Notes
Carboxylative Cyclization of a Propargylic Amine with CO2 Catalyzed by a Silica-Coated Magnetite
2021 Volume 69 Issue 7 Pages 698-701
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2021 Volume 69 Issue 7 Pages 698-701
By employing a silica-coated magnetite as a catalyst, a silica-catalyzed carboxylative cyclization of propargylic amines with carbon dioxide (CO2) proceeded to afford the corresponding 2-oxazolidinones. Moreover, after the reaction, the silica-coated magnetic catalyst was readily recovered by use of an external magnet and could be reused up to six times without deactivation.
The transformation of environmentally deleterious carbon dioxide (CO2) into valuable organic molecules has drawn much attention in synthetic organic chemistry in the last two decades, because CO2 can be used as a nontoxic, non-flammable, and renewable resource.1) Moreover, CO2 is one of the most attractive C1 building blocks to displace toxic reagents such as phosgene and carbon monoxide.2) However, most of the chemical transformations of CO2 require high pressure and high reaction temperatures due to the thermodynamic stability and chemical inertness of CO2.
One of the useful methods for chemical transformation of CO2 is the carboxylative cyclization of propargylic amines with CO2 to provide 2-oxazolidinones.3) Recently, various studies have described the synthesis of the 2-oxazolidinone from the propargylic amine and CO2 with homogeneous catalysts such as organometallic complexes of transition metals4–7) and metal-free activators.8–10) In addition, some recent studies have employed heterogeneous catalysts such as a silica (SiO2)-bound organic base and basic alumina for the synthesis of the 2-oxazolidinone from the propargylic amine and CO2, and showed that these catalysts could be recovered by filtration after the reaction.11,12) Very recently, our group reported that the carboxylative cyclization of propargylic amines with CO2 could be catalyzed even by using silica alone.13) We report herein that a silica-coated magnetite can be used to catalyze the carboxylative cyclization of propargylic amines with CO2 to provide 2-oxazolidinones.14,15) Because this catalyst is magnetic, it could be easily separated from the reaction medium by use of an external magnet without filtration. Moreover, the magnetically collected catalyst could be reused repeatedly without deactivation. From the perspective of green chemistry, the transformation of CO2 promoted by a magnetically recoverable catalyst is a very attractive field, because a magnetic catalyst is environmentally benign due to its recyclability.16)
We have examined the carboxylative cyclization of a propargylic amine 1a with CO2 for the synthesis of a 2-oxazolidinone 2a with a silica-coated magnetite 3 as a catalyst, as shown in Table 1. A toluene solution of propargylic amine 1a was stirred at 120 °C for 20 h in a sealed autoclave under 2 MPa of CO2 by employing silica-coated magnetite 3 as a catalyst. After the reaction, the catalyst 3 was collected using an external magnet, and the reaction mixture was then transferred out of the reaction vessel. The corresponding 2-oxazolidinone 2a was obtained in an 81% chemical yield (Table 1, entry 1). Also in the case of using silica alone, 2a was obtained in almost the same chemical yield (79%; Table 1, entry 2). On the other hand, when the reaction was performed using only magnetite (Fe3O4) as a catalyst, 2a was obtained in a poor chemical yield (1%; Table 1, entry 3). When no catalyst was used, 2a was not obtained (Table 1, entry 4). These results indicate that the carboxylative cyclization of 1a with CO2 promoted by silica-coated magnetite 3 could be catalyzed by a silica moiety of 3.
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|---|---|---|
| Entry | Catalyst | Yield (%)b) |
| 1 | SiO2/Fe3O4 3c) | 81 |
| 2 | SiO2 | 79 |
| 3 | Fe3O4 | 1 |
| 4 | None | 0 |
a) Reaction conditions: 1a (0.4 mmol), catalyst (80 mg), toluene (4 mL; 0.1 M based on 1a), carried out in the sealed autoclave at 120 °C for 20 h under the pressurized CO2 (2 MPa). b) Determined by 1H-NMR. c) SiO2/Fe3O4 = 1/4 (w/w).
Although silica in 3 is one-fifth as heavy as that in Table 1, entry 2 (referred from Table 1, footnotes a and c), it was found that 3 showed a good catalytic activity. In addition, in our previous report, the carboxylative cyclization of a propargylic amine 1a with CO2 catalyzed by silica alone was performed under 0.4 M reaction solution of 1a.13) On the other hand, the silica-coated magnetite 3-catalyzed carboxylative cyclization of 1a could smoothly proceed even under 0.1 M reaction solution of 1a. Thus, it is supposed that the magnetic catalyst 3 is also suitable for a small scale synthesis of the 2-oxazolidinone 2a.
Next, using silica-coated magnetite 3 as a catalyst, we examined two of the reaction conditions—namely, the reaction temperature and CO2 pressure—as shown in Table 2. We first performed the synthesis of the 2-oxazolidinone 2a from the propargylic amine 1a in toluene under CO2 pressure of 2 MPa for 20 h under various reaction temperatures. Even when the reaction was carried out at 130 °C, the chemical yield of the corresponding 2-oxazolidinone 2a was the same as that at 120 °C (81%; Table 2, entry 2). On the other hand, when the reaction was carried out at 140 °C, the chemical yield of 2a decreased slightly (72%; Table 2, entry 3). We then examined CO2 pressures at 120 °C. When the reaction was carried out under CO2 pressure of 1 MPa, 2a was obtained in a somewhat lower chemical yield (69%; Table 2, entry 4). On the other hand, when the reaction was carried out under CO2 pressure of 3 MPa, the chemical yield of 2a was almost the same as that under CO2 pressure of 2 MPa (82%; Table 2, entry 5).
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|---|---|---|---|
| Entry | CO2 (MPa) | Temp. (°C) | Yield (%)b) |
| 1 | 2 | 120 | 81 |
| 2 | 2 | 130 | 81 |
| 3 | 2 | 140 | 72 |
| 4 | 1 | 120 | 69 |
| 5 | 3 | 120 | 82 |
a) Reaction conditions: 1a (0.4 mmol), 3 (80 mg), toluene (4 mL; 0.1 M based on 1a), carried out in a sealed autoclave for 20 h under the indicated reaction conditions. b) Determined by 1H-NMR.
Next, the reusability of the silica-coated magnetite catalyst 3 was examined in the synthesis of the 2-oxazolidinone 2a from the propargylic amine 1a in toluene under CO2 pressure of 2 MPa at 120 °C for 20 h, as shown in Table 3. In this experiment, it was found that 3 was readily separated from the reaction mixture by magnetic decantation using an external magnet after the reaction, and could be reused six times without deactivation, and that the corresponding 2-oxazolidinone 2a could be consistently obtained in a good chemical yield.13,17)
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|---|---|---|---|---|---|---|---|
| Number of reuse | — | First | Second | Third | Fourth | Fifth | Sixth |
| Yield (%)b) | 81 | 89 | 89 | 88 | 87 | 82 | 84 |
a) The reaction conditions were the same as those indicated in entry 1 of Table 1. b) Determined by 1H-NMR.
Finally, we synthesized various 2-oxazolidinones 2 from the propargylic amines 1 and CO2 with the silica-coated magnetite 3 according to the reaction conditions indicated in Table 4. As shown in entry 1 of Table 4, by the stirring of a 0.1 M toluene solution of 1a under CO2 pressure of 2 MPa at 120 °C for 20 h, the corresponding 2-oxazolidinone 2a was obtained in an 81% chemical yield, which was already indicated in Tables 1–3. In the case of the cyclization of N-methyl propargylic amine 1b, which has a low boiling point, the cyclization reaction was performed at 90 °C. As a result, the corresponding 2-oxazolidinone 2b was obtained in a low chemical yield (33%; Table 4, entry 2). Then, by the stirring of a 0.4 M toluene solution of 1b under CO2 pressure of 3 MPa at 100 °C, the chemical yield of 2b was improved (60%; Table 4, entry 3). Even by the introduction of a methyl group at R2 or R3 in 1, the corresponding 2-oxazolidinone 2c was obtained in a fair chemical yield by carrying out the reaction at 140 °C in p-xylene (79%; Table 4, entry 4). In contrast, by the introduction of two methyl groups at R2 and R3 in 1, the corresponding 2-oxazolidinone 2d was obtained in an only 14% chemical yield even by carrying out the reaction at 140 °C (Table 4, entry 5). In addition, by the introduction of a phenyl group at R1 in 1, the carboxylative cyclization of 1e gave a low chemical yield (28%; Table 4, entry 6). On the other hand, when a cyano or a trifluoromethyl group was introduced at the phenyl group in R1, the chemical yields of the corresponding 2-oxazolidinones 2f and 2g increased slightly due to the high reactivity of the carbon–carbon triple bond owing to the introduction of electron-withdrawing groups8) (45 and 40%; Table 4, entries 7 and 8, respectively). Only in these cases, small amounts of the corresponding 2-oxazolones 4f and 4g were also obtained, respectively, just as in our previously reported reaction catalyzed by silica alone13) (Fig. 1; 4f: 2%, 4g: 1%). 2-Oxazolones 4f and 4g appeared to be obtained by the tautomerization of the generated 2-oxazolidinones 2f and 2g, respectively.8,18,19) By the introduction of the methyl group in R1, the carboxylative cyclization product 2h was obtained in a poor chemical yield (3%, Table 4, entry 9). By employing a primary amine 1i as a substrate, the corresponding 2-oxazolidinone 2i was not obtained (Table 4, entry 10).
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a) Carried out in a sealed autoclave under the indicated reaction condition. b) Determined by 1H-NMR. c) Isolated yield.
Figure 2 shows a proposed mechanism for the carboxylative cyclization of the propargylic amine 1 to afford the 2-oxazolidinone 2 catalyzed by a silica-coated magnetite 3. First, the propargylic amine 1 reacts with CO2 to form the corresponding carbamic acid 5 in situ.5) It is considered that the thus-obtained carbamic acid 5 was activated by silica-surface OH–π interaction with the carbon–carbon triple bond, as shown in 6.13) Then, the corresponding 2-oxazolidinone 2 was provided with the regeneration of a silica-coated magnetite 3.
In conclusion, by employing a silica-coated magnetite as a catalyst, a 2-oxazolidinone was synthesized from a propargylic amine and CO2. Moreover, after the reaction, the silica-coated magnetic catalyst was readily recovered by use of an external magnet and could be reused up to six times without deactivation.
A water dispersion of the silica-coated magnetite 3 was purchased from Bioneer, Korea (AccuBead™; 1 g/50 mL water, SiO2/Fe3O4 = 1/4 (w/w), size 1–5 μm). Silica (SiO2) was obtained from Fuji Silysia, Japan (CARiACT Q-6). Degassed toluene and 2-propanol were prepared by freeze-pump-thaw cycling of commercially available dry toluene and 2-propanol for the synthesis of the 2-oxazolidinone 2 and for the washing of a silica-coated magnetite 3, respectively. Propargylic amines 1a,20) 1c,21) and 1h7) were synthesized by the condensation of the corresponding propargylic halides and benzylamine.20) Propargylic amine 1d was synthesized by the condensation of the corresponding propargylic primary amine and benzyl chloride.10,22) Propargylic amines 1e,5) 1f,23) and 1g9) were synthesized by Sonogashira reaction of the corresponding aryl iodides and N-methylpropargylamine.9) Propargylic amines 1b and 1i and other reagents were commercially available and were used as received. Carbon dioxide (Showa Denko Gas Products Co., Ltd., Japan, purity >99.99%) was used without further purification.
1H- and 13C-NMR spectra were measured with Bruker Avance III 400 spectrometer (1H: 400 MHz, 13C: 100 MHz). Chemical shifts were reported as ppm downfield from tetramethylsilane (TMS) as an internal standard in δ units. Coupling constants (J) are given in hertz (Hz).
Synthesis of the 2-Oxazolidinone 2a from the Propargylic Amine 1a and CO2 Catalyzed by Silica-Coated Magnetite: Typical Procedure (Table 1, Entry 1)To a stainless steel autoclave 4 mL of a water dispersion of a silica-coated magnetite 3 (1 g/50 mL water) was added and water was removed by magnetic decantation using an external magnet. In order to remove the residual water, magnetic decantation was carried out by use of 2-propanol (3 times) and toluene (3 times). Toluene (4 mL) and the propargylic amine 1a (58.3 mg, 0.401 mmol) were subsequently added to the reaction vessel under an argon atmosphere. The autoclave was sealed, pressurized with CO2 to 2 MPa at room temperature, and then heated to 120 °C. The resulting mixture was stirred at 120 °C for 20 h. After the autoclave was cooled in an ice bath and depressurized, the magnetic catalyst 3 was separated by magnetic decantation using an external magnet, and then the reaction mixture was transferred out of the reaction vessel, followed by washing of the magnetic catalyst 3 with 2-propanol (4 times) and toluene (3 times) under an argon atmosphere successively, and the rinse solution was combined with the reaction mixture. The chemical yield of the 2-oxazolidinone 2a was determined by the integration of 1H-NMR absorption with reference to an internal standard (2-benzyloxynaphthalene), which was added to the combined mixture (81% yield). After the removal of the solvent under reduced pressure, the residues were purified with silica gel column chromatography (hexane–ethyl acetate as eluents) to obtain the isolated 2-oxazolidinone 2a in an 80% chemical yield, which was shown in entry 1 of Table 4.
3-Benzyl-5-methylidene-1,3-oxazolidin-2-one (2a)6)1H-NMR (400 MHz; CDCl3) δ: 7.40–7.30 (m, 3H), 7.30–7.25 (m, 2H), 4.74 (td, J = 2.7, 3.0 Hz, 1H), 4.47 (s, 2H), 4.24 (td, J = 2.2, 3.1 Hz, 1H), 4.02 (t, J = 2.4 Hz, 2H); 13C-NMR (100 MHz; CDCl3) δ: 155.6, 149.0, 135.0, 129.0, 128.23, 128.15, 86.7, 47.8, 47.2.
Catalyst Reusability in the Synthesis of the 2-Oxazolidinone 2a from the Propargylic Amine 1a and CO2 (Table 3)The magnetically recovered wet catalyst 3, which was washed with the solvent under an argon atmosphere following the synthesis of the 2-oxazolidinone 2a from the propargylic amine 1a and CO2, was reused for the subsequent reaction by the addition of 1a (0.4 mmol) and toluene (4 mL) to the reaction vessel and by the pressurization of CO2 according to the above reaction procedure.
This work was based on results obtained from a project (JPNP16010) commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
The authors declare no conflict of interest.
The online version of this article contains supplementary materials.
