Transition-Metal-Free Decarboxylative Amidation of Aryl α-Keto Acids with Diphenylphosphoryl Azide: New Avenue for the Preparation of Primary Aryl Amides

Jianwei Xie Li Shen Jie Zhang Shaofeng Gong

Citation:  Xie Jianwei, Shen Li, Zhang Jie, Gong Shaofeng. Transition-Metal-Free Decarboxylative Amidation of Aryl α-Keto Acids with Diphenylphosphoryl Azide: New Avenue for the Preparation of Primary Aryl Amides[J]. Chinese Journal of Organic Chemistry, 2020, 40(12): 4284-4289. doi: 10.6023/cjoc202006030 shu

无过渡金属条件下α-芳香酮酸与叠氮磷酸二苯酯脱羧酰胺化制备芳香伯酰胺

    通讯作者: 谢建伟, cesxjw@foxmail.com
    龚绍峰, simon.gong@huse.edu.cn
  • 基金项目:

    国家自然科学基金 21868032

    新疆兵团材料化工工程技术研究中心开放基金 2019BTRC001

    国家自然科学基金(No.21868032)和新疆兵团材料化工工程技术研究中心开放基金(No.2019BTRC001)资助项目

摘要: 报道了一种无过渡金属条件下,α-芳香酮酸和叠氮磷酸二苯酯(DPPA)发生脱羧偶联制备芳香伯酰胺的方法.该方法条件温和,无需惰性气体保护,各种取代芳香α-酮酸均可顺利反应得到优良的分离产率,体现了较好的官能团适应性.克量级实验在该反应条件下也可顺利实现,以高产率生成目标产物,同时对反应机理也进行了探究.

English

  • Primary aromatic amides are important structural motifs prevalent in a wide range of class of compounds in natural products, materials, pharmaceuticals, agrochemicals and polymers.[1] In addition, amides are very good functional groups in organic synthesis, because they can be easily transformed into nitriles, primary amines and heterocycles.[2] Because of their importance, numerous methodologies have been developed for their synthesis. Some examples include ammonolysis of carboxylic acids, rearrangement of aldoximes, metal-catalyzed carbonylation of organohalides with ammonia, direct oxidation of benzyl amines or benzyl alcohol, and hydration of nitriles.[3] Of these methods, the ammonolysis of activated carboxylic acid species is still the traditional method for synthesis of primary amides. However, this procedure requires pre-activation of the free carboxylic acid with stoichiometric activating reagents, and corrosive by-products are often generated during the reaction. Thus, the development of greener and more efficient methods to prepare amides from carboxylic acids is still a challenge for synthetic organic chemistry.

    On the other hand, carboxylic acids are readily available, and are usually used as key substrates for organic synthesis.[4] Currently, transition-metal-catalyzed decarboxylative couplings have attracted much attention due to readily accessible starting materials, simple operation, and clean byproduct (CO2 as the only byproduct). It should be noted that the preparation of functionalized acyl compounds via the decarboxylation of α-keto acids is of great interest.[5] Since the first report on Cu/Pd-catalyzed decarboxylative acylation of aryl bromides with α-keto acids, which was considered as an acyl anion equivalent to afford diaryl ketones, [5a] extensive studies have been carried out. So far, α-keto acids have been successfully utilized as acyl nucleophiles in transition-metal-catalyzed decarboxylative coupling reactions to form C—C bonds.[4b, 6] However, the formation of C—N bonds, which led to much more efficient protocols for the desired target molecules, has received less attention. In 2014, Lei's group[7] developed a visible-light-mediated decarboxylation/oxidative amidation of α-keto acids with amines for the formation of amides (Scheme 1, a). In 2016, Xu's group[8] reported two approaches for the same transformation by photocatalysis and Ag catalysis, respectively (Scheme 1, b). In the same year, Wang's group[9] described a decarboxylative amidation of α-keto acids with N-substituted N-heteroarene-2-car- boxamides in the presence of Pd(OAc)2 and K2S2O8 (Scheme 1, c). And then, Yotphan's group[10] demonstrated a copper-catalyzed oxidative decarboxylative coupling of α-keto acids with NH-sulfoximines (Scheme 1, d). In addition, Ilangovan and co-workers reported a one-pot procedure for the conversion of 2-iodo/2-nitro benzoic acids into the corresponding anilines using tosyl azide as the nitrogen resource. And the substituent of 2-iodo or 2-nitro in benzoic acids is the necessary group for the success of this reaction[11] (Scheme 1, e).

    Scheme 1

    Scheme 1.  Previous decarboxylative amidation of α-keto acids methods and our work

    These reactions are mostly suitable for preparation of secondary and tertiary amides or primary anilines via decarboxylative amidation strategies from α-keto acids or benzoic acids, but the direct decarboxylative coupling of α-keto acids with azide reagents to form primary amides has never been reported. Inspired and encouraged by these pioneering studies and with part of our continuous interest in C—N bond formation reaction, [12] we herein report a novel transition-metal-free decarboxylative amidation of α-keto acids with diphenylphosphoryl azide (DPPA) to preparation primary aryl amides under mild conditions (Scheme 1, f).

    In our initial experiment, phenylglyoxylic acid (1a) and DPPA were chosen as the model substrates to optimize reaction conditions including azide reagents and its dosage, bases, solvent, reaction temperature and time. As shown in Table 1, the reaction between 1a and DPPA in the presence of 2 equiv. of K2CO3 in N, N-dimethylformamide (DMF) at 100 ℃ for 24 h afforded benzamide (3a) in 75% yield (Entry 1). Azide reagents screening showed that DPPA gave the best isolate yield of the desired product, while tosyl azide and TMSN3 gave trace yield or no reaction (Entries 2 and 3). Decreasing the quantity of DPPA led to lower yield (Entry 4). Subsequently, the base was found to have an important effect on the reactivity (Entries 1 and 5~7). Among the bases tested, K3PO4 was found to be the best with 91% yield. Investigation of a variety of solvent, including DMF, DMSO, NMP, H2O and mixed solvent DMF/H2O (V:V=1:1), indicated DMF to be the best one (Entries 6 and 8~11). Furthermore, lower yields were observed when decreasing reaction temperature or shortening reaction time (Entries 12 and 13). In summary, the optimal conditions for this decarboxylative amidation were established from the reaction of 1a (1 equiv.) with DPPA (2 equiv.) in the presence K3PO4 (2.0 equiv.) in 2 mL of DMF at 100 ℃ for 24 h, affording 3a in 91% yield (Entry 6).

    Table 1

    Table 1.  Screening reaction conditions for decarboxylative ami- dation of phenylglyoxylic acid with azide reagentsa
    下载: 导出CSV
    Entry Azide Base Solvent Yieldb/%
    1 DPPA K2CO3 DMF 75
    2 TsN3 K2CO3 DMF trace
    3 TMSN3 K2CO3 DMF NR
    4 DPPA K2CO3 DMF 70c
    5 DPPA Cs2CO3 DMF 77
    6 DPPA K3PO4 DMF 91
    7 DPPA NaHCO3 DMF 73
    8 DPPA K3PO4 DMSO 75
    9 DPPA K3PO4 NMP 58
    10 DPPA K3PO4 H2O NR
    11 DPPA K3PO4 DMF/H2O (V:V=1:1) NR
    12 DPPA K3PO4 DMF 71d
    13 DPPA K3PO4 DMF 76e
    a Reaction conditions: 1a (0.5 mmol), 2 (1.0 mmol), base (1.0 mmol), solvent (2 mL), 100 ℃, 24 h; b Isolated yield; c DPPA (0.6 mmol); d 80 ℃; e 12 h.

    With the optimized reaction conditions in hand, the substrate scope of this decarboxylative amidation of α-keto acids with DPPA was next explored (Table 2). The results indicated that various functional groups, including both electron-donating and electron-withdrawing groups on the para-position of the benzene rings in 2-oxo-2-arylacetic acids were well tolerated, and the desired amidation products were obtained in good to excellent yields. Notably, the electron-donating groups, such as MeO and Me, exhibited higher activity, providing the corresponding products (3b~3c) in 90% and 92% yields. On the other hand, the electron-withdrawing groups, such as halogen groups (Cl, F, Br and I), CF3 and NO2 showed good reactivity to the reaction, affording the desired products (3d~3i) in 72%~95% yields, which provide useful handles for further transformations through traditional reactions. Furthermore, it was found that meta-substituted 2-oxo-2-phenylacetic acids were also reacted well to generate 3j~3m in 71%~93% yields. Notably, ortho-substituted substrate underwent this decarboxylation process smoothly to afford the corresponding product in 78% yields (3n). To our delight, 3, 4- or 2, 4-disubstituted 2-oxo-2-phenylacetic acids can react with DPPA in 78%~79% yields under the present conditions (3o~3q). It should be noted that when 2-(naphtha- len-1-yl)-2-oxoacetic acid and 2-(naphthalen-2-yl)-2-oxo- acetic acid were employed to react with DPPA, the corresponding imides 3r and 3s were afforded in 85% and 78% yields, respectively. Surprisingly, heterocyclic 2-oxo-2- (thiophen-2-yl)acetic acid could display good reactivity and provide the desired product 3t in 93% yield without prolonging the reaction time or raising the reaction temperature. Unfortunately, the reactions of aliphatic α-keto acids did not work well under the standard condition because of low conversion (3u and 3v).

    Table 2

    Table 2.  One pot synthesis of primary amides from α-keto acids and DPPAa
    下载: 导出CSV
    a Reaction conditions: 1 (0.5 mmol), DPPA (1.0 mmol), K3PO4 (1.0 mmol), DMF (2 mL), 100 ℃, 24 h; isolated yield; b DPPA (1.5 mmol); c 48 h.

    To evaluate the scalability of the catalytic system, 10 mmol scale of phenylglyoxylic acid was used to perform the decarboxylative amidation, and the product 3a was obtained in 81% yield, which provides a reference for industrial production (Scheme 2).

    Scheme 2

    Scheme 2.  Gram-scale synthesis of 3a

    In order to understand the mechanism of the present reaction, radical inhibition experiments were performed (Scheme 3). When 2 equiv. of radical scavenger such as 2, 2, 6, 6-tetramethyl-1-oxylpiper-idine (TEMPO) or 2, 6-bis- (1, 1-dimethylethyl)-4-methyl-phenol (BHT) was added to the mixture solution, the decarboxylative amidation reaction proceeded as normal with comparable efficiency and gave 3a in similar yield of the standard condition. These results suggested that the reaction might not proceed through a radical decarboxylation. Therefore, it is believed that the reaction is carried out through a Curtius rearrangement pathway (Scheme 4). First, α-keto acid reacts with DPPA to form acyl azide intermediate under the action of base, and then loses one molecule of N2 to form azobenzene intermediate under heating condition, rearranges to form benzoyl isocyanate intermediate through intramolecular rearrangement, followed by reaction with water to the unstable acylcarbamic acid derivative which will undergo decarboxylation to form benzamide and release one molecule of CO2.

    Scheme 3

    Scheme 3.  Validation experiments

    Scheme 4

    Scheme 4.  Plausible mechanism

    In conclusion, we have established a simple, transition-metal-free protocol for the decarboxylative amidation of α-keto acids with DPPA. This reaction provides a new synthetic method and idea for the synthesis of primary aryl amides, and twenty aromatic primary amides were obtained in 71%~95% yields. The convenient operation, no transition-metal catalysts and scalability are the salient features, which render this method viable for both academic research and industrial processes.

    Commercial reagents were purchased from Adamas, Ark Pharm, Innochem, Acros Organics and used as received unless otherwise noted. DMF was purchased from Fuyu Fine Chemical Co., Ltd, and was distilled under reduced pressure before used. The α-keto acids 1a, 1t, 1u and 1v were obtained from commercial suppliers, and the others were prepared from oxidation of corresponding methyl ketones by SeO2 according to the reported procedure.[13] All reactions were carried out under air atmosphere. NMR spectra were measured on a Bruker Avance Ⅲ HD 400 spectrometer. 1H NMR spectra were recorded at 400 MHz in NMR solvents and referenced internally to corresponding solvent resonance, and 13C NMR spectra were recorded at 100 MHz and referenced to corresponding solvent resonance. Column chromatography was performed with silica gel (200~300 mesh) purchased from Qingdao Haiyang Chemical Co. Ltd.

    α-Keto acid (0.5 mmol), DPPA (1.0 mmol), and K3PO4 (1.0 mmol) were placed in a schlenk tube equipped with a stirring bar. The solvent DMF (2 mL) was added under air atmosphere. The reaction was carried out at 100 ℃ for 24 or 48 h. After the reaction, the mixture was quenched with saturated sodium bicarbonate (10 mL) and extracted with ethyl acetate (10 mL×3). The organic layers were combined and concentrated in vacuo. The product was purified by flash column chromatography on silica gel [V(petro-leum ether)/V(ethyl acetate)=2/1] to afford target products 3a~3t.

    Benzamide (3a):[14] 91% yield. 1H NMR (400 MHz, DMSO-d6) δ: 7.98 (s, 1H), 7.90~7.86 (m, 2H), 7.57~7.49 (m, 1H), 7.49~7.42 (m, 2H), 7.37 (s, 1H); 13C NMR (100 MHz, DMSO-d6) δ: 168.4, 134.7, 131.7, 128.7, 127.9.

    4-Methoxybenzamide (3b):[15] 90% yield. 1H NMR (400 MHz, DMSO-d6) δ: 7.97~7.74 (m, 3H), 7.19 (s, 1H), 7.05~6.91 (m, 2H), 3.80 (s, 3H); 13C NMR (100 MHz, DMSO-d6) δ: 167.9, 162.0, 129.8, 127.0, 113.8, 55.8.

    4-Methylbenzamide (3c):[15] 92% yield. 1H NMR (400 MHz, DMSO-d6) δ: 7.90 (s, 1H), 7.78 (d, J=8.1 Hz, 2H), 7.25 (d, J=8.0 Hz, 3H), 2.35 (s, 3H); 13C NMR (100 MHz, DMSO-d6) δ: 168.3, 141.5, 132.0, 129.2, 128.0, 21.4.

    4-Fluorobenzamide (3d):[15] 95% yield. 1H NMR (400 MHz, DMSO-d6) δ: 8.00 (s, 1H), 7.97~7.92 (m, 2H), 7.40 (s, 1H), 7.31~7.25 (m, 2H); 13C NMR (100 MHz, DMSO- d6) δ: 167.3, 164.4 (J=246.8 Hz), 131.2 (J=2.9 Hz), 130.6 (J=9.0 Hz), 115.5 (J=21.6 Hz).

    4-Chlorobenzamide (3e):[15] 79% yield. 1H NMR (400 MHz, DMSO-d6) δ: 8.05 (s, 1H), 7.92~7.88 (m, 2H), 7.55~7.51 (m, 2H), 7.47 (s, 1H); 13C NMR (100 MHz, DMSO-d6) δ: 167.3, 136.5, 133.5, 129.9, 128.8.

    4-Bromobenzamide (3f):[15] 80% yield. 1H NMR (400 MHz, DMSO-d6) δ: 8.05 (s, 1H), 7.84~7.80 (m, 2H), 7.69~7.65 (m, 2H), 7.46 (s, 1H); 13C NMR (100 MHz, DMSO-d6) δ: 167.4, 133.9, 131.7, 130.1, 125.5.

    4-Iodobenzamide (3g):[15] 85% yield. 1H NMR (400 MHz, DMSO-d6) δ: 8.03 (s, 1H), 7.84 (d, J=8.4 Hz, 2H), 7.65 (d, J=8.4 Hz, 2H), 7.43 (s, 1H); 13C NMR (100 MHz, DMSO-d6) δ: 167.7, 137.6, 134.2, 129.9, 99.4.

    4-Trifluoromethylbenzamide (3h):[16] 72% yield. 1H NMR (400 MHz, DMSO-d6) δ: 8.24 (s, 1H), 8.11 (d, J=8.0 Hz, 2H), 7.88 (d, J=8.0 Hz, 2H), 7.67 (s, 1H); 13C NMR (100 MHz, DMSO-d6) δ: 167.2, 138.6 (d, J=0.9 Hz), 131.6 (q, J=1.6 Hz), 128.8, 125.7 (q, J=3.9 Hz), 123.1.

    4-Nitrobenzamide (3i):[17] 80% yield. 1H NMR (400 MHz, DMSO-d6) δ: 8.31 (d, J=8.8 Hz, 3H), 8.11 (d, J=8.8 Hz, 2H), 7.74 (s, 1H); 13C NMR (100 MHz, DMSO-d6) δ: 166.7, 149.5, 140.5, 129.4, 123.9.

    3-Methylbenzamide (3j):[14] 87% yield. 1H NMR (400 MHz, DMSO-d6) δ: 7.96 (s, 1H), 7.75 (s, 1H), 7.73~7.70 (m, 1H), 7.39~7.37 (m, 2H), 7.35 (s, 1H), 2.40 (s, 3H). 13C NMR (100 MHz, DMSO-d6) δ: 168.5, 137.9, 134.7, 132.2, 128.5, 125.1, 21.4.

    3-Iodobenzamide (3k):[18] 71% yield. 1H NMR (400 MHz, DMSO-d6) δ: 7.86 (s, 1H), 7.64 (d, J=8.0 Hz, 1H), 7.56 (s, 1H), 7.43~7.40 (m, 2H), 7.36~7.32 (m, 1H); 13C NMR (100 MHz, DMSO-d6) δ: 169.5, 139.8, 133.2, 131.1, 129.0, 128.0, 119.1.

    3-Fluorobenzamide (3l):[19] 93% yield. 1H NMR (400 MHz, DMSO-d6) δ: 8.07 (s, 1H), 7.73 (d, J=7.6 Hz, 1H), 7.69~7.65 (m, 1H), 7.54~7.48 (m, 2H), 7.40~7.35 (m, 2.4 Hz, 1H); 13C NMR (100 MHz, DMSO-d6) δ: 166.9 (d, J=2.4 Hz), 162.4 (d, J=242.5 Hz), 137.1 (d, J=6.6 Hz), 130.8 (d, J=7.9 Hz), 124.1 (d, J=2.7 Hz), 118.6 (d, J=21.1 Hz), 114.7 (d, J=22.4 Hz).

    3-Nitrobenzamide (3m):[20] 75% yield. 1H NMR (400 MHz, DMSO-d6) δ: 7.43 (s, 1H), 7.36~7.29 (m, 2H), 7.00 (d, J=7.6 Hz, 1H), 5.85 (s, 2H); 13C NMR (100 MHz, DMSO-d6) δ: 150.6, 149.2, 130.4, 120.4, 110.2, 107.5.

    2-Bromobenzamide (3n):[15] 78% yield. 1H NMR (400 MHz, DMSO-d6) δ: 7.86 (s, 1H), 7.65 (d, J=8.0 Hz, 1H), 7.56 (s, 1H), 7.44~7.40 (m, 2H), 7.37~7.30 (m, 1H); 13C NMR (100 MHz, DMSO-d6) δ: 169.5, 139.8, 133.2, 131.1, 129.0, 128.0, 119.1.

    3, 4-Dimethoxybenzamide (3o):[16] 78% yield. 1H NMR (400 MHz, DMSO-d6) δ: 7.90 (s, 1H), 7.49 (d, J=8.0 Hz, 2H), 7.26 (s, 1H), 7.02 (s, 1H), 3.82 (s, 6H); 13C NMR (100 MHz, DMSO-d6) δ: 167.9, 151.7, 148.6, 127.0, 121.2, 111.4, 111.2, 56.0.

    3, 4-Dimethylbenzamide (3p):[17] 79% yield. 1H NMR (400 MHz, DMSO-d6) δ: 7.84 (s, 1H), 7.67 (d, J=1.6 Hz, 1H), 7.59 (dd, J=7.6, 1.6 Hz, 1H), 7.21 (s, 1H), 7.19 (d, J=7.6 Hz, 1H), 2.25 (s, 6H); 13C NMR (100 MHz, DMSO-d6) δ: 168.4, 140.2, 136.5, 132.3, 129.7, 129.1, 125.4, 19.9, 19.8.

    2, 4-Dichlorobenzamide (3q):[21] 78% yield. 1H NMR (400 MHz, DMSO-d6) δ: 7.93 (s, 1H), 7.67 (t, J=1.2 Hz, 2H), 7.47 (d, J=1.2 Hz, 2H); 13C NMR (100 MHz, DMSO-d6) δ: 167.7, 136.5, 134.7, 131.4, 130.5, 129.6, 127.7.

    α-Naphthalamide (3r):[22] 85% yield. 1H NMR (400 MHz, DMSO-d6) δ: 8.37~8.35 (m, 1H), 8.05~7.99 (m, 3H), 7.96 (dd, J=7.2, 1.2 Hz, 1H), 7.63 (d, J=1.6 Hz, 1H), 7.62~7.55 (m, 3H); 13C NMR (100 MHz, DMSO-d6) δ: 171.1, 135.1, 133.7, 130.2, 130.1, 128.6, 127.1, 126.6, 126.1, 125.6, 125.4.

    β-Naphthalamide (3s):[22] 78% yield. 1H NMR (400 MHz, DMSO-d6) δ: 8.51 (s, 1H), 8.16 (s, 1H), 8.02 (dd, J=7.2, 2.4 Hz, 1H), 8.00~7.97 (m, 3H), 7.64~7.57 (m, 2H), 7.49 (s, 1H); 13C NMR (100 MHz, DMSO-d6) δ: 168.4, 134.6, 132.6, 132.1, 129.3, 128.3, 128.2, 128.1, 128.0, 127.1, 124.9.

    Thiophene-2-carboxamide (3t):[14] 93% yield. 1H NMR (400 MHz, DMSO-d6) δ: 7.96 (s, 1H), 7.75~7.73 (m, 2H), 7.38 (s, 1H), 7.13 (q, J=4.2 Hz, 1H); 13C NMR (100 MHz, DMSO-d6) δ: 163.4, 140.8, 131.4, 129.1, 128.3.

    Supporting Information   The 1H NMR and 13C NMR spectra of products 3a~3t. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn/.


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  • Scheme 1  Previous decarboxylative amidation of α-keto acids methods and our work

    Scheme 2  Gram-scale synthesis of 3a

    Scheme 3  Validation experiments

    Scheme 4  Plausible mechanism

    Table 1.  Screening reaction conditions for decarboxylative ami- dation of phenylglyoxylic acid with azide reagentsa

    Entry Azide Base Solvent Yieldb/%
    1 DPPA K2CO3 DMF 75
    2 TsN3 K2CO3 DMF trace
    3 TMSN3 K2CO3 DMF NR
    4 DPPA K2CO3 DMF 70c
    5 DPPA Cs2CO3 DMF 77
    6 DPPA K3PO4 DMF 91
    7 DPPA NaHCO3 DMF 73
    8 DPPA K3PO4 DMSO 75
    9 DPPA K3PO4 NMP 58
    10 DPPA K3PO4 H2O NR
    11 DPPA K3PO4 DMF/H2O (V:V=1:1) NR
    12 DPPA K3PO4 DMF 71d
    13 DPPA K3PO4 DMF 76e
    a Reaction conditions: 1a (0.5 mmol), 2 (1.0 mmol), base (1.0 mmol), solvent (2 mL), 100 ℃, 24 h; b Isolated yield; c DPPA (0.6 mmol); d 80 ℃; e 12 h.
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    Table 2.  One pot synthesis of primary amides from α-keto acids and DPPAa

    a Reaction conditions: 1 (0.5 mmol), DPPA (1.0 mmol), K3PO4 (1.0 mmol), DMF (2 mL), 100 ℃, 24 h; isolated yield; b DPPA (1.5 mmol); c 48 h.
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  • 发布日期:  2020-12-25
  • 收稿日期:  2020-06-17
  • 修回日期:  2020-07-13
  • 网络出版日期:  2020-07-30
通讯作者: 陈斌, bchen63@163.com
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