Electrochemical Radical Formyloxylation–Bromination,− Chlorination

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

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Electrochemical Radical Formyloxylation−Bromination, −Chlorination, and −Trifluoromethylation of Alkenes Xiang Sun,†,‡ Hong-Xing Ma,‡ Tian-Sheng Mei,‡ Ping Fang,*,‡ and Yulai Hu*,† †

College of Chemistry and Chemical Engineering, Northwest Normal University, 967 Anning East Road, Lanzhou 730070, China State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China

‡

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S Supporting Information *

ABSTRACT: Given the versatility and value of the structurally diverse organohalides and CF3-containing compounds in organic synthesis, we reported a green, oxidant-free electrochemical method using undivided electrochemical cells for radical bromination, chlorination and trifluoromethylation− formyloxylation of the various alkenes with readily available halogen radical (NaCl, NaBr), trifluoromethyl radical (CF3SO2Na) sources, and DMF as formyloxylation reagents. The protocol is operationally simple and robust and the Cl−, Br− or CF3− was directly oxidized at the anode, obviating the need for exogenous chemical oxidants. rganohalides (halohydrins, β-haloethers, and β-haloesters) are common structural units existing in numerous bioactive natural products and polymers which are widely used as intermediates in organic synthesis.1 The oxidative halofunctionalization of easily available alkenes remains a highly prevalent method for incorporating halogen atoms in organic molecules and constructing the ubiquitous scaffold.2 Furthermore, through the SN1 or SN2 reaction, the halogen atoms can be easily substituted by nucleophilic reagents to transform other significant compounds.3 Generally, this important class of compounds are prepared by employing electrophilic organohalogenating agents4 or combining nucleophilic halogen sources (Cl−, Br−) and a strong oxidant to generate highly active halogen sources in situ.5 In particular, the halofunctionalization of olefins, such as bromoformyloxylation6a−e,g,h and chloroformyloxylation,6f have attracted much research interest. Given the prevalence of CF3 groups in pharmaceuticals, materials, and agriculture chemicals, difunctionalization of unsaturated double bonds is considered as a useful protocol for the synthesis of diverse trifluoromethylated compounds, which adjusts its bioactivity (chemical and metabolic stabilities, lipophilicity, and binding selectivities).7 Over the past decade, various trifluoromethylation reactions of alkenes,8,15 like oxytrifluoromethylation,9 halotrifluoromethylation,10 carbotrifluoromethylation,11 hydrotrifluoromethylation,12 aminotrifluoromethylation,13 and thiotrifluoromethylation,14 have been reported using various trifluoromethylation reagents as CF3 radical sources. Among these reported approaches, visiblelight-induced photocatalysts9,10,11a−c,12a,b,13,14 have been demonstrated as an effective path for the difunctionalization of alkenes. In addition, DMF, despite being an effective polar

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© XXXX American Chemical Society

solvent, can also act as a formylation reagent in many organic transformation.16 However, current halo-functionalizations of olefins are limited by the need for expensive, toxic, and oxidative halogen sources or stoichiometric oxidants, and trifluoromethylation of alkenes are focused on photocatalytic or various trifluoromethylation reagents in conjunction with strong oxidants, which both reduces functional group compatibility and generates chemical waste (Scheme 1). Electrochemical anodic oxidation has a rich history in synthetic chemistry and has become a promising tool for the generation of reactive radical and radical ions intermediates due to its tunability over electron-transfer processes and use of electrons as traceless redox reagents.17 Recently, the Scheme 1. Electrochemical Oxidation, HaloFunctionalization, and Trifluoromethylation of Alkenes

Received: March 11, 2019

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DOI: 10.1021/acs.orglett.9b00867 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

oxidation conditions through the utilization of NaCl and CF3SO2Na as chlorine and trifluoromethyl radical source in solution. To our satisfaction, we found 83% isolated yield of the formyloxylation−chlorination product (3a) and 85% isolated yield of the formyloxylation−trifluoromethylation product (4a). The optimizations of reaction conditions have been discussed including different Cu(I), Mn(II), current, electrode material, electrolyte, acid, and the amount of acid and water (see Tables 2 and 3 in the Supporting Information). We then investigated the substrate scope of the electrochemical oxidation formyloxylation−bromination of the alkenes under the optimal conditions (Scheme 2). A variety

renaissance in the electrochemical oxidative difunctionalization of alkenes,18 Yoshida,18a Lin,18b−d Lei,18e,f Zeng,18g Li,18h and Xu18i−k demonstrated the advantages of electrochemistry oxidation for numerous challenging reactions. Given our research interests in electrochemical anodic oxidation, we aim to develop new versatile and environmentally friendly organic transformations. Encouraged by these reports, we report an electrochemically enabled regio- and chemoselective bromination−, chlorination−, and trifluoromethylation−formyloxylation of the alkenes by employing readily available haloride salts (NaCl, NaBr), Langlois reagent (CF3SO2Na), and DMF as formyloxylation reagents. Initially, we chose 4-vinylbiphenyl (1a) as the model substrate and NaBr as bromine radical source and screened a variety of different electrolysis conditions for the envisioned electrochemical oxidant halo-functionalization of alkenes in an undivided cell equipped with a carbon anode and a platinum cathode. After extensive optimizations, we found that 95% isolated yield of the desired product (2a) could be obtained under constant current electrolysis at 3.0 mA in the presence of HCOOH (1.5 mL) and n-Bu4NPF6 (2.0 equiv) as the electrolyte in DMF (4.0 mL) at room temperature for 24 h (Table 1, entry 1). Use of HOAc as the proton source led to

Scheme 2. Substrate Scope of Formyloxylation− Brominationa

Table 1. Reaction Optimization with Substrate 1aa

entry

variation from standard conditions

2a yieldb (%)

1 2 3 4 5 6 7 8 9 10

none HOAc (0.4 mL) HOAc (1.0 mL) HCOOH (0.8 mL) TFA (1.0 mL) n-Bu4NPF6 (1.5 equiv), HOAc (1.0 mL) n-Bu4OAc (2.0 equiv), HOAc (1.0 mL) n-Bu4NClO4 (2.0 equiv), HOAc (1.0 mL) C(−)-C(+), HOAc (1.0 mL) Pt(−)-Pt(+), HOAc (1.0 mL)

95c 67 97 93 79 87 67 77 84 84

Standard conditions: constant current = 3.0 mA (j anode ≅ 1.39 mAcm−2), 1a (0.3 mmol), NaBr (2.0 equiv), n-Bu4NPF6 (2.0 equiv), HCOOH (1.5 mL) and DMF (4.0 mL) in an undivided cell with one platinum electrode and one carbon electrode, rt, 24 h (9.0 Fmol−1). b The yield was determined by 1H NMR using CH2Br2 as an internal standard. cIsolated yield in parentheses. a

Standard reaction conditions: constant current = 3.0 mA (j anode ≅ 1.39 mAcm−2), 1a (0.3 mmol), NaBr (2.0 equiv), n-Bu4NPF6 (2.0 equiv), HCOOH(1.5 mL) and DMF (4.0 mL) in an undivided cell with one platinum electrode and one carbon electrode, rt, 24 h (9.0 Fmol−1) or 48 h (17.9 Fmol−1). Isolated yields are reported. bdr values are determined by crude 1H NMR. a

the formation of acetoxylation−bromination product in 5% yield (entries 2 and 3), and TFA led to an obvious loss in yield (entry 5). Further explorations showed that the yield was significantly effected by the amount of acid, primarily because the acid is probably important to protonate Me2NH released during the reaction. Otherwise, Me2NH may get oxidized at the anode. The supporting electrolyte had a major influence on this halo-functionalization reaction, and all displayed lower effectiveness than n-Bu4NPF6 (entries 6−8). The electrode materials has a slight impact on the reaction outcome, but the carbon anode and platinum cathode was the best choice (entries 9 and 10). Encouraged by the formyloxylation−bromination of the alkenes, we reasoned that it should be possible to incorporate chlorine and trifluoromethyl functional groups under anodic

of styrene-derived alkenes were competent substrates, affording the bromoformyloxylated products in moderate to excellent yields (2ab, 2ba−bb, 2ca−cb, 2d−j). In general, electron-rich styrenes with various substituents (t-Bu, Ph, OPh, OMe) at the para position reacted particularly well due to bromine radical reacting with alkene to produce benzyl radicals which are more compatible with oxidization (2aa, 2ac−ad, and 2cc). Moderately electron-deficient (F, Cl) styrenes provided the desired products in high yields (2ka−kb). However, the alkenes with strongly electron-withdrawing groups (CF3, COOMe, CH3CO, COOH, CN) all readily underwent the desired bromo-functionalization, but the reaction time needed to be 48 h (2kc−kf, 2l). It is worth noting that 1,1disubstituted acyclic alkenes are all suitable substrates for this B

DOI: 10.1021/acs.orglett.9b00867 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters reaction, giving the expected product in excellent yields (2m,n). In particular, cyclic alkenes and β-alkylstyrenes were converted to trans-bromoformyloxylated products in moderate yields and diastereoselectivity (2o−r). In addition, the steroid based natural products are valuable and useful drug compounds, and the alkenes derived from this natural products were halo-functionalized in good yield (2s). Thus, this electrochemical oxidation difunctionalization method can be used for the late-stage functionalization of medicinally and bioactive compounds. We next evaluated the scope and generality of the electrochemical oxidation formyloxylation−chlorination of the alkenes (Scheme 3). Several electron-rich styrenes were

Scheme 4. Substrate Scope of Formyloxylation− Thifluormethylationa

Scheme 3. Substrate Scope of Formyloxylation− Chlorinationa Standard reaction conditions: constant current = 3.0 mA (j anode ≅ 1.39 mAcm−2), 1a (0.3 mmol), NaSO2CF3 (2.0 equiv), n-Bu4NPF6 (2.0 equiv), CuOAc (10 mol %), HOAc (0.5 mL), H2O (0.1 mL) and DMF (4.0 mL) in an undivided cell with one platinum electrode and one carbon electrode, rt, 24 h (9.0 Fmol−1) or 48 h (17.9 Fmol−1). Isolated yields are reported. a

Electron-rich and moderately electron-deficient styrenes were proven to be suitable substrates, furnishing the corresponding products in good yields (4a−j). To our satisfaction, the alkenes with various strongly electron-withdrawing functional groups (COOMe, CH3CO, COOH) are tolerated, providing the desired trifluoromethyformyloxylation products in moderately yields (4k−m). Furthermore, the alkene derived from the steroid based natural products could also be trifluoromethylation under the optimal electrochemical oxidation conditions (4n). We evaluated the application potential of this electrochemical oxidative formyloxylation−bromination of alkenes by performing the reaction on a 6 mmol scale that resulted in a similarly high yield (90%) as that of the small-scale reaction (Scheme 5).

Standard reaction conditions: constant current = 3.0 mA (j anode ≅ 1.39 mAcm−2), 1a (0.3 mmol), NaCl (2.0 equiv), n-Bu4NPF6 (2.0 equiv), CuCl (10 mol %), HCOOH (0.5 mL), H2O (0.1 mL) and DMF (4.0 mL) in an undivided cell with one platinum electrode and one carbon electrode, rt, 24 h (9.0 Fmol−1) or 48 h (17.9 Fmol−1). Isolated yields are reported. bdr values are determined by crude 1H NMR. a

Scheme 5. Gram-Scale Experimenta

incompatible due to the extra step of elimination that takes place in this desired chloroformyloxylated products. Notably, the formyloxylation−chlorination of the various electron-rich alkenes afforded the desired products in moderate yields but along with a small amount of β-chlorinated side product likely stemming from the extra stability gained from the extended conjugation in these final products due to the push−pull feature of the β-chlorinated product in most cases (3a−f). Encouragingly, various styrenes with moderately (F, Cl, Br) and strongly electron-withdrawing (CF3, COOMe, CH3CO, COOH) substituents reacted particularly well, and no βchlorinated side product was observed (3g−m). In addition, cyclic alkenes also proved to be effective for transchloroformyloxylation, although with a slightly lower yield and diastereoselectivity (3n−p). To our delight, the steroid based natural products derived substrate all underwent the desired chloroformyloxylation smoothly (3q). Given the practicability of trifluoromethylation of alkenes, we next examined the scope of the formyloxylation− trifluoromethylation with various alkenes (Scheme 4).

a

The yield was isolated.

To gain insight into the reaction mechanism, a series of radical-trapping experiments and cyclic voltammetry (CV) experiments (see the Supporting Information, Scheme 2 and Figures A−H) was carried out. These results indicated that radical intermediates are possibly involved in this electrochemical oxidation system, and Br−, Cl−, or CF3− (without Cu(I)) was likely to be first oxidized at the anode. On the basis of the previous reports and the above experimental results, a plausible mechanism is presented for the electrochemical radical bromination−, chlorination−, and trifluoromethylation−formyloxylation of the alkenes (see the Supporting Information, Scheme 3). Initially, the Br−, Cl−, or CF3− (without Cu(I)) was likely to be first oxidized at the C

DOI: 10.1021/acs.orglett.9b00867 Org. Lett. XXXX, XXX, XXX−XXX

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anode to generate the Br, Cl, or CF3 radical. Then the Br, Cl, or CF3 radical rapidly combines with arylalkene to afford the benzyl radical intermediate. The radical intermediate can be oxidized to benzyl carbocation, which was subject to nucleophilic attack by DMF to give imine intermediate. Hydrolysis of the imine intermediate afforded the desired halofunctionalization and trifluoromethylation product as well as a hydrogen ion, which was reduced at the cathode to release hydrogen gas. In conclusion, we have developed a versatile and environmentally friendly method for electrochemical radical bromination−, chlorination−, and trifluoromethylation−formyloxylation of the alkenes with readily available halogen radical (NaCl, NaBr), trifluoromethyl radical (CF3SO2Na) sources, and DMF as formyloxylation reagents. The protocol is operationally simple, avoids the use of exogenous stoichiometric chemical oxidants, and exhibits a broad substrate scope for difunctionalization of various alkenes. Preliminary mechanistic studies revealed that Cl−, Br−, or CF3− was oxidized at the anode to generate Cl, Br, or CF3 radical. Further efforts are focused on extending the application of electrochemical oxidative method to other useful transformations with readily available haloride salts (NaCl, NaBr) and Langlois reagent (CF3SO2Na) as the halogen and trifluoromethyl radical sources.

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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.9b00867. Experimental procedures, compound characterization data, and crystallographic data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Tian-Sheng Mei: 0000-0002-4985-1071 Ping Fang: 0000-0002-3421-2613 Yulai Hu: 0000-0002-9630-4150 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB20000000), NSF of China (Grant 21572245, 21772222, 21772220, 21821002), and S&TCSM of Shanghai (Grant 17JC1401200, 18JC1415600).

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