Synthesis and Application of the First Chiral and Highly Lewis Acidic Silyl Cationic Catalyst Mogens Johannsen, ² Karl Anker Jørgensen,* ,² and Gu ¨nter Helmchen* ,‡ Center for Metal Catalyzed Reactions Department of Chemistry, Aarhus UniVersity DK-8000 Aarhus C, Denmark Organisch-Chemisches Institut der UniVersita ¨ t Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany ReceiVed March 26, 1998 Recently, great progress has been made in the field of silylium chemistry and it seems possible to prepare the long-desired silicon cation in condensed phase. 1-15 Although there is a widespread use of silicon-based Lewis acids in organic chemistry, only very little attention has been paid to the utilization of the more reactive cationic species. 16 To our knowledge, no attempts have ever been made to synthesize chiral silicon-based Lewis acids, nor has the expected high reactivity of these chiral cationic species as catalysts ever been investigated. The chemistry of chiral silicon catalysts can be of great importance to the synthetic chemist, as silicon complexes are some of the few metal complexes which catalyzes important reactions, such as addition to imines and Friedel-Crafts reactions. 17-21 Another important aspect is the application of these catalysts as chiral cationic polymerization initiators. 22 On the basis of the recent developments in silyl cation chemistry, it is now reasonable to assume that a very reactive Lewis acid catalyst can be obtained by a careful choice of anion and solvent. 3-5,14,16 This paper presents the first preparation, partial characterization, and catalytic reactions using a silylium-based chiral Lewis acid ((S)-1a,b). It appears from (S)-1a,b that the chirality is anchored in a C 2 -symmetric 2,2′-dimethyl-1,1′-binaphthyl backbone, related to the BINOL ligand. The synthesis of the chiral binaphthylic skeleton (S)-2 starts from 2-methylnaphthalene and involves a resolution step of the binaphthyl ligand. 23-25 The silylation of (S)-2 was carried out by a nucleophilic addition of the dilithiated ligand to methyltri- methoxysilane giving the methoxy substituted silane, 26 which could be converted to (S)-3 by reduction (Scheme 1). The chiral silane (S)-3 is an air and moisture stable solid which can be handled without any special precautions. Several attempts have been performed to convert (S)-3 into the corresponding silyl cationic complex. Protonolysis with various Brønsted acids were too harsh, cleaving both the Si-H and Si-C bonds. Attempts were also performed to convert the silane to the silyl chloride by heating with CuCl 2 . 27 The chiral silyl chloride was more moisture sensitive than the parent silane, and treatment with silver triflate gave the silyl triflate. According to NMR spectroscopy, this approach was more successful, although the catalyst was still more than 50% impure. The key method for the preparation of the silyl cationic species was a Corey hydride transfer 28,29 between the silyl hydride (S)-3 and trityl tetrakis(pentafluorophenyl)borate (TrTPFPB) (4) 30,31 or trityl tetrakis(3,5-bis(trifluoromethyl)- phenyl)borate (TrTFPB) (5). 32 After 5 min of reaction time, the yellow color of the trityl reagent had vanished and (S)-1a,b was formed as the only product according to NMR spectroscopy [Figure 1]. The choice of anion and solvent is crucial for the preparation and catalytical properties of (S)-1. The almost chemically inert and noncoordinating TPFPB and TFPB were chosen as they are more attractive than the triflate anions from a synthetic point of view. Not only are they more stable and easy to handle than trityl triflate but the complex formed is presumably also signifi- cantly more active than the triflate counterpart. 31,33-35 The solvent is also important, and even the relatively nonnucleophilic solvent CH 2 Cl 2 is fatal as the silyl cation immediately abstracts a chloride anion from the solvent and the catalyst is destroyed. 14 The optimal solvent is a perfectly nonnucleophilic solvent. Benzene and toluene have emerged as good candidates, although toluene has been shown to coordinate to the TMS cation in the solid. 7 The stability of the TMS cation in toluene is good; however, only trace amounts of the silyl cationic species (S)-1 were obtained in benzene and toluene. The focus were therefore turned to CH 3 - CN as the solvent. Compared to benzene, the 29 Si NMR shift of the TMS cation in CD 3 CN is at a higher field (see Table 1, entries ² Aarhus University. ‡ Organisch-Chemisches Institute der Universita ¨t. (1) Lambert, J. B.; Zhao, Y. Angew. Chem., Int. Ed. Engl. 1997, 36, 400. (2) Belzner, J. Angew. Chem., Int. Ed. Engl. 1997, 36, 1277. (3) Xie, Z.; Liston, D. J.; Jelinek, T.; Mitro, V.; Bau, R.; Reed, C. A. J. Chem. Soc., Chem. Commun. 1993, 384. (4) Lambert, J. B.; Zhang, S.; Ciro, S. M. Organometallics 1994, 13, 2430. (5) Lambert, J. B.; Zhang, S. J. Chem. Soc., Chem. Commun. 1993, 383. (6) Lambert, J. B.; Zhang, S. Science 1994, 263, 984. (7) Lambert, J. B.; Zhang, S.; Stern, C. L.; Huffman, J. C. Science 1993, 260, 1917. (8) Reed, C. A.; Xie, Z. Science 1994, 263, 985. (9) Olah, G. A.; Rasul, G.; Li, X.; Buchholz, H. A.; Sandford, G.; Prakash, G. K. S. Science 1994, 263, 983. (10) Pauling, L. Science 1994, 263, 983. (11) Schleyer, P. v. R.; Buzek, P.; Mu ¨ller, T.; Apeloig, Y.; Siehl, H.-U. Angew. Chem., Int. Ed. Engl. 1993, 32, 1471. (12) Arshadi, M.; Johnels, D.; Edlund, U.; Ottosson, C.-H.; Cremer, D. J. Am. Chem. Soc. 1996, 118, 5120. (13) Xie, Z.; Manning, J.; Reed, R. W.; Mathur, R.; Boyd, P. D. W.; Benesi, A.; Reed, C. A. J. Am. Chem. Soc. 1996, 118, 2922. (14) See also: Kira, M.; Hino, T.; Sakurai, H. J. Am. Chem. Soc. 1992, 114, 6697. (15) Reed, C. A.; Xie, Z.; Bau, R.; Benesi, A. Science 1993, 262, 402. (16) Olah, G. A.; Li, X.-Y.; Wang, Q.; Rasul, G.; Prakash, G. K. S. J. Am. Chem. Soc. 1995, 117, 8962. (17) Ishihara, K.; Funahashi, M.; Hanaki, N.; Miyata, M.; Yamamoto, H. Synlett 1994, 963. (18) Guanti, G.; Narisano, E.; Banfi, L. Tetrahedron Lett. 1987, 28, 4331. (19) Mukaiyama, T.; Akamatsu, H.; Han, J. S. Chem. Lett. 1990, 889. (20) Nogue, D.; Paugam, R.; Wartski, L. Tetrahedron Lett. 1991, 32, 1265. (21) Ikeda, K.; Achiwa, K.; Sekiya, M. Tetrahedron Lett. 1983, 24, 4707. (22) Okamoto, Y.; Nakano, T. Chem. ReV. 1994, 94, 349. (23) Maigrot, N.; Mazaleyrat, J. P.; Welvart, Z. J. Org. Chem. 1985, 50, 3916. (24) Mazaleyrat, J.-P.; Wakselman, M. J. Org. Chem. 1996, 61, 2695. (25) Adams, R.; Binder, L. O. J. Am. Chem. Soc. 1941, 63, 2773. (26) Jung, M. E.; Hogan, K. T. Tetrahedron Lett. 1988, 29, 6199. (27) Colvin, E. W. Silicon Reagents in Organic Synthesis; Academic Press: London, 1988. (28) Corey, J. Y. J. Am. Chem. Soc. 1975, 97, 3237. (29) Corey, J. Y.; West, R. J. Am. Chem. Soc. 1963, 85, 2430. (30) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245. (31) Chien, J. C. W.; Tsai, W. M.; Rausch, M. D. J. Am. Chem. Soc. 1991, 113, 8570. (32) Nishida, H.; Takada, N.; Yoshimura, M.; Sonoda, T.; Kobayashi, H. Bull. Chem. Soc. Jpn. 1984, 57, 2600. (33) Bochmann, M.; Dawson, D. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 2226. (34) Hayashi, Y.; Rohde, J. J.; Corey, E. J. J. Am. Chem. Soc. 1996, 118, 5502. (35) Vedejs, E.; Nguyen, T.; Powell, D. R.; Schrimpf, M. R. J. Chem. Soc., Chem. Commun. 1996, 2721. Scheme 1 7637 J. Am. Chem. Soc. 1998, 120, 7637-7638 S0002-7863(98)01021-X CCC: $15.00 © 1998 American Chemical Society Published on Web 07/17/1998