1 Introduction
Transition metal-catalyzed carbonylation of alkynes represents a high atom-economic, environmentally benign and efficient strategy for the preparation of carbonyl compounds and their derivatives.[1] The use of various nucleophiles (such as H2O, alcohols and amines) provides the corresponding unsaturated carboxylic acids, carboxylic esters and amides by the different routes of hydrocarboxylation[2], hydroesterification[3] and hydroaminocarbonylation[4]. As a significant type of carbonylation reaction, the hydrocarboxylation of alkynes could give the α,β-unsa- turated carboxylic acids, which serve as building blocks for synthesizing polymers, carbon rings and heterocycles. Since Walter Reppe[5] first reported the Ni(CO)4-catalyzed hydrocarboxylation of acetylene, Pd catalysts have garnered increased attention for this conversion.[6] By modulating the types of phosphine ligands, the catalytic activity and selectivity of hydrocarboxylation can be improved. In addition, the hydrocarboxylation of alkynes using alternative sources of CO (e.g., CO2[7], HCOOH[8], and oxalic acid[9]) has been developed.
Based on the homogeneous catalytic system, the development of the heterogeneous catalysts for hydrocarboxylation of alkynes has received a lot of research attention due to their advantages in facilitating the separation and recovery of catalysts. Bhattacharyya et al.[10] initially reported the Ni-silica gel-catalyzed hydrocarboxylation of alkynes, which resulted in a lower yield. Subsequently, catalysts of NiY,[11] CuY[12] and Ni-MCM-41[13] were prepared through ion exchange with various molecular sieves, resulting in enhanced catalytic activities. In addition to molecular sieves, silica core-shell and pseudoboehmite (AlOOH) could also serve as carriers to provide the catalysts of NiO@SiO2[14] and NiOAlOOH[15]. Despite these contributions, the heterogeneous nickel-based catalysts are plagued by high reaction temperature (220~250 ℃) as well as the gas pressure (3.0~4.5 MPa, Scheme 1a). Moreover, copper salts were employed as promoters to activate acetylene in these catalytic systems. Compared with the Ni catalysts, Pd catalysts could catalyze the reaction under milder conditions. However, heterogeneous Pd catalysts for hydrocarboxylation of alkynes have been little studied. The Liu group[16] successfully accomplished the recovery and recycling of a homogeneous Pd-catalyst by employing a zwitterionic ligand in room temperature ionic liquid (Scheme 1b).
In recent years, due to their stable covalent structure and high specific surface area, porous organic polymers (POPs) have gained significant attention for the preparation of catalysts and their application in heterogeneous catalytic reactions.[17] Herein, we developed a novel phosphine- containing POPs-supported palladium nanoparticle catalyst for the hydrocarboxylation of alkynes (Scheme 1c). In the carrier of POP-Nixantphos-PPh3-PhSO3Na, the Nixantphos ligand moieties play a significant role for enhancing the catalytic activity of palladium in the hydrocarboxylation reaction. The PPh3 ligand moieties are used for constructing the porous framework that facilitate the diffusion of reactants and products, as well as to disperse the Pd nanoparticles, while the PhSO3Na component improves the hydrophilicity of the polymers. Utilizing water as the solvent and under initial CO pressure of 0.1 MPa, Pd/POP-Nixantphos- PPh3-PhSO3Na effectively catalyzed the hydrocarboxylation of alkynes to afford the α,β-unsaturated carboxylic acids with high yields and a broad substrate scope.
2 Results and discussion
As shown in Scheme 2, the support of POP-Nixantphos- PPh3-PhSO3Na was prepared by free radical tri-copoly- merization of 4-vinylbenzylnixantphos, 3vPPh3 and sodium 4-vinylbenzenesulfonate in the presence of the azobisiso- butyronitrile (AIBN) initiator. With tetrahydrofuran (THF) as the solvent, the Pd(OAc)2 and the copolymer support were stirred for 24 h at 60 ℃ to give the Pd/POP-Nixant- phos-PPh3-PhSO3Na. ICP-MS indicated a Pd loading of 3.62% (w) in the material.
The catalyst of Pd/POP-Nixantphos-PPh3-PhSO3Na was further characterized by FT-IR (Figure 1a). The peaks of 2925 and 2854 cm-1 are assigned to the C—H stretching vibration of newly formed carbon-carbon bonds resulting from polymerization. The peaks at 1630, 1598, 1558 cm-1 are attributed to the C=C stretching vibration of aromatic ring. The peaks centers at 1117 and 1037 cm-1 can be attributed to the stretching vibrations of the S=O and C—P bonds, respectively. The TGA shows that the catalyst has high thermal stability and decomposition temperature up to 460 (Figure 1b). In the surface-wetting characterization, the 44.3° angle indicated that the catalyst has a good hydrophilicity (Figure 1c), while the 113.7° angle was observed in the case of hydrophobic Pd/POP-Nixantphos-PPh3 (Figure 1d). The above results demonstrated that the PhSO3Na moieties in the copolymerization could significantly improve the hydrophilicity of the catalyst.
The oxidation state of palladium in the Pd/POP-Nixant- phos-PPh3-PhSO3Na was examined through X-ray photoelectron spectroscopy (XPS). As shown in Figure 2a, the Pd 3d XPS spectrum exhibits two peaks at 341.2 and 335.8 eV, which are attributed to the 3d3/2 and 3d5/2 peaks of Pd(II), respectively. The binding energies of P 2p moved from 132.4 and 131.5 eV for POP-Nixantphos-PPh3-PhSO3Na to 132.7 and 131.7 eV for Pd/POP-Nixantphos-PPh3-PhSO3- Na (Figure 2b). The results indicated the successful coordination of phosphines ligand moieties and Pd(II) metals in the catalyst.
As shown in Figure 3a, the N2 adsorption-desorption isotherm gave a type-IV isotherm for Pd/POP-Nixantphos- PPh3-PhSO3Na. A high Brunauer-Emmett-Teller (BET) specific surface area (662.8 m3/g) and pore volume (1.32 cm3/g) were detected. Based on the calculations of nonlocal density functional theory (NLDFT), the pore sizes are dis-tributed in 1.4~2.0 and 2.0~41.4 nm, respectively. The pore-size distribution demonstrated the dominant presence of microporous and mesoporous in the catalyst. For Pd/ POP-Nixantphos-PhSO3Na, a type-III isotherm was observed (Figure 3b) along with low BET surface area (6.4 m3/g) and pore volume (0.06 cm3/g). The above results indicated that the PPh3 ligand moieties could effectively construct porous structures of catalyst. The hierarchical porosities of Pd/POP-Nixantphos-PPh3-PhSO3Na are also confirmed by scanning electron microscopy (SEM, Figure 3c) and transmission electron microscopy (TEM, Figure 3d) images. The energy-dispersive spectroscopy (EDS, Figure 3e) mapping shows that the Pd, P, C, S, O, N and Na elements are uniformly distributed in the catalyst of Pd/POP- Nixantphos-PPh3-PhSO3Na. In addition, TEM image reveals that the Pd nanoparticles have an average particle diameter of 7.68 nm. The high specific surface area and porous structure can effectively disperse Pd nanoparticles.
The reaction conditions for Pd/POP-Nixantphos-PPh3- PhSO3Na-catalyzed hydrocarboxylation were investigated with diphenylethyne as the model substrate. The reaction temperature, CO pressure and amount of H2O were carefully studied. At last, the preliminary optimal conditions (2 mL of H2O, 110 ℃, 0.1 MPa of CO, 24 h) were chosen. It is worth noting that only the cis-configuration product was obtained, and the trans-product was not detected.
Under the optimized reaction conditions, various catalyst for hydrocarboxylation of diphenylethyne were investigated (Figure 4). When using the hydrophobic Pd/POP-Nixant- phos-PPh3 as the catalyst, 56% yield of 2a was obtained. Compared with the catalytic performance of Pd/POP-Ni- xantphos-PPh3-PhSO3Na, the hydrophilicity facilitated the dispersion of the catalyst in water to improve the hydrocarboxylation reaction rate. The use of Pd/POP-Nixant- phos-PhSO3Na gave the product 2a in 12% yield. The low yield could be attributed to the relatively low specific surface area of the catalyst. In addition, only 18% yield of 2a was observed in case of the Pd/POP-PPh3-PhSO3Na catalyst. The results indicated that bidentate Nixantphos moieties were the crucial ligands for improving the catalytic hydrocarboxylation activity of palladium.
Figure 4 Various catalysts catalyzed the hydrocarboxylation of diphenylethyneReaction conditions: diphenylethyne 1a (0.5 mmol), H2O (2 mL), CO (0.1 MPa), 110 ℃, 24 h. (a) Pd/POP-Nixantphos-PPh3-PhSO3Na [14.6 mg, 3.62% (w) Pd], (b) Pd/POP-Nixantphos-PPh3 [14.2 mg, 3.73% (w) Pd], (c) Pd/POP- Nixantphos-PhSO3Na [12.9 mg, 4.12% (w) Pd], (d) Pd/POP-PPh3- PhSO3Na [15.9 mg, 3.32% (w) Pd]. |
Under the optimized reaction conditions, various alkynes were studied in the hydrocarboxylation process using Pd/ POP-Nixantphos-PPh3-PhSO3Na as the catalyst (Table 1). For symmetrical diaryl acetylenes, both electron-donating groups (Me and MeO) and electron-withdrawing group (CHO) were compatible with the reaction, resulting in the corresponding α,β-unsaturated carboxylic acids 2b~2d with high yields (80%~92%). The substrates containing furan and thiophene rings also worked well to yield products 2e and 2f. In the reactions involving asymmetric diaryl acetylenes, it was observed that carboxyl groups tended to form preferentially on the carbon atoms of alkynes located on one side of the electron-donating group (2g~2i). Similarly, the substrates of arylalkynes afforded the uniquely regioselective products 2j and 2k. However, when phenylacetylene and 2-pentyne were subjected to the reaction, the carboxyl groups were formed on the carbon of the alkynes with lower steric hindrance to give the products 2l and 2m.
Table 1 Study scope for hydrocarboxylation of various alkynes catalyzed by Pd/POP-Nixantphos-PPh3-PhSO3Naa |
 |
a Alkynes 1 (0.5 mmol), Pd/POP-Nixantphos-PPh3-PhSO3Na (14.6 mg), CO (0.1 MPa), H2O (2 mL), 110 ℃, 24 h, yield of isolated product after column chromatography. Regioselectivities were determined based on 1H NMR analysis. |
The recyclability of Pd/POP-Nixantphos-PPh3-PhSO3Na for hydrocarboxylation of diphenylethyne was further studied. After the reaction, the catalyst could be easily separated from the reaction mixture through centrifugation. The initial cycle of testing revealed a significant decrease in yield during the second use of the catalyst, with the product yield recorded at 76%. However, in the presence of p-tolu- enesulfonic acid, the catalyst could be reused up to five times without a significant decrease in yield (Figure 5a). The results indicated that the addition of acid could enhance the stability of Pd-H species, thereby preventing its deactivation.[18] During the sixth cycle of the catalyst, it was observed that the yield decreased to 81%. In addition, a hot filtration test was carried out. 46% yield of 2a was determined after 10 h of reaction. The catalyst was immediately filtered and the filtrate was conducted under 0.1 MPa of CO. No increase in yield was observed after 14 h. No aggregation of Pd nanoparticles was observed through TEM analysis of recovered catalyst (Figure 5b). The XPS characterization of the recovered catalyst (Figures 5c and 5d) revealed the presence of both Pd(II) and Pd(0) species in the catalyst. The ICP-MS analysis of the recovered catalyst revealed a palladium loading of 3.55% (w), demonstrating the stability of the catalyst.
Figure 5 (a) Recycling tests of the Pd/POP-Nixantphos-PPh3-PhSO3Na in hydrocarboxylation of diphenylethyneDiphenylethyne 1a (0.5 mmol), Pd/POP-Nixantphos-PPh3-PhSO3Na (14.6 mg), p-toluenesulfonic acid (8.6 mg), H2O (2 mL), CO (0.1 MPa), 110 ℃, 24 h. (b) TEM image, (c) Pd 3d XPS spectra, (d) P 2p of the recovered Pd/POP-Nixantphos-PPh3-PhSO3Na. |
3 Conclusions
In conclusion, a catalyst of Pd/POP-Nixantphos-PPh3- PhSO3Na has been developed for the hydrocarboxylation of alkynes. Under mild reaction condition, various alkynes performed well to afford the corresponding α,β-unsaturated carboxylic acids in excellent yields. Moreover, the catalyst has an excellent stability and can be reused for up to five cycles.
4 Experimental section
4.1 Instruments and reagents
NMR equipment (Bruker Avance 500 spectrometer), ICP-MS (Thermo iCAP Q), FT-IR (Thermo Scientific Nicolet iS10 FTIR Spectrometer), SEM (TESCAN MIRA LMS), TEM (JEOL JEM-2100Plus), N2 sorption isotherms (Micromeritics ASAP 2460), thermogravimetric analysis (Netzsch TG 209 F3), X-ray photoelectron spectroscopy (Thermo Scientific ESCALAB 250Xi), contact angles (SDC 350KS instrument). All the reagents used in the experiment were of analytical grade (Energy Chemical and Aladdin). The gas used in the experiment were supplied by Qingdao Ludong Gas Co., LTD.
4.2 Synthesis of 4-vinylbenzylnixantphos
Under nitrogen, 4,6-bis(diphenylphosphino)phenoxazine (110 mg, 0.2 mmol) and NaH (30 mg, 1.25 mmol) were added in an ice bath to a Schlenk bottle containing N,N- dimethylformamide (DMF, 6 mL). NaH was added to the bottle several times in batches, and the mixture was stirred for 1.5 h at 70 ℃. The reaction was cooled to room temperature, then 4-vinylbenzyl chloride (61 mg, 0.4 mmol) was added to the solution followed by stirring at 60 ℃ for 18 h. The mixture was extracted with dichloromethane and concentrated. The residue was purified by silica gel column chromatography to afford 4-vinylbenzylnixantphos (115 mg, 83.5% yield). 1H NMR (CDCl3, 500 MHz) δ: 7.40 (d, J=7 Hz, 2H), 7.27~7.21 (m, 22H), 6.73~6.89 (m, 1H), 6.57 (t, J=6.5 Hz, 2H), 6.29 (d, J=6 Hz,2H), 6.03~6.02 (m, 2H), 5.74 (d, J=14.5 Hz, 1H), 5.24 (d, J=9 Hz, 1H), 4.78 (s, 2H); 31P NMR (CDCl3, 201 MHz) δ: -18.8.[19]
4.3 Synthesis of POP-Nixantphos-PPh3-PhSO3Na
Under nitrogen, 4-vinylbenzylnixantphos (35 mg, 0.05 mmol), tris(4-vinylphenyl) phosphine (3vPPh3, 89 mg, 0.26 mmol), 4-vinyl-benzenesulfonic acid sodium (11 mg, 0.05 mmol) and AIBN (5.0 mg, 0.03 mmol) were dissolved in THF (10 mL) in Schlenk flask. After stirring for 10 min at room temperature, the mixture was heated to 100 ℃ for 24 h. After evaporation of THF at 40 ℃ under vacuum, the crude product was washed by THF (6 mL×3) and separated by centrifugation. The copolymer (120 mg) was obtained as a light white solid.
4.4 Synthesis of Pd/POP-Nixantphos-PPh3-PhSO3- Na
In glove box, POP-Nixantphos-PPh3-PhSO3Na (130.0 mg), Pd(OAc)2 (11.3 mg) and tetrahydrofuran (8 mL) were added into Schlenk flask. After stirring for 24 h under N2 at 60 ℃, the resulting product was separated by centrifugation. The crude product was washed by tetrahydrofuran (6 mL×3) and evaporation of tetrahydrofuran under vacuum. The catalyst Pd/POP-Nixantphos-PPh3-PhSO3Na (128 mg) was obtained.
4.5 General procedure for Pd/POP-Nixantphos- PPh3-PhSO3Na-catalyzed hydrocarboxylation of alkynes
In a glove box, an autoclave with a magnetic stirring bar was charged with alkyne (0.5 mmol), H2O (2.0 mL) and Pd/POP-Nixantphos-PPh3-PhSO3Na [14.6 mg, Pd loading at 3.62% (w)]. The mixture was purged with carbon monoxide for three times and subsequently charged with CO (0.1 MPa). The autoclave was then heated to 110 ℃ (oil bath) and was kept at this temperature for 24 h. The autoclave was cooled in ice water, and the gas was carefully released in a well-ventilated hood. After completion of the reaction, the reaction mixture was extracted with THF. The THF extracts were concentrated in vacuo, and the residue was purified by flash column chromatography on a silica gel to give the product.
(E)-2,3-Diphenylacrylic acid (2a): White solid, 108 mg, 96% yield. m.p. 179~180 ℃ (lit.[20] 180~182 ℃); 1H NMR (CDCl3, 500 MHz) δ: 7.88 (s, 1H), 7.31~7.29 (m, 3H), 7.18~7.15 (m, 3H), 7.11~7.08 (m, 2H), 7.01~6.99 (m, 2H).
(E)-2,3-Bis(3-formylphenyl)acrylic acid (2d):[6c] Yellow solid, 112 mg, 80% yield. m.p. 253~254 ℃; 1H NMR (CDCl3, 500 MHz) δ: 10.00 (s, 1H), 9.81 (s, 1H), 8.09 (s, 1H), 7.93~7.91 (m, 1H), 7.79~7.75 (m, 2H), 7.59~7.56 (m, 2H), 7.53~7.51 (m, 1H), 7.34 (t, J=8 Hz, 1H), 7.27 (s, 1H).
(E)-2,3-Di(furan-3-yl)acrylic acid (2e): White solid, 98 mg, 94% yield. m.p. 210~212 ℃; 1H NMR (CDCl3, 500 MHz) δ: 7.95 (s, 1H), 7.42~7.40 (m, 1H), 7.26~7.24 (m, 2H), 7.15~7.13 (m, 1H), 7.00 (d, J=5 Hz, 1H), 6.60 (d, J=5 Hz, 1H); 13C NMR (CDCl3, 125 MHz) δ: 172.2, 137.0, 136.6, 135.1, 130.3, 128.8, 128.1, 125.9, 125.7, 124.9, 124.6; FT-IR (neat) ν: 3102, 2938, 1688, 1590, 1517, 1424, 1402, 1385, 1219, 685 cm-1; HRMS (ESI) calcd for C11H9- O4 [M＋H＋] 205.0495, found 205.0455.
(E)-2,3-Di(thiophen-3-yl)acrylic acid (2f):[6c] Yellow solid, 109 mg, 93% yield. m.p. 229~230 ℃; 1H NMR (CDCl3, 500 MHz) δ: 7.95 (s, 1H), 7.41~7.40 (m, 1H), 7.26~7.23 (m, 2H), 7.14~7.13 (m, 1H), 7.01~7.00 (m, 1H), 6.60~6.59 (m, 1H).
(E)-2-(Naphthalen-2-yl)-3-phenylacrylic acid (2g) and (E)-3-(naphthalen-2-yl)-2-phenylacrylic acid (2g'):[6c] Whi- te solid, 119 mg, 87% yield. m.p. 183~185 ℃ (lit.[22] 222~224 ℃). For 2g: 1H NMR (CDCl3, 500 MHz) δ: 8.21 (s, 1H), 7.90 (d, J=8.5 Hz, 2H), 7.81 (d, J=8 Hz, 1H), 7.51~7.47 (m, 2H), 7.34~7.33 (m, 1H), 7.18~7.14 (m, 2H), 7.06 (t, J=8 Hz, 2H), 6.96~6.95 (m, 2H). For 2g': 1H NMR (CDCl3, 500 MHz) δ: 8.62 (s, 1H), 8.12 (d, J=8.5 Hz, 1H), 7.84 (d, J=8 Hz, 1H), 7.73 (d, J=8.5 Hz, 1H), 7.61~7.60 (m, 2H), 7.43~7.41 (m, 4H), 7.23~7.22 (m, 2H), 7.00 (d, J=7.5 Hz, 1H).
(E)-3-Phenyl-2-(m-tolyl)acrylic acid (2h) and (E)-2- phenyl-3-(m-tolyl)acrylic acid (2h'):[6c] White solid, 108 mg, 88% yield. m.p. 140~143 ℃ (lit.[20] 141~143 ℃); 1H NMR (CDCl3, 500 MHz) δ: 7.93 (s, 1H), 7.39~7.37 (m, 1H), 7.29~7.22 (m, 2H), 7.19~7.16 (m, 2H), 7.09~7.03 (m, 3H), 6.90~6.83 (m, 1H), 2.34 (s, 2H), 2.19 (s, 1H).
(E)-3-Phenyl-2-(o-tolyl)acrylic acid (2i) and (E)-2- phenyl-3-(o-tolyl)acrylic acid (2i'):[6c] White solid, 99 mg, 83% yield. m.p. 156~160 ℃ (lit.[20] 157~159 ℃); 1H NMR (CDCl3, 500 MHz) δ: 8.15~7.98 (m, 1H), 7.33~7.21 (m, 4H), 7.19~7.15 (m, 2H), 7.12~7.10 (m, 1H), 7.03 (d, J=7.5 Hz, 1H), 6.87~6.75 (m, 1H), 2.39~2.17 (m, 3H).
(E)-2-Phenylpent-2-enoic acid (2k):[6c] White solid, 84 mg, 95% yield. m.p. 156~157 ℃; 1H NMR (CDCl3, 500 MHz) δ: 7.38~7.32 (m, 3H), 7.21~7.18 (m, 3H), 2.16~2.10 (m, 2H), 1.03 (t, J=7.5 Hz, 3H).
Supporting Information Experimental procedures along with characterizing data and NMR spectra of products 2a~2m. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn/.
(Cheng, F.)