Mingming Yanga,
Yajun Jiana,
Weiqiang Zhang
a,
Huaming Suna,
Guofang Zhang
a,
Yanyan Wang
*a and
Ziwei Gao
*ab
aKey Laboratory of Applied Surface and Colloid Chemistry, Xi'an Key Laboratory of Organometallic Material Chemistry, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an 710119, P. R. China. E-mail: zwgao@snnu.edu.cn; yyw@snnu.edu.cn
bA School of Chemistry & Chemical Engineering, Xinjiang Normal University, Urumqi 830054, P. R. China
First published on 6th December 2021
An efficient one-pot approach for the synthesis of quinolines from o-aminothiophenol and 1,3-ynone under mild conditions is disclosed. With the aid of ESI-MS analysis and parallel experiments, a three-step mechanism is proposed—a two-step Michael addition–cyclization condensation step leading to intermediate 1,5-benzothiazepine catalyzed by zirconocene amino acid complex Cp2Zr(η1-C9H10NO2)2, followed by I2-mediated desulfurative step.
Recently, the ynones and amine synthons' combination toward the synthesis of quinolines have been verified as effective strategies,30,31 started by using preconnected starting materials containing both amine and ynone parts. For example, β-(2-aminophenyl)-α,β-ynones32–36 can undergo intramolecular cyclization to generate quinoline at room temperature (Scheme 1, entry a).37 In contrast, the reaction between anilines and ynones via 3 + 3 cycloaddition is more direct and step-economical, thus became more popular. Although C–H activation of aniline has less demanding for starting material, the reaction usually needs to be driven by noble metal catalysts, such as AgOTf, under high temperature (entry b).38 Therefore, using α-position preactivated aniline is more practical as cheap metal catalyst could be adopted. For example, 2-iodoanilines and ynones can be catalyzed by a nickel-catalyzed cyclization to generate quinolines (entry c).39 o-Sulfhydryl-aniline also proven to effective as preactivated 3-atom synthons,40,41 due to both the soft basic nature of sulfur atom for the ease to carry out 1,4-Michael addition reactions, and the viability to go through desulfurization via 7-membered ring's42–44 contraction from 1,5-benzothiazoles (entry d). However, such one-pot procedures are still suffering from negligible drawbacks, like incompatible solvent system for the thiol Michael addition and desulfurization processes, and high temperatures for desulfurization step. Therefore, it is still challenging to synthesize quinoline in one solvent under mild temperature.
In our previous work, by using titanocone complex as catalyst, 7-membered ring contained 1,5-benzodiazepines were successfully synthesized from o-phenylenediamine and 1,3-ynones through the [4 + 3] cyclization reaction under mild conditions, indicating such organometallic Lewis acid catalyst can facilitate the Michael addition process and cyclization reaction at room temperature.45 Additionally, the Lewis acidic catalytic activity46,47 and selectivity48 of such metallocene could be conveniently adjusted by changing carboxylic acid ligand bearing different electronic characteristic,49 based on our group's investigation accumulation of IVB metal based complex. In particular, the –NH2 group as a Brønsted base is potential to activate o-aminothiophenol through hydrogen bonding. In this work, we disclosed a one-pot approach for the synthesis of quinolines from o-aminothiophenol and 1,3-ynone, catalyzed and mediated by Cp2ZrCl2 and I2 at room temperature, via sequential thio-Michael addition, cyclization and desulfurization by ESI-MS and control experiments.
Run | Solvent | Ligand | Oxidant | Yieldb (%) |
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a Reaction conditions: 0.5 mmol of 1,3-ynone, 0.6 mmol of o-aminothiophenol, 5 mol% of Cp2ZrCl2 and 10 mol% of ligand in 1 mL DMF at room temperature for 5 h, then 0.6 mmol oxidant was added for 1 h.b Yield of the GC. | ||||
1 | Toluene | L-Phenylalanine | TBHP | 56 |
2 | CH2Cl2 | L-Phenylalanine | TBHP | 56 |
3 | THF | L-Phenylalanine | TBHP | 43 |
4 | 1,4-Dioxane | L-Phenylalanine | TBHP | 65 |
5 | CH3CN | L-Phenylalanine | TBHP | 48 |
6 | DMF | L-Phenylalanine | TBHP | 83 |
7 | DMSO | L-Phenylalanine | TBHP | 67 |
7 | EtOH | L-Phenylalanine | TBHP | 17 |
9 | DMF | Alanine | TBHP | 78 |
10 | DMF | Tyrosine | TBHP | 80 |
11 | DMF | Serine | TBHP | 77 |
12 | DMF | L-Proline | TBHP | 77 |
13 | DMF | Salicylic acid | TBHP | 62 |
14 | DMF | Benzoic acid | TBHP | 60 |
15 | DMF | 2-Hydroxynicotinic acid | TBHP | 52 |
16 | DMF | Camphorsulfonic acid | TBHP | 24 |
17 | DMF | L-Phenylalanine | H2O2 | 80 |
18 | DMF | L-Phenylalanine | DMP | 83 |
19 | DMF | L-Phenylalanine | I2 | 92 |
20 | DMF | L-Phenylalanine | — | 60 |
To further expand the substrate scope of this one-pot method, the utilization of 1,3-ynone derivatives was then explored (Table 2). The scope of this one-pot method concerning the substituents of the substrates was proven to be very wide. 1,3-Ynone bearing electron-donating and electron-withdrawing groups on the phenyl ring at the carbonyl group side were applied to the reaction. The introduction of electron-donating groups such as –Me and –OMe in different positions of the ring, affording quinolines in the yield range of 62–93% (3b, 3c, 3d and 3e). Electron-withdrawing halogen groups like p-fluoro, p-chloro and p-bromo led to the desired products in good yields of 90%, 84% and 80% respectively (3f–3h). The reaction of o-chloro and m-chloro substituted 1,3-ynones proceeded to afford quinolines in good yields of 64% (3i) and 81% (3j), respectively. Fortunately, heterocyclic thiophene- methylthiophen- and naphthyl-substituted 1,3-ynone were also compatible with this reaction, giving products in yield of 73%, 63%, and 50%, respectively (3k–3m). Then we turned attention to test the toleration with aliphatic 1,3-ynones. Ethoxy-substituted 1,3-ynone afforded the product in 84% yield (3n). Methyl-substituted 1,3-ynone assured a yield in 52% (3x).
a Reaction conditions: 0.5 mmol of 1,3-ynone, 0.6 mmol of o-aminothiophenol in the presence of 5 mol% Cp2ZrCl2, 10 mol% of L-phenylalanine and 1 mL DMF at room temperature for 5 h, then 0.6 mmol I2 was added for 1 h. Isolated yields were obtained after purification by column chromatography. |
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Secondly, we investigated the scope of this method concerning the impact of substitution of the alkyne side on 1,3-ynones. For p-methoxy, m-methoxy and o-methoxy substituents, the products were afforded in excellent yields of 99%, 94% and 99% (3o–3q), respectively. And p-methyl group also assured an excellent yield in 98% (3r). When electron-withdrawing substrates were subjected to this transformation, halogen groups like o-chloro, m-chloro and p-chloro also led to the product in good yields of 87%, 83% and 84% respectively (3s–3u). These results indicate that 1,3-ynones derivatives with whether electron-donating or electron-withdrawing groups can be used in this method to give excellent yields. Even for 1,3-ynone derivatives with aliphatic substituents, such as hexyl-, cyclopropyl- and diethoxymethyl-, were successfully applied to the reaction and gave the corresponding products in good yields (3v, 3y and 3z). Moreover, the p-chloro derivative of o-aminothiophenol provided the corresponding quinoline 3w in an excellent yield of 81%.
The next task is to map the route for the reaction. To elucidate the action mode of this one-pot tandem reaction, some reactions were performed (Table 3). Under the oxidant-free condition, the conversion of 1,3-ynone was almost 100%, but the yield of the target product was only 60%, and 1,5-benzothiazepines in 40% yield was obtained (Table 3, entry 1). This result indicates that both 1,5-benzothiazepines containing 7-membered ring and quinolines containing 6-membered ring could be formed from 1,3-ynones and o-aminothiophenol without I2. 0.1 equiv. of I2 prompted the reaction to give the quinoline in yield of 84% (entry 2), which indicated that a small amount of oxidant benefits the conversion from 1,5-benzothiazepine to quinoline. Meanwhile, the reaction couldn't be catalyzed by the oxidant alone, for that only 7% yield of quinoline was detected in Cp2ZrCl2-free and ligand-free condition (entry 3). However, 1,5-benzothiazepine could be almost completely converted to quinoline by I2, which further suggested the positive effect of I2 on the desulfurization of 1,5-benzothiazepine (entry 4). Regardless of the starting material, quinoline was not detectable after adding the radical scavenger TEMPO in the experiment (entries 5, 6). These results suggested that the iodine-mediated desulfurization is most likely a radical process. We also carried out ESI-MS experiment for the real reaction system, with important intermediates detected (proposed structures shown in Scheme 2), including zirconocene catalytic species Cp2Zr(η1-C9H10NO2)2 (II), and intermediate II′.
a Normal reaction conditions: 0.5 mmol of 1,3-ynone, 0.6 mmol of o-aminothiophenol in the presence of 5 mol% of Cp2ZrCl2, 10 mol% of L-phenylalanine and 1 mL DMF at room temperature for 5 h, then 0.6 mmol I2 was added and reaction for 1 h, yields were obtained after separation and purification of the product by column chromatography.b Adding 2 equiv. of TEMPO. |
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Taking all the aforementioned control experiment results and ESI-MS analysis together, a plausible reaction mechanism is proposed in Scheme 2. Initially, the zirconocene catalyst Cp2Zr(η1-C9H10NO2)2 is very likely to be formed in situ by the pre-catalyst and ligand in the presence of o-aminothiophenol.50,51 As one amino acid ligand is labile, the 1,3-ynone readily coordinated to the generated Cp2Zr(η1-C9H10NO2)2 to form the catalyst–substrate intermediate III, which facilitates a Michael addition with o-phenylenediamine to form an intermediate IV.45 Then the intermediate IV experiences an intramolecular cyclization condensation, followed by the removal of one molecule of H2O to form a seven-membered ring 1,5-benzothiazepine V, thus a catalytic cycle furnished. Finally, intermediate V experiences a desulfurization process mediated by I2.52–54 An intermediate VI is formed from the V and iodine molecular via I–S coordination, followed by a cleavage of I–I bond that generates an intermediate VII. Finally, the subsequent intramolecular cycloaddition followed by cleavage of the C–S bond affords the quinoline framework.
Footnote |
† Electronic supplementary information (ESI) available: NMR data and ESI(+)-MS spectra. See DOI: 10.1039/d1ra06976d |
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