Weimin Maa,
Jiawei Huanga,
Chao Lia,
Yueren Jianga,
Baolin Li*a,
Ting Qi
*a and
Xiaozhang Zhu
*b
aSchool of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, P. R. China. E-mail: libl@ucas.ac.cn; qiting@ucas.ac.cn
bBeijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. E-mail: xzzhu@iccas.ac.cn
First published on 1st March 2019
Based on the regioselective intermolecular Suzuki coupling and subsequent intramolecular Ullmann C–O coupling reactions, one-pot synthesis of benzo[4,5]thieno[3,2-b]benzofurans (BTBFs) was developed after optimization of the reaction conditions including catalysts, solvents, bases, ligands and reaction times. The one-pot reaction, with only 2 mol% Pd(PPh3)4 and 2 mol% copper(I) thiophene-2-carboxylate (CuTc) as the catalysts, K3PO4·3H2O as the base and tert-butanol as the solvent, afforded moderate to good yields (up to 70%) for a variety of substrates.
Over the years, some synthetic methods including transition metal catalyzed intramolecular C–O coupling16–18 and bis-C/H-activated ring closure of diaryl ethers19,20 as well as ring closure of ortho-hydroxyarylalkynes21–27 have been reported for the synthesis of furan-fused ring compounds. These methods are usually high-yielding and applicable to a variety of substrates. Nevertheless, they also suffer from some drawbacks, including the usage of expensive palladium catalysts, cumbersome steps for the preparation of the substrates, and so on. Therefore, these methods are not satisfactory from a synthetic chemistry viewpoint, and new cost-effective synthetic methods for furan-fused ring compounds are urgently needed.
The search for alternative safer, cleaner and environmentally friendly synthetic procedures is one of the major activities in synthetic chemistry. To this end, the reduction of wastes together with the use of renewable feedstocks, environmentally friendly reagents and inexpensive catalysts are important pathways to access more sustainable processes.28,29 In this sense, one-pot procedure involving catalytic multistep reactions allows the decrease of energy consuming steps such as separation and purification of intermediates.30 These transformations known as tandem, domino or cascade reactions31,32 have become an important research area in organic chemistry33–35 since they can reduce raw materials and energy consumption and improve atom economy.
As shown in Scheme 1a, thieno[3,2-b]benzofurans have been synthesized by palladium-catalyzed double C–H activation from diaryl ethers.19,20 However, the reactions were performed under rather hash condition (110 or 150 °C), and the precursor diaryl ethers need to be synthesized by a multiple-step reaction. Recently, our group has reported a new strategy for the synthesis of benzo[4,5]thieno[3,2-b]benzofurans (BTBFs) through intramolecular C–O coupling by using cheap copper catalyst (Scheme 1b) under mild condition (90 °C).8 We further noticed that both the Suzuki coupling and the subsequent Ullmann reaction require similar reaction conditions: transition metal catalysts, inorganic bases, and the reaction temperature at around 90 °C. In order to further simplify the synthetic route, we thus design one-pot method30 to synthesize the target product BTBFs. Fortunately, we successfully achieved this goal after screening the reaction conditions. The yields are moderate to good (up to 70%). Meanwhile, this method shows good functional group compatibility, halogenated compounds are also suitable substrates. On the other hand, we studied the photophysical properties of this type of compounds, representing the first example of thieno[3,2-b]furans.
Entry | Catalyst 1 | Catalyst 2 | Base | 18-crown-6/3 Å M.S.b | Solvents | GC yield |
---|---|---|---|---|---|---|
a Reaction conditions: the mixture of 1a (140 mg, 0.48 mmol), 1b (55 mg, 0.4 mmol), the first catalyst Pd(PPh3)4 and the base in the solvent (5.0 mL) under N2 was stirred at 90 °C for 24 hours. Then the second copper catalyst was added and heated for another 4 hours. 1,1,2,2-Tetrachloroethane was used as an internal standard to determine the GC yield.36b 3 Å molecular sieves were activated under vacuum condition at 250 °C for 5 hours before being used as water absorbent.c Different amount of water was added as additive.d The yield in parentheses is the isolated yield.e 2-Hydroxyphenylboronic acid (2a) was replaced by 2-hydroxyphenylboronic acid pinacol ester. | ||||||
1 | 5 mol% Pd(OAc)2 | 5 mol% CuI | K2CO3 | —/— | 1,4-Dioxane | 0 |
2 | 5 mol% Pd(OAc)2 | 5 mol% CuI | K2CO3 | —/— | DMF | 0 |
3 | 5 mol% Pd(PPh3)4 | 5 mol% CuI | K2CO3 | —/— | 1,4-Dioxane | 1.1% |
4 | 5 mol% Pd(PPh3)4 | 5 mol% CuI | K2CO3 | —/— | DMF | 3.7% |
5 | 5 mol% Pd(PPh3)4 | 5 mol% CuI | K2CO3 | —/— | NMP | 4.6% |
6 | 5 mol% Pd(PPh3)4 | 5 mol% CuOAc | K2CO3 | —/— | NMP | 5.6% |
7 | 5 mol% Pd(PPh3)4 | 5 mol% Cu(OAc)2 | K2CO3 | —/— | NMP | 5.1% |
8 | 5 mol% Pd(PPh3)4 | 5 mol% CuTc | K2CO3 | —/— | NMP | 6.8% |
9 | 5 mol% Pd(PPh3)4 | 5 mol% CuTc | K2CO3 | —/— | 1,4-Dioxane | 6.9% |
10 | 5 mol% Pd(PPh3)4 | 5 mol% CuTc | K2CO3 | —/— | t-BuOH | 6.4% |
11 | 5 mol% Pd(PPh3)4 | 5 mol% CuTc | Cs2CO3 | —/— | t-BuOH | 10% |
12 | 5 mol% Pd(PPh3)4 | 5 mol% CuTc | K3PO4 | —/— | t-BuOH | 29% |
13 | 5 mol% Pd(PPh3)4 | 5 mol% CuTc | K3PO4 | —/— | t-BuOH/10 μL H2Oc | 42% |
14 | 5 mol% Pd(PPh3)4 | 5 mol% CuTc | K3PO4 | —/— | t-BuOH/0.1 mL H2Oc | 35% |
15 | 5 mol% Pd(PPh3)4 | 5 mol% CuTc | K3PO4 | —/— | t-BuOH/1 mL H2Oc | 25% |
16 | 5 mol% Pd(PPh3)4 | 5 mol% CuTc | K3PO4·3H2O | —/— | t-BuOH | 33% |
17 | 5 mol% Pd(PPh3)4 | 5 mol% CuTc | K3PO4·3H2O | 5 mol%/— | t-BuOH | 36% |
18 | 5 mol% Pd(PPh3)4 | 5 mol% CuTc | K3PO4·3H2O | —/200 mg | t-BuOH | 43% |
19 | 5 mol% Pd(PPh3)4 | 5 mol% CuTc | K3PO4·3H2O | 5 mol%/200 mg | t-BuOH | 49% |
20 | 2 mol% Pd(PPh3)4 | 5 mol% CuTc | K3PO4·3H2O | 5 mol%/200 mg | t-BuOH | 56% |
21 | 0.5 mol% Pd(PPh3)4 | 5 mol% CuTc | K3PO4·3H2O | 5 mol%/200 mg | t-BuOH | 46% |
22 | 5 mol% Pd(PPh3)4 | 2 mol% CuTc | K3PO4·3H2O | 5 mol%/200 mg | t-BuOH | 43% |
23 | 2 mol% Pd(PPh3)4 | 2 mol% CuTc | K3PO4·3H2O | 5 mol%/200 mg | t-BuOH | 68% (62%)d |
24 | 2 mol% Pd(PPh3)4 | 2 mol% CuTc | K3PO4·3H2O | 5 mol%/200 mg | t-BuOH | 77% (70%)d,e |
We next screened a series of bases (Table 1, entries 10–12) and found that K3PO4 showed the best results (29% yield, Table 1, entry 12). After the screening of solvents and bases, we also explored the effect of the amount of water on reaction yield (Table 1, entries 13–15). To our delight, significant increase of the yield was observed when the reaction was carried out in 5 mL tert-butanol with 10 μL water (Table 1, entry 13). However, addition of excess amount of water reduces the reaction yield (Table 1, entry 15). This could be explained as followed: addition of small amount of water could increase the reaction rate of Suzuki coupling reaction since water and base are required to activate the boronic acid,37,38 and it doesn't do harm to the Ullmann reaction; while addition of large amount of water significantly reduces the catalytic activity of copper catalyst, thus does harm to the Ullmann reaction heavily and the intermediate remained when the reaction mixture was worked up. In order to simplify the experimental operation, we used K3PO4·3H2O as the source of base and water to avoid the need of addition of base and water separately (Table 1, entries 16–24).
On the other hand, we found that addition of 18-crown-6 as an additive to the reaction system can increase the solubility of K3PO4·3H2O37,39 in tert-butanol thus increase the reaction rate and improve the reaction yield (36%, Table 1, entry 17). It is well known that coordinated water in the base favors the previous Suzuki coupling,37 however, water will also coordinate with the copper catalyst in the subsequent Ullmann reaction and reduce its catalytic activity. Therefore, we introduced activated 3 Å molecular sieves as water absorbent into the reaction system after the Suzuki coupling has been carried out, and thus reaction yield has been further improved to 43% (Table 1, entry 18). By further screening the amount of two catalysts (Table 1, entries 19–23), we obtained the highest GC yield of 68% (isolated yield 62%, Table 1, entry 23). It is noted that the amount of two catalysts could be reduced to 2 mol%, which is significantly lower than our previous report [5 mol% Pd(PPh3)4, 10 mol% CuI]8 and the others' reports.19,20 Strikingly, when we used 2-hydroxyphenyl-boronic acid pinacol ester instead of 2-hydroxybenzeneboronic acid (Table 1, entry 24), GC yield can be further increased to 77% (isolated yield 70%), which is an improvement over the previously reported yield (67% total yield via two steps).
With the optimal reaction condition in hand, we next explored reaction scope. The reaction proved to be robust and versatile and allowed the synthesis of a variety of BTBFs as shown in Table 2. It worked well for various substrates bearing either electron-withdrawing or electron-donating groups on the aromatic rings, such as F, Cl, Me, C8H17, or t-Bu, affording the desired products in moderate to good yields (Table 2, 3a–3i). The yields are not high, it is understandable considering that both the Suzuki coupling and the subsequent Ullmann reaction are usually not very high yielding. To our delight, for the known compounds (3a–3b), we achieved higher yield than previous reports (70% vs. 67%, 61% for 3a; 60% vs. 46% for 3b).8,20 Furthermore, the π-extended product BTNF (3j) could be obtained through this approach in moderate yield (35%). This can extend the conjugation of this type of compounds and change their photophysical properties.
The UV-Vis absorption and fluorescence spectra of BTBF (3a), F-BTBF (3c), F,Cl-BTBF (3d), M-BTBF (3e), and BTNF (3j) in dilute ethanol solutions were investigated. As shown in Fig. 1, the absorption spectra of these compounds exhibits fine structures, and the maximum absorption peaks at 318, 320, 319, 319 and 355 nm respectively could be assigned to π–π* transitions of the conjugated backbones containing sulfur and oxygen heteroatoms.40,41 We can see that either halogen atoms on the F-BTBF and F,Cl-BTBF molecules or methyl group in the M-BTBF molecule have neglectable effect on the maximum absorption. But for BTNF, the introduction of naphthalene ring extends the conjugated system when compared to BTBFs, leading to a significant red shift of absorption peak.
As shown in Fig. 2, the maximum emission peaks of these compounds are located at 339, 342, 347, 340, 378 nm, respectively. The fluorescence quantum yields (Φf) of the compounds were determined to be 0.01, 0.03, 0.04, 0.10, 0.72 respectively in dilute ethanol solutions, with 9,10-diphenylanthracene (Φf = 0.95)42 as the reference standard. To be noted, BTNF exhibits significantly red-shifted fluorescence peak at 378 nm and much increased fluorescence quantum yield due to its extended π system provided by the rigid naphthalene ring. The introduction of rigid naphthalene ring may reduce the nonradiative decay rate, resulting in high fluorescence quantum yield. Thus, it is expected to be a good luminescent material. To our best knowledge, this is the first example of study on photophysical properties of thieno[3,2-b]furans.
To understand the frontier molecular orbitals and spectroscopic properties of these five compounds, the computation of molecular orbital geometries were performed by density functional theory (DFT) at the B3LYP/6-31G level of theory.43,44 In Fig. S2,† we can see that the highest occupied molecular orbitals (HOMOs) and lowest occupied molecular orbitals (LUMOs) of these compounds are homogeneously delocalized over the entire backbone. The HOMO levels (EHOMOs) of BTBF, F-BTBF, F,Cl-BTBF, M-BTBF and BTNF are calculated to be −5.71, −5.94, −6.22, −5.66, −5.45 eV, respectively, and the LUMO levels (ELUMOs) are −1.29, −1.54, −1.82, −1.24, −1.64 eV, respectively (Fig. S3†).
Compared with BTBF, F-BTBF and F,Cl-BTBF show much deeper HOMO and LUMO energy levels resulting from the existence of electron-withdrawing halogen atoms (Fig. S3†). On the contrary, the introduction of an electron-donating methyl group slightly raises the HOMO and LUMO energy levels of M-BTBF. Obviously, the tendency of the calculated energy gaps of these compounds are in agreement with those of the energy gaps calculated from the UV absorption onset (Table 3 and Fig. S3†).
Compounds | λabs,max (nm) | Ega (ev) | λFL (nm) | Stoke shift (nm) | Φfb |
---|---|---|---|---|---|
a The optical band gap Eg is calculated by the formula Eg = 1240/λonset. λonset represents the largest edge of the UV-Visible absorption spectrum.b 9,10-Diphenylanthracene (Φf = 0.95) as a reference in ethanol.42 | |||||
BTBF (3a) | 318 | 3.79 | 339 | 21 | 0.01 |
F-BTBF (3c) | 320 | 3.73 | 342 | 22 | 0.03 |
F,Cl-BTBF (3d) | 319 | 3.75 | 347 | 28 | 0.04 |
M-BTBF (3e) | 319 | 3.77 | 340 | 21 | 0.10 |
BTNF (3j) | 355 | 3.41 | 378 | 23 | 0.72 |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra00796b |
This journal is © The Royal Society of Chemistry 2019 |