Synthesis of N-Heterocyclic Carbene-Sulfonate Palladium Complexes

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Organometallics 2009, 28, 6131–6134 DOI: 10.1021/om9004252

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Synthesis of N-Heterocyclic Carbene-Sulfonate Palladium Complexes Yusuke Nagai, Takuya Kochi, and Kyoko Nozaki* Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan Received June 3, 2009 Summary: Syntheses of new NHC-sulfonate palladium complexes [N-(2,6-diisopropylphenyl)-N0 -sulfomethylNHC] Pd(2,6-lutidine) (7a) and [N-(2,6-diisopropylphenyl)-N0 -sulfomethylNHC]Pd(PPh3) (7b) are reported. The structure of complexes 7a and 7b were determined by single-crystal X-ray analysis as the first examples of bidentate coordination of imidazolium-sulfonate to a metal center.

Introduction Group 10 metals have been playing indispensable roles for decades in homogeneous catalysis, and numerous complexes of these metals have been developed as catalysts for a variety of C-C bond forming reactions. Particularly, an enormous amount of ligands has been invented to control the reactivity and selectivity of the reactions. Bidentate ligands are often used due to their stable coordination to the metal center by chelation when compared to monodentate ligands. On the basis of the charge of the ligands, they can be classified mainly into two groups: (1) neutral ones such as diphosphines, diimines, and bipyridine and (2) anionic ones such as imine-phenoxide,1 phosphine-carboxylate,2 and phosphineenolate.3 A phosphine-sulfonate ligand derived from phosphonium-sulfonate 1 was originally reported by a group of Shell for palladium-catalyzed polymerization of ethylene with methyl acrylate4a and CO.5a Since then, syntheses of novel

polymers with phosphine-sulfonate palladium catalysts have been intensively studied by several researchers including our group. The unique polymers produced by the catalysts include copolymers of ethylene with polar vinyl monomers,4 nonalternating copolymers of ethylene with CO,5 and alternating copolymers of polar vinyl monomers with CO.6 The phosphine-sulfonate ligand is considered to coordinate in a bidentate fashion, and the anionic and unsymmetrical nature of the ligand is anticipated to contribute the unique properties. Indeed, several palladium complexes bearing bidendate phosphine-sulfonate ligands such as 2 have been isolated and used for the polymerization reactions mentioned above.4d,e On the other hand, N-heterocyclic carbene (NHC) ligands have attracted much attention due to their strong σ-donating ability to stabilize the complexes and have been applied as ligands for a variety of catalytic reactions to achieve improved catalytic activities.7a-c Replacement of phosphine moieties in bidentate ligands by NHCs has also been examined, and several bidentate ligands containing an NHC substituted with enolate,8c phenoxide,8d and other donors have been prepared.8 The idea that substituting the phosphine part of 1 would make the ligand more electron donating prompted us to synthesize a new class of ligands, NHC-sulfonate bidentate ligands, which can be generated by deprotonation of sulfomethylimidazoliums 3 and 4 (Figure 1). Very recently, Hoveyda and his co-workers reported the synthesis of chiral imidazolium sulfonate inner salt 5 and NHC-sulfonate silver complex 6, which was successfully used as a catalyst precursor for copper-catalyzed asymmetric conjugate addition of organozinc species.9 It should be noted that the sole example of an isolated NHC-sulfonate complex, 6, is a dinuclear complex in which the ligand does not chelate one silver center but bridges two metal centers. Here we report the syntheses of new imidazolium sulfonate inner salts 3 and 4 and NHC-sulfonate palladium complexes 7.

*Corresponding author. E-mail: [email protected]. (1) (a) Wang, C.; Friedrich, S.; Younkin, T. R.; Li, R. T.; Bansleben, D. A.; Day, M. W.; Grubbs, R. H. Organometallics 1998, 17, 3149–3151. (b) Younkin, R. T.; Connor, F. E.; Henderson, I. J.; Friedrich, K. S.; Grubbs, H. R.; Bansleben, A. D. Science 2000, 287, 460–462. (c) Matsui, S.; Tohi, Y.; Mitani, M.; Saito, J.; Makio, H.; Tanaka, H.; Nitabaru, M.; Nakano, T.; Fujita, T. Chem. Lett. 1999, 28, 1065–1066. (2) Keim, W.; Schulz, R. P. J. Mol. Catal. 1994, 92, 21–33. (3) (a) Keim, W.; Kowaldt, F. H.; Goddard, R.; Kruger, C. Angew. Chem., Int. Ed. Engl. 1978, 17, 466–467. (b) Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123–134. (4) (a) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I. Chem. Commun. 2002, 744–745. (b) Luo, S.; Vela, J.; Lief, G. R.; Jordan, R. F. J. Am. Chem. Soc. 2007, 129, 8946–8947. (c) Weng, W.; Shen, Z.; Jordan, R. F. J. Am. Chem. Soc. 2007, 129, 15450–15451. (d) Kochi, T.; Yoshimura, K.; Nozaki, K. Dalton Trans. 2006, 25–27. (e) Kochi, T.; Noda, S.; Yoshimura, K.; Nozaki, K. J. Am. Chem. Soc. 2007, 129, 8948–8949. (f) Skupov, K. M.; Marella, P. R.; Simard, M.; Yap, G. P. A.; Allen, N.; Conner, D.; Goodall, B. L.; Claverie, J. P. Macromol. Rapid Commun. 2007, 28, 2033–2038. (g) Borkar, S.; Newsham, D. K.; Sen, A. Organometallics 2008, 27, 3331–3334. (h) Skupov, K. M.; Piche, L.; Claverie, J. P. Macromolecules 2008, 41, 2309–2310. (5) (a) Drent, E.; vsn Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I. Chem. Commun. 2002, 964–965. (b) Newsham, D. K.; Borkar, S.; Sen, A.; Corner, D. M.; Goodall, B. L. Organometallics 2007, 26, 3636–3638. (c) Hearley, A. K.; Nowack, R. J.; Rieger, B. Organometallics 2005, 24, 2775– 2763. (d) Bettucci, L.; Bianchini, C.; Claver, C.; Suarez, J. G.; Ruiz, A.; Meli, A.; Oberhauser, W. Dalton Trans. 2007, 5590–5602.

(6) (a) Kochi, T.; Nakamura, A.; Ida, H.; Nozaki, K. J. Am. Chem. Soc. 2007, 129, 7770–7771. (b) Nakamura, A.; Munakata, K.; Kochi, T.; Nozaki, K. J. Am. Chem. Soc. 2008, 130, 8128–8129. (7) (a) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122–3172. (b) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290– 1309. (c) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953–956. (d) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem. Soc. 1999, 121, 2674–2678. (8) (a) Herrmann, W. A.; Goossen, L. A.; Spiegler, M. Organometallics 1998, 17, 2162–2168. (b) Tsoureas, N.; Danopoulos, A. A.; Tulloch, Arran A. A.; Light, M. E. Organometallics 2003, 22, 4750–4758. (c) Ketz, B. E.; Cole, A. P.; Waymouth, R. M. Organometallics 2004, 23, 2835–2837. (d) Waltman, A. W.; Grubbs, R. H. Organometallics 2004, 23, 3105–3107. (e) Jahnke, M. C.; Pape, T.; Hahn, F. E. Eur. J. Inorg. Chem. 2009, 1960–1969. (f) Normand, A. T.; Cavell, K. J. Eur. J. Inorg. Chem. 2008, 2781–2800. (9) (a) Brown, M. K.; May, T. L.; Baxter, C. A.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2007, 46, 1097–1100.

r 2009 American Chemical Society

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Nagai et al. Scheme 2

Figure 1. Phosphine- and NHC-Sulfonate Complexes and Their Precursors. Scheme 1

Figure 2. ORTEP drawings of (a) 7a and (b) 7b. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. For 7a, the structure of one of two independent molecules in the unit cell is shown.

Single-crystal X-ray analysis of 7a,b revealed that the NHCsulfonate ligand coordinates to a palladium center in a bidentate fashion.

Results and Discussion Synthesis of Sulfomethylimidazoliums 3 and 4. Synthetic pathways for sulfomethylimidazoliums 3 and 4 are described in Scheme 1. Nucleophilic substitution on chloro(iodo)methane by N-arylimidazole, followed by recrystallization from CH2Cl2/toluene, provided N-aryl-N0 -halomethylimidazolium halide 8, a mixture of compounds containing chlorine or iodine atoms as the halogen (X and Y) atoms. In order to confirm the formation of the halomethylimidazolium core, all of the chloride ions in the mixture 8 were exchanged with iodide ions by treatment with sodium iodide, and pure N-aryl-N0 -iodomethylimidazolium iodide 9 was isolated. The structure of 9b was confirmed by X-ray diffraction analysis. The desired sulfomethylimidazolium salts 3 and 4 were obtained by the reaction of 9 with sodium bisulfate, and the molecular structure of 4 was also established by X-ray crystallography. Sulfomethylimidazolium salts 3 and 4 can also be obtained directly from 8 by treatment with sodium bisulfate.

Synthesis of Palladium Salts 7. First, direct synthesis of NHC-sulfonate/palladium complexes was attempted by deprotonation of imidazolium salt 3 in the presence of palladium precursors, but only complex mixtures were obtained (see Experimental Section). Alternatively, NHC silver salts were prepared and the carbene ligand was subsequently transferred to palladium (Scheme 2). A mixture of sulfomethylimidazolium 3 with silver oxide in chloroform provided NHC-silver complex 10 in 60% yield. Treatment of the silver complex 10 with chloro-bridged methylpalladium dimers10 [MePdCl(2,6-lutidine)]2 and [MePdCl(PPh3)]2 afforded NHC-palladium complexes 7a and 7b, respectively. The structures of complexes 7a and 7b were determined by single-crystal X-ray analysis (Figure 2). In both cases, the NHC-sulfonate ligand coordinates to the square-planar palladium center in a bidentate fashion, and the sulfonate locates at the trans position of the methyl group. 7a and 7b are the first complexes containing chelating bidentate NHCsulfonate ligands to metal centers. The X-ray structure of 7a is compared with its phosphinesulfonate analogue 2.4e The Pd-CH3 bond of 7a (1.976(7), 1.976(8) A˚) is shorter than that of 2 (2.107(3) A˚), and the Pd-O bond is elongated for the NHC complex 7a (2.231(5), 2.204(6) A˚ for 7a, 2.159(2) A˚ for 2). The lengths of the Pd-N bonds were similar and slightly shorter for 7a (2.128(7), 2.099(7) A˚ for 7a, 2.134(2) A˚ for 2), but it is hard to argue that the σ-donating ability of the NHC is actually lower than the phosphine, because there are other factors possibly affecting the structures. The larger steric bulk created by the 2-methoxyphenyl group on the phosphorus atom and the (10) Ladipo, F. T.; Anderson, G. K. Organometallics 1994, 13, 303– 306.

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ring strain in 7a consisting of an sp2 carbon atom rather than an sp3 phosphorus atom may contribute as well to the difference in the structures of 7a and 2.

Conclusion New sulfomethylimidazoliums 3 and 4 and NHC-sulfonate palladium complexes 7a,b were synthesized, and X-ray crystal structure analysis revealed that 7a,b are the first examples of chelating bidentate NHC-sulfonate complexes.

Experimental Section General Methods. All manipulations were carried out using the standard Schlenk technique under argon purified by passing through a hot column packed with BASF catalyst R3-11. 1H (500 MHz), 13C (126 MHz), and 31P NMR (202 MHz) spectra were recorded on a JEOL JNM-ECP-500 spectrometer. Elemental analysis was performed by the Microanalytical Laboratory, Department of Chemistry, Faculty of Science, The University of Tokyo. Preparation of N-(2,6-Diisopropylphenyl)-N0 -iodomethylimidazolium Iodide (9a). N-(2,6-Diisopropylphenyl)imidazole (212 mg, 0.928 mmol) and 50 equiv of chloroiodomethane (8.19 g, 46.4 mmol) were placed in a flask. The reaction mixture was stirred for 16 h at 80 °C. Volatiles were removed under vacuum, and the remaining residue was dissolved in CH2Cl2, reprecipitated by toluene to give 8a as a white solid, which was a mixture of chloro- and iodomethylimidazolium salts. The mixture (8a) and sodium iodide (1.34 g, 8.94 mmol) were placed in a flask and dissolved in 10 mL of acetone. The reaction mixture was stirred for 5 h at 80 °C. After removal of the solvent, the remaining residue was dissolved in CH2Cl2 and reprecipitated with hexane, giving 9a as a white solid (240 mg, 52%): 1H NMR (CDCl3) δ 1.17 (d, J=6.9 Hz, 6H, -CH(CH3)CH3), 1.27 (d, J=6.6 Hz, 6H, -CH(CH3)CH3), 2.35 (sept, J=6.6 Hz, 2H, -CH(CH3)2), 6.80 (s, 2H, I-CH2-N), 7.22-7.23 (m, 1H, N-CHdCH), 7.33 (d, J=8.0 Hz, 2H, Ar-H), 7.56 (t, J=8.0 Hz, 1H, Ar-H), 8.09-8.11 (m, 1H, N-CHdCH), 10.30-10.32 (m, 1H, N-CHdN); 13 C NMR (CDCl3) δ 11.4 (Ar-CH2-I), 24.3 (-CH(CH3)2), 24.5 ((-CH(CH3)2)), 28.8 (-CH(CH3)2), 124.2 (aromatic), 124.3 (aromatic), 124.9 (aromatic), 129.8 (aromatic), 132.3 (aromatic), 137.8 (aromatic), 145.2 (aromatic). Anal. Calcd for C16H22N2I2: C, 38.73; H, 4.47; N, 5.65. Found: C, 38.70; H, 4.49; N, 5.54. Preparation of N-(2,4,6-Trimethylphenyl)-N0 -iodomethylimidazolium Iodide (9b). The conditions and procedure for the synthesis of 9a were followed with N-(2,4,6-trimethylphenyl)imidazole (373 mg, 2.00 mmol) and chloroiodomethane (17.6 g, 99.8 mmol) to afford 206 mg (48%) of 9b: 1H NMR (CDCl3) δ 2.13 (s, 6H, Ar-CH3), 2.35 (s, 3H, Ar-CH3), 6.70 (s, 2H, I-CH2-N), 7.03 (s, 2H, Ar-H), 7.19-7.21 (m, 1H, N-CHdCH), 7.93-7.95 (m, 1H, N-CHdCH), 10.33-10.35 (m, 1H, N-CHdN); 13C NMR (CDCl3) δ 11.3 (Ar-CH2-I), 18.0 (Ar-CH3), 21.1 (Ar-CH3), 123.4 (aromatic), 124.2 (aromatic), 130.0 (aromatic), 130.3 (aromatic), 134.1 (aromatic), 137.7 (aromatic), 141.8 (aromatic). Anal. Calcd for C13H16N2I2: C, 34.39; H, 3.55; N, 6.17. Found: C, 34.22; H 3.52; N, 5.94. Preparation of N-(2,6-Diisopropylphenyl)-N0 -sulfomethylimidazolium (3). A mixture of 9a (221 mg, 0.445 mmol) and 10 equiv of sodium sulfite (561 mg, 4.45 mmol) in 10 mL of THF/water was stirred for 1 h at room temperature. After removal of the solvents, the remaining residue was extracted with methanol and reprecipitated from water to provide 3 as a white solid (96 mg, 67%): 1H NMR (DMSO-d6) δ 1.12 (d, J=6.9 Hz, 6H, -CH(CH3)CH3), 1.14 (d, J=6.9 Hz, 6H, -CH(CH3)CH3), 2.27 (sept, J=6.9 Hz, 2H, -CH(CH3)2), 5.05 (s, 2H, SO3-CH2-N), 7.45 (d, J = 7.8 Hz, 2H, Ar-H), 7.62 (t, J = 7.8 Hz, 1H, Ar-H), 8.00-8.02 (m, 1H, N-CHdCH), 8.03-8.04 (m, 1H,

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N-CHdCH), 9.54-9.56 (m, 1H, N-CHdN); 13C NMR (DMSO-d6) δ 23.7 (-CH(CH3)2), 23.9 (-CH(CH3)2), 28.0 (-CH(CH3)2), 63.1 (Ar-CH2-SO3), 124.1 (aromatic), 124.2 (aromatic), 124.4 (aromatic), 130.5 (aromatic), 131.4 (aromatic), 138.6 (aromatic), 145.1 (aromatic); HRMS (FAB) calcd for [M þ H]þ (C16H23N2O3S) m/z 323.1429, found 323.1380. Preparation of N-(2,4,6-Trimethylphenyl)-N0 -sulfomethylimidazolium (4). The conditions and procedure for the synthesis of 3 were followed with 9b (310 mg, 0.683 mmol) and sodium sulfite (860 mg, 6.82 mmol) to afford 122 mg (64%) of 9b: 1H NMR (DMSO-d6) δ 2.01 (s, 6H, Ar-CH3), 2.33 (s, 3H, Ar-CH3), 5.05 (s, 2H, SO3-CH2-N), 7.14 (s, 2H, Ar-H), 7.87-7.90 (m, 1H, N-CHdCH), 7.97-7.99 (m, 1H, N-CHdCH), 9.43-9.46 (m, 1H, N-CHdN); 13C NMR (DMSO-d6) δ 16.8 (Ar-CH3), 16.9 (Ar-CH3), 20.6 (Ar-CH3), 63.0 (Ar-CH2-SO3), 123.0 (aromatic), 124.3 (aromatic), 129.1 (aromatic), 129.3 (aromatic), 131.2 (aromatic), 134.2 (aromatic), 138.1 (aromatic), 138.3 (aromatic), 140.2 (aromatic); HRMS (FAB) calcd for [M þ H]þ (C13H17N2O3S) m/z 281.0960, found 281.0970. Attempted Synthesis of Pd Complex of N-(2,6-Diisopropylphenyl)-N0 -sulfomethylimidazolium with a Base and a Pd Source. In a Schlenk tube, 3 (14.0 mg, 0.043 mmol), 1 equiv of Pd source (PdMeCl(cod), Pd(OAc)2, or [PdMeCl(PPh3)]2), and 1 equiv of base (KHMDS, tBuOK, NaOAc) were placed, and 2.5 mL of solvent (THF, DMSO, benzene, or CHCl3) was added. The reaction mixture was stirred at room temperature, and the reaction was monitored by 1H NMR. In all cases, a complex mixture was detected possibly due to the multiple deprotonation from the imidazole ring. Preparation of N-(2,6-Diisopropylphenyl)-N0 -sulfomethylNHC Silver Complex (10). A mixture of 3 (226 mg, 0.700 mmol) and 3 equiv of silver oxide (490 mg, 2.10 mmol) was placed in a Schlenk tube and dissolved in 2.5 mL of chloroform. The reaction mixture was stirred for 18 h at 60 °C under dark. The reaction mixture was filtered through Celite and reprecipitated with hexane to give 10 as a white solid (181 mg, 60%). The solid was used in the subsequent step without further purification. 1H NMR (CDCl3) δ 0.70-1.12 (br m, 12H, -CH(CH3)CH3), 2.25-2.39 (br m, 2H, -CH(CH3)2), 5.53 (br s, 2H, SO3-CH2-N), 6.88 (br s, 1H, N-CHdCH), 7.00-7.17 (br m, 2H, Ar-H), 7.30-7.43 (br m, 1H, Ar-H), 7.75 (br s, 1H, N-CHdCH); 13C NMR (CDCl3) δ 23.1-25.6 (m, (-CH(CH3)2), 27.8 (-CH(CH3)2), 65.6 (Ar-CH2-SO3), 122.2-125.0 (m, aromatic), 129.0-130.7 (m, aromatic), 134.8 (aromatic), 145.8 (aromatic), 181.9-185.4 (m, N-C-N). A preliminary X-ray structural analysis also supports the formation of the NHC-sulfonate silver complex. Preparation of N-(2,6-Diisopropylphenyl)-N0 -sulfomethylNHC Palladium 2,6-Lutidine Complex (7a). Silver complex 10 (283 mg, 0.658 mmol of NHC) and 0.5 equiv of [(2,6-lutidine)PdMeCl]2 (174 mg, 0.329 mmol) were placed in a Schlenk tube, and 5 mL of chloroform was added. After stirring for 1 h at room temperature, the reaction mixture was filtered through Celite, and reprecipitation with hexane gave 8a as a white solid (154 mg, 42% (26% yield from 3)): 1H NMR (CDCl3) δ -0.15 (s, 3H, Pd-CH3), 1.08 (d, J=5.9 Hz, 6H, -CH(CH3)CH3), 1.37 (d, J=6.0 Hz, 6H, -CH(CH3)CH3), 2.90 (sept, J=6.0 Hz, 2H, -CH(CH3)2), 3.00 (s, 6H, C5H3N(CH3)2), 5.40 (s, 2H, SO3-CH2-N), 6.91 (d, J=1.8 Hz, 1H, N-CHdCH), 7.02 (d, J=7.5 Hz, 2H, Ar-H), 7.27-7.29 (m, 3H, N-CHdCH and Ar-H), 7.46 (t, J=7.8 Hz, 1H, Ar-H), 7.49 (t, J=7.5 Hz, 1H, Ar-H); 13C NMR (CDCl3) δ -13.2 (Pd-CH3), 22.6 (CH3), 25.8 (CH3), 26.5 (CH3), 28.5 (CH), 66.0 (Ar-CH2SO3), 122.3 (aromatic), 122.6 (aromatic), 123.7 (aromatic), 124.5 (aromatic), 130.1 (aromatic), 135.2 (aromatic), 138.0 (aromatic), 145.9 (aromatic), 159.0 (aromatic), 171.3 (N-C-N); HRMS (FAB) calcd for [M þ H]þ (C24H34N3O3PdS) m/z 550.1356, found 550.1349. Anal. Calcd for C24H33N3O3PdS 3 0.10CHCl3: C, 51.51; H, 5.94; N, 7.48. Found: C, 51.45; H 5.96; N, 7.03. The amount of CHCl3 contained in the sample was determined by 1H NMR analysis in CD2Cl2. Crystals suitable for X-ray diffraction analysis were obtained by recrystallization with CHCl3/hexane.

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Preparation of N-(2,6-Diisopropylphenyl)-N0 -sulfomethylNHC Palladium Triphenylphosphine Complex (7b). This complex was prepared by the same method as for 7a starting from silver complex 10 (48.6 mg, 0.113 mmol) and 0.5 equiv of [(PPh3)PdMeCl]2 (47.6 mg, 0.0568 mmol) to obtain 7b as a white solid (58 mg, 73% (44% yield from 3)): 1H NMR (CDCl3) δ -0.14 (d, J = 6.9 Hz, 3H, Pd-CH3), 1.10 (d, J=6.6 Hz, 6H, -CH(CH3)CH3), 1.32 (d, J= 6.6 Hz, 6H, -CH(CH3)CH3), 2.73 (sept, J=6.6 Hz, 2H, -CH(CH3)2), 5.33 (s, 2H, SO3-CH2-N), 6.95 (s, 1H, N-CHdCH), 7.24-7.48 (m, 19H, Ar-H and N-CHdCH); 13C NMR (CDCl3) δ -7.08 (Pd-CH3), 23.5 (CH3), 25.7 (CH3), 28.5 (CH), 65.7 (Ar-CH2-SO3), 122.5 (aromatic), 123.7-124.0 (m, aromatic), 128.4 (d, J = 10.7 Hz, aromatic), 129.9-130.5 (m, aromatic), 134.3-134.7 (m, aromatic), 146.0 (aromatic), 179.4 (d, J=140.1 Hz, N-C-N); 31P NMR (CDCl3) δ 27.3; HRMS (FAB) calcd for [M þ H]þ (C35H40N2O3PPdS) m/z 705.1532, found 705.1552. Anal. Calcd for C35H39N2O3PPdS: C, 59.61; H, 5.57; N, 3.95.

Nagai et al. Found: C, 59.24; H 5.59; N, 3.41. Crystals suitable for X-ray diffraction analysis were obtained by recrystallization with CH2Cl2/hexane.

Acknowledgment. This work was supported by KAKENHI (No. 21245023) from MEXT, Japan. We are grateful to Drs. Y. Nishibayashi and Y. Miyake (The Univ. of Tokyo) for high-resolution FAB-MS analysis and to Drs. Makoto Yamashita (The Univ. of Tokyo), N. Tokitoh and T. Sasamori (Kyoto Univ.) for X-ray analysis. Supporting Information Available: 1H and 13C NMR spectra of new compounds and X-ray crystallographic data of 4, 7a, 7b, and 9b (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.