Enantioselective Aldol Reactions of Trichlorosilyl Enol Ethers Catalyzed by the Chiral Phosphine Oxide BINAPO

Abstract

The enantioselective aldol reactions of aldehydes with trichlorosilyl enol ethers catalyzed by chiral phosphine oxide ­BINAPO afforded the corresponding aldol adducts with high dia­stereo- and enantioselectivities.

aldol reactions - asymmetric catalysis - Lewis bases - phosphine oxides - stereoselectivity

References and Notes

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14

In the case of the allylation of allyltrichlorosilane catalyzed by BINAPO, a combination of i-Pr₂NEt and TBAI had a beneficial effect on the reaction rate. In this aldol reaction, the addition of TBAI promoted dehydration, thus decreasing the yield of the aldol adduct [0.25 h, 75% yield, syn/anti, 1:7.6, 44% ee(syn), 82% ee(anti)].

15

E-Enol ether (E/Z, >20:1): 96% yield, syn/anti, 1:25, 28% ee(syn), 12% ee(anti); Z-enol ether (E/Z, 1:4): 92% yield, syn/anti, 3.8:1, 31% ee(syn), 13% ee(anti).

16

Typical Procedure: To a stirred solution of (S)-BINAPO (24.6 mg, 0.038 mmol), 4-nitrobenzaldehyde (57 mg, 0.38 mmol) and i-Pr₂NEt (0.45 mmol) in CH₂Cl₂ (3.5 mL), enol ether 1a in CH₂Cl₂ (0.9 M, 0.5 mL, 0.45 mmol) was added at -78 °C under an argon atmosphere. The mixture was stirred at the same temperature for 15 min. After quenching the reaction with a sat. aq solution of NaHCO₃ (2 mL), the mixture was filtered though celite, and the aqueous layer was extracted with CH₂Cl₂ (3 × 15 mL). The organic layer was washed with brine (30 mL), dried over Na₂SO₄, and concentrated. The crude material was purified by column chromatography (silica gel, 5 g, hexane-EtOAc, 6:1) to give the adduct as a syn/anti mixture (85 mg, 90% yield). Stereoselectivities were determined by HPLC (Chiralpak AD-H, hexane-i-PrOH, 4:1).