 contain formamides blocks. Furthermore, the application of formamides in Vilsmeier−Haack formylation reaction has been well documented.6 Various approaches have been developed for N-formylation of amines to access formamides. Besides the use of formic acid and its derivatives,7 other formylating reagents, including methanol,8 aldehydes,9 cyanides,10 carbon monoxide,11 and carbon dioxide,12 have also been focused on. Nevertheless, these methodologies suffer from one or more disadvantages, such as unwanted side reactions, the toxicity of the reagents, the necessity for the preactivation of the formylating agents, high temperatures, and anhydrous conditions. Nowadays, decarboxylative cross-coupling reaction has developed as an efficient method for the construction of carbon−carbon or carbon−heteroatom bonds.13 Formylation through decarboxylative cross-coupling of a glyoxylic acid is envisioned as a promising alternative protocol, since glyoxylic acid is stable and readily available. However, it was not explored until very recently; Wang and Xu et al. have developed several formylation protocols by employing diethoxyacetic acid14 or glyoxylic acid15 as a formyl equivalent. Nevertheless, C−C bond formations are targeted for these works. Since Kolbe electrolysis was discovered, anodic oxidative decarboxylation of a carboxylic acid has become a significant method allowing one-step C−C or C−heteroatom bond formations in electrooganic synthesis, which is difficult to be achieved through other routes.16 In spite of the convenience and the green aspects that electrochemistry contributes to contemporary organic synthesis,17,18 its utilizations in decarboxylative cross-coupling reactions remain challenging, and the present processes are yet limited. To the best of our knowledge, the electrochemical formylation through decarboxylation of © XXXX American Chemical Society
Received: March 1, 2018
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DOI: 10.1021/acs.orglett.8b00698 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 1. Optimization of Reaction Conditionsa
Scheme 1. Scope of the N-Formylation of Aminesa,b
entry
variation from the standard conditions
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13c 14c 15d 16 17 18
none in the absence of Cu(OAc)2·2H2O in the absence of NiCl2·6H2O CuCl instead of Cu(OAc)2·2H2O in the absence of Cs2CO3 NaOAc instead of Cs2CO3 DBU instead of Cs2CO3 NaBF4 instead of NaClO4 LiClO4 instead of NaClO4 n-Bu4NBF4 instead of NaClO4 DMF instead of DMSO MeCN instead of DMSO graphite rods as anode graphite rods as cathode Ni foam as cathode 10 mA instead of 5 mA, 5 h 2.5 mA instead of 5 mA, 20 h no electric current
95 65 79 63 trace 78 64 69 32 73 72 trace 65 61 76 55 82 N.R.
Standard conditions: 1a (0.3 mmol), 2 (0.9 mmol), Cu(OAc)2·2H2O (10 mol %), NiCl2·6H2O (20 mol %), Cs2CO3 (0.45 mmol), DMSO (5 mL) with 0.1 M NaClO4 as electrolyte, Pt foils (1.0 × 1.5 cm2) as anode and cathode, undivided cell, constant current = 5 mA, 10 h, room temperature. bThe yield of the product was determined by 1H NMR spectroscopy; N.R. = no reaction. cGraphite rods (diameter: 0.5 cm, height: 1.78 cm). dNi foam electrode (length, 1.5 cm; width, 1.0 cm; thickness, 0.01 cm). a
a Standard conditions: 1 or 4 (0.3 mmol), 2 (0.9 mmol), Cu(OAc)2· 2H2O (10 mol %), NiCl2·6H2O (20 mol %), Cs2CO3 (for secondary amines: 0.45 mmol; for primary amines: 0.3 mmol), DMSO (5 mL) with 0.1 M NaClO4 as electrolyte, Pt foils (1.0 × 1.5 cm2) as anode and cathode, undivided cell, constant current = 5 mA, 10 h, room temperature (see the Supporting Information for details). bIsolated yield.
Cs2CO3 was necessary for the N-formylation of primary amines (Table S5, entry 3; see SI for details). With the optimized conditions in hand, we examined the scope of the N-formylation reactions by testing a series of primary and secondary amines containing different substituents on the phenyl ring (Scheme 1). The reactions of secondary amines with both an electron-releasing group (ERG) and an electron-withdrawing group (EWG) could gain the desired products in satisfying yields. It is worthwhile to note that cyclic amines, such as indoline, piperazine, and tetrahydroisoquinoline, as well as morpholine could transform into the formylation products in good to excellent yields (3g−3j). For the Nisopropylaniline, unfortunately, no reaction occurred under the optimized conditions, which indicated that a bulky substituent on the nitrogen atom might play a negative impact. When it turned to primary amines, a variety of functionalities were tolerated, including methyl (5b), methoxyl (5c), trifluoromethoxyl (5d), trifluoromethyl (5e−5f), nitro (5g), cyano (5h), ester (5i), ketone (5j), and halogen groups (5k−5o). Heterocyclic amines, for instance, 2-aminopyridine and methyl 3-aminothiophene-2-carboxylate, were also compatible with the current protocol and afforded the products in good yields (5p− 5q). It is good to see that this protocol is also applicable to the aliphatic amines (5r). To understand the mechanism for this reaction, some control experiments were implemented. A reaction was conducted under N2 atmosphere, and no change was observed in terms of yield (Scheme 2A). This result indicated that the molecular oxygen was not crucial for this transformation. Then, a H218O
Scheme 2. Control Experimental Studies
control experiment was carried out (Scheme 2B). When 100 μL of H218O was added to the reaction system in N2 atmosphere, two products were obtained in a total yield of 95% (3a and 3a′, in the ratio of 5:6; see SI). This result demonstrated that the oxygen source in the final products was the water in the system. Next, when a radical scavenger (RS), TEMPO or 1,1diphenylethene, was added into the reaction mixture under the standard conditions, the desired product was obtained in 64% and 75% yields, respectively (Scheme 2C). Neither 1a nor any radical trapping adducts were detected by TLC, GC−MS, B
DOI: 10.1021/acs.orglett.8b00698 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
further removed rapidly due to the electron-donating substituent located at the α position. The proton released subsequently to produce the desired formylation product 10a. Meanwhile, the Cu(II) was regenerated by anodic oxidation. NiCl2 was proposed to act as a Lewis acid to promote the imine condensation. On the cathode, the proton was reduced into hydrogen. In conclusion, we have developed a highly efficient Nformylation of amines by electrochemical decarboxylation of glyoxylic acid. The method demonstrated a broad substrate scope, including primary/secondary amines and aliphatic/ aromatic amines, with excellent functional group tolerance. This electrochemical protocol, which proceeded smoothly under ambient conditions, provides a new and sustainable alternative for the N-formylating reaction. Further investigations into the mechanistic details and synthetic applications are currently underway in our laboratory.
and NMR analysis. At the current stage, the radical mechanism should not be ruled out (see SI). Cyclic voltammetry (CV) experiments were also implemented (Figure 1; for more details,
Figure 1. Cyclic voltammograms of 0.1 M NaClO4 solution in DMSO at room temperature. (a) None; (b) N-methylaniline (0.03 M); (c) glyoxylic acid (0.09 M) + Cs2CO3 (0.045 M); (d) b + c; Cu(OAc)2 (0.003 M); reaction mixture: glyoxylic acid (0.01 M) + Nmethylaniline (0.01 M) + Cs2CO3 (0.01 M) in DMSO. The voltammogram was obtained with Pt wire as auxiliary electrode and a saturated calomel electrode (SCE) as a reference electrode. The scan rate was 0.1 V/s on a platinum disk electrode (d = 2 mm).
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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/acs.orglett.8b00698. Experimental procedures and spectroscopic data (PDF)
see the SI). It was observed that the oxidation potential of the glyoxylic acid in the presence of cesium carbonate is Ep = 0.80 V vs SCE (Figure 1A, curve b), while the oxidation potential of N-methylaniline is Ep = 0.67 V vs SCE (Figure 1A, curve c). It is interesting to find that when the glyoxylic acid, aniline, and cesium carbonate were introduced into the solution, a new oxidation peak at Ep = 0.76 V vs SCE appeared (Figure 1A, curve d). This peak could be attributed to a condensation product of 1a and 2, which was oxidized at a relatively lower potential than glyoxylate to facilitate the process of decarboxylation.21 Meanwhile, the catalytic current of Cu(OAc)2 was observed (Figure 1B). This result indicated that the Cu(OAc)2 might act as an active oxidant and that the high valent copper is regenerated by anodic oxidation. Based on the mechanistic investigations above and the reported works,22 a plausible pathway for this decarboxylative formylation process is proposed (Scheme 3). Glyoxylic acid 2 was first transformed into carboxylate ion 6a by cesium carbonate and then condensed with the aniline to form 7a. The intermediate 7a was oxidized by cupric acetate, followed by the decarboxylation to generate 9a. Another electron of 9a was
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jing-Mei Huang: 0000-0003-2861-3856 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (Grant Nos. 21672074 and 21372089) for financial support.
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REFERENCES
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Scheme 3. Possible Reaction Pathway
C
DOI: 10.1021/acs.orglett.8b00698 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
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DOI: 10.1021/acs.orglett.8b00698 Org. Lett. XXXX, XXX, XXX−XXX
