English

• 1.   Introduction

  Halogen-containing compounds constitute an important part of the fine and application-oriented chemicals.^[1-4] Great efforts were made to develop efficient approaches for the synthesis of functionalized halogenated molecules under mild synthesis parameters in recent years.^[5-7] 1, 2, 2-Triio- dovinyl compounds are valuable synthons in organic chemistry and accessible intermediates for the preparation of tamoxifen analogues. Some of them have anti-fungal properties and can be used as supramolecular arrangements as well.^[8] On the other hand, efficient methods for the synthesis of 1, 2, 2-triiodovinyl derivatives are relatively rarely investigated in literatures.^[9-13] Morisawa et al.^[10] revealed the preparation of 1, 1, 2-triiodo-1-propene derivatives in the presence of iodine and potassium hydroxide. Madabhushi et al.^[11] developed an oxyhalogenation reaction of alkynylsilanes with oxone-KI to obtain 1, 2, 2- triiodovinyl derivatives in high yields. Liu et al.^[12] reported the preparation of tri-iodination products by combining the tetrabutylammonium iodide (TBAI)/(PIDA) (diacetoxyiodo)benzene and KI/PIDA systems in a one-pot synthesis. Recently, Ahmed et al.^[13] described the synthesis of 1, 2, 2-triiodovinyl compounds from terminal alkynes in the presence of iodine and dimethyl sulfoxide (DMSO). However, some of these protocols developed so far would rely on the use of strong bases, hypervalent iodine compounds, or strong oxidizing agents, which may result in low functional-group tolerance and low yields. Therefore, the development of efficient tri-iodination methods for the synthesis of 1, 2, 2-triiodo-vinyl derivatives is still highly desirable.

  It is well known that solids could be used in halogenations both as catalysts and as supporters for halogenation reagents. The unique environment created by the solids can affect the yield and product selectivity of various reactions.^{[14, 15]} Alumina (Al₂O₃) is a commonly used solid catalyst and was used in industrial processes such as hydro- desulfurization and some Ziegler-Natta polymerizations for many years.^{[16, 17]} Alumina can promote the chlorination, bromination and iodination of various substrates to produce the corresponding halogen-containing compounds.^[18-29] γ-Aluminum oxide is a common crystal form of Al₂O₃. Pagni et al.^[30-31] reported the essential role of γ-aluminum oxide in the iodination of alkynes with I₂ to obtain E-diiodoalkenes. Very recently, we have described the aluminum oxide-mediated iodination of terminal alkynes with N-iodosuccinimide to produce 1-iodoalkynes in moderate to high yields.^[32] Based on this protocol, herein, we further evaluate the tri-iodination of terminal alkynes with the activation of γ-aluminum oxide and present an efficient approach for the preparation of 1, 2, 2-triiodovinyl derivatives in high yields via twice iodination of terminal alkynes.

  2.   Results and discussion

  As it was already shown in our recent work, ^[32] the iodination of phenylacetylene (1a) with N-iodosuccinimide (NIS) in the presence of neutral γ-aluminum oxide could produce 1-(iodoethynyl)benzene (3) in high yield (98%) (Scheme 1). When 2.4 equiv. of iodine was added to the reaction mixture at 80 ℃, the tri-iodination product 2a was obtained in a 94% yield (Scheme 1). The mono-iodination of terminal alkynes in our previous work were carefully investigated.^[32] Therefore, we devoted our efforts to the tri-iodination of terminal alkynes by varying the amount of iodine and the temperature of the reaction (Table 1). As shown in Table 1, the best conditions for the second step were the treatment of the reaction mixture of the first step with 1. 5 equiv. of iodine at 80 ℃ for 3 h in considering the cost efficiency of the operation (Entry 2). The efforts to lower the reaction temperature to 60 or 45 ℃ in the presence of 1.5 equiv. of iodine only led to much longer reaction time and no increase of yield.

  Scheme 1

  

  Scheme 1.  The iodination of phenylacetylene in the presence of Al₂O₃

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  Table 1

  

  Table 1.  Optimization of the reaction parameters^a

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  Entry	I2/equiv.	Temp./℃	Time/h	Yieldb/%
  1	2.4	80	1	94
  2	1.5	80	3	94
  3	1.2	80	6	93
  4	1.5	60	5	93
  5	1.5	45	7	93
  a Unless noted otherwise, all the reactions were conducted in two steps in one pot. First, the mixture of phenylacetylene (1a, 2.0 mmol), N-iodosuccinimide (2.2 mmol), 200 mg of 4 Å MS, 10 mL of CH3CN and 265.0 mg of neutral Al2O3 was heated at 80 ℃ for 1 h. Then, iodine was added to the reaction mixture. b Isolated yield.

  Obtaining the enhanced conditions for the synthesis of (1, 2, 2-triiodovinyl)benzene (2a), we set out to examine the role of the substrate of the tri-iodination reaction described above (Table 2). Initially, a variety of 4-substituted terminal alkynes 1b~1k, bearing both electron-donating and electron-withdrawing groups, were evaluated considering their reactions with NIS and I₂ to afford the corresponding 1, 2, 2- triiodovinyl compounds 2b~2k in good to excellent yields (Entries 2~11). The yield of the reaction was not influenced significantly by the electronegativity of the substituents on the aromatic ring of the alkynes. Compared to the electron-deficient terminal alkynes, the tri-iodination of electron-rich terminal alkynes (eg. 1e, 1f) required longer reaction time (from 3 to 5 h). Subsequently, it was shown that the ortho- and meta-substituted aromatic alkynes 1l~1p could also be iodinated to obtain the desired products in high yields (Entry 12~16). Remarkably, the presence of ortho-fluoro moiety on the benzene ring gave the corresponding product 2l smoothly in the best yield (99%) (Entry 12).

  Table 2

  

  Table 2.  Iodination of alkynes 1a~1t to give 1, 1, 2-triiodoalkenes 2a~2t^a

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  Moreover, the heteroaromatic alkynes (1q and 1r) were also investigated and delivered the products 2q and 2r in 96% and 57% yields, respectively (Entries 17, 18). Although the high performance liquid chromatography (HPLC) chromatogram of the reaction mixture of 1r showed that the target 2r was the major product, and the reason for the relatively low yield of 2r remained unclear. Additionally, the tri-iodination product 2s could be obtained in 84% yield from the aliphatic alkyne 1s (Entry 19). Finally, the tri-iodination of the more reactive alkyne 1t was analyzed and provided the corresponding product of 2t in 96% yield (Entry 20). As shown in Table 2, the total yields of some tri-iodination reactions were higher than the yields of mono-iodination reactions due to the low boiling point of 1-iodoalkynes (the values in parentheses in Table 2). These results indicated that an extremely high efficiency was obtained during the second step of the synthesis.

  In order to explore the mechanism of this approach, a few controlled experiments were performed using phenylacetylene (1a) as the starting material (Scheme 2). Firstly, the reaction of phenylacetylene (1a) with I₂ was conducted in the presence of γ-Al₂O₃, and the di-iodination product 4 was generated exclusively in a yield of 97% (Eq. 1).^[30-32] Furthermore, 1.1 equiv. of NIS and 1.5 equiv. of I₂ were added to phenylacetylene (1a) in CH₃CN simultaneously, obtaining the mixture of corresponding di-iodination product 4^[33] and tri-iodination product 2a in a molar ratio of 47:53 (as estimated by analyzing the reaction mixture with HPLC) (Eq. 2). The direct reaction of 1-iodoalkyne 3 with I₂ resulted in the desired product 2a in almost quantitative yield (99%) (Eq. 3). These results further confirmed that the 1-iodoalkyne 3 was the pivotal intermediate during the tri-iodination of terminal alkynes. Besides, those selective iodinations did not occur without the activation of γ-Al₂O₃.^[30-32] Although the specified mechanism for Al₂O₃ mediated iodination reaction remains unclear, the should play a crucial role via activating the NIS and I₂ and influencing the chemoselectivity and yield of this new process.

  Scheme 2

  

  Scheme 2.  Control experiments

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  3.   Conclusions

  In the present work, a highly efficient, two-step, one-pot synthesis method of 1, 2, 2-triiodovinyl derivatives resulting good to excellent yields from terminal alkynes in the presence of γ-Al₂O₃ was investigated. Based on the formation of 1-iodoalkyne as the reaction intermediate, this approach generated the target products via a two-step reaction. The first step was the mono-iodination reaction of terminal alkynes with NIS. The second step was the iodination of 1-iodoalkynes with iodine. The control experiments showed that the yields of the second step were extremely high. Further application of this method is being investigated in our laboratory.

  4.   Experimental section

  4.1   General

  ¹H NMR and ¹³C NMR spectra were recorded on a Bruker Avance 400 FTNMR spectrometer using CDCl₃ as solvent. Tetramethylsilane (TMS) (δ 0.00) or residual solvent peak in CDCl₃ (δ 7.26) served as internal standard for recording. TLC analyses were performed on precoated GF254 silica gel plates and were visualized under UV254 nm light or by I₂ staining. Column chromatography was carried out using 200~300 mesh silica gel. All reagents and solvents were purchased from commercial suppliers and used as delivered.

  4.2   General procedure for the preparation of tri-iodination of alkynes

  Terminal alkynes 1a~1t (2.0 mmol, 1.0 equiv.), Al₂O₃ (2.6 mmol, 1.3 equiv.), 4Å MS (200 mg) and CH₃CN (10 mL) were added into a 25 mL round-bottomed flask. N-Iodosuccinimide (2.2 mmol, 1.1 equiv.) was added and the resulting mixture was heated to 80 ℃ and stirred for 1 h at this temperature. Then I₂ (3.0 mmol, 1.5 equiv.) was added to the reaction mixture. The reaction mixture was stirred at 80 ℃ for 3~4 h (determined by HPLC). After completion of the reaction, the mixture was quenched with saturated aqueous sodium thiosulfate and extracted with ethyl acetate (10 mL×3). The combined organic layers were washed with aqueous sodium chloride and dried over anhydrous sodium sulfate, filtered and concentrated under vacuum. The residue was purified by flash column chromatography on silica gel (petroleum ether and ethyl acetate).

  (1, 2, 2-Triiodovinyl)benzene (2a): purified by silica gel column chromatography, eluting with petroleum ether afforded the title compound as pale yellow solid (905.8 mg, 94% yield). m.p. 106~107 ℃ (Lit.^[10] 107~110 ℃); ¹H NMR (CDCl₃, 400 MHz) δ: 7.39~7.33 (m, 3H), 7.27~7.25 (m, 2H); ¹³C NMR (CDCl₃, 101 MHz) δ: 147.81, 128.86, 128.73, 127.52, 112.58, 22.65.

  Fluoro-4-(1, 2, 2-triiodovinyl)benzene (2b):^[13] purified by silica gel column chromatography, eluting with petroleum ether afforded the title compound as pale yellow solid (969.7 mg, 97% yield). m.p. 84~85 ℃; ¹H NMR (CDCl₃, 400 MHz) δ: 7.27~7.24 (m, 2H), 7.07~7.03 (m, 2H); ¹³C NMR (CDCl₃, 101 MHz) δ: 162.47 (d, J＝250.9 Hz), 143. 90 (d, J＝3.5 Hz), 129. 60 (d, J＝8.5 Hz), 115. 85 (d, J＝22.2 Hz), 111.32, 23.66.

  Chloro-4-(1, 2, 2-triiodovinyl)benzene (2c):^[13] purified by silica gel column chromatography, eluting with petroleum ether afforded the title compound as pale yellow solid (908.6 mg, 88% yield). m.p. 51~53 ℃; ¹H NMR (CDCl₃, 400 MHz) δ: 7.34 (d, J＝8.4 Hz, 2H), 7.21 (d, J＝8.4 Hz, 2H); ¹³C NMR (CDCl₃, 101 MHz) δ: 146.18, 134.78, 129.04, 128.98, 110.98, 23.58.

  Bromo-4-(1, 2, 2-triiodovinyl)benzene (2d): purified by silica gel column chromatography, eluting with petroleum ether afforded the title compound as pale yellow solid (1.0878 g, yield 97%). m.p. 75~76 ℃; ¹H NMR (CDCl₃, 400 MHz) δ: 7.52~7.48 (m, 2H), 7.16~7.12 (m, 2H); ¹³C NMR (CDCl₃, 101 MHz) δ: 146.63, 131.99, 129.19, 123.04, 110.95, 23.55; HRMS (EI) calcd for C₈H₄BrI₃ 559.6638, found 559.6630.

  Methyl-4-(1, 2, 2-triiodovinyl)benzene (2e):^[11] purified by silica gel column chromatography, eluting with petroleum ether afforded the title compound as pale yellow solid (932.2 mg, yield 94%). ¹H NMR (CDCl₃, 400 MHz) δ: 7.17 (brs, 4H), 2.34 (s, 3H); ¹³C NMR (CDCl₃, 101 MHz) δ: 145.03, 138.99, 129.39, 127.47, 113.00, 22.33, 21.60.

  Methoxy-4-(1, 2, 2-triiodovinyl)benzene (2f): purified by silica gel column chromatography, eluting with petroleum ether/ethyl acetate (V:V＝20:1) afforded the title compound as yellow solid (921.3 mg, 90% yield). m.p. 100~101 ℃ (Lit.^[11] 100~102 ℃); ¹H NMR (CDCl₃, 400 MHz) δ: 7.21 (d, J＝8.8 Hz, 2H), 6. 86 (d, J＝8.8 Hz, 2H), 3.82 (s, 3H); ¹³C NMR (CDCl₃, 101 MHz) δ: 159.73, 140.30, 129.18, 113.98, 112.99, 55.47, 22.70.

  (4-(1, 2, 2-Triiodovinyl)phenyl)acetonitrile (2g): purified by silica gel column chromatography, eluting with petroleum ether/ethyl acetate (V:V＝4:1) afforded the title compound as yellow solid (989.6 mg, 95% yield). ¹H NMR (CDCl₃, 400 MHz) δ: 7. 34 (d, J＝8.4 Hz, 2H), 7.29 (d, J＝8.4 Hz, 2H), 3.76 (s, 2H); ¹³C NMR (CDCl₃, 101 MHz) δ: 147.69, 130.52, 128.36, 117.44, 111.23, 23.66, 23.30; HRMS (EI) calcd for C₁₀H₆NI₃ 520.7630, found 520.7634.

  (1, 2, 2-Triiodovinyl)benzonitrile (2h):^[11] purified by silica gel column chromatography, eluting with petroleum ether/ethyl acetate (V:V＝10:1) afforded the title compound as yellow oil (983.3 mg, 97% yield). ¹H NMR (CDCl₃, 400 MHz) δ: 7.67 (d, J＝8. 4 Hz, 2H), 7.37 (d, J＝8.4 Hz, 2H); ¹³C NMR (CDCl₃, 101 MHz) δ: 151.86, 132.65, 128.40, 118.38, 112.52, 109.37, 24.28.

  (Trifluoromethyl)-4-(1, 2, 2-triiodovinyl)benzene (2i): purified by silica gel column chromatography, eluting with petroleum ether afforded the title compound as pale brown solid (1.0777 g, 98% yield). m.p. 52~53 ℃ (Lit.^[12]: 50~52 ℃); ¹H NMR (CDCl₃, 400 MHz) δ: 7.63 (d, J＝8.0 Hz, 2H), 7.38 (d, J＝8.0 Hz, 2H); ¹³C NMR (CDCl₃, 101 MHz) δ: 151.04, 130.71 (q, J＝32.7 Hz), 128.01, 125.86 (q, J＝3.7 Hz), 123.86 (q, J＝273.2 Hz), 110.20, 23.75.

  (1, 2, 2-Triiodovinyl)benzaldehyde (2j): purified by silica gel column chromatography, eluting with petroleum ether/ethyl acetate (V:V＝10:1) afforded the title compound as yellow solid (846.4 mg, 83% yield). ¹H NMR (CDCl₃, 400 MHz) δ: 10.03 (s, 1H), 7.89 (d, J＝8.4 Hz, 2H), 7.43 (d, J＝8.4 Hz, 2H); ¹³C NMR (CDCl₃, 101 MHz) δ: 191.50, 153.20, 135.98, 130.14, 128.30, 110.33, 23.64; HRMS (EI) calcd for C₉H₅OI₃ 509.7480, found 509.7474.

  Methyl 4-(1, 2, 2-triiodovinyl)benzoate (2k): purified by silica gel column chromatography, eluting with petroleum ether/ethyl acetate (V:V＝15:1) afforded the title compound as white solid (982.5 mg, 91% yield). m.p. 135~137 ℃ (Lit.^[12] 129~131 ℃); ¹H NMR (CDCl₃, 400 MHz) δ: 8.04 (d, J＝8.4 Hz, 2H), 7.33 (d, J＝8.4 Hz, 2H), 3.92 (s, 3H); ¹³C NMR (CDCl₃, 101 MHz) δ: 166.44, 151.88, 130.26, 130.08, 127.64, 110.83, 52.47, 23.22.

  Fluoro-2-(1, 2, 2-triiodovinyl)benzene (2l): purified by silica gel column chromatography, eluting with petroleum ether afforded the title compound as white solid (989.7 mg, 99% yield). m.p. 66~67 ℃; ¹H NMR (CDCl₃, 400 MHz) δ: 7.39~7.33 (m, 1H), 7.23~7.14 (m, 2H), 7.09~7.05 (m, 1H); ¹³C NMR (CDCl₃, 101 MHz) δ: 156.92 (d, J＝251.4 Hz), 135.39 (d, J＝15.4 Hz), 131.03 (d, J＝8.1 Hz), 129.19 (d, J＝2.1 Hz), 124.54 (d, J＝3.6 Hz), 116.60 (d, J＝21 Hz), 104.83, 26.07; HRMS (EI) calcd for C₈H₄FI₃ 499.7439, found 499.7431.

  Methoxy-2-(1, 2, 2-triiodovinyl)benzene (2m): purified by silica gel column chromatography, eluting with petroleum ether/ethyl acetate (V:V＝20:1) afforded the title compound as yellow solid (962.3 mg, 94% yield). m.p. 137~139 ℃; ¹H NMR (CDCl₃, 400 MHz) δ: 7.36~7.32 (m, 1H), 7.12 (dd, J＝7.6, 1.6 Hz), 6.98~6.94 (m, 1H), 6.89 (d, J＝8.4 Hz, 1H), 3.90 (s, 3H); ¹³C NMR (CDCl₃, 101 MHz) δ: 154.57, 136.10, 130.57, 128.65, 120.78, 111.82, 109.36, 55.90, 24.56; HRMS (EI) calcd for C₉H₇OI₃ 511.7636, found 511.7631.

  Fluoro-3-(1, 2, 2-triiodovinyl)benzene (2n): purified by silica gel column chromatography, eluting with petroleum ether afforded the title compound as white solid (959.6 mg, 96% yield). m.p. 63~65 ℃; ¹H NMR (CDCl₃, 400 MHz) δ: 7.37~7.30 (m, 1H), 7.05~6.95 (m, 3H); ¹³C NMR (CDCl₃, 101 MHz) δ: 162.31 (d, J＝149.2 Hz), 149.48 (d, J＝8.1 Hz), 130.38 (d, J＝8.4 Hz), 123.35 (d, J＝3.1 Hz), 115.95 (d, J＝21.1 Hz), 114.75 (d, J＝22.7 Hz), 110.45 (d, J＝2.3 Hz), 23.63; HRMS (EI) calcd for C₈H₄FI₃ 499.7437, found 499.7431.

  Chloro-3-(1, 2, 2-triiodovinyl)benzene (2o): purified by silica gel column chromatography, eluting with petroleum ether afforded the title compound as pale yellow solid (960.2 mg, 93% yield). m.p. 71~72 ℃; ¹H NMR (CDCl₃, 400 MHz) δ: 7.31~7.28 (m, 2H), 7.26~7.25 (m, 1H), 7.16~7.13 (m, 1H); ¹³C NMR (CDCl₃, 101 MHz) δ: 149.17, 134.31, 130.03, 129.01, 127.61, 125.76, 110.28, 23.91; HRMS (EI) calcd for C₈H₄ClI₃ 515.7142, found 515.7136.

  Methyl-3-(1, 2, 2-triiodovinyl)benzene (2p):^[13] purified by silica gel column chromatography, eluting with petroleum ether afforded the title compound as white solid (917.4 mg, 95% yield). m.p. 89~90 ℃; ¹H NMR (CDCl₃, 400 MHz) δ: 7.27~7.23 (m, 1H), 7.15~7.12 (m, 1H), 7.08~7.04 (m, 2H), 2.36 (s, 3H); ¹³C NMR (CDCl₃, 101 MHz) δ: 147.67, 138.50, 129.65, 128.56, 128.00, 124.54, 112.85, 22.41, 21.55.

  (1, 2, 2-Triiodovinyl)thiophene (2q): purified by silica gel column chromatography, eluting with petroleum ether afforded the title compound as pale yellow solid (yield 936. 7 mg, 96%). m.p. 126~127 ℃; ¹H NMR (CDCl₃, 400 MHz) δ: 7.36 (dd, J＝2.8, 1.2 Hz, 1H), 7.30 (dd, J＝5.0, 2.8 Hz, 1H), 7.07 (dd, J＝5.0, 1.2 Hz, 1H); ¹³C NMR (CDCl₃, 101 MHz) δ: 146.93, 127.27, 125.60, 124.65, 106.92, 23.44; HRMS (EI) calcd for C₆H₃SI₃ 487.7096, found 487.7089.

  (1, 2, 2-Triiodovinyl)pyridine (2r): purified by silica gel column chromatography, eluting with petroleum ether/ ethyl acetate (V:V＝5:1) afforded the title compound as pale yellow solid (550.4 mg, 57% yield). m.p. 130~131 ℃; ¹H NMR (CDCl₃, 400 MHz) δ: 8.57~8.55 (m, 2H), 7.59 (dt, J＝8.0, 2.0 Hz, 1H), 7.32~7.29 (m, 1H); ¹³C NMR (CDCl₃, 101 MHz) δ: 149.52, 148.24, 143.89, 135.09, 123.44, 108.02, 25.02; HRMS (EI) calcd for C₇H₄NI₃ 482.7479, found 482.7478, .

  (3, 4, 4-Triiodobut-3-en-1-yl)benzene (2s): purified by silica gel column chromatography, eluting with petroleum ether afforded the title compound as pale brown oil (833.0 mg, 84% yield). ¹H NMR (CDCl₃, 400 MHz) δ: 7.34~7.29 (m, 2H), 7.26~7.22 (m, 3H), 2.94~2.83 (m, 4H); ¹³C NMR (CDCl₃, 101 MHz) δ: 139.55, 128.81, 128.65, 126.60, 117.95, 52.94, 34.42, 19.56; HRMS (EI) calcd for C₁₀H₉I₃ 509.7841, found 509.7838.

  Ethyl 2, 3, 3-triiodoacrylate (2t): purified by silica gel column chromatography, eluting with petroleum ether/ethyl acetate (V:V＝15:1) afforded the title compound as white solid (917.3 mg, 96% yield). m.p. 64~66 ℃; ¹H NMR (CDCl₃, 400 MHz) δ: 4.32 (t, J＝7.2 Hz, 2H), 1.37 (t, J＝7.0 Hz, 3H); ¹³C NMR (CDCl₃, 101 MHz) δ: 166.85, 101.61, 63.28, 23.24, 14.03; HRMS (EI) calcd for C₅H₅O₂I₃ 477.7429, found 477.7424.

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

  
  
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  Scheme 1  The iodination of phenylacetylene in the presence of Al₂O₃

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  Scheme 2  Control experiments

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  Table 1.  Optimization of the reaction parameters^a

  Entry	I2/equiv.	Temp./℃	Time/h	Yieldb/%
  1	2.4	80	1	94
  2	1.5	80	3	94
  3	1.2	80	6	93
  4	1.5	60	5	93
  5	1.5	45	7	93
  a Unless noted otherwise, all the reactions were conducted in two steps in one pot. First, the mixture of phenylacetylene (1a, 2.0 mmol), N-iodosuccinimide (2.2 mmol), 200 mg of 4 Å MS, 10 mL of CH3CN and 265.0 mg of neutral Al2O3 was heated at 80 ℃ for 1 h. Then, iodine was added to the reaction mixture. b Isolated yield.

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  Table 2.  Iodination of alkynes 1a~1t to give 1, 1, 2-triiodoalkenes 2a~2t^a

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