Synthesis of Oxygen-and Sulfur-Bridged Dirhodium Complexes and

Apr 9, 2010 - Tire Material Development, Bridgestone Corporation, Tokyo 187-8531, Japan. Received January 27, 2010. Oxygen-bridged and sulfur-bridged ...
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Organometallics 2010, 29, 2098–2103 DOI: 10.1021/om100067r

Synthesis of Oxygen- and Sulfur-Bridged Dirhodium Complexes and Their Use As Catalysts in the Chemoselective Hydrogenation of Alkenes Chuan Zhu, Noriaki Yukimura,† and Motoki Yamane* Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore. †Current address: Tire Material Development, Bridgestone Corporation, Tokyo 187-8531, Japan. Received January 27, 2010

Oxygen-bridged and sulfur-bridged rhodium homobimetallic complexes were synthesized as airstable crystals by using 2,6-bis(phosphanylmethyl)phenolate and -thiophenolate as the ligands, respectively. The oxygen-bridged dirhodium complex has a symmetrical structure where the carbon atom at the ipso position, oxygen, and two rhodium atoms are located in the same plane. It is thermally stable compared to the sulfur-bridged dirhodium complex and shows catalytic activity for hydrogenation of alkenes with high chemoselectivity. Introduction Bimetallic catalysts have attracted synthetic organic chemists because it is expected that two metal centers in the same molecule cooperatively take part in a reaction and achieve an efficient or novel chemical transformation.1 Many reactions involving bimetallic catalysis have been reported; however, bimetallic catalysis that surpasses conventional monometallic catalysis is still rare, and we need more efficient reactions that are applicable for practical organic synthesis. We have studied heteroatom-bridged bimetallic catalysts2,3 because it is supposed that the heteroatom captures and brings the two metal atoms close to each other, and the electronic properties of one of the metal centers can be controlled by the other *Corresponding author. Tel: (þ65) 6513 8014. Fax: (þ65) 6791 1961. E-mail: [email protected]. (1) For the review of bimetallic catalysis, see: (a) Multimetallic Catalysts in Organic Synthesis; Shibasaki, M.; Yamamoto, Y., Eds.; WielyVCH Verlag GmbH & Co. KGaA: Weinheim, 2004. Braunstein, P.; Rose, J. Catalysis and Related Reactions with Compounds Containing Heteronuclear Metal-Metal Bonds. In Comprehensive Organometallic Chemistry II; Abel, E. W.; Willkinson, G.; Stone, F. G. A., Eds.; Elsevier: Oxford, U.K., 1995, Vol. 10, p 351. (b) Wheatley, N.; Kalck, P. Chem. Rev. 1999, 99, 3379. For a recent article, see: (c) Tsukada, N.; Mitsuboshi, T.; Setoguchi, H.; Inoue, Y. J. Am. Chem. Soc. 2003, 125, 12102. (d) Parimala, S.; Kandaswamy, M. Inorg. Chem. Commun. 2003, 6, 1252. (e) Tsukada, N.; Setoguchi, H.; Mitsuboshi, T.; Inoue, Y. Chem. Lett. 2006, 35, 1164. (f) Man, M. L.; Lam, K. C.; Sit, W. N.; Ng, S. M.; Zhou, Z.; Lin, Z.; Lau, C. P. Chem.;Eur. J. 2006, 12, 1004. (g) Chen, Q.; Yu, J.; Huang, J. Organometallics 2007, 26, 617. (h) Biffis, A.; Lobbia, G. G.; Papini, G.; Pellei, M.; Santini, C.; Scattolin, E.; Tubaro, C. J. Organomet. Chem. 2008, 693, 3760. (i) Rodriguez, B. A.; Delferro, M.; Marks, T. J. Organometallics 2008, 27, 2166. (j) Singh, A. K.; Dwivedi, S. D.; Dubey, S. K.; Singh, S. K.; Sharma, S.; Pandey, D. S.; Zou, R.-K.; Xu, Q. J. Organomet. Chem. 2009, 694, 3084. (k) Fang, F.; Xie, F.; Yu, H.; Zhang, H.; Yang, B.; Zhang, W. Tetrahedron Lett. 2009, 50, 6672. (2) (a) Loose, C.; Ruiz, E.; Kersting, B.; Kortus, J. Chem. Phys. Lett. 2008, 452, 38. (b) Punji, B.; Mague, J. T.; Balakrishna, M. S. Inorg. Chem. 2007, 46, 11316. (c) Sarkar, S.; Mondal, A.; Banerjee, A.; Chopra, D.; Ribas, J.; Rajak, K. K. Polyhedron 2006, 25, 2284. (d) Konno, T.; Usami, M.; Toyota, A.; Hirotsu, M.; Kawamoto, T. Chem. Lett. 2005, 34, 1046. (e) Xie, Y.; Jiang, H.; Chan, A. S.-C.; Liu, Q.; Xu, X.; Du, C.; Zhu, Y. Inorg. Chim. Acta 2002, 333, 138. (f) Palmer, M. S.; Harris, S. Organometallics 2000, 19, 2114. (3) Yamane, M.; Yukimura, N.; Ishiai, H.; Narasaka, K. Chem. Lett. 2006, 35, 540. (4) (a) Nakamura, E.; Yoshikai, N; Yamanaka, M. J. Am. Chem. Soc. 2002, 124, 7181. (b) Nishibayashi, Y.; Milton, M. D.; Inada, Y.; Yoshikawa, M.; Wakiji, I.; Hidai, M.; Uemura, S. Chem.;Eur. J. 2005, 11, 1433. pubs.acs.org/Organometallics

Published on Web 04/09/2010

Scheme 1. Heteroatom-Bridged Bimetallic Complex

metal center by donating or accepting electrons through the heteroatom (Scheme 1).4 For the metal center, we have been interested in rhodium because many unique reactions including activation of inert bonds were recently reported.5 In this paper, we describe the design and preparation of oxygen- or sulfur-bridged dirhodium complexes and their application for chemoselective hydrogenation of alkenes.

Results and Discussion Design and Preparation of Heteroatom-Bridged Dirhodium Complexes. We chose 2,6-bis(phosphanylmethyl)phenol 2 and -thiophenol 3 as the ligands and designed dirhodium complexes as depicted in Figure 1.6 The tridentate ligands are expected to coordinate to rhodium centers strongly from one side, making the structure of the complex rigid and leaving the opposite side free for reactions. Bis[(diphenylphosphanyl)methyl]phenol (2) was prepared from commercially available 2,6-dimethylphenol (4) in four steps (Scheme 2). The hydroxy group was protected by silylation (5) For the review of rhodium-catalyzed reactions, see: (a) Fagnou, K.; Lautens, M. Chem. Rev. 2003, 103, 169. (b) Lewis, J. C.; Bergman, R. G.; Ellman, J. A. Acc. Chem. Res. 2008, 41, 1013. For recent articles, see: (c) Yamane, M.; Uera, K.; Narasaka, K. Chem. Lett. 2004, 33, 424. (d) Yamane, M.; Uera, K.; Narasaka, K. Bull. Chem. Soc. Jpn. 2005, 78, 477. (e) Joo, J. M.; Yuan, Y.; Lee, C. J. Am. Chem. Soc. 2006, 128, 14818. (f) Cook, M. J.; Rovis, T. J. Am. Chem. Soc. 2007, 129, 9302. (g) Osborne, J. D.; Randell-Sly, H. E.; Currie, G. S.; Cowley, A. R.; Willis, M. C. J. Am. Chem. Soc. 2008, 130, 17232. (h) Phan, D. H. T.; Kim, B.; Dong, V. M. J. Am. Chem. Soc. 2009, 131, 15608. (i) Hojo, D.; Noguchi, K.; Tanaka, K. Angew. Chem., Int. Ed. 2009, 48, 8129. (j) Shibata, Y.; Tanaka, K. J. Am. Chem. Soc. 2009, 131, 12552. (6) For sulfur-bridged dirhodium complexes with the same ligand, see: Dilworth, J. R.; Zheng, Y.; Griffths, D. V. J. Chem. Soc., Dalton. Trans. 1999, 1877. r 2010 American Chemical Society

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Scheme 4. Preparation of O- and S-Bridged Dirhodium Complexes

Figure 1. Designed O- and S-bridged dirhodium complexes. Scheme 2. Preparation of O-Ligand 2a

a (a) Me2tBuSiCl, imidazole, DMF, 50 °C, 4 h, 5 86%, (b) cat. AIBN, NBS, CCl4, reflux, 8 h, 6 48%, (c) Ph2PH 3 BH3, cat. nBu4NBr, KOH, toluene-H2O, 50 °C, 7 h, 7 81%, (d) DABCO, 50 °C, 12 h, 2 78%.

Scheme 3. Preparation of S-Ligand 3a

a (a) Ph2PH 3 BH3, cat. nBu4NBr, KOH, toluene-H2O, rt, 16 h, 9 81%, (b) Et2NH, 50 °C, 4 h, 10 95%, (c) nBuLi then S8, THF, -78 °C, 12 h, 11 80%, (d) nBu3P, 195 °C, 3 h, 3 39%.

to give silyl ether 5. Radical dibromination afforded dibromide 6 in 41% yield in two steps. Introduction of a phosphanyl group was performed by using Ph2PH 3 BH3 with a base,7 and borane parts were removed by heating with DABCO,8 to give phenol 2 in 63% yield in two steps. Bis[(diphenylphosphanyl)methyl]thiophenol (3) is a known compound and was prepared according to the literature6 with modification (Scheme 3). Two phosphanyl groups were introduced by using Ph2PH 3 BH3, and borane parts were removed by heating with diethylamine9 to give bisphosphane 2 in 77% yield in two steps. Lithium-bromine exchange and successive treatment with S8 gave bis(phosphane sulfide), and successive desulfurization10 by nBu3P afforded tridentate ligand 3 in 31% yield in two steps. With tridentate ligands in hand, we tried complexation with [RhCl(cod)2] (Scheme 4). To a solution of O-tridentate ligand 2 in THF, NaH and [RhCl(cod)2] were added at 0 °C. After stirring for 4 h at room temperature, AgClO4 was added for counteranion exchange. The crude material was purified by column chromatography on silica gel to afford the desired O-bridged dirhodium complex μ-O-1 in 70% yield. The same procedure was applied with tridentate ligand 3 and gave μ-S-1 in 43% yield. Both complexes were obtained as yellow air-stable crystals that were suitable for X-ray crystallographic analysis. The ORTEP diagrams of O- and S-bridged dirhodium complexes μ-O-1 and μ-S-1 are shown in Figures 2 and 3, respectively. X-ray diffraction of complex μ-O-1 shows a quite symmetrical structure. The sum of three angles, Rh(1)-O(1)-Rh(2) 126.1°, C(1)-O(1)-Rh(1) 116.4°, and C(1)-O(1)-Rh(2) 117.4°, is (7) (a) Imamoto, T.; Oshiki, T.; Onozawa, T.; Kusumoto, T.; Sato, K. J. Am. Chem. Soc. 1990, 112, 5244. (b) Wyatt, P.; Eley, H.; Charmant, J.; Daniel, B. J.; Kantacha, A. Eur. J. Org. Chem. 2003, 4216. (c) Clark, T. J.; Rodezno, J. M.; Clendenning, S. B.; Aouba, S.; Brodersen, P. M.; Lough, A. J.; Ruda, H. E.; Manners, I. Chem.;Eur. J. 2005, 11, 4526. (8) Anderson, B. J.; Glueck, D. S.; DiPasquale, A. G.; Rheingold, A. L. Organometallics 2008, 27, 4992. (9) Speiser, F.; Braunstein, P. Organometallics 2004, 23, 2625. (10) Gorla, F.; Venanzil, L. M. Organometallics 1994, 13, 43.

almost planar (359.9°), which means the structure around oxygen is distorted trigonal planar. Rh(1)-O(1) and Rh(2)-O(1) bond distances of 2.1237(19) and 2.1191 A˚ are almost identical and similar to data of reported μ-O dirhodium complexes.11 On the contrary, the X-ray diffraction of complex μ-S-1 reveals its less symmetrical structure. The sum of three angles, Rh(1)-S(1)-Rh(2) 123.7°, C(1)-S(1)-Rh(1) 98.4°, and C(1)-S(1)-Rh(2) 118.0°, is 340.1°, and the structure around sulfur is distorted tetrahedral. Application for Chemoselective Hydrogenation of Alkenes. We examined the catalytic ability of the synthesized bimetallic complexes μ-O-1 and μ-S-1 by analyzing its catalytic effects on the hydrogenation of alkenes.12 We found that both complexes μ-O-1 and μ-S-1 had good catalytic activity for the hydrogenation of monosubstituted alkenes. When 1-(3-butenyl)naphthalene 12a was treated with a catalytic amount of μ-O-1 under the atmospheric pressure of H2, reduced product 13a was obtained in 96% yield (eq 1). To confirm whether the μ-Obimetallic structure was maintained during the reaction, monitoring by 31P NMR analysis was conducted. For both analyses during the reaction (2 h) and after the reaction (6 h), only the doublet peak corresponding to the phosphorus atom in μ-O-1 was observed (δ (ppm) = 46 (d, 1J31P-103Rh = 165 Hz)). This observation suggests that the bimetallic structure is rigid and would not release monometallic complexes. Compared to complex μ-O-1, complex μ-S-1 shows less stability under the hydrogenation conditions. Although the reduction proceeded and alkane 13a was obtained in good yield, precipitation of rhodium black was observed during the reaction (eq 1). Formation of cyclooctane and cyclooctene was confirmed by 1H NMR analysis when μ-S-1 was dissolved in CDCl3 under an atmospheric pressure of H2. This hydrogenation of the ligand cyclooctadiene (COD) generated unstable dirhodium intermediates that gradually decomposed to form rhodium black. Meanwhile, μ-O-1 was found stable even in solution in the presence of H2, and no hydrogenation of ligand was observed.

Complex μ-O-1 was also active for hydrosilylation of a terminal alkene.13 The reaction of alkene 12a and (11) Fryzuk, M. D.; Jang, M.-L. Can. J. Chem. 1986, 64, 174. (12) For reviews of transition metal-catalyzed hydrogenation of alkenes, see: (a) Chessum, N.; Couty, S.; Jones, K. Reduction by Heterogeneous Catalysis. In Science of Synthsis; Hiemstra, H., Ed.; Georg Thieme Verlag: Stuttgart, NY, 2002; Vol. 48, p 275. (b) Tsukamoto, M.; M. Kitamura, M. Reduction by Homogeneous Catalysis or Biocatalysis. In Science of Synthsis; Hiemstra, H., Ed.; Georg Thieme Verlag: Stuttgart, NY, 2002; Vol. 48, p 341. (c) Diesen, J. S.; Andersson, P. G. Hydrogenation of Unfunctionalized Alkenes. In Modern Reduction Methods; Andersson, P. G.; Munslow, I. J., Eds.; Wiely-VCH Verlag GmbH & Co. KGaA: Weinheim, 2008; p 39. (13) For review of hydrosilylation, see: Mayes, P. A.; Perlmutter, P. Alkene Reduction: Hydrosilylation. In Modern Reduction Methods; Andersson, P. G.; Munslow, I. J., Eds.; Wiely-VCH Verlag GmbH & Co. KGaA: Weinheim, 2008; p 87.

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Figure 2. ORTEP drawing of μ-O-1. All hydrogen atoms, solvent molecule, and counteranion are omitted for clarity. Key atoms are labeled. Selected bond lengths (A˚) and angles (deg): P(1)-Rh(1) 2.2743(9), P(2)-Rh(2) 2.2791(9), O(1)-Rh(1) 2.1237(19), O(1)-Rh(2) 2.1191(19), C(1)-O(1) 1.370(3); Rh(1)-O(1)-Rh(2) 126.14(9), C(1)-O(1)-Rh(1) 116.47(16), C(1)-O(1)-Rh(2) 117.39(15), P(1)-Rh(1)-O(1) 88.34(6), P(2)-Rh(2)-O(1) 88.62(6), C(6)-C(7)-P(1) 106.83(19), C(2)-C(8)-P(2) 108.14(19).

Figure 3. ORTEP drawing of μ-S-1. All hydrogen atoms, solvent molecule, and counteranion are omitted for clarity. Key atoms are labeled. Selected bond lengths (A˚) and angles (deg): P(1)-Rh(1) 2.3315(14), P(2)-Rh(2) 2.2851(14), S(1)-Rh(1) 2.3549(13), S(1)-Rh(2) 2.3178(13), C(1)-S(1) 1.780(4); Rh(1)-S(1)-Rh(2) 123.79(5), C(1)-S(1)-Rh(1) 98.43(14), C(1)-S(1)-Rh(2) 118.01(14), P(1)-Rh(1)-S(1) 83.34(5), P(2)-Rh(2)-S(1) 91.65(5), C(6)-C(7)-P(1) 111.6(3), C(2)-C(8)-P(2) 115.3(3).

dimethyl(phenyl)silane gave silane 14 in 39% yield along with internal alkenes that were obtained by the alkene isomerization of 12a (34%) (eq 2).

When we mixed μ-O-1 with phenylacetylene, the terminal sp-C-H bond was activated and polymerization proceeded to give all-trans polyacetylene as the whole product (eq 3).14

We tried the reactions with a variety of compounds, such as aryl iodide and acid anhydride; unfortunately, complex

μ-O-1 did not show any reactivity toward them. In conclusion, complex μ-O-1 seemed to be reactive only to compounds that had a simple chemical bond involving a hydrogen atom: H-H, Si-H, sp-C-H. The above trials and failures led us to think of chemoselective hydrogenation.15 If the catalyst μ-O-1 does not affect any other functional groups but H2, chemoselective hydrogenation of functionalized alkenes is possible. With such an idea, we examined μ-O-1-catalyzed hydrogenation of styrenes with a functional group that is normally affected in the conventional hydrogenation conditions. The results of the reaction are summarized in Table 1. Oxidative addition of aryl bromide to a low-valent transition metal is an important key step for many kinds of (14) Yashima, E.; Matsushima, T.; Okamoto, Y. J. Am. Chem. Soc. 1997, 119, 6345.

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Table 1. Chemoselective Hydrogenation of Substituted Styrene

entry

R

1 2 3 4 5

Br I NO2 OCH2Ph -CtC-SiMe3

12b 12c 12d 12e 12f

temp

time (h)

yield (%)

rt 50 °C rt rt rt

20 60 22 27 22

98 97 98 99 83

transition metal-catalyzed reactions such as cross-coupling reactions.16 Interestingly, μ-O-1-catalyzed hydrogenation of alkene did not affect the aryl bromide functionality (entry 1).15a-d Surprisingly, aryl iodide functionality was also found to be tolerant in the reaction conditions. This is a very rare example of chemoselective hydrogenation of alkenes that possess an sp2-C-I bond in the molecule (entry 2).15f Chemoselective hydrogenation was also achieved for styrenes that have a nitro,15d-f,17 benzyloxy,15f-j,18 or alkynyl group and gave the corresponding alkanes in satisfactory yields (entries 3-5). The allyl group is used for protecting hydroxy compounds such as alcohol and carboxylic acid.19 Deprotection by a transition metal catalyst is one of the convenient ways to remove the allyl moiety. However, μ-O-1-catalyzed hydrogenation was (15) For chemoselective hydrogenation, see: (a) Ikawa, T.; Sajiki, H.; Hirota, K. Tetrahedron 2005, 61, 2217. (b) Sajiki, H.; Ikawa, T.; Yamada, H.; Tsubouchi, K.; Hirota, K. Tetrahedron Lett. 2003, 44, 171. (c) Kitamura, Y.; Tanaka, A.; Sato, M.; Oono, K.; Ikawa, T.; Maegawa, T.; Monguchi, Y.; Sajiki, H. Synth. Commun. 2007, 37, 4381. (d) Burk, M. J.; Gerlach, A.; Semmeril, D. J. Org. Chem. 2000, 65, 8933. (e) Amer, I.; Bravdo, T.; Blum, J. Tetrahedron Lett. 1987, 28, 1321. (f) Jourdant, A.; Gonzalez-Zamora, E.; Zhu, J. J. Org. Chem. 2002, 67, 3163. (g) Sajiki, H.; Hirota, K. Tetrahedron 1998, 54, 13981. (h) Sajiki, H.; Hattori, K.; Hirota, K. J. Org. Chem. 1998, 63, 7990. (i) Maki, S.; Okawa, M.; Matsui, R.; Hirano, T.; Niwa, H. Synlett 2001, 1590. (j) Callis, N. M.; Thiery, E.; Bras, J. L.; Muzart, J. Tetrahedron Lett. 2007, 48, 8128. (k) Ghosh, A. K.; Krishnan, K. Tetrahedron Lett. 1998, 39, 947. (l) Misiti, D.; Zappia, G.; Monache, G. D. Synthesis 1999, 873. (m) Maki, S.; Harada, Y.; Hirano, T.; Niwa, H.; Yoshida, Y.; Ogata, S.; Nakamatsu, S.; Inoue, H.; Iwakura, C. Synth. Commun. 2000, 30, 3575. (n) Shinada, T.; Hayashi, K.; Yoshida, Y.; Ohfune, Y. Synlett 2000, 1506. (o) Hattori, K.; Sajiki, H.; Hirota, K. Tetrahedron 2000, 56, 8433. (p) Maki, S.; Harada, Y.; Matsui, R.; Okawa, M.; Hirano, T.; Niwa, H.; Koizumi, M.; Nishiki, Y.; Furuta, T.; Inouec, H.; Iwakura, C. Tetrahedron Lett. 2001, 42, 8323. (q) Maki, S.; Okawa, M.; Makii, T.; Hirano, T.; Niwa, H. Tetrahedron Lett. 2003, 44, 3717. (r) Sajiki, H.; Ikawa, T.; Hirota, K. Tetrahedron Lett. 2003, 44, 8437. (s) Bras, J. L.; Mukherjee, D. K.; Gonzalez, S.; Tristany, M.; Ganchegui, B.; Moreno-Ma~nas, M.; Pleixats, R.; Henin, F.; Muzart, J. New J. Chem. 2004, 28, 1550. (t) Miyazaki, Y.; Hagio, H.; Kobayashi, S. Org. Biomol. Chem. 2006, 4, 2529. (u) Mori, A.; Miyakawa, Y.; Ohashi, E.; Haga, T.; Maegawa, T.; Sajiki, H. Org. Lett. 2006, 8, 3279. (v) Mori, A.; Mizusaki, T.; Miyakawa, Y.; Ohashi, E.; Haga, T.; Maegawa, T.; Monguchi, Y.; Sajiki, H. Tetrahedron 2006, 62, 11925. (w) Sajiki, H.; Hattori, K.; Hirota, K. Chem.;Eur. J. 2006, 6, 2200. (x) Erathodiyil, N.; Ooi, S.; Seayad, A. M.; Han, Y.; Lee, S. S.; Ying, J. Y. Chem.;Eur. J. 2008, 14, 3118. (y) Akao, A.; Sato, K.; Nonoyama, N.; Mase, T.; Yasuda, N. Tetrahedron. Lett. 2006, 47, 969. (z) Corma, A.; Gonza-Arellano, G.; Iglesias, M.; Sanchez, F. Appl. Catal., A 2009, 356, 99. (16) (a) Metal-Catalyzed Cross-Coupling Reactions; De Meijere, A.; Diederich, F., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2004. (b) Buchwald, S. L. Acc. Chem. Res. 2008, 41, 1439. (17) The nitro group is reduced in the normal hydrogenation conditions using palladium complexes as the catalysts; see: (a) Erion, M. D.; Dang, Q.; Reddy, M. R.; Kasibhatla, S. R.; Huang, J.; Lipscomb, W. N.; Van Poelje, P. D. J. Am. Chem. Soc. 2007, 129, 15480. (b) Takasaki, M.; Motoyama, Y.; Higashi, K.; Yoon, S.-H.; Mochida, I.; Nagashima, H. Org. Lett. 2008, 10, 1601. (18) The benzyl group is used as the protecting group for hydroxy compounds and easily removed in hydrogenation conditions; see: (a) Buchi, G.; Weinreb, S. M. J. Am. Chem. Soc. 1971, 93, 746. (b) Barrero, A. F.; AlvarezManzaneda, E. J.; Chahboun, R. Tetrahedron Lett. 1997, 38, 8101. (19) (a) Dufour, M.; Gramain, J.-C.; Husson, H.-P.; Sinibaldi, M.-E.; Troin, Y. Tetrahedron Lett. 1989, 30, 3429. (b) Ziegler, F. E.; Brown, E. G.; Sobolov, S. B. J. Org. Chem. 1990, 55, 3961. (c) Mereyala, H. B.; Guntha, S. Tetrahedron Lett. 1993, 34, 6929.

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so specific to alkenes that the allyl group was reduced without cleavage of the C-O bond.15a,b,j,n,u,v,x For example, benzyl allyl carbonate 12g was subjected to the hydrogenation conditions, and benzyl propyl carbonate 13g was obtained in 98% yield (eq 4). The carbonyl group of aldehyde 12h was also tolerant of chemoselective hydrogenation, giving propyloxybenzaldehyde 13h in 97% yield (eq 5).

Conclusion Air-stable O- and S-bridged dirhodium complexes μ-X-1 (X = O, S) were designed and synthesized by using bis[(diphenylphosphanyl)methyl]phenol and -thiophenol, respectively. Complex μ-O-1 has a symmetrical structure and shows catalytic activity for chemoselective hydrogenation of alkenes.

Experimental Section General Methods. All reactions were performed under a nitrogen or argon atmosphere. Solvents (toluene, tetrahydrofuran, Et2O) were purified with a PS-MD-5 slovent purification system (Innovative Technology, Inc.) prior to use. The compounds [RhCl(cod)]2,20 3,6 5,21 6,21 9,6 10,6 and 116 were prepared by literature methods. 1H, 13C, and 31P NMR spectra were recorded on 400 and 500 MHz spectrometers. The NMR spectra were recorded in CDCl3. The chemical shift are reported in ppm against TMS (δ = 0) for 1H and the residual solvent signal (using CDCl3, δ = 77) for 13C as internal standard, and 85% H3PO4 (δ = 0) for 31P as external standard. Melting points were measured in sealed glass tubes without correction. tert-Butyl(2,6-dimethylphenoxy)dimethylsilane (5). 21 To a solution of 2,6-dimethylphenol (7.50 g, 61.4 mol) and imidazole (5.81 g, 85.4 mmol) in DMF (250 mL) at 0 °C was added t BuMe2SiCl (11.7 g, 77.7 mmol). The solution was stirred for 12 h at 50 °C. Then the solution was quenched with saturated aqueous NH4Cl, and the organic layer was extracted with Et2O. The combined organic extracts were washed with water and dried over MgSO4. The solvent was removed under reduced pressure, and the residue was purified by column chromatography on silica gel (hexane-ethyl acetate, 24:1) to afford tert-butyl(2,6-dimethylphenoxy)dimethylsilane as colorless oil (13.6 g, 57.4 mmol) in 93% yield.: 1H NMR (400 MHz, CDCl3) δ 0.19 (6H, s), 1.03 (9H, s), 2.12 (6H, s), 6.79 (1H, t, J = 7.6 Hz), 6.96 (2H, d, J = 7.6 Hz). [2,6-Bis(bromomethyl)phenoxy](tert-butyl)dimethylsilane (6).21 To a CCl4 solution (90 mL) of tert-butyl(2,6-dimethylphenoxy)dimethylsilane (13.6 g, 57.4 mol) were added NBS (20.8 g, 114.9 mmol) and a catalytic amount of AIBN (0.94 g, 5.7 mmol), and the mixture was refluxed for 6 h. Then the solution was filtered by Celite and the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel (hexane-ethyl acetate, 19:1) to afford [2,6-bis(bromomethyl)phenoxy](tert-butyl)dimethylsilane as colorless oil (10.9 g, 27.6 mol) in (20) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1979, 19, 218. (21) Weng, X.; Ren, L.; Weng, L.; Huang, J.; Zhu, S.; Zhou, X.; Weng, L. Angew. Chem., Int. Ed. 2007, 46, 8020.

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48% yield: 1H NMR (500 MHz, CDCl3) δ 0.29 (6H, s), 1.09 (9H, s), 4.52 (4H, s), 6.99 (1H, t, J = 7.6 Hz), 7.37 (2H, d, J = 7.6 Hz); 13 C NMR (125 MHz, CDCl3) δ -3.4, 18.9, 26.0, 29.2, 122.5, 129.2, 132.3, 150.8. Anal. Found: C, 42.72; H, 5.63. Calcd for C14H22Br2OSi: C, 42.65; H, 5.62. 2,6-Bis[(boronatediphenylphosphanyl)methyl]phenol (7). To a stirred solution of [2,6-bis(bromomethyl)phenoxy](tert-butyl)dimethylsilane (4.14 g, 10.5 mmol) in toulene-H2O (200 mL, 1:1) were added diphenylphosphine-borane (4.46 g, 22.3 mmol), KOH (100 g, 1.53 mol), and a catalytic amount of tetrabutylammonium bromide (338 mg, 1.05 mmol). The solution was stirred for 7 h at 50 °C. Then the solution was quenched with saturated aqueous NH4Cl, and the organic layer was extracted with Et2O. The combined organic extracts were washed with water and dried over MgSO4. The solvent was removed under reduced pressure, and the residue was purified by column chromatography on silica gel (hexane-ethyl acetate, 7:1) to afford 2,6-bis[(boronatediphenylphosphanyl)methyl]phenol as white crystals (4.68 g, 9.0 mmol) in 86% yield: mp 148-150 °C (dec); IR (ZnSe) 3419, 2382, 1589, 1464, 1437, 1107, 1057, 904, 860, 723 cm-1; 1H NMR (500 MHz, CDCl3) δ 1.00 (6H, br), 3.65 (4H, d, J = 12.1 Hz), 5.82 (1H, brs), 6.49 (1H, t, J = 7.6 Hz), 6.65 (2H, dt, J = 1.9, 7.6 Hz), 7.38-7.41 (8H, m), 7.45-7.48 (4H, m), 7.61-7.65 (8H, m); 13C NMR (125 MHz, CDCl3) δ = 28.1 (d, J = 34.1 Hz), 121.0 (t, J = 2.3 Hz), 122.3 (dd, J = 2.6, 4.2 Hz), 128.3 (d, J = 55.2 Hz), 128.6 (t, J = 9.8 Hz), 130.6 (t, J = 3.4 Hz), 131.3 (d, J = 1.9 Hz), 132.5 (d, J = 8.8 Hz), 152.4 (t, J = 4.7 Hz); 31P NMR (202 MHz, CDCl3) δ 17.5. Anal. Found: C, 74.12; H, 6.85. Calcd for C32H34B2OP2: C, 74.17; H, 6.61. 2,6-Bis[(diphenylphosphanyl)methyl]phenol (2). To a solution of 2,6-bis[(boronatediphenylphosphanyl)methyl]phenol (1.31 g, 2.5 mmol) in CH2Cl2 (10 mL) at 0 °C was added [2.2.2]-diazabicyclooctane (1.10 g, 9.8 mmol). The solution was stirred for 12 h at 50 °C. The solvent was removed under reduced pressure, and the residue was purified by column chromatography on silica gel (hexane-ethyl acetate, 7:1) under an Ar atmosphere at 0 °C to afford 2,6-bis[(diphenylphosphanyl)methyl]phenol as a white solid (966 mg, 1.97 mmol) in 78% yield: IR (NaCl) 3361, 3016, 1461, 1434, 1216, 908, 747 cm-1; 1H NMR (500 MHz, CDCl3) δ 3.44 (4H, s), 6.51 (1H, t, 7.5 Hz), 6.64 (2H, d, J = 7.5 Hz), 7.30-7.35 (12H, m), 7.38-7.42 (8H, m); 13C (125 MHz, CDCl3) δ 31.1 (d, J = 12.5 Hz), 120.6 (s, J = 2.6 Hz), 124.5 (d, J = 5.0 Hz), 128.4 (d, J = 6.8 Hz), 128.8 (t, J = 6.0 Hz), 129.1 (dd, J = 2.0 Hz, 6.3 Hz), 132.9 (d, J = 2.4 Hz), 137.6 (d, J = 12.4 Hz), 152.1 (t, J = 3.5 Hz); 31 P (202 MHz, CDCl3) δ -17.5; HRMS (ESI) m/z calcd for C32H28OP2 (M þ H) 491.1694, found 491.1689. {Rh2(1,5-cyclooctadiene)2[2,6-(Ph 2PCH 2)2 C6H 3O]}(ClO 4) (μ-O-1). To a solution of 2,6-bis[(diphenylphosphanyl)methyl]phenol (929 mg, 1.89 mmol) in THF (20 mL) at 0 °C were added sodium hydride (54 mg, 2.27 mmol) and [RhCl(cod)]2 (932 mg, 1.89 mmol). The solution was stirred for 4 h at room temperature. To the solution was added AgClO4 (431 mg, 2.08 mmol), and the reaction mixture was stirred for 1 h. Then the solution was filtered by Celite and washed with CH2Cl2 (40 mL), and the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel (hexane-ethyl acetate, 3:1, to ethyl acetate only) to afford {Rh2(1,5-cyclooctadiene)2[2,6-(Ph2PCH2)2C6H3O]}(ClO4) as a yellow crystals in 70% yield (1.34 g, 1.3 mol): mp 201-202 °C (dec); IR (NaCl) 3019, 2401, 1445, 1437, 1212, 1102757, 669 cm-1; 1H NMR (500 MHz, CDCl3) δ 1.98-2.65 (16H, m, CH2 (cod)), 3.11 (2H, dd, J = 12.9, 15.7 Hz, CHHPPh2), 3.26 (2H, br, dCH (cod)), 3.31 (2H, br, dCH (cod)), 3.44 (2H, dd, J = 4.1, 12.9 Hz, CHHPPh2), 5.67 (2H, br, dCH (cod)), 6.22 (1H, t, J = 7.5 Hz, OC6H3), 6.32 (2H, d, J = 7.5 Hz, OC6H3), 6.62 (2H, br, dCH (cod)), 7.29-7.56 (20H, m, P(C6H5)2); 13C NMR (125 MHz, CDCl3, selected) δ 27.9 (2C, s, CH2 (cod)), 29.0 (2C, s, CH2 (cod)), 30.5 (2C, d, J = 21 Hz, CH2PPh2), 32.4 (2C, s, CH2 (cod)), 32.4 (2C, s, CH2 (cod)), 67.5 (2C, d, J = 13.5 Hz, dCH (cod)), 73.1 (2C, d, J = 14.1 Hz, dCH (cod)), 107.1 (2C, dd, J = 12.8, 21.6 Hz, dCH (cod)), 109.6 (2C, dd, J = 15.5, 15.5 Hz, dCH (cod)); 31P NMR

Zhu et al. (202 MHz, CDCl3) δ 46.2 (2P, d, J = 165 Hz, CH2PPh2). For μ-O1 3 AcOEt, Anal. Found: C, 56.61; H, 5.28. Calcd for C52H59O7ClRh2P2: C, 56.82; H, 5.41. Crystal data for μ-O-1 3 2CHCl3: C50H53Cl7O5P2Rh2, fw = 1249.83, triclinic, space group P1, a = 13.389(4) A˚, b = 13.783(4) A˚, c = 14.458(4) A˚, β = 79.395(8)°, V = 2564.1(14) A˚3, Z = 2, Dcalcd = 1.619 g/cm3, temperature -153 °C, μ(Mo KR) = 1.117 mm-1, R1=0.0308, wR2=0.0751 for 8705 reflections with I >2σ(I). 2,6-Bis[(boronatediphenylphosphanyl)methyl]bormobenzene (9).6 To a stirred solution of 2-bromo-1,3-bis(bromomethyl)benzene (2.06 g, 6.0 mmol) in toulene-H2O (60 mL, 5:1) were added phosphineborane (2.64 g, 13.2 mmol), KOH (20.0 g, 30.6 mmol), and a catalytic amount of terabutylammonium bromide (193 mg, 0.60 mmol). The solution was stirred for 20 h at room temperature. Then the organic phase was separated, and the aqueous layer was extracted with Et2O. The combined organic extracts were washed with water and dried over MgSO4. The solvent was removed under reduced pressure, and the residue was purified by column chromatography on silica gel (hexane-ethyl acetate, 8:1) to afford 2,6bis[(boronatediphenylphosphanyl)methyl]bromobenzene (2.64 g, 4.3 mmol) as white crystals in 72% yield: 1H NMR (500 MHz, CDCl3) δ 0.60-1.40 (6H, br), 3.82 (4H, d, J = 12.0 Hz), 6.96-7.02 (3H, m), 7.37-7.41 (8H, m), 7.46-7.49 (4H, m), 7.51-7.61 (8H, m); 13C NMR (125 MHz, CDCl3) δ 34.4 (d, J = 17.0 Hz), 126.2 (d, J = 2.6 Hz), 128.3 (d, J = 54.1 Hz), 128.6 (d, J = 9.7 Hz), 128.9 (t, J = 6.0 Hz), 130.6 (t, J = 3.5 Hz), 131.3 (d, J = 2.4 Hz), 132.7 (d, J = 8.9 Hz), 133.3 (dd, J = 1.9, 3.5 Hz); 31P NMR (202 MHz, CDCl3) δ 17.1. Anal. Found: C, 66.26; H, 5.94. Calcd for C32H33B2BrP2: C, 66.14; H, 5.72. 2,6-Bis[(diphenylphosphanyl)methyl]bromobenzene (10). 6 A solution of 2,6-bis[(boronatediphenylphosphanyl)methyl]bromobenzene (2.11 g, 3.6 mmol) in degassed Et2NH (30 mL) was stirred for 2 h at 50 °C. The solvent was removed under reduced pressure, and to the residue was added hexane. The resulting white solid was filtered off under Ar and dried to afford 2,6-bis[(diphenylphosphanyl)methyl]bromobenzene as a colorless oil (1.81 g, 3.3 mmol) in 91% yield: 1H NMR (500 MHz, CDCl3) δ 3.60 (4H, s), 6.55 (2H, d, J = 7.5 Hz), 6.72 (1H, t, J = 7.5 Hz), 7.31-7.45 (20H, m); 31P NMR (202 MHz, CDCl3) δ -16.0. 2,6-Bis[(diphenylthioxophosphanyl)methyl]benzenethiol (11).6 To a stirred solution of 2,6-bis[(diphenylphosphanyl)methyl]bromobenzene (1.81 g, 3.3 mmol) in THF (10 mL) was added n BuLi (2.5 mL, 1.6 M in hexane) at -78 °C. The solution was stirred for 6 h at -78 °C. To the solution was added S8 (839 mg, 3.3 mmol), and the mixture was warmed to room temperature and stirred overnight. The solution was added to a suspension of LiAlH4 (1.74 g, 46.0 mmol) in Et2O (5 mL). The suspension was stirred 4 h, and any excess LiAlH4 was filtered off. To the filtrate were added water and concentrated aqueous HCl at 0 °C, the organic phase was separated, and the aqueous layer was extracted with Et2O. The combined organic extracts were washed with water and dried over MgSO4. The solvent was removed under reduced pressure to afford 2,6-bis[(diphenylthioxophosphanyl)methyl]benzenethiol (1.53 g, 2.7 mmol) in 82% yield: 1H NMR (500 MHz, CDCl3) δ 4.35 (4H, d, J = 13.6 Hz), 6.72-6.76 (3H, m), 7.43-7.48 (8H, m), 7.51-7.54 (4H, m), 7.34-7.82 (8H, m); 31P NMR (202 MHz, CDCl3) δ 39.2. 2,6-Bis[(diphenylphosphanyl)methyl]benzenethiol (3). 6 To 2,6bis[(diphenylthioxophosphanyl)methyl]benzenethiol (640 mg, 1.12 mmol) was added nBu3P (2 mL) under Ar, and the solution was stirred for 3 h at 195 °C. The excess nBu3P was removed under reduced pressure, and to the residue was added dry degassed methanol. The resulting white solid was filtered off under Ar to afford 2,6-bis[(diphenylphosphanyl)methyl]benzenethiol (230 mg, 0.45 mmol) in 40% yield: 1H NMR (500 MHz, CDCl3) δ 3.65 (4H, s), 6.50 (2H, d, J = 7.55 Hz), 6.64 (1H, t, J = 7.55 Hz), 7.31-7.41 (20H, m); 31P NMR (202 MHz, CDCl3) δ -15.8. {Rh2(1,5-cyclooctadiene)2[2,6-(Ph2PCH2)2C6H3S]}(ClO4) (μ-S-1). To a solution of 2,6-bis[(diphenylphosphanyl)methyl]benzenethiol (77.0 mg, 0.15 mmol) in THF (5 mL) at 0 °C were added

Article sodium hydride (4.4 mg, 0.18 mmol) and [RhCl(cod)]2 (75.0 mg, 0.15 mmol). The solution was stirred for 4 h at room temperature. To the solution was added AgClO4 (35.0 mg, 0.17 mmol), and the mixture was stirred for 1 h. Then the solution was filtered by Celite and washed with THF, and the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel (hexane-ethyl acetate, 3:1, to ethyl acetate only) to afford {Rh2(1,5-cyclooctadiene)2[2,6-(Ph2PCH2)2C6H3S]}(ClO4) (57.3 mg, 0.06 mol) in 43% yield as a yellow crystal: mp 211 °C (dec); IR (ZnSe) 3055, 2881, 1571, 1481, 1432, 1087, 694 cm-1; 1H NMR (500 MHz, CDCl3) δ 2.10-2.51 (16H, m, CH2 (cod)), 3.41 (2H, br, dCH (cod)), 3.46-3.53 (2H, m, CHHPPh2), 3.72-3.77 (2H, m, CHHPPh2), 3.72-3.77 (2H, br, dCH (cod)), 5.35 (2H, br, dCH (cod)), 5.70 (2H, br, dCH (cod)), 6.69-6.72 (3H, m, SC6H3), 7.28-7.68 (20H, m, P(C6H5)2); 13C NMR (125 MHz, CDCl3, selected) δ 28.7 (2C, s, CH2 (cod)), 30.1 (2C, s, CH2 (cod)), 31.8 (2C, s, CH2 (cod)), 32.4 (2C, s, CH2 (cod)), 35.9 (2C, d, J = 27.2 Hz, CH2PPh2), 78.8 (2C, d, J = 11.3 Hz, dCH (cod)), 83.3 (2C, d, J = 12.1 Hz, dCH (cod)), 103.6 (2C, dd, J = 6.8, 12.2 Hz, dCH (cod)), 107.7 (2C, dd, J = 6.7, 10.6 Hz, dCH (cod)); 31P NMR (202 MHz, CDCl3) δ 51.0 (2P, d, J = 155 Hz, CH2PPh2). Crystal data for μ-S-1 3 CH2Cl2: C49H53Cl3O4P2Rh2S, fw =1112.08, monoclinic, space group P21/n, a = 13.427(6) A˚, b = 24.331(10) A˚, c = 14.036(6) A˚, β = 99.584(2)°, V = 4522(3) A˚3, Z = 4, Dcalcd = 1.634 g/cm3, temperature -153 °C, μ(Mo KR) = 1.070 mm-1, R1 = 0.0450, wR2 = 0.1076 for 26 570 reflections with I >2σ(I). 1-Butylnapthalene (13a).22 Under 1 atm of H2, to a toluene solution (1.5 mL) of 1-(but-3-en-1-yl)naphthalene (18.2 mg, 0.10 mmol) was added {Rh2(1,5-cyclooctadiene)2[2,6-Ph2PCH2)2C6H3O]} (ClO4) (10.1 mg, 0.01 mmol), and the mixture was stirred at room temperature for 5 h. After evaporation of the solvent, the crude products were purified by column chromatography on silica gel (hexane only) to afford 1-butylnapthalene (17.6 mg, 0.095 mmol) as a colorless oil in 96% yield: IR (NaCl) 2956, 2870, 1596, 1510, 1466, 1396, 1165, 1013 cm-1; 1H NMR (400 MHz, CDCl3) δ 0.99 (3H, t, J = 7.6 Hz), 1.48 (2H, m), 1.75 (2H, m), 3.09 (2H, t, J = 7.6 Hz), 7.34 (1H, d, J = 6.8 Hz), 7.41 (1H, t, J = 8 Hz), 7.51 (2H, m), 7.72 (1H, d, J = 8.4 Hz), 7.86 (1H, t, J = 1.2 Hz), 8.07 (1H, d, J = 8.4 Hz); 13C NMR (100 MHz, CDCl3) δ 14.0, 22.9, 33.0, 123.9, 125.3, 125.5, 125.6, 125.8, 126.4, 128.7, 131.9, 133.9, 139.0. Dimethyl[4-(naphthalen-1-yl)butyl]phenylsilane (14). To a toluene solution (1.5 mL) of 1-(but-3-en-1-yl)naphthalene (18.2 mg, 0.10 mmol) was added {Rh2(1,5-cyclooctadiene)2[2,6-(Ph2PCH2)2C6H3O]}(ClO4) (10.1 mg, 0.01 mmol) and dimethyphenylsilane (15.0 mg, 0.11 mmol), and the mixture was stirred at room temperature for 3 days. After evaporation of the solvent, the crude products were purified by colummn chromatography on silica gel (hexane only) to afford dimethyl(4-(naphthalen-1-yl)butyl)(phenyl)silane (13.0 mg, 0.04 mmol) as a colorless oil in 43% yield: IR (NaCl) 3018, 2931, 1249, 1216, 1113, 750, 668 cm-1; 1H NMR (400 MHz, CDCl3) δ 0.99 (3H, t, J = 7.6 Hz), 1.48 (2H, m), 1.75 (2H, m), 3.09 (2H, t, J = 7.6 Hz), 7.34 (1H, d, J = 6.8), 7.41 (1H, t, J= 8.0 Hz), 7.52 (2H, m), 7.72 (1H, d, J = 8.4 Hz), 7.86 (1H, d, J=7.6 Hz), 8.07 (1H, d, J = 8.4 Hz); 13C NMR (100 MHz, CDCl3) δ -3.0, 15.6, 24.1, 32.8, 34.6, 123.9, 125.3, 125.5, 125.6, 125.8, 126.3, 127.7, 128.7, 128.8, 131.9, 133.6, 133.8, 138.9, 139.5; HRMS (ESI) m/z calcd for C22H26Si (M þ H) 305.1726, found 305.1726. Polyphenylacetylene (15). 23 To a CDCl3 solution (0.5 mL) of {Rh2(1,5-cyclooctadiene)2[2,6-(Ph2PCH2)2C6H3O]}(ClO4) (10.1 mg, 0.01 mmol) was added phenylacetylene (10.2 mg, 0.10 mmol), and the mixture was stirred at room temperature for 3 h. After evaporation of the solvent, the crude products were purified by column chromatography on neutral aluminum (22) Xu, H.; Ekoue-Kovi, K.; Wolf, C. J. Org. Chem. 2008, 73, 7638. (23) Falcon, M.; Farnetti, E.; Marsich, N. J. Organomet. Chem. 2001, 629, 187.

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oxide (dichloromethane only) to afford polyphenylacetylene as a sticky brown oil (9.5 mg) in 93% yield: IR (NaCl) 3019, 1598, 1492, 1216, 1027, 756, 699 cm-1; 1H NMR (400 MHz, CDCl3) δ 5.85 (1H, s), 6.63-6.65 (2H, m), 6.94-7.01 (3H, m); 13C NMR (100 MHz, CDCl3) δ 126.7, 127.5, 127.8, 131.8, 139.3, 142.9. General Procedure of {Rh2(1,5-cyclooctadiene)2[2,6-(Ph2PCH2)2C6H3O]}(ClO4)-Catalyzed Hydrogenation. Under 1 atm of H2, to a THF solution (3 mL) of 4-bromostyrene (54.9 mg, 0.30 mmol) was added {Rh2(1,5-cyclooctadiene)2[2,6-(Ph2PCH2)2C6H3O]}(ClO4) (30.3 mg, 0.03 mmol), and the mixture was stirred at room temperature for 20 h. After evaporation of the solvent, the crude products were purified by column chromatography on silica gel (hexane only) to afford 1-bromo-4-ethylbenzene (54.4 mg, 0.29 mmol) in 98% yield. 1-Bromo-4-ethylbenzene (13b): 15c colorless oil; IR (ZnSe) 2966, 1487, 1072, 1011, 820, cm-1; 1H NMR (500 MHz, CDCl3) δ 1.22 (3H, t, J = 7.6 Hz), 2.60 (2H, q, J = 7.6 Hz), 7.07 (2H, d, J = 8.3 Hz), 7.39 (2H, d, J = 8.3 Hz); 13C NMR (125 MHz, CDCl3) δ 15.5, 28.3, 119.2, 129.6, 131.3, 143.1. 1-Ethyl-4-iodobenzene (13c):24 colorless oil; IR (ZnSe) 2964, 1646, 1483, 1400, 1061, 1007, 817, cm-1; 1H NMR (500 MHz, CDCl3) δ 1.21 (3H, t, J = 7.6 Hz), 2.59 (2H, d, J = 7.6 Hz), 6.95 (2H, d, J = 7.1 Hz), 7.59 (2H, d, J = 7.1 Hz); 13C NMR (125 MHz, CDCl3) δ 15.4, 28.4, 90.5, 130.0, 137.3, 143.8. 1-(Benzyloxy)-4-ethylbenzene (13d): 25 colorless oil; IR (ZnSe) 2962, 1610, 1508, 1234, 1174, 1024, 827, 694 cm-1; 1H NMR (500 MHz, CDCl3) δ 1.21 (3H, t, J = 7.6 Hz), 2.59 (2H, d, J = 7.6 Hz), 5.04 (1H, s), 6.90 (2H, d, J = 8.7 Hz), 7.11 (2H, d, J = 8.7 Hz), 7.30-7.33 (1H, m), 6.95 (2H, d, J = 7.1 Hz), 7.59 (2H, d, J = 7.1 Hz); 13C NMR (125 MHz, CDCl3) δ 15.9, 28.0, 70.0, 114.7, 127.5, 127.9, 128.5, 128.7, 136.7, 137.2, 156.8. 1-Ethyl-4-nitrobenzene (13e):26 colorless oil; IR (ZnSe) 2970, 2360, 1599, 1512, 1340, 1109, 849, 694 cm-1; 1H NMR (500 MHz, CDCl3) δ 1.28 (3H, t, J=7.6 Hz), 2.76 (2H, q, J=7.6 Hz), 7.35 (2H, d, J = 8.8 Hz), 8.15 (2H, d, J = 8.8 Hz); 13C NMR (125 MHz, CDCl3) δ 15.0, 28.8, 123.6, 128.6, 146.2, 152.0. 1-Ethyl-4-trimethylsilylethynylbenzene (13f): 27 colorless oil; IR (NaCl) 2963, 2156, 1607, 1504, 1250, 760, 689 cm-1; 1H NMR (400 MHz, CDCl3) δ 0.26 (9H, s), 1.23 (3H, t, J = 7.6 Hz), 2.64 (2H, q, J = 7.6 Hz), 7.13 (2H, d, J = 8.0 Hz), 7.39 (2H, d, J = 8.0 Hz); 13 C NMR (100 MHz, CDCl3) δ 0.0, 15.3, 28.8, 93.2, 105.4, 120.3, 127.7, 131.9, 144.9. Benzyl propyl carbonate (13g):28 colorless oil; IR (ZnSe) 2970, 2360, 1739, 1238, cm-1; 1H NMR (500 MHz, CDCl3) δ 0.96 (3H, t, J = 7.4 Hz), 1.70 (2H, tq, J = 6.7, 7.4 Hz), 4.11 (2H, t, J = 6.7 Hz), 5.16 (2H, s), 7.26-7.40 (5H, m); 13C NMR (125 MHz, CDCl3) δ 10.2, 22.0, 69.4, 69.7, 128.3, 128.5, 128.6, 135.3, 155.2. 4-Propoxybenzaldehyde (13h): 29 colorless oil; IR (ZnSe) 2966, 2360, 1685, 1597, 1577, 1508, 1254, 1213, 1155, 972, 829 cm-1; 1 H NMR (500 MHz, CDCl3) δ 1.58 (3H, t, J = 7.4 Hz), 1.84 (2H, tq, J = 6.6, 7.4 Hz), 4.01 (2H, t, J = 6.6 Hz), 7.00 (2H, d, J = 8.8 Hz), 7.83 (2H, d, J = 8.8 Hz), 9.88 (1H, s); 13C NMR (125 MHz, CDCl3) δ 10.4, 22.4, 69.8, 114.7, 129.7, 132.0, 164.2, 190.8.

Acknowledgment. We thank Nanyang Technological University for their generous financial support. Supporting Information Available: Representative NMR spectra and crystallographic data are available free of charge via the Internet at http://pubs.acs.org. (24) Abe, T.; Yamaji, T.; Kitamura, T. Bull. Chem. Soc. Jpn. 2003, 76, 2175. (25) Baggaley, K. H.; Fears, R.; Hindley, R. M.; Morgan, B.; Murrell, E.; Thorne, D. E. J. Med. Chem. 1977, 20, 1388. (26) Shi, M.; Cui, S.-C. Adv. Synth. Catal. 2003, 345, 1329. (27) Gottardo, C.; Kraft, T. M.; Hossain, M. S.; Zawada, P. V.; Muchall, H. M. Can. J. Chem. 2008, 86, 410. (28) Bratt, M. O.; Taylor, P. C. J. Org. Chem. 2003, 68, 5439. (29) Bertran, J. F.; Rodrı´ guez, M. Org. Magn. Reson. 1983, 21, 2.