English

• 1.   Introduction

  The functionalization of alkynes represents an extremely powerful tool for the construction of chemical bonds, which has widespread application in organic synthesis.^[1] Alkynes usually are used as feedstock to afford a series of valuable commodity chemicals, such as acetaldehydes, ketones, vinyl esters and some unique compounds that are not easy to obtain by conventional synthetic methods.^[2] Particularly, in the case of functionalized alkynes, a number of vinyl esters can be prepared from the addition reaction of carbon-carbon triple bond, in which the pursuit of good selectivity remains a significant challenge. In recent years, considerable effort has been devoted to explore the versatile additions of alkynes toward achieving high selectivity.^[3] For example, Jiang's group^[4] has realized the difunctionalization of terminal alkynes by silver catalysis, generating a variety of 1-halo-1-alkenes in good yields and selectivity (Scheme 1a). Hou and coworkers^[5] have established the halogenation of internal alkynes using quaternary ammonium salt as mediators. Then it is found that both phenols and 4-hydroxyl coumarins are effective O-containing nucleophiles, which can enable the stereoselective addition of alkynes (Schemes 1b and 1c).^[6] Very recently, Xu et al.^[7] reported an efficient addition of sulfonic acids to haloalkynes, in which the reactivity of sulfonic acids can be tuned by varying the hydrogen bonding cluster (Scheme 1, d). Moreover, it should be noted that the direct addition of carboxyl acid to propiolates also proceeds well under palladium catalysis (Scheme 1, e).^[8] Although the reported strategies have been well developed for the alkyne addition, most of which suffer from poor selectivity or critical reaction conditions. Hence, the development of an efficient and alternative methodology for the alkyne addition with good selectivity is highly desirable.

  Scheme 1

  

  Scheme 1.  Stereoselective addition reactions of alkynes with O-containing nucleophiles

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  Organic hypervalent iodine reagents (HIRs) have gained increasing interest as coupling partners, oxidants and both of them in organic synthesis, mainly due to their selective, versatile and environment-benign properties.^[9] Several typical organoiodine(III) reagents have been successfully employed for the construction of organic molecules.^[9-10] Among them, [bis(acyloxy)iodo]arenes (DIB) and [hydroxy(tosyloxy)iodo]benzene (HTIB, Koser's reagent), belong to one such reagent that has been frequently explored in organic transformations.^[10] For example, we recently have utilized the organoiodine(III) reagents to realize the selective C(sp³)—H acetoxylation of 2H-aziri-dines^[11c] and radical cyclization of unsaturated oximes, ^[11d] respectively. During the course of the investigation, it was found that the cleavage of I—O bond counld be facilitated by iron(II/III) salts. We then postulated the possible cleavage model of I—O bond mediated by other transition metals, such as silver salts. In this contribution, we will report an example of silver-catalyzed I—O cleavage within HIRs derived from DIB and the corresponding addition to functionalized alkynes (Scheme 1f).

  2.   Results and discussion

  Based on previous work, ^[11] we commenced this study with the reaction of (bromoethynyl)benzene 1a with hypervalent iodine reagent 2a to evaluate the feasibility of the process, and the results are summarized in Table 1. Initially, Ag₂O was selected as a catalyst for the catalytic system.^[6b] The model reaction proceeded slightly in CH₃CN at 90 ℃ for 6 h, and the desired product 3a was generated in 21% isolated yield (Entry 1). It shoud be noted that the NOSEY spectra of 3a further supported the observed stereoslectivity. Other silver salts, such as AgOAc, AgOTf and AgNO₃ were then tried for the addition reaction of 1a, but failed to give the product 3a (Entries 2~4). Gratifyingly, it was found that Ag₂CO₃ exhibited high efficiency in this reaction system, affording 3a in 86% yield (Entry 5). Instead of Ag₂CO₃, the use of some other catalysts including CuO and Cu₂O, Fe(acac)₂, Fe(acac)₃ and Pd(OAc)₂ did not promote this addition process (Entries 6~10). No product 3a was isolated in absence of Ag₂CO₃ (Entry 11). Following, it was found that the reaction of 1a with 2a was considerably affected by the reaction medium. For instance, using ethanol or acetic acid as a solvent did not remarkably accelerate the addition process, and only 15% and 22% yields were achieved under the above conditions, respectively (Entries 12 and 13). Moreover, some other polar solvents were also examined in the model reaction, and resulted into inferior yields (Entries 13~17). The reaction did proceed in toluene (Entry 18). Finally, the evaluation of reaction time, temperature and catalyst loading gave the optimal reaction conditions for the addition reaction of 1a with 2a. The experiment results indicated that longer reaction time and higher temperature would lead to partial decomposition of 3a with lower isolated yield (Entries 19~24).

  Table 1

  

  Table 1.  Optimization of reaction conditions^a

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  Entry	Catalyst	Solvent	Yieldb/%
  1	Ag2O	CH3CN	21
  2	AgOAc	CH3CN	0
  3	AgOTf	CH3CN	0
  4	AgNO3	CH3CN	0
  5	Ag2CO3	CH3CN	86
  6	CuO	CH3CN	0
  7	Cu2O	CH3CN	Trace
  8	Fe(acac)2	CH3CN	0
  9	Fe(acac)3	CH3CN	0
  10	Pd(OAc)2	CH3CN	0
  11	—	CH3CN	0
  12	Ag2CO3	EtOH	15
  13	Ag2CO3	AcOH	22
  14	Ag2CO3	THF	Trace
  15	Ag2CO3	DMF	n.r.
  16	Ag2CO3	DMSO	17
  17	Ag2CO3	1, 4-Dioxane	Trace
  18	Ag2CO3	PhMe	0
  19	Ag2CO3	CH3CN	65c
  20	Ag2CO3	CH3CN	83d
  21	Ag2CO3	CH3CN	72e
  22	Ag2CO3	CH3CN	58f
  23	Ag2CO3	CH3CN	78g
  24	Ag2CO3	CH3CN	64h
  a Reaction conditions: 1a (0.2 mmol), 2a (0.2 mmol) and catalyst (2.5 mol%) in CH3CN (1 mL) at 90 ℃ under ambient air for 6 h.b Isolated yield.cAg2CO3 (1 mol%). dAg2CO3 (5 mol%). e 80 ℃. f100 ℃. g 8 h. h5 h.

  With the optimal conditions in hand, we next investigated the applicability of this method across a series of bromoalkynes, and the result is listed in Table 2. The bromoalkynes bearing aryl group could undergo the reaction with 2a to deliver the related products in good yields. Notably, the para-substitued substrates with an electron-donating group such as Me, ⁿPr, ⁿBu, ⁿPent, MeO and EtO, were well tolerated under the standard conditions (3b~3g). The incorporation of F and Br into the para position did not affect the silver-catalyzed system, generating the desired product in 89% (3h) and 86% (3i) yields, respectively. We then found that the bromoalkynes with electron-deficient groups including CN, NO₂ and CF₃ afforded the products in excellent yields (3j~3l). It was found that 4-(bromoethynyl)-1, 1'-biphenyl could react with 2a to give 76% yield of 3m. The use of meta-substituted bromoalkynes gave the according products in comparable yields, indicating weak steric hindrance exists in this process (3n and 3o). Introducing thienyl group into bromoalkynes still generated the desired products, albeit with moderate yields (3p~3q). Finally, it was found that 1-bromopent-1-yne still reacted with 2a to afford the desired product 3r in 75% isolated yield.

  Table 2

  

  Table 2.  Scope of bromoalkynes^a

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  a Reaction conditions: 1a (0.2 mmol), 2 (0.2 mmol) and Ag2CO3 (2.5 mol%) in CH3CN (1 mL) at 90 ℃ under ambient air for 6 h. b Isolated yield.

  To explore the scope of this method, we next turned our attention to the HIRs, and the result is shown in Table 3. Firstly, a number of PhI(O₂CR)₂ were readily synthesized through a ligand exchange of DIB with carboxylate acids in boiling toluene. Notably, most of the HIRs derived from aryl carboxylic acid reacted with (bromoethynyl)benzene 1a under optimized conditions to afford the addition products in satisfactory yields (4a~4d). The scope was also expanded to HIRs derived from alkyl carboxylic acid, and which produced the addition products in lower yields (4e~4f). This result can be explained by an intramolecular competition experiment of BI-OAc with bromoalkyne 1a under standard reaction conditions, generating 5 as a major product in 76% yield (Scheme 2). It was found that replacing the bromide with Cl or I within alkynes still gave the corresponding products in acceptable yields (4g and 4h).

  Table 3

  

  Table 3.  Scope of HIRs and alkynes^a

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  a Reaction conditions: 1 (0.2 mmol), 2 (0.2 mmol) and Ag2CO3 (2.5 mol%) in CH3CN (1 mL) at 90 ℃ under ambient air for 6 h. bIsolated yield. c24 h. d10 h.

  Scheme 2

  

  Scheme 2.  Intramolecular competition experiment

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  Furthermore, methyl 3-phenylpropiolate was employed to react with HIRs under the modified reaction conditions, and which delivered the vinyl esters in excellent yields (4i~4l). Then, a gram-scale reaction of 1a with 2a was performed under standard conditions, which generated 3a in 72% yield (Scheme 3a). Finally, a transformation of 3i with 1-phenylethyne by Sonogashira reaction was realized, and produced the product 6 in 70% yield (Scheme 3b).

  Scheme 3

  

  Scheme 3.  Gram-scale reaction and late-stage transformation

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  To reveal the reaction mechanism, several control experiments were subsequently designed and performed under given conditions (Scheme 4). It was found that the reaction of 1-phenylethyne with DIB under air condition did not give the desired product 7, which demonstrated the critical role of halogens (Scheme 4a).^[12] The addition of a radical quencher, 2, 2, 6, 6-tetramethyl-1-piperidinyloxy (TEMPO), into the reaction of 1a with DIB did not completely inhibit the process under air atmosphere, generating 4e in 32% yield, which indicated a radical pathway was not involved therein (Scheme 4b). The model reaction was carried out in CH₃CN at 90 ℃ under a N₂ atmosphere for 6 h, and generated 4e in 59% yield, showing the presence of oxygen did not affect the addition event (Scheme 4c). Further, it was found that the selected reaction almost did not proceed in anhydrous CH₃CN (Scheme 4, d). We then treated this reaction with heavy water, and a 1.86:1 ratio of (vinylic) D/H was observed by the ¹H NMR analysis (Scheme 4e). The above result demonstrates that the vinylic hydrogen comes from H₂O and a protonation step also takes part in this catalytic system.

  Scheme 4

  

  Scheme 4.  Mechanistic studies

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  Based on the above results and previous report, ^[4-7] a possible reaction mechanism for the silver-catalyzed stereoselective addition reaction of functionalized alkynes with DIB is depicted in Scheme 5. Initially, in the presence of H₂O, DIB interacts with silver(I) via a I—O bond cleavage to generate species A and cationic iodine intermediate B.^[12] Then bromoalkyne 1 is activated by weak coordination with A or B to afford the complex C or C', ^{[4, 6b, 13]} followed by a nucleophilic attack of OAc^－ on the carbon- carbon triple bond to give intermediate D.^[13] Finally, the hydrolysis of D releases a hypervalent iodine compound E and desired product 3.

  Scheme 5

  

  Scheme 5.  Possible reaction mechanism

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  3.   Conclusions

  In summary, a silver-catalyzed stereoselective addition reactions of functionalized alkynes with hypervalent iodine(III) reagents via I—O bond cleavage has been developed, which provides an efficient access to vinyl esters in high yields. The control experiment supports that the presence of aryl group considerably affects the cleavage model of I—O bond within hypervalent iodine(III) reagents. The further exploration of this strategy using HIRs and transition metal catalysis in synthetic chemistry and mechanistic studies is underway in our laboratory.

  4.   Experimental section

  4.1   General information

  All ¹H NMR and ¹³C NMR spectra were recorded on 400, 600 MHz Bruker FT-NMR spectrometers (400 MHz and 100 MHz, 600 MHz and 150 MHz, respectively). All chemical shifts are given with reference to tetramethylsilane (TMS) as an internal standard. High resolution mass spectroscopy data of the products were collected on a Thermo Fisher Scientific LTQ FTICR-MS instrument.

  The starting materials, such as (bromoethynyl)benzene and hypervalent iodine(III) reagents, are prepared according to the reported methods, respectively.^[1-2] All chemical reagents used in this work were purchased from commercial suppliers, Huawei Ruike Chemical company, Energy Chemical company, or Shanghai Chemical Company of China and Aldrich of USA. All solvents were dried and freshly distilled prior to use. Products were purified by flash chromatography on silica gels with petroleum ether/ethyl acetate (V : V＝10:1 to 20:1) as eluent.

  4.2   General procedure for the Ag-catalyzed stereoselective addition reaction

  A 10 mL oven-dried reaction tube equipped with a magnetic stirrer bar was charged with alkyne 1 (0.20 mmol), HIRs 2 (0.20 mmol), silver carbonate (2.5 mol%), and freshly distilled CH₃CN (1.0 mL) under air atmosphere. The reaction tube was placed in an oil bath and stirred at 90 ℃ for 6 h. After the reaction was completed, the mixture was cooled to room temperature, detected by thin-layer chromatography (TLC), and extracted with ethyl acetate (5 mL×3). The organic layers were combined, dried over anhydrous MgSO₄, and concentrated under reduced pressure to yield the crude product, which was further purified by flash chromatography [silica gel, V (petroleum ether): V (ethyl acetate)＝10:1 to 20:1], affording the desired product 3.

  (Z)-2-Bromo-1-phenylvinyl 4-methylbenzoate (3a): Yellow liquid, 51.8 mg, 82% yield. ¹H NMR (600 MHz, CDCl₃) δ: 8.13 (d, J＝8.0 Hz, 2H), 7.49~7.48 (m, 2H), 7.36~7.32 (m, 5H), 6.66 (s, 1H), 2.47 (s, 3H); ¹³C NMR (150 MHz, CDCl₃) δ: 163.17, 150.65, 144.81, 133.41, 130.40, 129.39, 129.34, 128.78, 125.96, 124.92, 96.65, 21.77; HRMS (ESI) calcd for C₁₆H₁₃BrNaO₂ [M+Na]⁺ 338.9991, found 338.9991.

  (Z)-2-Bromo-1-(p-tolyl)vinyl 4-methylbenzoate (3b): Pale yellow solid, 52.7 mg, 80% yield. m.p. 46.8~47.4 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.13 (t, J＝8.4 Hz, 2H), 7.36 (dd, J＝16.0, 8.0 Hz, 4H), 7.16 (d, J＝8.0 Hz, 2H), 6.60 (s, 1H), 2.47 (s, 3H), 2.34 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.18, 150.69, 144.72, 139.46, 130.62, 130.37, 129.45, 129.36, 126.02, 124.82, 95.61, 21.75, 21.25; HRMS (ESI) calcd for C₁₇H₁₄BrNaO₂ [M+Na]⁺ 353.0148, found 353.0150.

  (Z)-2-Bromo-1-(4-propylphenyl)vinyl 4-methylbenzoate (3c): Pale yellow solid, 60.2 mg, 84% yield. m.p. 98.7~100.4 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.13 (d, J＝8.0 Hz, 2H), 7.38 (d, J＝8.0 Hz, 2H), 7.32 (d, J＝8.0 Hz, 2H), 7.15 (d, J＝8.0 Hz, 2H), 6.61 (s, 1H), 2.59~2.55 (m, 2H), 2.47 (s, 3H), 1.65~1.59 (m, 2H), 0.93 (t, J＝7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.19, 150.71, 144.72, 144.24, 130.82, 130.39, 129.36, 128.86, 126.04, 124.82, 95.66, 37.75, 24.25, 21.75, 13.73; HRMS (ESI) calcd for C₁₉H₁₉BrNaO₂ [M+Na]⁺ 381.0461, found 381.0460.

  (Z)-2-Bromo-1-(4-butylphenyl)vinyl 4-methylbenzoate (3d): Pale yellow solid, 61.8 mg, 83% yield. m.p. 89.2~91.3 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.13 (d, J＝8.4 Hz, 2H), 7.39 (d, J＝8.0 Hz, 2H), 7.33 (d, J＝8.0 Hz, 2H), 7.16 (d, J＝8.0 Hz, 2H), 6.61 (s, 1H), 2.59 (t, J＝7.6 Hz, 2H), 2.46 (s, 3H), 1.58 (t, J＝8.0 Hz, 2H), 1.34 (dd, J＝15.2, 7.6 Hz, 2H), 0.92 (t, J＝7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.19, 150.73, 144.72, 144.48, 130.79, 130.39, 129.36, 128.82, 126.06, 124.84, 95.65, 35.37, 33.30, 22.27, 21.76, 13.87; HRMS (ESI) calcd for C₂₀H₂₁- BrNaO₂ [M+Na]⁺ 395.0617, found 395.0617.

  (Z)-2-Bromo-1-(4-pentylphenyl)vinyl 4-methylbenzoate (3e): Pale yellow solid, 65.6 mg, 85% yield. m.p. 101.4~103.2 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.14 (d, J＝8.0 Hz, 2H), 7.39 (d, J＝8.0 Hz, 2H), 7.34 (d, J＝8.0 Hz, 2H), 7.16 (d, J＝8.0 Hz, 2H), 6.61 (s, 1H), 2.59 (t, J＝7.6 Hz, 2H), 2.47 (s, 3H), 1.61~1.58 (m, 2H), 1.36~1.30 (m, 4H), 0.90 (t, J＝6.9 Hz, 3H); ¹³C NMR (150 MHz, CDCl₃) δ: 163.25, 150.78, 144.77, 144.56, 130.84, 130.45, 129.42, 128.87, 126.12, 124.90, 95.71, 35.71, 31.45, 30.90, 22.53, 21.81, 14.02; HRMS (ESI) calcd for C₂₁H₂₃BrNaO₂ [M+Na]⁺ 409.0774, found 409.0774.

  (Z)-2-Bromo-1-(4-methoxyphenyl)vinyl 4-methyl-benzoate (3f): Pale yellow solid, 57.5 mg, 83% yield. m.p. 101.6~102.6 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.13 (d, J＝8.0 Hz, 2H), 7.42 (d, J＝8.0 Hz, 2H), 7.33 (d, J＝7.6 Hz, 2H), 6.87 (d, J＝7.6 Hz, 2H), 6.51 (s, 1H), 3.80 (s, 3H); 2.46 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ:164.45, 161.73, 151.68, 145.97, 131.63, 130.62, 127.68, 127.34, 115.44, 95.72, 56.56, 23.00; HRMS (ESI) calcd for C₁₇H₁₆BrNaO₃ [M+Na]⁺ 369.0097, found 369.0097.

  (Z)-2-Bromo-1-(4-ethoxyphenyl)vinyl 4-methylbenzoate (3g): Pale yellow solid, 61.9 mg, 86% yield. m.p. 102.7~103.2 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.13 (d, J＝8.0 Hz, 2H), 7.40 (d, J＝8.8 Hz, 2H), 7.33 (d, J＝8.0 Hz, 2H), 6.85 (d, J＝8.8 Hz, 2H), 6.51 (s, 1H), 4.02 (dd, J＝13.0, 8.4 Hz, 2H), 2.46 (s, 3H), 1.40 (t, J＝6.8 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.19, 159.85, 150.45, 144.70, 130.38, 129.36, 126.39, 126.08, 125.86, 114.67, 94.35, 63.53, 21.76, 14.70; HRMS (ESI) calcd for C₁₈H₁₇- BrNaO₃ [M+Na]⁺ 383.0253, found 383.0253.

  (Z)-2-Bromo-1-(4-fluorophenyl)vinyl 4-methylbenzoate (3h): Pale yellow solid, 59.5 mg, 89% yield. m.p. 76.2~77.5 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.12 (d, J＝8.0 Hz, 2H), 7.48~7.45 (m, 2H), 7.33 (d, J＝7.8 Hz, 2H), 7.04 (t, J＝9.0 Hz, 2H), 6.59 (s, 1H), 2.46 (s, 3H); ¹³C NMR (150 MHz, CDCl₃) δ: 163.25 (d, J＝249.9 Hz), 162.42, 149.81, 144.95, 130.40, 129.75 (d, J＝3.2 Hz), 129.43, 126.94 (d, J＝8.1 Hz), 125.80, 115.89 (d, J＝21.8 Hz), 96.38, 21.77; ¹⁹F NMR (600 MHz, CDCl₃) δ: －111.2; HRMS (ESI) calcd for C₁₆H₁₂BrFNaO₂ [M+ Na]⁺ 356.9897, found 356.9899.

  (Z)-2-Bromo-1-(4-bromophenyl)vinyl 4-methylbenzoate (3i): Pale yellow solid, 67.7 mg, 86% yield. m.p. 83.3~84.8 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.11 (d, J＝8.0 Hz, 2H), 7.48 (d, J＝8.8 Hz, 2H), 7.34 (dd, J＝8.5, 2.5 Hz, 4H), 6.67 (s, 1H), 2.47 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.08, 149.81, 144.99, 132.45, 131.98, 130.40, 129.44, 126.44, 125.73, 123.53, 97.33, 21.77; HRMS (ESI) calcd for C₁₆H₁₂Br₂NaO₂ [M+Na]⁺ 416.9096, found 416.9097.

  (Z)-2-Bromo-1-(4-cyanophenyl)vinyl 4-methylbenzoate (3j): Pale yellow solid, 61.4 mg, 90% yield. m.p. 156.3~157.7 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.11 (d, J＝8.0 Hz, 2H), 7.64 (d, J＝8.4 Hz, 2H), 7.57 (d, J＝8.4 Hz, 2H), 7.34 (d, J＝8.0 Hz, 2H), 6.84 (s, 1H), 2.47 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.02, 149.14, 145.30, 137.62, 132.62, 130.44, 129.53, 125.38, 118.24, 112.80, 100.42, 21.78; HRMS (ESI) calcd for C₁₇H₁₂BrNNaO₂ [M+Na]⁺ 363.9944, found 363.9942.

  (Z)-2-Bromo-1-(4-nitrophenyl)vinyl 4-methylbenzoate (3k): Pale yellow solid, 66.4 mg, 92% yield. m.p. 148.0~149.9 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.21 (d, J＝8.8 Hz, 2H), 8.12 (d, J＝8.4 Hz, 2H), 7.64 (d, J＝9.2 Hz, 2H), 7.35 (d, J＝8.0 Hz, 2H), 6.91 (s, 1H), 2.48 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.00, 148.91, 147.92, 145.35, 139.34, 130.43, 129.54, 125.58, 125.30, 124.12, 101.10, 21.78; HRMS (ESI) calcd for C₁₆H₁₂BrNNaO₄ [M+Na]⁺ 383.9842, found 383.9842.

  (Z)-2-Bromo-1-(4-(trifluoromethyl)phenyl)vinyl 4-methylbenzoate (3l): Pale yellow solid, 66.8 mg, 87% yield. m.p. 75.3~77.2 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.12 (d, J＝8.0 Hz, 2H), 7.63~7.57 (m, 4H), 7.35 (d, J＝8.0 Hz, 2H), 6.79 (s, 1H), 2.47 (s, 3H); ¹³C NMR (150 MHz, CDCl₃) δ: 163.11, 149.53, 145.17, 136.83, 131.12 (q, J＝32.4 Hz), 130.78, 130.44, 129.50, 123.79 (q, J＝270.4 Hz), 125.85 (q, J＝3.0 Hz), 125.188, 99.15, 21.79; ¹⁹F NMR (600 MHz, CDCl₃) δ: －62.8; HRMS (ESI) calcd for C₁₇H₁₃BrF₃NaO₂ [M+Na]⁺ 406.9865, found 406.9865.

  (Z)-1-([1, 1'-Biphenyl]-4-yl)-2-bromovinyl 4-methyl-benzoate (3m): Pale yellow solid, 59.9 mg, 76% yield. m.p. 123.3~124.6 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.16 (d, J＝8.4 Hz, 2H), 7.60~7.54 (m, 6H), 7.44 (t, J＝7.2 Hz, 2H), 7.36 (t, J＝8.8 Hz, 3H), 6.72 (s, 1H), 2.48 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.24, 150.42, 144.86, 142.16, 140.18, 132.26, 130.44, 129.43, 128.84, 127.68, 127.49, 127.00, 125.96, 125.31, 96.62, 21.78; HRMS (ESI) calcd for C₂₂H₁₇BrNaO₂ [M+Na]⁺ 415.0304, found 415.0306.

  (Z)-2-Bromo-1-(m-tolyl)vinyl 4-methylbenzoate (3n): Pale yellow solid, 52.1 mg, 79% yield. m.p. 45.8~47.9 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.18 (d, J＝8.4 Hz, 2H), 7.38 (d, J＝8.0 Hz, 2H), 7.33 (d, J＝6.4 Hz, 2H), 7.29 (d, J＝7.6 Hz, 1H), 7.21 (d, J＝7.2 Hz, 1H), 6.68 (s, 1H), 2.51 (s, 3H), 2.39 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.19, 150.79, 144.75, 138.47, 133.36, 130.39, 130.18, 129.37, 128.66, 126.01, 125.53, 122.09, 96.38, 21.74, 21.39; HRMS (ESI) calcd for C₁₇H₁₅BrNaO₂ [M+Na]⁺ 353.0148, found 353.0148.

  (Z)-2-Bromo-1-(3-fluorophenyl)vinyl 4-methylbenzoate (3o): pale yellow solid, 54.1 mg, 81% yield. m.p. 78.0~79.2 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.13 (d, J＝8.0 Hz, 2H), 7.42 (td, J＝7.7, 1.6 Hz, 1H), 7.36~7.29 (m, 3H), 7.14~7.09 (m, 2H), 6.91 (s, 1H), 2.46 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.08, 159.48 (d, J＝250.3 Hz), 145.07 (d, J＝4.3 Hz), 144.86, 130.58 (d, J＝8.7 Hz), 130.40, 129.39, 127.72 (d, J＝1.7 Hz), 125.80, 124.36 (d, J＝3.5 Hz), 121.26 (d, J＝11.4 Hz), 116.39 (d, J＝22.5 Hz), 101.81 (d, J＝13.7 Hz), 21.75; ¹⁹F NMR (600 MHz, CDCl₃) δ: －111.9; HRMS (ESI) calcd for C₁₆H₁₂Br- FNaO₂ [M+Na]⁺ 356.9897, found 356.9897.

  (Z)-2-Bromo-1-(thiophen-2-yl)vinyl 4-methylbenzoate (3p): Pale yellow solid, 43.8 mg, 68% yield. m.p. 86.1~87.9 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.12 (d, J＝8.4 Hz, 2H), 7.33 (d, J＝8.0 Hz, 2H), 7.28 (dd, J＝4.8, 0.8 Hz, 1H), 7.15 (dd, J＝4.0, 0.8 Hz, 1H), 6.98 (dd, J＝5.2, 4.0 Hz, 1H), 6.59 (s, 1H), 2.46 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 162.87, 145.65, 144.91, 136.56, 130.44, 129.39, 127.57, 126.36, 125.74, 125.15, 95.51, 21.77; HRMS (ESI) calcd for C₁₄H₁₁Br NaO₂S [M+Na]⁺ 344.9555, found 344.9556.

  (Z)-2-Bromo-1-(thiophen-3-yl)vinyl 4-methylbenzoate (3q): Pale yellow solid, 39.3 mg, 61% yield. m.p. 88.6~89.3 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.12 (d, J＝8.4 Hz, 2H), 7.34~7.31 (m, 4H), 7.19 (dd, J＝4.8, 1.2 Hz, 1H), 6.61 (s, 1H), 2.47 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.03, 146.88, 144.85, 135.01, 130.39, 129.39, 126.93, 125.87, 124.20, 122.36, 96.27, 21.76; HRMS (ESI) calcd for C₁₄H₁₁BrNaO₂S [M+Na]⁺ 344.9555, found 344.9555.

  (Z)-1-Bromohept-1-en-2-yl 4-methylbenzoate (3r): Pale yellow liquid, 46.5 mg, 75% yield. ¹H NMR (600 MHz, CDCl₃) δ: 8.03 (d, J＝8.4 Hz, 2H), 7.29 (d, J＝7.8 Hz, 2H), 5.91 (s, 1H), 2.44 (s, 3H), 2.42~2.39 (m, 2H), 1.53~1.51 (m, 2H), 1.33~1.31 (m, 4H), 0.90~0.87 (m, 3H); ¹³C NMR (150 MHz, CDCl₃) δ: 163.19, 153.27, 144.48, 130.20, 129.26, 126.31, 93.70, 33.66, 31.09, 25.85, 22.30, 21.73, 13.91; HRMS (ESI) calcd for C₁₅H₁₉BrNaO₂ [M+Na]⁺ 333.0461, found 333.0463.

  For the synthesis of compound 4, see the general procedure for compound 3.

  (Z)-2-Bromo-1-phenylvinyl benzoate (4a):^[14] Pale yellow solid, 44.7 mg, 74% yield. m.p. 45.8~47.3 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.25 (dd, J＝8.0, 0.8 Hz, 2H), 7.67 (t, J＝7.6 Hz, 1H), 7.54 (t, J＝7.8 Hz, 2H), 7.51~7.48 (m, 2H), 7.38~7.36 (m, 3H), 6.68 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.10, 150.63, 133.86, 133.31, 130.33, 129.39, 128.79, 128.74, 128.67, 124.92, 96.71.

  (Z)-2-Bromo-1-phenylvinyl 4-(tert-butyl)benzoate (4b): White solid, 58.0 mg, 81% yield. m.p. 82.4~83.7 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.17 (d, J＝8.8 Hz, 2H), 7.55 (d, J＝8.4 Hz, 2H), 7.50~7.47 (m, 2H), 7.36~7.33 (m, 3H), 6.66 (s, 1H), 1.38 (s, 9H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.09, 157.76, 150.68, 133.46, 130.29, 129.35, 128.78, 125.95, 125.70, 124.95, 96.64, 35.24, 31.09; HRMS (ESI) calcd for C₁₉H₁₉BrNaO₂ [M+Na]⁺ 381.0461, found 381.0461.

  (Z)-2-Bromo-1-phenylvinyl 3-chlorobenzoate (4c): Pale yellow solid, 51.7 mg, 77% yield. m.p. 79.9~80.9 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.23 (t, J＝1.6 Hz, 1H), 8.13 (dt, J＝8.0 Hz, 1.2 Hz, 1H), 7.66~7.63 (m, 1H), 7.50~7.46 (m, 3H), 7.37 (dd, J＝6.0, 2.4 Hz, 3H), 6.69 (s, 1H); ¹³C NMR (100 MHz, CDCl₃) δ: 161.91, 150.47, 134.87, 133.90, 133.01, 130.44, 130.26, 130.01, 129.51, 128.83, 128.42, 124.89, 96.89; HRMS (ESI) calcd for C₁₅H₁₀Br- ClNaO₂ [M+Na]⁺ 358.9445, found 358.9445.

  (Z)-2-Bromo-1-phenylvinyl 2-methylbenzoate (4d): Pale yellow solid, 50.5 mg, 80% yield. m.p. 47.8~48.3 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.29 (d, J＝8.0 Hz, 1H), 7.54~7.50 (m, 3H), 7.38~7.33 (m, 5H), 6.67 (s, 1H), 2.68 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 163.50, 150.68, 141.66, 133.51, 132.97, 131.98, 131.33, 129.34, 128.80, 127.75, 125.96, 124.92, 96.63, 21.86; HRMS (ESI) calcd for C₁₆H₁₃BrNaO₂ [M+Na]⁺ 338.9991, found 338.9990.

  (Z)-2-Bromo-1-phenylvinyl acetate (4e)^[4a]: Yellow liquid, 25.4 mg, 53% yield. ¹H NMR (400 MHz, CDCl₃) δ: 7.44~7.40 (m, 2H), 7.38~7.35 (m, 3H), 6.55 (s, 1H), 2.34 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 167.23, 150.50, 133.29, 129.39, 128.77, 124.91, 96.55, 20.57.

  (Z)-2-Bromo-1-(p-tolyl)vinyl heptanoate (4f): Yellow liquid, 23.9 mg, 37% yield. ¹H NMR (400 MHz, CDCl₃) δ: 7.32 (d, J＝8.8 Hz, 2H), 6.85 (d, J＝8.8 Hz, 2H), 6.39 (s, 1H), 4.03 (q, J＝7.2 Hz, 2H), 2.59 (t, J＝7.6 Hz, 2H), 1.78 (q, J＝7.2 Hz, 2H), 1.44~1.34 (m, 7H), 0.93 (t, J＝7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ: 170.11, 159.83, 150.28, 126.37, 125.89, 114.62, 94.17, 63.53, 33.99, 31.25, 24.52, 22.27, 14.70, 13.87; HRMS (ESI) calcd for C₁₆H₂₁BrNaO₂ [M+Na]⁺ 347.0617, found 347.0616.

  (Z)-2-Chloro-1-phenylvinyl acetate (4g):^[4a] Pale yellow liquid, 21.5 mg, 55% yield. ¹H NMR (600 MHz, CDCl₃) δ: 7.39~7.36 (m, 5H), 6.45 (s, 1H), 2.34 (s, 3H); ¹³C NMR (150 MHz, CDCl₃) δ: 167.33, 148.41, 132.76, 129.35, 128.76, 124.70, 107.98, 20.46.

  (Z)-2-Iodo-1-phenylvinyl acetate (4h):^[4a] Pale yellow liquid, 35.7 mg, 62% yield. ¹H NMR (600 MHz, CDCl₃) δ: 7.42 (s, 2H), 7.35 (s, 3H), 6.63 (s, 1H), 2.35 (s, 3H); ¹³C NMR (150 MHz, CDCl₃) δ: 167.33, 155.04, 133.66, 129.39, 128.73, 125.15, 68.28, 20.98.

  (Z)-Methyl-3-acetoxy-3-phenylacrylate (4i): Pale yellow liquid, 41.8 mg, 95% yield. ¹H NMR (600 MHz, CDCl₃) δ: 7.59 (d, J＝7.8 Hz, 2H), 7.44 (d, J＝6.6 Hz, 1H), 7.41 (t, J＝7.2 Hz, 2H), 6.28 (s, 1H), 3.74 (s, 3H), 2.40 (s, 3H); ¹³C NMR (150 MHz, CDCl₃) δ: 168.12, 164.60, 158.31, 133.20, 131.06, 128.80, 125.90, 105.57, 51.49, 20.92; HRMS (ESI) calcd for C₁₂H₁₃O₄ [M+H]⁺ 221.0808, found 221.0808.

  (Z)-3-Methoxy-3-oxo-1-phenylprop-1-en-1-yl butyrate (4j): Pale yellow liquid, 45.6 mg, 92% yield. ¹H NMR (600 MHz, CDCl₃) δ: 7.58 (d, J＝7.2 Hz, 2H), 7.45~7.39 (m, 3H), 6.28 (s, 1H), 3.74 (s, 3H), 2.68 (t, J＝7.8 Hz, 2H), 1.86~1.80 (m, 2H), 1.06 (t, J＝7.2 Hz, 3H); ¹³C NMR (150 MHz, CDCl₃) δ: 170.64, 164.56, 158.29, 133.46, 130.95, 128.77, 125.89, 105.73, 51.43, 36.01, 18.04, 13.70; HRMS (ESI) calcd for C₁₄H₁₇O₄ [M+H]⁺ 249.1121, found 249.1120.

  (Z)-3-Methoxy-3-oxo-1-phenylprop-1-en-1-yl 3-chloro-benzoate (4k): White solid, 60.7 mg, 96% yield. m.p. 146.1~147.4 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 7.74 (s, 1H), 7.73 (d, J＝7.8 Hz, 1H), 7.25~7.22 (m, 2H), 7.10~7.00 (m, 5H), 6.02 (s, 1H), 3.30 (s, 3H); ¹³C NMR (150 MHz, CDCl₃)δ: 164.29, 162.61, 158.03, 134.73, 133.69, 132.91, 131.15, 130.71, 130.24, 129.95, 128.85, 128.41, 125.86, 106.00, 51.51; HRMS (ESI) calcd for C₁₇H₁₄ClO₄ [M+H]⁺ 317.0575, found 317.0576.

  (Z)-3-Methoxy-3-oxo-1-phenylprop-1-en-1-yl 4-(tert-butyl)benzoate (4l): White solid, 60.8 mg, 90% yield. m.p. 149.1~149.8 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.18 (d, J＝7.8 Hz, 2H), 7.66 (d, J＝7.8 Hz, 2H), 7.55 (d, J＝7.8 Hz, 2H), 7.43 (d, J＝6.6 Hz, 1H), 7.40 (t, J＝7.8 Hz, 2H), 6.41 (s, 1H), 3.68 (s, 3H), 1.37 (s, 9H); ¹³C NMR (150 MHz, CDCl₃) δ: 164.46, 163.80, 158.38, 157.44, 133.41, 130.97, 130.24, 128.79, 126.17, 125.95, 125.68, 106.04, 51.49, 35.16, 31.06; HRMS (ESI) calcd for C₂₁H₂₃O₄ [M+H]⁺ 339.1591, found 339.1590.

  4.3   General procedure for the intramolecular competition experiment

  A 10 mL oven-dried reaction tube equipped with a magnetic stirrer bar was charged with bromoalkyne 1a (0.20 mmol), BI-OAc (0.20 mmol), silver carbonate (5 mol%) and freshly distilled CH₃CN (1.0 mL) under air atmosphere. The reaction tube was placed in an oil bath and stirred at 90 ℃ for 6 h. After the reaction was completed (detected by TLC), the mixture was cooled to room temperature, and extracted with ethyl acetate (5 mL×3). The organic layers were combined, dried over anhydrous MgSO₄, and concentrated under reduced pressure to yield the crude product, which was further purified by flash chromatography [silica gel, V (petroleum ether):V (ethyl acetate)＝10:1 to 20:1], affording the desired product (Z)-2-bromo-1-phenylvinyl 2-iodobenzoate (5) as white solid (65.0 mg, 76% yield). m.p. 61.4~62.8 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.26 (dd, J＝7.8, 1.8 Hz, 1H), 8.11 (d, J＝7.8 Hz, 1H), 7.54~7.52 (m, 3H), 7.41~7.39 (m, 3H), 7.28 (t, J＝7.8 Hz, 1H), 6.70 (s, 1H); ¹³C NMR (150 MHz, CDCl₃) δ: 162.40, 150.40, 141.91, 133.59, 132.92, 132.74, 131.85, 129.48, 128.81, 128.08, 124.99, 96.98, 95.11; HRMS (ESI) calcd for C₁₅H₁₀BrINaO₂ [M+Na]⁺ 450.8801, found 450.8801.

  4.4   General procedure for the gram-scale synthesis of 3a

  A 25 mL oven-dried reaction tube equipped with a magnetic stirrer bar was charged with 1a (5.0 mmol), HIRs 2a (5.00 mmol), silver carbonate (2.5 mol%) and freshly distilled CH₃CN (10 mL) under air atmosphere. The reaction tube was placed in an oil bath and stirred at 90 ℃ for 10 h. After the reaction was completed, the mixture was cooled to room temperature, and extracted with ethyl acetate (20 mL×3). The organic layers were combined, dried over anhydrous MgSO₄, and concentrated under reduced pressure to yield the crude product, which was further purified by flash chromatography [silica gel, V (petroleum ether): V (ethyl acetate)＝10:1 to 20:1], affording the desired product 3a as pale yellow liquid (1.14 g, 72%).

  4.5   General procedure for the late-stage transformation

  Under nitrogen atmosphere, a 10 mL oven-dried reaction tube equipped with a magnetic stirrer bar was charged with 3i (0.40 mmol), 1-phenylethyne (0.84 mmol), Pd(dppf)₂Cl₂ (1 mol %), CuI (10 mol%), triethylamine (2.0 mol) and anhydrous N, N-dimethylformamide (DMF, 1.0 mL). The reaction tube was placed in an oil bath and stirred at 80 ℃ for 12 h. After the reaction was completed, the mixture was cooled to room temperature, and extracted with ethyl acetate (5 mL×3). The organic layers were combined, dried over anhydrous MgSO₄, and concentrated under reduced pressure to yield the crude product, which was further purified by flash chromatography [silica gel, V (petroleum ether): V (ethyl acetate)＝9:1], affording the desired product (Z)-4-phenyl-1-(4-(phenyl- ethynyl)phenyl)but-1-en-3-yn-1-yl 4-methylbenzoate (6) as white solid (122.7 mg, 70%). m.p. 185.1~186.3 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.19 (d, J＝7.8 Hz, 2H), 7.56~7.51 (m, 6H), 7.36~7.34 (m, 5H), 7.24~7.14 (m, 6H), 6.30 (s, 1H), 2.49 (s, 3H); ¹³C NMR (150 MHz, CDCl₃) δ: 163.93, 155.42, 144.70, 133.22, 131.91, 131.80, 131.66, 131.41, 130.45, 129.42, 128.48, 128.37, 128.32, 128.16, 126.31, 124.49, 123.04, 123.00, 98.61, 98.04, 91.37, 89.05, 84.39, 21.80; HRMS (ESI) calcd for C₃₂H₂₃O₂ [M+H]⁺ 439.1693, found 439.1693.

  4.6   General procedure for the radical trapping experiments with TEMPO

  A 10 mL oven-dried reaction tube equipped with a magnetic stirrer bar was charged with 1a (0.20 mmol), (diacetoxyiodo)benzene (DIB, 0.20 mmol), silver carbonate (2.5 mol%), TEMPO (0.40 mmol) and freshly distilled CH₃CN (1.0 mL) under air atmosphere. The reaction tube was placed in an oil bath and stirred at 90 ℃ for 6 h. After that, the reaction mixture was cooled to room temperature, and extracted with ethyl acetate (5 mL×3). The organic layers were combined, dried over anhydrous MgSO₄, and concentrated under reduced pressure to yield the crude product, which was further purified by flash chromatography [silica gel, V (petroleum ether): V (ethyl acetate)＝10:1 to 20:1], affording the desired product 4e in 32% yield. Therefore, a radical pathway can be ruled out for this silver-catalyzed system.

  4.7   General procedure for deuterium-labeled experiments

  A 10 mL oven-dried reaction tube equipped with a magnetic stirrer bar was charged with 1a (0.20 mmol), (diacetoxyiodo)benzene (DIB, 0.20 mmol), silver carbonate (2.5 mol%), D₂O (0.20 mmol) and freshly distilled CH₃CN (1.0 mL) under N₂ atmosphere. The reaction tube was placed in an oil bath and stirred at 90 ℃ for 6 h. After the reaction was completed, the mixture was cooled to room temperature, and extracted with ethyl acetate (5 mL×3). The organic layers were combined, dried over anhydrous MgSO₄, and concentrated under reduced pressure to yield the crude product, which was further purified by flash chromatography [silica gel, V (petroleum ether): V (ethyl acetate)＝10:1 to 20:1], affording the desired product 4e/[D]-4e (ca. 29% total yield). The ratio of mixture ([D]-4e/4e＝1.86) was determined by ¹H NMR analysis of the obtained mixture, which indicates the pronation step was involved therein.

  Supporting Information  ¹H NMR and ¹³C NMR spectra of new compounds 3~6. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn/.

  
  
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      (a) Tan, H.; Li, H.; Ji, W.; Wang, L. Angew. Chem., Int. Ed. 2015, 54, 8374.
      (b) Zha, D.; Li, H.; Li, S.; Wang, L. Adv. Synth. Catal. 2017, 359, 467.
      (c) Wang, L.; Li, H.; Wang, L. Org. Lett. 2018, 20, 1663.
      (d) Yang, S.; Li, H.; Li, P.; Yang, J.; Wang, L. Org. Biomol. Chem 2020, 18, 715.
      

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      Deng's group reported a reaction of terminal alkynes with DIB using AgOAc as a catalyst, but which only forms α-acetoxy ketones as sole products. Under Deng's conditions, this stereoselective addition reaction of functionalized alkynes with hypervalent iodine(Ⅲ) reagents does not give the desired product (Entry 2, Table 1). Based on these experiment results, a different pathway for this silver-catalyzed addition is proposed in Scheme 5.
      

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  Scheme 1  Stereoselective addition reactions of alkynes with O-containing nucleophiles

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  Scheme 2  Intramolecular competition experiment

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  Scheme 3  Gram-scale reaction and late-stage transformation

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  Scheme 4  Mechanistic studies

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  Scheme 5  Possible reaction mechanism

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  Table 1.  Optimization of reaction conditions^a

  Entry	Catalyst	Solvent	Yieldb/%
  1	Ag2O	CH3CN	21
  2	AgOAc	CH3CN	0
  3	AgOTf	CH3CN	0
  4	AgNO3	CH3CN	0
  5	Ag2CO3	CH3CN	86
  6	CuO	CH3CN	0
  7	Cu2O	CH3CN	Trace
  8	Fe(acac)2	CH3CN	0
  9	Fe(acac)3	CH3CN	0
  10	Pd(OAc)2	CH3CN	0
  11	—	CH3CN	0
  12	Ag2CO3	EtOH	15
  13	Ag2CO3	AcOH	22
  14	Ag2CO3	THF	Trace
  15	Ag2CO3	DMF	n.r.
  16	Ag2CO3	DMSO	17
  17	Ag2CO3	1, 4-Dioxane	Trace
  18	Ag2CO3	PhMe	0
  19	Ag2CO3	CH3CN	65c
  20	Ag2CO3	CH3CN	83d
  21	Ag2CO3	CH3CN	72e
  22	Ag2CO3	CH3CN	58f
  23	Ag2CO3	CH3CN	78g
  24	Ag2CO3	CH3CN	64h
  a Reaction conditions: 1a (0.2 mmol), 2a (0.2 mmol) and catalyst (2.5 mol%) in CH3CN (1 mL) at 90 ℃ under ambient air for 6 h.b Isolated yield.cAg2CO3 (1 mol%). dAg2CO3 (5 mol%). e 80 ℃. f100 ℃. g 8 h. h5 h.

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  Table 2.  Scope of bromoalkynes^a

  a Reaction conditions: 1a (0.2 mmol), 2 (0.2 mmol) and Ag2CO3 (2.5 mol%) in CH3CN (1 mL) at 90 ℃ under ambient air for 6 h. b Isolated yield.

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  Table 3.  Scope of HIRs and alkynes^a

  a Reaction conditions: 1 (0.2 mmol), 2 (0.2 mmol) and Ag2CO3 (2.5 mol%) in CH3CN (1 mL) at 90 ℃ under ambient air for 6 h. bIsolated yield. c24 h. d10 h.

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