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Full Paper Received: 2 February 2012
Revised: 4 April 2012
Accepted: 4 April 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2875
Efficient salicylaldimine ligands for a palladium-catalyzed Suzuki–Miyaura cross-coupling reaction Feng-Shou Liu*, Ying-Tang Huang, Chao Lu, Dong-Sheng Shen* and Tao Cheng A series of salicylaldimine ligands were designed to promote palladium-catalyzed Suzuki–Miyaura cross-coupling reaction. After a screening process, a ligand with a bulky 2,4-di-tert-butyl substituent on the salicyaldehyde backbone and cyclohexylamine moiety was found to serve as a good combination for this reaction in aqueous solutions of DMF. The protocol demonstrated a significant advance in the efficiency of the cross-coupling of aryl bromides and aryl chlorides with arylboronic acids to produce the desired biaryl products. Copyright © 2012 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article. Keywords: salicylaldimine ligands; Suzuki–Miyaura reaction; aryl chlorides; arylboronic acids
Introduction Palladium-catalyzed cross-coupling reactions are powerful synthetic tools for carbon–carbon bond formation, which plays a very important role in the synthesis of natural products, pharmaceuticals, and functional polymer materials.[1–3] Hence the Suzuki–Miyaura cross-coupling reaction is one of the most widely used methods for the construction of biaryl compounds, owing to the stability and low toxicity of the organoboranes relative to other organometallic reagents.[4–11] Currently, looking for efficient ligands has become an important topic. Electron-rich and sterically demanding ligands for palladium-catalyzed reactions, such as phosphines[12–15] and N-heterocyclic carbenes,[16,17] have been shown to be very effective. Although remarkable advances have been achieved, some limitations still remain. For example, most of the ligands are sensitive or of high cost, limiting their use in large-scale applications. The development of highly efficient and general catalysts, which may overcome these limitations and further expand the substrate scope, is still an important area of research. The easily obtained and stable phosphine-free ligands have enabled Suzuki–Miyaura cross-coupling to be applied with a broad substrate scope, wide functional group tolerance, and low catalyst loadings.[18–34] For instance, commercial amines have been developed for the coupling reaction of aryl bromides with arylboronic acids to give the coupling products in good to high yields.[31,32] Other nitrogen ligands such as salicylaldimines[27–30] have also shown their advantage in cross-coupling reactions due to their strong s-donating abilities. The salicylaldimine ligands can be readily prepared by condensation of salicylaldehyde with amines. Moreover, the steric and electronic properties of these ligands can be easily tuned using an appropriate choice of substituents. Usually, aryl bromides are employed in these coupling protocols. However, the more economical and more readily available aryl chlorides have rarely been used as coupling partners owing to the greater difficulty in activating aryl chlorides relative to aryl bromides
Appl. Organometal. Chem. (2012)
or iodides.[7] As part of our ongoing research interest in crosscoupling using phosphine-free ligands,[35,36] we report herein the use of salicylaldimines as the ligand source (Fig. 1) and the subsequent evaluation of the reactivity of aryl bromides and aryl chlorides with arylboronic acids in the Suzuki–Miyaura cross-coupling reaction.
Results and Discussion In the initial study of the performance of salicylaldimine ligands, the Suzuki–Miyaura cross-coupling of 4-chlorobenzaldehyde and phenylboronic acid under aerobic conditions in the presence of K2CO3 was selected as a model reaction. As shown in Table 1, the data revealed that salicylaldimines were effective for this cross-coupling. For a comparison, in the absence of ligands only 20–81% gas chromatographic conversions of 4-phenylbenzaldehyde were produced in the presence of 1 mol% of PdCl2 under different solvents such as H2O, DMF, or DMF/H2O (runs 1–3, Table 1). In contrast, when adding L1, the yields of product increased to a range of 89–92% (runs 7 and 15, Table 1), indicating that the catalyst was formed in situ. As can also be seen in Table 1, a profound effect of solvent on the reaction was observed. For instance, the nonpolar solvent toluene produced a poor gas chromatographic conversion (5%), On the other hand, polar solvents such as N,N-dimethylacetamide (DMA) and N,N-dimethylformamide (DMF) provided conversions of 78% and 89%, respectively (runs 6 and 7, Table 1). However, readily available nontoxic solvents such as water did not exhibit much efficiency. This is probably the result of low solubility of substrates and the
* Correspondence to: Feng-Shou Liu, School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical University, Zhongshan, Guangdong 528458, People’s Republic of China. E-mail: [email protected], Dong-Sheng Shen, E-mail: [email protected] School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical University, Zhongshan, Guangdong, 528458, People’s Republic of China
Copyright © 2012 John Wiley & Sons, Ltd.
F.-S. Liu et al.
Figure 1. The structure of salicylaldimine ligands
stability of the palladium active center.[11] When the reaction was conducted in organic/water co-solvents (runs 9–12, Table 1), significantly improved activities were observed. For instance,
DMA/water showed a good conversion (90%). Moreover, the investigation also showed that the ratio of organic and water exerts an effect on the reactivity. As can be seen from runs 12–17, a ratio of 4 of DMF/water exhibited the highest activity. These results could be ascribed to the co-solvent system playing an important role in the solubility of the reagents (such as inorganic bases and substrates) and an easier reduction of Pd2+ to Pd(0), facilitating entry into the catalytic cycle.[37] The catalytic activity of L1–10 in the Suzuki–Miyaura crosscoupling reaction was then evaluated. In an effort to understand how the ligands would promote the coupling reaction most efficiently, different steric and electronic effects of imine moieties and salicyaldehyde backbones were examined in our laboratory, and these reactions were typically run under similar reaction conditions. Among the salicylaldimine ligands investigated, L1, with a non-substituted salicylaldehyde backbone and a phenyl group on the imine moiety, showed good efficiency and provided the coupling product in 92% conversion (run 15, Table 1). Meanwhile,
Table 1. Screening of ligands on Suzuki–Miyaura cross-coupling reaction
Run
Ligand
Pd (mol%)a
Solvent
T ( C)
Cov. (%)b
Yield (%)c
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
None None None L1 L1 L1 L1 L1 L1 L1 L1 L1 L1 L1 L1 L1 L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 L10 L10
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.5 1
H2O DMF DMF/H2O (4/1) Toluene Dioxane DMA DMF H2O Toluene/H2O (4/1) Dioxane/H2O (4/1) DMA/H2O (4/1) DMF/H2O (20/1) DMF/H2O (15/1) DMF/H2O (8/1) DMF/H2O (4/1) DMF/H2O (1/2) DMF/H2O (1/2) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) H2O
100 110 110 110 100 110 110 100 110 100 110 110 110 110 110 110 110 110 110 110 110 110 110 110 110 110 110 100
20 76 81 5 14 78 89 12 6 25 90 85 86 90 92 81 15 95 55 73 90 93 96 73 79 100 94 18
17 54 78 —d 10 72 85 7 — 20 88 82 81 87 89 77 11 91 52 70 86 89 92 68 74 96 91 14
Reaction conditions: The molar ratio of ligand and Pd is 1:1; 4-chlorobenzaldehyde (1.0 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2.0 mmol); the total volume of solvent is 5 ml. Reaction time was 4 h, under aerobic atmosphere. a The molar ratio of Ligand and Pd is 1:1. b Conversions determined by GC. c Isolated yields. d Not determined.
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Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2012)
Salicylaldimine ligands for palladium-catalyzed Suzuki–Miyaura reaction L2, with an anisole group, was found to be an even more efficient ligand and afforded the coupling product in 95% conversion (run 18, Table 1). Conversely, L3 with an electron-withdrawing nitro group at the para position of aniline, was considerably less active under the same conditions (55% conversion). These results presumably could be ascribed to the increase in electron-donating ability of the ligand, leading to an increase in the rate of oxidative addition and stabilization of the palladium species, and therefore favoring the cross-coupling reaction.[16] In addition to the electronic effect, the steric effect of the substituents on the ligand also had important effects on the catalyst properties. Generally, the bulky ligand would increase catalytic activities.[13,16] However, L4, bearing 2,6-diisopropylphenyl on the imine moiety, only provided a gas chromatographic conversion of 73%, which was much lower than L1 or L2. The lower yield obtained was probably due mainly to the steric hindrance of the groups on the imine moiety, with coordination directed to the palladium center, leading to an increase in the energy barrier for the oxidative addition step. However, introducing the tBu group at the 2,4-position of the salicylaldehyde backbone would lead to enhanced activities. For example, with the same phenyl-, 4-methoxyphenyl-, nitrosubstituents on the imine moieties, the conversion values of L6, L7 and L8 were 93%, 96% and 73% (runs 22–24, Table 1),
respectively, which was higher than that of the corresponding non-substituted L1, L2 and L3. It seems reasonable that the bulky backbone substituents can play an important role in improving catalytic behavior. The backbone substituents are remote from the palladium center; thus the steric hindrance on the ligand could facilitate reductive elimination and improve the cross-coupling reaction.[13] Among the ligands evaluated, L10, which bears a cyclohexyl group on the imine moiety and a bulky backbone, turned out to be the most efficient and exhibited nearly quantitative conversion (run 26, Table 1). Even at a catalyst loading of 0.5 mol% and a reaction temperature of 90 C, the conversion to the product reached 94% (run 27, Table 1). It is important to note that previous research on salicylaldimine ligands was exclusively focused on the imine moiety, and less information has been obtained from the change in the salicyaldehyde backbone. Comparatively, this study strongly suggests that the bulky backbones shielding of the metal center can greatly benefit from enhancement of the catalytic activities, and shows the superiority to the reported salicylaldimine ligands. Using the available optimal conditions, the Suzuki–Miyaura cross-coupling reaction of aryl bromide substrates was tested with several representative arylboronic acids. As shown in Table 2, using L10 as the ligand, K2CO3 as a base, DMF/water as the
Table 2. Suzuki–Miyaura cross-coupling reaction of aryl bromides with arylboronic acid under aerobic conditions
Run
ArBr
ArB(OH)2
Product
T ( C)
t (h)
Cov. (%)a
Yield (%)b
1 2 3c 4 5 6 7 8 9c 10 11 12 13c 14 15d 16 17 18 19 20 21 22 23
4-O2NPhBr 4-ClPhBr 4-ClPhBr 4-OHCPhBr 4-CH3OCPhBr PhBr 4-CH3PhBr 4-CH3PhBr 4-CH3PhBr NaphtylBr 4-tBuPhBr 4-CH3OPhBr 4-CH3OPhBr 2-CH3PhBr 2-NH2PhBr 4-OHCPhBr 4-CH3OCPhBr 4-tBuPhBr 4-CH3OPhBr 4-CH3PhBr 4-OHCPhBr 4-CH3OCPhBr 4-CH3PhBr
PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 2-CH3PhB(OH)2 2-CH3PhB(OH)2 2-CH3PhB(OH)2 2-CH3PhB(OH)2 2-CH3PhB(OH)2 4-ClPhB(OH)2 4-ClPhB(OH)2 4-ClPhB(OH)2
4-O2NPh-Ph 4-ClPh-Ph 4-ClPh-Ph 4-OHCPh-Ph 4-CH3OCPh-Ph Ph-Ph 4-CH3Ph-Ph 4-CH3Ph-Ph 4-CH3Ph-Ph Naphtyl-Ph 4-tBuPh-Ph 4-CH3OPh-Ph 4-CH3OPh-Ph 2-CH3Ph-Ph 2-NH2Ph-Ph 4-OHCPh-20 -CH3Ph 4-CH3OCPh-20 -CH3Ph 4-tBuPh-20 -CH3Ph 40 -CH3OPh-2-CH3Ph 40 -CH3Ph-2-CH3Ph 4-OHCPh-40 -ClPh 4-CH3OCPh-40 -ClPh 4-CH3Ph-40 -ClPh
RT RT RT RT RT RT RT 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60
0.1 2 2 2 2 2 2 2 2 2 4 4 4 2 4 6 6 6 6 6 6 6 6
98 94 87 95 97 96 42 98 73 95 95 96 83 92 98 97 96 88 92 93 98 96 97
95 90 83 89 92 93 37 95 69 88 93 94 80 86 94 87 85 83 84 87 95 94 93
Reaction conditions: PdCl2 (0.5% mmol); the molar ratio of ligand and Pd is 1:1; aryl bromide (1.0 mmol), arylboronic acid (1.2 mmol), K2CO3 (2.0 mmol), DMF: H2O, 4:1 ml, under aerobic atmosphere. a GC conversions. b Isolated yields. c Water used as solvent. d Under nitrogen atmosphere.
Appl. Organometal. Chem. (2012)
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F.-S. Liu et al. co-solvent, and with 0.5 mol% of palladium loading, high catalytic efficiency was observed in the coupling of activated bromides with phenylboronic acid at room temperature (runs 1–6, Table 2). Conversely, increasing the electron density on aryl bromides lowered the activity at room temperature. For instance, 4-methylphenyl bromide gives a yield of only 42% at room temperature for 2 h (run 7, Table 2). In contrast, an excellent conversion (98%) was found at an elevated reaction temperature of 60 C after 2 h (run 8, Table 2). Additionally, results were also observed for 1-naphthyl bromide, 4-tertbutyl bromide, and 4-bromoanisole (runs 10–12, Table 2). Substrates with ortho-methyl and ortho-amino substituents, which slow down the oxidative addition and transmetallation processes, have proven to be less active in other catalytic systems.[22,38] However, the present optimal conditions could achieve high yields using these prescribed substrates (runs 14 and 15, Table 2). Moreover, the steric (runs 16–20, Table 2) and electron-withdrawing (runs 21–23, Table 2) substituents of arylboronic acids did not have a substantial effect on the overall performance of the reaction. With a slightly extended reaction time of 6 h, the arylboronic acids and aryl bromides underwent the coupling reaction to obtain high product yields. Use of the Suzuki–Miyaura cross-coupling reaction to synthesize biaryls from aryl chlorides has been received with concern for its readily availability and low cost. While the aryliminebased salicylaldimine palladium complexes displayed relatively low activity towards aryl chlorides,[27–30] we found that L10/PdCl2 was an effective mediator of this coupling reaction. For example, activated aryl chlorides, such as 1-chloro-4nitrobenzene, 4-chlorobenzonitrile and 4-chloroacetophenone, coupled with phenylboronic acid in high yield at a reaction temperature of 90 C and 0.5 mol% palladium loading (runs 1–3, Table 3). As expected, electron-rich substrates such as 4-chlorotoluene and 4-chloroanisole are not electronically activated and led to a lower activity. However, an elevated temperature of 110 C and 1 mol% palladium loading (runs 5 and 6, Table 3) provided moderate yields with these substrates. It is noteworthy that the cross-coupling of 2-nitrochlorobenzene and 4-chlorophenylboronic acids proceeded at a conversion as
high as 88% to provide an important intermediate of the synthesis of Boscalid, which is an active ingredient of preventive fungicide (run 9, Table 3). Additionally, 2-chlorobenzonitrile and 4-methylphenylboronic acids were coupled to provide the key intermediate of the Sartan family in 87% conversion (run 10, Table 3).
Conclusions In summary, a systematic investigation on the effects of steric and electronic substituted salicylaldimine ligands and reaction conditions were screened in the Suzuki–Miyaura cross-coupling reaction. We speculate that the catalytic system with an electrondonating imine moiety, and a bulky substituted backbone on the salicylaldehyde, benefits from enhancing the catalytic activity. The results provide a significant improvement in the efficiency of the cross-coupling of aryl bromides and aryl chlorides with arylboronic acids to produce the desired biaryls.
Experimental Instruments and Reagents All the chemical reagents (AR grade) were from commercial resources and used without any further purification. Gas chromatographic analysis was performed on an Agilent GC-6820 chromatograph with a 30 m (column height) 0.32 mm (column diameter) 0.5 mm (column coating thickness) OV-17 capillary column with a flame ionization detector. NMR spectra were recorded on a Bruker 300 MHz spectrometer using CDCl3 as the solvent with tetramethylsilane (TMS) as an internal standard. All the salicylaldimine ligands used in the present work were easily prepared by the condensation of salicyaldehydes with amines according to previous reports.[39–45] L1[39] L2,[40] L3,[41] L4,[39] L5,[42] L6,[43] L7,[44] L8,[45] L9,[43] and L10[43] were obtained with yields of 87%, 92%, 97%, 84%, 72%, 92%, 87%, 93%, 75%, and 91%, respectively.
Table 3. Suzuki–Miyaura cross-coupling reaction of aryl chlorides with arylboronic acid under aerobic conditions
Run 1 2 3 4 5 6 7 8 9 10
ArCl
ArB(OH)2
Product
Pd (mol%)
T ( C)
Cov. (%)a
Yield (%)b
4-O2NPhCl 4-NCPhCl 4-CH3OCPhCl 4-CH3OCPhCl 4-CH3PhCl 4-CH3OPhCl 2-O2NPhCl 2-O2NPhCl 2-O2NPhCl 2-NCPhCl
PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 4-ClPhB(OH)2 4-CH3PhB(OH)2
4-O2NPh-Ph 4-NCPh-Ph 4-CH3OCPh-Ph 4-CH3OCPh-Ph 4-CH3Ph-Ph 4-CH3OPh-Ph 2-O2NPh-Ph 2-O2NPh-Ph 2-O2NPh-40 ClPh 2-NCPh-40 CH3Ph
0.5 0.5 0.5 0.5 1 1 0.5 1 1 1
90 90 90 110 110 110 110 110 110 110
99 95 78 99 73 43 87 99 88 87
96 92 73 94 68 36 83 95 83 82
Reaction conditions: The molar ratio of Ligand and Pd is 1:1; aryl chloride (1.0 mmol), arylboronic acid (1.2 mmol), K2CO3 (2.0 mmol), DMF: H2O = 4:1 ml, reaction time = 4 h, under aerobic atmosphere. a GC conversions. b Isolated yields.
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Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2012)
Salicylaldimine ligands for palladium-catalyzed Suzuki–Miyaura reaction General Procedure for Suzuki–Miyaura Reaction To a round bottle with a magnetic stir bar, ligand (1% mmol), PdCl2 (1% mmol), aryl halides (1 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2 mmol) and 5 ml of solvent were added. The reaction mixture was heated to the described temperature for the required time, and then the solvent was removed under reduced pressure. The residual was diluted with Et2O (5 ml), followed by extraction twice (2 5 ml) with Et2O. The organic layer was dried with anhydrous MgSO4, filtered and evaporated under vacuum. The conversions rates were analyzed by gas chromatography, based on the peak area normalization method. The corrected factor was determined by samples against a standard of n-heptane. The crude products were purified by silica-gel column chromatography using petroleum ether–ethyl acetate (20:1) as an eluent, and the isolated yield was then calculated based on the feeding of the aryl halide. The isolated corresponding products were characterized by 1H NMR and 13C NMR.
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
Acknowledgments The National Natural Science Foundation of China (No. 21004014), Foundation for Distinguished Young Talents in Higher Education of Guangdong, China (No. LYM10091), and Key Laboratory of Designed Synthesis and Application of Polymer Material, are gratefully acknowledged for financial support.
[32] [33] [34] [35] [36]
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Copyright © 2012 John Wiley & Sons, Ltd.
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Revised: 4 April 2012
Accepted: 4 April 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2875
Efficient salicylaldimine ligands for a palladium-catalyzed Suzuki–Miyaura cross-coupling reaction Feng-Shou Liu*, Ying-Tang Huang, Chao Lu, Dong-Sheng Shen* and Tao Cheng A series of salicylaldimine ligands were designed to promote palladium-catalyzed Suzuki–Miyaura cross-coupling reaction. After a screening process, a ligand with a bulky 2,4-di-tert-butyl substituent on the salicyaldehyde backbone and cyclohexylamine moiety was found to serve as a good combination for this reaction in aqueous solutions of DMF. The protocol demonstrated a significant advance in the efficiency of the cross-coupling of aryl bromides and aryl chlorides with arylboronic acids to produce the desired biaryl products. Copyright © 2012 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article. Keywords: salicylaldimine ligands; Suzuki–Miyaura reaction; aryl chlorides; arylboronic acids
Introduction Palladium-catalyzed cross-coupling reactions are powerful synthetic tools for carbon–carbon bond formation, which plays a very important role in the synthesis of natural products, pharmaceuticals, and functional polymer materials.[1–3] Hence the Suzuki–Miyaura cross-coupling reaction is one of the most widely used methods for the construction of biaryl compounds, owing to the stability and low toxicity of the organoboranes relative to other organometallic reagents.[4–11] Currently, looking for efficient ligands has become an important topic. Electron-rich and sterically demanding ligands for palladium-catalyzed reactions, such as phosphines[12–15] and N-heterocyclic carbenes,[16,17] have been shown to be very effective. Although remarkable advances have been achieved, some limitations still remain. For example, most of the ligands are sensitive or of high cost, limiting their use in large-scale applications. The development of highly efficient and general catalysts, which may overcome these limitations and further expand the substrate scope, is still an important area of research. The easily obtained and stable phosphine-free ligands have enabled Suzuki–Miyaura cross-coupling to be applied with a broad substrate scope, wide functional group tolerance, and low catalyst loadings.[18–34] For instance, commercial amines have been developed for the coupling reaction of aryl bromides with arylboronic acids to give the coupling products in good to high yields.[31,32] Other nitrogen ligands such as salicylaldimines[27–30] have also shown their advantage in cross-coupling reactions due to their strong s-donating abilities. The salicylaldimine ligands can be readily prepared by condensation of salicylaldehyde with amines. Moreover, the steric and electronic properties of these ligands can be easily tuned using an appropriate choice of substituents. Usually, aryl bromides are employed in these coupling protocols. However, the more economical and more readily available aryl chlorides have rarely been used as coupling partners owing to the greater difficulty in activating aryl chlorides relative to aryl bromides
Appl. Organometal. Chem. (2012)
or iodides.[7] As part of our ongoing research interest in crosscoupling using phosphine-free ligands,[35,36] we report herein the use of salicylaldimines as the ligand source (Fig. 1) and the subsequent evaluation of the reactivity of aryl bromides and aryl chlorides with arylboronic acids in the Suzuki–Miyaura cross-coupling reaction.
Results and Discussion In the initial study of the performance of salicylaldimine ligands, the Suzuki–Miyaura cross-coupling of 4-chlorobenzaldehyde and phenylboronic acid under aerobic conditions in the presence of K2CO3 was selected as a model reaction. As shown in Table 1, the data revealed that salicylaldimines were effective for this cross-coupling. For a comparison, in the absence of ligands only 20–81% gas chromatographic conversions of 4-phenylbenzaldehyde were produced in the presence of 1 mol% of PdCl2 under different solvents such as H2O, DMF, or DMF/H2O (runs 1–3, Table 1). In contrast, when adding L1, the yields of product increased to a range of 89–92% (runs 7 and 15, Table 1), indicating that the catalyst was formed in situ. As can also be seen in Table 1, a profound effect of solvent on the reaction was observed. For instance, the nonpolar solvent toluene produced a poor gas chromatographic conversion (5%), On the other hand, polar solvents such as N,N-dimethylacetamide (DMA) and N,N-dimethylformamide (DMF) provided conversions of 78% and 89%, respectively (runs 6 and 7, Table 1). However, readily available nontoxic solvents such as water did not exhibit much efficiency. This is probably the result of low solubility of substrates and the
* Correspondence to: Feng-Shou Liu, School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical University, Zhongshan, Guangdong 528458, People’s Republic of China. E-mail: [email protected], Dong-Sheng Shen, E-mail: [email protected] School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical University, Zhongshan, Guangdong, 528458, People’s Republic of China
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F.-S. Liu et al.
Figure 1. The structure of salicylaldimine ligands
stability of the palladium active center.[11] When the reaction was conducted in organic/water co-solvents (runs 9–12, Table 1), significantly improved activities were observed. For instance,
DMA/water showed a good conversion (90%). Moreover, the investigation also showed that the ratio of organic and water exerts an effect on the reactivity. As can be seen from runs 12–17, a ratio of 4 of DMF/water exhibited the highest activity. These results could be ascribed to the co-solvent system playing an important role in the solubility of the reagents (such as inorganic bases and substrates) and an easier reduction of Pd2+ to Pd(0), facilitating entry into the catalytic cycle.[37] The catalytic activity of L1–10 in the Suzuki–Miyaura crosscoupling reaction was then evaluated. In an effort to understand how the ligands would promote the coupling reaction most efficiently, different steric and electronic effects of imine moieties and salicyaldehyde backbones were examined in our laboratory, and these reactions were typically run under similar reaction conditions. Among the salicylaldimine ligands investigated, L1, with a non-substituted salicylaldehyde backbone and a phenyl group on the imine moiety, showed good efficiency and provided the coupling product in 92% conversion (run 15, Table 1). Meanwhile,
Table 1. Screening of ligands on Suzuki–Miyaura cross-coupling reaction
Run
Ligand
Pd (mol%)a
Solvent
T ( C)
Cov. (%)b
Yield (%)c
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
None None None L1 L1 L1 L1 L1 L1 L1 L1 L1 L1 L1 L1 L1 L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 L10 L10
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.5 1
H2O DMF DMF/H2O (4/1) Toluene Dioxane DMA DMF H2O Toluene/H2O (4/1) Dioxane/H2O (4/1) DMA/H2O (4/1) DMF/H2O (20/1) DMF/H2O (15/1) DMF/H2O (8/1) DMF/H2O (4/1) DMF/H2O (1/2) DMF/H2O (1/2) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) DMF/H2O (4/1) H2O
100 110 110 110 100 110 110 100 110 100 110 110 110 110 110 110 110 110 110 110 110 110 110 110 110 110 110 100
20 76 81 5 14 78 89 12 6 25 90 85 86 90 92 81 15 95 55 73 90 93 96 73 79 100 94 18
17 54 78 —d 10 72 85 7 — 20 88 82 81 87 89 77 11 91 52 70 86 89 92 68 74 96 91 14
Reaction conditions: The molar ratio of ligand and Pd is 1:1; 4-chlorobenzaldehyde (1.0 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2.0 mmol); the total volume of solvent is 5 ml. Reaction time was 4 h, under aerobic atmosphere. a The molar ratio of Ligand and Pd is 1:1. b Conversions determined by GC. c Isolated yields. d Not determined.
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Salicylaldimine ligands for palladium-catalyzed Suzuki–Miyaura reaction L2, with an anisole group, was found to be an even more efficient ligand and afforded the coupling product in 95% conversion (run 18, Table 1). Conversely, L3 with an electron-withdrawing nitro group at the para position of aniline, was considerably less active under the same conditions (55% conversion). These results presumably could be ascribed to the increase in electron-donating ability of the ligand, leading to an increase in the rate of oxidative addition and stabilization of the palladium species, and therefore favoring the cross-coupling reaction.[16] In addition to the electronic effect, the steric effect of the substituents on the ligand also had important effects on the catalyst properties. Generally, the bulky ligand would increase catalytic activities.[13,16] However, L4, bearing 2,6-diisopropylphenyl on the imine moiety, only provided a gas chromatographic conversion of 73%, which was much lower than L1 or L2. The lower yield obtained was probably due mainly to the steric hindrance of the groups on the imine moiety, with coordination directed to the palladium center, leading to an increase in the energy barrier for the oxidative addition step. However, introducing the tBu group at the 2,4-position of the salicylaldehyde backbone would lead to enhanced activities. For example, with the same phenyl-, 4-methoxyphenyl-, nitrosubstituents on the imine moieties, the conversion values of L6, L7 and L8 were 93%, 96% and 73% (runs 22–24, Table 1),
respectively, which was higher than that of the corresponding non-substituted L1, L2 and L3. It seems reasonable that the bulky backbone substituents can play an important role in improving catalytic behavior. The backbone substituents are remote from the palladium center; thus the steric hindrance on the ligand could facilitate reductive elimination and improve the cross-coupling reaction.[13] Among the ligands evaluated, L10, which bears a cyclohexyl group on the imine moiety and a bulky backbone, turned out to be the most efficient and exhibited nearly quantitative conversion (run 26, Table 1). Even at a catalyst loading of 0.5 mol% and a reaction temperature of 90 C, the conversion to the product reached 94% (run 27, Table 1). It is important to note that previous research on salicylaldimine ligands was exclusively focused on the imine moiety, and less information has been obtained from the change in the salicyaldehyde backbone. Comparatively, this study strongly suggests that the bulky backbones shielding of the metal center can greatly benefit from enhancement of the catalytic activities, and shows the superiority to the reported salicylaldimine ligands. Using the available optimal conditions, the Suzuki–Miyaura cross-coupling reaction of aryl bromide substrates was tested with several representative arylboronic acids. As shown in Table 2, using L10 as the ligand, K2CO3 as a base, DMF/water as the
Table 2. Suzuki–Miyaura cross-coupling reaction of aryl bromides with arylboronic acid under aerobic conditions
Run
ArBr
ArB(OH)2
Product
T ( C)
t (h)
Cov. (%)a
Yield (%)b
1 2 3c 4 5 6 7 8 9c 10 11 12 13c 14 15d 16 17 18 19 20 21 22 23
4-O2NPhBr 4-ClPhBr 4-ClPhBr 4-OHCPhBr 4-CH3OCPhBr PhBr 4-CH3PhBr 4-CH3PhBr 4-CH3PhBr NaphtylBr 4-tBuPhBr 4-CH3OPhBr 4-CH3OPhBr 2-CH3PhBr 2-NH2PhBr 4-OHCPhBr 4-CH3OCPhBr 4-tBuPhBr 4-CH3OPhBr 4-CH3PhBr 4-OHCPhBr 4-CH3OCPhBr 4-CH3PhBr
PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 2-CH3PhB(OH)2 2-CH3PhB(OH)2 2-CH3PhB(OH)2 2-CH3PhB(OH)2 2-CH3PhB(OH)2 4-ClPhB(OH)2 4-ClPhB(OH)2 4-ClPhB(OH)2
4-O2NPh-Ph 4-ClPh-Ph 4-ClPh-Ph 4-OHCPh-Ph 4-CH3OCPh-Ph Ph-Ph 4-CH3Ph-Ph 4-CH3Ph-Ph 4-CH3Ph-Ph Naphtyl-Ph 4-tBuPh-Ph 4-CH3OPh-Ph 4-CH3OPh-Ph 2-CH3Ph-Ph 2-NH2Ph-Ph 4-OHCPh-20 -CH3Ph 4-CH3OCPh-20 -CH3Ph 4-tBuPh-20 -CH3Ph 40 -CH3OPh-2-CH3Ph 40 -CH3Ph-2-CH3Ph 4-OHCPh-40 -ClPh 4-CH3OCPh-40 -ClPh 4-CH3Ph-40 -ClPh
RT RT RT RT RT RT RT 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60
0.1 2 2 2 2 2 2 2 2 2 4 4 4 2 4 6 6 6 6 6 6 6 6
98 94 87 95 97 96 42 98 73 95 95 96 83 92 98 97 96 88 92 93 98 96 97
95 90 83 89 92 93 37 95 69 88 93 94 80 86 94 87 85 83 84 87 95 94 93
Reaction conditions: PdCl2 (0.5% mmol); the molar ratio of ligand and Pd is 1:1; aryl bromide (1.0 mmol), arylboronic acid (1.2 mmol), K2CO3 (2.0 mmol), DMF: H2O, 4:1 ml, under aerobic atmosphere. a GC conversions. b Isolated yields. c Water used as solvent. d Under nitrogen atmosphere.
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F.-S. Liu et al. co-solvent, and with 0.5 mol% of palladium loading, high catalytic efficiency was observed in the coupling of activated bromides with phenylboronic acid at room temperature (runs 1–6, Table 2). Conversely, increasing the electron density on aryl bromides lowered the activity at room temperature. For instance, 4-methylphenyl bromide gives a yield of only 42% at room temperature for 2 h (run 7, Table 2). In contrast, an excellent conversion (98%) was found at an elevated reaction temperature of 60 C after 2 h (run 8, Table 2). Additionally, results were also observed for 1-naphthyl bromide, 4-tertbutyl bromide, and 4-bromoanisole (runs 10–12, Table 2). Substrates with ortho-methyl and ortho-amino substituents, which slow down the oxidative addition and transmetallation processes, have proven to be less active in other catalytic systems.[22,38] However, the present optimal conditions could achieve high yields using these prescribed substrates (runs 14 and 15, Table 2). Moreover, the steric (runs 16–20, Table 2) and electron-withdrawing (runs 21–23, Table 2) substituents of arylboronic acids did not have a substantial effect on the overall performance of the reaction. With a slightly extended reaction time of 6 h, the arylboronic acids and aryl bromides underwent the coupling reaction to obtain high product yields. Use of the Suzuki–Miyaura cross-coupling reaction to synthesize biaryls from aryl chlorides has been received with concern for its readily availability and low cost. While the aryliminebased salicylaldimine palladium complexes displayed relatively low activity towards aryl chlorides,[27–30] we found that L10/PdCl2 was an effective mediator of this coupling reaction. For example, activated aryl chlorides, such as 1-chloro-4nitrobenzene, 4-chlorobenzonitrile and 4-chloroacetophenone, coupled with phenylboronic acid in high yield at a reaction temperature of 90 C and 0.5 mol% palladium loading (runs 1–3, Table 3). As expected, electron-rich substrates such as 4-chlorotoluene and 4-chloroanisole are not electronically activated and led to a lower activity. However, an elevated temperature of 110 C and 1 mol% palladium loading (runs 5 and 6, Table 3) provided moderate yields with these substrates. It is noteworthy that the cross-coupling of 2-nitrochlorobenzene and 4-chlorophenylboronic acids proceeded at a conversion as
high as 88% to provide an important intermediate of the synthesis of Boscalid, which is an active ingredient of preventive fungicide (run 9, Table 3). Additionally, 2-chlorobenzonitrile and 4-methylphenylboronic acids were coupled to provide the key intermediate of the Sartan family in 87% conversion (run 10, Table 3).
Conclusions In summary, a systematic investigation on the effects of steric and electronic substituted salicylaldimine ligands and reaction conditions were screened in the Suzuki–Miyaura cross-coupling reaction. We speculate that the catalytic system with an electrondonating imine moiety, and a bulky substituted backbone on the salicylaldehyde, benefits from enhancing the catalytic activity. The results provide a significant improvement in the efficiency of the cross-coupling of aryl bromides and aryl chlorides with arylboronic acids to produce the desired biaryls.
Experimental Instruments and Reagents All the chemical reagents (AR grade) were from commercial resources and used without any further purification. Gas chromatographic analysis was performed on an Agilent GC-6820 chromatograph with a 30 m (column height) 0.32 mm (column diameter) 0.5 mm (column coating thickness) OV-17 capillary column with a flame ionization detector. NMR spectra were recorded on a Bruker 300 MHz spectrometer using CDCl3 as the solvent with tetramethylsilane (TMS) as an internal standard. All the salicylaldimine ligands used in the present work were easily prepared by the condensation of salicyaldehydes with amines according to previous reports.[39–45] L1[39] L2,[40] L3,[41] L4,[39] L5,[42] L6,[43] L7,[44] L8,[45] L9,[43] and L10[43] were obtained with yields of 87%, 92%, 97%, 84%, 72%, 92%, 87%, 93%, 75%, and 91%, respectively.
Table 3. Suzuki–Miyaura cross-coupling reaction of aryl chlorides with arylboronic acid under aerobic conditions
Run 1 2 3 4 5 6 7 8 9 10
ArCl
ArB(OH)2
Product
Pd (mol%)
T ( C)
Cov. (%)a
Yield (%)b
4-O2NPhCl 4-NCPhCl 4-CH3OCPhCl 4-CH3OCPhCl 4-CH3PhCl 4-CH3OPhCl 2-O2NPhCl 2-O2NPhCl 2-O2NPhCl 2-NCPhCl
PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 PhB(OH)2 4-ClPhB(OH)2 4-CH3PhB(OH)2
4-O2NPh-Ph 4-NCPh-Ph 4-CH3OCPh-Ph 4-CH3OCPh-Ph 4-CH3Ph-Ph 4-CH3OPh-Ph 2-O2NPh-Ph 2-O2NPh-Ph 2-O2NPh-40 ClPh 2-NCPh-40 CH3Ph
0.5 0.5 0.5 0.5 1 1 0.5 1 1 1
90 90 90 110 110 110 110 110 110 110
99 95 78 99 73 43 87 99 88 87
96 92 73 94 68 36 83 95 83 82
Reaction conditions: The molar ratio of Ligand and Pd is 1:1; aryl chloride (1.0 mmol), arylboronic acid (1.2 mmol), K2CO3 (2.0 mmol), DMF: H2O = 4:1 ml, reaction time = 4 h, under aerobic atmosphere. a GC conversions. b Isolated yields.
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Salicylaldimine ligands for palladium-catalyzed Suzuki–Miyaura reaction General Procedure for Suzuki–Miyaura Reaction To a round bottle with a magnetic stir bar, ligand (1% mmol), PdCl2 (1% mmol), aryl halides (1 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2 mmol) and 5 ml of solvent were added. The reaction mixture was heated to the described temperature for the required time, and then the solvent was removed under reduced pressure. The residual was diluted with Et2O (5 ml), followed by extraction twice (2 5 ml) with Et2O. The organic layer was dried with anhydrous MgSO4, filtered and evaporated under vacuum. The conversions rates were analyzed by gas chromatography, based on the peak area normalization method. The corrected factor was determined by samples against a standard of n-heptane. The crude products were purified by silica-gel column chromatography using petroleum ether–ethyl acetate (20:1) as an eluent, and the isolated yield was then calculated based on the feeding of the aryl halide. The isolated corresponding products were characterized by 1H NMR and 13C NMR.
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
Acknowledgments The National Natural Science Foundation of China (No. 21004014), Foundation for Distinguished Young Talents in Higher Education of Guangdong, China (No. LYM10091), and Key Laboratory of Designed Synthesis and Application of Polymer Material, are gratefully acknowledged for financial support.
[32] [33] [34] [35] [36]
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