Juliá-Colonna Asymmetric Epoxidation in a Continuously Operated Chemzyme Membrane Reactor

Abstract

Two novel soluble polymer-bound oligo-l-leucines 2 and 5, which can be retained by a membrane reactor system, have been prepared and used as catalysts for the continuously operated asymmetric epoxidation of chalcone. The optimized batch reaction conditions yield epoxychalcone in high enantioselectivities (up to 94%) and conversions (over 99%) after 15 minutes.

Key words

enantioselective epoxidation - homogeneous catalysis - trans-chalcone epoxides - continuously-operated CMR - oligo(l-leucine)s

References

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• 7 To a stirred solution of O,O-bis(2-aminoethyl)-polyethylenglycol 20000 1 (3.05 g, 0.152 mmol, 1 equiv) in CHCl₃ (60 mL) was added a solution of l-leucine-N-carboxyanhydride (384 mg, 2.44 mmol, 16 equiv) in CHCl₃ (27 mL). The reaction mixture was stirred under nitrogen at r.t. for 20 h. After solvent evaporation under vacuum, addition of diethyl ether caused precipitation of a white solid (3.3 g)
• 9a

  4-vinylbenzylamine (0.80 g, 6.08 mmol; prepared according to the literature, see ref. [9b] ), styrene (12.7 g, 13.9 mL; 122 mmol) and AIBN (119 mg, 0.72 mmol) were dissolved in cyclohexane (15 mL). The solution was degassed three times by evaporation (with stirring) and re-pressurizing with N₂, followed by heating at 50 °C for 90 h. The reaction mixture was cooled to r.t., dropped into methanol (600 mL), and the polymer 4 (11 g) was filtered off.

• 9b Zhou W.-J. Kurth MJ. Hsieh Y.-L. Krochta JM. Macromolecules  1999,  32:  5507 
• 11a

  Continuous epoxidation of trans -chalcone using catalyst 2. The nanofiltration membrane (MPF-50 of Koch Membrane Systems, Düsseldorf, Germany. The cutoff is defined by a substance with a certain molecular weight, leading to 99.8% retention) was placed in a 10 mL chemzyme membrane reactor (CMR), and THF was transported through the reactor (20 mL/h). The mixture of polymer catalyst 2 (0.089 mmol of active centers), dissolved in of THF (6 mL), and NaOH (0.775 mmol) was stirred at r.t. for 20 min. The solution was filtered, and the filtrate was pumped into the membrane reactor using a Pharmacia P-500 pump. Afterwards, a sat. solution of urea-H₂O₂ (1.4 × 10^-4 mol/L) was pumped through the membrane reactor with a flow rate of 7 mL/h. At the same time a solution of chalcone (48 mmol/L in THF) (which was previously stirred with NaOH (0.052 mol) at r.t. for 30 min and then filtered) was pumped in the reactor (13 mL/h) using another Pharmacia P-500 pump. The product and unconverted chalcone were collected in a fraction collector (Pharmacia Frac 100) with a residence time of 30 min.

• 11b

  Continuous epoxidation of trans -chalcone using catalyst 5. The same equipment and conditions were used as described above. The following concentration of polymer 5 was used: 0.190 mmol of active centers.

8

Asymmetric epoxidation of trans -chalcone (batch reaction) . The following procedure is representative of these reactions. To a stirred solution of polymer 2 (3.89 × 10^-3 mmol; 32 mol% of active centers) in THF
(0.5 mL) was added NaOH (0.065 mmol) and chalcone (0.024 mmol). Stirring at r.t. was continued for 20 min before the addition, (over 15 min) of a sat. solution of
urea-H₂O₂ (0.213 mmol, 98% pure from Aldrich) in THF (1.5 mL; prepared using an ultrasound bath over 45 min followed by filtration).

10

To a solution of styrene/aminomethyl styrene copolymer 4 (3.51 g, 0.88 mmol, 1 equiv) in THF (115 mL) was added a solution of l-leucine-N-carboxyanhydride (829 mg, 5.28 mmol, 6 equiv) in THF (60 mL). The reaction mixture was stirred under nitrogen at r.t. for 20 h, then concentrated to ca. 20 mL and dropped into methanol (400 mL); 3.5 g of polymer was filtered off.