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A Ligand-Accelerated Chiral Lewis Acid Catalyst in Asymmetric

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A Ligand-Accelerated Chiral Lewis Acid Catalyst in Asymmetric LETTER 983 A Ligand-Accelerated Chiral Lewis Acid Catalyst in Asymmetric Michael Addition of Thiols to a,b-Unsaturated Carbonyls Sh Kobayashi,* Chikako Ogawa, Mikako Kawamura, Masaharu Sugiura Graduate School of Pharmaceutical Sciences, The University of Tokyo, CREST, Japan Science and Technology Corporation (JST), Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Received 30 January 2001 Dedicated to Professor Ryoji Noyori in recognition of his remarkable contribution to Synthetic Organic Chemistry. BnS Abstract: A novel chiral hafnium catalyst, which was readily pre- O O MXn + Ligand O O (10 mol%) pared from Hf(OTf) and chiral ligand 1, has been developed in + 4 N O BnSH NO asymmetric Michael reactions of thiols with 3-(2-alkenoyl)-2-ox- CH2Cl2, MS 4Å azolidinones, affording the corresponding adducts in high yields 2 and enantiomeric excesses. Although chiral Lewis acids are less re- active than their original Lewis acids in many cases, ligand-acceler- Table Effect of Ligands and Lewis Acids ation has been demonstrated in this asymmetric Michael addition reaction. MXn Ligand Temp (ºC) Yield (%) ee (%) Key words: Lewis acid, Michael reaction, asymmetric catalysis Yb(OTf)3 1a -45 75 56 Yb(OTf)3 1b -45 82 61 Yb(OTf)3 1c -45 80 62 Ligand-acceleration is essential for catalytic asymmetric Sc(OTf)3 1c -45 73 57 1 reactions. In some asymmetric oxidations and alkyla- Hf(OTf)4 1c 0 82 67 tions, chiral ligands coordinate to metals and accelerate ScCl3 1d 0 15 45 the catalyzed reactions in a highly enantioselective man- Hf(OTf)4 1d 0 82 69 ner. On the other hand, asymmetric reactions using chiral Hf(OTf)4 1e 0 76 60 Lewis acids are now popular and recognized as one of the Hf(OTf)4 1f 0 79 71 most efficient methods for the preparation of chiral mole- Hf(OTf)4 1g 0 85 69 cules, especially via enantioselective carbon-carbon Hf(OTf)4 1h 0 82 65 2 bond-forming processes. From the viewpoint of ligand- Hf(OTf)4 1i 0 72 21 acceleration, however, this is not the case with most chiral HfCl4 1d 0 68 59 Lewis acid catalysis. Chiral Lewis acids are less reactive TiCl(OTf)3 1d 0 44 0 than their original Lewis acids in many cases, because ba- TiCl4 1d 0 92 6 sic chiral ligands coordinate to Lewis acids to form chiral MgBr2 1d 0 25 25 Lewis acids, and in this coordination Lewis acids are neu- 1a: R1 = H, R2 = Ph, R3 = OH tralized to decrease Lewis acidity compared to the original R2 R2 1b: R1 = COPh, R2 = Ph, R3 = OH 3 1c: R1 = COPh, R2 = Ph, R3 = OMe Lewis acids. Development of a ligand-accelerated pro- R3 cess in chiral Lewis acid catalysis is important to realize a N 1d: R1 = COtBu, R2 = Ph, R3 = OMe 1e: R1 = COMe, R2 = Ph, R3 = OMe R1 truly efficient catalytic system. In this paper, we describe 1f: R1 = COtBu, R2 = α-Nap, R3 = OMe such examples of chiral hafnium-catalyzed asymmetric 1g: R1 = COtBu, R2 = β-Nap, R3 = OMe 1 t 2 3 Michael addition of thiols to a,b-unsaturated carbonyl 1h: R = CO Bu, R = c-C6H11, R = OMe 1 t 2 3 compounds. 1i: R = CO Bu, R = Pr, R = OMe Catalytic Michael addition of thiols to a,b-unsaturated carbonyls provides a useful route for the synthesis of chiral sulfur-containing molecules.4 Bearing in mind the Hf(OTf)4. While the desired Michael adduct was obtained ligand-accelerated chiral Lewis acids, we examined suit- in 58% yield in the addition of benzyl mercaptan to 2 in able combinations of Lewis acids and chiral ligands (1) in the presence of 10 mol% of Hf(OTf)4 and MS 4 Å in the asymmetric Michael reaction of 3-crotonoyl-2-oxazo- dichloromethane (CH2Cl2) at 0 °C, 82% yield of the ad- lidinone (2) with benzyl mercaptan. Several Lewis acids duct was obtained using the chiral hafnium under the and ligands were examined and the results are shown in same reaction conditions. The difference between the two the Table. Among the Lewis acids screened, hafnium catalyst systems is the presence and absence of the chiral triflate (Hf(OTf) )5 gave better results, while ytterbium ligand. This is indeed an example of ligand-accelerated 4 Lewis acid catalysis. We then carefully followed the reac- triflate (Yb(OTf)3) or scandium triflate (Sc(OTf)3) was promising.6 tion course in the presence and absence of the chiral ligand in the model Michael addition (Figure 1). It was re- During this investigation, it was found that the catalytic vealed that the initial rate constant of the chiral hafnium- activity of the chiral hafnium is higher than that of catalyzed reaction was 1.5 times larger than that of the Synlett 2001 SI, 983–985 ISSN 0936-5214 © Thieme Stuttgart · New York 984 S. Kobayashi et al. LETTER achiral reaction. Although the value is not very satisfacto- ry, the origin of the ligand acceleration is interesting from the viewpoint of very rare ligand-accelerated Lewis acid catalysis as well as development of more efficient catalyt- ic systems. We performed kinetic experiments in the reac- tion of 2 with benzyl mercaptan. Rate constants (kobs) were determined over a range of catalyst concentrations (5-15 mol%), and a linear correlation between kobs vs 2 [Hf(OTf)4] was obtained, which reflected a second-order dependence on the catalyst (Figure 2). These kinetic data provide strong support for a mechanism involving catalyst activation of both the nucleophile and the electrophile7 that would explain the ligand-accelerated Lewis acid ca- talysis, although the detailed activation form is not clear 2 at this stage. While Hf(OTf) is a white solid and does not Figure 2 Plot kobs vs [Hf(OTf)4] for the Michael Reaction of 2 4 with Benzyl Mercaptan dissolve in CH2Cl2, a clear solution was obtained after 1 13 mixing with the ligand in CH2Cl2. H and C NMR anal- yses showed formation of a novel chiral hafnium Lewis acid.8 Hf(OTf)4 (10 mol%) O O R2S O O Ligand (12 mol%) + R2SH 1 N O 1 N R CH2Cl2, MS 4A R O 0 °C Table 2 Chiral Hafnium Complex-Catalyzed Asymmetric Michael Additions of Thiols R1 R2SHLigand Time (h) Yieid (%) ee (%) CH3 C2H5SH 1d 15 90 67 a,c) CH3 SH 1d 15 60 76 CH3 PhCH2SH 1d 15 82 69 CH3 PhCH2SH 1j 22 53 92 CH3 PhCH2SH 1j 48 74 88 CH3 C4H9SH 1d 15 92 71 b) CH3 C4H9SH 1j 15 80 90 c,d) CH3 CH2=CHCH2SH 1d 20 71 91 c,d) CH3 CH2=CHCH2SH 1k 20 83 94 d) C4H9 SH 1d 20 85 50 Ph SH 1d 30 70 43 a) Twenty mol% of hafnium triflate was used. b) Fifteen mol% of haf- Figure 1 Plot of Isolated Yield vs Time in the Presence and Ab- c) d) sence of the Ligand for the Michael Reaction of 2 with Benzyl Mer- nium triflate was used. Thiol was added slowly over 8 h. The re- captan action was carried out at 5 °C. Ph Ph MOMO Ph Ph Several examples of the catalytic asymmetric Michael re- N OMe actions are summarized in Table 2. In the presence of a N OMe O catalytic amount of the chiral hafnium compound, aliphat- tBu O ic thiols reacted with 3-(2-alkenoyl)-2-oxazolidinones to 1j 1k afford the corresponding Michael adducts in high yields with high enantiomeric excesses. In some cases, ligands 1j and 1k gave better selectivity. Benzenethiols showed To this mixture, 2 (0.5 mmol) in CH2Cl2 (1 mL) and ben- lower selectivity under these conditions. zyl mercaptan (0.55 mmol) in CH2Cl2 (1 mL) were added A typical experimental procedure is described for the re- successively at 0 °C. After the mixture was stirred for 22 h at the same temperature, saturated aqueous NaHCO action of 2 with benzyl mercaptan: To a CH2Cl2 suspen- 3 was added to quench the reaction. After the usual work- sion (1 mL) of Hf(OTf)4 (0.05 mmol) and MS 4 Å (125 mg) was added chiral ligand 1j (0.06 mmol) at 0 °C. The up, the crude material was purified by preparative TLC to mixture was stirred for 30 min at the same temperature. afford the desired Michael adduct. The enantiomeric ex- Synlett 2001, SI, 983–985 ISSN 0936-5214 © Thieme Stuttgart · New York LETTER A Ligand-Accelerated Chiral Lewis Acid Catalyst 985 cess of the adduct was determined by HPLC analysis us- M.; Korenaga, T.; Matsumoto, Y.; Ueki, M.; Angelaud, R. ing a chiral column. The absolute configuration was Angew. Chem., Int. Ed. 2000, 112, 3532, and Ref. 1. determined to be S compared with the authentic sample.4f (4) (a) Hiemstra, H.; Wynberg, J. Am. Chem. Soc. 1981, 103, 417. (b) Suzuki, K.; Ikegawa, A.; Mukaiyama, T. Bull. Chem. Soc. In summary, a novel chiral hafnium catalyst has been de- Jpn. 1982, 55, 3277. (c) Yamashita, H.; Mukaiyama, T. Chem. veloped in asymmetric Michael reactions of thiols with Lett. 1985, 363. (d) Nishimura, K.; Ono, M.; Nagaoka, Y.; 3-(2-alkenoyl)-2-oxazolidinones. The hafnium catalyst Tomioka, K. J. Am. Chem. Soc. 1997, 119, 12974. (e) Emori, E.; Arai, T.; Sasai, H.; Shibasaki, M. J. Am. Chem. Soc. 1998, was readily prepared from Hf(OTf)4 and chiral ligand 1, and to the best of our knowledge, this is the first success- 120, 4043. (f) Kanemasa, S.; Oderaotoshi, Y.; Wada, E. J. Am. Chem. Soc. 1999, 121, 8675. (g) Saito, M.; Nakajima, M,; ful example of the use of chiral hafnium Lewis acids in Hashimoto, S.
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