English

• Functionalized nitrogen heterocycles commonly exist in chemicals that are widely used in agrochemicals, pharmaceuticals, and energetic materials.^[1] In particular, N-vinylazoles^[2] often form the core building blocks of antitumor, antifungal, antimicrobial, and other bioactive compounds.^[3] Furthermore, they are important monomers for synthesizing polymers that are utilized in photochromics and semiconductive materials.^[4] Thus, the synthesis of vinylazoles is of great interest to organic chemists.

  The preparation of functional azoles can be performed through the N—C coupling of azoles with organic halides. This usually requires the expensive catalyst that contains noble metal, such as silver, palladium and gold.^[5] Another typical method is the addition of azoles with alkynes (Scheme 1a), and the coupling of azoles with the active bromo-alkenes bearing electron-withdrawing substituents (Scheme 1b)^{[5a, 5b, 5f]} also works. It is noticed that all of these procedures require the noble metal catalyst. However, the expense of the noble metal is troublesome, and the metal-containing catalysts tend to be toxic.^{[2b, 6]}

  Scheme 1

  

  Scheme 1.  Methods for preparation of N-alkenyl azoles

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  Consequently, the facile metal-free methods for the vinylation have been designed to release the necessity of the precious metal catalyst. The construction of vinylazoles is realized via the activation of the halogen esters and the straightforward C—N coupling. This procedure takes advantages of metal-free and mild reaction conditions, and it also exhibits broad availability. In addition, using a α-halo Michael acceptor as the vinylating reagent produces vinylazoles with good regioselectivity, and the sequential Zaitsev elimination of hydrogen halide generates the double bond with good stereoselectivity. In this paper, we report the metal-free N—C coupling at room temperature, and a wide spectrum of stereoselective vinylazoles are synthesized (Scheme 1c).

  1.   Result and discussion

  Initially, we chose the coupling between 5-phenyl-1H- tetrazole (0.2 mmol) and methyl 2, 3-dibromopropionate (0.24 mmol) as a probing reaction to identify the efficiency of various nucleophilic promoters. The reactions are carried out in aprotic solvent (2 mL) and under room temperature. The structure of 3a was firstly confirmed by MS and NMR. As shown in Table 1, Et₃N displayed the highest activity with 79% chromatographic yield. The results showed that the tertiary amines (Table 1, Entries 2～7), including tetramethylethylenediamine (TMEDA), trimethylamine, tributylamine, benzyldimethylamine (BDMA), 1, 4-diazabicyclo[2.2.2]octane (DABCO) and urotropine were more positive than ethylenediamine (EDA, Entry 1), which is a kind of primary amines. BDMA (Table 1, Entry 5) and DABCO (Table 1, Entry 6) produced the same optimal yields as Et₃N, but urotropine gave a poor yield because of its poor solubility in dimethyl sulfoxide (DMSO). The tributylamine was also less active (Table 1, Entry 4). So Et₃N is the best choice especially for its ready availability. Reducing the loading amount of Et₃N from 10 equiv. to 5 equiv. produced similar yield. In the further refinement, controlled experiments were employed in various solvents. Only DMSO gave the acceptable yield of vinylazoles. Using Et₃N as the only solvent, the reaction displayed a lower yield (33%, Table 1, Entry 13), but no vinylazole was produced using only DMSO or N-methyl pyrrolidone (NMP) in the absence of bases even at 80 ℃ (Table 1, Entry 14～16). We deduced that DMSO facilitated the base-mediated dehydrohalogenation because of its dielectric nature, ^[7] and the coupling reaction might undergo a selective DMSO-mediated dehalogenative process which converted α, β-dihalopro- panoate derivatives into haloacrylate analogues.^[8]

  Table 1

  

  Table 1.  The condition optimization for the metal-free vinylation of 5-phenyl-1H-tetrazole^a

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  Entry	Base	Solvent	T/℃	Yield a /%
  1	EDA	DMSO	25	N.R.
  2	TMEDA	DMSO	25	42
  3	Et3N	DMSO	25	79
  4	Bu3N	DMSO	25	37
  5	BDMA	DMSO	25	79
  6	DABCO	DMSO	25	78
  7	Urotropine	DMSO	25	16
  8	Et3N	Ethanol	25	12
  9	Et3N	Acetone	25	43
  10	Et3N	CH2Cl2	25	34
  11	Et3N	1, 4-Dioxane	25	35
  12	Et3N	CH3Ph	25	42
  13	Et3N	—	25	33
  14	—	DMSO	25	N.R. b
  15	—	DMSO	80	N.R.
  16	—	NMP	80	N.R.
  17	Et3N	DMSO	15	45
  18	Et3N	DMSO	35	94
  19	Et3N	DMSO	80	95
  a Reaction conditions: 1a (0.24 mmol), 2a (0.2 mmol) and base (5 equiv.) in 2 mL of solvent for 2 h; chromtographic yield of 3a; b N.R.: No reaction.

  Furthermore, it has been found that protonic solvent such as ethanol (Table 1, Entry 8) has no positive effect. In addition, when the coupling was carried out in acetone, dichloromethane, dioxane or methylbenzene, the reactions did not provide high yield (Table 1, Entries 9～12). The vinylation gave low conversion at a temperature lower than 25 ℃ (Table 1, Entry 17). A slight raising temperature to 35 ℃ and extending reaction to 4 h improved the yield of target product to 94% (Table 1, Entry 18), however, a higher temperature 80 ℃ did not show obvious promotion anymore (Table 1, Entry 19). This indicated that the reaction was not a temperature dependent one. Considering the ease of operation and the good selectivity, near room temperature would be the best choice.

  Under the optimized conditions, a series of functionalized tetrazoles were investigated with 2, 3-dibromopro- pionate, and all of the results are shown in Table 2. The structure of 3a was further confirmed by the single crystal XRD, which explicitly authenticated that the vinyl substituent was at the N2 position, and the C＝C double bond was in trans-form (Figure 1).

  Table 2

  

  Table 2.  Metal-free N-vinylation of 1H-tetrazoles with 2, 3-dibromopropionate^{a, b}

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  Figure 1

  

  Figure 1.  Structure of 5-phenyl-1H-tetrazole from single crystal XRD

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  We were pleased to find that the various 5-substituted tetrazoles reacted with 2a to afford the vinylazoles with good to excellent isolated yields, including electron-defi- cient, electron-neutral and electron-rich tetrazoles. Especially, all the phenyl tetrazoles gave the 2-substituted products as the single isomers (3e～3k). Besides, N-vinylation of 5-pyridyl-1H-tetrazole with 1a provided the considerable isomers (3d: 3d'＝71: 9). However, the reaction of 1H-tetrazole with 5-methyl-1H-tetrazoles gave different results in the immiscible Et₃N/DMSO. The 5-methyl-1H-tetrazole reacted with 1a to give an overall yield of 86%, and the two regioisomers 3b and 3b' are in approximate 1: 2.5 ratio, while the N1-substituted is the primary product. These results indicate that the hindrance of tetrazoles guides the regioselectivity of the N1 or N2 substituted position.

  Inspired by the extensive N-vinylated-tetrazoles, we charged a series of azoles in this metal free vinylation, including triazoles, imidazoles and 2-substituted 1H-imi- dazolines. They were similarly treated with α-C(sp³)—H containing halogen esters (1a). All of the tested azoles gave the expected vinylated products (Table 3, 4a～4g), merely a relatively longer time and higher temperature were required because these less-nitrogen heterocycles showed slower coupling rate. To our delight, they produced a spectrum of pure and single products (except 4e). Besides, the more stable 2, 3-dibromopropionamide as the alternative of 1a also showed desirable result (4h). These results promise the generality of this metal-free methodology.

  Table 3

  

  Table 3.  Metal-freeN-vinylation of azoles with 2, 3-dibromo- propionate^{a, b}

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  In order to further discover the nature of the reaction, we made some efforts to elucidate the mechanism of the excellent stereoselectivity. By analyzing the structural isomer distribution, we deduced the reaction pathway as illustrated in Figure 2.

  Figure 2

  

  Figure 2.  Proposed mechanism

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  The reaction was initialled by the nucleophilic base β-dehydrohalogenation to form the α-bromo acrylate (1a'). Meanwhile, due to the acidity of azoles, it was also depro- tonated by the base to generate the anionic species (2a') and easily transform into the corresponding isomer (2a'').After that, 1a' reacted with 2a'' as Michael addition to form N-alkylated intermediate 3a', which underwent a Zaitsev elimination to give the target products with perfect trans-stereoselectity. This pathway was confirmed by the capture of 3a', which is detectable by MS promptly, and could easily remove HBr in Et₃N and transform into 3a. Similarly, another intermediate 1a' is also traceable during the initial time of reaction under Et₃N (5 min, GC-MS). We obtained the same product 3a when we used α-bromo acrylate as the starting material instead of 2, 3-dibromopro- pionate. Besides, the vinylation mainly occurred at N2-position which could be assigned to the steric hindrance of the 5-substituted group of tetrazoles. These results support the pathway that we have deduced.

  2.   Conclusion

  In summary, a metal-free method was developed for N-vinylation of NH-azoles, and the corresponding N-vinylazoles were obtained from the coupling of tetrazoles, imidazoles, imidazolines and other heterocycles with α-C(sp³)—H containing dihalogen esters. The reaction conditions are quite mild, and the products are highly selective as trans-isomers.

  3.   Experimental

  3.1   Synthesis of (2E)-2-(2-propenoic acid methyl ester)-5-phenyl-tetrazole

  In a 25 mL of sealed tube, the mixture of methyl 2, 3-dibromopropinate (0.24 mmol), 5-phenyl-1H-tetrazole (0.2 mmol), Et₃N (1 mmol) and DMSO (2 mL) was added. The mixture was stirred at room temperature for 2 h (imidazolines were at 60 ℃ for 12 h). When the reaction completed (monitored by TLC), the reaction mixture was poured into 20 mL of water, then extracted with ethyl acetate. The organic layer was separated, dried over anhydrous Na₂SO₄, and then evaporated under reduced pressure. The crude product was purified on silica gel (ethyl acetate/petroleum ether), and the target product was obtained as a write solid (37 mg, 79%). Other azoles were synthesized following the similar procedure.

  3.3   Synthesis of (2E)-1-(2-propenoic acid methyl ester)-imidazoline

  In a 25 mL sealed tube, the mixture of 1H-imidazoline (0.2 mmol), methyl 2, 3-dibromopropinate (0.24 mmol), Et₃N (1 mmol) and DMSO (2 mL) was added. The mixture was stirred at 60 ℃ for 12 h. When the reaction completed (monitored by TLC), the reaction mixture was poured into 20 mL of water, then extracted with ethyl acetate (10 mL×3). The organic layer was separated, dried over anhydrous Na₂SO₄, then evaporated under reduced pressure. The crude product was purified on silica gel (EA/PE), and the target product was obtained as a white solid (22 mg, 72%).

  Other tetrazoles were synthesized following the similar procedure.

  (2E)-2-(2-Propenoic acid methyl ester)-5-phenyl-tetra- zole (3a): White solid (379 mg, 94%); m.p. 145.3～146.6 ℃ (Lit.^[9] 144.5～145.5 ℃); ¹H NMR (600 MHz, CDCl₃) δ: 8.42 (d, J＝14.1 Hz, 1H), 8.27～8.17 (m, 2H), 7.55～7.51 (m, 3H), 6.93 (d, J＝14.1 Hz, 1H), 3.88 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 165.90, 165.33, 135.49, 131.35, 129.22, 127.47, 126.34, 113.55, 52.57; GCMS (70 eV) m/z (%): 202.1 (59, [M－N₂]⁺), 171.1 (100, [M－C₂H₃O₂]⁺).

  (2E)-2-(2-Propenoic acid methyl ester)-5-methyl-tetra- zole (3b): White solid (67 mg, 24%); m.p. 84.2～85.1 ℃ (Lit.^[9a] 83.0～84.0 ℃); ¹H NMR (600 MHz, CDCl₃) δ: 8.33 (d, J＝14.4 Hz, 1H), 6.82 (d, J＝14.4Hz, 1H), 3.86 (s, 3H), 2.62 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 165.34, 164.23, 135.38, 113.19, 52.52, 11.10; GCMS (70 eV) m/z (%): 140.1 (67, [M－N₂]⁺), 109.0 (56, [M－C₂H₃O₂]⁺).

  (2E)-1-(2-Propenoic acid methyl ester)-5-methyl-tetra- zole (3b'): White solid (173 mg, 62%): m.p. 63.0～64.2 ℃ (Lit.^[9a] 61.0 ℃); ¹H NMR (600 MHz, CDCl₃) δ: 7.89 (d, J＝13.8Hz, 1H), 6.85 (d, J＝13.8 Hz, 1H), 3.87 (s, 3H), 2.71 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 165.34, 151.97, 131.00, 114.42, 52.60, 9.21; GCMS (70 eV) m/z (%): 140.1 (67, [M－N₂]⁺), 109.0 (56, [M－C₂H₃O₂]⁺).

  (2E)-2-(2-Propenoic acid methyl ester)-tetrazole (3c): White solid (60 mg, 24%); m.p. 110.2～111.5 ℃ (Lit.^[10] 111.5～112.5 ℃); ¹H NMR (600 MHz, CDCl₃) δ: 8.63 (s, 1H), 8.42 (d, J＝14.1 Hz, 1H), 6.93 (d, J＝14.1 Hz, 1H), 3.88 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 165.07, 153.64, 135.36, 114.53, 52.64; GCMS (70 eV) m/z (%): 126.0 (28, [M－N₂]⁺), 95.0 [M－C₂H₃O₂]⁺.

  (2E)-1-(2-Propenoic acid methyl ester)-tetrazole (3c'): White solid (131 mg, 52%); m.p. 61.3～62.6 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.89 (s, 1H), 8.17 (d, J＝14.4 Hz, 1H), 6.80 (d, J＝14.4 Hz, 1H), 3.88 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 164.89, 141.83, 132.12, 114.58, 52.74; GCMS (70 eV) m/z (%): 126.0 (28, [M－N₂]⁺), 95.0 [M－C₂H₃O₂]⁺.

  (2E)-2-(2-Propenoic acid methyl ester)-5-(2-pyridyl)- tetrazole (3d): White solid (329 mg, 71%); m.p. 92.2～93.6 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 9.37 (d, J＝14.1 Hz, 1H), 8.82 (d, J＝4.8 Hz, 1H), 8.42 (d, J＝7.9 Hz, 1H), 7.96 (td, J＝7.8, 1.8 Hz, 1H), 7.52 (ddd, J＝7.7, 4.8, 1.2 Hz, 1H), 7.00 (d, J＝14.2 Hz, 1H), 3.87 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 165.84, 151.43, 150.02, 144.16, 137.93, 135.34, 126.27, 125.46, 114.82, 52.47; HRMS (ESI) calcd for C₁₀H₁₀N₅O₂ [M+H]⁺ 232.0829, found 232.0824.

  (2E)-1-(2-Propenoic acid methyl ester)-5-(2-pyridyl)- tetrazole (3d'): White solid (42 mg, 9%); m.p. 47.5～48.3 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.83 (dd, J＝4.8, 0.9 Hz, 1H), 8.45 (d, J＝14.1 Hz, 1H), 8.32 (d, J＝7.9 Hz, 1H), 7.91 (td, J＝7.7, 1.7 Hz, 1H), 7.46 (ddd, J＝7.6, 4.8, 1.2 Hz, 1H), 7.08 (d, J＝14.1 Hz, 1H), 3.88 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 165.42, 165.18, 150.78, 145.88, 137.44, 135.36, 125.70, 123.36, 114.60, 52.65; HRMS (ESI) calcd for C₁₀H₁₀N₅O₂ [M+H]⁺ 232.0829, found 232.0825.

  (2E)-2-(2-Propenoic acid methyl ester)-5-benzyl-tetra- zole (3e): White solid (308 mg, 63%); m.p. 56.7～57.9 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.33 (d, J＝14.1 Hz, 1H), 7.35～7.33 (m, 4H), 7.32～7.27 (m, 1H), 6.83 (d, J＝14.1 Hz, 1H), 4.30 (s, 2H), 3.85 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 166.76, 165.26, 135.86, 135.41, 129.01, 128.93, 127.33, 113.55, 52.53, 31.98; HRMS (ESI) calcd for C₁₂H₁₃N₄O₂ [M+H]⁺ 245.1033, found 245.1028.

  (2E)-2-(2-Propenoic acid methyl ester)-5-ethylsuleenyl- tetrazole (3f): White solid (214 mg, 50%); m.p. 74.4～75.5 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.31 (d, J＝14.0 Hz, 1H), 6.79 (d, J＝14.1 Hz, 1H), 3.86 (s, 3H), 3.27 (q, J＝7.4 Hz, 2H), 1.46 (t, J＝7.3 Hz, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 166.78, 165.32, 134.97, 113.18, 52.57, 26.48, 14.93; HRMS (ESI) calcd for C₇H₁₁N₄O₂S [M+ H]⁺ 215.0597, found 215.0594.

  (2E)-2-(2-Propenoic acid ethyl ester)-5-phenyl-tetra- zole^[10](3g): White solid (328 mg, 76%); m.p. 62.3～63.2 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.41 (d, J＝14.4 Hz, 1H), 8.21～8.20 (m, 2H), 7.53～7.52 (m, 3H), 6.93 (d, J＝14.1 Hz, 1H), 4.37～4.25 (m, 2H), 1.38～1.33 (m, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 165.86, 164.87, 135.30, 131.33, 129.21, 127.45, 126.37, 114.05, 61.62, 14.36; GCMS (70 eV) m/z (%): 216.1 (52, [M－N₂]⁺), 171.1 (100, [M－C₂H₃O₂]⁺).

  (2E)-2-(2-Propenoic acid methyl ester)-5-(3-methoxyl) phenyl-tetrazole (3h): White solid (332 mg, 62%); m.p. 102.2～103.5 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.42 (d, J＝14.1 Hz, 1H), 7.81 (dt, J＝7.7, 1.2 Hz, 1H), 7.74 (dd, J＝2.7, 1.5 Hz, 1H), 7.44 (t, J＝7.9 Hz, 1H), 7.07 (ddd, J＝8.3, 2.7, 1.0 Hz, 1H), 6.94 (d, J＝14.1 Hz, 1H), 3.91 (s, 3H), 3.88 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 165.83, 165.34, 135.48, 130.38, 119.89, 117.80, 113.60, 112.04, 55.63, 52.60. HRMS (ESI) calcd for C₁₂H₁₃N₄O₃ [M+H]⁺ 261.0982, found 261.0978.

  (2E)-2-(2-Propenoic acid methyl ester)-5-(3-bromo)- phenyl-tetrazole (3i): White solid (466 mg, 76%); m.p. 123.6～124.8 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.42 (d, J＝14.1 Hz, 1H), 8.37 (t, J＝1.8 Hz, 1H), 8.17～8.12 (m, 1H), 7.66 (ddd, J＝8.0, 2.0, 1.0 Hz, 1H), 7.41 (t, J＝7.9 Hz, 1H), 6.95 (d, J＝14.1 Hz, 1H), 3.89 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 165.20, 164.64, 135.37, 134.32, 130.81, 130.39, 128.25, 125.99, 123.29, 114.02, 52.65; HRMS (ESI) calcd for C₁₁H₁₀BrN₄O₂ [M+H]⁺ 308.9982, found 308.9977.

  (2E)-2-(2-Propenoic acid methyl ester)-5-(2-bromo)- phenyl-tetrazole (3j): White solid (380 mg, 62%); m.p. 84.5～85.5 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.45 (d, J＝14.4 Hz, 1H), 7.96 (dd, J＝7.8, 1.8 Hz, 1H), 7.77 (dd, J＝8.4, 1.2 Hz, 1H), 7.47～7.46 (m, 1H), 7.38～7.36 (m, 1H), 6.97 (d, J＝13.8 Hz, 1H), 3.88 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 165.20, 164.88, 135.46, 134.56, 132.03, 131.96, 127.73, 127.49, 122.30, 114.15, 52.60; HRMS (ESI) calcd for C₁₁H₁₀N₄O₂Br [M+H]⁺ 308.9982, found 308.9981.

  (2E)-2-(2-Propenoic acid methyl ester)-5-(3-chloro)- phenyl-tetrazole (3k): White solid (485 mg, 92%); m.p. 168.5～169.5 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.41 (d, J＝14.1 Hz, 1H), 8.18～8.13 (m, 2H), 7.53～7.49 (m, 2H), 6.93 (d, J＝14.1 Hz, 1H), 3.88 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 165.25, 165.05, 137.54, 135.40, 129.58, 128.74, 124.83, 113.82, 52.64; HRMS (ESI) calcd for C₁₁H₁₀N₄O₂Cl [M+H]⁺ 265.0487, found 265.0484.

  (2E)-1-(2-Propenoic acid methyl ester)-2-phenyl-imida- zoline (4a): White solid (140 mg, 27%); m.p. 109.0～110.6 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 7.70 (d, J＝13.3 Hz, 1H), 7.28～7.25 (m, 2H), 7.23～7.19 (m, 2H), 7.17 (s, 1H), 4.82 (d, J＝13.3 Hz, 1H), 4.05～4.00 (m, 2H), 3.67～3.63 (m, 2H), 3.57 (s, 3H), 2.30 (d, J＝0.8 Hz, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 168.71, 160.84, 140.99, 139.04, 131.79, 129.50, 128.90, 128.61, 125.78, 93.80, 53.91, 51.25, 47.18, 21.50, 21.49; HRMS (ESI) calcd for C₁₄H₁₅N₂O₃ [M－H]⁺ 259.1183, found 259.1184.

  (2E)-1-(2-Propenoic acid methyl ester)-2-(3-methyl)- phenyl-imidazoline (4b): White solid (283 mg, 58%); m.p. 106.2～107.5 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 7.79 (d, J＝13.3 Hz, 1H), 7.37～7.27 (m, 4H), 4.91 (d, J＝13.3 Hz, 1H), 4.14～4.09 (m, 2H), 3.74 (t, J＝9.3 Hz, 2H), 3.66 (s, 3H), 2.40 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 168.71, 160.82, 140.98, 139.03, 131.78, 129.49, 128.89, 128.61, 125.77, 93.78, 53.91, 51.24, 47.17, 21.50; HRMS (ESI) calcd for C₁₄H₁₇N₂O₂ [M+H]⁺ 245.1285, found 245.1280

  (2E)-1-(2-Propenoic acid methyl ester)-2-(4-trifluoro- methyl)phenyl-imidazoline (4c): White solid (149 mg, 25%). m.p. 126.7～127.9 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 7.76 (d, J＝8.1 Hz, 2H), 7.69 (d, J＝13.3 Hz, 1H), 7.67 (d, J＝8.2 Hz, 2H), 4.97 (d, J＝13.3 Hz, 1H), 4.16 (t, J＝9.4 Hz, 2H), 3.78 (t, J＝9.3 Hz, 2H), 3.67 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 168.47, 159.46, 140.15, 133.11, 132.89, 132.38, 129.41, 126.16, 94.76, 54.18, 51.38, 47.36; HRMS (ESI) calcd for C₁₄H₁₄N₂O₂F₃ [M+H]⁺ 299.0997, found 299.1002.

  (2E)-1-(2-Propenoic acid methyl ester)-2-phenyl-imid- azoline (4d): White solid (163 mg, 64%); m.p. 142.3～143.5 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 7.79 (d, J＝13.3 Hz, 1H), 7.56～7.43 (m, 5H), 4.93 (d, J＝13.3 Hz, 1H), 4.13 (t, J＝9.3 Hz, 2H), 3.75 (t, J＝9.3 Hz, 2H), 3.66 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 168.68, 160.64, 140.85, 131.04, 129.09, 128.86, 128.71, 93.93, 53.94, 51.26, 47.22. HRMS (ESI) calcd for C₁₃H₁₅N₂O₂ [M+H]⁺ 231.1128, found 231.1129.

  (2E)-1-(2-Propenoic acid methyl ester)-5-methyl-benzo- triazole (4e): White solid (282 mg, 65%). m.p. 94.0～95.2 ℃; ¹H NMR (600 MHz, CDCl₃) δ: 8.49 (dd, J＝14.3, 10.6 Hz, 1H), 8.05～7.87 (m, 1H), 7.63～7.50 (m, 1H), 7.48～7.29 (m, 1H), 6.73 (dd, J＝14.3, 12.8 Hz, 1H), 3.87 (d, J＝2.2 Hz, 3H), 2.57 (d, J＝22.4 Hz, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 166.53, 135.82, 135.53, 131.42, 127.59, 120.27, 119.97, 109.65, 107.36, 52.16, 21.47. HRMS (ESI) calcd for C₁₁H₁₂N₃O₂ [M+H]⁺ 218.0924, found 218.0921.

  (2E)-1-(2-Propenoic acid methyl ester)-1, 3, 5-triazole (4f): White solid (192 mg, 63%); m.p 149.0～151.0 ℃ (Lit.^[11] 155.0 ℃); ¹H NMR (600 MHz, CDCl₃) δ: 8.34 (s, 1H), 8.07 (s, 1H), 8.01 (dd, J＝13.8, 0.9 Hz, 1H), 6.60 (d, J＝13.8 Hz, 1H), 3.82 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 166.20, 153.78, 144.71, 135.27, 110.14, 52.30; GCMS (70 eV) m/z (%): 153.0 (70, [M]⁺), 122.0 (100, [M－CH₃O]⁺), 95.0 (16, [M+H－C₂H₃O₂]⁺).

  (2E)-1-(2-Propenoic acid methyl ester)-imidazole (4g): White solid (218 mg, 72%); m.p. 121.0～122.0 ℃ (Lit.^[12] 118.0～119.0 ℃); ¹H NMR (600 MHz, CDCl₃) δ: 7.90 (d, J＝14.2 Hz, 1H), 7.76 (s, 1H), 7.23 (t, J＝1.4 Hz, 1H), 7.17 (s, 1H), 6.05 (d, J＝14.2 Hz, 1H), 3.81 (s, 3H); ¹³C NMR (151 MHz, CDCl₃) δ: 166.49, 138.01, 136.71, 132.07, 116.25, 106.70, 52.17; GCMS (70 eV) m/z (%): 152.1 (100, [M]⁺), 121.0 (78, [M－CH₃O]⁺), 93.1 (28, [M－C₂H₃O₂]⁺).

  (2E)-2-(2-Propanamide)-5-phenyl-tetrazole (4h): White solid (211 mg, 49%); m.p. 240 ℃ (dec.); ¹H NMR (600 MHz, DMSO-d₆) δ: 8.37 (d, J＝14.0 Hz, 1H), 8.22 (dd, J＝6.7, 3.0 Hz, 2H), 8.07 (s, 1H), 7.71 (dd, J＝5.0, 2.0 Hz, 2H), 7.20 (d, J＝13.9 Hz, 1H), 2.64 (s, 2H); ¹³C NMR (151 MHz, DMSO-d₆) δ: 164.96, 164.63, 132.36, 131.77, 129.93, 127.24, 126.43, 118.57, 40.88. HRMS (ESI) calcd for C₆H₁₀NO₂ [M+H⁺－N₂] 188.0824, found 188.0816.

  2-((2-Bromo)-methylpropionate)-5-pheny-tetrzole (3a' reactive intermediate): GCMS (70 eV) m/z (%): 204.0 (7, [M－N₂－Br]⁺), 131.1 (100, [M－C₂H₃O₂－N₂]⁺), 104.1 (78, [M－C₂H₃O₂－C₇H₅N₄]⁺).

  2-Bromo-2-propenoic acid methyl ester (1a' reactive intermediate): GCMS (70 eV) m/z (%): 164.0 (80, [M]⁺), 104.9 (72, [M－C₂H₃O₂]⁺), 85.0 (67, [M－Br]⁺), 59.0 (24, [M－C₂H₂Br]⁺).

  Supporting Information X-ray crystallographic structure, crystal data and copies of NMR spectra. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn.

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• 

  Scheme 1  Methods for preparation of N-alkenyl azoles

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  Figure 1  Structure of 5-phenyl-1H-tetrazole from single crystal XRD

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  Figure 2  Proposed mechanism

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  Table 1.  The condition optimization for the metal-free vinylation of 5-phenyl-1H-tetrazole^a

  Entry	Base	Solvent	T/℃	Yield a /%
  1	EDA	DMSO	25	N.R.
  2	TMEDA	DMSO	25	42
  3	Et3N	DMSO	25	79
  4	Bu3N	DMSO	25	37
  5	BDMA	DMSO	25	79
  6	DABCO	DMSO	25	78
  7	Urotropine	DMSO	25	16
  8	Et3N	Ethanol	25	12
  9	Et3N	Acetone	25	43
  10	Et3N	CH2Cl2	25	34
  11	Et3N	1, 4-Dioxane	25	35
  12	Et3N	CH3Ph	25	42
  13	Et3N	—	25	33
  14	—	DMSO	25	N.R. b
  15	—	DMSO	80	N.R.
  16	—	NMP	80	N.R.
  17	Et3N	DMSO	15	45
  18	Et3N	DMSO	35	94
  19	Et3N	DMSO	80	95
  a Reaction conditions: 1a (0.24 mmol), 2a (0.2 mmol) and base (5 equiv.) in 2 mL of solvent for 2 h; chromtographic yield of 3a; b N.R.: No reaction.

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  Table 2.  Metal-free N-vinylation of 1H-tetrazoles with 2, 3-dibromopropionate^{a, b}

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  Table 3.  Metal-freeN-vinylation of azoles with 2, 3-dibromo- propionate^{a, b}

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•