Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Synthesis of 3‑Formylindoles via Electrochemical Decarboxylation of Glyoxylic Acid with an Amine as a Dual Function Organocatalyst 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
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S Supporting Information *
ABSTRACT: A new method for 3-formalytion of indoles has been developed through electrochemical decarboxylation of glyoxylic acid with the amine as a dual function organocatalyst. The amine facilitated both the electrochemical decarboxylation and the nucleophilic reaction efficiently, whose loading can be as low as 1 mol %. This protocol has a broad range of functional group tolerance under ambient conditions. The gram-scale experiment has shown great potential in the synthetic application of this strategy. t has been proved that α-iminocarboxylate (or α-iminium carboxylate) is more favorable to undergo decarboxylation than the corresponding α-oxocarboxylate salt.1 By employing this strategy, our group has recently developed an electrochemical route for the synthesis of formamides from glyoxylic acid and amines.2 In addition, iminium ion formation is a classic mode for carbonyl activation toward nucleophilic attack.3 Inspired by the earlier work, we anticipated that an amine could serve as a dual function catalyst to accomplish a new C−C bond formation: the amine first condenses with the glyoxylic acid to form the α-iminium carboxylate species, followed by the anodic decarboxylation, the loss of another electron, attack of a nucleophile, and then the hydrolysis to afford the formylation product. Meanwhile, the amine is released to furnish the catalytic cycle (Scheme 1). In this process, it is envisioned that by the formation of the α-iminium carboxylate, the activation barrier of the decarboxylation could be lowered and the reactivity of the nucleophilic attack could be increased at the same time. However, two challenges arising from the introduction of the aminocatalyst have to be overcome: (1) the oxidative stability of the aminocatalyst and (2) outcompetition of homogeneous side reactions (such as hydrolysis) with heterogeneous oxidations at the anode of the catalytically generated electro-active species which is at a low concentration. 3-Formylindoles and their derivatives are key intermediates for the preparation of biologically active molecules since the formyl group can undergo a variety of transformations to other functionalities.4 Traditional methods for C-3 formylation of indoles, for example, Vilsmeier−Haack,5 Reimer−Tiemann,6 Rieche,7 and Duff8 reaction, suffer from one or more drawbacks, including the usage of an environmentally
I
© XXXX American Chemical Society
Scheme 1. Hypothesis of Amino-Catalyzed Formylation via Decarboxylation of Glyoxylic Acid
unfriendly reagent4f,9 (i.e., POCl3), harsh reaction conditions (e.g., elevated temperature, workup processes with excess strong bases4f,9a,d,10 or acids11), low selectivity,9b,c and lack of functionality tolerance.4d−f,9d,11,12 Although several new strategies were developed to produce 3-formylindoles in recent years,13 a more general and milder method to access 3formylindoles using environmentally benign reagents is still desired. Due to its reactive and readily available properties, glyoxylic acid has great potential in organic synthesis. However, the usage of glyoxylic acid as a C1 synthon through decarboxReceived: June 7, 2019
A
DOI: 10.1021/acs.orglett.9b01971 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters ylation was seldom reported.3,14 On the other hand, electroorganic synthesis has been demonstrated to be an efficient and eco-friendly synthetic strategy over the past decades.15 In continuation of our interest in organic electrosynthesis and our previous work,3,16 we herein report a highly efficient 3formylation of indoles by the combination of aminocatalysis with electrochemical decarboxylative C−C bond formation using glyoxylic acid as a formyl source. We started our investigations by treating N-methylindole (1a) with glyoxylic acid (2) in the presence of the aminocatalyst (A) under current density at j = 3.3 mA/cm2 in an undivided cell (Table 1; please refer to Supporting
1-methyl-3-formylindole (3a) increased to 71% when aniline (A7) was utilized as a catalyst (Table 1, entry 7). It was interesting to find that 1 mol % dimethylamine (40 wt % solution in water, A8) gained an efficient result on this reaction, with a 94% yield of the formylating product (Table 1, entry 8). However, the yield of the desired product dropped sharply to 14% when the aqueous solution was replaced by a THF solution (Table 1, entry 9), which indicated that the addition of H2O might be of significance in this approach.17 Hence, for the reaction with aniline as an aminocatalyst, 100 μL of H2O were added into the reaction system. As we expected, the yield of the desired product increased to 85% (Table 1, entry 10). Herein, the dimethylamine showed greater efficiency than other screened aliphatic amines, while aniline showed better catalytic ability than other screened aromatic amines (see SI for details). Overall, the dimethylamine and aniline showed relatively stronger nucleophilicity, less steric hindrance, and higher oxidation potential. Next, further studies were carried out on the dimethylamine catalyst system. When DMSO was replaced by DMF or MeCN, no desired product could be collected (Table 1, entries 11−12). The reaction efficiency decreased when the graphite rod was used as the anode or cathode (Table 1, entries 13−14). The supporting electrolyte also affected the reaction. A perchlorate salt showed a better result than a tetrafluoroborate salt (Table 1, entries 15−16). The yield of the 3a dropped slightly when the electric current density was increased to j = 4.67 mA/cm2 (Table 1, entry 17), while the yield of the 3a was not obviously affected when the current density was decreased to j = 2 mA/cm2 (Table 1, entry 18). Predictably, no reaction occurred in the absence of an electric current or an aminocatalyst (Table 1, entries 19−20). With the optimized conditions in hand, we examined the scope of the 3-formylation reactions by testing a series of indoles containing different substituents (Scheme 2). The reactions of N-substituted indoles as well as free NH-indoles with both an electron-releasing group (ERG) and an electronwithdrawing group (EWG) could produce the desired products in satisfying yields. It is worthy to note that steric substituents had little influence in this transformation (3b−3c, 3j). A variety of functionalities on the phenyl ring of indoles including methyl (3d, 3k), methoxyl (3e−3f), nitro (3h, 3l), ester (3m), and halogen groups (3g, 3n−3p) were tolerated, with yields up to 94%. To our delight, indoles with different substituents on the nitrogen atom, such as the butyl group, phenyl group, and benzyl group, could participate in this reaction to afford the corresponding products in 63%, 76%, and 94% yield, respectively (3q−3s). Notably, 7-azaindole could also be transformed into the corresponding formylation product, albeit with a yield of 21% (3t). A gram-scale synthesis was then performed to demonstrate the synthetic utility of this transformation. The formylation of 1a could be scaled up to gram scale smoothly to afford 3a in 74% yield on a 9 mmol scale (Scheme 3; see SI for details). Next, several control experiments were implemented to gain an understanding of the mechanism for this transformation. When the reaction was conducted under a N2 atmosphere, no change was observed in terms of yield (Scheme 4, eq 1). This result suggested that the molecular oxygen was not critical for this transformation. Radical trap experiments indicated that a radical pathway could not be ruled out (see SI for details). Two possible intermediates in the reaction, 4 and 5, were synthesized followed by further control studies. It was found
Table 1. Optimization of Reaction Conditionsa
entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
A (x mol %) A1 (10 A2 (10 A3 (10 A4 (10 A5 (10 A6 (10 A7 (10 A8b (1 A8d (1 A7 (10 A8b (1 A8b (1 A8b (1 A8b (1 A8b (1 A8b (1 A8b (1 A8b (1 A8b (1 none
mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol
%) %) %) %) %) %) %) %) %) %) %) %) %) %) %) %) %) %) %)
variation from the above conditions
yield (%)c
none none none none none none none none none DMSO/H2O (v/v = 50:1) DMF instead of DMSO MeCN instead of DMSO graphite rod as anode graphite rod as cathode KClO4 as electrolyte NaBF4 as electrolyte j = 4.67 mA/cm2, 5 h j = 2.0 mA/cm2, 11 h no electric current none
N.R. N.R. 28 49 13 51 71 94 14 85 N.R. N.R. 73 64 81 45 80 89 N.R. N.R.
a
General conditions: 1a (0.3 mmol), 2 (0.9 mmol), A (10 mol %), DMSO (5 mL) with 0.1 M NaClO4 as electrolyte, Pt foils (1.0 × 1.5 cm2) as anode and cathode, undivided cell, j = 3.3 mA/cm2, 7 h, room temperature. b40 wt % solution in water. cThe yield of the product was determined by 1H NMR spectroscopy; N.R. = no reaction. d2 mol/L in THF solution.
Information (SI) for details). Various factors, such as the nucleophilicity, the steric effect, the oxidation potential of the amine, and so on should be considered during the course of screening the aminocatalysts. Generally, the stronger nucleophilicity, the less steric hindrance, and the higher oxidation potential the amine has, the better the catalytic effect it will have. When the proline (A1) or piperidine (A2) was used as an aminocatalyst, no desired product was detected (Table 1, entries 1−2). Other amines, such as piperazine (A3), morpholine (A4), thiomorpholine (A5), and N-phenylpiperazine (A6) showed inefficiency on this transformation, with the yield from 13% to 51% (Table 1, entries 2−6). The yield of B
DOI: 10.1021/acs.orglett.9b01971 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Scheme 2. Substrate Scopea,b,c
be afforded in the yield of 51% (Scheme 4, eq 3). These results demonstrated that the methanimine species might be the intermediate in the transformation. In other words, the decarboxylation of the α-iminium carboxylate occurred before the nucleophilic attack. Cyclic voltammetry (CV) experiments were also executed to compare the oxidation potential between glyoxylic acid and the corresponding iminiumcarboxylate. Given that the iminiumcarboxylate of dimethylamine with glyoxylic acid was unable to be isolated, the iminocarboxylate (6a) of aniline with glyoxylic acid was synthesized for the CV study. It was observed that the anodic peak potential of 6a was Ep = 0.58 V vs. SCE (Figure 1,
a
Method A: 1 (0.3 mmol), 2 (0.9 mmol), dimethylamine (1 mol %, 40 wt % solution in water), DMSO (5 mL) with 0.1 M NaClO4 as electrolyte, Pt foils (1.0 × 1.5 cm2) as anode and cathode, undivided cell under constant current, room temperature. bMethod B: 1 (0.3 mmol), 2 (0.9 mmol), aniline (10 mol %), DMSO/H2O (v/v = 50:1, 5 mL) with 0.1 M NaClO4 as electrolyte, Pt foils (1.0 × 1.5 cm2) as anode and cathode, undivided cell under constant current, room temperature. cIsolated yield. dj = 3.3 mA/cm2, 7 h. ej = 2.0 mA/cm2, 11 h.
Scheme 3. Gram-Scale Synthesis Figure 1. Cyclic voltammograms of 0.1 M NaClO4 solution in DMSO at room temperature. (a) None; (b) glyoxylic acid (0.03 M); (c) 6a (0.003 M). The voltammogram was obtained with Pt wire as an 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). a
Conditions: 1a (9 mmol, 1.0 equiv), 2 (27 mmol, 3.0 equiv), dimethylamine (1 mol %, 40 wt % solution in water), DMSO (150 mL) with 0.1 M NaClO4 as electrolyte, Pt foils (2.0 × 3.0 cm2) as anode and cathode, undivided cell under 20 mA (j = 3.3 mA/cm2), 25 h, room temperature.
c), while the anodic peak potential of the glyoxylic acid was Ep = 1.2 V vs SCE (Figure 1, b).18 These results have further verified that the formation of the α-iminium carboxylate could promote the decarboxylation process. On the basis of the mechanistic investigations above and the reported works,1,2,19 a plausible pathway for this decarboxylative formylation process is proposed (Scheme 5). Glyoxylic acid 2 first condensed with the aminocatalyst to form the αiminium carboxylate 7a. 7a subsequently went through anodic oxidative decarboxylation to generate 8a. Another electron of 8a was further removed rapidly due to the electron-donating substituent located at the α position, followed by the nucleophilic attack of N-methylindole 1a to produce 9a (path A). 9a then underwent hydrolysis to gain the formylation product 3a and release the amine to furnish the catalytic cycle. A radical addition route was also a possibility in this transformation (path B). After the decarboxylation, 8a underwent addition to the N-methylindole 1a, followed by deprotonation, loss of an electron, and hydrolysis to gain the final formylation product 3a. Since the molecular oxygen did not contribute to this conversion, all the oxidation processes occurred at the anode. In conclusion, we have developed a highly efficient methodology for the synthesis of 3-formylindoles by electrochemical decarboxylation of glyoxylic acid using amine as a dual function catalyst. The method demonstrated a broad
Scheme 4. Control Experimental Studies
that when 4 was utilized as a substrate, only a trace of 3a could be collected (Scheme 4, eq 2). When the reaction started from N-methylindole and 5, the desired formylation product could C
DOI: 10.1021/acs.orglett.9b01971 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
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Scheme 5. Possible Reaction Pathway
substrate scope, including N-substituted indoles as well as free NH-indoles, with an excellent functional group tolerance. The gram-scale experiment has shown great potential in the synthetic utility of this transformation. Further investigations into the mechanistic details and synthetic applications are currently underway in our laboratory.
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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.9b01971. Experimental procedures and spectroscopic data (PDF)
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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 the financial support.
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REFERENCES
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DOI: 10.1021/acs.orglett.9b01971 Org. Lett. XXXX, XXX, XXX−XXX
