Aerobic Ru-Catalyzed Epoxidations in Fluorous Biphasic System Using New Fluorous Benzimidazolic Ligands

Abstract

An efficient ruthenium catalyzed fluorous biphasic ­epoxidation of alkenes with oxygen in the presence of a pyridine-benzimidazole ligand bearing perfluorinated ponytails is described. Excellent yields and reaction rates were obtained and the fluorous phase could be recycled up to ten times without any loss of activity.

Key words

aerobic epoxidations - catalysis - ruthenium - fluorous biphasic system - benzimidazoles

References

• 1a Gerhartz W. Yamamoto YS. Kandy L. Rounsaville JF. Schulz G. Ullmann’s Encyclopedia of Industrial Chemistry   5th ed., Vol. A9:  Verlag Chemie; Weinheim: 1987.  p.531 
• 1b Rao AS. In Comprehensive Organic Synthesis   Vol. 7:  Trost BM. Fleming I. Ley SV. Pergamon; Oxford: 1991.  p.357 
• 2a De Vos D. Bein T. Chem. Commun.  1996,  917 
• 2b De Vos DE. Sels BF. Reynaers M. Subba RaoYV. Jacobs PA. Tetrahedron Lett.  1998,  39:  3221 
• 2c Wentzel BB. Gosling PA. Feiters MC. Nolte RJM. J. Chem. Soc., Dalton Trans.  1998,  2241 ; and references cited therein
• 3a Groves JT. Quinn R. J. Am. Chem. Soc.  1985,  107:  5790 
• 3b Liu C.-J. Yu W.-Y. Che C.-M. Yeung C.-H. J. Org. Chem.  1999,  64:  7365 
• 3c Cheung W.-H. Yu W.-Y. Yip W.-P. Zhu N.-Y. Che C.-M. J. Org. Chem.  2002,  67:  7716 ; and references cited therein
• 4a Horváth IT. Rabái J. Science  1994,  266:  72 
• 4b Juliette JJJ. Horváth IT. Gladysz JA. Angew. Chem., Int. Ed. Engl.  1997,  36:  1610 
• For some excellent reviews on fluorous catalysis see:
• 5a Cornils B. Angew. Chem., Int. Ed. Engl.  1997,  36:  2057 
• 5b Curran DP. Chemtracts Org. Chem.  1996,  9:  75 
• 5c Horváth IT. Acc. Chem. Res.  1998,  31:  641 
• 5d de Wolf E. van Koten G. Deelman B.-J. Chem. Soc. Rev.  1999,  28:  37 
• 5e Betzemeier B. Knochel P. Top. Curr. Chem.  1999,  206:  61 
• 5f Fish RH. Chem.-Eur. J.  1999,  5:  1677 
• 5g Hope EG. Stuart AM. J. Fluorine Chem.  1999,  100:  75 
• 5h Rocaboy C. Gladysz JA. Actualité Chimique  2000,  9:  47 
• For some examples of oxidations in FBC see:
• 6a Klement I. Lütjens H. Knochel P. Angew. Chem.  1997,  109:  1605 
• 6b Betzemeier B. Lhermitte F. Knochel P. Synlett  1999,  489 
• 6c Crich D. Neelamkavil S. J. Am. Chem. Soc.  2001,  123:  7449 
• 6d Cavazzini M. Manfredi A. Montanari F. Quici S. Pozzi G. Eur. J. Org. Chem.  2001,  24:  4639 
• 6e ten Brink G.-J. Vis JM. Arends IWCE. Sheldon RA. Tetrahedron  2002,  58:  3977 
• 6f Legros J. Crousse B. Bonnet-Delpon D. Bégué J.-P. Tetrahedron  2002,  58:  3993 
• 6g Ragagnin G. Betzemeier B. Quici S. Knochel P. Tetrahedron  2002,  58:  3985 
• 7a Pozzi G. Montanari F. Quici S. Chem. Commun.  1997,  69 
• 7b Yamada T. Takai T. Rholde O. Mukaiyama T. Chem. Lett.  1991,  1 
• 7c Yamada T. Takai T. Rholde O. Mukaiyama T. Chem. Lett.  1991,  5 
• 8a Höger S. Bonrad K. Mourran A. Beginn U. Möller M. J. Am. Chem. Soc.  2001,  123:  5651 
• 8b Tsubata Y. Suzuki T. Miyashi T. Yamashita Y. J. Org. Chem.  1992,  57:  6749 
• 9 Singh MP. Sasmal S. Lu W. Chatterjee MN. Synthesis  2000,  1380 

10

Main isomer 7: 1-5% of the 5,7-diiodo isomer was also detected.

11

Analytical data for C₂₉H₉N₃F₃₄ (1): ¹H NMR (400 MHz, CDCl₃): δ = 8.73 (d, J = 4 Hz, 1 H), 8.51 (d, J = 7.8 Hz, 1 H), 7.89 (td, J = 7.8 and 1.6 Hz, 1 H), 7.87 (s, 1 H), 7.73 (s, 1 H), 7.42 (m, 1 H), 4.40 (s, 3 H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 148.6, 147.7, 137.2, 136.1, 136.0, 122.0-113.0(bm), 32.3, 28.7 ppm. ¹⁹F NMR (400 MHz, CDCl₃): δ = -79.7 (s, 6 F), -107.4 (m, 4 F), -120.1 (s, 8 F), -120.7 (s, 8 F), -121.6 (s, 4 F), -125.0 (s, 4 F) ppm. IR (KBr, pellets): 3440 (m), 2922 (w), 2856 (w), 1635 (w), 1208 (s), 1150 (s), 666 (w) cm^-1. MS (EI, 70 eV): m/e (rel. int.) = 1045 (100), 1025 (19), 967 (1), 706 (6), 676 (23), 626 (6), 338 (2), 307 (3), 258 (7), 168 (1). HRMS: calcd 1045.0254; found: 1045.0230. Mp 134 °C.
Analytical data for C₃₃H₁₇N₃F₃₄ (1a): ¹H NMR (400 MHz, CDCl₃): δ = 8.68 (d, J = 3.6 Hz, 1 H), 8.37 (d, J = 6.0 Hz, 1 H), 7.84 (td, J = 5.4 and 0.9 Hz, 1 H), 7.34 (m, 1 H), 7.15 (s, 1 H), 6.98 (s, 1 H), 4.25 (s, 3 H), 3.37 (m, 2 H), 3.06 (m, 2 H), 2.73 (m, 2 H), 2.45 (m, 2 H) ppm. ¹³C NMR (75 MHz, CDCl₃): δ = 149.7, 149.6, 147.5, 139.5, 136.7, 135.8, 133.7, 130.4, 123.9, 123.8, 122.6, 121.6, 122.0-108.0 (m), 107.0, 32.7 (t), 31.8, 30.5 (t), 28.7 ppm. ¹⁹F NMR (400 MHz, CDCl₃): δ = -79.7 (s, 6 F), -113.4 (s, 4 F), -120.5 (s, 4 F), -120.6 (s, 8 F), -120.8 (s, 4 F), -121.6 (s, 4 F), -122.3 (s, 4 F), -125.0 (s, 4 F). IR (KBr, pellets): 2961 (w), 1591 (w), 1470 (w), 1204 (s), 1150 (s), 724 (w), 659 (w) cm^-1. MS (EI, 70 eV): m/e (rel. int.) = 1101 (100), 1082 (24), 732 (55), 682 (23), 668 (25), 355 (2), 334 (3), 281 (4), 207 (3). HRMS: calcd.1101.088; found: 1101.044. Mp 104 °C.

12

Typical Procedure: Preparation of cyclooctene oxide (3a):
A 50 mL Schlenk tube, equipped with a stirrer and a O₂-inlet was charged with the fluorous benzimidazole 1 (52.0 mg, 50 µmol, 2 mol%) dissolved in perfluorooctyl bromide (2.5 mL), and RuCl₃·xH₂O (36% Ru, 7 mg, 25 µmol, 1 mol%) dissolved in a few drops of acetone leading to a red solution. After stirring for 0.5 h, a solution of cis-cyclooctene (220 mg, 2.0 mmol) and i-PrCHO (288 mg, 4.0 mmol, 2 equiv) in chlorobenzene (2 mL) was added. The biphasic reaction mixture was stirred at 40 °C while a gentle stream of oxygen from a balloon was passing. The color of the reaction mixture changes from red to deep blue within 15 min. At the end of the reaction, the mixture was cooled to 10 °C, the organic layer was decanted and the fluorous phase was washed with chlorobenzene (4 × 2 mL). The chlorobenzene is removed in vacuo and the residue diluted with CH₂Cl₂ (20 mL). The organic phase is treated with cold NaOH (0.1 M, 20 mL) and washed with brine. After drying (MgSO₄), filtration and evaporation of the solvent in vacuo the crude product was purified by flash chromatography (eluent: Et₂O-pentane), yielding 224 mg (89%) of analytically pure cyclooctene oxide. The blue fluorous phase containing the Ru-catalyst was used directly for further reaction runs without loss of activity.