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Organometallics 2009, 28, 2334–2337
Notes Suzuki Coupling Reaction of Aryl Halides Catalyzed by an N-Heterocyclic Carbene-PdCl2 Species Based on a Porphyrin at Room Temperature Jun-Wen Wang,* Fan-Hui Meng, and Li-Fang Zhang Department of Chemistry, Shanxi Normal UniVersity, Linfen 041004, People’s Republic of China ReceiVed September 7, 2008 Summary: A noVel tetraimidazolium salt based on a porphyrin as a ligand was synthesized and eValuated as a precursor of an N-heterocyclic carbene ligand and PdCl2(CH3CN)2 as the palladium source in Suzuki cross-coupling reactions with aryl halides and arylboronic acids. Among the bases and solVents tested, KOH and ethanol were the most effectiVe. The tetraimidazolium salt exhibited excellent actiVity in Suzuki coupling reactions at room temperature oVer 1 h. The results indicate that catalytic actiVities are dependent on the collectiVe effect of the porphyrin core and the NHC. The new porphyrin-based imidazolium salt shows promise as a supermolecular skeleton for the construction of highly actiVe catalysts. Transition-metal-catalyzed cross-coupling reactions now provide a poupular means of adding a variety of carbon species to aryl halides.1 During the past years, the development of efficient and selective catalytic reactions for C-C bond formation have been a significant focus of organic chemistry.2 The palladiumcatalyzed Suzuki coupling reaction with aryl halides and arylboronic acid is one of the most widely employed transition metal catalyst reactions.3 The N-heterocyclic carbene (NHC) system is now regarded as a prospective replacement catalyst
* To whom correspondence should be addressed. E-mail:
[email protected]. (1) (a) Corbet, J.-P.; Mignani, G. Chem. ReV. 2006, 106, 2651. (b) Arduengo, A. J. R.; Harlow, L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361. (c) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (d) Weskamp, T.; Bo¨hm, V. P. W.; Herrmann, W. A. J. Organomet. Chem. 2000, 600, 12. (e) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 4176. (f) Lillo, V.; Mata, J. A.; Segarra, A. M.; Peris, E.; Fernandez, E. Chem. Commun. 2007, 2184. (g) Maerten, E.; Sauthier, M.; Mortreux, A.; Castanet, Y. Tetrahedron 2007, 63, 682. (h) Navarro, O.; Marion, N.; Oonishi, Y.; Kelly, R. A.; Nolan, S. P. J. Org. Chem. 2006, 71, 685. (i) Viciu, M. S.; Stevens, E. D.; Peterson, J. L.; Nolan, S. P. Organometallics 2004, 23, 3752. (2) (a) Nunes, R. C.; Araujo, M. H.; Santos, E. N. Catal. Commun. 2007, 8, 1798. (b) Taige, M. A.; Zeller, A.; Ahrens, S.; Goutal, S.; Herdtweck, E.; Strassner, T. J. Organomet. Chem. 2007, 692, 1519. (c) Kakiuchi, F.; Chatani, N. AdV. Synth. Catal. 2003, 345, 1077. (d) Kakiuchi, F.; Murai, S. Acc. Chem. Res. 2002, 35, 826. (e) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. ReV. 2002, 102, 1731. (f) Crabtree, R. H. Dalton Trans. 2001, 2437. (3) (a) Hillier, A. C.; Grasa, G. A.; Viciu, M. S.; Lee, H. M.; Yang, C.; Nolan, S. P. J. Organomet. Chem. 2002, 653, 69. (b) Miura, M. Angew. Chem., Int. Ed. 2004, 43, 2201. (c) Fliedel, C.; Francois, A. M.; Laponnaz, S. B. Inorg. Chim. Acta 2007, 360, 143. (d) Grasa, G. A.; Viciu, M. S.; Huang, J.; Zhang, C.; Trudell, M. L.; Nolan, S. P. Organometallics 2002, 21, 2866. (e) Vargas, V. C.; Rubio, R. J.; Hollis, T. K.; Salcido, M. E. Org. Lett. 2003, 5, 4847.
Figure 1. Structures of the complexes A and B.
for phosphines in the Suzuki reaction.4 In recent years, the Suzuki cross-coupling reaction of aryl halides under mild conditions has been extensively investigated.5 In order to search for the starting materials and catalytic system, we were interested in tetradentate ligands based on a porphyrin scaffold. Surprisingly, there are no examples of porphyrins bearing N-heterocyclic carbene tetradentate ligands. To the best of our knowledge, only very few reports have published on N-hetetrocyclic carbene complexes fused to a porphyrin. In 1995, Ghosh discussed the analogy between the “N-disordered porphyrins” (A).6 Notably, Richeter reported on the synthesis of an NHC-porphyrin (B) in 2007 (Figure 1). They presented a new synthetic procedure to obtain a palladium-NHC complex fused to a porphyrin.7 There have been no reports on the Suzuki-Miyaura cross-coupling reaction using a palladium-NHC-porphyrin catalyst. In our studies on homogeneous catalysis using metal-NHC complexes, we became interested in developing the synthesis of the supramolecular catalysts. Herein, we report the modular synthesis of a tetradentate ligand based on the porphyrin 3 and an initial catalytic study of its use in the Suzuki-Miyaura coupling reaction (Figure 2). (4) (a) Zhang, C.; Huang, J.; Trudell, M. L.; Nolan, S. P. J. Org. Chem. 1999, 64, 3804. (b) Bo¨hm, V. P. W.; Gsto¨ttmayr, C. W. K.; Weskamp, T.; Herrmann, W. A. J. Organomet. Chem. 2000, 595, 186. (c) Viciu, M. S.; Germaneau, R. F.; Navarro-Fernandez, O.; Stevens, E. D.; Nolan, S. P. Organometallics 2002, 21, 5470. (5) (a) Yen, S. K.; Koh, L. L.; Huynh, H. V.; Hor, T. S. A. Dalton Trans. 2008, 699. (b) Ohta, H.; Fujihara, T.; Tsuji, Y. Dalton Trans. 2008, 379. (c) Yen, S. K.; Koh, L. L.; Huynh, H. V.; Hor, T. S. A. Dalton Trans. 2007, 3952. (d) Fleckenstein, C. A.; Plenio, H. Green Chem. 2007, 12, 1287. (e) Ray, L.; Shaikh, M. M.; Ghosh, P. Dalton Trans. 2007, 4546. (f) Wass, D. F.; Haddow, M. F.; Hey, T. W.; Orpen, A. G.; Russell, C. A.; Wingad, R. L.; Green, M. Chem. Commun. 2007, 2704. (g) Hahn, F. E.; Volker, L.; Tania, P. Chem. Commun. 2005, 5390. (h) Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am. Chem. Soc. 2004, 126, 8717. (6) Ghosh, A. Angew. Chem., Int. Ed. Engl. 1995, 34, 1028. (7) Richeter, S.; Hadj-Aı¨ssa, A.; Taffin, C.; Leclercq, D. Chem. Commun. 2007, 2148.
10.1021/om800870b CCC: $40.75 2009 American Chemical Society Publication on Web 03/06/2009
Notes
Organometallics, Vol. 28, No. 7, 2009 2335 Table 1. Screening the Reaction Conditions for the Suzuki-Miyaura Cross-Coupling Reactiona
entry
Figure 2. Structure of the NHC-porphyrin 3. Scheme 1. Synthesis of NHC Ligand 3
1 2 3 4 5 6 7 8 9 10 11 12 13
solvent
base
temp (°C)
time (h)
yield (%)b
1,4-dioxane 1,4-dioxane 1,4-dioxane 1,4-dioxane 1,4-dioxane DMF CH3CN CH3CN CH3CN DMF DMF/H2O (1:1) ethanol ethanol
t
75 75 75 75 75 75 75 75 75 75 75 75 room temp
5 5 5 5 5 5 5 5 5 5 5 5 1
31 30 11 32 61 65 42 26 9 60 50 99 96
KO Bu NaOtBu Na2CO3 LiOH KOH KOH KOH LiOH K2CO3 K2CO3 K2CO3 KOH KOH
a Reaction conditions: 0.5 mmol of PhBr, 0.6 mmol of PhB(OH)2, 1.0 mmol of base, 0.1 mol % of 3, 0.2 mol % of PdCl2(CH3CN)2, 75 °C. b Isolated yields of homogeneous samples after chromatographic purification.
The synthetic route of the NHC-porphyrin 3 is demonstrated in Scheme 1. Following the literature procedure, the porphyrin bromide 2 was prepared by simple alkylation of meso-5,10,15,20tetrakis(4-hydroxylphenyl)porphyrin (1) with 1,3-dibromopropane.8 By using an excess (48 equiv) of 1,3-dibromopropane, the main product porphyrin 2 was formed in DMF. The ligand precursor NHC-porphyrin 3 was prepared by refluxing the porphyrin bromide 2 with 1-(9-anthracenylmethyl)imidazole in THF, followed by exchange of the anions with PF6-,9 as indicated by UV-vis, 1H NMR, and TLC of the crude reaction mixture. In the 1H NMR spectra of 3, the imidazolium proton signals (NCHN) appear at δ 9.16 ppm, which is consistent with the chemical shifts of known imidazolium salts, and the pyrrole proton signals at -2.91 ppm. The UV-vis spectrum in acetone shows the Soret band at 413 nm and four Q-bands at 516, 552, 592, and 649 nm. NHC-transition-metal catalysts have been successfully employed in the Suzuki-Miyaura reaction. Recently, many chelating carbene complexes have shown promising properties as efficient catalysts.10 To test the catalytic ability of the NHC-porphyrin ligand, a few test reactions were used. As (8) (a) Bettelheimm, A.; White, B. A.; Raybuck, S. A Inorg. Chem. 1987, 26, 1009. (b) Li, X.-X.; Wang, J.-W.; Guo, Y.-J.; Kong, L.-H.; Pan, J.-H. J. Incl. Phenom. Macrocycl. Chem. 2007, 58, 307. (c) Li, X.-X.; Guo, Y.-J.; Wang, J.-W.; Kong, L.-H.; Pan, J.-H. Supramol. Chem. 2008, 20, 243. (d) Lan, Z.-W.; Yuan, X.-Q.; Chen, J.-T. Chinese J. Org. Chem. 1995, 15, 47. (9) (a) Wang, J.-W.; Song, H.-B.; Li, Q.-S.; Xu, F.-B.; Zhang, Z.-Z. Inorg. Chim. Acta 2005, 358, 3653. (b) Wang, J.-W.; Li, Q.-S.; Xu, F.-B.; Song, H.-B.; Zhang, Z.-Z. Eur. J. Org. Chem. 2006, 1310. (10) (a) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus, G. R. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 2371. (b) Gardiner, M. G.; Herrmann, W. A.; Reisinger, C.-P.; Schwarz, J.; Spiegler, M. J. Organomet. Chem. 1999, 572, 239. (c) Mas-Marza, E.; Segarra, A. M.; Claver, C.; Peris, E.; Fernandez, E. Tetrahedron Lett. 2003, 44, 6595. (d) Miecznikowski, J. R.; Crabtree, R. H. Organometallics 2004, 23, 629. (e) Moncada, A. I.; Manne, S.; Tanski, J. M.; Slaughter, L. M. Organometallics 2006, 25, 491.
shown in Table 1, under unoptimized reaction conditions, coupling reactions were carried out by using bromobenzene and phenylboronic acid as the prototypical substrate combination. The results show that the nature of the initial precatalyst, solvent, and base is especially crucial for the success of the Suzuki crosscoupling reaction. The catalytic activity of 3 is highly dependent on solvent and base. Among the several bases examined, the use of NatOBu and KtOBu as the bases in 1,4-dioxane at 75 °C gave biphenyl in about 30% yield after 5 h (Table 1, entries 1 and 2). We then turned our attention to some inorganic bases that are known to be useful in Suzuki coupling. Here, the inorganic bases Na2CO3 and LiOH gave lower yields after 5 h at 75 °C (Table 1, entries 3 and 4). It is noteworthy that the biphenyl product was obtained in 61% yields using the base KOH in 1,4-dioxane at 75 °C (Table 1, entry 5); therefore, KOH was used for the Suzuki cross-coupling reaction. When the solvent was changed to ethanol, an excellent yield was obtained, while DMF and acetonitrile gave moderate yields (Table 1, entries 6, 7, and 12). Previously, Nolan and co-workers reported that the catalytic system with an alcoholic solvent could be beneficial in the coupling reaction of aryl halides with phenylboronic acid.11 Indeed, we also employed the Suzuki reaction using ethanol as solvent and KOH as base at room temperature over 1 h. The most significant improvement was found in this case, with the coupled product biphenyl obtained in an excellent yield of 96% at room temperature in 1 h (Table 1, entry 13). Using the NHC-porphyrin ligand 3 and PdCl2(CH3CN)2 as the catalyst and KOH as the base, Suzuki reactions of aryl bromides and chlorides with arylboronic acid were carried out in ethanol at room temperature (Table 2). With 0.2 mol % of PdCl2(CH3CN)2, all of the aryl bromides and arylboronic acids were converted to the corresponding biaryls in excellent yields within 0.5 h, regardless of the functionality substituents (Table 2, entries 1-8). In a similar fashion, by using the NHC-porphyrin 3 as the ligand precursor, the effect of different substrates was tested. As expected, with the optimized reaction conditions (0.1 mol % of 3, 0.2 mol % of PdCl2(CH3CN)2 · EtOH/KOH), aryl chloride gave moderate yields of biaryls at room temperature over 1 h (Table 2, entries 9-24). Here, we performed the Suzuki (11) Marion, N.; Navarro, O.; Mei, J.; Scott, N. M.; Stevens, E. D.; Nolan, S. P. J. Am. Chem. Soc. 2006, 128, 4101. (b) Diebolt, O.; Braunstein, P.; Nolan, S. P.; Cazin, C. S. J. Chem. Commun. 2008, 3190.
2336 Organometallics, Vol. 28, No. 7, 2009
Notes
Table 2. Suzuki-Miyaura Cross-Coupling of Aryl Halide with Different Boronic Acidsa
entry
R1
R2
X
time (h)
yield with cat. 3 (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
H H H H p-OMe p-OMe p-OMe p-OMe H H H H o-CH3 o-CH3 o-CH3 o-CH3 p-CH3 p-CH3 p-CH3 p-CH3 p-NO2 p-NO2 p-NO2 p-NO2
H Me OMe Cl H Me OMe Cl H OMe Me Cl H OMe Me Cl H OMe Me Cl H OMe Me Cl
Br Br Br Br Br Br Br Br Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
96 92 91 95 92 93 92 94 40 31 27 28 21 22 21 26 26 24 25 30 68 63 61 64
a Reaction conditions: 0.5 mmol of aryl halide, 0.6 mmol of arylboronic acid, 1.0 mmol of KOH, 0.1 mol % of 3, 0.2 mol % of PdCl2(CH3CN)2, room temperature. b Isolated yields of homogeneous samples after chromatographic purification.
Figure 3. Structure of the mono-NHC ligand 4.
reaction with different substrates; the results are given in Table 2. As shown in entries 13-20, o-methyl- and p-methylsubstituted chlorobenzenes gave similarly good isolated yields at room temperature over 1 h. Again, we found that under identical conditions the coupled products of p-nitrochlorobenzene with arylboronic acid were obtained in higher yield within 1 h. As expected, the catalytic activity depended on the halide, while electron-withdrawing groups on the aryl ring increased the reaction rate (entries 21-24). Recently, Kostas and co-workers first reported the synthesis of a palladium complex with a phosphine-free and water-soluble potassium carboxylate salt of a porphyrin and its evaluation in the Suzuki-Miyaura reaction of phenylboronic acid with aryl bromides, under mild conditions (H2O, K2CO3, 100 °C, 4 h, 0.1 mol % of catalyst).12 The cross-coupling reaction product was obtained in efficient yield (about 80%, GC yield). To further explore the effect from the porphyrin core and the corresponding mono-NHC ligand on catalytic activity, four catalysts, the porphyrin 1, the porphyrin bromide 2, the porphyrin-based imidazolium salt 3, and the mono-NHC precatalyst 4 (Figure 3), were compared in the coupling of bromobenzene with arylboronic acid (Table 3). As shown in Table 3, all the catalysts (12) Kostas, I. D.; Coutsolelos, A. G.; Charalambidisb, G.; Skondrab, A. Tetrahedron Lett. 2007, 48, 6688.
Table 3. Suzuki-Miyaura Cross-Coupling of Bromobenzene with Different Catalystsa
entry 1 2 3 4 5 6 7 8 9 10 11 12
R Me Me Me Me H H H H OMe OMe OMe OMe
cat.
solvent
time (h)
yield (%)b
1 2 3 4 1 2 3 4 1 2 3 4
ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol
1 1 1 1 1 1 1 1 1 1 1 1
75 63 92 81 80 69 96 86 73 60 91 80
a Reaction conditions: 0.5 mmol of bromobenzene, 0.6 mmol of arylboronic acid, 1.0 mmol of KOH, 0.1 mol % of catalyst, 0.2 mol % of PdCl2(CH3CN)2, room temperature. b Isolated yields of homogeneous samples after chromatographic purification.
(1-4) gave interesting results in this simple coupling reaction of bromobenzene and arylboronic acid at room temperature. Catalysts 1-4 exhibited efficient catalytic activity (entries 1-12), with the catalysts 1 and 4 having similar catalytic activity, activity greater than that of porphyrin bromide 2 and less than that of the porphyrin-based imidazolium salt 3 under the same conditions. These facts suggest that the higher yields are built upon the collective effect of the porphyrin core and the NHC. In summary, a novel NHC-porphyrin supramolecular catalyst has been developed to construct transition-metal catalysts. Catalytic activities have been evaluated by using as precursor an N-heterocyclic carbene ligand and PdCl2(CH3CN)2 as the palladium source in Suzuki cross-coupling reactions with aryl halides and arylboronic acids. These present NHC ligands exhibited good catalytic activity in Suzuki-Miyaura reactions of aryl bromide and arylboronic acid at room temperature over 1 h. This novel ligand shows great promise for the construction of highly active supramolecular catalytic systems.
Experimental Section General Procedures. All manipulations were performed using Schlenk techniques, and solvents were thoroughly dried and deoxygenated by standard methods. All the reagents for syntheses and analyses were of analytical grade and used without further purification. 1H NMR spectra were recorded on a Bruker AC-400 NMR spectrometer. Elemental analyses were measured using a Perkin-Elmer 2400C Elemental Analyzer. Preparation of the Porphyrin 2. The precursor, 5,10,15,20tetrakis(4-hydroxylphenyl)porphyrin, was synthesized according to the literature procedures. 5,10,15,20-Tetrakis(4-hydroxylphenyl)porphyrin (6.78 g, 10 mmol) was dissolved in 500 mL of DMF along with potassium carbonate (8.28 g, 60 mmol) and 1,3-dibromopropane (48 mL, 480 mmol). The resulting solution was stirred for 24 h at room temperature. After DMF was removed, the resulting solid was dissolved in CH2Cl2 and then washed with water. The crude product was purified by column chromatography with CH2Cl2 to give a purple solid (934 mg, yield 80%). Anal. Calcd for C56H50Br4N4O4: C, 57.85, H, 4.33, N, 4.82. Found: C, 57.66, H, 4.22, N, 4.68. 1H NMR (400 MHz, CDCl3, 25 °C): 8.98 (s, 8 H), 8.12-8.14 (d, 8 H), 7.32-7.34 (d, 8 H), 4.35 (t, 2H), 3.57 (t, 2H), 2.3 (m, 2H), -2.79 (s, 2 H) ppm. Preparation of the Porphyrin 3. A solution of porphyrin 2 (1.16 g, 1 mmol) and 1-(1-naphthylmethyl)imidazole (1.14 g, 4.4 mmol)
Notes in THF (60 mL) was stirred and refluxed for 3 days; a purple precipitate was formed gradually in the solvent. After removal of the upper organic phase, the raw tetraimidazolium salt was purified by recrystallization with DMF and water. Yield: 2.04 g (93%). NH4PF6 (0.62 g, 3.8 mmol) was added to a DMF (15 mL) solution of the obtained imidaolium salt with stirring, and then 30 mL of water was added to the DMF solution; a purple precipitate formed immediately. The purple powder was collected by filtration, washed with small portions of cold water, and dried in vacuo. The pure imidazolium salt was obtained by recystallization with acetone and water. Yield: 2.2 g (97%). Anal. Calcd for C130H112F24N12O4P4: C, 62.80, H, 4.54, N, 6.76; Found: C, 62.72, H, 4.40, N, 6.69. 1H NMR (400 MHz, [D6]DMSO, 25 °C): 9.16 (s, 4H), 8.98 (s, 8 H), 8.82 (m, 4H), 8.54 (s, 8H), 7.61-8.18 (m, 40 H), 7.17-7.34 (m, 8 H), 6.57 (s, 8H), 4.47 (t, 8H), 4.24 (t, 8H), 2.41 (s, 8H), -2.91 (s, 2 H) ppm. UV: 350, 368, 413, 516, 552, 592, 649 nm.
Organometallics, Vol. 28, No. 7, 2009 2337 General Procedure for the Suzuki Coupling Reactions. In a typical run, a mixture of aryl bromide (0.5 mmol), phenylboronic acid (0.6 mmol), KOH (1.0 mmol), PdCl2(CH3CN)2 (0.2 mol %), and the imidazolium salt (0.1 mol %) in 3 mL of ethanol was stirred at room temperature for 1 h under nitrogen. Water was added to the reaction mixture, the organic layer was extracted with diethyl ether and dried over magnesium sulfate, and the solvent was removed completely under high vacuum to give a crude product. The pure product was isolated by column chromatography on silica.
Acknowledgment. This work was supported by the Doctor Foundation of Shanxi Normal University and the Shanxi Natural Science Foundation of China under Project No. 2006011069. We thank our technical staff, in particular Dr. Hongwang Du and Jin Lin, for assistance. OM800870B
