English

• 1.   Introduction

  To develop general and effective methods for the synthesis of complex molecular skeletons is one of the focuses of modern organic chemistry. The haloethylene carboxylates are important intermediates in organic synthesis because the vinyl halide moiety is often employed as the parteners of transition-metal-catalyzed cross-coupling reactions and halogen-metal exchange reactions.^[1] Particully, haloethylene carboxylates are frequently used as the important intermediates in the synthetic chemistry and pharmaceutical chemistry.^[2-3] For the preparation of haloethylene carboxylates, Barluenga et al.^[4] describled a reaction of terminal alkynes with bis(pyridine)iodonium(I) tetrafluoroborate and acetic acid to (E)-β-iodoenol acetates in 1990. Twenty years later, Jiang et al.^[5] developed an elegant Ag-catalyzed difunctionalization of terminal alkynes with acetic anhydride and N-halosuccinimide (NXS, X＝Cl, Br and I) to (Z)-β-haloenol acetates with high regio- and stereo- selectivity in 2010 (Scheme 1a). Recently, Jiang et al.^[6] further reported a Pd-catalyzed sequential nucleophilic addition/oxidative annulation of bromoalkynes with benzoic acids to isocoumarins in one-pot, suggesting via a haloethylene carboxylate intermediate. Although other methods for the preparation of haloethylene carboxylates from terminal alkynes have been reported, ^[7] there are some drawbacks of poor yields, harsh reaction conditions, complicated procedure or low stereoselectivity to limit the applications. Therefore, to develop efficient method from the com- mercialy available starting materials for the synthesis of halogenated vinyl carboxylic acid esters is highly desired.

  Scheme 1

  

  Scheme 1.  Transformations of alkynes and 1-bromoalkynes

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  During the past decades, haloalkynes have become powerful and highly versatile building blocks in organic chemistry, which exhibit abundant and tunable reactivities, particularly in the presence of transition-metal catalyst.^[8] So, the remarkable efforts have been devoted to the synthesis of a valuable scaffold from haloalkynes, and a variety of achievements have been obtained.^[9] For examples, a series of nucleophilic additions of fluoride, ^[10] iodide, ^[11] acetate, ^{[5, 12]} isocyanide, ^[13] sulfide, ^[14] and phenol^[15] to haloalkynes, have been investigated, providing the corresponding trisubstituted alkenes in exclusive Z-type with high yields. Moreover, several organic transformations using haloalkynes as strating materials under visible light irradiation were estalished, including an alkynylation of α-keto acids with bromoacetylenes to ynones, an oxidative amidation of bromoalkynes with anilines to α-ketoamides, and a three-component cycliza- tion of 2H-azirines, alkynyl bromides and molecular oxygen to oxazoles (Scheme 1b and 1c).^[16] However, most of the above reactions are involved in the cleavage of C—X bonds to complete the further transformations. As part of continuing our study on the chemistry of 1-bromoalkynes, we wish to report a silver-catalyzed functionalization 1-bromoalkynes with aryl carboxylic acids to afford (Z)-β-bromo-1-arylvinyl aryl esters in high regio- and stereo-selectivity with good yields under mild reaction conditions (Scheme 1d).

  2.   Results and discussion

  Initially, a model reaction of phenylethynyl bromide (1a) with benzoic acid (2a) was chosen to optimize reaction conditions. When the model reaction was conducted in the presence of Ag₂O (10 mol%) and KOAc (1.5 equiv.) in N, N-dimethylformamide (DMF) under air atmosphere at 80 ℃ for 12 h, a nucleophilic addition of 1a proceeded selectively and delivered the desired product (Z)-2-bromo-1-phenylvinyl benzoate (3a) in 80% yield, and no (E)-isomer of 3a was detected (Table 1, Entry 1). It was found that the base played an important role in the reaction. Screening of the bases indicated that Et₃N was the best one for the reaction among the tested inorganic and organic bases, providing 3a in 94% yield (Table 1, Entry 2). Other bases exhibited the less reactivity and showed the following order, Et₃N > KOAc > NaOAc > Cs₂CO₃ > K₂CO₃ > Na₂CO₃ > DABCO > K₃PO₄ > DBU > LiO^tBu＝KO^tBu to the model reaction (Table 1, Entries 3~11). It was noteworthy that the reaction could not occur when LiOtBu and KO^tBu were used as bases (Table 1, Entries 10, 11). Then the effect of solvents on the reaction was explored, and the results were also shown in Table 1. When N, N-dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), N-methyl pyrrolidone (NMP), CH₃CN, CH₃NO₂ and 1, 2-dichloroethane (DCE) were used as reaction media, the reactions provided 3a in 27%~78% yields (Table 1, Entries 12~17), indicating the inferior reactivity to the functionalization of 1-bromoalkyne. It should be noted that Ag-catalyst played an important role in the reaction and it was fond that Ag was essential to the reaction (Table 1, Entry 18). The effect of a series of Ag-catalysts on the yield of 3a was explored, and Ag₂O showed highest reactivity. Meanwhile, Ag₂CO₃ and AgOAc used as catalysts, 3a was obtained in 72% and 68% yield, respectively (Table 1, Entries 19, 20). In addition, Ag₂SO₄ and AgCl used as catalysts gave trace amounts of 3a (Table 1, Entries 21, 22). Finally, the optimal amount of Ag₂O and Et₃N were found to be 7.5 mol% and 1.2 equiv., respectively for 0.50 mmol scale reaction (Table 1, Entries 23~26).

  Table 1

  

  Table 1.  Optimization of the reaction conditions^a

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  Entry	Solvent	Base	Ag source	Yieldb/%
  1	DMF	KOAc	Ag2O	80
  2	DMF	Et3N	Ag2O	94
  3	DMF	NaOAc	Ag2O	78
  4	DMF	Cs2CO3	Ag2O	60
  5	DMF	K2CO3	Ag2O	47
  6	DMF	Na2CO3	Ag2O	39
  7	DMF	DABCO	Ag2O	33
  8	DMF	K3PO4	Ag2O	23
  9	DMF	DBU	Ag2O	21
  10	DMF	LiOtBu	Ag2O	NR
  11	DMF	KOtBu	Ag2O	NR
  12	DMA	Et3N	Ag2O	78
  13	DMSO	Et3N	Ag2O	56
  14	NMP	Et3N	Ag 2O	31
  15	CH3CN	Et3N	Ag2O	28
  16	CH3NO2	Et3N	Ag 2O	27
  17	DCE	Et3N	Ag2O	44
  18	DMF	Et3N	—	NR
  19	DMF	Et3N	Ag2CO3	72
  20	DMF	Et3N	AgOAc	68
  21	DMF	Et3N	Ag2SO4	Trace
  22	DMF	Et3N	AgCl	Trace
  23	DMF	Et3N	Ag2O	94c
  24	DMF	Et3N	Ag2O	93d
  25	DMF	Et3N	Ag2O	94e
  26	DMF	Et3N	Ag2O	92f
  a Reaction conditions: phenylethynyl bromide (1a, 0.50 mmol), benzoic acid (2a, 1.0 mmol), Ag-catalyst (10 mol%), base (0.75 mmol, 1.5 equiv.), solvent (2.0 mL), 80 ℃, sealed tube, air, 12 h. b Isolated yield. c Ag2O (7.5 mol%) was used. d Ag2O (5 mol%) was added. e Et3N (0.60 mmol, 1.2 equiv.) was used. f Et3N (0.50 mmol 1.0 equiv.) was added. NR＝No reaction.

  Based on the optimized conditions in our hand, the generality of (Z)-β-bromo-1-phenylvinyl aryl esters prepa- ration from phenylethynyl bromide (1a) was investigated, as shown in Table 2. When aromatic carboxylic acids with an electron-donating group, such as methyl group on the ortho- or meta-position of the benzene rings, the corresponding products (3b and 3c) were obtained in 88% and 84% yields, respectively. It is obvious that the reactions of 1a and substituted benzoic acids with a moderate electron-withdrawing group including Cl, Br, and I on the phenyl rings underwent the nucleophilic addition smoothly to generate the desired products (3d~3i) in 83%~90% yields, neglecting steric effect. It should be noted that halogens such as Cl, Br, and I on the benzoic acid moiety were well tolerated and the obtained products provided further opportunity for organic transformations via C(sp²)X (X＝Cl, Br and I) bonds activation and functionalization. When one or two NO₂ (nitro group), a strong electron-withdrawing group, was introduced into the benzene rings of aromatic acids, they also reacted with 1a to afford the anticipated products (3j~3l) in 67%~81% yields. The structure of 3j was unambiguously confirmed by X-ray crystallography.^[17] The electronic effect and steric effect of the substituted benzonic acids was found to be neglected in the reactions. Moreover, furan-2-carbo- xylic acid and acrylic acid could also be transformed into the corresponding products (3m and3n) in 91% and 72% yields, respectively.

  Table 2

  

  Table 2.  Scope of aromatic carboxylic acids^a

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  a Reaction conditions: phenylethynyl bromide (1a, 0.50 mmol), aryl carboxylic acid (2, 1.0 mmol), Ag2O (7.5 mol%), Et3N (0.60 mmol, 1.2 equiv.), DMF (2.0 mL), 80 ℃, sealed tube, air, 12 h; isolated yields of the products.

  Subsequently, we turned our attention to extend the scope of acetylene bromides, as shown in Table 3. A number of arylethynyl bromides with an electron- withdrawing or an electron-donating substituent at the para-position of the benzene rings, including F, Cl, Br, Me and n-C₃H₇, were used to react with benzoic acid (2a) under the standard conditions, affording the according products (3o~3p and 3r~3t) in 88%~91% yields, indicating little electronic effect. On the other hand, meta-substituted phenylethynyl bromide also reacted with 2a smoothly to generate the anticipated product (3q) in 87% yield. In addition, aliphatic terminal acetylene bromide, such as 1-bromoheptyne was used as substrate, and the expected product (3u) was obtained in 69% yield.

  Table 3

  

  Table 3.  Scope of acetylene bromides^a

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  a Reaction conditions: acetylene bromide (1, 0.50 mmol), benzoic acid (2a, 1.0 mmol), Ag2O (7.5 mol%), Et3N (0.60 mmol, 1.2 equiv.), DMF (2.0 mL), 80 ℃, sealed tube, air, 12 h; isolated yields of the products.

  On the basis of our result and the previous report, ^[6] a possible mechanism is proposed as shown in Scheme 2. Firstly, Ag₂O reacts with Et₃N to generate an Ag(I)-com-plex along with the formation of OH^－. Then, the obtained Ag(NEt₃)₄-complex undergoes the reaction with phenyl- ethynyl bromide (1a) to afford π-complex A, which is subsequently converted to the corresponding σ-complex C by nucleophilic attack of the benzonic anion (B) from the reaction of benzonic acid (2a) with generated OH^－. Finally, the silver complex C takes a reaction with Et₃N and 2a to provide the desired product (Z)-β-bromo-1- phenylvinyl benzoate (3a) in high regio- and stereo-selectivity, and regeneration of Ag/NEt₃-complex and benzonic anion (B) for the next run.

  Scheme 2

  

  Scheme 2.  Proposed mechanism

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  3.   Conclusions

  In summary, a silver-catalyzed method for the nucleophilic addition of 1-bromoalkyne with aromatic carboxylic acids to afford (Z)-β-bromo-1-arylvinyl aryl esters has been developed. This reaction uses commercially available aryl acids as nucleophiles and shows a wide range of substrates, which can provide corresponding Z-form addition products in high yields under mild reaction conditions. The reaction has a variety of advantages, including a wide range of substrates, simple operation, easy raw materials, good yields, excellent regio- and stereo-selectivity. A detail mechanistic study is currently underway in our laboratory.

  4.   Experimental section

  4.1   General experimental

  All reactions were carried out under air. ¹H NMR and ¹³C NMR spectra were measured on a Bruker Avance NMR spectrometer (400 MHz or 100 MHz, respectively) in CDCl₃ or DMSO-d₆ as solvent with internal tetramethylsilane standard. Ethyl acetate and petroleum ether were used for column chromatography without further purification. All solvents and chemicals were obtained from commercial sources and used as received unless otherwise noted.

  4.2   General procedure

  A 10 mL of reaction tube was charged with phenylethynyl bromide (1a, 0.50 mmol), benzoic acid (2a, 1.0 mmol), Ag₂O (0.0375 mmol, 7.5 mol%), Et₃N (0.60 mmol, 1.2 equiv.), and DMF (2.0 mL). The reaction vessel was placed in an oil bath. After the reaction was carried out at 80 ℃ for 12 h, it was cooled to room temperature, extracted with EtOAc (5.0 mL×3). The organic layers were combined, dried over MgSO₄, and concentrated to yield the crude product, which was further purified by flash chromatography (silica gel, petroleum ether/ethyl acetate, V:V＝7:1 to 12:1) to give the desired product 3a.

  4.3   Characterization data

  (Z)-2-Bromo-1-phenylvinyl benzoate (3a):^[6] White solid. m.p. 46.1~47.4 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.25 (d, J＝8.0 Hz, 2H), 7.70~7.66 (m, 1H), 7.57~7.53 (m, 2H), 7.50~7.49 (m, 2H), 7.37~7.32 (m, 3H), 6.68 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.0, 150.6, 133.8, 133.3, 130.3, 129.3, 128.7, 128.7, 128.6, 124.9, 96.6. HRMS (ESI) calcd for C₁₅H₁₂BrO₂ [M+H]⁺ 303.0015, found 303.0018.

  (Z)-2-Bromo-1-phenylvinyl 3-methylbenzoate (3b): White solid. m.p. 54.6~55.9 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.08~8.07 (m, 2H), 7.53~7.49 (m, 3H), 7.46~7.42 (m, 1H), 7.38~7.37 (m, 3H), 6.70 (s, 1H), 2.48 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.2, 150.6, 138.5, 134.6, 133.3, 130.7, 129.3, 128.7, 128.6, 128.5, 127.5, 124.9, 96.6, 21.2. HRMS (ESI) calcd for C₁₆H₁₄BrO₂ [M+H]⁺ 317.0172, found 317.0169.

  (Z)-2-Bromo-1-phenylvinyl 2-methylbenzoate (3c): White solid. m.p. 52.8~53.7 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.30 (d, J＝7.6 Hz, 2H), 7.53~7.51 (m, 3H), 7.40~7.38 (m, 3H), 7.36~7.35 (m, 2H), 6.69 (s, 1H), 2.70 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.5, 150.7, 141.6, 133.5, 132.9, 131.9, 131.3, 129.3, 128.8, 127.7, 125.9, 124.9, 96.6, 21.8. HRMS (ESI) calcd for C₁₆H₁₄BrO₂ [M+H]⁺ 317.0172, found 317.0170.

  (Z)-2-Bromo-1-phenylvinyl 2-chlorobenzoate (3d): White solid. m.p. 62.1~64.2 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.20 (d, J＝8.0 Hz, 2H), 7.56~7.54 (m, 2H), 7.54~7.52 (m, 2H), 7.45~7.42 (m, 1H), 7.40~7.39 (m, 3H), 6.69 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 161.5, 150.5, 134.6, 133.4, 133.0, 132.1, 131.4, 129.4, 128.8, 128.3, 126.7, 125.0, 96.8. HRMS (ESI) calcd for C₁₅H₁₁BrClO₂ [M+H]⁺ 336.9625, found 336.9622.

  (Z)-2-Bromo-1-phenylvinyl 4-bromobenzoate (3e): White solid. m.p. 68.9~70.2 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.11 (d, J＝8.4 Hz, 2H), 7.69 (d, J＝8.0 Hz, 2H), 7.50~7.48 (m, 2H), 7.38~7.37 (m, 3H), 6.69 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 162.4, 150.5, 133.1, 132.0, 131.7, 129.4, 129.2, 128.8, 127.6, 124.9, 96.8. HRMS (ESI) calcd for C₁₅H₁₁Br₂O₂ [M+H]⁺ 382.9100, found 382.9104.

  (Z)-2-Bromo-1-phenylvinyl 2-bromobenzoate (3f): White solid. m.p. 65.6~67.9 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.21~7.19 (m, 2H), 7.77~7.75 (m, 1H), 7.55~7.53 (m, 2H), 7.50~7.43 (m, 2H), 7.40~7.39 (m, 3H), 6.69 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 161.9, 150.5, 134.8, 133.4, 133.0, 132.1, 130.3, 129.4, 128.8, 127.3, 125.0, 122.5, 96.9. HRMS (ESI) calcd for C₁₅H₁₁Br₂O₂ [M+H]⁺ 382.9100, found 382.9101.

  (Z)-2-Bromo-1-phenylvinyl 4-iodobenzoate (3g): White solid. m.p. 66.8~68.2 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.11 (d, J＝8.8 Hz, 2H), 7.69 (d, J＝8.4 Hz, 2H), 7.50~7.48 (m, 2H), 7.38~7.37 (m, 3H), 6.69 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 162.6, 150.4, 138.0, 133.0, 131.6, 129.4, 128.8, 128.1, 124.8, 101.9, 96.8. HRMS (ESI) calcd for C₁₅H₁₁BrIO₂ [M+H]⁺ 428.8982, found 428.8981.

  (Z)-2-Bromo-1-phenylvinyl 3-iodobenzoate (3h): White solid. m.p. 56.6~58.3 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.60 (s, 1H), 8.22 (d, J＝7.6 Hz, 1H), 8.00 (d, J＝8.0 Hz, 1H), 7.50~7.48 (m, 2H), 7.39~7.38 (m, 3H), 7.31~7.27 (m, 1H), 6.70 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 161.6, 150.4, 142.7, 139.0, 133.0, 130.6, 130.3, 129.5, 129.4, 128.8, 124.9, 96.9, 94.0. HRMS (ESI) calcd for C₁₅H₁₁BrIO₂ [M+H]⁺ 428.8982, found 428.8983.

  (Z)-2-Bromo-1-phenylvinyl 2-iodobenzoate (3i): White solid. m.p. 61.4~62.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.24~8.23 (m, 1H), 8.10 (d, J＝8.0 Hz, 1H), 7.53~7.50 (m, 3H), 7.40~7.38 (m, 3H), 7.28~7.23 (m, 1H), 6.69 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 162.3, 150.5, 141.9, 133.5, 133.0, 133.0, 131.8, 129.4, 128.8, 128.0, 96.9, 94.9. HRMS (ESI) calcd for C₁₅H₁₁BrIO₂ [M+H]⁺ 428.8982, found 428.8980.

  (Z)-2-Bromo-1-phenylvinyl 4-nitrobenzoate (3j): Pale yellow solid. m.p. 149.1~149.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.42 (d, J＝8.8 Hz, 2H), 8.37 (d, J＝8.8 Hz, 2H), 7.49~7.47 (m, 2H), 7.40~7.38 (m, 3H), 6.71 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 161.2, 150.0, 150.4, 134.0, 132.7, 131.4, 129.6, 128.8, 124.8, 123.7, 97.0. HRMS (ESI) calcd for C₁₅H₁₁BrNO₄ [M+H]⁺ 347.9866, found 347.9866.

  (Z)-2-Bromo-1-phenylvinyl 2-nitrobenzoate (3k): Pale yellow solid. m.p. 146.1~147.4 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.11~8.09 (m, 1H), 7.98~7.96 (m, 1H), 7.78~7.69 (m, 2H), 7.5~7.56 (m, 2H), 7.43~7.41 (m, 3H), 6.67 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 161.3, 150.5, 148.2, 132.9, 132.5, 132.5, 130.4, 129.7, 128.8, 126.1, 125.3, 124.1, 97.1. HRMS (ESI) calcd for C₁₅H₁₁BrNO₄ [M+H]⁺ 347.9866, found 347.9869.

  (Z)-2-Bromo-1-phenylvinyl 3, 5-dinitrobenzoate (3l): Yellow solid. m.p. 157.9~158.3 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 9.34~9.33 (m, 3H), 7.48~7.47 (m, 2H), 7.42~7.40 (m, 3H), 6.67 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 159.2, 150.2, 148.8, 132.4, 132.2, 129.9, 129.8, 128.9, 124.9, 123.1, 97.4. HRMS (ESI) calcd for C₁₅H₁₀BrN₂O₆ [M+H]⁺ 392.9717, found 392.9718.

  (Z)-2-Bromo-1-phenylvinyl furan-2-carboxylate (3m): White solid. m.p. 86.1~87.2 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.70 (s, 1H), 7.49~7.46 (m, 3H), 7.37~7.36 (m, 3H), 6.68 (s, 1H), 6.62~6.61 (m, 1H). ¹³C NMR (100 MHz, CDCl₃) δ: 154.6, 149.8, 147.5, 143.2, 133.0, 129.4, 128.7, 124.9, 120.0, 112.2, 97.1. HRMS (ESI) calcd for C₁₃H₁₀BrO₃ [M+H]⁺ 292.9808, found 292.9811.

  (Z)-2-Bromo-1-phenylvinyl acrylate (3n): Pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 7.44~7.43 (m, 2H), 7.38~7.36 (m, 3H), 6.70~6.66 (m, 1H), 6.61 (s, 1H), 6.41~6.34 (m, 1H), 6.10~6.07 (m, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 162.3, 150.2, 133.3, 133.1, 129.3, 128.7, 126.9, 124.8, 96.5. HRMS (ESI) calcd for C₁₁H₁₀BrO₂ [M+H]⁺ 252.9859, found 252.9862.

  (Z)-2-Bromo-1-(p-tolyl)vinyl benzoate (3o): White solid. m.p. 76.2~77.2 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.27 (d, J＝8.0 Hz, 2H), 7.70~7.66 (m, 1H), 7.57~7.53 (m, 2H), 7.40 (d, J＝8.0 Hz, 2H), 718 (d, J＝8.0 Hz, 2H), 6.63 (s, 1H), 2.36 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.1, 150.7, 139.5, 133.7, 130.5, 130.3, 129.4, 128.8. 128.6, 124.8, 95.6, 21.2. HRMS (ESI) calcd for C₁₆H₁₄Br- O₂ [M+H]⁺ 317.0172, found 317.0174.

  (Z)-2-Bromo-1-(4-fluorophenyl)vinyl benzoate (3p): White solid. m.p. 79.4~80.9 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.26 (d, J＝8.0 Hz, 2H), 7.70~7.67 (m, 1H), 7.57~7.53 (m, 2H), 7.50~7.47 (m, 2H), 7.08~7.04 (m, 2H), 6.62 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.2 (d, J_CF＝250.8 Hz), 163.0, 149.7, 133.9, 130.3, 129.6 (d, J_CF＝3.2 Hz), 128.6, 128.5, 126.9 (d, J_CF＝8.4 Hz), 115.8 (d, J_CF＝21.9 Hz), 96.4 (d, J_CF＝1.6 Hz). HRMS (ESI) calcd for C₁₅H₁₁BrFO₂ [M+H]⁺ 320.9921, found 320.9925.

  (Z)-2-Bromo-1-(3-fluorophenyl)vinyl benzoate (3q): White solid. m.p. 75.6~76.2 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.06 (d, J＝7.6 Hz, 2H), 7.94 (d, J＝8.8 Hz, 2H), 7.56~7.49 (m, 3H), 7.42~7.36 (m, 4H), 6.70 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 162.6 (d, J_CF＝248.2 Hz), 161.9 (d, J_CF＝3.2 Hz), 150.5, 133.0, 130.8 (d, J_CF＝7.7 Hz), 130.3 (d, J_CF＝7.9 Hz), 129.4, 128.8, 126.0 (d, J_CF＝3.2 Hz), 124.9, 120.1 (d, J_CF＝21.2 Hz), 117.1 (d, J_CF＝23.1 Hz), 96.8. HRMS (ESI) calcd for C₁₅H₁₁BrFO₂ [M+H]⁺ 320.9921, found 320.9922.

  (Z)-2-Bromo-1-(4-chlorophenyl)vinyl benzoate (3r): White solid. m.p. 86.3~88.2 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.25 (d, J＝8.0 Hz, 2H), 7.70~7.66 (m, 1H), 7.57~7.53 (m, 2H), 7.43 (d, J＝8.4 Hz, 2H), 7.34 (d, J＝8.0 Hz, 2H), 6.69 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.0, 149.7, 135.3, 133.9, 131.8, 130.3, 129.0, 128.7, 128.5, 126.2, 97.3. HRMS (ESI) calcd for C₁₅H₁₁BrClO₂ [M+H]⁺ 336.9625, found 336.9628.

  (Z)-2-Bromo-1-(4-bromophenyl)vinyl benzoate (3s): White solid. m.p. 83.3~84.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.24 (d, J＝7.6 Hz, 2H), 7.70~7.67 (m, 1H), 7.57~7.53 (m, 2H), 7.49 (d, J＝8.0 Hz, 2H), 7.36 (d, J＝8.4 Hz, 2H), 6.69 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.0, 149.7, 133.9, 132.3, 131.9, 130.3, 128.7, 128.4, 126.4, 123.5, 97.4. HRMS (ESI) calcd for C₁₅H₁₁Br₂O₂ [M+H]⁺ 382.9100, found 382.9101.

  (Z)-2-Bromo-1-(4-propylphenyl)vinyl benzoate (3t): White solid. m.p. 98.7~100.3 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.25 (d, J＝7.6 Hz, 2H), 7.69~7.66 (m, 1H), 7.56~7.53 (m, 2H), 7.40 (d, J＝8.0 Hz, 2H), 7.17 (d, J＝8.0 Hz, 2H), 6.63 (s, 1H), 2.59 (t, J＝7.6 Hz, 2H), 1.65 (q, J＝7.2 Hz, 2H), 0.95 (t, J＝7.6 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.1, 150.6, 144.2, 133.7, 130.7, 130.3, 128.8, 128.8, 28.6, 124.8, 95.6, 37.7, 24.2, 13.6. HRMS (ESI) calcd for C₁₈H₁₈BrO₂ [M+H]⁺ 345.0485, found 345.0487.

  (Z)-1-Bromohept-1-en-2-yl benzoate (3u):^[6] pale yellow oil. ¹H NMR (400 MHz, CDCl₃) δ: 8.17~8.15 (m, 2H), 7.65~7.61 (m, 1H), 7.52~7.48 (m, 2H), 5.94 (s, 1H), 2.44 (t, J＝7.6 Hz, 2H), 1.57~1.54 (m, 2H), 1.38~1.32 (m, 4H), 0.92~0.89 (m, 2H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.0, 153.2, 133.5, 130.0, 129.0, 128.5, 93.7, 33.5, 31.0, 25.8, 22.2, 13.8. HRMS (ESI) calcd for C₁₄H₁₈BrO₂ [M+H]⁺ 297.0485, found 297.0486.

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

  
  
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• 

  Scheme 1  Transformations of alkynes and 1-bromoalkynes

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  Scheme 2  Proposed mechanism

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  Table 1.  Optimization of the reaction conditions^a

  Entry	Solvent	Base	Ag source	Yieldb/%
  1	DMF	KOAc	Ag2O	80
  2	DMF	Et3N	Ag2O	94
  3	DMF	NaOAc	Ag2O	78
  4	DMF	Cs2CO3	Ag2O	60
  5	DMF	K2CO3	Ag2O	47
  6	DMF	Na2CO3	Ag2O	39
  7	DMF	DABCO	Ag2O	33
  8	DMF	K3PO4	Ag2O	23
  9	DMF	DBU	Ag2O	21
  10	DMF	LiOtBu	Ag2O	NR
  11	DMF	KOtBu	Ag2O	NR
  12	DMA	Et3N	Ag2O	78
  13	DMSO	Et3N	Ag2O	56
  14	NMP	Et3N	Ag 2O	31
  15	CH3CN	Et3N	Ag2O	28
  16	CH3NO2	Et3N	Ag 2O	27
  17	DCE	Et3N	Ag2O	44
  18	DMF	Et3N	—	NR
  19	DMF	Et3N	Ag2CO3	72
  20	DMF	Et3N	AgOAc	68
  21	DMF	Et3N	Ag2SO4	Trace
  22	DMF	Et3N	AgCl	Trace
  23	DMF	Et3N	Ag2O	94c
  24	DMF	Et3N	Ag2O	93d
  25	DMF	Et3N	Ag2O	94e
  26	DMF	Et3N	Ag2O	92f
  a Reaction conditions: phenylethynyl bromide (1a, 0.50 mmol), benzoic acid (2a, 1.0 mmol), Ag-catalyst (10 mol%), base (0.75 mmol, 1.5 equiv.), solvent (2.0 mL), 80 ℃, sealed tube, air, 12 h. b Isolated yield. c Ag2O (7.5 mol%) was used. d Ag2O (5 mol%) was added. e Et3N (0.60 mmol, 1.2 equiv.) was used. f Et3N (0.50 mmol 1.0 equiv.) was added. NR＝No reaction.

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  Table 2.  Scope of aromatic carboxylic acids^a

  a Reaction conditions: phenylethynyl bromide (1a, 0.50 mmol), aryl carboxylic acid (2, 1.0 mmol), Ag2O (7.5 mol%), Et3N (0.60 mmol, 1.2 equiv.), DMF (2.0 mL), 80 ℃, sealed tube, air, 12 h; isolated yields of the products.

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  Table 3.  Scope of acetylene bromides^a

  a Reaction conditions: acetylene bromide (1, 0.50 mmol), benzoic acid (2a, 1.0 mmol), Ag2O (7.5 mol%), Et3N (0.60 mmol, 1.2 equiv.), DMF (2.0 mL), 80 ℃, sealed tube, air, 12 h; isolated yields of the products.

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•