Green recyclable supported-metal catalyst for useful organic transformations

Current Organic Synthesis

Volumn 8, Issue 2, 2011, Pages 146-171

  Ranu, Brindaban C ^a   Bhadra, Sukalyan ^a   Saha, Debasree ^a  

^a DEPARTMENT OF MATERIALS SCIENCE   (India)

Author keywords

Catalysis; Catalyst recyclability; Green procedure; Organic reactions; Supported metal

Indexed keywords

EID: 79952744479     PISSN: 15701794     EISSN: None     Source Type: Journal    
DOI: 10.2174/157017911794697312     Document Type: Article

References (94)

1. Yin, L.; Liebscher, J. Carbon-carbon coupling reactions catalyzed by heterogeneous palladium catalysts. Chem. Rev. 2007, 107, 133
2. Felpin, F. -X.; Fouquet, E. Heterogeneous multifunctional catalysts for tandem processes: an approach toward sustainability. Chem Sus Chem 2008, 1, 718;
3. Polshettiwar V.; Len, C.; Fihri, A. Silica-supported palladium sustainable catalysts for cross-coupling reactions; Coord. Chem. Rev. 2009, 253, 2599;
4. Lamblin, M.; Nassar-Hardy, L.; Hierso, J. -C.; Fouquet, E.; Felpin, F. -X. Recyclable heterogeneous palladium catalysts in pure water: sustainable developments in Suzuki, Heck, Sonogashira and Tsuji-Trost reactions. Adv. Synth. Catal. 2010, 352, 33.
5. Miyaura, N.; Yanada, K.; Suzuki, A. A new stereospecific cross-coupling by the palladium-catalyzed reaction of 1-alkenylborane with 1-alkenyl or 1-alkynyl halides. Tetrahedron Lett. 1979, 20, 3437.
6. Miyaura, N.; Suzuki, A. Palladium-catalyzed cross-coupling reactions of organoboron compounds. Chem. Rev. 1995, 95, 2457;
7. Hassan, J.; Sevignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M. Aryl-aryl bond formation one century after the discovery of the Ullmann reaction. Chem. Rev. 2002, 102, 1359;
8. Farina, V. High-turnover palladium catalysts in cross-coupling and Heck chemistry: a critical overview. Adv. Synth. Catal. 2004, 346, 1553;
9. Nakamura, I.; Yamamoto, Y. Transition-metal-catalyzed reactions in heterocyclic synthesis. Chem. Rev. 2004, 104, 2127;
10. Phan, N. T. S.; Vander Sluys, M.; Jones, C. W. On the nature of the active species in palladium catalyzed Mizoroki-Heck and Suzuki-Miyaura couplings - homogeneous or heterogeneous catalysis, a critical review. Adv. Synth. Catal. 2006, 348, 609.
11. Mora, M.; Jiménez-Sanchidrián, C.; Ruiz, J. R. Suzuki cross-coupling reactions over Pd(II)-hydrotalcite in water. J. Mol. Catal. A Chem. 2008, 285, 79.
12. Phan, N. T. S.; Styring, P. Supported phosphine-free palladium catalysts for the Suzuki-Miyaura reaction in aqueous media. Green Chem. 2008, 10, 1055.
13. Felpin, F. -X.; Fouquet, E. Efficient and practical cross-couling of arenediazonium tetrafluoroborate salts with boronic acids catalyzed by palladium(0)/Barium carbonate. Adv. Synth. Catal. 2008, 350, 863.
14. Bandini, M.; Pietrangelo, A.; Sinisi, R.; Umani-Ronchi, A.; Wolf, M. O. New electrochemically generated polymeric Pd complexes as heterogeneous catalysts for Suzuki cross-coupling reactions. Eur. J. Org. Chem. 2009, 3554.
15. Zhang, P. -P.; Zhang, X. -X.; Sun, H. -X.; Liu, R. -H.; Wang, B.; Lin, Y. -H. Pd-CNT-catalyzed ligandless and additive-free heterogeneous Suzuki-Miyaura cross-coupling of aryl bromides. Tetrahedron Lett., 2009, 50, 4445.
16. Choi, M.; Lee, D. -H.; Na, K.; Yu, B. -W.; Ryoo, R. High catalytic activity of palladium(II)-exchanged mesoporous sodalite and Na A zeolite for bulky aryl coupling reactions: reusabilty under aerobic conditions. Angew. Chem. Int. Ed. 2009, 48, 3673.
17. Chattopadhyay, K.; Dey, R.; Ranu, B. C. Shape-dependent catalytic activity of copper oxide-supported Pd(0) nanoparticles for Suzuki and cyanation reactions. Tetrahedron Lett., 2009, 50, 3164.
18. Irfan, M.; Fuchs M.; Glasnov, T. N.; Kappe C. O. Microwave-assisted crosscoupling and hydrogenation chemistry by using heterogeneous transitionmetal catalyst: an evaluation of the role of selective catalyst heating; Chem. Eur. J. 2009, 15, 11608.
19. Hoshiya, N.; Isomura, N.; Shimoda, M.; Yoshikawa, H.; Yamashita, Y.; Iizuka, K.; Tsukamoto, S.; Shuto, S.; Arisawa M. Development of a recyclable and low-leaching palladium catalyst supported on sulfur-modified gallium arsenide (001) for use in Suzuki-Miyaura coupling. Chem. Catal. Chem. 2009, 1, 279.
20. Dey, R.; Sreedhar, B.; Ranu, B. C. Molecular sieves-supported palladium(II) catalyst: Suzuki coupling of chloroarenes and an easy access to useful intermediates for the synthesis of irbesartan, losartan and boscalid. Tetrahedron 2010, 66, 2301.
21. Polshettiwar, V.; Nadagouda, M. N.; Varma, R. S. The synthesis and applications of a micro-pine structured nanocatalyst. Chem. Commun., 2008, 6318;
22. Polshettiwar, V.; Baruwati, B.; Varma, R. S. Self-assembly of metal oxides into three-dimentional nanostructures: synthesis and application in catalysis. ACS Nano, 2009, 3, 728.
23. Blaser, H.-U.; Indolese, A.; Naud, F.; Nettekoven, U.; Schnyder, A. Industrial R&D on catalytic C-C and C-N coupling reactions:a personal account on goals, approaches and results; Adv. Synth. Catal. 2004, 346, 1583;
24. Zapf, A.; Beller, M. The development of efficient catalysts for palladium-catalyzed coupling reactions of aryl halides. Chem. Commun. 2005, 431;
25. Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Palladium-catalyzed cross-coupling reactions in total synthesis. Angew. Chem. 2005, 117, 4516; Angew. Chem. Int. Ed. 2005, 44, 4442;
26. Zeni, G.; Larock, R. C. Synthesis of heterocycles via palladium-catalyzed oxidative addition. Chem. Rev. 2006, 106, 4644.
27. Bernini, R.; Cacchi, S.; Fabrizi, G.; Forte, G.; Niembro, S.; Petrucci, F.; Pleixats, R.; Prastaro, A.; Sebastián, R. M.; Soler, R.; Tristany, M.; Vallribera, A. Phosphine-free perfluoro-tagged palladium nanoparticles supported on fluorous silica gel: application to the Heck reaction. Org. Lett. 2008, 10, 561;
28. Niembro, S.; Shafir, A.; Vallribera A.; Alíbes, R. Palladium nanoparticles supported on an organic-inorganic fluorinated hybrid material. Application to microwave-based Heck reaction. Org. Lett. 2008, 10, 3215.
29. Demel, J.; Sujandi Park, S. -E.; Cejka, J.; Stepnicka, P. Preparation of heterogeneous catalysts supported on mesoporous molecular seives modified with various N-groups and their use in the Heck reaction. J. Mol. Catal. A Chem. 2009, 302, 28.
30. Wan, Y.; Wang, H.; Zhao, Q.; Klingstedt, M.; Terasaki, O.; Zhao, D. Ordered mesoporous Pd/Silica/Carbon as a highly active heterogeneous catalyst for coupling reaction of chlorobenzene in aqueous media. J. Am. Chem. Soc. 2009, 131, 4541.
31. Sonogashira, K. Metal-catalyzed cross-coupling reactions; Diederich, F.; Stang, P. J.; Eds.; Wiley-VCH: New York, NY, 1998; Chapter 5;
32. Sonogashira, K. Comprehensive organic synthesis; Trost, B. M.; Ed.; Pergamon: New York, NY, 1991; Vol. 3, Chapter 2.4;
33. Hundertmark, T.; Littke, A. F.; Buchwald, S. L.; Fu, G. C. Pd (Ph CN) 2Cl2/P(t-Bu)3: A versatile catalyst for sonogashira reactions of aryl bromides at room temperature. Org. Lett. 2000, 2, 1729;
34. Erdelyi, M.; Gogoll, A. Rapid homogeneous-phase sonogashira coupling reactions using controlled microwave heating. J. Org. Chem. 2001, 66, 4165.
35. Cai, M.; Sha, J.; Xu, Q. MCM-41-supported bidentate phosphine palladium(0) complex: a highly active and recyclable catalyst for the Sonogashira reaction of aryl iodides. Tetrahedron 2007, 63, 4642.
36. Komáromi, A.; Novák, Z. Efficient copper-free Sonogashira coupling of aryl chlorides with palladium on charcoal. Chem. Commun. 2008, 4968.
37. Mori, S.; Yanase, T.; Aoyagi, S.; Monguchi, Y.; Maegawa, T.; Sajiki, H. Ligand-free Sonogashira coupling reactions with heterogeneous Pd/C as the catalyst. Chem. Eur. J. 2008, 14, 6994.
38. Zhao, H.; Wang, Y.; Sha, J.; Sheng, S.; Cai, M. MCM-41-supported bidentate phosphine palladium(0) complex as an efficient catalyst for heterogeneous Stille reaction. Tetrahedron 2008, 64, 7517;
39. Cai, M.; Zheng, G.; Ding, G. The first heterogeneous carbonylative Stille coupling of organostannanes with aryl iodides catalyzed by MCM-41-supported bidentate phosphine palladium(0) complex. Green Chem. 2009, 11, 1687.
40. Ranu, B. C.; Adak, L.; Chattopadhyay, K. Hydroxyapatite supported palladium-catalyzed efficient synthesis of (E)-2-alkene-4-ynecarboxylic esters. Intense fluorescene emission of selected compounds. J. Org. Chem. 2008, 73, 5609.
41. Kantam, M. L.; Chakravarti, R.; Pal, U.; Sreedhar, B.; Bhargava, S. Nanocrystalline magnesium oxide-stabilized palladium(0): an efficient and reusable catalyst for selective reduction of nitro compounds. Adv. Synth. Catal. 2008, 350, 822.
42. Maegawa, T.; Takahashi, T.; Yoshimura, M.; Suzuka, H.; Monguchi, Y.; Sajiki, H. Development of molecular sieves-supported palladium catalyst and chemoselective hydrogenation of unsaturated bonds in the presence of nitro groups. Adv. Synth. Catal. 2009, 351, 2091.
43. Jiang, Z.; She, J.; Lin, X. Palladium on charcoal as a recyclable catalyst for C-S cross-coupling of thiols with aryl halides under ligand-free conditions. Adv. Synth. Catal. 2009, 351, 2558.
44. Hirai, Y.; Uozumi, Y. Clean synthesis of triarylamines: Buchwald-Hartwig reaction in water with amphiphilic resin supported palladium complexes. Chem. Commun. 2010, 46, 1103.
45. Batail, N.; Bendjeriou, A.; Lomberget, T.; Barret, R.; Dufaud, V.; Djakovitch, L. First heterogeneous ligand-and salt-free Larock indole synthesis. Adv. Synth. Catal. 2009, 351, 2055.
46. Polshettiwar, V.; Varma, R. S. Nanoparticle-supported and magnetically recoverable palladium (Pd) catalyst: a selective and sustainable oxidation protocol with high turnover number. Org. Biomol. Chem., 2009, 7, 37.
47. Wu, Q.; Wang, L. Immobilization of copper(II) in organic-inorganic hybrid materials: a highly efficient and reusable catalyst for the classic Ullmann reaction. Synthesis 2008, 2007.
48. Benyahya, S.; Monnier, F.; Taillefer, M.; Man, M. W. C.; Bied, C.; Ouazzani, F. Efficient and versatile sol-gel immobilized copper catalyst for Ullmann arylation of phenols. Adv. Synth. Catal. 2008, 350, 2205.
49. Bhadra, S.; Sreedhar, B.; Ranu, B. C. Recyclable heterogeneous supported copper-catalyzed coupling of thiols with aryl halides: base-controlled differential arylthiolation of bromoiodobenzenes. Adv. Synth. Catal. 2009, 351, 2369.
50. Namitharan, K.; Kumarraja, M.; Pitchumani, K. CuII-hydrotalcite as an efficient heterogeneous catalyst for Huisgen [3+2] cycloaddition. Chem. Eur. J. 2009, 15, 2755.
51. Yamaguchi, K.; Oishi, T.; Katayama, T.; Mizuno, N. A supported copper hydroxide on titanium oxide as an efficient reusable heterogeneous catalyst for 1,3-dipolar cycloaddition of organic azides to terminal alkynes. Chem. Eur. J. 2009, 15, 10464.
52. Sharghi, H.; Khalifeh, R.; Doroodmand, M. M. Copper nanoparticles on charcoal for multicomponent catalytic synthesis of 1,2,3-triazole derivatives from benzyl halides or alkyl halides, terminal alkynes and sodium azide in water as a 'green' solvent. Adv. Synth. Catal. 2009, 351, 207.
53. Xiao, B.; Hou, H.; Fana, Y. Syntheses and structural characteristics of copper(II)-organic polymers based on N-heterocyclic ligands: a study on the importance of steric factors in design of potent catalysts. J. Mol. Catal. A: Chem. 2008, 288, 42.
54. Wu, J.; Hou, H.-W.; Guo, Y.-X.; Fan, Y.-T.; Wang X. Construction of two discrete molecular high-nuclearity copper(II) complexes as heterogeneous catalysts for oxidative coupling polymerization of 2,6-dimethylphenol. Eur. J. Inorg. Chem. 2009, 2796.
55. Choudary, B. M.; Sridhar, C.; Kantam, M. L.; Venkanna, G. T.; Sreedhar, B. Design and evolution of copper apatite catalysts for N-arylation of heterocycles with chloro-and fluoroarenes. J. Am. Chem. Soc. 2005, 127, 9948.
56. Kantam, M.L.; Venkanna, G. T.; Sridhar, C.; Kumar, K. B. S. Copper fluorapatite catalyzed N-arylation of heterocycles with bromo and iodoarenes. Tetrahedron Lett. 2006, 47, 3897.
57. Kantam, M. L.; Venkanna, G. T.; Sridhar, C.; Sreedhar, B.; Choudary B. M. An efficient base-free N-arylation of imidazoles and amines with arylboronic acids using copper-exchanged fluorapatite. J. Org. Chem. 2006, 71, 9522.
58. Zaccheria, F.; Ravasio, N.; Psaro R.; Fusia A. Heterogeneous selective catalytic hydorgenation of aryl ketones to alcohols without additives. Tetrahedron Lett., 2005, 46, 3695.
59. Huang, L.; Zhu, Y.; Huo, C.; Zheng, H.; Feng, G.; Zhang, C.; Li, Y. Mechanistic insight into the heterogeneous catalytic transfer hydrogenation over Cu/Al2O3: Direct evidence for the assistant role of support. J. Mol. Catal. A Chem. 2008, 288, 109.
60. Kantam, M. L.; Venkanna, G. T.; Kumar, K. B. S.; Balasubrahmanyam, V.; Venkateswarlu, G.; Sreedhar, B. Mild and efficient allylation of aldehydes by using copper fluorapatite as catalyst. Adv. Synth. Catal. 2008, 350, 1497.
61. Li, P.; Wang, L. A highly efficient three-component coupling of aldehyde, terminal alkyne, and amine via C-H activation catalyzed by reusable immobilized copper in organic-inorganic hybrid materials under solvent-free reaction conditions. Tetrahedron 2007, 63, 5455.
62. Matsumoto, T.; Ueno, M.; Wang, N.; Kobayashi, S. Immobilization of ruthenium in organic-inorganic hybrid copolymers: a reusable heterogeneous catalyst for oxidation of alcohols with molecular oxygen. Chem. Asian J. 2008, 3, 239.
63. Yamaguchi, K.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K. Creation of monomeric Ru species on the surface of Hydroxyapatite as an efficient heterogeneous catalyst for aerobic alcohol oxidation. J. Am. Chem. Soc. 2000, 122, 7144.
64. Mondelli, C.; Ferri, D.; Baiker, A. Ruthenium at work in Ru-hydroxyapatite during the aerobic oxidation of benzyl alcohol: An in situ ATR-IR spectroscopy study. J. Catal. 2008, 258, 170.
65. Mori, K.; Tano, M.; Mizugaki, T.; Ebitani, K.; Kaneda, K. Efficient heterogeneous oxidation of organosilanes to silanols catalysed by hydroxyapatitebound Ru complex in the presence of water and molecular oxygen. New J. Chem., 2002, 26, 1536.
66. Kim, J. W.; Yamaguchi, K.; Mizuno, N. Heterogeneously catalyzed efficient oxygenation of primary amines to amides by a supported ruthenium hydroxide catalyst. Angew. Chem. Int. Ed. 2008, 47, 9249.
67. Oishi, T.; Yamaguchi, K.; Mizuno, N. Catalytic oxidative synthesis of nitriles directed from primary alcohols and ammonia. Angew. Chem. Int. Ed. 2009, 48, 6286.
68. Takasaki, M.; Motoyama, Y.; Higashi, K.; Yoon, S.-H.; Mochida, I.; Nagashima, H. Ruthenium nanoparticles on nano-level-control carbon supports as highly effective catalysts for arene hydrogenation. Chem. Asian J. 2007, 2, 1524.
69. Baruwati, B.; Polshettiwar, V.; Varma, R. S. Magnetically recoverable supported ruthenium catalyst for hydrogenation of alkynes and transfer hydrogenation of carbonyl compounds. Tetrahedron Lett. 2009, 50, 1215.
70. Polshettiwar, V.; Varma, R. S. Nanoparticle-supported and magnetically recoverable ruthenium hydroxide catalyst: efficient hydration of nitriles to amides in aqueous medium. Chem. Eur. J. 2009, 15, 1582.
71. Kantam, M. L.; Reddy, R. S.; Pal, U.; Sreedhar, B.; Bhargava, S. Transfer hydrogenation of carbonyl compounds catalyzed by ruthenium nanoparticles stabilized on nanocrystalline magnesium oxide by ionic liquids. Adv. Synth. Catal. 2008, 350, 2231.
72. Caballero, V.; Bautista, F. M.; Campelo, J. M.; Luna, D.; Luque, R.; Marinas, J. M.; Romero, A. A.; Romero, I.; Rodríguez, M.; Serrano, I.; Hidalgo, J. M.; Llobet, A. Efficient hydrogenation of alkenes using a highly active and reusable immobilised Ru complex on AlPO4. J. Mol. Catal. A Chem. 2009, 308, 41.
73. Li, L.; Shi, J.-L. A highly active and reusable heterogeneous ruthenium catalyst for olefin metathesis. Adv. Synth. Catal. 2005, 347, 1745.
74. Allen, D. P.; Wingerden, M. M. V.; Grubbs, R. H. Well-defined silicasupported olefin metathesis catalysts. Org. Lett., 2009, 11, 1261.
75. Jafarpour, L.; Heck, M.-P.; Baylon, C.; Lee, H. M.; Mioskowski, C.; Nolan, S. P. Preparation and activity of recyclable polymer-supported ruthenium olefin metathesis catalysts. Organometallics 2002, 21, 671.
76. Miura, H.; Wada, K.; Hosokawa, S.; Sai, M.; Kondo, T.; Inoue, M. A heterogeneous Ru/CeO2 catalyst effective for transfer-allylation from homoallyl alcohols to aldehydes. Chem. Commun. 2009, 4112.
77. Kim, J. W.; Yamaguchi, K.; Mizuno, N. Heterogeneously catalyzed selective N-alkylation of aromatic and heteroaromatic amines with alcohols by supported ruthenium hydroxide. J. Catal. 2009, 263, 205.
78. Oe, Y.; Uozumi, Y. Highly efficient heterogeneous aqueous Kharasch reaction with an amphiphilic resin-supported ruthenium catalyst. Adv. Synth. Catal. 2008, 350, 1771.
79. Fujita, N.; Motokura, K.; Mori, K.; Mizugaki, T.; Ebitani, K.; Jitsukawac, K; Kaneda, K. A rhodium-grafted hydrotalcite as a highly efficient heterogeneous catalyst for 1,4-addition of organoboron reagents to α,β-unsaturated carbonyl compounds. Tetrahedron Lett. 2006, 47, 5083.
80. Neatu, F.; Besnea, M.; Komvokis, V. G.; Genêt, J.-P.; Michelet, V.; Triantafyllidis, K. S.; Pârvulescu, V. I. Hydrotalcite docked Rh-TPPTS complexes as efficient catalysts for the arylation of 2-cyclohexen-1-one in neat water. Catal. Today 2008, 139, 161.
81. Fujiwara, H.; Ogasawara, Y.; Yamaguchi, K.; Mizuno, N. A one-pot synthesis of primary amines from aldoximes or aldehydes in water in the presence of a supported rhodium catalyst. Angew. Chem. Int. Ed. 2007, 46, 5202.
82. Marciniec, B.; Szubert, K.; Potrzebowski, M. J.; Kownacki, I.; Lȩszczak, K. Synthesis, characterization, and catalytic activity of a well-defined rhodium siloxide complex immoboilized on silica. Angew. Chem. Int. Ed. 2008, 47, 541.
83. Park, K. H.; Son, S.U.; Chung, Y. K. Sequential actions of palladium and cobalt nanoparticles immobilized on silica: one-pot synthesis of bicyclic enones by catalytic allylic alkylation and Pauson-Khand reaction. Org. Lett. 2002, 4, 4361.
84. Thiot, C.; Schmutz, M.; Wagner, A.; Mioskowski, C. A one-pot synthesis of (E)-disubstituted alkenes by a bimetallic [Rh-Pd]-catalyzed hydrosilylation/Hiyama cross-coupling sequence. Chem. Eur. J. 2007, 13, 8971.
85. Lipshutz, B. H.; Nihan, D. M.; Vinogradova, E.; Taft, B. R.; Boskovic, Z. V. Copper + nickel-in-charcoal (Cu-Ni/C): a bimetallic, heterogeneous catalyst for cross-couplings. Org. Lett. 2008, 10, 4279.
86. Djakovitch, L.; Rouge, P. New homogeneously and heterogeneously [Pd/Cu]-catalysed C3-alkenylation of free NH-indoles. J. Mol. Catal. A Chem. 2007, 273, 230.
87. Mihalcik, D. J.; Lin, W. Mesoporous silica nanosphere supported ruthenium catalysts for asymmetric hydrogenation. Angew. Chem. Int. Ed. 2008, 47, 6229.
88. Hu, A.; Ngo, H. L.; Lin, W. Chiral, porous, hybrid solids for highly enantioselective heterogeneous asymmetric hydrogenation of β-keto esters. Angew. Chem. Int. Ed. 2003, 42, 6000.
89. Jiang, H.; Yang, C.; Li C.; Fu, H.; Chen, H.; Li, R.; Li, X. Heterogeneous enantioselective hydrogenation of aromatic ketones catalyzed by cinchonaand phosphine-modified iridium catalysts. Angew. Chem. Int. Ed. 2008, 47, 9240.
90. Hems, W. P.; McMorn, P.; Riddel, S.; Watson, S.; Hancock, F. E.; Hutchings, G. J. Asymmetric hydrogenation using chiral Rh complexes immobilized with a new ion-exchange strategy. Org. Biomol. Chem., 2005, 3, 1547.
91. Uozumi, Y.; Matsura, Y.; Arakawa, T.; Yamada, Y. M. A. Asymmetric Suzuki-Miyaura coupling in water with a chiral palladium catalyst supported on an amphiphilic resin. Angew. Chem. Int. Ed. 2009, 48, 2708.
92. Choudary, B. M.; Chowdari, N. S.; Madhi, S.; Kantam, M. L. A trifunctional catalyst for the synthesis of chiral diols. Angew. Chem. Int. Ed. 2001, 40, 4619.
93. Uozumi, Y.; Tanaka, H.; Shibatomi, K. Asymmetric allylic amination in water catalyzed by an amphiphilic resin-supported chiral palladium complex. Org, Lett., 2004, 6, 281.
94. Lipshutz, B. H.; Frieman, B. A.; Tomaso, Jr., A. E. Copper-in-charcoal (Cu/C): heterogeneous copper-catalyzed asymmetric hydrosilylations. Angew. Chem. Int. Ed. 2006, 45, 1259.