Catalytic N-methyl amidation of carboxylic acids under cooperative conditions期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索


Catalytic N-methyl amidation of carboxylic acids under cooperative conditions

Authors:	Li Yingxian Chen Wei Zhao Linchun Zhang Ji-Quan Zhao Yonglong Li Chun Guo Bing Tang Lei Yang Yuan-Yong

Affiliation:	State Key Laboratory of Functions and Applications of Medicinal Plants, School of Pharmacy, Guizhou Provincial Engineering Technology Research Center for Chemical Drug R&D, Guizhou Medical University, 550014 Guiyang P. R. China.; Guizhou Provincial Key Laboratory of Pathogenesis and Drug Research on Common Chronic Diseases, Guizhou Medical University, 550004 Guiyang P. R. China

Abstract:	Amide is a fundamental group that is present in molecular structures of all domains of organic chemistry and the construction of this motif with high atom economy is the focus of the current research. Specifically, N-methyl amides are valuable building blocks in natural products and pharmaceutical science. Due to the volatile nature of methyl amine, the generation of N-methyl amides using simple acids with high atom economy is rare. Herein, we disclose an atom economic protocol to prepare this valuable motif under DABCO/Fe₃O₄ cooperative catalysis. This protocol is operationally simple and compatible with a range of aliphatic and (hetero)aromatic acids with very good yields (60–99%). Moreover, the Fe₃O₄ can be easily recovered and high efficiency is maintained for up to ten cycles. The generation of N-methyl amides using simple acids with high atom economy is rare owning to the volatile nature of methyl amine. Herein, an atom economic protocol was disclosed to prepare this valuable motif under DABCO/Fe_₃O_₄ cooperative catalysis. Amide is a fundamental group that is present in molecular structures of all domains of organic chemistry.¹ It is widely distributed in natural products, synthetic drugs and functional polymers, and is also the key chemical connection in proteins.² It has been shown that amide bond formation alone accounts for 65% of all preliminary screening reactions in the pharmaceutical industry.³ This means the generation of amide bonds with high atom efficiency is of high practical importance. And not surprisingly, ‘amide formation avoiding poor atom economy reagents’ was voted as the top challenge for organic chemistry by the ACS Green Chemistry Institute in 2007.³From synthetic point of view, the ideal way to produce amide bonds would be the direct coupling of readily available carboxylic acids and amines, but this process is thermodynamically unfavourable due to the formation of the corresponding carboxylate-ammonium salt,⁴ therefore, stoichiometric amount of coupling reagents, such as DCC, DIC, EDCI, HATU, HBTU, HCTU, SOCl₂, BOP, acid chloride etc, are generally required to sidestep thermal conditions for amide bond formation.⁵ These reagents are highly successful, but the process generally suffers from poor atom economy and side products removal issue especially in the large-scale applications.⁵ To overcome these drawbacks, “nonclassical” amide bonds formation routes were investigated.⁶ In these processes, the catalyst takes the role of a coupling reagent in generating an active ester suitable for amidation in a waste-free manner. However, these processes have not been applied in the preparation of N-methyl amides, probably because the methyl amine was delivered in its hydrochloride salt, alcoholic or aqueous form due to its volatile nature.On a different note, N-methyl amides are extensively presented in numerous natural products and pharmaceutical molecules, as shown in Fig. 1,⁷ and the methylation of amides is a promising way to improve the pharmacological property of molecules.⁸ However, the synthesis of N-methyl amides compounds relies heavily on non-catalytic approaches.^5,9 Catalytic approaches were also investigated by Hisaeda,¹⁰ Kundu,¹¹ Li,¹² Guo,¹³ Yu,¹⁴ Maruoka,¹⁵ Wang,¹⁶ Chen,¹⁷ Lamaty¹⁸ and their co-workers starting from nitriles, primiary amides, aldoximes, aldehydes, lignin, carbamoylsilane and alcohols. Until recently, Thakur,¹⁹ Marce,²⁰ Sadeghzadeh²¹ and their co-workers developed elegant N-methyl amidation approach starting from carboxylic acids under nano-MgO, diatomite Earth@IL/ZrCl₄ and Mg(NO₃)₂·6H₂O catalysis respectively, while limitations like poor substrate scope or sophisticated tailored catalyst still persist. Mindful of all the above issues, developing an N-methyl amidation process of simple carboxylic acids, which is still of great challenge in synthesis, and establishing a broad (hetero)aryl scope with high atom economy from commercial available reagents and catalysts were critical considerations in this study. Moreover, the significance of N-methyl amides combined with our interests in the development of green synthetic approaches motivated us to explore the direct coupling of the carboxylic acids and isothiocyanates. To the best of our knowledge, this is the first successful work using isothiocyanatomethane to prepare N-methyl amides.Open in a separate window Fig. 1Marketed drugs bearing N-methyl amide group.Our initial investigation begins with phenylacetic acid and isothiocyanatomethane as model substrate for condition optimization. Using acetonitrile as solvent, only trace amount of product was detected under catalyst free or p-toluenesulfonic acid (PTSA) catalysis conditions (
Entry	Additive	Time (h)	Catalyst	Yield (%)
1	—	24	—	5
2	—	24	PTSA	—
3	—	48	TEA	17
4	—	48	DBU	45
5	—	48	DMAP	43
6	—	48	DBN	51
7	—	48	DABCO	65
8	LiBr	48	DABCO	71
9	Mn(OAc)₂	48	DABCO	75
10	MnO	48	DABCO	79
11	MgO	48	DABCO	88
12	Al₂O₃	48	DABCO	85
13	Fe₃O₄	48	DABCO	98
14	Fe₃O₄	24	DABCO	75
15^b	Fe₃O₄	48	DABCO	80

Open in a separate window^aReactions were run on 1 mmol 1a and 1.1 mmol 2a with 10 mol% catalyst and 10 mol% additive in 1 mL of MeCN at 85 °C for 48 hours unless otherwise noted.^bReaction was conducted at 60 °C.Firstly, different acids were employed to react with isothiocyanatomethane and the results were summarized in

Open in a separate window^aReactions were run on 1 mmol 1 and 1.1 mmol 2 with 10 mol% DABCO and 10 mol% Fe₃O₄ in 1 mL of MeCN for 48 hours at 85 °C unless otherwise noted.Subsequently, aromatic and heteroaromatic acids were tested for their compatibility with current reaction conditions and the results were summarized in Fig. 1) respectively, and all could be convenient prepared using current procedure with excellent yields.Substrate scope for the amidation reaction^a

Open in a separate window^aReactions were run on 1 mmol 1 and 1.1 mmol 2 with 10 mol% DABCO and 10 mol% Fe₃O₄ in 1 mL of MeCN for 48 hours at 85 °C unless otherwise noted.Furthermore, to demonstrate the synthetic utilization of our methodology, the preparation of bioactive compounds was demonstrated Scheme 1. Compounds 3as is a patent HDAC₄ inhibitor.²² Under the standard reaction conditions, 3as could be obtained from commercial available 1as in 92% yield. Our procedure is much more atom economy as it excluded the usage of activating reagent and excess amount of base. In a recent report, Yang group reported their pilot-scale synthesis of substituted phenylacetamides to tetrahydroisoquinoline-2-ones.²³ In their practice, corrosive thionyl chloride was applied as activating reagent and large excess amount of methyl amine was required, however, moderate yield of 3a was obtained, while our method can achieve better yield along with the exclusion of corrosive thionyl chloride. Following this report, tetrahydroisoquinoline-2-one 4a could be obtained in 82% yield, which could be used in the preparation of various bioactive 4-aryl-tetrahydroisoquinolines 5a with known procedure.²⁴ Open in a separate window Scheme 1Application of N-methyl amide.Finally, owing to the magnetic nature of Fe₃O₄, we try to recover the Fe₃O₄ from the reaction system and test its efficiency. As the Fe₃O₄ is always stick to the magnetic stir bar, after the termination of the reaction, the reaction solution was pour out and the tub along with the magnetic stir bar was rinsed with MeCN three times, oven dried and used for the next cycle. The results shown that the Fe₃O₄ could be used 10 times and still maintained very good efficiency (Fig. 2).Open in a separate window Fig. 2The efficiency of recovered Fe₃O₄.Combined with the literature reports and experimantal observation,²⁵ a plausable mechanism was proposed in Scheme 2. Firstly, the carboxylic acid reacts with the Fe₃O₄ to get iron (II and III) carboxylate A, which will coordinate to the intermediate B generated from DABCO and isothiocyanate to get intermediate C. Then, one of the carboxylate attack intermediate B to release DABCO and generates intermediate D. Intermediate D go through an intramolecular addition to generate intermediate E, which go through a rearrangement reaction to get intermediate F with the release of carbonyl sulfide. Finally, the protonation of F with carboxylic acid to get the final product and regenerate the iron (II and III) carboxylate A.Open in a separate window Scheme 2Proposed reaction mechanism. Keywords:

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