Abstract
Efficient hydrochlorination of 2-ethynylpyridines was achieved without the use of special reagents. Ethynylpyridine readily reacts with hydrochloric acid to form a pyridinium salt. The salt formation considerably enhances the electrophilicity of the ethynyl group and attracts a chloride ion as the counteranion. The spatial proximity facilitates the nucleophilic addition of the halide anion to the ethynyl group, producing 2-(2-chloroethenyl)pyridine in high yields. This protocol could also be applied for hydrobromination and hydroiodination using hydrobromic and hydroiodic acids, respectively. In the case of acetic acid, the reaction did not proceed because of the low acidity and lack of salt formation. This problem was overcome by exchanging the counteranion using silver acetate; the resultant pyridinium acetate underwent hydroacetoxylation.
Introduction
Hydrohalogenation of a multiple bond is a fundamental approach for the synthesis of halogenated compounds. Although the carbon–carbon double bonds of alkenes undergo electrophilic addition of hydrogen halides to afford alkyl halides, the carbon–carbon triple bonds of alkynes cannot undergo similar reactions efficiently,1,2 primarily because of the lower stability of the cationic intermediate derived from the electronegative sp carbon as compared with that from an sp2 carbon. Moreover, the produced haloalkene exhibits a higher reactivity when compared with the starting alkyne and undergoes a second hydrohalogenation reaction.3 To avoid this undesired second reaction, metal catalysts or reagents other than hydrohalic acids are often used.4 On the other hand, alkynes undergo nucleophilic addition fairly easily to yield substituted alkenes; however, the addition of hydrogen halides initiated by the nucleophilic attack of a halide ion has not been reported to date because of the low nucleophilicity.
To this end, we focused on 2-ethynylpyridine 1, which has both a basic ring nitrogen and an electrophilic ethynyl group (Scheme 1), as it was considered to be a suitable substrate for hydrohalogenation via nucleophilic addition. In other words, the nitrogen of 1 forms a pyridinium salt 3 upon treatment with hydrohalic acid 2, which enhances the electrophilicity of the ethynyl group. Furthermore, the spatial proximity between the ethynyl group and the counteranion facilitates the nucleophilic addition to afford 2-(2-haloethenyl)pyridine 4.
Scheme 1. Reaction Design of Hydrohalogenation of 2-Ethynylpyridine 1.
The 2-(2-haloethenyl)pyridine framework is often found in biologically active compounds5 and is also used as a synthetic intermediate for the synthesis of versatile compounds.6 Despite their high utility, only a few methods are available for the preparation of ethenylpyridines. Specifically, Guinchard et al. synthesized 2-(2-chloroethenyl)pyridine by the cross-coupling reaction of potassium(2-chloroethenyl)trifluoroborates with 2-bromopyridine.7 Wittig bromoolefination of pyridinecarbaldehyde leading to the synthesis of 2-(2-bromoethenyl)pyridine was also reported by Kaloko and Hayford.8 However, these approaches suffer from the poor availability of the starting materials and low reaction efficiencies. There is no report regarding the preparation of 4 upon addition of hydrogen halide 2 to 1, except for a single description,9 which prompted us to study this reaction.
Results and Discussion
When a suspension of pyridinium salt 3Aa, derived from 1A (R = Ph) and hydrochloric acid 2a, was heated in benzene at 100 °C for 14 h, a small amount of 2-(2-chloro-2-phenylethenyl)pyridine hydrochloride (5Aa) was detected as a single isomer (Table 1, entry 1). The regio- and stereochemistries of the product were confirmed using X-ray crystallography for the hydrobrominated product 5Ab (Table 3, entry 1). Ethenylpyridine 4Aa was readily liberated upon treatment of 5Aa with an equimolar amount of triethylamine. Among the several tested solvents, aprotic polar solvents such as tetrahydrofuran (THF) and acetonitrile were found to be effective; however, the yield was rather poor when methanol was used (Table 1, entries 2–4). Although acetonitrile afforded the best result, the yield of 5Aa was still 50%, and 50% of 1A was recovered by the proton transfer from 3Aa to the more basic ethenylpyridine 4Aa formed in situ (entry 4). Hence, the reaction was conducted in the presence of additional 1 equiv of 2a. Although the reaction did not proceed at room temperature, heating at higher than 60 °C resulted in hydrochlorination (entries 5–7). Heating at 100 °C could shorten the reaction time, and all 3Aa was consumed within 14 h to afford 5Aa in 82% yield (entry 8–10). Even though large amounts of hydrochloric acid were used, the second hydrohalogenation was not observed because the sp2 carbon of 5Aa is less electrophilic than the sp carbon of 3Aa (entry 11).
Table 1. Optimization of the Reaction Conditions for the Hydrochlorination of 1A.
| entry | x | solv. | temp (°C) | time (h) | yield (%) | recovery (%) |
|---|---|---|---|---|---|---|
| 1 | 0 | PhH | 100 | 14 | 7 | 90 |
| 2 | 0 | THF | 100 | 14 | 27 | 0 |
| 3 | 0 | MeOH | 100 | 14 | 10 | 61 |
| 4 | 0 | MeCN | 100 | 14 | 50 | 0a |
| 5 | 1 | MeCN | rt | 24 | 0 | quant. |
| 6 | 1 | MeCN | 60 | 24 | 20 | 66 |
| 7 | 1 | MeCN | 80 | 24 | 51 | 22 |
| 8 | 1 | MeCN | 100 | 3 | 16 | 71 |
| 9 | 1 | MeCN | 100 | 8 | 51 | 22 |
| 10 | 1 | MeCN | 100 | 14 | 82 | 0 |
| 11 | 10 | MeCN | 100 | 14 | 84 | 0 |
50% of 1A was recovered.
Table 3. Reactions of 1A with Other Acids.
| entry | HX | product | yield (%) | |
|---|---|---|---|---|
| 1 | HBr | 2b | 5Ab | 70 |
| 2a | HI | 2c | 5Ac | 97 |
| 3 | HF | 2d | 5Ad | 0 |
| 4 | MeSO3H | 2e | 5Ae | 0 |
| 5 | PhSO3H | 2f | 5Af | 0 |
| 6 | CF3COOH | 2g | 5Ag | 0 |
| 7 | t-BuCOOH | 2h | 5Ah | 0 |
| 8 | AcOH | 2i | 5Ai | 0 |
At 60 °C.
Several alkynes were used for this reaction under optimized conditions (Table 2). Diphenylacetylene (6A) and 2-(phenylethynyl)nitrobenzene (6B) caused no change (entries 1 and 2). These results indicate that the 2-pyridyl group is crucial for this reaction, as the pyridine ring serves not only as an electron-withdrawing group but also as a base to attract hydrogen chloride by forming pyridinium salt 3.
Table 2. Study on the Ring Nitrogen and Position of the Ethynyl Group.
| alkyne |
|
|||||
|---|---|---|---|---|---|---|
| entry | Y | position | yield of product (%) | recovery of alkyne (%) | ||
| 1 | CH | 6A | 9Aa | 0 | 100 | |
| 2 | C–NO2 | 6B | 9Ba | 0 | 100 | |
| 3 | N | 3 | 7 | 10 | 0 | 100 |
| 4 | N | 4 | 8 | 11 | 22 | 78 |
Ethynylpyridines 7 and 8, positional isomers of 1A, revealed different reactivities. 3-Ethynylpyridine 7 was inert and was recovered quantitatively (entry 3). On the other hand, 4-ethynylpyridine 8 underwent hydrochlorination, although the reaction efficiency was low (entry 4). Although the nitrogen activates the ethynyl group by an electron-withdrawing resonance effect, the distance cannot diminish the electron density of the ethynyl group because of the lack of electron-withdrawing inductive effects. Thus, the distance between the basic site and the reaction sites is also important for the hydrochlorination. In other words, the chloride anion attacks the ethynyl group intramolecularly rather than intermolecularly. Indeed, the reaction of 3Aa afforded 5Aa in a similar yield, even though the reaction mixture was diluted from 0.5 to 0.05 M.
Next, other hydrohalic acids 2b–d were subjected to the reaction (Table 3). Hydrobromic acid 2b and hydroiodic acid 2c underwent the reaction smoothly to give 2-(bromoethenyl)pyridine 5Ab and 2-(iodoethenyl)pyridine 5Ac in excellent yields (entries 1 and 2). Although hydrofluoric acid 2d formed pyridinium salt 3Ad, the subsequent addition reaction did not proceed, presumably because of the low nucleophilicity of the fluoride anion (entry 3). Sulfonic acids 2e and 2f and trifluoroacetic acid 2g exhibited similar reactivities and afforded salts 3Ae, 3Af, and 3Ag, respectively (entries 4–6). On the other hand, the less acidic carboxylic acids 2h and 2i could not form pyridinium acetate 3Ah and 3Ai (entries 7 and 8). The issue was resolved by anion exchange using silver acetate. When 2-(phenylethynyl)pyridinium chloride (3Aa) and 2 equiv of silver acetate were heated at 120 °C in acetonitrile, hydroacetoxylated product 4Ai was obtained, albeit in a low yield, and considerable amounts of 1A were obtained owing to the deprotonation of 3Aa by the more basic acetate anion (Scheme 2).10 Although the formation of 4Ai was confirmed using 1H NMR and high-resolution mass spectrometry (HRMS), it could not be isolated using column chromatography because it hydrolyzed to form phenacylpyridine, the hydrated form of 1A, on silica gel. However, all attempts to exchange the chloride ion of 3Aa with the other halide ion using tetrabutylammonium fluoride, cesium fluoride, potassium bromide, and potassium iodide failed under the same reaction conditions.
Scheme 2. Hydroacetoxylation of Ethynylpyridine 1A.
Several ethynylpyridine hydrochlorides 3Ba–3Oa were used to expand the scope of substrates (Table 4). Hydrochlorides of ethynylpyridines 3Ba–3Da, having an electron-donating or an electron-withdrawing group on the benzene ring, underwent the desired reaction to afford the corresponding chloroethenylpyridines 5Ba–5Da (entries 1–3). The ethynyl group is not required to have an aryl group, as hexynylpyridine 3E furnished the hydrochlorinated product 5Ea in high yield (entry 4). Although a bulky tert-butyl group diminished the reaction efficiency, the hydrochlorinated product 5Fa was obtained in a high yield at a higher reaction temperature (entry 5). Substrates 3G–I, possessing functional groups such as trimethylsilyl, methoxycarbonyl, or a hydroxymethyl, afforded the corresponding products 5Ga–5Ia in moderate to excellent yields, highlighting the functional group tolerance of the reaction under the employed conditions (entries 6–8). This reaction was applicable to terminal alkyne 3J, leading to the formation of 5Ja (entry 9). Because olefinic protons have a coupling constant of 13.6 Hz, the E-form might be formed to avoid the steric repulsion between a chloro and a pyridyl group. In cases of substrates possessing an electron-withdrawing group, a small amount of inseparable isomer was also formed (entries 3, 7, and 12). Although the structure of the minor isomer has not been determined, it is considered to be a regioisomer of 5 because the electron density at the α-carbon of the pyridine moiety is also diminished.
Table 4. Hydrochlorination of Other Ethynylpyridines.
| entry | R1 | R2 | product | yield (%) | |
|---|---|---|---|---|---|
| 1 | 4-EtOC6H4 | H | 3Ba | 5Ba | 69 |
| 2 | 4-BuC6H4 | H | 3Ca | 5Ca | 86 |
| 3 | 4-CF3C6H4 | H | 3Da | 5Da | 59 |
| 4 | Bu | H | 3Ea | 5Ea | 84 |
| 5a | t-Bu | H | 3Fa | 5Fa | 89 |
| 6 | Me3Si | H | 3Ga | 5Ga | 40 |
| 7 | MeOCO | H | 3Ha | 5Ha | 46 |
| 8b | HOCH2 | H | 3Ia | 5Ia(9) | 80 |
| 9 | H | H | 3Ja | 5Ja | 44 |
| 10c | Ph | 4-Me | 3Ka | 5Ka | 15 |
| 11c | Ph | 3-Me | 3La | 5La | 80 |
| 12b | Ph | 4-CN | 3Ma | 5Ma | 60 |
| 13 | Ph | 3-Br | 3Na | 5Na | 63 |
| 14 | Ph | 3-C≡CPh | 3Oa | 5Oa | 77 |
At 150 °C for 5 h.
At 65 °C for 14 h.
At 150 °C for 3 h.
Next, substituent effects of the pyridine ring were studied because electron-donating groups increase the basicity of the ring nitrogen and electron-withdrawing groups diminish the electron density of the ethynyl group.11 In the case of 3Ka, the reaction was suppressed, and severe conditions were required because stabilization of the pyridinium cation by the electron-donating group, especially in the case of 3Ka, could not diminish the electron density of the ethynyl group sufficiently (entries 10 and 11, Scheme 3). On the other hand, electron-withdrawing groups promoted the reactivity for hydrochlorination to afford the corresponding products 5Ma and 5Na in moderate yields (entries 12 and 13). Hence, the activation of the ethynyl group was found to be an influential factor. When 2,3-bis(phenylethynyl)pyridinium salt 3Oa was used as a substrate, only the ethynyl group at the 2-position was hydrochlorinated without any reaction on the ethynyl group at the 3-position (entry 14).
Scheme 3. Resonance Structures of the Protonated 3Ja.
Furthermore, this protocol was applicable to other ethynylated azaheterocycles 12 (Table 5). Although ethynylquinoline 12a and ethynylisoquinoline 12d underwent the hydrohalogenation to afford the corresponding haloalkenes 13a–d in high yields, 8-ethynylquinoline 12e afforded a complex mixture because the ethynyl group was not activated by the resonance effects of the ring nitrogen. Pyrimidine 12f and benzimidazole 12g also underwent the reaction efficiently to afford the corresponding haloalkenes 13f and 13g, respectively.
Table 5. Hydrohalogenation of Other Ethynylated Azaheterocycles.
The obtained 2-(2-bromoethenyl)pyridine 4Ab was subjected to the Suzuki coupling. Both electron-rich and electron-poor boronic acids underwent the cross-coupling reactions to afford triarylated alkenes 14h–j in high yields (Scheme 4).12
Scheme 4. Synthesis of Triarylated Alkenes 14.
Haloalkene 4Eb was a viable substrate for Sonogashira coupling, affording corresponding enynes 15k and 15l (Scheme 5).8,13 On the other hand, when trimethylsilylacetylene was used as a coupling partner, enyne 15m was not obtained, and desilylation occurred under the employed conditions. Instead, 3,3′-bis(indolizine) 16 was isolated as a result of double coupling and tandem cyclization (Scheme 6).14 Modification of the 3,3′-positions of the bis(indolizine) framework was easily achieved by changing the starting 2-(bromoethenyl)pyridines 4.
Scheme 5. Synthesis of Enynes 15.
Scheme 6. Synthesis of 3,3′-Bis(indolizine)s 16.
Conclusions
We established a nucleophilic hydrohalogenation protocol using ethynylpyridines containing a basic nitrogen and electrophilic ethynyl group. The initial salt formation is crucial to attract a halide ion and facilitate the nucleophilic addition to the ethynyl group. Each hydrohalogenation afforded single stereoisomer, which is presumably a Z form, although regioisomers were obtained in some cases. The obtained 2-(haloethenyl)pyridines serve as substrates for cross-coupling reactions to afford triarylalkenes and enynes, which will be useful scaffolds for the development of new functional materials.
Experimental Section
General Procedure for Hydrohalogenation
To a solution of 2-phenylethynylpyridine 1A (0.5 mmol, 89.5 mg) in MeCN (10 mL), 12 M HCl (0.5 mmol, 0.042 mL) was added at room temperature, and the mixture was stirred for 10 min. Then, the mixture was heated at 100 °C for 14 h in a sealed tube. Triethylamine (0.5 mmol, 0.07 mL) was added, and the mixture was extracted with CHCl3 (10 mL × 3), dried over MgSO4, filtered, and concentrated. Purification was achieved using flash column chromatography on silica gel to afford 2-(2-chloro-2-phenylethenyl)pyridine 4Aa (94 mg, 82%, 0.41 mmol eluted with hexane/ethyl acetate = 9/1) as yellow oil.
When other substrates were used, reactions were performed in a similar way.
2-(2-Chloro-2-phenylethenyl)pyridine (4Aa)
1H NMR (400 MHz, CDCl3): δ 8.67 (ddd, J = 4.8, 1.8, 0.9 Hz, 1H), 8.06 (ddd, J = 7.7, 1.1, 0.9 Hz, 1H), 7.74 (ddd, J = 7.7, 7.7, 1.8 Hz, 1H), 7.76–7.73 (m, 2H), 7.40 (s, 1H), 7.44–7.37 (m, 3H), 7.22 (ddd, J = 7.7, 4.8, 1.1 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 122.3 (CH), 124.1 (CH), 126.7 (CH), 126.9 (CH), 128.4 (CH), 129.2 (CH), 134.8 (C), 136.0 (CH), 138.7 (C), 149.5 (CH), 154.4 (C); IR (ATR, cm–1): 1620; HRMS (ESI/TOF): calcd for (M + H+) C13H12ClN, 216.0575; found, 216.0575.
2-(2-Bromo-2-phenylethenyl)pyridine (4Ab)
Yellow oil (91 mg, 70%, 0.35 mmol). 1H NMR (400 MHz, CDCl3): δ 8.66 (ddd, J = 4.8, 1.8, 0.9 Hz, 1H), 8.11 (ddd, J = 7.7, 1.1, 0.9 Hz, 1H), 7.71–7.69 (m, 2H), 7.68 (ddd, J = 7.7, 7.7, 1.8 Hz, 1H), 7.42–7.35 (m, 3H), 7.39 (s, 1H), 7.20 (ddd, J = 7.7, 4.8, 1.1 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 122.4 (CH), 123.9 (CH), 126.6 (C), 127.9 (CH), 128.4 (CH), 129.2 (CH), 130.3 (CH), 135.9 (CH), 140.6 (C), 149.5 (CH), 155.1 (C); IR (ATR, cm–1): 1581; HRMS (ESI/TOF): calcd for (M + H+) C13H11BrN2, 260.0069; found, 260.0081.
2-(2-Iodo-2-phenylethenyl)pyridine (4Ac)
Yellowish green oil (149 mg, 97%, 0.485 mmol). 1H NMR (400 MHz, CDCl3): δ 8.71 (ddd, J = 4.8, 1.8, 0.9 Hz, 1H), 7.91 (ddd, J = 7.7, 1.1, 0.9 Hz, 1H), 7.87 (ddd, J = 7.7, 7.7, 1.8 Hz, 1H), 7.62–7.60 (m, 2H), 7.39–7.33 (m, 3H), 7.36 (ddd, J = 7.7, 4.8, 1.1 Hz, 1H), 7.26 (s, 1H); 13C NMR (100 MHz, CDCl3): δ 105.2 (C), 122.7 (CH), 123.9 (CH), 128.3 (CH), 128.7 (CH), 128.9 (CH), 136.1 (CH), 136.7 (CH), 144.1 (C), 149.3 (CH), 156.4 (C); IR (ATR, cm–1): 1584; HRMS (ESI/TOF): calcd for (M + H+) C13H11IN2, 307.9931; found, 307.9942.
2-[2-Chloro-4-(ethoxyphenyl)ethenyl]pyridine (4Ba)
Yellow oil (90 mg, 69%, 0.345 mmol). 1H NMR (400 MHz, CDCl3): δ 8.63 (ddd, J = 5.6, 1.7, 0.7 Hz, 1H), 8.09 (d, J = 7.7 Hz, 1H), 7.71 (ddd, J = 1.7, 7.7, 7.7 Hz, 1H), 7.68 (d, J = 8.1 Hz, 2H), 7.25 (s, 1H), 7.16 (ddd, J = 0.7, 5.6, 7.7 Hz, 1H), 6.91 (d, J = 8.1 Hz, 2H), 4.07 (q, J = 7.0 Hz, 2H), 1.42 (t, J = 7.0 Hz, 3H); 13C NMR (100 MHz, CDCl3): δ 14.7 (CH3), 63.5 (CH2), 114.3 (CH), 122.0 (CH), 123.9 (CH), 124.9 (CH), 128.2 (CH), 130.5 (C), 130.9 (C), 135.9 (CH), 149.4 (CH), 154.6 (C), 159.9 (C); IR (ATR, cm–1): 1604; HRMS (ESI/TOF): calcd for (M + H+) C15H15ClNO, 260.0836; found, 260.0817.
2-[2-(4-Butylphenyl)-2-chloroethenyl]pyridine (4Ca)
Colorless oil (102 mg, 86%, 0.43 mmol). 1H NMR (400 MHz, CDCl3): δ 8.64 (ddd, J = 4.9, 1.8, 1.0 Hz, 1H), 8.11 (d, J = 7.8 Hz, 1H), 7.73 (ddd, J = 7.8, 7.8, 1.8 Hz, 1H), 7.66 (d, J = 8.3 Hz, 2H), 7.24 (d, J = 8.3 Hz, 2H), 7.20 (s, 1H), 7.18 (ddd, J = 7.8, 4.9, 1.0 Hz, 1H), 2.64 (t, J = 7.6 Hz, 2H), 1.62 (tt, J = 7.6, 7.4 Hz, 2H), 1.37 (tq, J = 7.6, 7.4 Hz, 2H), 0.94 (q, J = 7.4 Hz, 3H); 13C NMR (100 MHz, CDCl3): δ 13.9 (CH3), 22.3 (CH2), 33.4 (CH2), 35.3 (CH2), 122.2 (CH), 124.0 (C), 125.9 (CH), 126.8 (CH), 128.5 (CH), 134.9 (CH), 135.9 (C), 136.0 (CH), 144.4 (C), 149.5 (CH), 154.5 (C); IR (ATR, cm–1): 1633; HRMS (ESI/TOF): calcd for (M + H+) C17H19ClN, 272.1200; found, 272.1199.
2-[2-Chloro-4-(trifluoromethylphenyl)ethenyl]pyridine (4Da)
Brown oil [82 mg, 59%, 0.295 mmol, a mixture of regioisomers (93/7)]. Major isomer: 1H NMR (400 MHz, CDCl3): δ 8.64 (ddd, J = 8.0, 5.6, 1.0 Hz, 1H), 8.12 (d, J = 8.0 Hz, 1H), 7.86 (d, J = 8.0 Hz, 2H), 7.77 (ddd, J = 8.0, 8.0, 1.6 Hz, 1H), 7.67 (d, J = 8.0 Hz, 2H), 7.36 (s, 1H), 7.24 (ddd, J = 5.6, 1.6, 0.8 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 122.8 (CH), 124.3 (CH), 125.4 (C), 125.5 (C, q, J = 3.7 Hz), 127.2 (CH), 128.5 (CH), 133.1 (C), 136.0 (CH), 142.0 (C), 149.6 (CH), 153.7 (C); IR (ATR, cm–1): 1614; HRMS (ESI/TOF): calcd for (M + H+) C14H10F3ClN, 284.0448; found, 284.0445.
2-(2-Chloro-1-hexen-1-yl)pyridine (4Ea)
Brown oil (82 mg, 84%, 0.42 mmol). 1H NMR (400 MHz, CDCl3): δ 8.58 (ddd, J = 4.8, 1.8, 0.9 Hz, 1H), 7.67 (ddd, J = 7.7, 7.7, 1.8 Hz, 1H), 7.49 (ddd, J = 7.7, 1.1, 0.9 Hz, 1H), 7.14 (ddd, J = 7.7, 4.8, 1.1 Hz, 1H), 2.53 (t, J = 7.0 Hz, 2H), 1.67 (tt, J = 7.0, 7.0 Hz, 2H), 1.40 (tq, J = 7.0, 7.0 Hz, 2H), 0.95 (q, J = 7.0 Hz, 3H); 13C NMR (100 MHz, CDCl3): δ 13.9 (CH3), 21.6 (CH2), 30.4 (CH2), 43.2 (CH2), 122.1 (CH), 123.6 (CH), 128.2 (CH), 130.7 (C), 135.9 (CH), 149.2 (CH), 155.0 (C); IR (ATR, cm–1): 1651; HRMS (ESI/TOF): calcd for (M + H+) C17H19ClN, 196.0888; found, 196.0891.
2-(2-Chloro-3,3-dimethyl-1-buten-1-yl)pyridine (4Fa)
Brown oil (89 mg, 84%, 0.42 mmol). 1H NMR (400 MHz, CDCl3): δ 8.57 (ddd, J = 4.8, 1.8, 0.9 Hz, 1H), 7.90 (ddd, J = 7.7, 1.1, 0.9 Hz, 1H), 7.66 (ddd, J = 7.7, 7.7, 1.8 Hz, 1H), 7.14 (ddd, J = 7.7, 4.8, 1.1 Hz, 1H), 6.73 (s, 1H), 1.33 (s, 9H); 13C NMR (100 MHz, CDCl3): δ 28.9 (CH3), 39.8 (C), 121.8 (CH), 122.6 (CH), 124.2 (CH), 135.8 (CH), 147.8 (C), 149.2 (CH), 154.9 (C); IR (ATR, cm–1): 1631; HRMS (ESI/TOF): calcd for (M + H+) C11H15ClN, 196.0888; found, 196.0893.
2-[2-Chloro-2-(trimethylsilyl)ethenyl]pyridine (4Ga)
Brown oil (43 mg, 40% yield, 0.20 mmol). 1H NMR (400 MHz, CDCl3): δ 8.62 (ddd, J = 4.8, 1.8, 0.9 Hz, 1H), 8.13 (ddd, J = 7.7, 1.1, 0.9 Hz, 1H), 7.71 (ddd, J = 7.7, 7.7, 1.8 Hz, 1H), 7.19 (ddd, J = 7.7, 4.8, 1.1 Hz, 1H), 7.08 (s, 1H), 0.29 (s, 9H); 13C NMR (100 MHz, CDCl3): δ −0.004 (CH3), 124.8 (CH), 126.7 (CH), 138.4 (CH), 138.6 (CH), 143.5 (C), 151.7 (CH), 156.4 (C); IR (ATR, cm–1): 1581; HRMS (ESI/TOF): calcd for (M + H+) C10H15ClNSi, 212.0657; found, 212.0666.
Methyl 2-Chloro-3-(2-pyridyl)propenoate (4Ha)
Colorless oil [37 mg, 46% yield, 0.23 mmol, a mixture of regioisomers (90/10)]. Major isomer: 1H NMR (400 MHz, CDCl3): δ 8.62 (ddd, J = 4.8, 1.8, 0.9 Hz, 1H), 7.95 (ddd, J = 7.7, 1.1, 0.9 Hz, 1H), 7.80 (ddd, J = 7.7, 7.7, 1.8 Hz, 1H), 7.50 (s, 1H), 7.34 (ddd, J = 7.7, 4.8, 1.1 Hz, 1H), 3.83 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 51.8 (CH3), 118.0 (CH), 122.2 (CH), 124.8 (CH), 137.2 (CH), 143.2 (C), 149.3 (CH), 152.6 (C), 164.9 (C); IR (ATR, cm–1): 1625, 1728; HRMS (ESI/TOF): calcd for (M + H+) C9H19ClNO2, 198.0316; found, 198.0321.
2-(2-Chloro-2-hydroxy-1-propen-1-yl)pyridine (5Ia)9
Brown oil (77 mg, 80%, 0.4 mmol). 1H NMR (400 MHz, CDCl3): δ 8.56 (ddd, J = 8.0, 4.2, 1.8 Hz, 1H), 7.97 (d, J = 8.0 Hz, 1H), 7.72 (ddd, J = 8.0, 8.0, 1.8 Hz, 1H), 7.20 (ddd, J = 8.0, 4.2, 1.8 Hz, 1H), 7.16 (s, 1H), 5.30 (br s, 1H), 4.36 (s, 2H); 13C NMR (100 MHz, CDCl3): δ 66.5 (CH2), 122.5 (CH), 123.2 (CH), 124.1 (CH), 136.6 (CH), 137.5 (C), 140.8 (CH), 153.6 (C).
2-(2-Chloroethenyl)pyridine (4Ja)
Pale yellow oil (25 mg, 44%, 0.22 mmol). 1H NMR (400 MHz, CDCl3): δ 8.53 (dd, J = 4.2, 1.0 Hz, 1H), 7.63 (ddd, J = 7.7, 7.7, 1.8 Hz, 1H), 7.23 (d, J = 13.6 Hz, 1H), 7.14–7.17 (m, 2H), 6.86 (d, J = 13.6 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 121.7 (CH), 122.6 (CH), 124.3 (CH), 132.6 (CH), 136.7 (CH), 149.7 (CH), 153.4 (C); IR (ATR, cm–1): 1616; HRMS (ESI/TOF): calcd for (M + H+) C7H7ClN, 140.02615; found, 140.02557.
2-(2-Chloro-2-phenylethenyl)-4-methylpyridine (4Ka)
Colorless oil (18 mg, 15%, 0.075 mmol). 1H NMR (400 MHz, CDCl3): δ 8.47 (d, J = 4.9 Hz, 1H), 8.04 (s, 1H), 7.83 (d, J = 7.5 Hz, 2H), 7.70 (s, 1H), 7.41 (dd, J = 7.5, 7.5 Hz, 2H), 7.33 (t, J = 7.5 Hz, 1H), 7.07 (d, J = 4.9 Hz, 1H), 2.42 (CH3); 13C NMR (100 MHz, CDCl3): δ 21.2 (CH3), 121.9 (CH), 123.9 (CH), 128.0 (CH), 128.2 (CH), 128.3 (CH), 129.9 (CH), 130.1 (C), 134.9 (C), 148.2 (C), 148.8 (CH), 154.6 (C); IR (ATR, cm–1): 1641; HRMS (ESI/TOF): calcd for (M + H+) C14H13ClN, 230.0731; found, 230.0742.
2-(2-Chloro-2-phenylethenyl)-3-methylpyridine (4La)
Brown oil (93 mg, 80%, 0.40 mmol). 1H NMR (400 MHz, CDCl3): δ 8.56 (dd, J = 7.7, 4.8 Hz, 1H), 7.76–7.73 (m, 2H), 7.55 (dd, J = 7.7, 4.8 Hz, 1H), 7.43–7.38 (m, 3H), 7.19 (s, 1H), 7.17 (dd, J = 7.7, 4.8 Hz, 1H), 2.35 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 18.8 (CH3), 122.6 (CH), 124.6 (CH), 126.9 (CH), 127.9 (CH), 128.8 (CH), 129.2 (CH), 136.7 (CH), 137.9 (CH), 138.2 (CH), 146.6 (C), 153.7 (CH); IR (ATR, cm–1): 1620; HRMS (ESI/TOF): calcd for (M + H+) C14H13ClN, 230.0731; found, 230.0743.
2-(2-Chloro-2-phenylethenyl)-4-cyanopyridine (4Ma)
Brown oil [73 mg, 60%, 0.3 mmol, a mixture of regioisomers (89/11)]. Major isomer: 1H NMR (400 MHz, CDCl3): δ 8.82 (dd, J = 5.0, 0.8 Hz, 1H), 8.37 (s, 1H), 7.76–7.74 (m, 2H), 7.45–7.41 (m, 3H), 7.43 (dd, J = 5.0, 0.8 Hz, 1H), 7.28 (s, 1H); 13C NMR (100 MHz, CDCl3): δ 116.6 (C), 120.6 (C), 123.3 (CH), 124.9 (CH), 125.5 (CH), 126.9 (CH), 128.6 (CH), 129.9 (CH), 137.5 (C), 137.9 (C), 150.4 (CH), 155.7 (C); IR (ATR, cm–1): 1587, 2237; HRMS (ESI/TOF): calcd for (M + H+) C14H10ClN2, 241.0527; found, 241.0536.
3-Bromo-2-(2-chloro-2-phenylethenyl)pyridine (4Na)
Brown oil (93 mg, 63%, 0.315 mmol). 1H NMR (400 MHz, CDCl3): δ 8.65 (dd, J = 4.6, 1.4 Hz, 1H), 7.91 (dd, J = 8.1, 1.4 Hz, 1H), 7.77–7.75 (m, 2H), 7.43–7.40 (m, 3H), 7.31 (s, 1H), 7.11 (dd, J = 8.1, 4.6 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 121.9 (C), 123.6 (CH), 123.9 (CH), 127.1 (CH), 128.5 (CH), 129.5 (CH), 137.8 (C), 138.3 (C), 140.2 (CH), 147.8 (CH), 153.4 (C); IR (ATR, cm–1): 1607; HRMS (ESI/TOF): calcd for (M + H+) C13H11ClNBr, 293.9680; found, 293.9686.
2-(2-Chloro-2-phenylethenyl)-3-phenylethynylpyridine (4Oa)
Yellow oil (123 mg, 77%, 0.39 mmol). 1H NMR (400 MHz, CDCl3): δ 8.67 (dd, J = 1.6, 4.8 Hz, 1H), 7.85 (dd, J = 1.6, 7.9 Hz, 1H), 7.79 (dd, J = 1.9, 8.2 Hz, 2H), 7.61 (s, 1H), 7.50–7.47 (m, 2H), 7.44–7.38 (m, 3H), 7.37–7.32 (m, 3H), 7.22 (dd, J = 4.8, 7.9 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 85.7 (C), 96.8 (C), 120.4 (C), 121.8 (CH), 122.6 (C), 123.6 (CH), 127.1 (CH), 128.5 (CH), 128.9 (CH), 129.3 (CH), 131.6 (CH), 137.1 (C), 138.9 (C), 139.4 (CH), 148.1 (CH), 155.6 (C); IR (ATR, cm–1): 1620, 2214; HRMS (ESI/TOF): calcd for (M + H+) C21H16ClN, 316.0888; found, 316.0879.
2-(2-Chloro-2-phenylethenyl)quinoline (13a)
Yellow oil (85 mg, 64%, 0.32 mmol). 1H NMR (400 MHz, CDCl3): δ 8.23–8.07 (m, 2H), 8.08 (d, J = 8.5 Hz, 1H), 7.82–7.79 (m, 3H), 7.71 (ddd, J = 6.9, 6.9, 1.4 Hz, 1H), 7.53 (ddd, J = 8.0, 8.0, 1.0 Hz, 1H), 7.47 (s, 1H), 7.44–7.38 (m, 3H); 13C NMR (100 MHz, CDCl3): δ 121.8 (CH), 126.8 (CH), 126.9 (CH), 127.1 (CH), 127.2 (C), 127.5 (CH), 128.5 (CH), 129.3 (CH), 129.4 (CH), 129.7 (CH), 135.8 (CH), 136.0 (C), 138.6 (C), 148.1 (C), 154.8 (C); IR (ATR, cm–1): 1606; HRMS (ESI/TOF): calcd for (M + H+) C17H13ClN, 266.0731; found, 266.0740.
2-(2-Bromo-2-phenylethenyl)quinoline (13b)
Yellow oil (158 mg, 89%, 0.445 mmol). 1H NMR (400 MHz, CDCl3): δ 8.20–8.08 (m, 3H), 7.82 (d, J = 8.1 Hz, 1H), 7.77–7.74 (m, 2H), 7.71 (ddd, J = 1.4, 6.8, 6.8 Hz, 1H), 7.59 (s, 1H), 7.54 (ddd, J = 1.4, 6.8, 6.8 Hz, 1H), 7.43–7.37 (m, 3H); 13C NMR (100 MHz, CDCl3): δ 121.7 (CH), 126.8 (CH), 127.2 (C), 127.5 (CH), 127.7 (CH), 127.8 (C), 127.9 (CH), 129.3 (CH), 129.5 (CH), 129.7 (CH), 130.7 (CH), 135.8 (CH), 140.4 (C), 148.0 (C), 155.5 (C); IR (ATR, cm–1): 1593; HRMS (ESI/TOF): calcd for (M + H+) C17H13BrN, 310.0226; found, 310.0233.
2-(2-Iodo-2-phenylethenyl)quinoline (13c)
Yellow oil (174 mg, 97%, 0.49 mmol). 1H NMR (400 MHz, CDCl3): δ 8.20 (d, J = 8.5 Hz, 1H), 8.13 (d, J = 8.5 Hz, 1H), 7.87 (d, J = 8.5 Hz, 1H), 7.84 (dd, J = 1.0, 8.5 Hz, 1H), 7.73 (ddd, J = 1.5, 6.9, 6.9 Hz, 1H), 7.68–7.66 (m, 2H), 7.56 (ddd, J = 1.0, 6.9, 6.9 Hz, 1H), 7.39–7.33 (m, 4H); 13C NMR (400 MHz, CDCl3): δ 106.5 (C), 121.8 (CH), 126.8 (CH), 127.3 (C), 127.6 (CH), 128.3 (CH), 128.8 (CH), 129.0 (CH), 129.3 (CH), 129.8 (CH), 135.9 (CH), 136.9 (CH), 143.9 (C), 147.8 (C), 156.7 (C); IR (ATR, cm–1): 1591; HRMS (ESI/TOF): calcd for (M + H+) C17H13IN, 358.0087; found, 358.0089.
1-(2-Chloro-2-phenylethenyl)isoquinoline (13d)
Yellow oil (67 mg, 52%, 0.26 mmol). 1H NMR (400 MHz, CDCl3): δ 8.62 (d, J = 5.7 Hz, 1H), 8.10 (dd, J = 0.9, 8.4 Hz, 1H), 7.85–7.82 (m, 3H), 7.68 (ddd, J = 1.2, 6.8, 6.8 Hz, 1H), 7.62 (d, J = 5.7 Hz, 1H), 7.59 (s, 1H), 7.57 (ddd, J = 1.2, 6.8, 6.8 Hz, 1H), 7.46–7.39 (m, 3H); 13C NMR (100 MHz, CDCl3): δ 120.4 (CH), 123.5 (CH), 126.2 (CH), 126.9 (CH), 127.1 (C), 127.4 (CH), 128.5 (CH), 129.5 (CH), 130.2 (CH), 136.3 (C), 137.9 (C), 138.0 (C), 142.3 (CH), 155.4 (C); IR (ATR, cm–1): 1625; HRMS (ESI/TOF): calcd for (M + H+) C17H13ClN, 266.0731; found, 266.0719.
2-(2-Chloro-2-phenylethenyl)pyrimidine (13f)
Yellow oil (57 mg, 50%, 0.25 mmol). 1H NMR (400 MHz, CDCl3): δ 8.82 (d, J = 4.8 Hz, 2H), 7.79–7.71 (m, 2H), 7.44–7.40 (m, 3H), 7.39 (s, 1H), 7.15 (t, J = 4.8 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 118.7 (CH), 125.0 (CH), 127.2 (CH), 128.5 (CH), 129.7 (CH), 138.6 (C), 139.2 (C), 156.8 (CH), 163.7 (C); IR (ATR, cm–1): 1622; HRMS (ESI/TOF): calcd for (M + H+) C12H10ClN2, 217.0527; found, 217.0527.
2-(2-Chloro-2-phenylethenyl)-1-methylbenzimidazole (13g)
Brown solid (93 mg, 69%, 0.35 mmol). mp 145–146 °C. 1H NMR (400 MHz, CDCl3): δ 7.86–7.83 (m, 1H), 7.76–7.74 (m, 2H), 7.43–7.31 (m, 3H), 7.31–7.27 (m, 3H), 7.07 (s, 1H), 3.75 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 30.4 (CH3), 109.3 (CH), 113.4 (CH), 120.2 (CH), 122.5 (CH), 123.2 (CH), 127.0 (CH), 128.7 (CH), 129.9 (CH), 135.2 (C), 137.8 (C), 140.7 (C), 143.2 (C), 148.4 (C); IR (ATR, cm–1): 1634; HRMS (ESI/TOF): calcd for (M + H+) C16H14ClN2, 269.0840; found, 269.0838.
Hydroacetoxylation
To a solution of 2-(phenylethynyl)pyridinium chloride (0.5 mmol, 115 mg) in MeCN (10 mL), silver acetate (1.0 mmol, 167 mg) was added. After heating the resultant mixture at 120 °C for 6 h in a sealed tube, the solvent was removed under reduced pressure to afford the reaction mixture as a residue.
The formation of hydroacetoxylated product 4Ai was confirmed using 1H NMR, GC–MS, and HRMS spectra of the reaction mixture. However, the product could not be isolated because of the instability in the silica gel column chromatography.
HRMS (ESI/TOF): calcd for (M + H+) C15H14NO2, 240.1020; found, 240.1021.
Suzuki–Miyaura Coupling Reaction Using Hydrobrominated Product 4Ab
To a solution of 2-(2-bromo-2-phenylethenyl)pyridine 4Ab (2 mmol, 518 mg) in THF (10 mL) were added 4-chlorophenylboronic acid (3 mmol, 470 mg), CsCO3 (3 mmol, 977 mg), PPh3 (0.4 mmol, 105 mg), and Pd(OAc)2 (0.2 mmol, 45 mg) under argon. After being stirred at 60 °C for 1 day, the mixture was then poured into H2O (30 mL) and extracted with EtOAc (30 mL × 2). The organic layer was dried over MgSO4, filtered, and concentrated and subjected to flash chromatography on silica gel to afford coupling product 14h (497 mg, 57%, and 1.14 mmol eluted with hexane/EtOAc = 9/1) as yellow solid.
When other boronic acids were used, reactions were conducted in a similar way.
2-[2-(4-Chlorophenyl)-2-phenylethenyl]pyridine (14h)
1H NMR (400 MHz, CDCl3): δ 8.50 (ddd, J = 4.8, 1.8, 0.9 Hz, 1H), 7.36–7.29 (m, 9H), 7.13 (d, J = 8.2 Hz, 2H), 7.00 (ddd, J = 8.4, 4.8, 0.9 Hz, 1H), 6.75 (d, J = 8.0 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 121.4 (CH), 123.8 (CH), 127.8 (CH), 128.2 (CH), 128.4 (C), 129.1 (CH), 130.0 (CH), 131.7 (CH), 133.7 (C), 135.5 (CH), 142.2 (C), 144.6 (C), 149.4 (CH), 156.3 (C); IR (ATR, cm–1): 1581; HRMS (ESI/TOF): calcd for (M + H+) C19H15ClN, 292.0888; found, 292.0890.
2-[2-(4-Methoxylphenyl)-2-phenylethenyl]pyridine (14i)12a
Yellow oil (645 mg, 75%, 1.50 mmol). 1H NMR (400 MHz, CDCl3): δ 8.53 (ddd, J = 5.0, 1.9, 0.9 Hz, 1H), 7.39–7.29 (m, 6H), 7.16–7.08 (m, 3H), 7.04–6.96 (m, 1H), 6.92–6.85 (m, 2H), 6.76 (d, J = 7.8 Hz, 1H), 3.83 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 55.3 (CH3), 114.0 (CH), 121.0 (CH), 123.7 (CH), 127.8 (CH), 128.0 (CH), 128.1 (CH), 131.3 (CH), 131.9 (C), 135.3 (CH), 142.7 (C), 145.6 (C), 149.0 (CH), 156.7 (C), 159.1 (C).
2-[2-(4-Methylphenyl)-2-phenylethenyl]pyridine (14j)12a
1H NMR (400 MHz, CDCl3): δ 8.53 (ddd, J = 5.0, 1.9, 0.9 Hz, 1H), 7.39–7.29 (m, 6H), 7.16–7.08 (m, 3H), 7.04–6.96 (m, 1H), 6.92–6.85 (m, 2H), 6.76 (d, J = 7.8 Hz, 1H), 3.83 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 21.2 (CH3), 121.0 (CH), 123.6 (CH), 127.7 (CH), 127.9 (CH), 128.1 (CH), 128.4 (CH), 129.4 (CH), 129.9 (CH), 135.2 (CH), 136.8 (C), 137.4 (C), 142.7 (C), 149.1 (CH), 156.7 (C).
Sonogashira Coupling Reaction Using Hydrobrominated Product 4Eb
To a solution of bromoethenylpyridine 4Eb (0.2 mmol, 47.8 mg) in NEt3 (1 mL) were added PdCl2(PPh3)2 (0.02 mmol, 14 mg) and CuI (0.2 mmol, 38 mg) under argon. After adding 1-hexyne (0.4 mmol, 0.044 mL) dropwise, the resultant mixture was stirred at room temperature for 1 day. The reaction mixture was filtered using Celite (10 g) with washing with CH2Cl2 (10 mL). After the filtrate was concentrated, the residue was subjected to flash chromatography on silica gel to give enyne 15k (28.4 mg, 59%, 0.118 mmol, eluted with hexane/EtOAc = 9/1) as brown oil.
When 4-methylphenylacetylene was used as a coupling partner, the reaction was conducted in a similar way.
2-(2-Bromo-1-hexene-1-yl)pyridine (4Eb)
1H NMR (400 MHz, CDCl3): δ 8.59 (dd, J = 4.4, 1.6 Hz, 1H), 7.92 (d, J = 7.6 Hz, 1H), 7.68 (ddd, J = 7.6, 7.6, 1.6 Hz, 1H), 7.16 (dd, J = 7.6, 4.4 Hz, 1H), 6.92 (s, 1H), 2.66 (t, J = 7.2 Hz, 2H), 1.67 (tt, J = 7.2, 7.6 Hz, 2H), 1.37 (tq, J = 7.6, 7.2 Hz, 2H), 0.95 (t, J = 7.6 Hz, 3H); 13C NMR (100 MHz, CDCl3): δ 13.8 (CH3), 21.6 (CH2), 30.4 (CH2), 43.2 (CH2), 122.1 (CH), 123.6 (CH), 128.3 (CH), 130.6 (C), 135.8 (CH), 149.3 (CH), 155.0 (C); IR (ATR, cm–1): 1622; HRMS (ESI/TOF): calcd for (M + H+) C11H15BrN, 240.0382; found, 240.0385.
2-(2-Butyl-1-octene-3-yne-1-yl)pyridine (15k)
1H NMR (400 MHz, CDCl3): δ 9.22 (dd, J = 5.4, 1.5 Hz, 1H), 7.86 (ddd, J = 7.7, 7.7, 1.8 Hz, 1H), 7.35 (ddd, J = 7.7, 5.4, 1.3 Hz, 1H), 7.30 (d, J = 7.7 Hz, 1H), 6.62 (s, 1H), 2.94 (t, J = 7.1 Hz, 2H), 2.56 (t, J = 7.1 Hz, 2H), 1.66 (tt, J = 7.7, 7.2 Hz, 2H), 1.79 (tt, J = 7.7, 7.2 Hz, 2H), 1.59–1.36 (m, 4H), 0.96 (t, J = 7.3 Hz, 6H); 13C NMR (100 MHz, CDCl3): δ 13.6 (CH3), 13.8 (CH3), 22.0 (CH2), 22.1 (CH2), 24.7 (CH2), 30.9 (CH2), 31.1 (CH2), 39.1 (CH2), 110.8 (C), 123.2 (CH), 126.6 (CH), 126.8 (C), 129.8 (CH), 138.6 (CH), 151.1 (CH), 151.3 (C); IR (ATR, cm–1): 1634, 2328; HRMS (ESI/TOF): calcd for (M + H+) C17H24N, 242.1903; found, 242.1904.
2-[2-(4-Methylphenyl)-hex-1-ene-1-yl]pyridine (15l)
Brown solid (35 mg, 62%, 0.124 mmol). mp 105–106 °C. 1H NMR (400 MHz, CDCl3): δ 9.34 (d, J = 4.7 Hz, 1H), 7.89 (d, J = 8.0 Hz, 2H), 7.85–8.00 (m, 1H), 7.35–7.40 (m, 2H), 7.24 (d, J = 8.0 Hz, 1H), 6.69 (s, 1H), 2.44 (t, J = 7.6 Hz, 2H), 2.39 (s, 3H), 1.76 (tt, J = 7.6, 7.4 Hz, 2H), 1.45 (tq, J = 7.4, 7.3 Hz, 2H), 0.97 (t, J = 7.3 Hz, 3H); 13C NMR (100 MHz, CDCl3): δ 13.9 (CH3), 21.6 (CH3), 22.1 (CH2), 31.1 (CH2), 38.9 (CH2), 109.3 (C), 120.1 (C), 123.1 (CH), 126.3 (C), 127.1 (CH), 129.5 (CH), 133.3 (CH), 138.7 (CH), 140.5 (C), 151.3 (CH); IR (ATR, cm–1): 1643; HRMS (ESI/TOF): calcd for (M + H+) C20H22N, 276.1747; found, 276.1746.
Synthesis of Bis(indolizine)
To a solution of bromoethenylpyridine 4Eb (0.2 mmol, 48 mg) in NEt3 (1 mL) were added PdCl2(PPh3)2 (0.02 mmol, 14 mg) and CuI (0.2 mmol, 38 mg) under argon. After adding (trimethylsilyl)acetylene (0.4 mmol, 0.056 mL) dropwise, the resultant mixture was stirred at room temperature for 1 day. The reaction mixture was filtered using Celite (10 g) with washing with CH2Cl2 (10 mL). After the filtrate was concentrated, the residue was subjected to flash chromatography on silica gel to give enyne 16n (13 mg, 58%, 0.058 mmol, eluted with hexane/EtOAc = 8/2) as blue oil.
When other enynes were used, the experiments were conducted in a similar way.
3,3′-Bis(indolizine) (16n)13
1H NMR (400 MHz, CDCl3): δ 9.34 (d, J = 4.7 Hz, 1H), 7.89 (d, J = 8.0 Hz, 2H), 7.85–7.90 (m, 1H), 7.35–7.40 (m, 2H), 7.24 (d, J = 8.0 Hz, 1H), 6.69 (s, 1H), 2.44 (t, J = 7.6 Hz, 2H), 2.39 (s, 3H), 1.76 (tt, J = 7.6, 7.4 Hz, 2H), 1.45 (tq, J = 7.4, 7.3 Hz, 2H), 0.97 (t, J = 7.3 Hz, 3H); 13C NMR (100 MHz, CDCl3): δ 13.9 (CH3), 21.6 (CH3), 22.1 (CH2), 31.1 (CH2), 38.9 (CH2), 92.6 (C), 109.3 (C), 120.1 (C), 123.1 (CH), 126.3 (C), 127.1 (CH), 129.5 (CH), 133.3 (CH), 138.7 (CH), 140.5 (C), 151.3 (CH).
2,2′-Dibutyl-3,3′-bis(indolizine) (16o)
Blue oil (69 mg, quant., 0.10 mmol). 1H NMR (400 MHz, CDCl3): δ 7.34 (dd, J = 8.9, 0.9 Hz, 2H), 7.20 (dd, J = 6.4, 0.9 Hz, 2H), 6.69 (ddd, J = 0.9, 6.4, 8.9 Hz, 2H), 6.47 (s, 2H), 6.34 (ddd, J = 0.9, 7.0, 7.0 Hz, 2H), 2.46 (q, J = 7.8 Hz, 4H), 1.53 (tt, J = 7.8, 7.4 Hz, 4H), 1.25 (tq, J = 7.8, 7.4 Hz, 4H), 0.79 (t, J = 7.4 Hz, 6H); 13C NMR (100 MHz, CDCl3): δ 13.8 (CH3), 22.5 (CH2), 26.4 (CH2), 32.9 (CH2), 98.8 (CH), 109.6 (CH), 111.6 (C), 117.3 (CH), 118.4 (CH), 122.9 (CH), 132.5 (C), 133.6 (C); IR (ATR, cm–1): 1620; HRMS (ESI/TOF): calcd for (M + H+) C24H31N2, 345.2325; found, 345.2315.
2,2′-Diphenyl-3,3′-bis(indolizine) (16p)15
Blue oil (15 mg, 39%, 0.039 mmol). 1H NMR (400 MHz, CDCl3): δ 7.43 (d, J = 8.9 Hz, 2H), 7.30–7.27 (m, 6H), 7.16–7.07 (m, 6H), 6.90 (s, 2H), 6.70 (ddd, J = 8.9, 6.5, 0.9 Hz, 2H), 6.27 (ddd, J = 6.5, 6.5, 0.9 Hz, 2H); 13C NMR (100 MHZ, CDCl3): δ 98.5 (CH), 110.5 (C), 110.6 (CH), 118.5 (CH), 118.7 (CH), 123.0 (CH), 126.5 (C), 126.7 (CH), 128.5 (CH), 130.5 (C), 134.0 (C), 135.3 (C).
Supporting Information Available
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00133.
Author Present Address
§ Institute of Academic Initiatives, Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan (H.A.).
Author Contributions
All authors have given approval to the final version of the manuscript.
The authors declare no competing financial interest.
Supplementary Material
References
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