Electrooxidative 1,2-Bromoesterification of Alkenes with Acids and N

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Electrooxidative 1,2-Bromoesterification of Alkenes with Acids and N‑Bromosuccinimide Chao Wan,† Ren-Jie Song,*,† and Jin-Heng Li*,†,‡ †

Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, China ‡ State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China

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S Supporting Information *

ABSTRACT: A simple three-component 1,2-bromoesterification of alkenes with acids and N-bromosuccinimide under electrochemical oxidative conditions is described. This transformation enables the construction of β-bromoalkyl esters via oxidative C−Br/C−O difunctionalization, where a variety of alkenes, including styrenes and cycloolefins, were well tolerated to react efficiently with a wide range of acids, such as aromatic acids, aliphatic acids, and amino acids.

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have achieved enantioselective intermolecular bromoesterification of allylic sulfonamides with carboxylic aids and NBS as the bromogenation reagents, in which the sulfonamide NH directing group was necessary for both the reactivity and enantioselectivity. However, these transformations suffered from limited substrate scope, and examples of threecomponent bromoesterification of simple alkenes without a directing group are quite rare. Recently, electrochemistry has been established as a fundamental tool for molecular synthesis with efficient utilization of energy.5 Great success has been achieved in the construction of C−C,6 C−O,7 C−N,8 C−X (X = F, Cl, Br, I),9 other C−heteroatom bonds,10 and heteroatom−heteroatom bonds.11 In addition, there have been many examples on the difunctionalization of alkenes via electrochemistry.12 As the result of continuing interest in electrochemical difunctionalization of alkenes, herein, we report a metal-free, intermolecular 1,2-bromoesterification of alkenes with simple acids and Nbromosuccinimide (NBS) under electrochemical oxidative conditions. Notably, the features of these transformations include (a) being catalyst-free and (b) having exceedingly mild reactions at room temperature as well as (c) broad substrate scope including styrenes, cycloolefins, aromatic acids, aliphatic acids, and amino acids. We began this study by choosing ethene-1,1-diyldibenzene (1a), benzoic acid (2a), and 1-bromopyrrolidine-2,5-dione (3a) as the model substrates, nBu4NBF4 as the electrolyte, and TEMPO as the initiator. A mixture of these compounds in

romoesterification of olefins is one of the most important reactions; it allows the simultaneous introduction of a bromo group and an ester group across the C=C bond for the construction of complex bromo-containing esters.1 The versatile products of this reaction can be further employed in synthesis for accessing bioactive natural products and pharmaceutical agents.2 In the past few years, significant progress has been made in the intramolecular bromoesterification of alkenes (Scheme 1a).3 The development of new efficient ways for the transformations of alkenes still attracts much attention from chemists.4 For example, Tang’s group4a Scheme 1. 1,2-Bromoesterification of Alkenes

Received: March 1, 2019

© XXXX American Chemical Society

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DOI: 10.1021/acs.orglett.9b00771 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

best result (entry 1 versus entries 13−20). Gratifyingly, the reaction was successful in the construction of 4aaa in 70% yield under air atmosphere (entry 21). Notably, a 79% yield of 4aaa was obtained when 1 mmol of 1a was used (entry 22). With the optimal conditions in hand, the substrate scope regarding the acids was investigated (Scheme 2). The

CH3CN was then irradiated in an undivided cell (a threenecked round-bottomed flask) equipped with a carbon rod anode and a carbon plate cathode (Table 1). To our delight, Table 1. Screening of Optimal Conditionsa

Scheme 2. Variation of the Acids (2)a

entry

variation from the standard conditions

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22c

none without TEMPO K2S2O8 instead of TEMPO n Bu4NBF4 (1 equiv) Et4NBF4 instead of nBu4NBF4 n Bu4NPF6 instead of nBu4NBF4 LiClO4 instead of nBu4NBF4 3b instead of 3a 3c instead of 3a 3d instead of 3a 3e instead of 3a 3f, 3g, or 3h instead of 3a MeOH instead of MeCN DMF instead of MeCN H2O/MeCN (1:1) instead of MeCN I = 5 mA instead of I = 10 mA I = 15 mA instead of I = 10 mA no electricity (+)C/(−)Pt instead of (+)C/(−)C (+)Pt/(−)C instead of (+)C/(−)C under air none

88 58 59 65 33 62 3 79 63 26 trace 0 42 0 56 49 62 5 72 58 70 79

a

Reaction conditions: carbon rod anode, carbon plate cathode, constant current = 10 mA, 1a (0.5 mmol), 2 (2 equiv), 3a (2 equiv), TEMPO (50 mol %), nBu4NBF4 (0.1 M), MeCN, argon, rt, and 1 h.

introduction of electron-donating groups (Me, MeO), halogen substituents (F, Cl, Br), and an electron-withdrawing group (CN) on the phenyl ring of benzoic acid proved acceptable, providing 1,2-bromoesterification products 4aba−aia in moderate to good yields. For example, 2,3,4,5,6-pentafluorobenzoic acid 2i could react with ethene-1,1-diyldibenzene 1a and 1-bromopyrrolidine-2,5-dione 3a and provided the corresponding product 4aia in 72% yield. Particularly, 1naphthoic acid 2j and N-heterocyclic substrate 2k were compatible with the reaction conditions and provided products 4aja and 4aka in 76 and 88% yields, respectively. Cinnamic acid also delivered the corresponding product 4ala in 78% yield. Other aliphatic acids, such as adamantane-1-carboxylic acid, cyclohexanecarboxylic acid, cyclopentanecarboxylic acid, and tetrahydrofuran-2-carboxylic acid, were tolerated by the electrooxidative system, and the corresponding products were produced in moderate to good yields (4ama−apa, 46−73% yield). Notably, the reaction could also extend to amino acids 2q−s and afforded the 1,2-bromoesterification products 4aqa− asa in 40−48% yields. The scope of alkenes was subsequently explored using benzoic acid and 1-bromopyrrolidine-2,5-dione as the reaction partner (Scheme 3). A variety of alkenes including 1,1disubstituted ethylenes 1b−m, 1,2-disubstituted ethylene 1n, terminal arenes 1o−s, and cycloolefins 1t−u participated in the 1,2-bromoesterification reaction under optimized conditions and afforded the corresponding products 4baa−uaa in 50−83% yields. For example, symmetric 1,1-disubstituted

a

Reaction conditions: carbon rod anode, carbon plate cathode, constant current = 10 mA, 1a (0.5 mmol), 2a (2 equiv), 3a (2 equiv), TEMPO (50 mol %), nBu4NBF4 (0.1 M), MeCN, argon, rt, and 1 h. b Isolated yield. c1a (1 mmol) for 3 h.

the reaction proceeded very well under the initial conditions, leading to the formation of product 4aaa in 88% yield (entry 1). The yield was reduced to 58% in the absence of TEMPO, which showed that TEMPO promoted this transformation (entry 2). Another oxidant, K2S2O8 could not promote this reaction (entry 3). A lower amount of nBu4NBF4 had a negative effect on this transformation (entry 4). The replacement of nBu4NBF4 with other electrolytes, such as Et4NBF4, nBu4NPF6, and LiClO4, resulted in worse yields (entries 5−7). Subsequently, various organic bromine sources, such as 2-bromoisoindoline-1,3-dione (3b), 1,3-dibromo-5,5dimethylimidazolidine-2,4-dione (3c), 1-bromopyrrolidin-2one (3d), and 2,4,4,6-tetrabromocyclohexa-2,5-dien-1-one (3e) were tested, and the results showed that 1-bromopyrrolidine-2,5-dione was the best choice (entry 1 versus entries 8− 11). Other inorganic bromine sources, LiBr (3f), NH4Br (3g), and KBr (3h), were not suitable compounds (entry 12). Among the solvents, currents, and electrodes examined, the reaction in MeCN in an undivided cell (I = 10 mA) equipped with a carbon rod anode and a platinum plate cathode gave the B

DOI: 10.1021/acs.orglett.9b00771 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 3. Variation of the Alkenes (1)a

Scheme 4. Possible Mechanism

deprotonation formed the desired products 4. In addition, hydrogen cations are reduced on the cathode, providing molecular hydrogen. In conclusion, we have developed an intermolecular 1,2bromoesterification of alkenes with acids and NBS under electrochemical oxidative conditions, giving the functional 2bromo-1-phenylethyl benzoate derivatives in moderate to good yields. By employing a carbon rod anode and a carbon plate cathode in an undivided cell under constant-current electrolysis conditions, various alkenes, including 1,1-disubstituted ethylene, 1,2-disubstituted ethylene, terminal arenes, and cycloolefins, were transformed to the corresponding bromines via an oxidative C−Br/C−O difunctionalization process at room temperature with excellent functional group tolerance. Work on the application of the difunctionalization strategy is currently underway in our laboratory.

a

Reaction conditions: carbon rod anode, carbon plate cathode, constant current = 10 mA, 1 (0.5 mmol), 2a (2 equiv), 3a (2 equiv), TEMPO (50 mol %), nBu4NBF4 (0.1 M), MeCN, argon, rt, and 1 h.

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ASSOCIATED CONTENT

S Supporting Information *

ethylene, 4,4′-(ethene-1,1-diyl)bis(methylbenzene) 1b was compatible with the present conditions to afford the corresponding product 4baa in 83% yield. A range of functional groups on the aromatic ring of unsymmetric 1,1disubstituted ethylenes, including Me, Br, Cl, and F, were tolerated well (products 4eaa−jaa). 2-(1-Phenylvinyl)naphthalene 1k was also suitable for this conversion, giving the product 4kaa in 73% yield. The 1,2-bromoesterification protocol was applicable to 1-methyl-4-(prop-1-en-2-yl)benzene 1l and 2-(prop-1-en-2-yl)thiophene 1m, giving 4laa and 4maa in 79 and 60% yields, respectively. The reaction could also extend to 1,2-disubstituted ethylene 1n and afforded the desired product 2-bromo-1-phenylpropyl benzoate (4naa) in 52% yield. Furthermore, terminal styrenes 1o−s, with the substituents of OMe, OEt, Me, and Cl, respectively, gave the corresponding products 4oaa−saa in 50−60% yields. Notably, cycloolefins 1t and 1u underwent this transformation smoothly and gave the products 4taa and 4uaa in 76 and 68% yields, respectively. More interestingly, an estrone derivative 1v was employed to deliver product 4vaa in 49% yield. Finally, aliphatic alkene 1w also succeeded in constructing 4waa in 48% yield. On the basis of the current experiments and the previous reports,6−13 we propose a possible pathway for this electrochemical intermolecular 1,2-bromoesterification of alkenes with acids and NBS (Scheme 4). Initially, TEMPO+ is formed by the oxidation of TEMPO at the anode. Subsequently, the addition of N-bromosuccinimide to the C−C double bond of alkenes formed the cyclic bromonium ion intermediate with the aid of TEMPO+. Finally, nucleophilic attack as well as

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00771. Descriptions of experimental procedures for compounds and analytical characterization (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (R.-J.S.) *E-mail: [email protected]. (J.-H.L.) ORCID

Ren-Jie Song: 0000-0001-8708-7433 Jin-Heng Li: 0000-0001-7215-7152 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Natural Science Foundation of China (Nos. 51878326, 21762030, 21625203, and 21871126) and Jiangxi Province Science and Technology Project (Nos. 20171BCB23055 and 20171ACB21032) for financial support.

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DOI: 10.1021/acs.orglett.9b00771 Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.9b00771 Org. Lett. XXXX, XXX, XXX−XXX