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Ultrasonic irradiation accelerated cyclopalladated ferrocenylimines catalyzed Suzuki reaction in neat water
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Ultrasonics Sonochemistry 15 (2008) 115–118 www.elsevier.com/locate/ultsonch
Ultrasonic irradiation accelerated cyclopalladated ferrocenylimines catalyzed Suzuki reaction in neat water Jinli Zhang, Fan Yang, Gerui Ren, Thomas C.W. Mak 1, Maoping Song, Yangjie Wu
*
Department of Chemistry, Henan Key Laboratory of Chemical Biology and Organic Chemistry, Key Laboratory of Applied Chemistry of Henan Universities, Zhengzhou University, Zhengzhou 450052, PR China Received 3 November 2006; received in revised form 1 February 2007; accepted 15 February 2007 Available online 24 February 2007
Abstract Both conventional heating and ultrasound effect on the cyclopalladated ferrocenylimines catalyzed Suzuki reaction of phenylboronic acid with a range of arylhalides in neat water was investigated. Heterogenous reaction of electron-withdrawing arylchlorides with phenylboronic acid could also result in good yields by using Cat. 2. It was found that the ultrasonic irradiation could dramatically accelerate the Suzuki reaction to achieve comparable results. 2007 Elsevier B.V. All rights reserved. Keywords: Cyclopalladated ferrocenylimines; Suzuki reaction; Neat water; Ultrasonic irradiation
1. Introduction Palladium-catalyzed Suzuki cross-coupling reaction is a powerful, versatile and popular tool for selective construction of carbon–carbon bonds [1], in particular for the formation of biaryls [2] due to its importance as components of pharmaceuticals, herbicides, and natural products, as well as in the fields of engineering materials, such as conducting polymers, molecular wires, liquid crystals, and synthesis of ligands [3]. In recent years, various modifications involving the catalysts, solvents, bases, reaction conditions and synthetic technique have been developed [3a]. From the industrial point of view, some important goals need to be realized for the development of this process: the functionalization of inexpensive and readily accessible aryl chlorides [4] and the air-stable, efficient, and nontoxic cat-
* Corresponding author. Present address: Department of Chemistry, Zhengzhou University, Zhengzhou 450052, PR China. Tel.: +86 371 67767993; fax: +86 371 67979408. E-mail address: [email protected] (Y. Wu). 1 Present address: Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, PR China.
1350-4177/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2007.02.002
alysts. Furthermore, the use of water as a potential replacement for organic solvents are desired due to its low cost, nonflammability, nontoxicity, and the fact that it is a renewable resource [5]. Since Casalnuovo’s [6] initial report of TPPMS/ Pd(OAc)2 catalyzed cross-coupling reactions in aqueous solvents, there has been a great deal of effort devoted to the development of water soluble analogues of phosphines, particularly derivatives of triphenylphosphine [7,8]. Palladacycles have also attracted much attention for hosting the cross-coupling reactions of carbon–carbon and carbon–heteroatom bond formation owning to their availability, facile modification, insensitivity to air or moisture and easy handling as compared with the most other catalysts [9]. Subsequently, oxime-derived palladacycles [10], di-2pyridylmethylamine-based palladium complexes [11], and palladium N-heterocyclic carbene complexes [12], hydrophilic palladacycles in combination with t-Bu-amphos [13], and recent ligand free Pd-catalysts PdCl2 [14] and Pd(OAc)2 were employed to the aqueous Suzuki cross-coupling. However, most of them were limited to using aryl iodides and bromides [15]. Only scattered attentions have been focused on the coupling of arylchlorides in aqueous Suzuki reaction [16].
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J. Zhang et al. / Ultrasonics Sonochemistry 15 (2008) 115–118
The use of ultrasonic irradiation as a tool for synthetic chemistry is a fast growth area. Since the first reports of ultrasonic irradiation-assisted coupling reaction [17], this technique has been accepted as a method for reducing reaction times and increasing yields. Recently, Toma Stefan [18] also reported the ultrasound effect on Suzuki reaction catalyzed by 5% Pd/C and KF as base in methanol/water mixture. A few years ago, we found that cyclopalladated ferrocenylimine [19] was a kind of phosphine-free compounds and almost behaves all the properties of palladacycles catalysts. The synthetic convenience, facile modification and air, moisture stability made them easily to be handled. Subsequently, we discovered their wide applications for carbon–carbon bond formation in organic solvent [20]. Herein, we reported that the sonication of the reaction mixture could enhance the Suzuki cross-coupling reaction of the air stable, ligand-free cyclopalladated ferrocenylimines-catalyzed Suzuki reaction in neat water. The arylhalides including electron-withdrawing arylchlorides could be coupled with phenylboronic acid in moderate to good yields under both sonochemical and conventional heating. 2. Experimental 2.1. General Melting points were measured on a WC-1 microscopic apparatus and uncorrected. 1H NMR was recorded on a Bruker DPX 400 instrument using CDCl3 as the solvent and tetramethylsilane as the internal standard. Preparative TLC was performed on dry silica gel plates developed with dichloromethane/petroleum. Cyclopalladated ferrocenylimines were synthesized according to the literature procedures [19]. The arylhalides were obtained from commercial sources and were generally used without further purification. Phenylboronic acid was synthesized according to the literature [21]. 2.2. General procedure for Suzuki reaction in neat water using conventional heating A 25 mL round-bottom flask was charged with arylhalides (2.0 mmol), phenylboronic acid (3.0 mmol, 0.366 g), potassium phosphate (4.0 mmol, 0.849 g), TBAB (2.0 mmol, 0.645 g), catalyst, and 13 mL of H2O. The mixture was stirred at reflux temperature in air and was monitored by TLC. After the completion of the reaction, the mixture was extracted with CH2Cl2 (3 · 25 mL). The combined organic phases were dried over anhydrous Na2SO4, filtered, and the solvent was removed on a rotary evaporator. The residue was analyzed by GC (dissolved in CH2Cl2, undecane as internal standard) or purified by preparative TLC and the yield was calculated based on ArX. (the purified products were identified by comparison of melting points with the literature values [21] or by 1H NMR spectra).
2.3. General procedure for Suzuki reaction in neat water by ultrasonic wave A 25 mL round-bottom flask was charged with arylhalides (2.0 mmol), phenylboronic acid (3.0 mmol, 0.366 g), potassium phosphate (4.0 mmol, 0.849 g), TBAB (2.0 mmol, 0.645 g), catalyst, and 13 mL of H2O. The mixture was stirred at 70 C in air in a KQ-100B ultrasonic cleaner (40 KHz) and was monitored by TLC. After the completion of the reaction, the mixture was extracted with CH2Cl2 (3 · 25 mL). The combined organic phases were dried over anhydrous Na2SO4, filtered, and the solvent was removed on a rotary evaporator. The residue was analyzed by GC (dissolved in CH2Cl2, undecane as internal standard) or purified by preparative TLC and the yield was calculated based on ArX. (the purified products were identified by comparison of melting points with the literature values [21] or by 1H NMR spectra). Biphenyl [22] white plates, m.p. 70 C (lit. 70–71 C). 2-Methylbiphenyl [23] colorless oil. 1H NMR (CDCl3): d 2.27 (s, 3H, CH3), 7.25 (m, 4H, Ar-H), 7.33 (m, 3H, ArH), 7.40 (m, 2H, Ar-H). 4-Methoxybiphenyl [22] white solid. m.p. 88–90 C (lit. 91–92 C). 4-Acetylbiphenyl [22] white solids, m.p. 117 C (lit. 116– 118 C). 3-Nitrobiphenyl [22] yellow needles, m.p. 61 C (lit. 62 C). 4-Nitrobiphenyl [22] yellow needles, m.p. 113–114 C (lit. 112–114 C). 2,4-Dinitrobiphenyl [22] yellow plates, m.p. 110 C (lit. 110 C).
3. Results and discussion Previous reports have shown that the addition of phasetransfer catalysts, such as tetraalkylammonium salts, can greatly improve yields of Suzuki reactions in both water [24b,16a] and polar organic solvents [15c,25]. The role of ammonium salts is thought to be twofold. First, they facilitate dissolution of the organic substrates in the solvent medium. Second, they are supposed to enhance the rate of the coupling reaction by activating the boronic acid to form [ArB(OH)3] [R4N]+ [24b]. Based on our previous results in organic solvent [20], the first Suzuki coupling of phenylboronic acid with iodobenzene was conducted using K3PO4 as base at 0.5% catalyst loading of Cat. 1 with TBAB at room temperature for optimizing reaction conditions. Unfortunately, due to the poor solubility of catalyst and the substrates, the product was not observed when monitored by the TLC. Ramping the temperature from r.t. to reflux temperature, the isolated yields reached to 100% (Entry 1). Sonochemical reaction afforded 93% yield of the product when 1 mol% of catalyst was used at 70 C (Entry 2).
J. Zhang et al. / Ultrasonics Sonochemistry 15 (2008) 115–118
The substituent effect on arylhalides was also examined for both sonochemical and conventional heating and the results were given in Table 1. Sonochemical reaction has a crucial effect on the reaction time, compared with conventional heating (1 h/20 h) (Entry 3). We assume that the beneficial effect of ultrasonic irradiation on this heterogenerous reaction can be attributed to mass transfer in comparison with magnetically stirred reaction. It is well known that aryliodides do not give complete conversion in simple displacement reaction (i.e. SN2). Moreover, when using conventional heating, it is often the case that non-water-soluble aryliodides give incomplete conversion when reactions were performed in aqueous media, even in the presence of tetraalkylammonium salts [24b]. Representative aryliodides were also screened in the coupling reaction using our methodology. The good results were obtained for both sonochemical and conventional heating (Entries 1–3). Next, arylbromides bearing electron-donor, electro-neutral, and electron-withdrawing substituents were applied to examine the scope of this reaction. Moderate to good yields were obtained in both cases. In addition, steric effect was also observed, but not very significant (Entry 7). Sonochemical reaction gave lower yield when reaction stopped after 3 h.
117
Attempts have also been made to couple some arylchlorides with phenylboronic acid under the optimized reaction conditions. However, the electron-withdrawing arylchlorides only gave 57% isolated yield under conventional heating (Entry 10). Remarkably, for this Suzuki reaction under conventional heating, when Cat. 2 [26] was employed at the same catalyst loading as of 1 mol%, the yield reached 86% (Entry 11). Therefore, the electron-withdrawing arylchlorides could be coupled with phenylboronic acid in good yields in the presence of 1 mol% catalyst loading (Cat. 2), as shown in Table 1, Entries 10–14. The good results were also obtained when arylchlorides were employed under ultrasonic irradiation. 4. Conclusion In conclusion, we have proved that ultrasound can facilitate the heterogenous reaction of Suzuki coupling of phenylboronic acid with arylhalides in the presence of TBAB by means of easily obtained ligand-free cyclopalladated ferrocenylimines in neat water. Moderate to good yields was also obtained for the coupling of arylchlorides. Compared with conventional heating, the reaction was accelerated by ultrasound irradiation drastically. This methodology has
Table 1 Ultrasonic irradiation and conventionally heated Suzuki coupling of arylhalides with phenylboronic acid in water CH3
X
Cat. K3PO4
+
B(OH)2
R
H2O, Reflux, T BA B
Fe
Arylhalides
1d 2d 3d 4d 5d 6d 7d 8d 9d 10d 11e 12e 13e 14e
PhI PhI 4-CH3OC6H4I PhBr 4-CH3OC6H4Br 4-CH3OCC6H4Br 2-CH3C6H4Br 4-CH3OC6H4Cl 3-NO2C6H4Cl 4-NO2C6H4Cl 4-NO2C6H4Cl 4-CH3OCC6H4Cl 2,4-N2O4C6H3Cl 2,4-N2O4C6H3Cl
a b c d e
Cl
N Fe
Pd
I
2
2
Cat.1
Ultrasonic irradiation with heating (70 C)
CH3
Pd
Cl
R
X=I, Br, Cl
Entrya
CH3
N
Cat.2
Conventional heating (100 C)
Reaction time (h)
Catalyst loading (mol% Pd)
Yields
Reaction time (h)
Catalyst loading (mol% Pd)
Yields
1 0.5 1 3 1.5 2.5 3 8 8 8 8 8 7 7
0.5 1 1 0.5 1 1 1 1 1 1 1 1 1 0.1
94c 93c 86c 73c 75c 96c 83c trace <5b 30c 80c 75c 96c 94c
5 3 20 6 20 4 10 40 18 24 24 24 10 10
0.5 1 1 0.5 1 1 1 1 1 1 1 1 1 0.1
100c 100c 83c 100c 95c 94c 88c 13b 24b 57c 86c 82c 96c 91c
Reaction stoichiometry: PhX 2.0 mmol, PhB(OH)2 3.0 mmol, K3PO4 4.0 mmol, H2O 13 mL, catalyst, TBAB for conventional heating (2.0 mmol). Average results of two runs determined by GC based on ArX. Isolated yields based on ArX. Cat. 1 was used and TBAB as emulsifying agent for conventional heating. Cat. 2 and TBAB as emulsifying agent were used.
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J. Zhang et al. / Ultrasonics Sonochemistry 15 (2008) 115–118
the advantage of use of a nontoxic, nonflammable solvent and easily handled. Acknowledgements We are grateful to the National Natural Science Foundation of China (Project 20472074) and the Innovation Fund for Outstanding Scholar of Henan Province (Project 0621001100) for the financial support given to this research. References [1] N. Miyaura, A. Suzuki, Chem. Rev. 95 (1995) 2457. [2] For a review, see: S.P. Stanforth, Tetrahedron 54 (1998) 263. [3] (a) J. Hassan, M. Sevignon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev. 102 (2002) 1359; (b) M.B. Goldfinger, K.B. Crawford, T.M. Swager, J. Am. Chem. Soc. 119 (1997) 4578. [4] (a) For highlights of the palladium activation of chloroarenes, see: H. Groger, J. Prakt, Chem. (Leipzig) 342 (2000) 334; (b) R. Sturmer, Angew. Chem. 111 (1999) 3509–3510, Angew. Chem. Int. Ed. 38 (1999) 3307. [5] R.A. Sheldon, Green Chem. 7 (2005) 267. [6] A.L. Casalnuovo, J.C. Calabrese, J. Am. Chem. Soc. 112 (1990) 4324. [7] M. Nishimura, M. Ueda, N. Miyaura, Tetrahedron 58 (2002) 5779. [8] A. Konovets, A. Penciu, E. Framery, N. Percina, C. Goux-Henry, D. Sinou, Tetrahedron Lett. 46 (2005) 3205. [9] J. Dupont, M. Pfeffer, J. Spencer, Eur. J. Inorg. Chem. (2001) 1917. [10] L. Botella, C. Najera, J. Organomet. Chem. 663 (2002) 46. [11] C. Najera, J. Gil-Molto, S. Karlstrom, L.R. Falvello, Org. Lett. 5 (2003) 1451. [12] J.-W. Kim, J.-H. Kim, D.-H. Lee, Y.-S. Lee, Tetrahedron Lett. 47 (2006) 4745. [13] R. Huang, K.H. Shaughnessy, Organometallics 25 (2006) 4105.
[14] (a) D.N. Korolev, N.A. Bumagin, Tetrahedron Lett. 47 (2006) 4225; (b) N.E. Leadbeater, M. Marco, J. Org. Chem. 68 (2003) 888. [15] (a) Y. Li, X.M. Hong, D.M. Collard, M.A. El-Sayed, Org. Lett. 2 (2000) 2385; (b) S. Bhattacharya, A. Srivastava, S. Sengupta, Tetrahedron Lett. 46 (2005) 3560; (c) K.H. Shaughnessy, R.S. Booth, Org. Lett. 3 (2001) 2757. [16] (a) L. Botella, C. Najera, Angew. Chem. Int. Ed. 41 (2002) 179; (b) I. Osdemir, S. Demir, S. Yasar, B. Cetinkaya, Appl. Organomet. Chem. 19 (2005) 55. [17] H.J. Lim, G. Keum, S.B. Kang, B.Y. Chung, Y. Kim, Tetrahedron Lett. 39 (1998) 4367. [18] V. Polackova, M. Hut’ka, S. Toma, Ultrason. Sonochem. 12 (2005) 99. [19] S.Q. Huo, Y.J. Wu, C.X. Du, Y. Zhu, H.Z. Yuan, X.A. Mao, J. Orangomet. Chem. 483 (1994) 139. [20] (a) L.R. Yang, J.L. Zhang, M.P. Song, S.S. Zhang, N. Yu, Y.J. Wu, Acta Chim. Snica 61 (2003) 959; (b) Y.J. Wu, L.R. Yang, J.L. Zhang, M. Wang, L. Zhao, M.P. Song, J.F. Gong, Arkivoc ix (2004) 111; (c) J.L. Zhang, L. Zhao, M.P. Song, T.C.W. Mak, Y.J. Wu, J. Organomet. Chem. 691 (2006) 1301. [21] S.L. Chen, C.G. Xu, K.Q. Zhao, P.J. Hu, J. Sichuan Normal University (Natural Science) 23 (2000) 511. [22] (a) J. Huckingham, S.M. Donaghy, Dictionary of Organic Compounds, fifth ed., Chapman and Hall, New York, 1982; (b) D.W. Old, J.P. Wolfe, S.L. Buchwald, J. Am. Chem. Soc. 120 (1998) 9722. [23] D. Pan, L.C.T. Shoute, D.L. Phillips, Chem. Phys. Lett. 316 (2000) 395. [24] (a) H. Sakurai, T. Tsukuda, T. Hirao, J. Org. Chem. 67 (2002) 2721; (b) N.A. Bumagin, V.V. Bykov, Tetrahedron 53 (1997) 14437. [25] E.M. Campi, W.R. Jackson, S.M. Marcuccio, C.G.M. Naeslund, J. Chem. Soc., Chem. Commun. (1994) 2395. [26] J.-J. Hou, L.-R. Yang, X.-L. Cui, Y.-J. Wu, Chinese J. Chem. 21 (2003) 717.
Ultrasonics Sonochemistry 15 (2008) 115–118 www.elsevier.com/locate/ultsonch
Ultrasonic irradiation accelerated cyclopalladated ferrocenylimines catalyzed Suzuki reaction in neat water Jinli Zhang, Fan Yang, Gerui Ren, Thomas C.W. Mak 1, Maoping Song, Yangjie Wu
*
Department of Chemistry, Henan Key Laboratory of Chemical Biology and Organic Chemistry, Key Laboratory of Applied Chemistry of Henan Universities, Zhengzhou University, Zhengzhou 450052, PR China Received 3 November 2006; received in revised form 1 February 2007; accepted 15 February 2007 Available online 24 February 2007
Abstract Both conventional heating and ultrasound effect on the cyclopalladated ferrocenylimines catalyzed Suzuki reaction of phenylboronic acid with a range of arylhalides in neat water was investigated. Heterogenous reaction of electron-withdrawing arylchlorides with phenylboronic acid could also result in good yields by using Cat. 2. It was found that the ultrasonic irradiation could dramatically accelerate the Suzuki reaction to achieve comparable results. 2007 Elsevier B.V. All rights reserved. Keywords: Cyclopalladated ferrocenylimines; Suzuki reaction; Neat water; Ultrasonic irradiation
1. Introduction Palladium-catalyzed Suzuki cross-coupling reaction is a powerful, versatile and popular tool for selective construction of carbon–carbon bonds [1], in particular for the formation of biaryls [2] due to its importance as components of pharmaceuticals, herbicides, and natural products, as well as in the fields of engineering materials, such as conducting polymers, molecular wires, liquid crystals, and synthesis of ligands [3]. In recent years, various modifications involving the catalysts, solvents, bases, reaction conditions and synthetic technique have been developed [3a]. From the industrial point of view, some important goals need to be realized for the development of this process: the functionalization of inexpensive and readily accessible aryl chlorides [4] and the air-stable, efficient, and nontoxic cat-
* Corresponding author. Present address: Department of Chemistry, Zhengzhou University, Zhengzhou 450052, PR China. Tel.: +86 371 67767993; fax: +86 371 67979408. E-mail address: [email protected] (Y. Wu). 1 Present address: Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, PR China.
1350-4177/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2007.02.002
alysts. Furthermore, the use of water as a potential replacement for organic solvents are desired due to its low cost, nonflammability, nontoxicity, and the fact that it is a renewable resource [5]. Since Casalnuovo’s [6] initial report of TPPMS/ Pd(OAc)2 catalyzed cross-coupling reactions in aqueous solvents, there has been a great deal of effort devoted to the development of water soluble analogues of phosphines, particularly derivatives of triphenylphosphine [7,8]. Palladacycles have also attracted much attention for hosting the cross-coupling reactions of carbon–carbon and carbon–heteroatom bond formation owning to their availability, facile modification, insensitivity to air or moisture and easy handling as compared with the most other catalysts [9]. Subsequently, oxime-derived palladacycles [10], di-2pyridylmethylamine-based palladium complexes [11], and palladium N-heterocyclic carbene complexes [12], hydrophilic palladacycles in combination with t-Bu-amphos [13], and recent ligand free Pd-catalysts PdCl2 [14] and Pd(OAc)2 were employed to the aqueous Suzuki cross-coupling. However, most of them were limited to using aryl iodides and bromides [15]. Only scattered attentions have been focused on the coupling of arylchlorides in aqueous Suzuki reaction [16].
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J. Zhang et al. / Ultrasonics Sonochemistry 15 (2008) 115–118
The use of ultrasonic irradiation as a tool for synthetic chemistry is a fast growth area. Since the first reports of ultrasonic irradiation-assisted coupling reaction [17], this technique has been accepted as a method for reducing reaction times and increasing yields. Recently, Toma Stefan [18] also reported the ultrasound effect on Suzuki reaction catalyzed by 5% Pd/C and KF as base in methanol/water mixture. A few years ago, we found that cyclopalladated ferrocenylimine [19] was a kind of phosphine-free compounds and almost behaves all the properties of palladacycles catalysts. The synthetic convenience, facile modification and air, moisture stability made them easily to be handled. Subsequently, we discovered their wide applications for carbon–carbon bond formation in organic solvent [20]. Herein, we reported that the sonication of the reaction mixture could enhance the Suzuki cross-coupling reaction of the air stable, ligand-free cyclopalladated ferrocenylimines-catalyzed Suzuki reaction in neat water. The arylhalides including electron-withdrawing arylchlorides could be coupled with phenylboronic acid in moderate to good yields under both sonochemical and conventional heating. 2. Experimental 2.1. General Melting points were measured on a WC-1 microscopic apparatus and uncorrected. 1H NMR was recorded on a Bruker DPX 400 instrument using CDCl3 as the solvent and tetramethylsilane as the internal standard. Preparative TLC was performed on dry silica gel plates developed with dichloromethane/petroleum. Cyclopalladated ferrocenylimines were synthesized according to the literature procedures [19]. The arylhalides were obtained from commercial sources and were generally used without further purification. Phenylboronic acid was synthesized according to the literature [21]. 2.2. General procedure for Suzuki reaction in neat water using conventional heating A 25 mL round-bottom flask was charged with arylhalides (2.0 mmol), phenylboronic acid (3.0 mmol, 0.366 g), potassium phosphate (4.0 mmol, 0.849 g), TBAB (2.0 mmol, 0.645 g), catalyst, and 13 mL of H2O. The mixture was stirred at reflux temperature in air and was monitored by TLC. After the completion of the reaction, the mixture was extracted with CH2Cl2 (3 · 25 mL). The combined organic phases were dried over anhydrous Na2SO4, filtered, and the solvent was removed on a rotary evaporator. The residue was analyzed by GC (dissolved in CH2Cl2, undecane as internal standard) or purified by preparative TLC and the yield was calculated based on ArX. (the purified products were identified by comparison of melting points with the literature values [21] or by 1H NMR spectra).
2.3. General procedure for Suzuki reaction in neat water by ultrasonic wave A 25 mL round-bottom flask was charged with arylhalides (2.0 mmol), phenylboronic acid (3.0 mmol, 0.366 g), potassium phosphate (4.0 mmol, 0.849 g), TBAB (2.0 mmol, 0.645 g), catalyst, and 13 mL of H2O. The mixture was stirred at 70 C in air in a KQ-100B ultrasonic cleaner (40 KHz) and was monitored by TLC. After the completion of the reaction, the mixture was extracted with CH2Cl2 (3 · 25 mL). The combined organic phases were dried over anhydrous Na2SO4, filtered, and the solvent was removed on a rotary evaporator. The residue was analyzed by GC (dissolved in CH2Cl2, undecane as internal standard) or purified by preparative TLC and the yield was calculated based on ArX. (the purified products were identified by comparison of melting points with the literature values [21] or by 1H NMR spectra). Biphenyl [22] white plates, m.p. 70 C (lit. 70–71 C). 2-Methylbiphenyl [23] colorless oil. 1H NMR (CDCl3): d 2.27 (s, 3H, CH3), 7.25 (m, 4H, Ar-H), 7.33 (m, 3H, ArH), 7.40 (m, 2H, Ar-H). 4-Methoxybiphenyl [22] white solid. m.p. 88–90 C (lit. 91–92 C). 4-Acetylbiphenyl [22] white solids, m.p. 117 C (lit. 116– 118 C). 3-Nitrobiphenyl [22] yellow needles, m.p. 61 C (lit. 62 C). 4-Nitrobiphenyl [22] yellow needles, m.p. 113–114 C (lit. 112–114 C). 2,4-Dinitrobiphenyl [22] yellow plates, m.p. 110 C (lit. 110 C).
3. Results and discussion Previous reports have shown that the addition of phasetransfer catalysts, such as tetraalkylammonium salts, can greatly improve yields of Suzuki reactions in both water [24b,16a] and polar organic solvents [15c,25]. The role of ammonium salts is thought to be twofold. First, they facilitate dissolution of the organic substrates in the solvent medium. Second, they are supposed to enhance the rate of the coupling reaction by activating the boronic acid to form [ArB(OH)3] [R4N]+ [24b]. Based on our previous results in organic solvent [20], the first Suzuki coupling of phenylboronic acid with iodobenzene was conducted using K3PO4 as base at 0.5% catalyst loading of Cat. 1 with TBAB at room temperature for optimizing reaction conditions. Unfortunately, due to the poor solubility of catalyst and the substrates, the product was not observed when monitored by the TLC. Ramping the temperature from r.t. to reflux temperature, the isolated yields reached to 100% (Entry 1). Sonochemical reaction afforded 93% yield of the product when 1 mol% of catalyst was used at 70 C (Entry 2).
J. Zhang et al. / Ultrasonics Sonochemistry 15 (2008) 115–118
The substituent effect on arylhalides was also examined for both sonochemical and conventional heating and the results were given in Table 1. Sonochemical reaction has a crucial effect on the reaction time, compared with conventional heating (1 h/20 h) (Entry 3). We assume that the beneficial effect of ultrasonic irradiation on this heterogenerous reaction can be attributed to mass transfer in comparison with magnetically stirred reaction. It is well known that aryliodides do not give complete conversion in simple displacement reaction (i.e. SN2). Moreover, when using conventional heating, it is often the case that non-water-soluble aryliodides give incomplete conversion when reactions were performed in aqueous media, even in the presence of tetraalkylammonium salts [24b]. Representative aryliodides were also screened in the coupling reaction using our methodology. The good results were obtained for both sonochemical and conventional heating (Entries 1–3). Next, arylbromides bearing electron-donor, electro-neutral, and electron-withdrawing substituents were applied to examine the scope of this reaction. Moderate to good yields were obtained in both cases. In addition, steric effect was also observed, but not very significant (Entry 7). Sonochemical reaction gave lower yield when reaction stopped after 3 h.
117
Attempts have also been made to couple some arylchlorides with phenylboronic acid under the optimized reaction conditions. However, the electron-withdrawing arylchlorides only gave 57% isolated yield under conventional heating (Entry 10). Remarkably, for this Suzuki reaction under conventional heating, when Cat. 2 [26] was employed at the same catalyst loading as of 1 mol%, the yield reached 86% (Entry 11). Therefore, the electron-withdrawing arylchlorides could be coupled with phenylboronic acid in good yields in the presence of 1 mol% catalyst loading (Cat. 2), as shown in Table 1, Entries 10–14. The good results were also obtained when arylchlorides were employed under ultrasonic irradiation. 4. Conclusion In conclusion, we have proved that ultrasound can facilitate the heterogenous reaction of Suzuki coupling of phenylboronic acid with arylhalides in the presence of TBAB by means of easily obtained ligand-free cyclopalladated ferrocenylimines in neat water. Moderate to good yields was also obtained for the coupling of arylchlorides. Compared with conventional heating, the reaction was accelerated by ultrasound irradiation drastically. This methodology has
Table 1 Ultrasonic irradiation and conventionally heated Suzuki coupling of arylhalides with phenylboronic acid in water CH3
X
Cat. K3PO4
+
B(OH)2
R
H2O, Reflux, T BA B
Fe
Arylhalides
1d 2d 3d 4d 5d 6d 7d 8d 9d 10d 11e 12e 13e 14e
PhI PhI 4-CH3OC6H4I PhBr 4-CH3OC6H4Br 4-CH3OCC6H4Br 2-CH3C6H4Br 4-CH3OC6H4Cl 3-NO2C6H4Cl 4-NO2C6H4Cl 4-NO2C6H4Cl 4-CH3OCC6H4Cl 2,4-N2O4C6H3Cl 2,4-N2O4C6H3Cl
a b c d e
Cl
N Fe
Pd
I
2
2
Cat.1
Ultrasonic irradiation with heating (70 C)
CH3
Pd
Cl
R
X=I, Br, Cl
Entrya
CH3
N
Cat.2
Conventional heating (100 C)
Reaction time (h)
Catalyst loading (mol% Pd)
Yields
Reaction time (h)
Catalyst loading (mol% Pd)
Yields
1 0.5 1 3 1.5 2.5 3 8 8 8 8 8 7 7
0.5 1 1 0.5 1 1 1 1 1 1 1 1 1 0.1
94c 93c 86c 73c 75c 96c 83c trace <5b 30c 80c 75c 96c 94c
5 3 20 6 20 4 10 40 18 24 24 24 10 10
0.5 1 1 0.5 1 1 1 1 1 1 1 1 1 0.1
100c 100c 83c 100c 95c 94c 88c 13b 24b 57c 86c 82c 96c 91c
Reaction stoichiometry: PhX 2.0 mmol, PhB(OH)2 3.0 mmol, K3PO4 4.0 mmol, H2O 13 mL, catalyst, TBAB for conventional heating (2.0 mmol). Average results of two runs determined by GC based on ArX. Isolated yields based on ArX. Cat. 1 was used and TBAB as emulsifying agent for conventional heating. Cat. 2 and TBAB as emulsifying agent were used.
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